Study objectives: Chronic hypercapnia in patients with COPD has been associated with a poor prognosis. We hypothesized that, within this group of chronic hypercapnic COPD patients, factors that could mediate this hypercapnia, such as decreased maximum inspiratory mouth pressure (P(I(max))), decreased maximum expiratory mouth pressure (P(E(max))), and low hypercapnic ventilatory response (HCVR), could be related to survival. Other parameters, such as arterial blood gas values, airway obstruction (FEV1), body mass index (BMI), current smoking status, and the presence of comorbidity were studied as well.
Methods: A cohort of 47 chronic hypercapnic COPD patients recruited for short-term trials (1 to 3 weeks) in our institute was followed up for 3.8 years on average. Survival was analyzed using a Cox proportional hazards model. The risk factors considered were analyzed, optimally adjusted for age and gender.
Results: At the time of analysis 18 patients (10 male) were deceased. After adjusting for age and gender, P(I(max)), P(E(max)), and HCVR were not correlated with survival within this hypercapnic group. Current smoking (hazard ratio [HR], 7.0; 95% confidence interval [CI], 1.4 to 35.3) and the presence of comorbidity (HR, 5.5; 95% CI, 1.7 to 18.7) were associated with increased mortality. A higher Pa(O2) affected survival positively (HR, 0.6 per 5 mm Hg; 95% CI, 0.4 to 1.0). Pa(CO2) tended to be lower in survivors, but this did not reach statistical significance (HR, 2.0 per 5 mm Hg; 95% CI, 0.9 to 4.3). FEV1 and BMI were not significantly related with survival in hypercapnic COPD patients.
Conclusion: In patients with chronic hypercapnia, only smoking status, the presence of comorbidity, and Pa(O2) level are significantly associated with survival. Airway obstruction, age, and BMI are known to be predictors of survival in COPD patients in general. However, these parameters do not seem to significantly affect survival once chronic hypercapnia has developed.