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Leak compensation in positive pressure ventilators: a lung model study

^a Mt. Sinai Hospital, University of Toronto, Toronto, Canada, ^b Memorial Hospital of Rhode Island and ^c Rhode Island Hospital and Brown University School of Medicine, Providence, RI, USA

CORRESPONDENCE: N.S. Hill, Division of Pulmonary, Sleep, and Critical Care Medicine, Rhode Island Hospital, 593 Eddy Street, Providence, RI 02903, USA. Fax: 1 4014446665

Keywords: mask ventilation, mechanical ventilators, noninvasive ventilation, portable ventilators

Received: September 30, 1999
Accepted August 21, 2000

Leak compensating abilities of six different positive pressure ventilators commonly used to deliver noninvasive positive pressure ventilation, including the bilevel positive airway pressure (BiPAP) S/T-D and Quantum (Respironics Inc, Murrysville, PA, USA), 335 and O'NYX (Mallinckrodt Inc, St Louis, MO, USA), PLV 102 (Respironics), and Siemens Servo 900C (Siemens Inc, Danvers, MA, USA).

Using a test lung model, compensatory capabilities of the ventilators were tested for smaller and larger leaks using the assist/control or timed modes. Back-up rate was 20·min^–1, inspiratory pressure was 18 cmH₂O, and expiratory pressure was 5 cmH₂O.

It was found that even in the absence of air leaking, delivered tidal volume differed substantially between the ventilators during use of pressure-targeted modes, depending on inspiratory flows, inaccuracies in set versus delivered pressures, and inspiratory duration. Also during pressure-targeted ventilation, increasing the t_I/t_tot up to, but not beyond, 0.5 improved compensation by lengthening inspiratory duration, whereas use of a sensitive flow trigger setting tended to cause autocycling during leaking, interfering with compensation. Leaking interfered with cycling of the BiPAP S/T, inverting the I:E ratio, shortening expiratory time, and reducing delivered tidal volume. Volume-targeted modes achieved limited compensation for small air leaks, but compensated poorly for large leaks.

To conclude, leak-compensating capabilities differ markedly between ventilators but pressure-targeted ventilators are preferred for noninvasive positive pressure ventilation in patients with substantial air leaking. Adequate inspiratory flows and durations should be used, triggering sensitivity should be adjusted to prevent autocycling, and a mechanism should be available to limit inspiratory time and avoid I:E ratio inversion.

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