Proportional assist ventilation, a new approach to ventilatory support. Theory.

The relation between inspiratory effort and ventilatory return (flow and volume) is usually abnormal in patients who require ventilatory support because of respiratory distress. Although all available support methods provide the patient with greater ventilation than would obtain with the same effort while unsupported, the relation between instantaneous effort and ventilatory consequences is not normalized. We describe an approach with which the ventilator simply amplifies patient instantaneous effort throughout inspiration while leaving the patient with complete control over all aspects of breathing pattern (tidal volume, inspiratory and expiratory durations, and flow patterns). This approach is implemented by monitoring the instantaneous rate (V) and volume (V) of gas flow from ventilator to patient and causing applied pressure (P) to change according to the equation of motion P = f1(V) + f2(V)], where f1 and f2 are appropriately selected functions for the relation between pressure and volume (elastic assist) and pressure and flow (resistive assist). There are several potential advantages to this approach: (1) greater comfort; (2) reduction of peak airway pressure required to sustain ventilation and, hence, the potential for avoiding intubation; (3) less likelihood of overventilation; (4) preservation and enhancement of patient's own reflex, behavioral, and homeostatic control mechanisms since the ventilator essentially becomes an extension of the patient's own muscles; and (5) improved efficiency of negative pressure ventilation.