The role of chemical (CO₂) drive in the apnea induced by high frequency ventilation in the cat

The purpose of the present study was to compare the relative roles of Pa_CO2 and pulmonary mechanoreceptors in the generation of apnea during high frequency ventilation (HFV) and conventional mechanical ventilation (CMV) in the anesthetized cat. Our hypothesis was that Pa_CO2 primarily determines the appearance of apnea, while mechanoreceptor input plays an important but secondary role. We calculated the tidal volume which would have a 50% chance of inducing apnea (Vt₅₀) for each combination of ventilator settings, and used it as a standard for comparison of mechanical dynamic inputs. When either 2% or 5% CO₂ was added to the bias flow during HFV, the Vt₅₀ was significantly increased over that seen with a room air bias flow. Apnea was observed during either mode of ventilation only when Pa_CO2 was reduced below normocapnic levels. The mean tidal volume associated with each apnea group was larger than that for the corresponding nonapnea group. There was no difference in the Pa_CO2 for apnea between HFV and CMV, but the EMG of the diaphragm did differ. With HFV-apnea tonic activity was observed, while CMV-apnea showed abolition of all activity. We conclude that both HFV- and CMV-induced apnea depend primarily on lowering the chemical drive (Pa_CO2), and secondarily on inhibitory mechanical input (Vt). The excitation of rapidly adapting receptors during HFV could explain the tonic EMG activity.