Proportional assist ventilation is provided through an experimental prototype—the Winnipeg ventilator (University of Manitoba, Winnipeg, Canada). The gas delivery system consists of a freely moving piston coupled to a motor. As the patient pulls, the piston moves freely into the cylinder and generates force proportional to the current applied to it. This current is proportional to the signals of volume or flow. These signals are amplified by gain controls and are then summed to give a certain amount of current to the motor.
The flow gain signal determines how much pressure (resistive pressure or resistance units) will be given per unit flow. The volume gain signal determines how much pressure (elastic pressure or elastance units) will be given per unit volume. When the piston is activated it creates a pressure in the piston chamber and the forward movement of the piston produces airflow. According to the equation of motion of the respiratory system, the total pressure generated by the ventilator during proportional assist ventilation (the airway pressure ( Paw)) will be a function of the instantaneous airflow and volume, and the respective gains (resistive units and elastic units):
where KR and KE are respectively the gain values for resistance (in cmH 2O/l/s) and elastance (in cmH2O/l) set on the ventilator.
The difference between chamber pressure and proximal airway pressure creates a trigger or assist signal. Gas transfer is terminated when the patient makes an expiratory effort or there is no longer muscular pressure and flow stops. This creates a positive pressure relative to the chamber pressure. When patient pressure exceeds chamber pressure the trigger signal causes the exhalation valve to open, allowing passive deflation. The piston then returns to the starting position. Typical tracings obtained during proportional assist ventilation are shown in Fig 4.
Fig. 4 Tracings of airflow (flow), airway pressure (Paw), esophageal pressure (Pes), and tidal volume obtained in a patient ventilated with proportional assist ventilation. There is a typical breath-by-breath variability in Paw, and a proportionality between patient effort (esophageal pressure swings) and Paw. The stronger the patient effort, the higher is Paw.
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