An Exploratory Analysis of the Association between Hypercapnia and Hospital Mortality in Critically Ill Patients with Sepsis

Partial pressure of carbon dioxide/pH interaction and its association with mortality among patients mechanically ventilated in the emergency department

OBJECTIVES

There is currently conflicting data as to the effects of hypercapnia on clinical outcomes among mechanically ventilated patients in the emergency department (ED). These conflicting results may be explained by the degree of acidosis. We sought to test the hypothesis that hypercapnia is associated with increased in-hospital mortality and decreased ventilator-free days at lower pH, but associated with decreased in-hospital mortality and increased ventilator-free days at higher pH, among patients requiring mechanical ventilation in the emergency department (ED).

METHODS

Secondary analysis of patient level data from prior clinical trials and cohort studies that enrolled adult patients who required mechanical ventilation in the ED. Patients who had a documented blood gas while on mechanical ventilation in the ED were included in these analyses. The primary outcome was in-hospital mortality, and secondary outcome was ventilator-free days. Mixed-effects logistic, linear, and survival-time regression models were used to test if pH modified the association between partial pressure of carbon dioxide (pCO2) and outcome measures.

RESULTS

Of the 2348 subjects included, the median [interquartile range (IQR)] pCO2 was 43 (35-54) and pH was 7.31 (7.22-7.39). Overall, in-hospital mortality was 27%. We found pH modified the association between pCO2 and outcomes, with higher pCO2 associated with increased probability of in-hospital mortality when pH is below 7.00, and decreased probability of in-hospital mortality when pH is above 7.10. These results remained consistent across multiple sensitivity and subgroup analyses. A similar relationship was found with ventilator-free days.

CONCLUSIONS

Higher pCO2 is associated with decreased mortality and greater ventilator-free days when pH is >7.10; however, it is associated with increased mortality and fewer ventilator-free days when the pH is below 7.00. Targeting pCO2 based on pH in the ED may be a potential intervention target for future clinical trials to improve clinical outcomes.

Cardiovascular changes induced by targeted mild hypercapnia after out of hospital cardiac arrest. A sub-study of the TAME cardiac arrest trial

AIM

Hypercapnia may elicit detrimental haemodynamic effects in critically ill patients. We aimed to investigate the consequences of targeted mild hypercapnia versus targeted normocapnia on pulmonary vascular resistance and right ventricular function in patients resuscitated from out-of-hospital cardiac arrest (OHCA).

METHODS

Pre-planned, single-centre, prospective, sub-study of the Targeted Therapeutic Mild Hypercapnia After Resuscitated Cardiac Arrest (TAME) trial. Patients were randomised to mild hypercapnia (PaCO2 = 6.7-7.3 kPa) or normocapnia (PaCO2 = 4.7-6.0 kPa) for 24 hours. Haemodynamic assessment was performed with right heart catheterisation and serial blood-gas analyses every4th hour for 48 hours.

RESULTS

We studied 84 patients. Mean pH was 7.24 (95% CI 7.22-7.30) and 7.32 (95% CI 7.31-7.34) with hypercapnia and normocapnia, respectively (P-group < 0.001). Pulmonary vascular resistance index (PVRI), pulmonary artery pulsatility index, and right atrial pressure did not differ between groups (P-group > 0.05). Mean cardiac index was higher with mild hypercapnia (P-group < 0.001): 2.0 (95% CI 1.85-2.1) vs 1.6 (95% CI 1.52-1.76) L/min/m2. Systemic vascular resistance index was 2579 dyne-sec/cm-5/ m2 (95% CI 2356-2830) with hypercapnia, and 3249 dyne-sec/cm-5/ m2 (95% CI 2930-3368) with normocapnia (P-group < 0.001). Stroke volumes (P-group = 0.013) and mixed venous oxygen saturation (P-group < 0.001) were higher in the hypercapnic group.

CONCLUSION

In resuscitated OHCA patients, targeting mild hypercapnia did not increase PVRI or worsen right ventricular function compared to normocapnia. Mild hypercapnia comparatively improved cardiac performance and mixed venous oxygen saturation.

Extracorporeal Carbon Dioxide Removal With the Hemolung in Patients With Acute Respiratory Failure: A Multicenter Retrospective Cohort Study

OBJECTIVES

Extracorporeal carbon dioxide removal (ECCO 2 R) devices are effective in reducing hypercapnia and mechanical ventilation support but have not been shown to reduce mortality. This may be due to case selection, device performance, familiarity, or the management. The objective of this study is to investigate the effectiveness and safety of a single ECCO 2 R device (Hemolung) in patients with acute respiratory failure and identify variables associated with survival that could help case selection in clinical practice as well as future research.

DESIGN

Multicenter, multinational, retrospective review.

SETTING

Data from the Hemolung Registry between April 2013 and June 2021, where 57 ICUs contributed deidentified data.

PATIENTS

Patients with acute respiratory failure treated with the Hemolung. The characteristics of patients who survived to ICU discharge were compared with those who died. Multivariable logistical regression analysis was used to identify variables associated with ICU survival.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Of the 159 patients included, 65 (41%) survived to ICU discharge. The survival was highest in status asthmaticus (86%), followed by acute respiratory distress syndrome (ARDS) (52%) and COVID-19 ARDS (31%). All patients had a significant reduction in Pa co2 and improvement in pH with reduction in mechanical ventilation support. Patients who died were older, had a lower Pa o2 :F io2 (P/F) and higher use of adjunctive therapies. There was no difference in the complications between patients who survived to those who died. Multivariable regression analysis showed non-COVID-19 ARDS, age less than 65 years, and P/F at initiation of ECCO 2 R to be independently associated with survival to ICU discharge (P/F 100-200 vs <100: odds ratio, 6.57; 95% CI, 2.03-21.33).

CONCLUSIONS

Significant improvement in hypercapnic acidosis along with reduction in ventilation supports was noted within 4 hours of initiating ECCO 2 R. Non-COVID-19 ARDS, age, and P/F at commencement of ECCO 2 R were independently associated with survival.

Association between the time-varying arterial carbon dioxide pressure and 28-day mortality in mechanically ventilated patients with acute respiratory distress syndrome

BACKGROUND

Recent studies have shown an association between baseline arterial carbon dioxide pressure (PaCO₂) and outcomes in patients with acute respiratory distress syndrome (ARDS). However, PaCO₂ probably varies throughout the disease, and few studies have assessed the effect of longitudinal PaCO₂ on prognosis. We thus aimed to investigate the association between time-varying PaCO₂ and 28-day mortality in mechanically ventilated ARDS patients.

METHODS

In this retrospective study, we included all adult (≥ 18 years) patients diagnosed with ARDS who received mechanical ventilation for at least 24 h at a tertiary teaching hospital between January 2014 and March 2021. Patients were excluded if they received extracorporeal membrane oxygenation (ECMO). Demographic data, respiratory variables, and daily PaCO₂ were extracted. The primary outcome was 28-day mortality. Time-varying Cox models were used to estimate the association between longitudinal PaCO₂ measurements and 28-day mortality.

RESULTS

A total of 709 patients were eligible for inclusion in the final cohort, with an average age of 65 years, of whom 70.7% were male, and the overall 28-day mortality was 35.5%. After adjustment for baseline confounders, including age and severity of disease, a significant increase in the hazard of death was found to be associated with both time-varying PaCO₂ (HR 1.07, 95% CI 1.03-1.11, p<0.001) and the time-varying coefficient of variation for PaCO₂ (HR 1.24 per 10% increase, 95% CI 1.10-1.40, p<0.001) during the first five days of invasive mechanical ventilation. The cumulative proportion of exposure to normal PaCO₂ (HR 0.72 per 10% increase, 95% CI 0.58-0.89, p = 0.002) was associated with 28-day mortality.

CONCLUSION

PaCO₂ should be closely monitored in mechanically ventilated ARDS patients. The association between PaCO₂ and 28-day mortality persisted over time. Increased cumulative exposure to normal PaCO₂ was associated with a decreased risk of death.