Endurance of Hyperventilation in Chronic Airflow Limitation

Abstract

The capacity to sustain an increase in ventilation (VE) sufficient to decrease the end-tidal partial pressure of carbon dioxide (PETCO2) by about 10 mm Hg was studied in six hypercapnic patients with moderate to severe chronic airflow limitation (CAL). Patients could continue such an increased VE for a finite time (range 5 to 54 minutes). During hyperventilation (H), ventilation was approximately doubled and represented 77.1 +/- 8.4 (mean +/- SE) percent of maximum voluntary ventilation, mean oxygen consumption (VO2) increased 44 percent (p less than 0.005) and mean inspiratory pleural pressure (Ppl) swings were 43.8 +/- 10.5 percent of maximum Ppl. Four patients achieved reductions of PETCO2 less than 10 mm Hg, and two patients achieved or exceeded the target decrease in PETCO2. The decrease in PaCO2 was correlated with the wasted ventilation ratios (VD/VT) during H, the greatest decrease in PaCO2 being related to the lowest VD/VT (p less than 0.05). Electromyographic (EMG) evidence of inspiratory muscle fatigue developed in four of the six patients during H. Five normal subjects achieved an equal or greater decrease in PETCO2, and none showed EMG evidence of inspiratory muscle fatigue. We conclude that, although impaired gas exchange limits the capacity to voluntarily reduce the PaCO2, the development of respiratory muscle fatigue in some patients with CAL may also contribute by limiting the capacity to sustain the substantial increase in respiratory muscle work done in the attempt.