Mortality Increase Seen in Patients With Heart Failure, Hypercapnia

Outcomes of in-hospital mortality, 7-day mortality, and emergency department length of stay were investigated among a cohort of patients with acute heart failure and hypercapnia, or excessive levels of CO2 in the blood.

Patients with acute heart failure (AHF) and comorbid hypercapnia, or excessive levels of CO2 in the blood from poor respiration, were shown to have higher rates of both in-hospital mortality and 7-day mortality, according to new study findings published in BMC Emergency Medicine.

Factors associated with acute AHF that are linked with a negative prognosis among persons with the condition include hypotension, ischemic or parainfectious etiologies, renal dysfunction, and comorbidities. Hypercapnia, or excessive levels of CO2 in the blood, also has a negative association among patients with AHF, because it has been linked with greater rates of intensive care unit (ICU) admission. However, little is known regarding its effect on in-hospital mortality, according to the study authors.

“The goal of this study was to determine whether prehospital hypercapnia was associated with mortality in AHF,” they wrote.

Their retrospective cohort study utilized electronic patient data from July 1, 2016, through January 31, 2020, from the prehospital medical mobile unit (SMUR, Service Mobile d’Urgence et de Réanimation) of the Geneva University Hospitals in Switzerland, a unit that provides critical care to patients with AHF in the field prior to hospital admission, including prehospital noninvasive ventilation (NIV) and arterial blood gas (ABG) analysis. Patients requiring a SMUR intervention after January 31 were excluded to avoid COVID-19–related bias, as were those missing ABG data or who had a cardiac arrest prior to SMUR team arrival.

Of the 225 patients included in the final analysis, 58.7% had a diagnosis of prehospital hypercapnia, their median (interquartile range) age was 86 (78-90) years, and 63.6% were female. These patients had an in-hospital mortality rate of 17.4%, which was more than 2.5 times that of patients without prehospital hypercapnia (6.5%; P = .016). In addition, both the crude and adjusted odds ratios (ORs) were higher in the hypercapnia groups:

  • Crude OR: 3.06 (95% CI, 1.19-7.85)
  • Adjusted OR: 3.18 (95% CI, 1.22-8.26)

Following univariate and multivariate analysis, respectively, mortality odds for both groups were similar for chronic obstructive pulmonary disease (1.05 and 1.06), hypertension (0.87 and 0.63), and chronic renal failure (1.45 and 1.53). However, 7-day mortality was also more than twice as high in the hypercapnia group, at 13.6% compared with 5.6% (P = .044), while emergency department length of stay was shorter: 5.6 vs 7.1 hours (P = .018).

The most common comorbidities were hypertension (84.4%), chronic renal failure (50.7%), and atrial fibrillation (44.4%).

In addition, compared with the prehospital nonhypercapnia group, at baseline the hypercapnia groups had higher mean heart rate, systolic and diastolic blood pressures, respiratory rate, oxygen saturation, partial pressure of CO2 (PaCO2), bircarbonate levels, and prehospital NIV. More were also female (69.7% vs 54.8%)—but in-hospital mortality was still higher in male patients (17.1% vs 10.5%; P = .810).

Possible reasons for the increased mortality seen in the patients with hypercapnia include that their higher blood pressures may increase risk of pulmonary edema, which subsequently prevents optimal CO2 clearance and oxygen uptake, and that the sympathetic stress seen in sicker patients increases their risk for higher blood pressure.

Areas for future research should include a randomized, controlled trial that investigates the use of prehospital ABG as evaluation criteria “to improve patient disposition and reduce the time spent in the emergency department”; a multicenter study with a larger sample size; and interventions that target greater use of ABG in the prehospital setting should other studies confirm its utility.

“Prehospital hypercapnia is associated with an increase in in-hospital and 7-day mortality in patients with AHF,” the authors concluded. “Prospective randomized studies should be performed before systematic prehospital analysis can be recommended in these patients.”


Fabre M, Fehlmann CA, Boczar KE, et al. Association between prehospital arterial hypercapnia and mortality in acute heart failure: a retrospective cohort study. BMC Emerg Med. Published online November 6, 2021. doi:10.1186/s12873-021-00527-y