Enhancing ventilatory strategies for the critically ill--proportional assist ventilation.

PROPORTIONAL assist ventilation (PAV) is a new mode of synchronized partial ventilatory support designed to optimize adaptation of the ventilator to the patient.1 Proportional assist ventilation differs markedly from other modes of partial assistance. Rather than providing a fixed volume or pressure assistance to the patient, PAV generates pressure assistance in proportion to the patient’s own respiratory effort. This unique characteristic has the potential to facilitate optimal adaptation of the ventilator to the patient’s changing respiratory demand. An appreciation of the physiological basis of assisted ventilation helps our understanding of the working principles of PAV. If one considers a patient’s respiratory system impedance with its resistive and elastic components, pressure or effort must be generated by the patient (Pmus) and/or the ventilator (Pvent) for any given inspiration. The amount of effort required is dependent upon the elastance of the respiratory system (E = 1/compliance) and the cumulative inspired volume, and on the total resistance of the patient’s respiratory system and breathing apparatus (R) and the inspiratory flow rate. Partition of the respiratory effort between the patient and the ventilator varies according to the mode of ventilation. In the spontaneously breathing patient (e.g., continuous positive airway pressure mode), inspiratory Pvent is negligible, and all the inspiratory work is generated by the patient’s respiratory muscles.