Hypercapnic acidosis and compensated hypercapnia in control and pulmonary hypertensive piglets

Is It the pH That Matters? Challenging the Pathophysiology of Acidemia in a Case of Severe Hypercapnia Secondary to Intraoperative CO2 Insufflation

Background

Acidemia has been long thought to lead to hemodynamic compromise. While some literature to date challenges this idea, there is no consensus on this topic.

Case Summary

To our knowledge, this is the most severe case of hypercapnia and acidosis due to carbon dioxide (CO₂) insufflation during laparoscopy reported in the literature. Remarkably, this patient remained hemodynamically normal despite having a blood pH below 6.81. This prompts a wider discussion about the effects of blood pH on human physiology. Most patients who present acidotic are critically ill and have confounding underlying metabolic or respiratory pathophysiology driving their illness. In this case, the patient experienced no respiratory insult leading to an increase in blood CO₂ but rather had CO₂ iatrogenically introduced into the circulatory system, effectively detaching the deleterious effects of CO₂ from the respiratory pathologies that so often cause its accumulation.

Conclusion

This raises the question, in patients with severe acidosis and hemodynamic compromise, is acidosis a symptom of the underlying process, or is the acidosis itself causing harm?

Home Oxygen Therapy for Children. An Official American Thoracic Society Clinical Practice Guideline

Background: Home oxygen therapy is often required in children with chronic respiratory conditions. This document provides an evidence-based clinical practice guideline on the implementation, monitoring, and discontinuation of home oxygen therapy for the pediatric population.

Methods: A multidisciplinary panel identified pertinent questions regarding home oxygen therapy in children, conducted systematic reviews of the relevant literature, and applied the Grading of Recommendations, Assessment, Development, and Evaluation approach to rate the quality of evidence and strength of clinical recommendations.

Results: After considering the panel’s confidence in the estimated effects, the balance of desirable (benefits) and undesirable (harms and burdens) consequences of treatment, patient values and preferences, cost, and feasibility, recommendations were developed for or against home oxygen therapy specific to pediatric lung and pulmonary vascular diseases.

Conclusions: Although home oxygen therapy is commonly required in the care of children, there is a striking lack of empirical evidence regarding implementation, monitoring, and discontinuation of supplemental oxygen therapy. The panel formulated and provided the rationale for clinical recommendations for home oxygen therapy based on scant empirical evidence, expert opinion, and clinical experience to aid clinicians in the management of these complex pediatric patients and identified important areas for future research.

Borderline pulmonary hypertension associated with chronic hypercapnia in chronic pulmonary disease

Pulmonary hypertension (PH) due to lung diseases is classified as group 3 by the Dana Point classification. Given the basic pathophysiological conditions of group 3 lung diseases and the previously well-known concept of hypercapnic pulmonary vasoconstriction, chronic hypercapnia besides alveolar hypoxia might be another causative factor to increase mean pulmonary arterial pressure (PAm). Two hundred twenty-five subjects with chronic pulmonary diseases were assessed by a right heart catheterization and blood gas parameters. The subjects were classified into the following 4 groups: Hypercapnic Hypoxia (HCHX), Hypercapnic Normoxia (HCnx), Normocapnic Hypoxia (ncHX), and Normocapnic Normoxia (ncnx). Compared with ncnx, the HCHX, HCnx and ncHX groups all showed significantly higher PAm and met the criteria of borderline PH. Multiple regression analysis showed that PaCO₂, as well as SaO₂, was an independent variable for PAm. Given the poor prognosis with borderline PH, the elimination of excess pulmonary carbon dioxide in hypercapnia could be a considerable treatment strategy in chronic pulmonary disease.

Pulmonary hemodynamics responses to hypoxia and/or CO2 inhalation during moderate exercise in humans

In this study, we hypothesized that adding CO₂ to an inhaled hypoxic gas mixture will limit the rise of pulmonary artery pressure (PAP) induced by a moderate exercise. Eight 20-year-old males performed four constant-load exercise tests on cycle at 40% of maximal oxygen consumption in four conditions: ambient air, normobaric hypoxia (12.5% O₂), inhaled CO₂ (4.5% CO₂), and combination of hypoxia and inhaled CO₂. Doppler echocardiography was used to measure systolic (s)PAP, cardiac output (CO). Total pulmonary resistance (TPR) was calculated. Arterialized blood pH was 7.40 at exercise in ambient and hypoxia conditions, whereas CO₂ inhalation and combined conditions showed acidosis. sPAP increases from rest in ambient air to exercise ranged as follows: ambient + 110%, CO₂ inhalation + 135%, combined + 184%, hypoxia + 217% (p < 0.001). CO was higher when inhaling O₂-poor gas mixtures with or without CO₂ (~ 17 L min^-1) than in the other conditions (~ 14 L min^-1, p < 0.001). Exercise induced a significant decrease in TPR in the four conditions (p < 0.05) but less marked in hypoxia (- 19% of the resting value in ambient air) than in ambient (- 33%) and in both CO₂ inhalation and combined condition (- 29%). We conclude that (1) acute CO₂ inhalation did not significantly modify pulmonary hemodynamics during moderate exercise. (2) CO₂ adjunction to hypoxic gas mixture did not modify CO, despite a higher CaO₂ in combined condition than in hypoxia. (3) TPR was lower in combined than in hypoxia condition, limiting sPAP increase in combined condition.

Influence of PCO2 Control on Clinical and Neurodevelopmental Outcomes of Extremely Low Birth Weight Infants

BACKGROUND

Levels or fluctuations in the partial pressure of CO2 (PCO2) may affect outcomes for extremely low birth weight infants.

OBJECTIVES

In an exploratory analysis of a randomized trial, we hypothesized that the PCO2 values achieved could be related to significant outcomes.

METHODS

On each treatment day, infants were divided into 4 groups: relative hypocapnia, normocapnia, hypercapnia, or fluctuating PCO2. Ultimate assignment to a group for the purpose of this analysis was made according to the group in which an infant spent the most days. Statistical analyses were performed with analysis of variance (ANOVA), the Kruskal-Wallis test, the χ2 test, and the Fisher exact test as well as by multiple logistic regression.

RESULTS

Of the 359 infants, 57 were classified as hypocapnic, 230 as normocapnic, 70 as hypercapnic, and 2 as fluctuating PCO2. Hypercapnic infants had a higher average product of mean airway pressure and fraction of inspired oxygen (MAP × FiO2). For this group, mortality was higher, as was the likelihood of having moderate/severe bronchopulmonary dysplasia (BPD), necrotizing enterocolitis (NEC), and poorer neurodevelopment. Multiple logistic regression analyses showed an increased risk for BPD or death associated with birth weight (p < 0.001) and MAP × FiO2 (p < 0.01). The incidence of adverse neurodevelopment was associated with birth weight (p < 0.001) and intraventricular hemorrhage (IVH; p < 0.01).

CONCLUSIONS

Birth weight and respiratory morbidity, as measured by MAP × FiO2, were the most predictive of death or BPD and NEC, whereas poor neurodevelopmental outcome was associated with low birth weight and IVH. Univariate models also identified PCO2. Thus, hypercapnia seems to reflect greater disease severity, a likely contributor to differences in outcomes.

Acidosis in the critically ill - balancing risks and benefits to optimize outcome

Acidosis is associated with poor outcome in critical illness. However, acidosis - both hypercapnic and metabolic - has direct effects that can limit tissue injury induced by many causes. There is also a clear potential for off-target harm with acute exposure (for example, raised intracranial pressure, pulmonary hypertension), and with exposure for prolonged periods (for example, increased risk of infection) or at high doses. Ongoing comprehensive determination of molecular, cellular and physiologic impact across a range of representative pathologies will allow us to understand better the risks and benefits of hypercapnia and acidosis during critical illness.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Stable mixed acidemia in anesthetized pigs--a model for research on biocompatibility of hemofilters under a deteriorated acid-base balance

In recent years, acidosis has been of growing interest in intensive care medicine. Most animal models only provide a short-term investigation of the effects of acidosis. They are not suitable for research on interactions with extracorporeal organ support (here continuous venovenous hemofiltration, CVVH). The rationale for this study was to establish a porcine model of prolonged mixed acidemia, which is suitable for research on the interactions of acidemia and CVVH. After the induction of anesthesia in pigs (40 kg), acidemia was induced and maintained in one group with a bolus of 0.4 mol/L lactic acid followed by continuous infusion and a reduced respiratory frequency (lactic acid-group, n = 4). In another group, mixed acidemia was induced with a 0.4 mol/L acid solution (lactic and hydrochloric acid) and low tidal volume ventilation (mixed acidemia-group, n = 8). To get first proof of the model's suitability to operate over an extracorporeal circuit, CVVH was additionally performed in seven pigs (mixed acidemia/CVVH-group, n = 7). The target for the pH was 7.19-7.24. The targeted pH was constantly missed in the lactic acid group, whereas it was successfully maintained for 3.5 h in four out of eight pigs of the mixed acidemia group, and in five out of seven pigs of the mixed acidemia/CVVH group. The CVVH was performed successfully for 3 h in all pigs of the respective group. The mixed acidemia model was sufficient to maintain a low pH within a narrow range for some hours and enabled research on hemofilters in vivo.

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.

Upregulation of vascular calcium channels in neonatal piglets with hypoxia-induced pulmonary hypertension

Inhibition of voltage-gated, L-type Ca(2+) (Ca(L)) channels by clinical calcium channel blockers provides symptomatic improvement to some pediatric patients with pulmonary arterial hypertension (PAH). The present study investigated whether abnormalities of vascular Ca(L) channels contribute to the pathogenesis of neonatal PAH using a newborn piglet model of hypoxia-induced PAH. Neonatal piglets exposed to chronic hypoxia (CH) developed PAH by 21 days, which was evident as a 2.1-fold increase in pulmonary vascular resistance in vivo compared with piglets raised in normoxia (N). Transpulmonary pressures (DeltaPtp) in the corresponding isolated perfused lungs were 20.5 +/- 2.1 mmHg (CH) and 11.6 +/- 0.8 mmHg (N). Nifedipine reduced the elevated DeltaPtp in isolated lungs of CH piglets by 6.4 +/- 1.3 mmHg but only reduced DeltaPtp in lungs of N piglets by 1.9 +/- 0.2 mmHg. Small pulmonary arteries from CH piglets also demonstrated accentuated Ca(2+)-dependent contraction, and Ca(2+) channel current was 3.94-fold higher in the resident vascular muscle cells. Finally, although the level of mRNA encoding the pore-forming alpha(1C)-subunit of the Ca(L) channel was similar between small pulmonary arteries from N and CH piglets, a profound and persistent upregulation of the vascular alpha(1C) protein was detected by 10 days in CH piglets at a time when pulmonary vascular resistance was only mildly elevated. Thus chronic hypoxia in the neonate is associated with the anomalous upregulation of Ca(L) channels in small pulmonary arteries in vivo and the resulting abnormal Ca(2+)-dependent resistance may contribute to the pathogenesis of PAH.

Permissive hypercapnia: role in protective lung ventilatory strategies

PURPOSE OF REVIEW

Hypercapnia is a central component of current protective ventilatory strategies. This review aims to present and interpret data from recent clinical and experimental studies relating to hypercapnia and its role in protective ventilatory strategies.

RECENT FINDINGS

Increasing clinical evidence supports the use of permissive hypercapnia, particularly in acute lung injury/acute respiratory distress syndrome, status asthmaticus, and neonatal respiratory failure. However, there are no clinical data examining the contribution of hypercapnia per se to protective ventilatory strategies. Recent experimental studies provide further support for the concept of therapeutic hypercapnia, whereby deliberately elevated PaCO2 may attenuate lung and systemic organ injury. CO2 administration attenuates experimental acute lung injury because of adverse ventilatory strategies, mesenteric ischemia reperfusion, and pulmonary endotoxin instillation. Hypercapnic acidosis attenuates key effectors of the inflammatory response and reduces lung neutrophil infiltration. At the genomic level, hypercapnic acidosis attenuates the activation of nuclear factor-kappaB, a key regulator of the expression of multiple genes involved in the inflammatory response. The physiologic effects of hypercapnia, both beneficial and potentially deleterious, are increasingly well understood. In addition, reports suggest that humans can tolerate extreme levels of hypercapnia for relatively prolonged periods without adverse effects.

SUMMARY

The potential for hypercapnia to contribute to the beneficial effects of protective lung ventilatory strategies is clear from experimental studies. However, the optimal ventilatory strategy and the precise contribution of hypercapnia to this strategy remain unclear. A clearer understanding of its effects and mechanisms of action is central to determining the safety and therapeutic utility of hypercapnia in protective lung ventilatory strategies.