Expiratory Asynchrony in Proportional Assist Ventilation

Abstract

One of the proposed advantages of proportional assist ventilation (PAV) has been the automatic synchrony between the end of the patient's inspiratory effort and the ventilator cycle (i.e., expiratory synchrony). However, recent clinical studies have shown a prolonged ventilator inspiratory time or even a "runaway" phenomenon with the normal use of PAV. We hypothesize that control-system delay may account for this, because in reality there is always some degree of delays between control-system's input and output in all ventilators. Computer simulation study to date has not taken into account the potential effect of control-system delay on expiratory synchrony. We therefore created a computer model in which the parameter of control-system delay time was introduced. We found that significant expiratory asynchrony may occur with this more realistic model of PAV. The ventilator flow termination may fall behind the completion of the patient inspiration by as long as 0.33 seconds under the selected simulation conditions. The inspiratory termination delay time is in proportion to the control-system delay time, the respiratory time constant, and the assist gain settings. In conclusion, this model indicates that due to the unavoidable control-system delay in the ventilators, expiratory asynchrony may be an inherent shortcoming associated with PAV.

A physiologic comparison of proportional assist ventilation with load-adjustable gain factors (PAV+) versus pressure support ventilation (PSV)

“…During PAV, the ventilator assistance is delivered in proportion to the patient's instantaneous flow and volume, by compensating patient resistive and elastic loads, thus amplifying the patient's own effort breath by breath [10]. Several studies showed that PAV improves patient-ventilator synchrony at the start of inspiration [11][12][13] but not necessarily at the end of inspiration [13][14][15]. Moreover, its widespread clinical use was limited by the necessity of regular measurements of respiratory mechanics (elastance and resistance).…”

Cycling of the Mechanical Ventilator Breath

“…[9][10][11][12][13][14][15][16] The reason for multiple studies of flow cycling is attributed to the universal adoption of spontaneous breathing modes such as pressure-support ventilation (PSV). 17 During PSV the patient both triggers and cycles the breath (Fig.…”

Tracheal pressure regulated volume assist ventilation in acute respiratory failure

“…7,10 Failure to take this into account during FAV may lead to under-assistance as well as over-assistance, resulting in patient-ventilator asynchrony, frequent alarms, and increased caregiver intervention. 1,2,5,11,12 Clinical acceptance of PAV has been disappointing as a result of such considerations. 13,14 Automatic tube compensation (ATC) is a ventilatory modality designed to overcome the added flow resistance of an artificial airway by applying positive pressure in relationship to flow based on in vitro determination of tube resistance and continuous measurement of inspiratory flow and airway opening pressure (Pao) during pressure preset modes of ventilation.…”