Permissive hypercapnia

Exploring Alternate Targets for Respiratory Resuscitation in Patients With Sepsis and Septic Shock

Despite limited evidence to support it, resuscitation in sepsis has primarily targeted aggressive fluid administration and liberal administration of oxygen. In 2024, new thought paradigms emerged to suggest that dysregulation of aerobic metabolism are essential underpinnings of sepsis, and that in fact, aggressive resuscitation with fluids liberal oxygen could potentially aggravate oxidative stress and organ failure in sepsis. As sepsis continues to be shaped and molded by the latest research; therapies targeting sepsis and septic shock management warrant similar scrutiny.

METHODS

We searched literature pertaining to what is known about metabolic dysregulation in sepsis, to consider approaches to identifying new targets for resuscitation and management in sepsis.

RESULTS

Therapeutic hypoxemic targets of 88-92% have been shown to have some benefit in sepsis resuscitation in a limited number of studies. The benefit is believed to result from protection from excessive accumulation of harmful reactive oxygen species.

CONCLUSION

Limited supporting evidence exists in the literature to recommend targeted hypoxemia or hypercapnia in patients with sepsis. Mixed results have been observed in the literature, including minimal benefit to mortality. New research designs with consideration to the dysregulated metabolic sequelae in sepsis could improve the meaningfulness of these therapies in sepsis.

Optimal Maternal Ventilation During Laparotomy with General Anesthesia in Pregnancy in the Ovine Model

BACKGROUND

General anesthesia during pregnancy is not uncommon, for example, for trauma surgery, cerclage, or cesarean delivery. Current recommendations are to maintain maternal partial pressure of carbon dioxide in arterial blood (paCO2) at 30 mm Hg, which is based solely on the average maternal paCO2 in awake pregnant women. However, there is no evidence that this target, compared to other targets, would enable optimal conditions for the fetus during general anesthesia. Maternal paCO2 can affect uterine blood flow, affinity of hemoglobin for oxygen, and fetal CO2 elimination. In this study, a range of potential targets of maternal paCO2 was investigated in the ovine model, aiming to determine which target is most conducive to physiological fetal blood gas values during laparotomy with general anesthesia.

METHODS

Ten time-mated pregnant Swifter ewes with a gestational age of 93 to 104 days were used. During the first phase of the experiment, anesthesia was induced, all ewes were ventilated to target a physiological maternal paCO2 of 30 mm Hg, a maternal laparotomy was performed, and a fetal microcatheter was inserted surgically to enable blood sampling from the fetal aorta. Thereafter, in the second phase of the experiment, the 10 pregnant ewes were randomized to 10 different targets of maternal paCO2 between 27 and 50 mm Hg (1 target for each ewe), and maternal ventilation was adjusted accordingly. Forty-five minutes later, maternal and fetal arterial blood gas samples were analyzed. Linear regression models were used to estimate maternal paCO2 enabling physiologic fetal parameters, including fetal paCO2 (primary outcome).

RESULTS

A maternal paCO2 of 27.4 mm Hg (95% confidence interval, 23.1-30.3) enabled physiological fetal paCO2. Each increase in maternal paCO2 by 1 mm Hg, on average, increased fetal paCO2 by 0.94 mm Hg (0.69-1.19). This relationship had a strong correlation (r² = 0.906). No fetuses died during the experiment.

CONCLUSIONS

This study provides experimental support for the clinical recommendation to maintain maternal paCO2 close to the physiologic value of 30 mm Hg during general anesthesia for maternal laparotomy in pregnancy as it is conducive to physiological fetal blood gas values. Given the lower bound of the 95% confidence interval, the possibility that a lower maternal paCO2 would improve fetal gas exchange cannot be excluded.

Review of pediatric hypercarbia and intraoperative management

PURPOSE OF REVIEW

Hypercarbia in pediatric patients is an important component of intraoperative management. Despite marked advances in medicine and technology, it is uncertain what the physiological CO2 range in neonates, infants and small children. This data is extrapolated from the adult population. We are going to review advantages and disadvantages of CO2 measurement techniques, causes and systemic effects of hypercarbia. We are going to discuss how to approach management of intraoperative hypercarbia.

RECENT FINDINGS

Although physiological range in this patient population may not be fully understood, it is known that any rapid change from a child's baseline increases risks of complications. Any derangements in CO2 are further compromised by hypoxia, hypotension, hypothermia, anemia, all of which may occur in a dynamic operating room environment.

SUMMARY

Pediatric anesthesiologists and their teams must remain vigilant and anticipate these developments. Care must be taken to avoid any rapid changes in these vulnerable patients to minimize risks of adverse outcomes.

Carbon dioxide levels in neonates: what are safe parameters?

There is no consensus on the optimal pCO₂ levels in the newborn. We reviewed the effects of hypercapnia and hypocapnia and existing carbon dioxide thresholds in neonates. A systematic review was conducted in accordance with the PRISMA statement and MOOSE guidelines. Two hundred and ninety-nine studies were screened and 37 studies included. Covidence online software was employed to streamline relevant articles. Hypocapnia was associated with predominantly neurological side effects while hypercapnia was linked with neurological, respiratory and gastrointestinal outcomes and Retinpathy of prematurity (ROP). Permissive hypercapnia did not decrease periventricular leukomalacia (PVL), ROP, hydrocephalus or air leaks. As safe pCO₂ ranges were not explicitly concluded in the studies chosen, it was indirectly extrapolated with reference to pCO₂ levels that were found to increase the risk of neonatal disease. Although PaCO₂ ranges were reported from 2.6 to 8.7 kPa (19.5-64.3 mmHg) in both term and preterm infants, there are little data on the safety of these ranges. For permissive hypercapnia, parameters described for bronchopulmonary dysplasia (BPD; PaCO₂ 6.0-7.3 kPa: 45.0-54.8 mmHg) and congenital diaphragmatic hernia (CDH; PaCO₂ ≤ 8.7 kPa: ≤65.3 mmHg) were identified. Contradictory findings on the effectiveness of permissive hypercapnia highlight the need for further data on appropriate CO₂ parameters and correlation with outcomes. IMPACT: There is no consensus on the optimal pCO₂ levels in the newborn. There is no consensus on the effectiveness of permissive hypercapnia in neonates. A safe range of pCO₂ of 5-7 kPa was inferred following systematic review.

Capillary partial pressure of carbon dioxide for predicting rehospitalization in preterm infants under noninvasive respiratory support with severe bronchopulmonary dysplasia

BACKGROUND

The severity of bronchopulmonary dysplasia (BPD) is an important predictor of prognosis in preterm infants. However, the severity of BPD was determined mainly by the degree of oxygen supplementation and mode of respiratory support.

OBJECTIVES

This retrospective study aimed to examine the role of partial pressure of carbon dioxide (pCO₂ ) in predicting rehospitalization among preterm infants with severe BPD without invasive ventilation at 36 weeks' postmenstrual age (PMA).

METHODS

We assessed preterm infants aged <32 gestational weeks with severe BPD who were receiving noninvasive respiratory support at 36 weeks' PMA. Patients were compared after stratifying them according to the history of rehospitalization owing to respiratory infection before a corrected age (CA) of 1 year and pCO₂ measured by capillary blood gas analysis at 36 weeks' PMA.

RESULTS

Among 54 infants who had severe BPD with noninvasive respiratory support at 36 weeks' PMA, 16 (29.6%) experienced rehospitalization due to respiratory problems. At 36 weeks' PMA, the amount of oxygen supplementation (0.30 vs. 0.28, p = 0.021) and pCO₂ (62.1 vs. 53.6 mmHg, p = 0.006) were higher in the rehospitalization group than in the no rehospitalization group. Multivariate logistic analysis findings indicated that pCO₂  ≥ 57.4 mmHg was the only factor associated with rehospitalization (adjusted odds ratio: 8.017, 95% confidence interval 1.239-51.859).

CONCLUSION

High pCO₂ during noninvasive respiratory support at 36 weeks' PMA in severe BPD was associated with rehospitalization. Consideration of the degree of impairment in ventilatory capacity may improve the prediction of later respiratory outcomes in infants with BPD.

Transcutaneous carbon dioxide pattern and trend over time in preterm infants

BACKGROUND

Chronic lung disease remains a burden for extremely preterm infants. The changes in ventilation over time and optimal ventilatory management remains unknown. Newer, non-invasive technologies provide insight into these patterns.

METHODS

This single-center prospective cohort study enrolled infants ≤32 0/7 weeks. We obtained epochs of transcutaneous carbon dioxide (TcCO₂) measurements twice each week to describe the pattern of hypercarbia throughout their hospitalization.

RESULTS

Patterns of hypercarbia varied based on birth gestational age and post-menstrual age (PMA) (p = 0.03), regardless of respiratory support. Infants receiving the most respiratory support had values 16-21 mmHg higher than those on room air (p < 0.001). Infants born at the youngest gestational ages had the greatest total change but the rate of change was slower (p = 0.049) compared to infants born at later gestational ages. All infants had TcCO₂ values stabilize by 31-33 weeks PMA, when values were not significantly different compared to discharge. No rebound was observed when infants weaned off invasive support.

CONCLUSIONS

Hypercarbia improves as infants approached 31-33 weeks PMA. Hypercarbia was the highest in the most immature infants and improved with age and growth despite weaning respiratory support.

IMPACT

This study describes the evolution of hypercarbia as very preterm infants grow and develop. The pattern of ventilation is significantly different depending on the gestational age at birth and post-menstrual age. Average transcutaneous carbon dioxide (TCO₂) decreased over time as infants became more mature despite weaning respiratory support. This improvement was most significant in infants born at the lowest gestational ages.

Diffusion tensor imaging in extremely low birth weight infants managed with hypercapnic vs. normocapnic ventilation

BACKGROUND

Permissive hypercapnia is a ventilatory strategy used to prevent lung injury in ventilated extremely low birth weight (ELBW, birth weight ≤1,000 g) infants. However, there is retrospective evidence showing that high CO2 is associated with brain injury.

OBJECTIVE

The objective of this study was to compare brain white matter development at term-equivalent age in ELBW infants randomized to hypercapnic vs. normocapnic ventilation during the first week of life and in healthy non-ventilated term newborns.

MATERIALS AND METHODS

Twenty-two ELBW infants from a randomized controlled trial were included in this study; 11 received hypercapnic (transcutaneous PCO2 [tcPCO2] 50-60 mmHg) ventilation and 11 normocapnic (tcPCO2 35-45 mmHg) ventilation during the first week of life while still intubated. In addition, ten term healthy newborns served as controls. Magnetic resonance imaging (MRI) with diffusion tensor imaging (DTI) was performed at term-equivalent age for the ELBW infants and at approximately 2 weeks of age for the control infants. White matter injury on conventional MRI was graded in the ELBW and control infants using a scoring system adopted from literature. Tract-based spatial statistics (TBSS) was used to evaluate for differences in DTI measured fractional anisotropy (FA, spatially normalized to a customized template) among the ELBW and term control infants.

RESULTS

Conventional MRI white matter scores were not different (7.3 ± 1.7 vs. 6.9 ± 1.4, P = 0.65) between the hypercapnic and normocapnic ELBW infants. TBSS analysis did not show significant differences (P < 0.05, corrected) between the two ELBW infant groups, although before multiple comparisons correction, hypercapnic infants had many regions with lower FA and no regions with higher FA (P < 0.05, uncorrected) compared to normocapnic infants. When compared to the control infants, normocapnic ELBW infants had a few small regions with significantly lower FA, while hypercapnic ELBW infants had more widespread regions with significantly lower FA (P < 0.05, fully corrected for multiple comparisons).

CONCLUSIONS

Normocapnic ventilation vs. permissive hypercapnia may be associated with improved white matter development at term-equivalent age in ELBW infants. This effect, however, was small and was not apparent on conventional MRI. Further research is needed using larger sample sizes to assess if permissive hypercapnic ventilation in ELBW infants is associated with worse white matter development.

Incidence of hypo- and hyper-capnia in a cross-sectional European cohort of ventilated newborn infants

OBJECTIVE

To determine the incidence of hypo- and hyper-capnia in a European cohort of ventilated newborn infants.

DESIGN AND SETTING

Two-point cross-sectional prospective study in 173 European neonatal intensive care units.

PATIENTS AND METHODS

Patient characteristics, ventilator settings and measurements, and blood gas analyses were collected for endotracheally ventilated newborn infants on two separate dates.

RESULTS

A total of 1569 blood gas analyses were performed in 508 included patients with a mean±SD Pco2 of 48±12 mm Hg or 6.4±1.6 kPa (range 17-104 mm Hg or 2.3-13.9 kPa). Hypocapnia (Pco2<30 mm Hg or 4 kPa) and hypercapnia (Pco2>52 mm Hg or 7 kPa) was present in, respectively, 69 (4%) and 492 (31%) of the blood gases. Hypocapnia was most common in the first 3 days of life (7.3%) and hypercapnia after the first week of life (42.6%). Pco2 was significantly higher in preterm infants (49 mm Hg or 6.5 kPa) than term infants (43 mm Hg or 5.7 kPa) and significantly lower during pressure-limited ventilation (47 mm Hg or 6.3±1.6 kPa) compared with volume-targeted ventilation (51 mm Hg or 6.8±1.7 kPa) and high-frequency ventilation (50 mm Hg or 6.7±1.7 kPa).

CONCLUSIONS

This study shows that hypocapnia is a relatively uncommon finding during neonatal ventilation. The higher incidence of hypercapnia may suggest that permissive hypercapnia has found its way into daily clinical practice.

Maternal carbon dioxide level during labor and its possible effect on fetal cerebral oxygenation: mini review

During pregnancy, and especially during labor, the maternal carbon dioxide level declines considerably. Maternal carbon dioxide levels show a close relation with fetal carbon dioxide levels. The latter affects fetal cerebral oxygenation by regulating cerebral blood flow and shifting the oxyhemoglobin dissociation curve. In addition, maternal hypocapnia appears to impair placental oxygen transfer. Thus, maternal hyperventilation may interfere with optimal fetal cerebral oxygenation. Here, we provide a brief overview of the literature relevant to this issue.

A comprehensive approach to the prevention of bronchopulmonary dysplasia

The current bronchopulmonary dysplasia (BPD) is seen in infants born extremely premature, with less severe respiratory distress syndrome (RDS) and who received prenatal steroids-"new BPD". The pathophysiology of BPD is based on an impairment of lung maturation with prenatal and postnatal multi-hit insults and genetic susceptibility. This multifactorial pathophysiology of BPD suggests that no single "magic bullet" will prevent it. Thus, to avoid BPD we need to implement a complex and comprehensive strategy. This strategy is based on ventilatory and non-ventilatory measures. The ventilatory route allows an individualized endotracheal intubation approach. Early lung recruitment with nasal respiratory support (nasal continuous positive airway pressure [NCPAP] or nasal intermittent positive pressure ventilation [NIPPV] / synchronized NIPPV [SNIPPV]) and the INSURE (intubation, surfactant and early extubation) approach are discussed. Initial treatment with NCPAP did not reduce the rate of BPD compared to endotracheal ventilation and surfactant administration. While NIPPV/SNIPPV may have short-term advantages over NCPAP, the effect on BPD needs to be further studied. During hospitalization the respiratory goals should aim for adequate oxygenation, permissive hypercapnia, and gentle ventilation. However, these goals were found to have short-term benefits but did not reduce significantly the rate of BPD. Selective use of a short course of low dose corticosteroids can be considered after the first or second week of life in infants who are unable to be weaned from the ventilator and are at high risk for BPD. Non-ventilatory measures include early nutritional support with fluid restriction, caffeine and consideration of vitamin A. Hemodynamic significant patent ductus arteriosus (PDA) may be associated with BPD, but medical or surgical treatment of PDA were not shown to decrease BPD. Each component and the strategy as a whole needs to be further studied in large randomized prospective studies or by meta-analyses, especially in the target population of extremely premature infants who are the most prone to BPD.

Association of bronchopulmonary dysplasia and hypercarbia in ventilated infants with birth weights of 500-1,499 g

Bronchopulmonary dysplasia (BPD) continues to be a major pulmonary complication in very low birth weight (VLBW) and extremely low birth weight (ELBW) survivors of neonatal intensive care units (NICUs). Many factors including partial pressures of carbon dioxide (PaCO: (2)) have been implicated as possible causes. Permissive hypercapnia has become a more common practice in ventilated infants, but its effect on BPD is unclear. The hypothesis of this study was that hypercarbia is associated with increased BPD in infants with birth weights of 500-1,499 g. Nine hospitals were involved in this observational cohort study. Maternal and infant information including socio-demographics, antenatal steroids, gender, race, gestational age, birth weight, intubation and ventilator status, physiologic variables and data on therapies were collected by chart abstraction. SNAP scores were assigned. Candidate BPD risk factors, including cumulative exposures derived from blood gas and ventilation data in the first 6 days of life, were identified. Risk models were developed for 425 preterm infants who survived to 36 weeks post-menstrual age. BPD occurrence was associated with the cumulative burden of MAP >0 cm H(2)O in the first 6 days of life (P < 0.0001). After adjustment for the burden of MAP, the occurrence of hypercarbia (PaCO: (2) >50 torr) was associated with a greater incidence of BPD (P = 0.024). Among 293 intubated, mechanically ventilated infants, those with hypercarbia occurring only when MAP ≤ 8 cm H(2)O, a scenario more comparable to permissive hypercapnia, also had increased BPD incidence compared to infants without hypercarbia (P = 0.0003). Hypercarbia during the first 6 days of life was associated with increased incidence of BPD in these infants. Mechanically ventilated infants with hypercarbia during low MAP also had a significant increase in BPD. Permissive hypercapnia in ventilated infants needs further close review before the practice becomes even more widespread.

PaCO2 and neurodevelopment in extremely low birth weight infants

OBJECTIVE

To determine the relationship between PaCO2 in the first 4 days of life and neurodevelopment at 18 to 22 months.

STUDY DESIGN

Stepwise regression and exhaustive CHAID (Chi-squared Automatic Interaction Detector) analyses were done for neurodevelopmental impairment (NDI), mental developmental index (MDI), and psychomotor developmental index (PDI) using clinical variables in combination with the maximum (max), time-weighted average (avg), and max-minimum (max-min) PaCO2 in 400 infants of 401 to 1000 g birth weight (BW).

RESULTS

By regression, NDI predictors were male sex, non-Caucasian race, premature prolonged rupture of membranes (PPROM), lower BW, IVH 3 to 4, and lower 1-minute Apgar score. For lower MDI, predictors were male sex, non-Caucasian race, PPROM, IVH 3 to 4, sepsis, and higher max-min PaCO2. For lower PDI, predictors were male sex, PPROM, lower BW, IVH 3 to 4, sepsis, and higher avg PaCO2. By CHAID, the most important predictor of NDI was sex. For MDI, sex was most important followed by max-min PaCO2 >42 mm Hg in boys. For PDI, IVH was most important, followed by max-min PaCO2 >42 mm Hg for grade < or = 2 IVH.

CONCLUSIONS

Extreme fluctuations in PaCO2 and higher max PaCO2 are associated with worse neurodevelopmental outcomes and may indicate either a greater severity of illness or contribution of PaCO2 to pathophysiology of adverse neurodevelopment.

Effects of chronic hypercapnia in the neonatal mouse lung and brain

BACKGROUND

Permissive hypercapnia is increasingly utilized in the care of premature infants to prevent bronchopulmonary dysplasia. In a previous investigation, we described gene expression changes in the neonatal mouse lung exposed to chronic hypercapnia that might contribute to lung protection and accelerated maturation. However, it is unknown whether chronic hypercapnia increases alveolar formation, nor if it has detrimental effects in other developing organs such as the brain.

OBJECTIVE

To determine whether chronic hypercapnia accelerates early alveolar formation and increases neuronal cell injury in the developing mouse lung and brain, respectively.

DESIGN

Mouse pups were exposed to 8% CO(2) + 21% O(2) starting at postnatal day (P) 2 until P7. Control animals were maintained in room air. Animals were sacrificed at P4 or P7, and lungs and brains were excised and analyzed.

RESULTS

Exposure to 8% CO(2) resulted in an increased expression of alpha-smooth muscle actin (alpha-sma) which localized to the tips of developing alveolar buds, and also an increased number of alveolar buds at P7. Importantly, hypercapnic animals also demonstrated evidence of increased TUNEL-positive cells in the brain.

CONCLUSIONS

Exposure to chronic hypercapnia may lead to early initiation of alveolar budding in the neonatal mouse, but may also lead to increased TUNEL-positive cells in the developing brain.

Permissive hypercapnia to decrease lung injury in ventilated preterm neonates

Lung injury in ventilated premature infants occurs primarily through the mechanism of volutrauma, often due to the combination of high tidal volumes in association with a high end-inspiratory volume and occasionally end-expiratory alveolar collapse. Tolerating a higher level of arterial partial pressure of carbon dioxide (PaCO2) is considered as 'permissive hypercapnia' and when combined with the use of low tidal volumes may reduce volutrauma and lead to improved pulmonary outcomes. Permissive hypercapnia may also protect against hypocapnia-induced brain hypoperfusion and subsequent periventricular leukomalacia. However, extreme hypercapnia may be associated with an increased risk of intracranial hemorrhage. It may therefore be important to avoid large fluctuations in PaCO2 values. Recent randomized clinical trials in preterm infants have demonstrated that mild permissive hypercapnia is safe, but clinical benefits are modest. The optimal PaCO2 goal in clinical practice has not been determined, and the available evidence does not currently support a general recommendation for permissive hypercapnia in preterm infants.

Hypercapnia and the neonate

'Permissive hypercapnia' is a familiar term in neonatal intensive care, given the widespread adoption of low-tidal-volume ventilation strategies applied with the goal of decreasing respiratory morbidity. Recent evidence suggesting that hypercapnic acidosis may itself have protective effects on the lung and other organs has led to the coining of a new phrase, 'therapeutic hypercapnia', which also encompasses the use of supplemental inspired CO(2).

CONCLUSION

Experimental evidence suggests that mild-moderate hypercapnia can improve tissue oxygenation and perfusion, which may ameliorate injury to the immature lung and brain. However, hypercapnia may also be associated with adverse outcomes, and the range of PaCO(2) levels that are both safe and effective for specific subsets of neonates has yet to be determined.

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.

Safety and effectiveness of permissive hypercapnia in the preterm infant

PURPOSE OF REVIEW

Bronchopulmonary dysplasia continues to be an important cause of morbidity in premature infants who require mechanical ventilation. Management strategies have historically focused on normalizing blood gases but new research suggests that a higher PCO2 level may be well tolerated in premature infants. There are physiologic rationale and recent experimental data to support the potential benefits of permissive hypercapnia.

RECENT FINDINGS

Higher PCO2 levels may allow a reduction in ventilatory support which reduces the risk of lung injury in intubated patients. Targeting PCO2 levels above 45 mmHg has been tested in randomized controlled trials. These trials report that neonates managed with permissive hypercapnia have a shorter duration of mechanical ventilation and reduced severity of bronchopulmonary dysplasia without an increase in adverse events.

SUMMARY

Permissive hypercapnia appears as a safe and effective management strategy to decrease morbidity from bronchopulmonary dysplasia in premature infants. Although the preliminary results are promising, further research is needed to determine whether this strategy improves pulmonary outcomes without adverse effects.

Fetal cerebral oxygenation: the role of maternal hyperoxia with supplemental CO2 in sheep

OBJECTIVE

We tested the hypothesis that supplemental CO2 can enhance the effect of maternal oxygen administration on fetal cerebral oxygenation.

STUDY DESIGN

In near-term fetal sheep (n = 6), we instrumented the cerebral cortex with tissue PO2-laser Doppler flow probes, and placed arterial catheters. Following a 30-minute control period, the ewe breathed 50% O2 for 15 minutes, followed by added 6% CO2 for 15 minutes. We examined fetal cortical tissue PO2, cerebral blood flow (CBF), and fetal and maternal blood gases and related variables.

RESULTS

In response to maternal O2 administration, fetal arterial PO2, O2 content, cerebral O2 delivery, and cortical tissue PO2 increased significantly. In response to supplemental CO2 inhalation, fetal cortical tissue PO2 increased further. Fetal CBF also increased in response to the elevated arterial CO2 level.

CONCLUSION

CO2 supplementation of maternal O2 administration enhanced fetal cerebral oxygenation. In contrast, it was considered that during labor maternal hyperventilation with hypocapnia may blunt the effect of maternal O2 inhalation.

Inhaled nitric oxide therapy might reduce the need for hyperventilation therapy in infants with persistent pulmonary hypertension of the newborn

AIM

To determine whether inhaled nitric oxide might reduce the need for excessive respiratory alkalosis to maintain systemic oxygenation in infants with persistent pulmonary hypertension of the newborn (PPHN).

MATERIALS AND METHODS

A retrospective historical cohort study of 34 infants with PPHN with oxygenation index (OI) of 25 or more, including 19 infants without inhaled nitric oxide (i-NO) therapy (control group) and 15 infants with inhaled nitric oxide therapy (i-NO group) was performed. The initial dose of 10 ppm of i-NO was administered and no responders received the maximum dose of 25 ppm. We evaluated the mortality rate and the change of OI index and PaCO(2) during the first 6 days.

RESULTS

There were no significant differences in characteristics between groups. Two of 15 in the i-NO group and 6 of 19 infants in the control group died during the first 48 h. Baseline OI, PaCO(2) and arterial pH were similar in the two groups. OI in the i-NO group was significantly higher than in the control group between 12 and 96 h. PaCO(2) in the i-NO group was higher than in the control group between 24 and 144 h.

CONCLUSION

i-NO therapy for PPHN might improve systemic oxygenation without excessive hypocapnia. However there was no reduction in duration of ventilation support or oxygen supply.

Fetal hypercapnia and cerebral tissue oxygenation: studies in near-term sheep

The precise role of CO2 in cerebral oxygenation is not as well defined as O2, especially in the immature brain. In the ovine fetus, we tested the hypotheses that arterial Pco2 (Paco2) plays a critical role not only in the regulation of cerebral blood flow but also in the regulation of cerebral tissue oxygenation. By use of a fluorescent O2 probe with a laser Doppler flowmeter and the placement of sagittal sinus catheter in six near-term fetal sheep, we measured values of cortical tissue O2 tension (tPo2), sagittal sinus oxyhemoglobin saturation ([HbO2]), and laser Doppler cerebral blood flow (LD-CBF) in response to 20 min hypercapnia induced by having the ewe breathe CO2. In response to moderate to severe hypercapnia, LD-CBF increased above baseline in a curvilinear fashion, cortical tPo2 increased linearly (1 torr per 3.2 torr Paco2), and sagittal sinus [HbO2] increased significantly in a curvilinear manner. Hypercapnia favored cerebral tissue oxygenation of the fetal brain; and cortical tPo2 and sagittal sinus [HbO2] complement or support one another as indices of cerebral oxygenation under hypercapnic conditions.

Therapeutic hypercapnia prevents chronic hypoxia-induced pulmonary hypertension in the newborn rat

Induction of hypercapnia by breathing high concentrations of carbon dioxide (CO(2)) may have beneficial effects on the pulmonary circulation. We tested the hypothesis that exposure to CO(2) would protect against chronic pulmonary hypertension in newborn rats. Atmospheric CO(2) was maintained at <0.5% (normocapnia), 5.5%, or 10% during exposure from birth for 14 days to normoxia (21% O(2)) or moderate hypoxia (13% O(2)). Pulmonary vascular and hemodynamic abnormalities in animals exposed to chronic hypoxia included increased pulmonary arterial resistance, right ventricular hypertrophy and dysfunction, medial thickening of pulmonary resistance arteries, and distal arterial muscularization. Exposure to 10% CO(2) (but not to 5.5% CO(2)) significantly attenuated pulmonary vascular remodeling and increased pulmonary arterial resistance in hypoxia-exposed animals (P < 0.05), whereas both concentrations of CO(2) normalized right ventricular performance. Exposure to 10% CO(2) attenuated increased oxidant stress induced by hypoxia, as quantified by 8-isoprostane content in the lung, and prevented upregulation of endothelin-1, a critical mediator of pulmonary vascular remodeling. We conclude that hypercapnic acidosis has beneficial effects on pulmonary hypertension and vascular remodeling induced by chronic hypoxia, which we speculate derives from antioxidant properties of CO(2) on the lung and consequent modulating effects on the endothelin pathway.

Respiratory plasticity after perinatal hypercapnia in rats

Environmental conditions during early life may have profound effects on respiratory control development. We hypothesized that perinatal hypercapnia would exert lasting effects on the mammalian hypercapnic ventilatory response, but that these effects would differ between males and females. Rats were exposed to 5% CO2 from 1 to 3 days before birth through postnatal week 2 and ventilation was subsequently measured by whole-body plethysmography. In both male and female rats exposed to perinatal hypercapnia, a rapid, shallow breathing pattern was observed for the first 2 weeks after return to normocapnia, but ventilation was unchanged. Acute hypercapnic ventilatory responses (3% and 5% CO2) were reduced 27% immediately following perinatal hypercapnia, but these responses were normal after 2 weeks of recovery in both sexes and remained normal as adults. Collectively, these data suggest that perinatal hypercapnia elicits only transient respiratory plasticity in both male and female rats. This plasticity appears similar to that observed after chronic hypercapnia in adult animals and, therefore, is not unique to development.

Acute respiratory distress syndrome. Computed tomography findings and their applications to mechanical ventilation therapy

In acute respiratory distress syndrome, computed tomography (CT) typically demonstrates symmetric ground-glass opacity and gravity-dependent consolidation when patients are positioned supine. Moreover, CT findings may vary according to the evolutional stage of the disease over time. The slope of the pressure-volume curve, which is a direct or indirect measure of lung recruitment rather than a measure of the characteristics of the respiratory mechanics of a portion of the aerated lung, indicates the potential for recruitment. The lung recruitment maneuver is performed by maintaining a sustained increase in airway pressure with the goal of opening collapsed alveoli, after which sufficient positive end-expiratory pressure is applied to maintain the alveoli in an open state. Alveolar recruitment resulting from continuous positive airway pressure occurs predominantly in nondependent and cephalic lung regions and is more limited in the diaphragmatic region or even negative (alveolar derecruitment) caudal to the diaphragmatic cupola. By partially relieving cardiac and abdominal compression, positioning patients in prone and semirecumbent positions may reopen dependent and caudal lung regions and improve gas exchange. The mean CT attenuation of a given region is equivalent to its aeration. Computed tomography, especially helical CT, may help to assess alveolar recruitment and lung overinflation by providing information on whole-lung attenuation.

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.

Tracheal gas insufflation as a lung-protective strategy: physiologic, histologic, and biochemical markers

OBJECTIVE

Conventional mechanical ventilation in acute lung failure potentiates lung injury, which can be assessed by physiologic, histologic, and biochemical markers. Thus, new ventilation strategies are directed at reducing lung injury. Tracheal gas insufflation has been shown to reduce endotracheal tube prosthetic deadspace and peak inspiratory pressure during conventional mechanical ventilation. Our objective was to use physiologic, histologic, and biochemical markers to test the hypothesis that tracheal gas insufflation in acute lung injury is lung protective.

DESIGN

Animal experiment.

SETTING

University setting.

SUBJECTS

Juvenile rabbits (n = 12; 1.95 +/- 0.1 SE kg).

INTERVENTIONS

Rabbits were anesthetized, instrumented, paralyzed, and ventilated with Fio(2) = 1.0. Lung injury was induced with repeated saline lavage (10 mL/kg per lavage until Pao(2) </=150 mm Hg and compliance </=0.50 mL/cm H(2)O/kg for 30 mins). Animals were randomized to conventional mechanical ventilation with and without 0.5 lpm of continuous tracheal gas insufflation (Vygon endotracheal tube) for 4 hrs to maintain Paco(2) at 45-55 mm Hg by adjusting the peak inspiratory pressure; other conventional mechanical ventilation settings remained constant.

MEASUREMENTS AND MAIN RESULTS

Gas exchange and pulmonary mechanics were measured every 30 mins; plasma and pulmonary tissue were taken for cytokine and histologic evaluation after 4 hrs. Peak inspiratory pressure, tidal volume, and physiologic deadspace were significantly less (p < .05) in the tracheal gas insufflation animals when compared with conventional mechanical ventilation animals. Pao(2), positive end-expiratory pressure, mean airway pressure, vital signs, Paco(2), and respiratory resistance and compliance were not statistically different between the two groups. There was a difference (p < .05) in interleukin-8 tissue (pg/mug protein; dependent = 52.4 +/- 7.6 vs. nondependent = 32.8 +/- 4.2) and plasma levels (pg/mL; preinjury = 7.2 +/- 2.3 vs. postinjury = 118 +/- 58). Histology showed a trend toward protection of alveolar structures for tracheal gas insufflation.

CONCLUSIONS

Tracheal gas insufflation resulted in lower ventilatory requirements (peak inspiratory pressure, tidal volume, and deadspace) and a more favorable histologic trend than conventional mechanical ventilation. Tracheal gas insufflation offers potential as a lung-protective strategy for acute lung injury in the developing rabbit lung and may be a useful clinical adjunct to neonatal respiratory management.