The effects of variations in PaCO2 on brain blood flow and cardiac output in the newborn piglet

Study of the relationship between regional cerebral saturation and pCO2 changes during mechanical ventilation to evaluate modifications in cerebral perfusion in a newborn piglet model

Near-infrared spectroscopy (NIRS) could be a useful continuous, non-invasive technique for monitoring the effect of partial pressure of carbon dioxide (PaCO₂) fluctuations in the cerebral circulation during ventilation. The aim of this study was to examine the efficacy of NIRS to detect acute changes in cerebral blood flow following PaCO₂ fluctuations after confirming the autoregulation physiology in piglets. Fourteen piglets (<72 h of life) were studied. Mean arterial blood pressure, oxygen saturation, pH, glycemia, hemoglobin, electrolytes, and temperature were monitored. Eight animals were used to evaluate brain autoregulation, assessing superior cava vein Doppler as a proxy of cerebral blood flow changing mean arterial blood pressure. Another 6 animals were used to assess hypercapnia generated by decreasing ventilatory settings and complementary CO₂ through the ventilator circuit and hypocapnia due to increasing ventilatory settings. Cerebral blood flow was determined by jugular vein blood flow by Doppler and continuously monitored with NIRS. A decrease in PaCO₂ was observed after hyperventilation (47.6±2.4 to 29.0±4.9 mmHg). An increase in PaCO₂ was observed after hypoventilation (48.5±5.5 to 90.4±25.1 mmHg). A decrease in cerebral blood flow after hyperventilation (21.8±10.4 to 15.1±11.0 mL/min) and an increase after hypoventilation (23.4±8.4 to 38.3±10.5 mL/min) were detected by Doppler ultrasound. A significant correlation was found between cerebral oxygenation and Doppler-derived parameters of blood flow and PaCO₂. Although cerebral NIRS monitoring is mainly used to detect changes in regional brain oxygenation, modifications in cerebral blood flow following experimental PaCO₂ changes were detected in newborn piglets when no other important variables were modified.

Non-invasive optical neuromonitoring of the temperature-dependence of cerebral oxygen metabolism during deep hypothermic cardiopulmonary bypass in neonatal swine

Management of deep hypothermic (DH) cardiopulmonary bypass (CPB), a critical neuroprotective strategy, currently relies on non-invasive temperature to guide cerebral metabolic suppression during complex cardiac surgery in neonates. Considerable inter-subject variability in temperature response and residual metabolism may contribute to the persisting risk for postoperative neurological injury. To characterize and mitigate this variability, we assess the sufficiency of conventional nasopharyngeal temperature (NPT) guidance, and in the process, validate combined non-invasive frequency-domain diffuse optical spectroscopy (FD-DOS) and diffuse correlation spectroscopy (DCS) for direct measurement of cerebral metabolic rate of oxygen (CMRO₂). During CPB, n = 8 neonatal swine underwent cooling from normothermia to 18℃, sustained DH perfusion for 40 min, and then rewarming to simulate cardiac surgery. Continuous non-invasive and invasive measurements of intracranial temperature (ICT) and CMRO₂ were acquired. Significant hysteresis (p < 0.001) between cooling and rewarming periods in the NPT versus ICT and NPT versus CMRO₂ relationships were found. Resolution of this hysteresis in the ICT versus CMRO₂ relationship identified a crucial insufficiency of conventional NPT guidance. Non-invasive CMRO₂ temperature coefficients with respect to NPT (Q₁₀ = 2.0) and ICT (Q₁₀ = 2.5) are consistent with previous reports and provide further validation of FD-DOS/DCS CMRO₂ monitoring during DH CPB to optimize management.

Neonatal Encephalopathy: Current Management and Future Trends

It is well-documented in the literature that infants who suffer from hypoxic ischemic encephalopathy are at high risk for neurologic sequelae or even death. With the addition of therapeutic hypothermia into the treatment regimen for neonatal hypoxic ischemic encephalopathy, newborns afflicted with hypoxic ischemic encephalopathy were given the opportunity for a better outcome. Questions linger as to the most optimal treatment strategy of therapeutic hypothermia for these newborns. The goal of this article is to discuss current management strategies, as well as future trends, for infants with hypoxic ischemic encephalopathy.

Underlying mechanism of subcortical brain protection during hypoxia and reoxygenation in a sheep model - Influence of α1-adrenergic signalling

While the cerebral autoregulation sufficiently protects subcortical brain regions during hypoxia or asphyxia, the cerebral cortex is not as adequately protected, which suggests that regulation of the cerebral blood flow (CBF) is area-specific. Hypoxia was induced by inhalation of 5% oxygen, for reoxygenation 100% oxygen was used. Cortical and subcortical CBF (by laser Doppler flowmetry), blood gases, mean arterial blood pressure (MABP), heart rate and renal blood flow were constantly monitored. Low dosed urapidil was used for α1A-adrenergic receptor blockade. Western blotting was used to determine adrenergic receptor signalling mediators in brain arterioles. During hypoxia cortical CBF decreased to 72 ± 11% (mean reduction 11 ± 3%, p < 0.001) of baseline, whereas subcortical CBF increased to 168±18% (mean increase 43 ± 5%, p < 0.001). Reoxygenation led to peak CBF of 194 ± 27% in the subcortex, and restored cortical CBF. α1A-Adrenergic blockade led to minor changes in cortical CBF, but massively reduced subcortical CBF during hypoxia and reoxygenation–almost aligning CBF in both brain regions. Correlation analyses revealed that α1A-adrenergic blockade renders all CBF-responses pressure-passive during hypoxia and reoxygenation, and confirmed the necessity of α1A-adrenergic signalling for coupling of CBF-responses to oxygen saturation. Expression levels and activation state of key signalling-mediators of α1-receptors (NOSs, CREB, ERK1/2) did not differ between cortex and subcortex. The dichotomy between subcortical and cortical CBF during hypoxia and reoxygenation critically depends on α1A-adrenergic receptors, but not on differential expression of signalling-mediators: signalling through the α1A-subtype is a prerequisite for cortical/subcortical redistribution of CBF.

Carbon Dioxide Fluctuations Are Associated with Changes in Cerebral Oxygenation and Electrical Activity in Infants Born Preterm

OBJECTIVES

To evaluate the effects of acute arterial carbon dioxide partial pressure changes on cerebral oxygenation and electrical activity in infants born preterm.

STUDY DESIGN

This retrospective observational study included ventilated infants born preterm with acute fluctuations of continuous end-tidal CO₂ (etCO₂) as a surrogate marker for arterial carbon dioxide partial pressure, during the first 72 hours of life. Regional cerebral oxygen saturation and fractional tissue oxygen extraction were monitored with near-infrared spectroscopy. Brain activity was monitored with 2-channel electroencephalography. Spontaneous activity transients (SATs) rate (SATs/minute) and interval between SATs (in seconds) were calculated. Ten-minute periods were selected for analysis: before, during, and after etCO₂ fluctuations of ≥5  mm Hg.

RESULTS

Thirty-eight patients (mean ± SD gestational age of 29 ± 1.8 weeks) were included, with 60 episodes of etCO₂ increase and 70 episodes of etCO₂ decrease. During etCO₂ increases, brain oxygenation increased (regional cerebral oxygen saturation increased, fractional tissue oxygen extraction decreased; P < .01) and electrical activity decreased (SATs/minute decreased, interval between SATs increased; P < .01). All measures recovered when etCO₂ returned to baseline. During etCO₂ decreases, brain oxygenation decreased (regional cerebral oxygen saturation decreased, fractional tissue oxygen extraction decreased; P < .01) and brain activity increased (SATs/minute increased, P < .05), also with recovery after return of etCO₂ to baseline.

CONCLUSION

An acute increase in etCO₂ is associated with increased cerebral oxygenation and decreased brain activity, whereas an acute decrease is associated with decreased cerebral oxygenation and slightly increased brain activity. Combining continuous CO₂ monitoring with near-infrared spectroscopy may enable the detection of otherwise undetected fluctuations in arterial carbon dioxide partial pressure that may be harmful to the neonatal brain.

Monitoring Cerebral Oxygenation in Neonates: An Update

Cerebral oxygenation is not always reflected by systemic arterial oxygenation. Therefore, regional cerebral oxygen saturation (rScO₂) monitoring with near-infrared spectroscopy (NIRS) is of added value in neonatal intensive care. rScO₂ represents oxygen supply to the brain, while cerebral fractional tissue oxygen extraction, which is the ratio between rScO₂ and systemic arterial oxygen saturation, reflects cerebral oxygen utilization. The balance between oxygen supply and utilization provides insight in neonatal cerebral (patho-)physiology. This review highlights the potential and limitations of cerebral oxygenation monitoring with NIRS in the neonatal intensive care unit.

Continuous End-Tidal Carbon Dioxide Monitoring during Resuscitation of Asphyxiated Term Lambs

BACKGROUND

The Neonatal Resuscitation Program (NRP) recommends close monitoring of oxygenation during the resuscitation of newborns using a pulse oximeter. However, there are no guidelines for monitoring carbon dioxide (CO2) to assess ventilation. Considering that cerebral blood flow (CBF) correlates directly with PaCO2, continuous capnography monitoring of end-tidal CO2 (ETCO2) may limit fluctuations in PaCO2 and, therefore, CBF during resuscitation of asphyxiated infants.

OBJECTIVE

To evaluate whether continuous monitoring of ETCO2 with capnography during resuscitation of asphyxiated term lambs with meconium aspiration will prevent fluctuations in PaCO2 and carotid arterial blood flow (CABF).

METHODS

Fifty-four asphyxiated term lambs with meconium aspiration syndrome were mechanically ventilated from birth to 60 min of age. Ventilatory parameters were adjusted based on clinical observation (chest excursion) and frequent arterial blood gas analysis in 24 lambs (control group) and 30 lambs (capnography group) received additional continuous ETCO2 monitoring. Left CABF was monitored. We aimed to maintain PaCO2 between 35 and 50 mm Hg and ETCO2 between 30 and 45 mm Hg.

RESULTS

There was a significant correlation between ETCO2 and PaCO2 (R = 0.7, p < 0.001), between PaCO2 and carotid flow (R = 0.52, p < 0.001) and between ETCO2 and carotid flow (R = 0.5, p < 0.001). PaCO2 and CABF during the first 60 min of age showed significantly higher fluctuation in the control group compared to the capnography group.

CONCLUSION

Continuous monitoring of ETCO2 using capnography with mechanical ventilation during and after resuscitation in asphyxiated term lambs with meconium aspiration limits fluctuations in PaCO2 and CABF and may potentially limit brain injury.

The relationship between systemic hemodynamic perturbations and periventricular-intraventricular hemorrhage--a historical perspective

Periventricular-intraventricular hemorrhage (PV-IVH) remains the major cause of injury to the developing brain. Predisposing factors include a germinal matrix with an immature vasculature, a pressure passive cerebral circulation, and hemodynamic perturbations in sick premature infants. Intact cerebral autoregulation has been documented in stable premature infants; however, it functions within a limited blood pressure range and is likely to be absent in the sick hypotensive infant, which increases the risk for PV-IVH with perturbations in blood pressure. The risk for PV-IVH is markedly increased in the absence of antenatal glucocorticoid exposure in the intubated low birthweight infant <1000 g with respiratory distress syndrome; +/- other complications. Although surfactant administration reduces the severity of respiratory distress syndrome, it has not led to a reduction in PV-IVH. Early postnatal administration of indomethacin has been associated with a reduction in PV-IVH, although this has not translated into long-term neurocognitive benefits.

Pharmacotherapy for meconium aspiration

In this article we have attempted to review the current pharmacological treatment options for infants with meconium aspiration syndrome with or without persistent pulmonary hypertension. These treatments include ventilatory support, surfactant treatment and inhaled nitric oxide (INO), in addition to older and newer pharmacological treatments. These include sedatives, muscle relaxants, alkali infusion, antibiotics and the newer vasodilators. Many aspects of treatment, including ventilatory care, surfactant treatment and the use of INO, are reviewed in great detail in this issue. On the other hand, many newer pharmacological modalities of treatment described here have not been evaluated with randomized control trials. We have given an overview of these emerging therapies.

Both extremes of arterial carbon dioxide pressure and the magnitude of fluctuations in arterial carbon dioxide pressure are associated with severe intraventricular hemorrhage in preterm infants

OBJECTIVE

The goal was to test the hypothesis that extremes of PaCO2 during the first 4 days after birth are associated with severe intraventricular hemorrhage (grades 3 and 4).

METHODS

A single-center retrospective review of clinical and blood gas data in the first 4 postnatal days for 849 infants with birth weights of 401 to 1250 g was performed. The univariate and multivariate relationships of severe intraventricular hemorrhage with maximal and minimal PaCO2, PaCO2 averaged over time (time-weighted PaCO2), and measures of PaCO2 fluctuation (SD of PaCO2 and difference in PaCO2 [maximum minus minimum]) were assessed.

RESULTS

Birth weight (mean +/- SD) was 848 +/- 212 g, and the median gestational age was 26 weeks. Infants with severe intraventricular hemorrhage had higher maximal PaCO2 (median: 72 vs 59 mm Hg) and time-weighted PaCO2 (mean: 49 vs 47 mm Hg) values but lower minimal PaCO2 values (32 vs 37 mm Hg). High PaCO2, low PaCO2, SD of PaCO2, and difference in PaCO2 predicted severe intraventricular hemorrhage, but time-weighted average PaCO2 was not as predictive.

CONCLUSIONS

Both extremes and fluctuations of PaCO2 are associated with severe intraventricular hemorrhage. It may be prudent to avoid extreme hypocapnia and hypercapnia during the period of risk for intraventricular hemorrhage.

Air versus ground transportation of artificially ventilated neonates: comparative differences in selected cardiopulmonary parameters

OBJECTIVE

To assess if cardiopulmonary interventions and abnormal CO2 tension are more likely in intubated neonates transported by air versus ground.

METHODS

We reviewed the transport records of all ventilated neonates retrieved to a pediatric teaching hospital in the United States within a 12-month period. Demographic data, underlying diagnosis, pretransport and posttransport ventilation settings and blood gas data, and transport data were recorded.

RESULTS

Seventy-five intubated neonates were transported by ground (n = 43), helicopter (n = 29) and by fixed-wing aircraft (n = 3). Thirty-nine patients (52%) received interventions, including adjustments of ventilator settings (36 patients) and increase in the rate of dopamine infusion or boluses infusion (volume expanders or sodium bicarbonate) in 9 patients. There were no overt pneumothoraces, endotracheal tube complications, arrhythmias, or cardiopulmonary resuscitation en route. The posttransport blood gas analysis revealed 7 patients with hypercapnia greater than 55 mm Hg and 17 patients with hypocapnia of less than 30 mm Hg. When compared with patients with Pco2 30 to 55 mm Hg, all patients with posttransport Pco2 greater than 55 mm Hg had interventions en route (P = 0.01). No significant difference between the mode of transport and stabilization time, return time, diagnostic groups, interventions, or the occurrence of hypercapnia and hypocapnia was identified. Additional adjustments of ventilatory settings were retrospectively considered necessary in many of these patients with Pco2 greater than 55 mm Hg or less than 30 mm Hg.

CONCLUSIONS

There were no cardiopulmonary disasters (such as overt pneumothoraces, endotracheal tube complications, arrhythmias, or cardiopulmonary resuscitation en route) in the various modes of neonatal transport. Adjustments of ventilation, inotropes, and volume infusion are often required for stabilization of patients during the dynamic process of transport. When compared with ground transport, there is no increase in the risk of cardiopulmonary interventions or abnormal CO2 tension in air transport of intubated neonates. Additional adjustments of ventilatory settings were retrospectively considered necessary in many of these patients with significant hypercapnia or hypocapnia.

Meconium aspiration induces neuronal injury in piglets

AIM

Meconium aspiration-induced hypertensive lung injury, especially when connected with perinatal asphyxia, has been associated with brain damage. We aimed to determine the neuronal injury induced by pulmonary meconium contamination alone and with concurrent asphyxia.

METHODS

36 anaesthetized and ventilated newborn piglets were haemodynamically monitored for 6 h. Seven piglets without concurrent asphyxia and seven piglets with asphyxia were instilled with a bolus of human meconium intratracheally. Seven piglets had only asphyxia and 15 piglets served as controls. The brains were studied histologically.

RESULTS

Meconium aspiration did not change systemic haemodynamics acutely, while its combination with asphyxia diminished the abrupt postasphyxic systemic hypertensive peak and resulted in a transient increase in carotid artery flow, not seen after isolated asphyxia. Systemic pressure declined after 4 h in all insulted groups, but only isolated asphyxia was associated with a sustained decrease in carotid artery flow. Arterial oxygenation remained normal, except during the acute insults. Brain examination after meconium instillation indicated neuronal injury, especially in the CA3 region of the hippocampus. Asphyxia resulted in neuronal injury in the cortical, cerebellar and hippocampal hilus regions.

CONCLUSION

Severe meconium aspiration itself may result in hippocampal neuronal injury.

Hyperventilation at referring hospitals is common before transport in intubated children with neurological diseases

OBJECTIVE

To assess if cardiopulmonary complications and abnormal carbon dioxide tension are more likely in intubated children with neurological diseases undergoing transport.

METHODS

We reviewed the transport records of all ventilated children retrieved to a pediatric teaching hospital in the United States within a 12-month period.

RESULTS

Twenty-seven children were transported by ground (n = 11), helicopter (n = 10), and fixed-wing aircraft (n = 6). Adjustments of ventilator settings were made in 17 (63%). There were no pneumothoraces, endotracheal tube complications, arrhythmias, or cardiopulmonary resuscitation en route. Twelve patients (44%) had a primary neurological condition. In the neurological category, the pretransport blood gases revealed 7 patients with hyperventilation (Pco2, 20-29 mm Hg), and the posttransport blood gases showed 4 patients with hyperventilation (Pco2, 15-28 mm Hg). In the nonneurological category, hyperventilation occurred only in one patient before and another after transport. No significant difference between the mode of transport, stabilization time, return time, and the occurrence of hypercapnia and hypocapnia was identified. Patients who had a neurological condition were more likely to be hyperventilated at the referring hospitals (P = 0.007). Additional maneuvers were considered necessary in 3 of the 6 neurological patients and 2 of the 5 nonneurological patients with DeltapH greater than +/-0.1, whereas the management of all but one patient with DeltapH less than +/-0.1 was considered appropriate (DeltapH defined as the difference between posttransport and pretransport pH values).

CONCLUSION

There is no cardiopulmonary disaster in the various modes of pediatric transport. When compared with ground transport, there is no significant increase in the risk for cardiopulmonary complications or abnormal CO2 tension in air transport of intubated children. DeltapH, in conjunction with clinical data and PCO2 values, may be a simple index for evaluation of cardiopulmonary management during transport.

Risk factors for cerebral palsy in preterm infants

OBJECTIVE

To identify crucial factors that precipitate cerebral palsy by controlling confounding factors in logistic regression analyses.

DESIGN AND PATIENTS

We retrospectively investigated a cohort of all 922 infants with gestational ages of less than 34 weeks (22-33 weeks), who were admitted to our neonatal intensive care unit between 1990 and 1998. Thirty (3.7%) were diagnosed to have cerebral palsy. We analyzed the prenatal and postnatal clinical variables of the cerebral palsy cases and compared them with 150 randomly selected controls.

RESULTS

Risk factors for cerebral palsy identified in univariate analysis were: twin pregnancy, long-term ritodrine tocolysis, respiratory distress syndrome, air leak, surfactant administration, intermittent mandatory ventilation, high frequency oscillation, lowest PaCO2 levels, prolonged hypocarbia during the first 72 h of life, and postnatal steroid therapy. In a conditional multiple logistic model, long-term ritodrine tocolysis, prolonged hypocarbia and postnatal steroid therapy remained associated with an increased risk of cerebral palsy after adjustment for other antenatal and postnatal variables (OR [Odds Ratio] = 8.62, 95% CI [Confidence Interval], 2.18-33.97; OR = 7.81, 95% CI, 1.42-42.92; OR = 21.37, 95% CI, 2.01-227.29, respectively).

CONCLUSIONS

Our results suggest that long-term ritodrine tocolysis underlines the development of cerebral palsy. Further assessments of the effect of ritodrine on fetal circulation and nervous system are required. Moreover, possible alternatives to systemic postnatal steroids are needed, and carbon dioxide levels should be more strictly controlled.

Early hypocarbia of preterm infants: its relationship to periventricular leukomalacia and cerebral palsy, and its perinatal risk factors

AIM

To clarify clinical interactions between early hypocarbia, periventricular leukomalacia (PVL) and cerebral palsy of preterm infants.

METHODS

Serial measurements of PaCO2 using arterial blood samples at 3, 6, 12, 24 and 48 h of life were performed for 115 very-low-birthweight infants admitted between 1995 and 1999. Severe early hypocarbia, defined when at least two of five PaCO2 values showed 3.3 kPa or less, was observed in eight infants (hypocarbia group). Preterm PVL was diagnosed by serial ultrasonographic examinations and cranial magnetic resonance imaging, and subsequently classified into early-onset PVL diagnosed within 14 d, or late-onset PVL after 14 d. Perinatal risk factors for early hypocarbia were investigated from the mothers' records and interviews with obstetricians.

RESULTS

The average birthweight in the hypocarbia group was significantly smaller than that in the control group (p < 0.01). The occurrence of PVL in the hypocarbia group was not different from that in the control group. Early hypocarbia was significantly related to late-onset PVL (p < 0.001), but not related to early-onset PVL. The incidence of cerebral palsy in the hypocarbia group was significantly higher than that in the control group (p < 0.001). Multivariate analysis showed that both low birthweight and number of maternal previous abortions were predictive for early hypocarbia.

CONCLUSION

Early hypocarbia of preterm infants in our hospital was significantly associated with both cerebral palsy and late-onset PVL, but not with early-onset PVL. The background of the three clinical events, early hypocarbia, PVL, and cerebral palsy, may not be identical in human newborns.

Mechanisms of bilirubin toxicity: clinical implications

The basic mechanism of kernicterus and bilirubin encephalopathy has not been unequivocally determined. Much knowledge has been gained about phenomena that contribute to bilirubin neurotoxicity, and this knowledge has implications for clinical practice. Conditions that impact on blood-brain barrier function, increase brain blood flow, or impact on bilirubin metabolism, including its transport in serum, should be avoided, if possible. Such conditions include drugs and drug stabilizers that compete with bilirubin binding to albumin, or that inhibit P-glycoprotein in the blood-brain barrier, prematurity/immaturity, and clinically significant illness in the infant that involves hemolysis, respiratory and metabolic acidosis, infection, asphyxia, hypoxia and (perhaps) hyperoxia, and hyperosmolality. If these conditions are not avoidable then there should be a more aggressive approach to the treatment of hyperbilirubinemia. The limits of tolerance for hyperbilirubinemia varies among neonates and there are no tools to determine with certainty when a particular infant is approaching the danger zone. Neurological symptoms in a jaundiced infant require extreme vigilance, and, in most cases, immediate intervention.

Rescue high frequency oscillation and predictors of adverse neurodevelopmental outcome in preterm infants

BACKGROUND

High frequency oscillation (HFO) is now frequently used as rescue support, but it has been suggested that as many as one-third of survivors have abnormal neurodevelopmental findings at follow-up.

OBJECTIVE

To identify risk factors for adverse neurodevelopmental outcome at 1 and 2 years in very prematurely born patients, who, because of severe neonatal respiratory failure, had required transfer to high frequency oscillation (HFO).

METHODS

A case control study was performed. Controls were supported by conventional mechanical ventilation (CMV) only and matched to HFO infants for gestational age. At 1 and 2 years, neurodevelopmental status was assessed in both groups. Abnormal neurodevelopmental outcome was diagnosed if infants had impairment with or without disability or a Griffiths developmental quotient of at least two standard deviations below the mean.

PATIENTS

Fifty-six infants were studied, median gestation age of 28 weeks (range 23--31).

RESULTS

At 2 years of age, a greater proportion of the HFO infants compared to the controls had an abnormal outcome (p<0.05). HFO infants with an abnormal outcome compared to those with a normal outcome had poorer oxygenation prior to transfer to HFO (p=0.05), but did not have a lower initial improvement in oxygenation or longer duration of hypocarbia on HFO. Logistic regression demonstrated adverse outcomes significantly related to HFO use and gestational age in the whole study population and to gestational age in the HFO infants.

CONCLUSION

An initial response to HFO does not guarantee normal neurodevelopmental outcome. Rescue HFO in very immature infants should be used cautiously.

Permissive hypercapnia in neonates: the case of the good, the bad, and the ugly

Advances in neonatology have resulted in an increase in the absolute number of survivors with chronic lung disease (CLD), though its overall incidence has not changed. Though the single most important high-risk factor for CLD is prematurity, the focus of attention has recently changed over to minimizing the impact of other two risk factors: baro/volutrauma related to mechanical ventilation, and oxygen toxicity. Permissive hypercapnia (PHC) or controlled ventilation is a strategy that minimizes baro/volutrauma by allowing relatively high levels of arterial CO(2), provided the arterial pH does not fall below a preset minimal value. The benefits of PHC are primarily mediated by the reduction of lung stretch that occurs when tidal volumes are minimized. PHC can be a deliberate choice to restrict ventilation in order to avoid overdistention, while application of high airway pressures and large tidal volumes would permit normocapnia, or relative hypocapnia (PaCO(2), < or = 25-30 mmHg), but may result in CLD and be harmful to the developing lung. The current concept that PaCO(2) levels of 45-55 mmHg in high-risk neonates are "safe" and "well tolerated" is based on limited data. Further prospective trials are needed to study the definition, safety and efficacy of PHC in ventilated preterm and term neonates. However, designing disease/gestational-postnatal age-specific clinical trials of PHC will be difficult in neonates, given the diverse pathophysiology of their diseases and the various ventilatory modes/variables currently available. The potential benefits and adverse effects of PHC are reviewed, and its relationship to current ventilatory strategies like synchronized mechanical ventilation and high-frequency ventilation in high-risk neonates is briefly discussed.

Hypocapnia and hypercapnia in respiratory management of newborn infants

Recent experimental and clinical data demonstrate that both hypocapnia and hypercapnia during the neonatal period may result in beneficial or adverse consequences. Multiple retrospective studies report a strong association between PaCO2 levels less than 25 to 30 mm Hg and an increased incidence of cystic PVL and CP in preterm infants. Prolonged exposure to PaCO2 values less than 25 to 30 mm Hg is also associated with hearing loss in term and near-term infants. A low tidal volume strategy combined with permissive hypercapnia is potentially a strategy that could prevent lung injury. Clearly, more randomized, controlled trials are needed before this latter strategy or that of permissive hypercapnia can be recommended routinely for preterm, near-term, or term gestation infants with respiratory disorders.

Hypocarbia in preterm infants with periventricular leukomalacia: the relation between hypocarbia and mechanical ventilation

OBJECTIVE

The aim of this study was to elucidate the relationship between mechanical ventilation and hypocarbia in infants with periventricular leukomalacia (PVL).

STUDY DESIGN

Matched pair analysis was conducted for 26 infants with PVL and 26 with normal development, who were born between 27 and 32 weeks' gestational age and required mechanical ventilation. The time-averaged carbon dioxide (CO(2)) index, PaCO(2), and pH were calculated every 24 hours for samples obtained from indwelling arterial catheters within the first 72 hours of life. The time-averaged respiratory rate of the ventilator (RR), peak inspiratory pressure (PIP), mean airway pressure (MAP), and ventilator index (VI) were also determined. The time-averaged total respiratory rate (TRR) was determined by observing the movement of the chest wall. The patients' characteristics, antenatal and neonatal variables, and electroencephalographic findings were also compared.

RESULTS

The time-averaged CO(2) index was larger, the time-averaged CO(2) lower and the time-averaged pH higher in infants with PVL than in those with normal development on the third day of life. There was no significant difference in the time-averaged RR, PIP, MAP, or VI on any day. TRR was larger in the PVL group than in the control group on each day, but there was no significant difference. No significant difference was observed in the clinical characteristics or neonatal variables. Electroencephalographic abnormalities within 48 hours of life were more frequent in infants with PVL than in those with normal development.

CONCLUSION

Hypocarbia was associated with PVL because the time-averaged CO(2) index was larger and the time-averaged PaCO(2) lower in infants with PVL than in those with normal development. However, the ventilator settings were similar among the infants with and without PVL.

Cerebral blood flow and metabolism during and after prolonged hypercapnia in newborn lambs

OBJECTIVE

To study the effects of prolonged (6 hrs) hypercapnia on cerebral blood flow and cerebral metabolism in newborn lambs and to evaluate the effects on cerebral blood flow and cerebral metabolism on return to normocapnia after prolonged hypercapnia.

DESIGN

Animal studies, using the newborn lamb, with comparison to control group.

SUBJECTS

Newborn lambs of mixed breed, 1-7 days of age, were used for the study. Two groups of animals were studied: a hypercapnic group (n = 10) and a normocapnic control group (n = 5).

SETTING

Work was conducted in the research laboratories at Children's National Medical Center, Washington, DC.

INTERVENTIONS

Animals were anesthetized with pentobarbital, intubated, paralyzed, and mechanically ventilated. After baseline measurements were made, CO2 was blended into the ventilator gas until a PaCO2 of 75-80 torr (10-10.6 kPa) was obtained. Measurements were made 1 hr after the desired PaCO2 was achieved and after 6 hrs of hypercapnia. After 6 hrs of hypercapnia, the ventilator gas was returned to the baseline value, that is, normocapnia. Measurements were made 30, 60, and 90 mins after PaCO2 returned to baseline.

MEASUREMENTS

Six measurements were made during the study. For each measurement, blood samples were drawn from the sagittal sinus and brachiocephalic artery catheters and were analyzed for pH, hemoglobin concentration, oxygen saturation, and blood gas values. Cerebral blood flow (CBF) was measured by using the radiolabeled microsphere technique. Cerebral oxygen consumption, fractional oxygen extraction, and oxygen transport values were calculated at each study period.

MAIN RESULTS

Increasing PaCO2 from 37 +/- 3 torr to 78 +/- 6 torr (4.9 +/- 0.4 kPa to 10.3 +/- 0.8 kPa) for 1 hr increased CBF by 355%. After 6 hrs of PaCO2 at 78 +/- 3 torr (10.3 +/- 0.4 kPa), CBF remained 195% above baseline. At 30 mins of normocapnia, CBF had returned to baseline and remained at baseline until the conclusion of the study, a total of 90 mins of normocapnia. Cerebral oxygen consumption did not change during hypercapnia or with return to normocapnia. Oxygen transport increased 331% above baseline after 1 hr of hypercapnia and stayed 180% above baseline after 6 hrs of hypercapnia. Fractional oxygen extraction decreased by 55% at 1 hr of hypercapnia and stayed 39% below baseline at 6 hrs of hypercapnia.

CONCLUSIONS

Healthy lambs seem to tolerate undergoing hypercapnia for 6 hrs with a return to normocapnia. The return to baseline of CBF and cerebral metabolism at normocapnia seen in our study with lambs may explain why prolonged hypercapnia appears to be well tolerated in mechanically ventilated patients. If these results can be extrapolated to human subjects, our study in lambs supports evidence that patients who have undergone permissive hypercapnia seem to be neurologically unaffected.

Hypocapnia under hypotension induces apoptotic neuronal cell death in the hippocampus of newborn rabbits

We investigated the adverse effect of hypocapnia on the neonatal rabbit brain. Two-week-old Japanese white rabbits were assigned to three groups, hyperventilation (H group), ischemia (I group), or hypocapnia with ischemia (HI group) and then subjected for 1.5 h with simultaneous measurement of the mean arterial blood pressure (MABP) and intracranial Hb concentration changes. Marked reductions of PaCO2 and MABP were induced in the hyperventilation-loaded groups and the ischemia-loaded groups, respectively. The intracranial oxyhemoglobin and total Hb concentrations decreased slightly in the H group and markedly in the I and HI groups after the start of experimental protocols, although there were no statistical differences between the I and HI groups. Animals were killed at 24 h after experiments and then subjected to pathologic examination. Damaged neurons with shrunken cell bodies and nuclear changes were found on light microscopic examination, mainly in the pyramidal cell layer of the subiculum and cornu ammonis 1. The numerical density of damaged neurons was significantly higher in the HI group than those in the H or I groups (p < 0.05). These damaged neurons were positive on DNA nick end labeling. A DNA ladder was detected on electrophoresis with a DNA sample extracted from hippocampal tissue in the HI group, but not in the other two groups. On electron microscopic examination, not only condensation of the nucleus but also disruption of mitochondria and the cell membrane were detected. These results suggested that hypocapnia under hypotension might cause neuronal cell death in the hippocampus of neonatal rabbit. Not only ischemia but also a metabolic change induced by hypocapnia might contribute to this apoptotic neuronal cell damage.

Acute changes in arterial carbon dioxide tension and acid-base status and early neurologic characteristics in term infants following perinatal asphyxia

BACKGROUND

Marked acute changes in arterial carbon dioxide tension (PaCO2) and acid-base status occur in the immediate postnatal period in infants delivered in the presence,of pathologic fetal acidemia (FA) in whom the risk for hypoxic-ischemic cerebral injury is high. The cerebral vasculature is extremely sensitive to changes in PaCO2. However, the relationship between the acute changes in PaCO2 and subsequent neonatal neurologic characteristics remains unclear.

OBJECTIVES

(1) To determine the extent of the acute changes in PaCO2 and acid-base status following birth in infants delivered in the presence of pathologic FA and (2) to determine the potential relationship of the initial changes in PaCO2 and neonatal neurologic characteristics.

METHODS

PaCO2 and acid base status of cord umbilical arterial blood and initial postnatal arterial blood were studied in 73 term infants admitted to the Neonatal Intensive Care Unit. Infants were categorized in three groups: I, no FA, no respiratory support and normal neonatal neurologic examination (n = 49); II, pathologic FA (umbilical artery pH < or = 7.00, base deficit > or = 12 mEq/l), no respiratory support and normal neonatal neurologic examination (n = 17); III, FA, intubated and with evidence of hypoxic ischemic encephalopathy (HIE) including seizures (n = 7).

RESULTS

Demographic characteristics were similar among the three groups, although 5-min Apgar score < or = 5 was more common in group II (47%) and group III (100%) than in group I (4%). Umbilical arterial pH was lower in group III (6.75 +/- 0.18) vs. group II (6.90 +/- 0.09) and in group II vs. group I (6.90 +/- 0.09 vs. 7.19 +/- 0.09) (P < 0.005) and the PaCO2 was higher in group III (141 +/- 37 mmHg) vs. group II (94 +/- 22 mmHg) and in group II vs. group I (94 +/- 22 vs. 60 +/- 13 mmHg) (P < 0.05). The mean base deficit was large but comparable between groups III and II, i.e. 18 +/- 6 vs. 18 +/- 5 mEq/l, respectively, and higher than in group I infants (6 +/- 4 mEq/l) (P < 0.00). At 1 h postnatal age, the mean arterial pH had increased in all groups, i.e. 7.06 +/- 0.15 (group III), 7.25 +/- 0.09 (group II), and 7.31 +/- 0.06 (group I); however, the differences amongst the groups remained significant (P < 0.005). The mean PaCO2 decreased from 94 +/- 22 mmHg (12.5 +/- 2.9 kPa) to 30 +/- 6 mmHg (4.0 +/- 0.8 kPa) for the spontaneously ventilating group II infants and from 141 +/- 37 mmHg (18.8 +/- 4.9 kPa) to 45 +/- 14 mmHg (6.0 +/- 1.9 kPa) in the intubated group III infants (P < 0.005). A repeat PaCO2 at 2 h of age in group III infants had decreased to 29 + 2 mmHg (3.9 +/- 0.3 kPa),which was not different from the PaCO2 at 2 h in group II infants (30 +/- 8 mmHg; 4.0 +/- 1.1 kPa). No significant differences were observed for pH or base deficit at this time.

CONCLUSIONS

Marked and rapid changes in PaCO2 and pH were observed in term infants delivered in the presence of pathologic FA. Initial postnatal PaCO2 values varied significantly with the lowest values noted in those infants breathing spontaneously and who exhibited an uneventful neonatal course; higher initial postnatal values, despite mechanical ventilation, were noted in infants with HIE including seizures. Further investigation in this area is imperative in order to better define the optimal respiratory management of the neurologically at-risk infant.

Lactate attenuates neuron specific enolase elevation in newborn rats

This study was undertaken to investigate the protective role of lactate on the hypoxic brain in newborn rats. A total of 107 7-day-old Wistar rats were divided into three groups. The lactate accumulation group was given 5% oxygen and 95% nitrogen for 30 minutes. The lactate elimination group was given 5% oxygen, a concentration of 7.5% carbon dioxide, and 87.5% nitrogen for 30 minutes. The control rats were placed in room air. Lactate levels in the brain tissue were higher in the lactate accumulation group than in those of the control group (control: 1.78 +/- 0.91, lactate accumulation: 11.42 +/- 1.64 mmol/kg) and significantly decreased in the lactate elimination group (4.10 +/- 1.73 mmol/kg). Blood pH remained at the same levels in the two groups. Neuron specific enolase in the cerebrospinal fluid, which is the initial neurocyte damage marker, was significantly elevated in the lactate elimination group (control: 18.3 +/- 7.5, lactate accumulation: 18.8 +/- 7.9, lactate elimination: 63.1 +/- 61.3 ng/mL). Brain adenosine 5'-triphosphate levels were significantly decreased in the lactate elimination group. Histologic findings of the brain at 72 hours after the load revealed no abnormal changes in any of the groups examined. The authors conclude that lactate accumulation plays a protective role on the hypoxic brain in newborn rats.

Neonatal tolerance to hypoxia: a comparative-physiological approach

Newborn mammals exhibit a number of physiological reactions which differ from normal adult physiology and are often regarded as signs of immaturity. However, when looked upon from a comparative point of view, it becomes obvious that some of these 'physiological peculiarities' bear striking similarity to adaptation mechanisms known from hypoxia-tolerant animals and may thus contribute to the well-established, yet poorly understood, phenomenon of neonatal hypoxia tolerance. As the mammalian fetus lives at oxygen partial pressures corresponding to 8000 m altitude, the first line of perinatal hypoxia defense consists of long-term adaptations to limited intrauterine oxygen supply: (1) improved O2 transport by fetal acclimatization to high altitude, (2) reduced metabolic rate by hibernation-like deviation from metabolic size allometry, (3) diminished cerebral vulnerability by functional analogies to diving turtle brain, and (4) enhanced metabolic flexibility by optional repartitioning of energy supply from growth to maintenance metabolism. In the case of birth asphyxia, these background mechanisms are complemented by short-term responses to acute oxygen lack: (1) reduction of body temperature as in natural torpor, (2) reduction of heart rate and redistribution of circulation as in diving mammals, (3) reduction of respiration rate typical of 'hypoxic hypometabolism', and (4) reduction of blood pH according to the concept of 'acidotic torpidity'. Although anaerobic metabolism is improved in neonatal mammals by increased glycogen stores, reduced metabolic demands, and sustained wash-out of acid metabolites, neonatal hypoxia tolerance seems to be primarily based on the ability to maintain tissue aerobiosis as long as possible. This is even reflected by isoenzyme patterns which do not consistently favour anaerobic glycolysis and, thus, are reminiscent of the 'lactate paradox' found in high altitude adaptation. Altogether, from a biological point of view, the perinatal period appears as a source of adaptive mechanisms that can be refound, in varying combinations, in many survival strategies. From a clinical point of view, the interplay of long- and short-term mechanisms offers a novel approach to estimation of the newborn's ability to withstand temporary oxygen lack. However, most of these mechanisms are not unambiguous and, above all, not unlimited in their protective effect so that they do not release obstetricians or neonatologists from their obligation to counteract fetal or neonatal hypoxia without delay.

Correlation of flow probe determinations of common carotid artery blood flow and internal carotid artery blood flow with microsphere determinations of cerebral blood flow in piglets

We investigated whether blood flow determined by a flow probe situated on one common carotid artery provided an accurate estimation of unilateral cerebral blood flow (CBF) in piglets. In eight anesthetized, mechanically ventilated piglets, blood flow determined by an ultrasonic flow probe placed on the right common carotid artery was correlated with CBF determined by microspheres under two experimental conditions: 1) before ligation of the right external carotid artery with both the right external and internal carotid circulations intact [common carotid artery blood flow (CCABF) condition], and 2) after ligation of the right external carotid artery (ipsilateral to the flow probe) with all residual right-sided carotid artery blood flow directed through the right internal carotid artery [internal carotid artery blood flow (ICABF) condition]. The left carotid artery was not manipulated in any way in either protocol. Independent correlations of unilateral CCABF and ICABF with microsphere-determined unilateral CBF were highly significant over a 5-fold range of CBF induced by hypercarbia or hypoxia (r = 0.94 and 0.92, respectively; both p < 0.001). The slope of the correlation of unilateral CCABF versus unilateral CBF was 1.68 +/- 0.19 (SEM), suggesting that CCABF overestimated CBF by 68%. The slope of the correlation of unilateral ICABF versus unilateral CBF did not differ significantly from unity (1.06 +/- 0.15), and the y intercept did not differ significantly from zero [-1.3 +/- 5.2 (SEM) mL]. Consequently, unilateral ICABF determined by flow probe accurately reflected unilateral CBF determined by microspheres under these conditions. Flow probe assessments of CCABF and ICABF in piglets may provide information about dynamic aspects of vascular control in the cerebral circulation that has heretofore been unavailable.

Sleep state changes associated with cerebral blood volume changes in healthy term newborn infants

In order to assess the possible effects of sleep states on cerebral haemodynamics in healthy term infants, we measured cerebral oxyhaemoglobin, deoxyhaemoglobin and total haemoglobin concentration using near infrared spectroscopy. Thirty-seven sleep state changes in seventeen infants (gestational age: 37 to 41 4/7 weeks), aged between two and eight days were continuously registrated during 1-3 h. Transcutaneous PaO2, PaCO2, arterial O2 saturation and heart rate were simultaneously recorded and sleep states were clinically defined. There was a close relationship between sleep state changes and changes in total cerebral haemoglobin concentration, which increased from active to quiet sleep and decreased from quiet to active sleep. Changes in total cerebral haemoglobin were due, in the most part, to changes in the cerebral oxyhaemoglobin concentration. In conclusion, sleep states influence the cerebral haemoglobin concentration. Studies on cerebral haemodynamics should take sleep state into account in term newborn infants.

A multicenter randomized masked comparison trial of synthetic surfactant versus calf lung surfactant extract in the prevention of neonatal respiratory distress syndrome

OBJECTIVE

To compare the efficacy and safety of a synthetic surfactant (Exosurf Neonatal, Burroughs Wellcome Co) and a surfactant extract of calf lung lavage (Infasurf, IND #27,169, ONY, Inc) in the prevention of neonatal respiratory distress syndrome (RDS).

DESIGN AND SETTING

Ten-center randomized masked comparison trial.

PATIENTS

Premature infants (n = 871) <29 weeks gestational age by best obstetric estimate.

INTERVENTIONS

Infants were randomly assigned to a course of treatment with Exosurf Neonatal (n = 438) or Infasurf (n = 433) at birth, and if still intubated, at 12 and 24 hours of age. Crossover treatment was allowed within 72 hours of age if severe respiratory failure (defined as two consecutive a/A PO2 ratios </=.10) persisted after three doses of the randomized surfactant.

PRIMARY OUTCOME MEASURES

Three primary outcome measures of efficacy [the incidence of RDS; the incidence of RDS death; and the incidence of survival without bronchopulmonary dysplasia at 28 days after birth] were compared using linear regression techniques.

RESULTS

Of 871 randomized infants, 18 infants did not receive treatment with a study surfactant, and 25 infants did not meet all eligibility criteria. The primary analysis of efficacy was performed in the 846 eligible infants and analysis of safety outcomes in the 853 infants who received study surfactant. Demographic characteristics did not differ between the two treatment groups. Compared with Exosurf, Infasurf treatment resulted in a 62% decrease in the incidence of RDS (Infasurf, 16% vs Exosurf, 42%) and a 70% decrease in RDS death (Infasurf, 1.7% vs Exosurf, 5.4%) but did not increase the incidence of survival without bronchopulmonary dysplasia at 28 days. Treatment with Infasurf resulted in significant improvement in several secondary outcome measures. Infasurf-treated infants had lower average FIO2 (Infasurf, .33 [SEM] vs Exosurf, .42; difference .08; 95% confidence interval [CI], .06 to .11) and average mean airway pressure (Infasurf, 6.0 cm H2O vs Exosurf, 7.1 cm H2O; difference 1.1 cm H2O; 95% CI, .7 to 1.6 cm H2O) for the first 72 hours of life. Crossover surfactant treatment was significantly less frequent in the Infasurf compared with the Exosurf group (Infasurf, 1% vs Exosurf, 6%). Complications (bradycardia, clinical airway obstruction, and transcutaneous arterial desaturation) associated with second and third, but not initial, surfactant treatments were observed more frequently in the Infasurf treatment group. Infasurf-treated infants had significantly less air leak (</=7 days) (Infasurf, 8% vs Exosurf, 14%; adjusted relative risk [ARR] .55; 95% CI, .37 to .81). Severe intraventricular hemorrhage (IVH) (grade 3 and 4) did not differ between the two groups (Infasurf, 11.8% vs Exosurf, 8.3%; ARR 1.41; 95% CI, .94 to 2.09) but total IVH occurred more frequently in Infasurf-treated infants (Infasurf, 39.0% vs Exosurf, 29.9%; ARR, 1.30; 95% CI, 1.08 to 1.57).

CONCLUSION

Significant reductions in the incidence of RDS, the severity of early respiratory disease, the incidence of pulmonary air leaks associated with RDS, and the mortality attributable to RDS suggest that Infasurf is a more effective surfactant preparation than Exosurf Neonatal in the prophylaxis of RDS. However, Infasurf prophylaxis as used in this study was also associated with a greater risk of total but not severe IVH.

Postnatal development of the cerebral blood flow velocity response to changes in CO2 and mean arterial blood pressure in the piglet

Cerebral blood flow velocity was studied during changes (haemorrhage) in mean arterial blood pressure or P(a)CO2 in 56 (aged 0-26 days) anaesthetized and ventilated piglets. The CO2 reactivity increased with age from 6.5% kPa-1 (< 1 day) to adult levels of 25% kPa-1 for piglets over 4 days old. The mean arterial blood pressure reactivity was reduced from 1.3% mmHg-1 (< 1 day old) to 0.0%/mmHg (> 4 days old). The reactivities were similar with two different anesthetics: chloralose/urethane or pentobarbital. To validate the cerebral blood flow velocity data, both electromagnetic flow and precerebral Doppler ultrasound velocity were recorded from the same common carotid artery with extracranial branches tied off. There were no differences between the results with these two methods nor between these results and those obtained when the cerebral blood flow velocities were recorded from an intracerebral artery and the electromagnetic flowmeter recorded from the carotid artery. The vessel diameter appears stable during these interventions. In conclusion, the autoregulatory response and the reaction to P(a)CO2 appear poorly developed in the newborn piglet, but rapidly mature during the first 4 days of life.

The influence of arterial carbon dioxide on cerebral oxygenation and haemodynamics during ECMO in normoxaemic and hypoxaemic piglets

OBJECTIVE

To investigate the cerebrovascular response to changes in arterial CO2 tension during extracorporeal membrane oxygenation (ECMO) in normoxaemic and hypoxaemic piglets.

METHODS

Four groups of six anaesthetized, paralysed and mechanically ventilated piglets: group 1-normoxaemia without ECMO, group 2-ECMO after normoxaemia, group 3-hypoxaemia without ECMO, and group 4-ECMO after hypoxaemia, were exposed successively to hypercapnia and hypocapnia. Changes in cerebral concentrations of oxyhaemoglobin (cO2Hb), deoxyhaemoglobin (cHHb), (oxidized-reduced) cytochrome aa3 (cCyt.aa3) and blood volume (CBV) were continuously measured using near infrared spectrophotometry. Heart rate, arterial O2 saturation, arterial blood pressure, central venous pressure, intracranial pressure (ICP) and left common carotid artery blood flow (LCaBF) were measured simultaneously.

RESULTS

Hypercapnia resulted in increased CBV, cO2Hb and ICP in all groups, while cHHb was decreased. No changes in LCaBF were found. Hypocapnia resulted in decreased cO2Hb and increased cHHb except in group 3. LCaBF decreased in all groups except group 2. CBV decreased only in groups 2 and 4. No effect on ICP was observed in any of the groups. The other variables showed no important changes either during hypercapnia or hypocapnia. ECMO after hypoxaemia resulted in a greater response of cO2Hb and cO2Hb and cHHb during hypocapnia. The effect of hypercapnia on CBV while on ECMO was greater than without ECMO.

CONCLUSION

Since cerebrovascular reactivity to CO2 remains intact during ECMO in piglets, it is important to keep arterial CO2 tension stable and in normal range during clinical ECMO.

Postnatal maturation of the blood-brain barrier for unbound bilirubin in newborn piglets

The postnatal maturation of the blood-brain barrier (BBB) for unbound bilirubin was studied in 2-day- and 2-week-old piglets. Hyperbilirubinemia was induced by bolus infusion of bilirubin at 25 mg/kg followed by continuous infusion of 20 mg/kg/h for 3 h. During the study period, arterial blood pH and blood gas tensions, serum osmolarity, and mean arterial blood pressures were within the physiologic range. Brain bilirubin content and the brain/blood distribution ratio for bilirubin were higher in the 2-day-old than in the 2-week-old piglets. In both age groups, regional brain bilirubin concentration and brain/blood ratios were higher in subcortical regions (cerebellum and brainstem) than in the cerebral cortex. We conclude that in newborn piglets the blood-brain barrier for unbound bilirubin matures with increasing postnatal age and that irrespective of maturity the relative permeability of the BBB for bilirubin appears higher in subcortical than in cortical regions.

Pulmonary vascular resistance in infants after cardiac surgery: role of carbon dioxide and hydrogen ion

OBJECTIVE

The objective of this study was to describe the effects of altering arterial PCO2 and pH on pulmonary vascular resistance in infants after cardiopulmonary bypass for cardiac surgery.

DESIGN

Prospective study (with each patient as his or her own control).

SETTING

Cardiac intensive care unit in a children's hospital.

PATIENTS

We studied 15 infants (ages ranging from 0.4 to 15.6 months; median 5.7) who were mechanically ventilated during the immediate postoperative period after corrective cardiac surgery.

INTERVENTIONS

The study was designed to have the following three stages: In the baseline stage, the initial postoperative hemodynamic parameters were stabilized and body temperature was normalized. In the hypercarbia stage, with FIO2 at 0.40, the rate of ventilation was decreased to produce an end-tidal CO2 level of > 55 torr (> 7.3 kPa). This stage established a clinical model of increased pulmonary vascular resistance. With the minute ventilation held constant in order to maintain a constant PaCO2, the arterial pH was increased by administration of a 4-mEq/kg iv dose of sodium bicarbonate (sodium bicarbonate stage). Arterial blood gas and hemodynamic determinations were obtained after a 10-min stabilization period at each stage. Drug infusions were not altered during the study period.

MEASUREMENTS AND MAIN RESULTS

In the hypercarbia stage (stage 2), the mean PaCO2 increased from 36 +/- 5 torr (4.8 +/- 0.7 kPa) (at baseline) to 55 +/- 16 torr (7.3 +/- 2.1 kPa) (p < .01). As a result, the mean arterial pH decreased from 7.48 +/- 0.05 to 7.31 +/- 0.03 (p < .01). During this stage, the mean pulmonary arterial pressure increased from 21 +/- 6 to 30 +/- 8 mm Hg (p < .01) but the cardiac index remained unchanged (3.7 +/- 1.2 to 3.8 +/- 1.2 L/min/m2). Pulmonary vascular resistance index increased from 4.1 +/- 2.0 to 6.0 +/- 3.1 U.m2 (p < .01). After the administration of sodium bicarbonate (stage 3), the arterial pH increased to 7.44 +/- 0.06 (p < .05), while the PaCO2 was unchanged. The pulmonary vascular resistance index decreased to 3.1 +/- 1.5 U.m2 (from 6.0 +/- 3.1 U.m2; p < .01) as a result of both a decrease in mean pulmonary arterial pressure (to 26 +/- 6 mm Hg; p < .01) and a concomitant increase in cardiac index to 5.1 +/- 1.6 L/min/m2 (p < .01).

CONCLUSIONS

Increasing the arterial pH by the administration of sodium bicarbonate both lowers the pulmonary arterial pressure and increases the cardiac index, resulting in a decrease in pulmonary vascular resistance. These changes were observed without alteration in PaCO2. Metabolic alkalosis may have a role in the treatment of increased pulmonary vascular resistance in infants after cardiopulmonary bypass for cardiac surgery.

Hypocarbia and cystic periventricular leukomalacia in premature infants

One hundred sixty seven survivors among very low birthweight infants with a gestational age of less than 35 weeks have been studied prospectively. The purpose of this study was to clarify the relationship of severe prenatal and perinatal complications and hypocarbic alkalosis, defined as a carbon dioxide tension (PaCO2) of less than or equal to 2.67 kPa and a pH of 7.50 or greater during the first 24 hours of life, to cystic periventricular leukomalacia (PVL) depicted by serial cranial ultrasonographic examinations. Complications occurred in 16 infants, five of whom presented with PVL, while eight of 151 infants without complications had PVL. Twenty six of the infants had hypocarbic alkalosis, six with evidence of PVL, and seven of the 136 infants without hypocarbic alkalosis had PVL. These results suggest a significant relationship of complications and hypocarbic alkalosis to PVL. Mechanical ventilation should be managed carefully in premature infants to avoid PaCO2 of lower than 2.67 kPa.

Regional cerebral blood flow response to acute hypoxia changes with postnatal age in the rat

The quantitative autoradiographic [14C]iodoantipyrine technique was applied to measure the effects of an acute hypoxic exposure on rates of local cerebral blood flow (LCBF) in the 10 (P10)-, 14 (P14)- and 21 (P21)-day-old rat. The animals were exposed to hypoxic (7% O2/93% N2) or control gas mixtures (21% O2/79% N2) for 40 min before the initiation of the 1-min LCBF measurement. At P10, hypoxia induced a 142-415% increase in LCBF over control levels, which affected the 45 structures studied. The highest increases in LCBF were noticed in posterior midbrain and brainstem regions. These increases are in good accordance with hypoxia-induced increases in LCBF recorded during acute hypoxia exposure in both newborn and adult animals. At P14 and P21, rates of LCBF decreased with hypoxia. These decreases were significant in 23 and 21 brain regions, respectively, belonging to all systems studied. These changes in LCBF are in quite good correlation with our previous data on the effects of acute hypoxia exposure on cerebral glucose utilization but the decrease in LCBF is of higher amplitude than the one in cerebral glucose utilization translating into a relative hypoperfusion at a constant metabolic level at P14 and P21. However, arterial blood pressure was reduced by 16 mmHg and arterial pCO2 was significantly decreased at the two latter ages in hypoxic animals compared to controls. These two systemic factors, and mainly hypocapnia, are rather responsible for the cerebral hypoperfusion recorded at P14 and P21 in hypoxic rats whereas the circulatory response seems to be predominantly hypoxic at P10.

Regional blood flow in brain and peripheral tissues during acute experimental arterial subdural bleeding

The effects of a large intracranial arterial subdural bleeding on regional blood flow in the brain (rCBF) and in other body organs were studied, using a porcine model. The bleeding was produced by leading blood through a catheter from the abdominal aorta via an electronic drop recorder into the subdural compartment (SDC) over the left cerebral hemisphere. Pressures in the right lateral cerebral ventricle and in the cisterna magna were recorded along with 15 other vital parameters. Measurements of rCBF were carried out using radioactive microspheres 1) before the start of bleeding, 2) during the early bleeding phase, and 3) during the late bleeding phase. When the bleeding was initiated, the intracranial pressures rose within one minute to a level approximately 40 mmHg below the systemic arterial pressure, whilst the latter usually decreased 30-40 mmHg. In the subsequent early bleeding phase the cerebral perfusion pressure and the bleeding pressure fluctuated at a level of approximately 40 mmHg for several minutes. In the late bleeding phase, the perfusion pressure decreased maximally, even when a Cushing reaction was activated. During the early bleeding phase the changes in rCBF varied between the cerebral regions. However, the mean flow remained largely constant in the presence of a decreasing cerebrovascular resistance, indicating that autoregulation of CBF was intact. Concomitantly, cardiac output and heart rate decreased, whilst regional blood flow in extracerebral organs tended to increase, possibly due to an intracranial effect on the autonomic nervous system. In the late bleeding phase, rCBF was critically reduced in all regions, in spite of a marked rise in systemic arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of carbon dioxide on cerebral arteries

The constancy of cerebral blood flow and volume relies heavily upon the cerebral arteries' intrinsic ability to respond to changes in the partial pressure of arterial CO2. The physiologic mechanisms underlying these responses have not been determined, although changes in extracellular and intracellular pH, mediation by prostanoids and neural activity have been suggested. CO2 reactivity can be influenced by oxygen status and blood pressure and can vary according to age and brain region. In certain pathological conditions or diseases, it can be severely altered. Modern techniques, which measure CBF in cases of cerebral hemodynamic insufficiency, head injury or tumor, rely on the inherent ability of the cerebral circulation to respond to changing levels of CO2.

Cerebral haemodynamic and electrocortical CO2 reactivity in pigs anaesthetized with fentanyl, nitrous oxide and pancuronium

Cerebral haemodynamic, metabolic and electrocortical reactivity to alterations in arterial CO2 tension (PaCO2) was assessed in seven mechanically ventilated juvenile pigs to test an experimental model designed for cerebral pharmacodynamic and pharmacokinetic studies. The animals were anaesthetized with fentanyl, nitrous oxide and pancuronium and sequentially normo- and hyperventilated over a 100-min period. Five measurements were made at 25-min intervals. The cerebral blood flow (CBF) was measured with the intra-arterial 133Xe technique and the cerebral metabolic rate for oxygen (CMRO2) determined from CBF and the cerebral arteriovenous oxygen content difference. A linear correlation (r = 0.845) was found between CBF and PaCO2. The cerebrovascular reactivity to hypocapnia (delta CBF/delta PaCO2) was maintained throughout the experimental period and amounted to (95% confidence interval) 9.1 (7.1-11.1) ml x 100 g-1 x min-1 x kPa-1 within the PaCO2 range 3.3-6.3 kPa. The CMRO2 was not influenced by hyperventilation. The baseline electroencephalographic (EEG) pattern was stable at normocapnia (mean PaCO2 5.6 kPa), whereas spectral values for delta and total average voltage increased significantly (P < 0.05) at extensive hypocapnia (3.5 kPa). Maintenance of cerebral CO2 reactivity and spectral EEG voltage at a stable plasma level of fentanyl is complementary to the cerebral haemodynamic and metabolic stability previously found at sustained normocapnia in this model.

Cerebral vasoconstriction in response to hypocapnia is maintained after ischemia/reperfusion injury in newborn pigs

BACKGROUND AND PURPOSE

Hypocapnic cerebral vasoconstriction is used therapeutically to reduce elevated intracranial pressure caused by cerebral edema. Because cerebral ischemia/reperfusion injury causes a selective loss of prostanoid-dependent responses, including vasodilation to hypercapnia, we designed these experiments to examine the effect of ischemia/reperfusion on hypocapnic cerebral vasoconstriction.

METHODS

Microvascular responses were studied in 10 newborn pigs (closed cranial window) in response to hyperventilation-induced hypocapnia (PaCO2, 22 +/- 2 mm Hg) both before and 45 minutes after 20 minutes of global cerebral ischemia. Responses to hypercapnia (PaCO2, 63 +/- 3 mm Hg), topical isoproterenol (10(-7) M), and norepinephrine (10(-4) M) were also studied before and after ischemia in the same animals for comparison.

RESULTS

Before ischemia/reperfusion, pial arterioles vasoconstricted to hypocapnia (-17 +/- 2%) and norepinephrine (-35 +/- 4%) and vasodilated to CO2 (37 +/- 7%) and isoproterenol (25 +/- 2%). After ischemia/reperfusion, the constriction of pial arterioles to hypocapnia (-19 +/- 2%) was similar to that before ischemia. This is in contrast to the loss of dilation to hypercapnia. Dilation to isoproterenol and constriction to norepinephrine were not affected by ischemia.

CONCLUSIONS

Hypocapnic cerebral vasoconstriction is maintained after ischemia/reperfusion. Since prostanoid-dependent responses, such as hypercapnic dilation, are lost following cerebral ischemia, these data suggest that hypocapnic constriction is not dependent on an intact prostanoid system and that cerebral vascular responses to CO2 involve multiple mechanisms, depending on whether CO2 is increasing or decreasing from baseline.

The effects of hypoxia on the ventilatory response to sudden changes in CO2 in newborn piglets

1. The ventilatory response to square-wave challenges in end-tidal partial pressure of CO2 (PCO2) was investigated at three levels of arterial PO2 (Pa,O2) in nineteen anaesthetized 2- to 11-day-old piglets. 2. The ventilatory responses, measured on a breath-to-breath basis, were separated into a peripheral and a central component using a two-compartment model. Both components were described by a CO2 sensitivity, a time constant, a time delay and a single offset. 3. Fifty-six responses were analysed against a background of normoxaemia (Pa,O2 = 12.70 +/- 0.72 kPa, mean +/- S.D.), fifty-three against a background of moderate hypoxaemia (Pa,O2 = 8.63 +/- 0.34 kPa) and fifty-one against a background of severe hypoxaemia (Pa,O2 = 4.98 +/- 0.30 kPa). 4. The sensitivity of the peripheral chemoreceptors in mediating the response to CO2 increased from 38.3 +/- 17.0 ml min-1 kPa-1 kg-1 during normoxaemia to 48.8 +/- 15.3 ml min-1 kPa-1 kg-1 during moderate hypoxaemia and to 72.9 +/- 24.0 ml min-1 kPa-1 kg-1 at severe hypoxaemia. 5. As compared with the central CO2 sensitivity during moderate hypoxaemia and normoxaemia (104.0 +/- 39.0 and 100.8 +/- 41.6 ml min-1 kPa-1 kg-1, respectively) it decreased to 85.9 +/- 54.1 ml min-1 kPa-1 kg-1 at severe hypoxaemia. 6. We conclude that in newborn piglets there is a positive interaction between hypoxia and hypercapnia at the level of the peripheral chemoreceptors while severe hypoxaemia reduced the CO2 sensitivity centrally.

Effect of 2-chloroadenosine on cerebrovascular reactivity to hypercapnia in newborn pig

The effect of local administration of vasodilative concentrations of the adenosine receptor agonist 2-chloroadenosine (2-CADO) on the hyperemic responses of the pial and parenchymal microcirculations to graded hypercapnia was determined. The cranial window and brain microdialysis-hydrogen clearance techniques were utilized in two groups of isoflurane-anesthetized newborn pigs to measure changes in pial diameters and local CBF, respectively, in response to graded hypercapnia in the absence and presence of 2-CADO. Progressive size-dependent dilations of pial arterioles [small = 41 +/- 7 microns (mean +/- SD), intermediate = 78 +/- 13 microns, and large = 176 +/- 57 microns in diameter] occurred in response to graded hypercapnia alone (PaCO2 = 58 and 98 mm Hg) and to superfusions of 2-CADO (10(-5) M) during normocapnia; the magnitude of the dilative response to each of these stimuli was inversely proportional to vessel size. When hypercapnia was induced concomitantly with 2-CADO superfusion, the dilative effects of each stimulus were directly additive. Similarly, local microdialysis infusion of 10(-5) M 2-CADO, which doubled CBF during normocapnia, did not affect the hyperemic response of the parenchymal circulation to graded hypercapnia (PaCO2 = 69 and 101 mm Hg). Our findings are consistent with the participation of adenosine in the mediation of cerebral hypercapnic hyperemia. If, however, adenosine is not involved in this dilative response, our results indicate that concomitant vascular and neuromodulatory actions induced by adenosine receptor stimulation do not affect the mechanism responsible for the hypercapnic hyperemic response.

The effect of CO2- and O2-gas mixtures on laser Doppler measured cochlear and skin blood flow in guinea pigs

The effects of carbogen (5% CO2: 95% O2) 10% CO2-in-air and 100% O2 on cochlear blood flow (CBF), skin blood flow (SBP), blood pressure (BP) and arterial blood gases were investigated in the anesthetized, respired or self-respiring guinea pig. In respired animals, CBF and SBF were increased with carbogen and 10% CO2-in-air and decreased with O2. BP was elevated with each gas. In freely breathing animals, only 10% CO2-in-air caused a small increase in CBF; both carbogen and O2 caused CBF to decrease. SPF changes were similar in form, but larger than those seen in respirated subjects. No consistent change in BP was seen during breathing of these mixtures. Arterial PO2 was increased by carbogen and 10% CO2-in-air for both groups. PCO2 increased for both CO2 gas mixtures during forced respiration; but in free-breathing animals PCO2 only increased for 10% CO2-in-air (normal PCO2 values were maintained with carbogen thorough increased breathing rate). The observed changes in CBF were consistent with a balance between a combined vasoconstrictive effect of PO2 and vasodilation effect of PCO2 on cochlear vessels. Analysis of cochlear vascular conductivity (CBF/BP) indicated that vasodilation was significant only with 10% CO2-in-air in respirated animals. In all other conditions the increased CBF apparently reflects the increase profusion pressure associated with respiration of each gas. For clinical purposes, while carbogen does not appear to directly cause vasodilation of cochlear vessels it does lead to an increased oxygenation of the cochlea blood and would appear to avoid the cochlear vasoconstriction caused by 100% O2.

Local cerebral blood flow response to locally infused 2-chloroadenosine during hypotension in piglets

Brain interstitial adenosine increases during hypotension in piglets. If adenosine is to participate in the regulation of neonatal cerebral blood flow (CBF) during hypotension, it must retain its vasodilatory action under that condition. To examine this issue, we studied the effects of locally infused 2-chloroadenosine (2-CADO), a stable adenosine analog, on local CBF in the piglet frontal cortex during normotension and graded hemorrhagic hypotension. We used the modified brain microdialysis/hydrogen clearance technique to simultaneously infuse 2-CADO into the frontal cortex and measure local CBF from the same area. When 2-CADO from 10(-8) M to 10(-3) M was infused under control conditions (n = 7), CBF increased 61% at 10(-5) M, 167% at 10(-4) M, and 210% at 10(-3) M. In hypotension experiments, local infusion of 10(-5) M 2-CADO (n = 8) caused significant increases in CBF (P less than 0.05) under control conditions (MABP = 65 mmHg) and at hypotensive blood pressures of 55 mmHg and 44 mmHg, respectively. At a blood pressure of 33 mmHg, however, infusion of the analog failed to increase CBF. Local infusion of 10(-3) M 2-CADO also produced a similar change in CBF during graded hypotension. These results indicate that 2-CADO dilates intracerebral vessels during normotension, and mild and moderate hypotension, and support the hypothesis that endogenous adenosine mediates autoregulatory adjustments of CBF during hypotension in newborn piglets.

Cerebral blood flow during experimental epidural bleeding in swine

Regional cerebral blood flow (rCBF) was studied during an aggressive epidural bleed, using a ventilated swine model. rCBF, regional organ blood flow and cardiac output were measured using the radioactive microsphere technique. Blood flows were measured prior to the start of bleeding (Stage 1), when intracranial pressures had reached a plateau and supratentorial perfusion pressure was reduced by about 50% (Stage 2), and at isoelectric EEG (Stage 3). Supratentorial rCBF did not change significantly between stages 1 and 2 while rCVR decreased, implying autoregulatory activity. Cerebral ischaemia developed between stages 2 and 3 when rCBF values fell to levels between 20 and 50% of control values. Infratentorial rCBF changes were similar but less marked, so that adequate brain stem perfusion was maintained below the upper mesencephalon. The left temporal and left parietal cortex and upper mesencephalon suffered a greater reduction in rCBF than other regions, due to proximity to the haematoma and tentorial herniation. The supratentorial perfusion pressure at stage 2 was 60 mm Hg associated with a haematoma volume of 6% of the intracranial volume (ICV). The infratentorial perfusion pressure never fell below 60 mm Hg. The Cushing response was absent when the EEG became isoelectric. This is tentatively ascribed to the absence of hypoxia, because mechanical ventilation was used. Instead systemic arterial hypotension accompanied bleeding in this ventilated model. This hypotension was due to falling cardiac output and peripheral vasodilation.

Cerebral blood flow and metabolism during and after prolonged hypocapnia in newborn lambs

We studied the effects of prolonged (6 hours) hypocapnia and the abrupt termination thereof on cerebral blood flow and metabolism in six paralyzed, sedated (but not anesthetized) newborn lambs. Thirty minutes after institution of hyperventilation to an arterial carbon dioxide pressure of 15 +/- 2 torr, hyperventilation, cerebral blood flow had returned to baseline. Abrupt termination of hyperventilation after 6 hours resulted in a 110 +/- 71% increase in cerebral blood flow over baseline after 30 minutes of normocapnia. This cerebral hyperemia persisted for at least 90 minutes after hyperventilation was discontinued. Cerebral oxygen consumption did not change throughout the study. The posthypocapnia hyperemia noted in these animals after abrupt normalization of arterial carbon dioxide pressure may contribute to the increased risk of intracranial hemorrhage in newborn infants who are treated similarly in the management of pulmonary hypertension.

Does caffeine affect cerebral blood flow in the preterm infant?

Caffeine, used for treatment of idiopathic apnea in preterm infants, may have a vasoconstrictive effect on cerebral vessels. The ensuing reduction in cerebral blood flow may play a role in the pathogenesis of ischemic brain damage. In 25 preterm infants possible changes in cerebral blood flow due to caffeine administration were assessed using Doppler ultrasound. During caffeine treatment PaCO2 was reduced. However, no changes were found in cerebral blood flow velocity suggesting absence of major changes in cerebrovascular resistance and actual cerebral blood flow following caffeine medication.

Response of the cerebral circulation to profound hypocarbia in neonatal lambs

Hyperventilation to extremely low arterial carbon dioxide tension (PaCO2) has been used in the management of persistent pulmonary hypertension in newborn infants. With progressive hypocarbia, cerebral vasoconstriction occurs, raising the concern that extreme hypocarbia may result in cerebral oxygen deprivation. Therefore, I evaluated regulation of the cerebral circulation during acute hypocarbia in 10 newborn lambs. Whole-brain and regional blood flows measured using radioactive microspheres, arterial and venous (sagittal sinus) blood gases, and oxygen contents were measured in each lamb at four arterial carbon dioxide tensions. Whole-brain oxygen delivery, oxygen consumption, and fractional oxygen extraction were calculated. Finally, arterial and venous lactate concentrations were measured to assess cerebral lactate production. Whole-brain blood flow (CBF) decreased in a nonlinear fashion as PaCO2 ranged from 46 to 12 mm Hg [In(CBF) = 0.025(PaCO2) + 3.38; r = 0.70, p less than 0.001]. Similar responses were demonstrated for all regional blood flows examined. Cerebral fractional oxygen extraction (E) increased in a nonlinear fashion [In(1-E) = 0.023(PaCO2)-1.37; r = 0.80, p less than 0.001], and cerebral metabolic rate for oxygen was unchanged with hypocarbia. Cerebral venous lactate concentration increased significantly (3.49 +/- 0.23 vs. 2.01 +/- 0.22 mM, p less than 0.001) during severe hypocarbia (PaCO2 of less than 22 mm Hg), and the arterial-venous lactate concentration difference became negative. These results demonstrate uniform responses of whole-brain and regional blood flows and stable cerebral oxygen consumption during moderate and severe hypocarbia. Although there is evidence for cerebral lactate production during severe hypocarbia, this is not likely to indicate cerebral hypoxia as oxygen consumption does not change.

Persistence of impaired autoregulation of cerebral blood flow in the postictal period in piglets

Cerebral blood flow (CBF) autoregulation was evaluated in the postictal period in unanesthetized term newborn piglets with a mean age of 5.7 +/- 3 days. Seven animals (group 1) received 1 mg/kg bicuculline to induce brief generalized seizures, and six control animals (group II) received saline. Twenty to 90 min after the end of seizure activity in group I or saline injection in group II, CBF was measured by the radioactive labeled microspheres method at three levels of mean arterial blood pressure (MABP) obtained by controlled blood withdrawal within the normal range for autoregulation. In the postictal period, baseline CBF was higher in group I than in group II (85 +/- 21 vs. 48 +/- 7 ml/min/100 g, p less than 0.001). During hypotension, total CBF was positively correlated with variations of MABP in group I (r = 0.739, p less than 0.01) but not in group II. Regional CBF showed the same correlation with MABP. These data show that after seizures in piglets, the rise in CBF is associated with a persistent impairment of CBF autoregulation. These hemodynamic alterations may be relevant in the pathogenesis of hemorrhagic or ischemic brain lesions.

Effects of lactic acid infusions and pH on cerebral blood flow and metabolism

To determine the effects of lactic acidemia versus lactate on CBF, we infused lactic acid, either buffered with NaOH (L + NaOH) or with added NaCl (L + NaCl), to attain similar osmolalities in 18 piglets. CBF (microsphere technique), pH, blood gases, plasma osmolality, and cerebral arteriovenous differences of O2 content and lactic acid concentrations were measured prior to, at 30 min of a lactic acid infusion, and 15 and 90 min after completion of the infusion. Control arterial pH was comparable between groups (7.50 +/- 0.02 vs. 7.49 +/- 0.02, X +/- SE); during and following L + NaCl and L + NaOH, values were (p less than 0.05) 7.09 +/- 0.03, 7.35 +/- 0.02, and 7.46 +/- 0.02 vs. 7.58 +/- 0.03, 7.61 +/- 0.01, and 7.57 +/- 0.03, respectively. PaCO2 remained unchanged and osmolality rose by 15% in both groups during infusions and persisted throughout the study period. For L + NaCl piglets, CBF (ml/min.100 g) rose from 136 +/- 15 to 198 +/- 26 (p less than 0.05) at 30 min of infusion and remained elevated at 201 +/- 25 and 207 +/- 28 at 15 and 90 min following the infusion, respectively. Similarly, for L + NaOH piglets, CBF rose from 130 +/- 25 to 196 +/- 31 (p less than 0.05) with the infusion and was 174 +/- 17 and 166 +/- 21 at 15 and 90 min afterward, respectively. Although lactic acid infusion increases CBF, the associated metabolic acidemia is not responsible for changes in CBF.