Effects of ventilator resistance and compliance on phrenic nerve activity in spontaneously breathing cats

This study investigates the effects of respiratory mechanical loading and unloading on phrenic nerve activity and ventilation. Ten adult cats were anesthetized, intubated, and connected to a servocontrolled infant ventilator while breathing spontaneously in continuous positive airway pressure mode. The ventilator was then set to decrease the pressure at the airway opening in proportion to the inspiratory airflow or the volume of spontaneous breathing (resistive or elastic loading) or to increase the pressure at the airway opening accordingly (resistive or elastic unloading, also called proportional assist ventilation). When compared with continuous positive airway pressure, both resistive and elastic loading increased the number of phrenic nerve impulses per breath and the mean frequency of impulses (p < 0.01, ANOVA). In contrast, elastic unloading decreased phrenic nerve activity significantly in normal and injured lungs. Resistive unloading in normal lungs did not change phrenic nerve activity significantly. The compensatory response in phrenic nerve activity was not large enough to completely offset effects of loading and unloading on ventilation: elastic and resistive loading decreased ventilation. Unloading had the opposite effect on ventilation, particularly in injured lungs. We conclude that respiratory mechanical loading and unloading have statistically significant, opposite effects on phrenic nerve activity and opposite effects on ventilation.