The Effects of Lung Protective Ventilation or Hypercapnic Acidosis on Gas Exchange and Lung Injury in Surfactant Deficient Rabbits

Prospective Randomized Pilot Trial on the Effects of Mild Hypercapnia on Cerebral Oxygen Saturation in Patients Undergoing Off-Pump Coronary Artery Bypass Grafting

OBJECTIVE AND DESIGN

A single-center prospective randomized controlled study was conducted to assess the effect of targeted mild hypercapnia (TMH) on cerebral oxygen saturation (rSO2) in patients undergoing off-pump coronary artery bypass grafting (CABG).

SETTING AND PARTICIPANTS

A prospective randomized controlled study involving 100 patients undergoing off-pump CABG at U. N. Mehta Hospital, Ahmedabad, Gujarat, India.

INTERVENTION

Patients were randomized to either the TMH (PaCO2 45-55 mmHg) or the targeted normocapnia (TN; PaCO2 35-45 mmHg) group, containing 50 patients in each group.

MEASUREMENTS

Monitoring of rSO2, heart rate, mean arterial pressure (MAP), PaCO2, and peripheral oxygen saturation was done at baseline, after induction, after left internal mammary artery harvesting, at each grafting (distal and proximal), after protamine, and after shifting to the intensive care unit. The standardized minimental-state examination (SMMSE) was performed preoperatively and at 8, 12, and 24 hours postextubation. Data were analyzed using an independent sample t test.

RESULTS

The TMH group had higher MAP during grafting (p < 0.001) and higher rSO2 on both sides during distal and proximal grafting (p < 0.001) and after protamine (p < 0.05), as compared to the TN group. Compared to preoperative values, SMMSE scores in the TN group were significantly lower at 12 and 24 hours postextubation (p < 0.001).

CONCLUSION

TMH during grafting increased the cerebral blood flow and rSO2 when hemodynamic instability was very common. It has a protective role on the brain and helps maintain cognition postoperatively.

Inhaled CO2 vs. Hypercapnia Obtained by Low Tidal Volume or Instrumental Dead Space in Unilateral Pulmonary Artery Ligation: Any Difference for Lung Protection?

Background

Unilateral ligation of the pulmonary artery (UPAL) induces bilateral lung injury in pigs undergoing controlled mechanical ventilation. Possible mechanisms include redistribution of ventilation toward the non-ligated lung and hypoperfusion of the ligated lung. The addition of 5% CO₂ to the inspiratory gas (FiCO₂) prevents the injury, but it is not clear whether lung protection is a direct effect of CO₂ inhalation or it is mediated by plasmatic hypercapnia. This study aims to compare the effects and mechanisms of FiCO₂vs. hypercapnia induced by low tidal volume ventilation or instrumental dead space.

Methods

Healthy pigs underwent left UPAL and were allocated for 48 h to the following: Volume-controlled ventilation (VCV) with V_T 10 ml/kg (injury, n = 6); VCV plus 5% FiCO₂ (FiCO₂, n = 7); VCV with V_T 6 ml/kg (low V_T, n = 6); VCV plus additional circuit dead space (instrumental V_D, n = 6). Histological score, regional compliance, wet-to-dry ratio, and inflammatory infiltrate were assessed to evaluate lung injury at the end of the study. To investigate the mechanisms of protection, we quantified the redistribution of ventilation to the non-ligated lung, as the ratio between the percentage of tidal volume to the right and to the left lung (V_TRIGHT/LEFT), and the hypoperfusion of the ligated lung as the percentage of blood flow reaching the left lung (Perfusion_LEFT).

Results

In the left ligated lung, injury was prevented only in the FiCO₂ group, as indicated by lower histological score, higher regional compliance, lower wet-to-dry ratio and lower density of inflammatory cells compared to other groups. For the right lung, the histological score was lower both in the FiCO₂ and in the low V_T groups, but the other measures of injury showed lower intensity only in the FiCO₂ group. V_TRIGHT/LEFT was lower and Perfusion_LEFT was higher in the FiCO₂ group compared to other groups.

Conclusion

In a model of UPAL, inhaled CO₂ but not hypercapnia grants bilateral lung protection. Mechanisms of protection include reduced overdistension of the non-ligated and increased perfusion of the ligated lung.

Hypercapnia from Physiology to Practice

Acute hypercapnic ventilatory failure is becoming more frequent in critically ill patients. Hypercapnia is the elevation in the partial pressure of carbon dioxide (PaCO₂) above 45 mmHg in the bloodstream. The pathophysiological mechanisms of hypercapnia include the decrease in minute volume, an increase in dead space, or an increase in carbon dioxide (CO₂) production per sec. They generate a compromise at the cardiovascular, cerebral, metabolic, and respiratory levels with a high burden of morbidity and mortality. It is essential to know the triggers to provide therapy directed at the primary cause and avoid possible complications.

Addition of 5% CO2 to Inspiratory Gas Prevents Lung Injury in an Experimental Model of Pulmonary Artery Ligation

Rationale: Unilateral ligation of the pulmonary artery may induce lung injury through multiple mechanisms, which might be dampened by inhaled CO2. Objectives: This study aims to characterize bilateral lung injury owing to unilateral ligation of the pulmonary artery in healthy swine undergoing controlled mechanical ventilation and its prevention by 5% CO2 inhalation and to investigate relevant pathophysiological mechanisms. Methods: Sixteen healthy pigs were allocated to surgical ligation of the left pulmonary artery (ligation group), seven to surgical ligation of the left pulmonary artery and inhalation of 5% CO2 (ligation + FiCO2 5%), and six to no intervention (no ligation). Then, all animals received mechanical ventilation with Vt 10 ml/kg, positive end-expiratory pressure 5 cm H2O, respiratory rate 25 breaths/min, and FiO2 50% (±FiCO2 5%) for 48 hours or until development of severe lung injury. Measurements and Main Results: Histological, physiological, and quantitative computed tomography scan data were compared between groups to characterize lung injury. Electrical impedance tomography and immunohistochemistry analysis were performed in a subset of animals to explore mechanisms of injury. Animals from the ligation group developed bilateral lung injury as assessed by significantly higher histological score, larger increase in lung weight, poorer oxygenation, and worse respiratory mechanics compared with the ligation + FiCO2 5% group. In the ligation group, the right lung received a larger fraction of Vt and inflammation was more represented, whereas CO2 dampened both processes. Conclusions: Mechanical ventilation induces bilateral lung injury within 48 hours in healthy pigs undergoing left pulmonary artery ligation. Inhalation of 5% CO2 prevents injury, likely through decreased stress to the right lung and antiinflammatory effects.

Hypercapnia in the critically ill: insights from the bench to the bedside

Carbon dioxide (CO2) has long been considered, at best, a waste by-product of metabolism, and at worst, a toxic molecule with serious health consequences if physiological concentration is dysregulated. However, clinical observations have revealed that 'permissive' hypercapnia, the deliberate allowance of respiratory produced CO2 to remain in the patient, can have anti-inflammatory effects that may be beneficial in certain circumstances. In parallel, studies at the cell level have demonstrated the profound effect of CO2 on multiple diverse signalling pathways, be it the effect from CO2 itself specifically or from the associated acidosis it generates. At the whole organism level, it now appears likely that there are many biological sensing systems designed to respond to CO2 concentration and tailor respiratory and other responses to atmospheric or local levels. Animal models have been widely employed to study the changes in CO2 levels in various disease states and also to what extent permissive or even directly delivered CO2 can affect patient outcome. These findings have been advanced to the bedside at the same time that further clinical observations have been elucidated at the cell and animal level. Here we present a synopsis of the current understanding of how CO2 affects mammalian biological systems, with a particular emphasis on inflammatory pathways and diseases such as lung specific or systemic sepsis. We also explore some future directions and possibilities, such as direct control of blood CO2 levels, that could lead to improved clinical care in the future.

Humidified Warmed CO2 Treatment Therapy Strategies Can Save Lives With Mitigation and Suppression of SARS-CoV-2 Infection: An Evidence Review

The coronavirus disease (COVID-19) outbreak has presented enormous challenges for healthcare, societal, and economic systems worldwide. There is an urgent global need for a universal vaccine to cover all SARS-CoV-2 mutant strains to stop the current COVID-19 pandemic and the threat of an inevitable second wave of coronavirus. Carbon dioxide is safe and superior antimicrobial, which suggests it should be effective against coronaviruses and mutants thereof. Depending on the therapeutic regime, CO2 could also ameliorate other COVID-19 symptoms as it has also been reported to have antioxidant, anti-inflammation, anti-cytokine effects, and to stimulate the human immune system. Moreover, CO2 has beneficial effects on respiratory physiology, cardiovascular health, and human nervous systems. This article reviews the rationale of early treatment by inhaling safe doses of warmed humidified CO2 gas, either alone or as a carrier gas to deliver other inhaled drugs may help save lives by suppressing SARS-CoV-2 infections and excessive inflammatory responses. We suggest testing this somewhat counter-intuitive, but low tech and safe intervention for its suitability as a preventive measure and treatment against COVID-19. Overall, development and evaluation of this therapy now may provide a safe and economical tool for use not only during the current pandemic but also for any future outbreaks of respiratory diseases and related conditions.

Mechanical ventilation causes diaphragm dysfunction in newborn lambs

Background

Diaphragm weakness occurs rapidly in adult animals treated with mechanical ventilation (MV), but the effects of MV on the neonatal diaphragm have not been determined. Furthermore, it is unknown whether co-existent lung disease exacerbates ventilator-induced diaphragmatic dysfunction (VIDD). We investigated the impact of MV (mean duration = 7.65 h), either with or without co-existent respiratory failure caused by surfactant deficiency, on the development of VIDD in newborn lambs.

Methods

Newborn lambs (1–4 days) were assigned to control (CTL, non-ventilated), mechanically ventilated (MV), and MV + experimentally induced surfactant deficiency (MV+SD) groups. Immunoblotting and quantitative PCR assessed inflammatory signaling, the ubiquitin-proteasome system, autophagy, and oxidative stress. Immunostaining for myosin heavy chain (MyHC) isoforms and quantitative morphometry evaluated diaphragm atrophy. Contractile function of the diaphragm was determined in isolated myofibrils ex vivo.

Results

Equal decreases (25–30%) in myofibrillar force generation were found in MV and MV+SD diaphragms compared to CTL. In comparison to CTL, both MV and MV+SD diaphragms also demonstrated increased STAT3 transcription factor phosphorylation. Ubiquitin-proteasome system (Atrogin1 and MuRF1) transcripts and autophagy indices (Gabarapl1 transcripts and the ratio of LC3B-II/LC3B-I protein) were greater in MV+SD relative to MV alone, but fiber type atrophy was not observed in any group. Protein carbonylation and 4-hydroxynonenal levels (indices of oxidative stress) also did not differ among groups.

Conclusions

In newborn lambs undergoing controlled MV, there is a rapid onset of diaphragm dysfunction consistent with VIDD. Superimposed lung injury caused by surfactant deficiency did not influence the severity of early diaphragm weakness.

Electronic supplementary material

The online version of this article (10.1186/s13054-019-2409-6) contains supplementary material, which is available to authorized users.

Intermittent living; the use of ancient challenges as a vaccine against the deleterious effects of modern life - A hypothesis

Chronic non-communicable diseases (CNCD) are the leading cause of mortality in developed countries. They ensue from the sum of modern anthropogenic risk factors, including high calorie nutrition, malnutrition, sedentary lifestyle, social stress, environmental toxins, politics and economic factors. Many of these factors are beyond the span of control of individuals, suggesting that CNCD are inevitable. However, various studies, ours included, show that the use of intermittent challenges with hormetic effects improve subjective and objective wellbeing of individuals with CNCD, while having favourable effects on immunological, metabolic and behavioural indices. Intermittent cold, heat, fasting and hypoxia, together with phytochemicals in multiple food products, have widespread influence on many pathways related with overall health. Until recently, most of the employed challenges with hormetic effects belonged to the usual transient live experiences of our ancestors. Our hypothesis; we conclude that, whereas the total inflammatory load of multi-metabolic and psychological risk factors causes low grade inflammation and aging, the use of intermittent challenges, united in a 7-10 days lasting hormetic intervention, might serve as a vaccine against the deleterious effects of chronic low grade inflammation and it's metabolic and (premature) aging consequences.

Effects of Hypercapnia on Acute Cellular Rejection after Lung Transplantation in Rats

BACKGROUND

Hypercapnia alleviates pulmonary ischemia-reperfusion injury, regulates T lymphocytes, and inhibits immune reaction. This study aimed to evaluate the effect of hypercapnia on acute cellular rejection in a rat lung transplantation model.

METHODS

Recipient rats in sham-operated (Wistar), isograft (Wistar to Wistar), and allograft (Sprague-Dawley to Wistar) groups were ventilated with 50% oxygen, whereas rats in the hypercapnia (Sprague-Dawley to Wistar) group were administered 50% oxygen and 8% carbon dioxide for 90 min during reperfusion (n = 8). Recipients were euthanized 7 days after transplantation.

RESULTS

The hypercapnia group showed a higher oxygenation index (413 ± 78 vs. 223 ± 24), lower wet weight-to-dry weight ratio (4.23 ± 0.54 vs. 7.04 ± 0.80), lower rejection scores (2 ± 1 vs. 4 ± 1), and lower apoptosis index (31 ± 6 vs. 57 ± 4) as compared with the allograft group. The hypercapnia group showed lower CD8 (17 ± 4 vs. 31 ± 3) and CD68 (24 ± 3 vs. 43 ± 2), lower CD8 T cells (12 ± 2 vs. 35 ± 6), and higher CD4/CD8 ratio (2.2 ± 0.6 vs. 1.1 ± 0.4) compared to the allograft group. Tumor necrosis factor-α (208 ± 40 vs. 292 ± 49), interleukin-2 (30.6 ± 6.7 vs. 52.7 ± 8.3), and interferon-γ (28.1 ± 4.9 vs. 62.7 ± 10.1) levels in the hypercapnia group were lower than those in allograft group. CD4, CD4 T cells, and interleukin-10 levels were similar between groups.

CONCLUSIONS

Hypercapnia ameliorated acute cellular rejection in a rat lung transplantation model.

Permissive hypercapnia for severe acute respiratory distress syndrome in immunocompromised children: A single center experience

BACKGROUND

Controlled hypoventilation while accepting hypercapnia has been advocated to reduce ventilator-induced lung injury. The aim of the study was to analyze outcomes of a cohort of immunocompromised children with acute respiratory distress syndrome (ARDS) ventilated with a strategy of stepwise increasing PCO2 targets up to 140 mm Hg.

METHODS

Retrospective analysis of outcomes of a cohort of children with oncologic disease or after stem cell transplantation and severe respiratory failure in comparison with a historical control cohort.

RESULTS

Out of 150 episodes of admission to the PICU 88 children underwent invasive mechanical ventilation for >24h (overall survival 75%). In a subgroup of 38 children with high ventilator requirements the PCO2 target ranges were increased stepwise. Fifteen children survived and were discharged from the PICU. Severe pulmonary hypertension was seen in two patients and no case of cerebral edema was observed. Long term outcome was available in 15 patients and 10 of these patients survived without adverse neurological sequelae. With introduction of this strategy survival of immunocompromised children undergoing mechanical ventilation for >24h increased to 48% compared to 32% prior to introduction (historical cohort).

CONCLUSIONS

A ventilation strategy incorporating very high carbon dioxide levels to allow for low tidal volumes and limited inspiratory pressures is feasible in children. Even severe hypercapnia may be well tolerated. No severe side effects associated with hypercapnia were observed. This strategy could potentially increase survival in immunocompromised children with severe ARDS.