Measuring the oxygen cost of breathing in normal adults and patients with cystic fibrosis

Abstract

It has been suggested that the oxygen consumption of the respiratory muscles (V̇ O2resp) may play a role in limiting exercise performance in both healthy subjects and those with chronic airflow limitation (CAL). In order to measure V̇ O2resp reproducibly at both rest and on exercise, ventilation (V̇ E and total oxygen consumption (V̇ O2) in 3 normal subjects and in 3 patients with cystic fibrosis were measured while breathing air and again when V̇ E was stimulated by the addition of CO 2 to the inspired gas. Since external work was the same it was assumed that any changes in V̇ O2 would be due to the increase in V̇ O2resp during stimulated breathing allowing for the calculation of V̇ O2resp. The oxygen cost of breathing was higher in the patients with the increasing ventilation of exercise. These values were reproducible on repetitive measurements. It is concluded that the method employed is applicable in normal subjects and in patients with CAL, and that the O 2 cost of breathing is higher in patients with CAL. The O 2 cost of breathing increases as V̇ E increases but even during exercise the V̇ O2resp is only a small fraction of the total V̇ O2 and is unlikely to be a determining factor limiting exercise performance in either normal subjects or those with CAL.