Severe hypocarbia in preterm infants and neurodevelopmental deficit

Volume-Targeted Ventilation

Volume-targeted ventilation (VTV) has been increasingly used in neonatology. In systematic reviews, VTV has been shown to reduce the risk of neonatal morbidities and improve long-term outcomes. It is adaptive ventilation using complex computer algorithms to deliver ventilator inflations with expired tidal volumes close to a target set by clinicians. Significant endotracheal tube leak and patient-ventilator interactions may complicate VTV and make ventilator parameters and waveforms difficult to interpret. In this article, we review the rationale for using VTV and the evidence supporting its use and provide practical advice for clinicians ventilating newborn infants.

Challenges in respiratory management during therapeutic hypothermia for neonatal encephalopathy

Neonatal encephalopathy (NE) is a serious condition with devastating neurological outcomes that can impact oxygenation and ventilation. The currently recommended therapeutic hypothermia (TH) for these infants may also has several respiratory implications. It decreases metabolic rate and oxygen demands; however, it increases oxygen solubility in the blood and impacts its release to peripheral tissue including the brain. Respiratory management of infants treated with TH should aim for minimizing exposure to hypocapnia or hyperoxia. Inspiratory gas should be heated to 37 °C and humidified to prevent airway and alveolar injury. Blood gas values should be corrected to the core temperature during TH and the use of alkaline buffers is discouraged. While mild sedation/analgesia may ameliorate the discomfort related to cooling, paralytic agents/heavy sedation should be used with caution considering their side effects. Finally, the use of caffeine still needs careful investigation in this population.

Effects of High-Frequency Oscillatory Ventilation With Volume Guarantee During Surfactant Treatment in Extremely Low Gestational Age Newborns With Respiratory Distress Syndrome: An Observational Study

OBJECTIVE

To evaluate the effect of volume guarantee (VG) combined with high-frequency oscillatory ventilation (HFOV) on respiratory and other physiological parameters immediately after lung recruitment and surfactant administration in HFOV elective ventilated extremely low gestational age newborns (ELGAN) with respiratory distress syndrome (RDS).

DESIGN

Observational study.

SETTING

Tertiary neonatal intensive care unit.

PATIENTS

Twenty-two ELGANs of 25.5 ± 1.1 weeks of gestational age requiring invasive mechanical ventilation and surfactant administration for RDS during the first 6 h of life.

INTERVENTIONS

All infants intubated in delivery room, were managed with elective HFOV and received surfactant after a lung recruitment manoeuver. Eleven infants received HFOV + VG and were compared with a control group of 11 infants receiving HFOV alone. HFOV was delivered in both groups by Dräger Babylog VN500 ventilator (Dräger, Lubeck, Germany).

MAIN OUTCOME MEASURES

Variations and fluctuations of delivered high-frequency tidal volume (VT_hf), fluctuation of pressure amplitude (ΔP) and partial pressure of CO₂ (pCO₂) levels after recruitment manoeuver and immediately after surfactant administration, in HFOV + VG vs. HFOV ventilated infants.

RESULTS

There were no significant differences in the two groups at starting ventilation with or without VG. The mean applied VT_hf per kg was 1.7 ± 0.3 ml/kg in the HFOV group and 1.7 ± 0.1 ml/kg in the HFOV + VG group. Thirty minutes after surfactant administration, HFOV group had a significant higher VT_hf/Kg than HFOV + VG (2.1 ± 0.3 vs. 1.6 ± 0.1 ml/kg, p < 0.0001) with significantly lower pCO₂ levels (43.1 ± 3.8 vs. 46.8 ± 1.5 mmHg, p = 0.01), 54.4% of patients having pCO₂ below 45 mmHg. Measured post-surfactant ΔP values were higher in HFOV group (17 ± 3 cmH₂O) than in HFOV + VG group (13 ± 3 cmH₂O, p = 0.01).

CONCLUSION

HFOV + VG maintains pCO₂ levels within target range and reduces VT_hf delivered variations more consistently than HFOV alone after surfactant administration.

Volume guarantee ventilation in neonates treated with hypothermia for hypoxic-ischemic encephalopathy during interhospital transport

OBJECTIVE

We investigated if volume guarantee (VG) ventilation in babies with hypoxic-ischemic encephalopathy (HIE) during interhospital transport decreases tidal volumes and prevents hypocapnia.

STUDY DESIGN

We computationally collected and analyzed ventilator data of babies ventilated with synchronized intermittent mandatory ventilation (SIMV) with VG (n = 28) or without VG (n = 8).

RESULT

The expiratory tidal volume of ventilator inflations was lower with SIMV-VG (median [IQR]: 4.9 [4.6-5.3] mL/kg) than with SIMV only (median [IQR]: 7.1 [5.3-8.0] mL/kg, p = 0.01). Babies receiving SIMV-VG had lower peak inflating pressures (median: 10.7 cmH₂O, versus 17.5 cmH₂O, p = 0.01). There was no significant difference in minute ventilation or in pCO₂. Babies with strong spontaneous breathing had a mean PIP < 10 cmH₂O but this did not result in adverse events or worsening of acidosis.

CONCLUSIONS

The use of VG ventilation in babies with HIE reduces tidal volumes and frequently results in very low inflating pressures without affecting pCO₂.

Effects of Nasal Continuous Positive Airway Pressure on Cerebral Hemodynamics in Preterm Infants

Background: To evaluate the effects of pressure levels on cerebral hemodynamics in premature infants receiving nasal continuous positive airway pressure (nCPAP) during the first 3 days of life. Methods: Forty-four preterm infants treated with nCPAP were divided into two groups: very preterm infants [gestational age 1 (GA1), GA < 32 weeks, n = 24] and moderate/late preterm infants (GA2 group, GA 32-37 weeks, n = 20). During monitoring, pressure levels were set at 4 → 6 → 8 → 4 cmH₂O, and cerebral hemodynamics was assessed by near-infrared spectroscopy (NIRS). Vital signs, peripheral oxygen saturation (SpO₂) and transcutaneous carbon dioxide pressure (TcPCO₂) were simultaneously recorded. Results: Pressures of 4-8 cmH₂O had no significant influence on cerebral hemodynamics, TcPCO₂, SpO₂ or other vital signs. The tissue oxygenation index (TOI), the difference between oxygenated hemoglobin (ΔHbO₂) and deoxygenated hemoglobin (ΔHHb) (ΔHbD), and cerebral blood volume (ΔCBV) were all significantly positively correlated with gestational and post-natal age, with fluctuations being greater in the GA1 group. ΔHbD and ΔCBV were also significantly positively correlated with TcPCO₂. Conclusions: No significant differences were observed in cerebral hemodynamics when the nCPAP pressure was set to 4-8 cmH₂O.

Neonatal encephalopathy therapy optimization for better neuroprotection with inhalation of CO2: the HENRIC feasibility and safety trial

BACKGROUND

There is an association between hypocapnia and adverse neurodevelopmental outcome in infants with neonatal encephalopathy (NE). Our aim was to test the safety and feasibility of 5% CO₂ and 95% air inhalation to correct hypocapnia in mechanically ventilated infants with NE undergoing therapeutic hypothermia.

METHODS

Ten infants were assigned to this open-label, single-center trial. The gas mixture of 5% CO₂ and 95% air was administered through patient circuits if the temperature-corrected PCO₂ ≤40 mm Hg. The CO₂ inhalation was continued for 12 h or was stopped earlier if the base deficit (BD) level decreased <5 mmol/L. Follow-up was performed using Bayley Scales of Infant Development II.

RESULTS

The patients spent a median 95.1% (range 44.6-98.5%) of time in the desired PCO₂ range (40-60 mm Hg) during the inhalation. All PCO₂ values were >40 mm Hg, the lower value of the target range. Regression modeling revealed that BD and lactate had a tendency to decrease during the intervention (by 0.61 and 0.55 mmol/L/h, respectively), whereas pH remained stable. The rate of moderate disabilities and normal outcome was 50%.

CONCLUSIONS

Our results suggest that inhaled 5% CO₂ administration is a feasible and safe intervention for correcting hypocapnia.

Respiratory management during therapeutic hypothermia for hypoxic-ischemic encephalopathy

Therapeutic hypothermia (TH) has become the standard of care treatment to improve morbidity and mortality in infants with hypoxic-ischemic encephalopathy (HIE). Although TH has clearly proven to be beneficial, recent studies suggest optimization of respiratory management as an approach to prevent further damage and improve neurodevelopmental outcome. The ventilatory management of asphyxiated neonates presents a challenge because both the hypoxic insult and TH have an impact on respiratory functions. Although the danger of recurrence of hypocapnia is well recognized, a brief period of severe hyperoxia also can be detrimental to the previously compromised brain and have been shown to increase the risk of adverse neurodevelopmental outcomes. Therefore, judicious ventilatory management with rigorous monitoring is of particular importance in patients with HIE. In the present review, we provide an overview of the currently available evidence on pulmonary function, respiratory morbidities, and ventilation strategies in HIE and we highlight possible future research directions.

Inhaled Gases for Neuroprotection of Neonates: A Review

Importance: Hypoxic-ischemic encephalopathy (HIE) is a significant cause of morbidity and mortality in neonates. The incidence of HIE is 1-8 per 1,000 live births in developed countries. Whole-body hypothermia reduces the risk of disability or death, but 7 infants needed to be treated to prevent death or major neurodevelopmental disability. Inhalational gases may be promising synergistic agents due to their rapid onset and easy titratability. Objective: To review current data on different inhaled gases with neuroprotective properties that may serve as adjunct therapies to hypothermia. Evidence review: Literature review was performed using the PubMed database, google scholar, and ClinicalTrials.Gov. Results focused on articles published from January 1, 2005, through December 31, 2017. Articles published earlier than 2005 were included when appropriate for historical perspective. Our review emphasized preclinical and clinical studies relevant to the use of inhaled agents for neuroprotection. Findings: Based on the relevance to our topic, 111 articles were selected pertaining to the incidence of HIE, pathophysiology of HIE, therapeutic hypothermia, and emerging therapies for hypoxic-ischemic encephalopathy in preclinical and clinical settings. Supplemental tables summarizes highly relevant 49 publications that were included in this review. The selected publications emphasize the emergence of promising inhaled gases that may improve neurologic survival and alleviate neurodevelopmental disability when combined with therapeutic hypothermia in the future. Conclusions: Many inhaled agents have neuroprotective properties and could serve as an adjunct therapy to whole-body hypothermia. Inhaled agents are ideal due to their easy administration, titrability, and rapid onset and offset.

Asphyxiated neonates who received active therapeutic hypothermia during transport had higher rates of hypocapnia than controls

AIM

We investigated the association between active hypothermia and hypocapnia in neonates with moderate-to-severe hypoxic-ischaemic encephalopathy (HIE) transported after birth.

METHODS

This was a retrospective cohort study of neonates with HIE born between 2007 and 2011 and transported to Semmelweis University, Hungary, for hypothermia treatment before and after we introduced active cooling during transport in 2009. Of these, 71 received intensive care plus controlled active hypothermia during transport, while the 46 controls just received standard intensive care. Incident hypocapnia was defined as a partial pressure of carbon-dioxide (pCO₂ ) that decreased below 35 mm Hg during transport. Multivariable logistic regression investigated the relationship between hypothermia and incident hypocapnia.

RESULTS

Incident hypocapnia was more frequent in the actively cooled transport group (36.6%) than control group (17.4%; p = 0.025). pCO₂ decreased from a median of 45 to 35 mm Hg (p < 0.0001) in the intervention group, but remained unchanged in the controls. After adjusting for confounders, hypothermia remained an independent risk factor for hypocapnia with an odds ratio (OR) of 4.23 and 95% confidence interval (95% CI) of 1.30-13.79. Sedation was associated with a reduction in OR of hypocapnia, at 0.35 (95% CI 0.12-0.98).

CONCLUSIONS

Hypothermia increased the risk of hypocapnia in neonates with HIE during transport.

Acute Neonatal Respiratory Failure

Acute respiratory failure requiring assisted ventilation is one of the most common reasons for admission to the neonatal intensive care unit. Respiratory failure is the inability to maintain either normal delivery of oxygen to the tissues or normal removal of carbon dioxide from the tissues. It occurs when there is an imbalance between the respiratory workload and ventilatory strength and endurance. Definitions are somewhat arbitrary but suggested laboratory criteria for respiratory failure include two or more of the following: PaCO₂ > 60 mmHg, PaO₂ < 50 mmHg or O₂ saturation <80 % with an FiO₂ of 1.0 and pH < 7.25 (Wen et al. 2004).

Brain cell swelling during hypocapnia increases with hyperglycemia or ketosis

BACKGROUND

Severe hypocapnia reduces cerebral blood flow (CBF) and is known to be a risk factor for diabetic ketoacidosis (DKA)-related cerebral edema and cerebral injury in children. Reductions in CBF resulting from hypocapnia alone, however, would not be expected to cause substantial cerebral injury. We hypothesized that either hyperglycemia or ketosis might alter the effects of hypocapnia on CBF and/or cerebral edema associated with CBF reduction.

METHODS

We induced hypocapnia (pCO₂ 20 ± 3 mmHg) via mechanical ventilation in three groups of juvenile rats: 25 controls, 22 hyperglycemic rats (serum glucose 451 ± 78 mg/dL), and 15 ketotic rats (β-hydroxy butyrate 3.0 ± 1.0 mmol/L). We used magnetic resonance imaging to measure CBF and apparent diffusion coefficient (ADC) values in these groups and in 17 ventilated rats with normal pCO₂ (40 ± 3 mmHg). In a subset (n = 35), after 2 h of hypocapnia, pCO₂ levels were normalized (40 ± 3 mmHg) and ADC and CBF measurements were repeated.

RESULTS

Declines in CBF with hypocapnia occurred in all groups. Normalization of pCO₂ after hypocapnia resulted in hyperemia in the striatum. These effects were not substantially altered by hyperglycemia or ketosis. Declines in ADC (suggesting brain cell swelling) during hypocapnia, however, were greater during both hyperglycemia and ketosis.

CONCLUSIONS

We conclude that brain cell swelling associated with hypocapnia is increased by both hyperglycemia and ketosis, suggesting that these metabolic conditions may make the brain more vulnerable to injury during hypocapnia.

Intracranial dynamics in pre-term infants and neonates: implications for physiotherapists

The control of cerebral circulation and intracranial dynamics differs markedly in the pre-term and full term neonate from that in the adult. Immaturity can combine with several clinical conditions and iatrogenic factors to predispose the neonate to cerebral lesions, which may subsequently increase morbidity. As physiotherapists play an increasingly important role in neonatology, it is important to appreciate the immaturity of the nervous system and to recognise the risk factors for such conditions as peri-intraventricular haemorrhage and periventricular leucomalacia. This paper describes intracranial dynamics in the pre-term and full term infant and illustrates how these factors may interact with clinical conditions to cause cerebral lesions. Studies which examine the effect of respiratory physiotherapy on intracranial dynamics are reviewed and suggestions made for further research.

Cerebral vascular regulation and brain injury in preterm infants

Cerebrovascular lesions, mainly germinal matrix hemorrhage and ischemic injury to the periventricular white matter, are major causes of adverse neurodevelopmental outcome in preterm infants. Cerebrovascular lesions and neuromorbidity increase with decreasing gestational age, with the white matter predominantly affected. Developmental immaturity in the cerebral circulation, including ongoing angiogenesis and vasoregulatory immaturity, plays a major role in the severity and pattern of preterm brain injury. Prevention of this injury requires insight into pathogenesis. Cerebral blood flow (CBF) is low in the preterm white matter, which also has blunted vasoreactivity compared with other brain regions. Vasoreactivity in the preterm brain to cerebral perfusion pressure, oxygen, carbon dioxide, and neuronal metabolism is also immature. This could be related to immaturity of both the vasculature and vasoactive signaling. Other pathologies arising from preterm birth and the neonatal intensive care environment itself may contribute to impaired vasoreactivity and ineffective CBF regulation, resulting in the marked variations in cerebral hemodynamics reported both within and between infants depending on their clinical condition. Many gaps exist in our understanding of how neonatal treatment procedures and medications have an impact on cerebral hemodynamics and preterm brain injury. Future research directions for neuroprotective strategies include establishing cotside, real-time clinical reference values for cerebral hemodynamics and vasoregulatory capacity and to demonstrate that these thresholds improve long-term outcomes for the preterm infant. In addition, stimulation of vascular development and repair with growth factor and cell-based therapies also hold promise.

Association between initial prescribed minute ventilation and post-resuscitation partial pressure of arterial carbon dioxide in patients with post-cardiac arrest syndrome

BACKGROUND

Post-cardiac arrest hypocapnia/hypercapnia have been associated with poor neurological outcome. However, the impact of arterial carbon dioxide (CO2) derangements during the immediate post-resuscitation period following cardiac arrest remains uncertain. We sought to test the correlation between prescribed minute ventilation and post-resuscitation partial pressure of CO2 (PaCO2), and to test the association between early PaCO2 and neurological outcome.

METHODS

We retrospectively analyzed a prospectively compiled single-center cardiac arrest registry. We included adult (age ≥ 18 years) patients who experienced a non-traumatic cardiac arrest and required mechanical ventilation. We analyzed initial post-resuscitation ventilator settings and initial arterial blood gas analysis (ABG) after initiation of post-resuscitation ventilator settings. We calculated prescribed minute ventilation:MVmL/kg/min=tidalvolumeTV/idealbodyweightIBWxrespiratoryrateRRfor each patient. We then used Pearson's correlation to test the correlations between prescribed MV and PaCO2. We also determined whether patients had normocapnia (PaCO2 between 30 and 50 mmHg) on initial ABG and tested the association between normocapnia and good neurological function (Cerebral Performance Category 1 or 2) at hospital discharge using logistic regression analyses.

RESULTS

Seventy-five patients were included. The majority of patients were in-hospital arrests (85%). Pulseless electrical activity/asystole was the initial rhythm in 75% of patients. The median (IQR) TV, RR, and MV were 7 (7 to 8) mL/kg, 14 (14 to 16) breaths/minute, and 106 (91 to 125) mL/kg/min, respectively. Hypocapnia, normocapnia, and hypercapnia were found in 15%, 62%, and 23% of patients, respectively. Good neurological function occurred in 32% of all patients, and 18%, 43%, and 12% of patients with hypocapnia, normocapnia, and hypercapnia respectively. We found prescribed MV had only a weak correlation with initial PaCO2, R = -0.40 (P < 0.001). Normocapnia was associated with good neurological function, odds ratio 4.44 (95% CI 1.33 to 14.85).

CONCLUSIONS

We found initial prescribed MV had only a weak correlation with subsequent PaCO2 and that early Normocapnia was associated with good neurological outcome. These data provide rationale for future research to determine the impact of PaCO2 management during mechanical ventilation in post-cardiac arrest patients.

Arterial blood carbonic Acid inversely determines lactic and organic acids

OBJECTIVE

To establish that arterial blood carbonic acid varies inversely with lactic acid in accordance with bicarbonate exchanging for lactate across cell membranes through the anion exchange mechanism to maintain the Gibbs-Donnan equilibrium.

STUDY DESIGN

Over 5 years, lactate was measured on all blood gases taken from neonatal admissions, as well as organic acid whenever electrolytes were required.

RESULTS

Arterial blood gases from 63 infants given high calcium TPN were analyzed. Twenty two needed continuous positive airways pressure (CPAP) only and 31 intermittent positive pressure ventilation (IPPV) and surfactant followed by CPAP to treat respiratory distress syndrome in 51 and meconium aspiration syndrome in 2. All survived and were free of infection. Excluded gases were those with high and falling lactate soon after delivery representing perinatal asphyxia, and those on dexamethasone. Strong inverse relations between carbonic and lactic acids were found at all gestational ages and, independent of glomerular filtration, between carbonic and organic acids. Lactate (mmol/L) = 62.53 X PCO2 (-0.96)(mmHg) r(2) 0.315, n 1232, p <0.001. Sixty divided by PCO2 is a convenient measure of physiological lactate at any given PCO2. In the first week, 9.13 ± 2.57% of arterial gases from infants on IPPV had lactates above 120/PCO2, significantly more than 4.74 ± 2.73% on CPAP (p<0.05) and 2.47 ± 2.39% on no support.

CONCLUSION

Changes in arterial blood carbonic acid cause immediate inverse changes in lactic acid, because their anions interchange across cell membranes according to the Gibbs -Donnan equilibrium. Increasing PCO2 from 40 to 120 mmHg decreased lactate from 1.5 mmol/L to 0.5 mmol/L, so that the sum of carbonic and lactic acids increased from 2.72 mmol/L to only 4.17 mmol/L. This helps explain the neuroprotective effect of hypercapnoea and highlights the importance of avoiding any degree of hypocapnoea in infants on IPPV.

Association Between Postresuscitation Partial Pressure of Arterial Carbon Dioxide and Neurological Outcome in Patients With Post–Cardiac Arrest Syndrome

Background—

Partial pressure of arterial CO 2 (Pa co 2 ) is a regulator of cerebral blood flow after brain injury. Recent guidelines for the management of cardiac arrest recommend maintaining Pa co 2 at 40 to 45 mm Hg after successful resuscitation; however, there is a paucity of data on the prevalence of Pa co 2 derangements during the post–cardiac arrest period and its association with outcome.

Methods and Results—

We analyzed a prospectively compiled and maintained cardiac arrest registry at a single academic medical center. Inclusion criteria are as follows: age ≥18, nontrauma arrest, and comatose after return of spontaneous circulation. We analyzed arterial blood gas data during 0 to 24 hours after the return of spontaneous circulation and determined whether patients had exposure to hypocapnia and hypercapnia (defined as Pa co 2 ≤30 mm Hg and Pa co 2 ≥50 mm Hg, respectively, based on previous literature). The primary outcome was poor neurological function at hospital discharge, defined as Cerebral Performance Category ≥3. We used multivariable logistic regression, with multiple sensitivity analyses, adjusted for factors known to predict poor outcome, to determine whether post–return of spontaneous circulation hypocapnia and hypercapnia were independent predictors of poor neurological function. Of 193 patients, 52 (27%) had hypocapnia only, 63 (33%) had hypercapnia only, 18 (9%) had both hypocapnia and hypercapnia exposure, and 60 (31%) had no exposure; 74% of patients had poor neurological outcome. Hypocapnia and hypercapnia were independently associated with poor neurological function, odds ratio 2.43 (95% confidence interval, 1.04–5.65) and 2.20 (95% confidence interval, 1.03–4.71), respectively.

Conclusions—

Hypocapnia and hypercapnia were common after cardiac arrest and were independently associated with poor neurological outcome. These data suggest that Pa co 2 derangements could be potentially harmful for patients after resuscitation from cardiac arrest.

Volume-limited and volume-targeted ventilation

This article shows that volume-targeted ventilation is physiologically more logical than pressure-limited ventilation, and is associated with a reduced risk of pneumothorax, hypocarbia, duration of ventilation, death or bronchopulmonary dysplasia, and severe intraventricular hemorrhage. Therefore, it should now be adopted as the main mode for mechanical ventilation of preterm neonates.

Current management of the infant who presents with neonatal encephalopathy

Neonatal encephalopathy after perinatal hypoxic-ischemic insult is a major contributor to global child mortality and morbidity. Brain injury in term infants in response to hypoxic-ischemic insult is a complex process evolving over hours to days, which provides a unique window of opportunity for neuroprotective treatment interventions. Advances in neuroimaging, brain monitoring techniques, and tissue biomarkers have improved the ability to diagnose, monitor, and care for newborn infants with neonatal encephalopathy as well as predict their outcome. However, challenges remain in early identification of infants at risk for neonatal encephalopathy, determination of timing and extent of hypoxic-ischemic brain injury, as well as optimal management and treatment duration. Therapeutic hypothermia is the most promising neuroprotective intervention to date for infants with moderate to severe neonatal encephalopathy after perinatal asphyxia and has currently been incorporated in many neonatal intensive care units in developed countries. However, only 1 in 6 babies with encephalopathy will benefit from hypothermia therapy; many infants still develop significant adverse outcomes. To enhance the outcome, specific diagnostic predictors are needed to identify patients likely to benefit from hypothermia treatment. Studies are needed to determine the efficacy of combined therapeutic strategies with hypothermia therapy to achieve maximal neuroprotective effect. This review focuses on important concepts in the pathophysiology, diagnosis, and management of infants with neonatal encephalopathy due to perinatal asphyxia, including an overview of recently introduced novel therapies.

Hyperventilation in panic disorder and asthma: empirical evidence and clinical strategies

Sustained or spontaneous hyperventilation has been associated with a variety of physical symptoms and has been linked to a number of organic illnesses and mental disorders. Theories of panic disorder hold that hyperventilation either produces feared symptoms of hypocapnia or protects against feared suffocation symptoms of hypercapnia. Although the evidence for both theories is inconclusive, findings from observational, experimental, and therapeutic studies suggest an important role of low carbon dioxide (CO2) levels in this disorder. Similarly, hypocapnia and associated hyperpnia are linked to bronchoconstriction, symptom exacerbation, and lower quality of life in patients with asthma. Raising CO2 levels by means of therapeutic capnometry has proven beneficial effects in both disorders, and the reversing of hyperventilation has emerged as a potent mediator for reductions in panic symptom severity and treatment success.

To autoregulate or not to autoregulate--that is no longer the question

In the late 1970s, high cerebral blood flow was perceived as a cause of intracranial hemorrhage in the preterm infant. Intracranial hemorrhage was diagnosed by computed tomography and ultrasound found to be frequent not only in babies who died. Hemorrhage was soon linked to cerebral palsy in survivors. The analogy was hypertensive hemorrhagic stroke in the adult. Cerebral hemorrhage was perceived as the major (preventable) cause of brain injury in the preterm baby. An immature cerebral autoregulation or a vulnerability of the autoregulation exposed by preceding hypoxia or ischemia therefore became a focus of neonatal brain research in the 1980s. Over the years the focus has changed, first to the pathogenesis of hypoxic-ischemic brain injury, then to the effects of pCO(2), and now 30 years later to a more comprehensive, less clearly hypothesis-driven exploration of the multitude of factors involved in cerebral blood flow and oxygenation. Meanwhile, some basic questions regarding autoregulation remain unanswered, and some concepts from the 1970s still direct clinical practice.

The role of systemic hemodynamic disturbances in prematurity-related brain injury

Premature infants who experience cerebrovascular injury frequently have acute and long-term neurologic complications. In this article, we explore the relationship between systemic hemodynamic insults and brain injury in this patient population and the mechanisms that might be at play.

ROP in three follow-up studies from Rigshospitalet during the period 1976-1987

Three studies of late outcome in groups of consecutively admitted very low birthweight or very pre-term infants have recently been reported from Rigshospitalet, Copenhagen. The periods were 1976-78, 1980-82, and 1984-87. The incidences of blindness due to retinopathy of prematurity (ROP) and of motor deficit due to cerebral palsy (CP) are analysed in this report. For the present purpose selection within the three groups made them more, although not entirely, comparable. In spite of marked changes in the use of mechanical ventilation, and in survival rate, the incidences of ROP as well as CP remained constant, at about 5% and 10% of the survivors, respectively. The actual numbers are too small for clearcut conclusions but the analysis supports two previously formed notions: 1) According to monitoring of blood gases deviating oxygen and carbon dioxide tensions do not appear as prime factors in inducing neither ROP nor CP, and 2) a very preterm infant of given size and maturity has become less exposed to ROP and CP over the years.

Treatment of the term newborn with brain injury: simplicity as the mother of invention

Neonatal brain injury remains a common cause of developmental disability, despite tremendously enhanced obstetrical and neonatal care. The timing of brain injury occurs throughout gestation, labor, and delivery, providing an evolving form of brain injury and a moving target for therapeutic intervention. Nonetheless, markedly improved methods are available to identify those infants injured at birth, via clinical presentation with neonatal encephalopathy and neuroimaging techniques. Postischemic hypothermia has been shown to be of tremendous clinical promise in several completed and ongoing trials. As part of this approach to the treatment of the newborn, other parameters of physiologic homeostasis can and should be attended to, with strong animal and clinical evidence that their correction will have dramatic influence on the outcome of the newborn infant. This review addresses aspects of newborn care to which we can direct our attention currently, and which should result in a safe and efficacious improvement in the prognosis of the newborn with neonatal encephalopathy.

Cerebrovascular injury in premature infants: current understanding and challenges for future prevention

Cerebrovascular insults are a leading cause of brain injury in premature infants, contributing to the high prevalence of motor, cognitive, and behavioral deficits. Understanding the complex pathways linking circulatory immaturity to brain injury in premature infants remains incomplete. These mechanisms are significantly different from those causing injury in the mature brain. The gaps in knowledge of normal and disturbed cerebral vasoregulation need to be addressed. This article reviews current understanding of cerebral perfusion, in the sick premature infant in particular, and discusses challenges that lie ahead.

Hypercapnia and hypocapnia in neonates

BACKGROUND

The arterial partial pressure of carbon dioxide (PaCO2) represents the balance between CO2 production and consumption. Abnormal increase or decrease in PaCO2 can affect the body's internal environment and function. Permissive hypercapnia has aroused more attention as a novel ventilatory therapy. The aim of this study was to elucidate the effects of hypercapnia and hypocapnia on the functions of such neonatal organs as the lung and brain.

DATA SOURCES

The PubMed database was searched with the keywords "hypocapnia", "hypercapnia" and "newborn".

RESULTS

Hypocapnia is a risk factor for potential damage to the central nervous system, such as periventricular leukomalacia, intraventricular hemorrhage, cerebral palsy, cognition developmental disorder, and auditory deficit. Hyperventilation can lessen pulmonary artery hypertension to certain extent, but hypocapnia can aggravate ischemia/reperfusion-induced acute lung injury. Severe hypercapnia can induce intracranial hemorrhage, even consciousness alterations, cataphora, and hyperspasmia. Permissive hypercapnia can improve lung injury caused by diseases of the respiratory system, lessen mechanical ventilation-associated lung injury, reduce the incidence of bronchopulmonary dysplasia and protect against ventilation-induced brain injury. In addition, permissive hypercapnia plays a role in expanding cerebral vessels and increasing cerebral blood flow.

CONCLUSIONS

Severe hypercapnia and hypocapnia can cause neonatal brain injury and lung injury. Permissive hypercapnia can increase the survival of neonates with brain injury or respiratory system disease, and lessen the brain injury and lung injury caused by mechanical ventilation. However, the mechanism of permissive hypercapnia needs further exploration to confirm its safety and therapeutic utility.

Pathogenesis of cerebral white matter injury of prematurity

Cerebral white matter injury, characterised by loss of premyelinating oligodendrocytes (pre-OLs), is the most common form of injury to the preterm brain and is associated with a high risk of neurodevelopmental impairment. The unique cerebrovascular anatomy and physiology of the premature baby underlies the exquisite sensitivity of white matter to the abnormal milieu of preterm extrauterine life, in particular ischaemia and inflammation. These two upstream mechanisms can coexist and amplify their effects, leading to activation of two principal downstream mechanisms: excitotoxicity and free radical attack. Upstream mechanisms trigger generation of reactive oxygen and nitrogen species. The pre-OL is intrinsically vulnerable to free radical attack due to immaturity of antioxidant enzyme systems and iron accumulation. Ischaemia and inflammation trigger glutamate receptor-mediated injury leading to maturation-dependent cell death and loss of cellular processes. This review looks at recent evidence for pathogenetic mechanisms in white matter injury with emphasis on targets for prevention and treatment of injury.

Matching ventilatory support strategies to respiratory pathophysiology

Neonates can suffer from various diseases that impact differently on lung function according to the specific pulmonary pathophysiology. As a consequence, the optimal respiratory support will vary according to disorder. Most randomized trials have only included prematurely born infants who have respiratory distress syndrome (RDS) or infants who have severe respiratory failure. Meta-analysis of the results has demonstrated that for the prematurely born infant who has RDS, prophylactic high-frequency oscillatory ventilation only results in a modest reduction in bronchopulmonary dysplasia, and patient-triggered ventilation (assist/control or synchronized intermittent mandatory ventilation) reduces the duration of ventilation if started in the recovery phase. Whether the newer triggered modes are more efficacious remains to be appropriately tested. In term infants who have severe respiratory failure, extracorporeal membrane oxygenation increases survival, but inhaled nitric oxide only reduces the need for extracorporeal membrane oxygenation. Research is required to identify the optimum respiratory strategy for infants who have other respiratory disorders, particularly bronchopulmonary dysplasia.

Impact of volume guarantee ventilation on arterial carbon dioxide tension in newborn infants: a randomised controlled trial

OBJECTIVE

To compare the effects of the two modes of ventilation, synchronous intermittent positive pressure ventilation (SIPPV) and SIPPV with Volume Guarantee (VG), on arterial carbon dioxide tension (PaCO(2)) immediately after neonatal unit admission.

STUDY DESIGN

Randomised study of ventilation mode for premature inborn infants admitted to two tertiary neonatal units. After admission, infants were randomised to receive either SIPPV or VG using a Dräger Babylog 8000 plus ventilator. In the SIPPV group, peak airway pressure was set clinically. In the VG group, desired tidal volume was set at 4 ml/kg, with the ventilator adjusting peak inspiratory pressure to deliver this volume. The study was completed once the first arterial PaCO(2) was available, with the desirable range defined as 5-7 kPa.

RESULTS

PaCO(2) was significantly higher in the VG group (VG: 5.7 kPa, SIPPV: 4.9 kPa; p=0.03). The VG group had fewer out-of-range PaCO(2) values (VG: 42%, SIPPV: 57%) and fewer instances of hypocarbia <5 kPa (VG: 32%, SIPPV: 57%) but neither difference achieved statistical significance. Regression analysis showed PaCO(2) was negatively correlated with gestation (r=-0.41, p=0.01) and also with the mode of ventilation (r=0.32, p<0.05). In the VG group, all infants 23-25 weeks' gestation had out-of-range PaCO(2) values. VG significantly reduced the incidence of out-of-range PaCO(2) and hypocarbia in infants over 25 weeks' gestation (VG: 27%, SIPPV: 61%; p<0.05).

CONCLUSION

Using this strategy, VG appears feasible in the initial stabilisation of infants over 25 weeks' gestation, with a halving of the incidence of hypocarbia. In the small number of babies studied below this gestation, VG was not found to be effective.

Minimising neonatal brain injury: how research in the past five years has changed my clinical practice

With improving neonatal survival for extremely premature babies, the challenge for neonatology is to improve outcome of surviving babies. This review concentrates on best evidence emerging in recent years on prevention of brain damage by early administration of drugs as well as avoidance of induced brain damage by hyperventilation and dexamethasone therapy given postnatally for chronic lung disease.

ADHD-like hyperactivity, with no attention deficit, in adult rats after repeated hypoxia during the equivalent of extreme prematurity

The most common behavioural disorder seen in children and adolescents born extremely prematurely is attention deficit hyperactivity disorder (ADHD). The hyperactive/impulsive sub-type of ADHD or the inattentive sub-type or the hyperactive/impulsive/inattentive sub-type can be evident. These sub-types of ADHD can persist into adulthood. The aim of this study was to investigate the relevance of a new immature rat model of repeated hypoxic exposure to these behavioural characteristics of extreme prematurity. More specifically, this study aimed to measure ADHD-like hyperactivity in response to delayed reward, and inattention, in repeated hypoxic versus repeated normoxic rats. Sprague-Dawley rats were exposed to either repeated hypoxia or repeated normoxia during postnatal days (PN) 1-3. The rat brain during PN1-3 is generally considered to be developmentally equivalent to the human brain during extreme prematurity. The rats were then behaviourally tested at 16 months-of-age on a multiple component fixed interval-extinction test. This test detects ADHD-like hyperactivity in response to delayed reward, as well as inattention. It was found that the repeated hypoxic rats exhibited ADHD-like hyperactivity in response to delayed reward, but no attention deficit, when compared to repeated normoxic rats. These findings provide a new animal model to investigate the biological mechanisms and treatment of ADHD-like hyperactivity due to repeated hypoxia during the equivalent of extreme prematurity.

Effects of severe hypocapnia on expression of bax and bcl-2 proteins, DNA fragmentation, and membrane peroxidation products in cerebral cortical mitochondria of newborn piglets

BACKGROUND

Hypocapnia occurs in the newborn infant inadvertently or as a therapeutic modality and may result in neuronal and mitochondrial alterations in the newborn brain. Since mitochondria regulate apoptosis, these alterations may initiate a cascade of reactions that lead to apoptotic cell death.

OBJECTIVES

This study tests the hypothesis that hypocapnia results in increased expression of the pro-apoptotic protein Bax, fragmentation of DNA and membrane lipid peroxidation in cerebral cortical mitochondria (mt) of newborn piglets.

METHODS

Studies were performed in three groups of anesthetized normoxic newborn piglets: hypocapnic (H, n = 5), ventilated at a PaCO(2) of 11-15 mm Hg; normocapnic (N, n = 5), ventilated at a PaCO(2) of 40 mm Hg; and corrected normocapnic (CN, n = 4), ventilated as H with CO(2) added to maintain normocapnia. Tissue ATP and phosphocreatine levels were determined. Mitochondrial membrane proteins were separated, transblotted and probed with antibodies to Bax and Bcl-2. Bands were detected by enhanced chemiluminescence and analyzed by imaging densitometry. mtDNA was isolated. Cell and mitochondrial membrane lipid peroxidation products were measured spectrofluorometrically.

RESULTS

ATP and PCr concentrations were similar in the 3 groups. The ratio of Bax/Bcl-2 increased significantly in H compared to N and CN. mtDNA fragmentation was also significantly greater in H compared to N or CN. Membrane lipid peroxidation was higher in H than in N or CN; and in CN compared to N.

CONCLUSIONS

The data demonstrate that severe hypocapnia results in increased Bax expression, DNA fragmentation, and membrane lipid peroxidation in mitochondria of cerebral cortical neurons of newborn piglets, and may result in apoptotic cell death.

Optimal strategies for newborn ventilation--a synthesis of the evidence

A variety of ventilation modes are available for the newborn. Although, there have been randomised trials assessing certain modes, these have generally only included prematurely born infants with RDS or infants with severe respiratory failure. Meta-analysis of the results of those trials has demonstrated that neither patient triggered nor high frequency oscillatory ventilation is advantageous for the prematurely born infant with RDS, but extracorporeal membrane oxygenation increases survival in infants with severe respiratory failure. Appropriately designed studies are required to determine the role of newer ventilation modes and whether forms of respiratory support avoiding intubation are less injurious to the lungs. Research should also focus on infants with other respiratory disorders, particularly BPD. Prior to further randomised trials being undertaken, it is essential that the optimum method of applying each ventilator mode is identified and it is clearly understood whether differences in ventilator/oscillator performance influence outcome.

Effects of tidal volume and positive end-expiratory pressure during resuscitation of very premature lambs

BACKGROUND

Guidelines recommend neonatal resuscitation without controlling tidal volume or positive end-expiratory pressure (PEEP). However, these may improve gas exchange, lung volume and outcome.

AIM

To investigate resuscitation of very premature lambs with a Laerdal bag without PEEP versus volume guarantee ventilation with PEEP.

METHODS

Anaesthetized lambs (n=20) delivered at 125 d gestation were randomized to three groups receiving 15 min resuscitation: (1) Laerdal bag and no PEEP; (2) ventilation with a tidal volume of 5 ml/kg and 8 cm H(2)O PEEP; (3) ventilation with 10 ml/kg and 8 cm H(2)O PEEP. They were then all ventilated for 2 h with tidal volumes of 5 or 10 ml/kg, and 8 cm H(2)O PEEP. Ventilation parameters and blood gases were recorded.

RESULTS

Different tidal volumes affected PaCO(2) within minutes, with 10 ml/kg causing severe hypocarbia. PEEP had little effect on PaCO(2). Oxygenation improved significantly with PEEP of 8 cm H(2)O, irrespective of tidal volume.

CONCLUSION

Very premature lambs can be resuscitated effectively using volume-guarantee ventilation and PEEP. Tidal volumes affected PaCO(2) within minutes but had little effect on oxygenation. PEEP halved the oxygen requirement compared with no PEEP. Resuscitating premature babies with controlled tidal volumes and PEEP might improve their outcome.

The effects of hypercapnia on cerebral autoregulation in ventilated very low birth weight infants

Permissive hypercapnia, a strategy allowing high Pa(CO2), is widely used by neonatologists to minimize lung damage in ventilated very low birth weight (VLBW) infants. While hypercapnia increases cerebral blood flow (CBF), its effects on cerebral autoregulation of VLBW infants are unknown. Monitoring of mean CBF velocity (mCBFv), Pa(CO2), and mean arterial blood pressure (MABP) from 43 ventilated VLBW infants during the first week of life was performed during and after 117 tracheal suctioning procedures. Autoregulation status was determined during tracheal suctioning because it perturbs cerebral and systemic hemodynamics. The slope of the relationship between mCBFv and MABP was estimated when Pa(CO2) was fixed at 30, 35, 40, 45, 50, 55, and 60 mm Hg. A slope near or equal to 0 suggests intact autoregulation, i.e. CBF is not influenced by MABP. Increasing values >0 indicate progressively impaired autoregulation. Infants weighed 905 +/- 259 g and were 26.9 +/- 2.3 wk gestation. The autoregulatory slope increased as Pa(CO2)) increased from 30 to 60 mm Hg. While the slopes for Pa(CO2) values of 30 to 40 mm Hg were not statistically different from 0, slopes for Pa(CO2) > or = 45 mm Hg indicated a progressive loss of cerebral autoregulation. The autoregulatory slope increases with increasing Pa(CO2), suggesting the cerebral circulation becomes progressively pressure passive with hypercapnia. These data raise concerns regarding the use of permissive hypercapnia in ventilated VLBW infants during the first week of life, as impaired autoregulation during this period may be associated with increased vulnerability to brain injury.

Postnatal laboratory timers of antenatal hypoxemic-ischemic brain damage

OBJECTIVE

Markers were sought to identify the antenatal starting times and rates at which brain damage advanced in children with hypoxemic-ischemic cerebral palsy.

STUDY DESIGN

Fetal bradycardia's onset marked the damage's start. Using this baseline, the following were tested as additional timers of the damage's onset: serial blood counts of neonates' normoblasts, platelets, lymphocytes,differences at birth between base excess values in umbilical arterial and venous bloods,brain damage patterns.

RESULTS

Each timer had a broad antenatal time frame within which it could identify specific damage starting times. The broad time frames are as follows: Blood lymphocyte counts: 0.45 to 13.8 hours before birth, blood normoblast counts: 0.45 to 55.0 hours before birth, blood platelet counts: 0.5 to >72 hours before birth. Brain damage patterns: 0.4 to >0.7 hour before birth. Hyperventilating and hyperoxygenating neonates greatly accelerated the damage's advance.

CONCLUSIONS

Commonly obtained laboratory values and brain images can identify when such brain damage began and the rate at which it advanced.

Effect of carbon dioxide on background cerebral electrical activity and fractional oxygen extraction in very low birth weight infants just after birth

Decreased arterial carbon dioxide tension (PaCO2) results in decreased cerebral blood flow, which is associated with diminished cerebral electrical activity. In such a situation, cerebral fractional oxygen extraction (CFOE) would be expected to increase to preserve cerebral oxygen delivery. This study aimed to determine whether changes in blood gases in infants less than 30 wk' gestation were associated with changes in background electroencephalograms (EEG) and CFOE. Thirty-two very low birth weight infants were studied daily for the first three days after birth. Digital EEG recordings were performed for 75 min each day. CFOE, mean blood pressure and arterial blood gases were measured midway through each recording. EEG was analysed by (a) spectral analysis and (b) manual calculation of interburst interval. Blood pressure, pH and PaCO2 did not have any effect on the EEG. On day one, only PaCO2 showed a relationship with the relative power of the delta frequency band (0.5-3.5 Hz) and the interburst interval. The relative power of the delta band remained within normal limits when PaCO2 was between 24 and 55 mmHg on day one. There was a negative association between PaCO2 and CFOE. The associations between PaCO2 and EEG measurements were strongest on day one, weaker on day two, and absent on day three. The slowing of EEG and increased CFOE at lower levels of PaCO2 are likely to be due to decreased cerebral oxygen delivery induced by hypocarbia. When PaCO2 was higher, there was suppression of the EEG.

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.

High-frequency oscillatory ventilation: Lessons from the neonatal/pediatric experience

Efforts to minimize ventilator-induced lung injury in adults with hypoxemic respiratory failure have recently focused on the potential role of high-frequency oscillatory ventilation (HFOV). However, HFOV has been studied in newborns with hypoxemic respiratory failure for nearly 3 decades. In this brief review, we attempt to summarize key physiological principles learned from this cumulative neonatal/pediatric experience with HFOV.

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.

Brain glucose and lactate levels during ventilator-induced hypo- and hypercapnia*

OBJECTIVE

Levels of glucose and lactate were measured in the brain by means of microdialysis in order to evaluate the effects of ventilator-induced hypocapnia and hypercapnia on brain metabolism in healthy non-brain-traumatized animals.

DESIGN AND SETTING

Prospective animal study in a university laboratory.

SUBJECTS

Eight adult Landrace/Yorkshire pigs.

INTERVENTIONS

The microdialysis probe was inserted in the brain along with a multiparameter sensor and intracranial pressure (ICP) probe. The animals were ventilated in a pressure-controlled mode according to the open lung concept with an inspired oxygen fraction of 0.4/1.0. Starting at normoventilation (PaCO(2) +/-40 mmHg) two steps of both hypercapnia (PCO(2) +/- 70 and 100 mmHg) and hypocapnia (PaCO(2) +/- 20 and 30 mmHg) were performed. Under these conditions, brain glucose and lactate levels as well as brain oxygen (PbrO(2)), brain carbon dioxide (PbrCO(2)), brain pH (brpH), brain temperature and ICP were measured.

RESULTS

At hypercapnia (PaCO(2) = 102.7 mmHg) there were no significant changes in brain glucose and lactate but there was a significant increase in PbrCO(2), PbrO(2) and ICP. In contrast, at hypocapnia (PCO(2) = 19.8 mmHg) there was a significant increase in brain lactate and a significant decrease in both brain glucose and PbrCO(2).

CONCLUSIONS

Hypocapnia decreases brain glucose and increases brain lactate concentration, indicating anaerobic metabolism, whereas hypercapnia has no influence on levels of brain glucose and brain lactate.

Cerebral fractional oxygen extraction in very low birth weight infants is high when there is low left ventricular output and hypocarbia but is unaffected by hypotension

This study examined the relationships between cerebral fractional oxygen extraction (FOE), mean arterial blood pressure (MABP), left ventricular output (LVO), blood gases, and other physiologic variables in 36 very-low-birth-weight preterm infants during the first 3 d after birth. There was a decrease in cerebral FOE (p = 0.008), and rises in LVO (p < 0.0001) and MABP (p = 0.02) during the 3 d. Between d 1 and 2, cerebral FOE decreased (p = 0.007) and LVO increased (p < 0.0001). There was no relationship between MABP and cerebral FOE. LVO correlated negatively with cerebral FOE on d 1 (p = 0.01), but not on d 2 (p = 0.07). On d 1, median pressure of arterial CO(2) was lower in infants with low LVO (<5(th) centile) and high cerebral FOE (>95(th) centile) than in infants with low LVO (<5(th) centile) but normal cerebral FOE (5(th)-95(th) centile) (p = 0.03). These findings suggest that cerebral FOE was increased only when LVO was low and there was hypocarbia. MABP had no demonstrable effect. It is likely that increased cerebral FOE is a normal physiologic response to maintain an adequate oxygen supply to the cerebral tissues when LVO is low and hypocarbia has caused vasoconstriction. It is possible that the cerebral hemispheres are low-priority vascular beds in the preterm infant, and that the high cerebral FOE is a result of reduced hemispheric blood flow to maintain MABP in the presence of low LVO.

Hemorrhagic-ischemic cerebral injury in the preterm infant: current concepts

PV-IVH and adjacent white matter injury remains a significant problem in the premature infant. The potential mechanisms contributing to injury are complex and involve factors related to blood flow and its regulation, as well as cellular mediators including cytokines, free radical formation, and excitotoxin release. Although a reduction in the occurrence of severe IVH can be achieved with indomethacin, it does translate into long-term neurodevelopmental benefit. This reinforces the concept of a more diffuse injury to brain in sick premature infants than is apparent from the appearance of current neuroimaging techniques.

Hypocarbia in the ventilated preterm infant and its effect on intraventricular haemorrhage and bronchopulmonary dysplasia

OBJECTIVE

To examine the relationship between PaCO2 levels in ventilated very preterm infants and (i) the incidence of severe intraventricular haemorrhage (IVH) and periventricular leukomalacia (PVL); and (ii) bronchopulmonary dysplasia (BPD).

METHODS

A retrospective cohort analysis of preterm infants comparing PaCO2 levels with the incidence of severe IVH/PVL and BPD was carried out on patients born at less than 29 weeks gestation from 1992 to 1994 and admitted to the tertiary neonatal intensive care unit at the King Edward Memorial Hospital (314 infants). During the first 96 h, PaCO2 levels were examined including lowest and highest PaCO2 levels, mean PaCO2 levels and duration of hypocarbia both pre- and post-surfactant administration.

RESULTS

Of the 314 infants, there were 40 early neonatal deaths (less than 48 h) who were not included in the analysis. Of the 274 surviving infants, 72 (26%) infants had severe IVH. Infants whose PaCO2 fell below 30 mmHg at any stage in the first 48 h of life had an increased risk of severe IVH or PVL (odds ratio 2.38; 95% CI 1.27-4.49; P = 0.007). Of the 265 survivors to 36 weeks corrected gestational age, 134 (51%) had BPD. Infants with at least three PaCO2 values less than 30 mmHg in the first 24 h of life had an increased risk of BPD (odds ratio 2.21; 95% CI 1.05-4.57; P = 0.036).

CONCLUSIONS

The risk of severe IVH/PVL was significantly increased by hypocarbia. There was also an association between hypocarbia and BPD, particularly when hypocarbia was prolonged. These findings suggest that avoidance of hypocarbia may reduce the incidence of severe IVH/PVL and BPD in preterm infants.

Cerebral palsy in very preterm infants: new epidemiological insights

The focus of this review is on new insights from recent epidemiological research on cerebral palsy in preterm infants. These include: 1) a better understanding of issues related to diagnosis and classification; 2) new information about the brain abnormalities underlying cerebral palsy in preterm infants; and 3) a better understanding of biological mechanisms that may underlie previously described epidemiological associations. Ongoing efforts to improve the diagnosis and classification of cerebral palsy have been enhanced by findings from serial examinations of cohorts of very preterm infants. Cranial ultrasonography through the anterior fontanelle of very preterm infants has provided information about grossly evident brain damage, found in about one-half of preterm infants who develop cerebral palsy. Insights into the pathophysiologic basis for certain epidemiologic associations have come from studies of experimental brain damage in animals and clinical studies of neurologic disorders in adults. Much of the current epidemiological research into the causes of cerebral palsy in preterm infants has focused on two potential mechanisms of brain damage. One mechanism involves insufficient cerebral perfusion; the other, cytokine-mediated damage, potentially triggered by events such as maternal infection (e.g., intrauterine or periodontal infection), neonatal infection (e.g., sepsis and necrotizing enterocolitis), and neonatal oxygen- or ventilator-induced lung injury. In addition to the preterm infant's increased exposure to such damaging factors, the high frequency of cerebral palsy in these infants might be due, in part, to insufficient levels of developmentally regulated protective substances, such as thyroid hormone and glucocorticoids. Models of causation currently are being investigated using recently developed methods for quantifying, with small quantities of blood, biomolecules that are suspected to either promote or protect against brain damage in the neonate. Clinical investigations now under way can be expected to identify strategies to be tested in clinical trials that could lower the risk of cerebral palsy in very preterm infants.

Permissive hypercapnia

Although lifesaving, mechanical ventilation can result in lung injury and contribute to the development of bronchopulmonary dysplasia. The most critical determinants of lung injury are tidal volume and end-inspiratory lung volume. Permissive hypercapnia offers to maintain gas exchange with lower tidal volumes and thus decrease lung injury. Further physiologic benefits include improved oxygen delivery and neuroprotection, the latter through both avoidance of accidental hypocapnia, which is associated with a poor neurologic outcome, and direct cellular effects. Clinical trials in adults with acute respiratory failure indicated improved survival and reduced incidence of organ failure in subjects managed with low tidal volumes and permissive hypercapnia. Retrospective studies in low birth weight infants found an association of bronchopulmonary dysplasia with low PaCO(2). Randomized clinical trials of low birth weight infants did not achieve sufficient statistical power to demonstrate a reduction of BPD by permissive hypercapnia, but strong trends indicated the possibility of important benefits without increased adverse events. Herein, we review the mechanisms leading to lung injury, the physiologic effects of hypercapnia, the dangers of hypocapnia, and the available clinical data.

Cerebral hemodynamics during early neonatal period in preterm infants with periventricular leukomalacia

We prospectively investigated the relation among cerebral blood flow, periventricular leukomalacia (PVL) and hypocarbia using Doppler ultrasonography in 53 preterm infants with gestational age between 27 and 34 weeks who required mechanical ventilation during the first 72 h of life. Cerebral blood flow of pericallosal artery was assessed by Doppler ultrasonography at the first and the third day of life. Mean velocity (MV) and Resistance index (RI) of anterior cerebral artery were calculated from the data obtained by Doppler ultrasonography. The diagnosis of PVL was made in 12 infants on the basis of the results of ultrasonography and MRI. Hypocarbia was judged as positive when both arterial blood gas analyses before and after the ultrasonography revealed PaCO(2) values < 25 mmHg. On the first day of life, RI was 0.62 +/- 0.022 in infants with PVL and 0.71 +/- 0.014 in those without PVL. On the third day of life, RI was 0.60 +/- 0.032 in infants with PVL and 0.66 +/- 0.013 in those without PVL. There was a significant difference in RI between the two groups at either point. MV was not significantly different between the two groups at either point. There was no significant difference in RI or MV between infants with and without hypocarbia at either point. RI was significantly lower in infants with PVL during the first 72 h of life, which is suggestive of vasoparalysis in such infants at the level of major cerebral arteries. However, RI or MV was no different between infants with and without hypocarbia.