Patient-ventilator asynchrony during noninvasive ventilation: the role of expiratory trigger

Objective:Air leaks around the mask are very likely to occur during noninva sive ventilation, in particular when prolonged ventilatory treatment is req uired. It has been suggested that leaks from the mask may impair the expira tory trigger cycling mechanism when inspiratory pressure support ventilatio n (PSV) is used, The aim of this study was to compare the short-term effect of two different expiratory cycling mechanisms (time-cycled vs now-cycled) during noninvasive inspiratory pressure support ventilation (NIPSV) on pat ient-ventilator synchronisation in severe hypoxemic respiratory failure. Study population: Six patients with acute lung injury (ALI) due to acquired immunodeficiency syndrome (AIDS)-related opportunistic pneumonia were enro lled in the protocol. Intervention: Each subject was first studied during spontaneous breathing w ith a Venturi oxygen mask (SB) and successively submitted to a randomly ass igned 20' conventional now-cycling (NIPSVfc) or time-cycling inspiratory pr essure support ventilation (NIPSVtc). The pre-set parameters were: inspirat ory pressure of 10 cm H2O, PEEP of 5 cm H2O for the same inspired oxygen fr action as during SE. A tight fit of the mask was avoided in order to facili tate air leaks around the mask. The esophageal pressure time product (PTPes ) and tidal swings (Delta Pes) were measured to evaluate the patient's resp iratory effort. A subjective "comfort score" and the difference between pat ient and machine respiratory rate [Delta RR(p-v)], calculated on esophageal and airway pressure curves, were used as indices of patient-machine intera ction. Results: Air leaks through the mask occurred in five out of six patients. T he values of PEEPi (< 1.9 cm H2O) excluded significant expiratory muscle ac tivity. NIPSVtc significantly reduced PTPes, Delta Pes, and Delta RR(p-v) w hen compared to NIPSVfc [230 +/- 41 (SE) vs 376 +/- 72 cm H2O . s . min(-1) : 8 +/- 2 vs 13 +/- 2 cm H2O; 1 +/- 1 vs 9 +/- 2 br.min(-1); respectively] with a concomitant significant improvement of the "comfort score". Conclusions: In the presence of air leaks a time-cycled expiratory trigger provides a better patient-machine interaction than a flow-cycled expiratory trigger during NIPSV.