Combining pressure-limiting and volume-cycling features in a patient-interactive mechanical breath

OBJECTIVESTo combine the patient synchrony effects of pressure-limited breath delivery strategies with the volume guarantee of volume-cycled breath delivery strategies, we designed a positive-pressure breath that incorporates both features. This breath is patient triggered and can be pressure limited. Breath termination (i.e., cycling) can either be flow- or volume-cycled, depending on whether a target volume has been attained. The pressure-limiting features are further enhanced by the capability to adjust demand-valve responsiveness at breath initiation. DESIGNProspective, mechanical simulation studies. SETTINGEngineering laboratory. INTERVENTIONSA mechanical lung model that could simulate patient ventilatory efforts was used. Vigorous and weak patient efforts were studied in conjunction with use of volume assist, pressure support, and a combination pressure-limited and volume-cycled breath. MEASUREMENTS AND MAIN RESULTSTwo clinical situations were simulated by the modela) a patient requiring total mechanical ventilatory support who had a very active ventilatory drive; and b) a patient receiving only partial ventilatory support who had an unstable ventilatory drive. A range of pressure-limiting and volume-cycling settings were given. Volume, pressure, and flow delivery were measured. Synchrony with patient effort was assessed by visual inspection of airway pressure graphics as well as by calculations of patient work during these support strategies. CONCLUSIONSFlow dyssynchrony during fixed-flow, volume-cycled assisted breaths in patients with active ventilatory drives can be improved with this breath design while a guaranteed tidal volume is maintained. In addition, this combination breath can provide a volume “safety net” for patients in whom partial support with pressure-support ventilation is desired. (Crit Care Med 1994; 22:353–357)