Patient-Ventilator Interactions during Volume-Support Ventilation: Asynchrony and Tidal Volume Instability-A Report of Three Cases

Abstract

During pressure-support ventilation, tidal volume (V_T) can vary according to the level of the patient's respiratory effort and modifications of the thoraco-pulmonary mechanics. To keep V_T as constant as possible, the Siemens Servo 300 ventilator proposes an original modification of pressure-support ven-tilation, called volume-support ventilation (VSV). VSV is a pressure-limited mode of ventilation that uses V_T as a feedback control: the pressure support level is continuously adjusted to deliver a preset V_T. Thus, the ventilator adapts the inspiratory pressure level, breath by breath, to changes in the patient's inspiratory effort and the mechanical thoraco-pulmonary properties. The clinician sets V_T and respi-ratory frequency, and the ventilator calculates a preset minute volume. It has been shown that ineffec-tive respiratory efforts can occur during pressure-support ventilation. A mismatch between the neural (ie, patient) and mechanical (ie, ventilator) timings is the main cause of missing breaths occurring while the ventilator is in the inspiratory phase: the reason is that the ventilator does not cycle from inspiration to expiration until the inspiratory flow decreases to a threshold value (5% of the peak inspiratory flow). The patient's ineffective efforts can also occur during the expiratory phase of the ventilator: in that situation, the inspiratory effort occurs before complete lung emptying and is not high enough to trigger the ventilator. The risk of the patient making ineffective efforts is increased by the algorithm included in the VSV mode. If the patient makes numerous ineffective efforts, the frequency of effective efforts (recorded by the ventilator) can be lower than the set frequency, in which case a new target V_T is calculated by the ventilator to achieve the preset minute volume. As V_T increases, the mismatch between the neural and mechanical timings also increases. I report 3 clinical observations showing numerous patient respiratory efforts not sensed by the ventilator and inducing V_T instability during VSV. These ineffective efforts can occur during inspiratory and expiratory phases. The mechanisms are discussed.