Regulation of PaCO2 and ventilation in humans inspiring low levels of CO2

This study was designed to determine whether 1) arterial PCO2 (PaCO2) increases when inspired PCO2 (PICO2) is increased from less than 0.4 Torr (eupnea) to 7 or 14 Torr, and 2) ventilatory sensitivity to CO2 (delta VE/ delta PaCO2) is greater at low levels of PICO2 (7-21 Torr) than it is at higher levels (28-42 Torr). Human subjects were studied while seated in an environmental chamber that permitted alteration of PICO2 by changing the chamber PCO2. In study 1, arterial blood was sampled over the final 5 min of a eupneic period and again 10-15 min later when PICO2 was 7 or 14 Torr. With this protocol, PACO2 was increased above eupnea by 0.7 (P less than 0.02) and 0.9 Torr (P less than 0.01) when PICO2 was 7 and 14 Torr, respectively. In study 2, arterial blood was sampled every 5 min during two 1-h periods of eupnea that were separated by 3 h during which PICO2 was increased by 7 Torr each 0.5 h. With this protocol there was no consistent difference in PACO2 between eupneic periods and periods when PICO2 was 7-14 Torr. There was a progressively increased hypercapnia as PICO2 was increased from 7 to 42 Torr. The delta VE/ delta PaCO2 was less than half for data obtained at low relative to high PICO2. The two studies demonstrated that measurement error and physiologic variation necessitate using a "powerful" experimental design (study 1) to detect small increases in PaCO2. On the basis of these results, we have concluded that there is no apparent reason to postulate a sensory mechanism other than the carotid and intracranial chemoreceptors to account for the hyperpnea during CO2 inhalation. Specifically, isocapnic hyperpnea probably does not occur.