Ventilation pulmonaire et alvéolaire, échanges gazeux et gaz du sang à l'exercice en rampe

In response to ramp exercise, changes in ventilation, gas exchange and arterial blood gases, which are closely interrelated, reflect the two roles of ventilation: 1) providing O2 and eliminating metabolic CO2 (from rest to maximal exercise); and 2) contributing to acid-base balance by eliminating non metabolic CO2 from the alkaline reserve (from the first ventilatory threshold [VT1] to maximal exercise). Hyperpnea before VT1 increases gas exchanges as needed for aerobic metabolism without large changes in ventilatory equivalent of O2 and CO2 (VE/VO2 or VE/VCO2), in PO2 and PCO2 in alveoli or arterial blood (except for a small widening of alveolo-arterial PO2 gradient), and in bicarbonate concentration. In contrast, above VT1, CO2 is washed-out from the alkaline reserve due to the combined effect of the fall in PACO2 (because of hyperventilation) and in pH, and this helps maintaining acid-base balance. PaCO2 and bicarbonate concentration decrease while PAO2 and VE/VO2 increase, and VCO2, which follows VE, becomes higher than VO2. In healthy young subjects, but very seldom in patients, the end of exercise can occur after a second ventilatory threshold (VT2), which is the zone where the increase in VCO2 fails to follow that in VE in spite of hyperventilation and acidosis because of the progressive depletion of the alkaline reserve.