Proportional-assist ventilation (PAV) is a form of synchronized ventilator support in which the ventilator generates pressure in proportion to instantaneous patient effort (Fig. 12-1).1 The ventilator simply amplifies inspiratory efforts. Unlike other modes of partial support, there is no target flow, tidal volume, or ventilation or airway pressure. Rather, PAV’s objective is to allow the patient to comfortably attain whatever ventilation and breathing pattern his or her control system desires.1 The main operational advantages of PAV are automatic synchrony with inspiratory efforts and adaptability of the assist to changes in ventilatory demand (Fig. 12-1).
Relationship between assist provided (airway pressure) and independently measured diaphragmatic pressure in proportional-assist ventilation. Note that amplitude and duration of assist correspond to amplitude and duration of inspiratory efforts.
A simple PAV delivery system illustrates how this happens (Fig. 12-2).2 Alveoli and chest wall are represented as an elastic compartment that opposes expansion. Elastic recoil pressure (Pel; hatched arrow in Fig. 12-2) is a function of how much lung volume deviates from passive functional residual capacity (FRC) and the stiffness of the system: Pel = V × E, where V is volume above FRC and E is respiratory system elastance. In a passive system, Pel increases alveolar pressure as the lung is artificially inflated. During assisted ventilation, inspiratory muscles are active. These muscles decrease alveolar pressure by an amount corresponding to their pressure output (Pmus) (Fig. 12-2). At any instant, alveolar pressure (Palv) is the difference between Pel (V × E), which tends to increase it, and Pmus, which tends to decrease it:
Diagram illustrating how a PAV delivery system generates pressure in proportion to effort. The gas-delivery system consists of a freely moving piston pressurized by a fast-acting motor. Force exerted by motor is a function of flow and volume leaving the ventilator. A stronger effort results in greater reduction in alveolar pressure (Palv), drawing more gas from the piston and resulting in more assist. If the volume-assist (VA) and flow-assist (FA) components are set to the same fraction of elastance and resistance, respectively, the pressure generated becomes proportional to effort (Pmus). See text.
The elastic compartment is connected to the external tubing via airways and the endotracheal tube. The ventilator controls pressure at the external airway (Paw). Air flows into the lungs when Paw exceeds Palv. Flow is a function of the difference between Paw and Palv (resistive pressure) and the resistance of the intervening tubing (R). Thus
Substituting equation (Eq.) 1 for Palv in Eq. 2 and rearranging, we get