Is hypercapnia necessary for the ventilatory response to exercise in man?

Continuous recordings and arterial pH, ventilation, airway CO₂ and heart rate were made during rest and during 3-4 min periods of rhythmic leg exercise in four renal patients with arteriovenous shunts. The patients were anaemic (haemoglobin 6.5-9.0 g/dl) but had a normal ventilatory response to exercise as judged by the ratio of the change in ventilation to the change in CO₂ production. Breath-by-breath oscillations in arterial pH disappeared for the majority of the exercise period in each patient. Changes in mean arterial pH and end-tidal CO₂ tension with exercise were inconsistent between subjects but consistent within a given subject. On average, mean arterial pH rose by 0.011 pH unit. Changes in end-tidal CO₂ tension reflected changes in mean pHa by falling on average by 1 mmHg (0.13 kPa). Hypercapnia and acidaemia were not found to be necessary for the ventilatory response to moderate exercise.