Carbon-dioxide oscillations during six-minute walk test in patients with very severe COPD

Abstract

Background: Carbon dioxide pressure (pCO2) can be reliably measured transcutaneously and may be of clinical relevance in COPD. Retention of CO2 and exercise-induced hypercapnia (EIH) has been scarcely studied as disease marker in COPD. The aims of this study were (1) to study pCO2 response during the 6MWT and (2) to compare clinical characteristics amongst patients with and without CO2 retention and those with and without EIH during the 6MWT. Methods: Sixty-two COPD patients (age: 63±8years, FEV1: 33±10%pred) participated in study. Transcutaneous carbon dioxide pressure (tcpCO2) was measured continuously during the 6MWT by the use of SenTec device. Measurements where repeated after 1 week and data from the longest 6MWT were analyzed. Results: Half of the patients (50%) had CO2 retention (ΔtcpCO2 >4mmHg from baseline) while the rest preserved or reduced tcpCO2 levels. Nineteen patients (31%) exhibited EIH (tcpCO2 >45mmHg) during the 6MWT. Physiological characteristics of CO2 retainers were similar with non-retainers but patients with EIH were characterized by higher resting tcpCO2 levels within the normal range, lower inspiratory pressures (Pimax), lower BMI, more severe FEV1 and higher dyspnoea at rest compared to non-EIH. The determinants of pCO2 retention were pre-walk dyspnoea and FEV1 while pre-walk dyspnoea and baseline pCO2 levels were crucial for EIH. Conclusions: The pCO2 response to 6MWT is highly heterogeneous in COPD. CO2 retention is related to more pre-walk dyspnoea and worse FEV1 while EIH is associated with more pre-walk dyspnoea and higher baseline PaCO2 levels. Monitoring of pCO2 changes during 6MWT might help to identify patients who need special attention regarding to EIH.