Time course of posthyperventilation breathing in humans depends on alveolar CO2 tension

The time course of the decrease in ventilation immediately after 1 min of voluntary isocapnic hyperventilation was studied in eight healthy subjects. The hyperventilation tests were done at three different levels of alveolar CO2 tension (PACO2), and at each PACO2 level with a fixed respiratory frequency and with a freely chosen (increased) frequency. The time constant of the decrease in ventilation after free-frequency hyperventilation was inversely related to the PACO2, with a regression equation of tau 1 (s) = 78.6 - 1.2 PACO2. After the fixed-frequency hyperventilation, the time constant of the decrease in ventilation was virtually zero and independent of PACO2. These results do not agree with earlier findings in anesthetized animals. The fact that hypocapnia increases the excitability of neurons might explain the findings. The extreme respiratory afterdischarge phenomenon in hypocapnic patients suffering from the hyperventilation syndrome is consistent with the outcome of this study.