Background We studied the cardiorespiratory effects of altering ventilator-set inspiratory flowrate (Vvent) during pressure support ventilation (PSV) in 15 patients using the IRISA microprocessor-controlled ventilator. Methods All patients were being ventilated at PSVmax (ie, PSV level adjusted to provide tidal volume VT = 10-12 mL/kg and f≤ 20 breaths/min) following a mean (SD) duration of ventilatory support of 16.1 (7) days. Patients were initially ventilated with the maximum Vvent for 1 hour. During this hour, tidal volume, inspiratory time (T₁), minute ventilation (VE), respiratory frequency (f), oxygen consumption (Vo₂), heart rate, and mean arterial blood pressure were monitored continuously. Additionally, at the end of the 1-hour period, arterial blood gas analysis and expired-gas collection for calculation of dead-space-to-tidal-volume ratio were performed. Patients then were ventilated at the lowest Vvent that would maintain PSVmax, and measurements were repeated. A third, midrange Vvent was then used, and measurements were repeated after 1 hour. Results A variable response to alterations in Vvent was observed. Four patients exhibited a lower f, longer T₁, and larger V₁ at the highest Vvent; four at the lowest Vvent; and seven patients at the midrange Vvent. Patients with the highest VE preferred the highest Vvent and decreasing Vvent in this population resulted in tachypnea and dysynchrony. When Vvent was increased in the group preferring the lowest Vvent, T₁ and V₁ fell while f increased. No clinically important changes were seen in pulse rate or mean arterial blood pressure in any patient during the Vvent manipulation. Conclusion Variable Vvent during PSV allows titration of ventilator output to patient demand, improves patient-ventilator synchrony, and should be incorporated into current PSV systems .
Branson et al. (Thu,) studied this question.