A bench study of 2 ventilator circuits during helmet noninvasive ventilation.

OBJECTIVE To compare helmet noninvasive ventilation (NIV), in terms of patient-ventilator interaction and performance, using 2 different circuits for connection: a double tube circuit (with one inspiratory and one expiratory line) and a standard circuit (a Y-piece connected only to one side of the helmet, closing the other side). METHODS A manikin, connected to a test lung set at 2 breathing frequencies (20 and 30 breaths/min), was ventilated in pressure support ventilation (PSV) mode with 2 different settings, randomly applied, of the ratio of pressurization time to expiratory trigger time (T(press)/T(exp-trigger)) 50%/25%, default setting, and T(press)/T(exp-trigger) 80%/60%, fast setting, through a helmet. The helmet was connected to the ventilator randomly with the double and the standard circuit. We measured inspiratory trigger delay (T(insp-delay)), expiratory trigger delay (T(exp-delay)), T(press)), time of synchrony (T(synch)), trigger pressure drop, inspiratory pressure-time product (PTP), PTP at 300 ms and 500 ms, and PTP at 500 ms expressed as percentage of an ideal PTP500 (PTP500 index). RESULTS At both breathing frequencies and ventilator settings, helmet NIV with the double tube circuit showed better patient-ventilator interaction, with shorter T(insp-delay), T(exp-delay), and T(press); longer T(synch); and higher PTP300, PTP500, and PTP500 index (all P < .01). CONCLUSIONS The double tube circuit had significantly better patient-ventilator interaction and a lower rate of wasted effort at 30 breaths/min.