The ventilatory response of the human respiratory system to sine waves of alveolar carbon dioxide and hypoxia.
Sine waves of alveolar CO2 at constant high alveolar O2, and sine waves of alveolar hypoxia (1/(PA, O2 -C), C congruent to 32 torr) at constant alveolar CO2 have been administered to three subjects in each case. Sine waves of six different periods were studied, ranging between 900 and 30 s for the CO2 sine waves and 300 and 20 s for the hypoxic sine waves. The sinusoidal variations in ventilation produced by these manoeuvres, expressed as amplitudes and phase shifts, were compared with values predicted from the dynamic responses to alveolar steps of gas tension already to be found in the literature. For the CO2 sine waves, the amplitudes of response agreed well with those predicted at the higher frequencies, but were less than predicted at the lower frequencies. For the hypoxic sine waves, the amplitude of response varied less with frequency than was predicted. For both the CO2 and the hypoxic sine waves, the phase shift of response was less than expected at the higher frequencies. An attempt was made to fit parameters to a simple model, based on the wash-in and wash-out of respiratory gases into and out of a tissue compartment, and used in the literature for describing the responses to step changes. No satisfactory fit was found. It is concluded that the simple model is unsatisfactory by itself for describing the responses to sinusoidal chemical stimulation; features additional to those included in the model are required to explain fully the responses seen. The possibilities for chemoreception at the higher frequencies are discussed in the light of the low phase shifts.