Effects of the inspiratory pressure waveform during patient-triggered ventilation on pulmonary stretch receptor and phrenic nerve activity in cats

Objective: To examine the effects of square wave, sinusoidal, and linear in spiratory pressure waveforms during pressure-controlled assist/control vent ilation on the firing pattern of pulmonary stretch receptors and phrenic ne rve activity. Design: Experimental, comparative study. Setting: Research laboratory at a university biomedical center. Subjects: Nine anesthetized, endotracheally intubated young cats (2.5-3.4 k g). Intervention: With interposed periods of continuous positive airway pressur e (0.2 kPa), each cat was exposed to periods of assist/control ventilation with three different pressure waveforms, where the peak inspiratory pressur e (0.74 +/- 0.13 kPa), end-expiratory pressure (0.2 +/- 0.02 kPa), and tida l volume (14.9 +/- 5.22 mL/kg) were kept constant. Preset controlled ventil ator rate was set below the rate of spontaneous breathing, and the mechanic al inflation time equaled the inspiratory time during spontaneous breathing on continuous positive airway pressure. Measurements and Main Results: Respiratory rate and arterial blood gases di d not change between the three pressure waveforms during assist/control ven tilation. Peak pulmonary stretch receptor activity was lower and mean phren ic nerve activity higher during continuous positive airway pressure than du ring assist/control ventilation (p < .05), Peak inspiratory pulmonary stret ch receptor activity was the same with all three pressure waveforms (82 +/- 17 impulses sec(-1)) but occurred earlier with square wave than with sinus oidal or linear pressure waveforms (p < .05). The total number of impulses in the phrenic nerve activity burst was smaller with square wave than with the other two pressure waveforms (0.21 +/- 0.17 vs. 0.33 +/- 0.27 and 0.42 +/- 0.30 arbitrary units; p < .05), and the phrenic nerve activity burst du ration was shorter with square wave (1.10 +/- 0.45 vs. 1.54 +/- 0.36 and 1. 64 +/- 0.25 sees; p < .05). Conclusion: Square wave pressure waveform during pressure-controlled assist /control ventilation strongly inhibits spontaneous inspiratory activity in cats. One mechanism for this inhibition is earlier and sustained peak pulmo nary stretch receptor activity during inspiration. These findings show that differences in inspiratory pressure waveforms influence the spontaneous br eathing effort during assist/control ventilation in cats.