The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

HUMIDIFICATION DURING NONINVASIVE POSITIVE PRESSURE VENTILATION

Matthew P. Branconnier, RRT, EMT; Kelly J. Gornnert, BS, RRT; Dean R. Hess, PhD, RRT, FAARC. Respiratory Care Department , Massachusetts General Hospital , Boston , MA .

Background. Noninvasive positive pressure ventilation (NPPV) is used increasingly in patients with acute respiratory failure. Humidification of the inspired gas is not provided universally during NPPV and little has been reported on this subject. We conducted this study to evaluate the amount of humidity delivered with several approaches to humidification using a model incorporating a ventilator designed for NPPV. Methods. All evaluations were conducted using a Respironics Vision BiPAP ventilator. The ventilator was set for an inspiratory pressure (IPAP) of 15 cm H2O, an expiratory positive airway pressure (EPAP) of 5 cm H2O, and a respiratory rate of 20/min. A standard BiPAP single limb circuit with an integrated leak port was used. The patient end of the circuit was connected to the hygrometer, which was attached to a Michigan Instruments TTL test lung. Psychometric measurements of relative humidity (RH) and absolute humidity (AH) were made using methodology previously described (Chest 1999; 115:1646). In brief, a device to separate inspiratory and expiratory gas flow using 2 unidirectional valves was inserted between the BiPAP circuit and the test lung. Two thermal probes, 1 dry and 1 wet (cotton wetted with water), were placed in the inspiratory part of the device. The temperatures were measured by the 2 probes (Fisher Scientific Dual Thermometer) and recorded after 30 min equilibrium. RH and AH were calculated using standard formulae. Five approaches to humidification were studied: 1) no humidification, 2) unheated passover humidification using a Hudson Concha Therm III, 3) heated passover humidification using a Concha Therm III with the temperature probe placed at the position of a water trap 60 cm from the outlet of the humidifier and adjusted to 30 C (trap), 4) heated passover humidification using a Concha Therm III with the temperature probe placed at the distal end of the BiPAP circuit and adjusted to 30 C (mask), 5) heated passover humidification using a Concha Therm IV and heated wire circuit with temperature adjusted to 31 C at the distal end of the BiPAP circuit and the humidity setting at the default position. Each approach to humidification was studied with 21% and 100% oxygen set on the BiPAP ventilator. Psychometric data were collected in triplicate. The amount of water that accumulated in the circuit was measured volumetrically. Results. There were significant differences in the delivered humidity between the 5 approaches that we tested (P < 0.001; see figure). With the temperature probe at the distal end of the circuit, the average rain-out in the circuit was 31 ± 4 mL/h. There was no appreciable rain-out for the heated wire circuit.

Conclusions. Without humidification, the absolute humidity of the gas delivered to the patient is very low with the BiPAP Vision ventilator. Use of a heated humidifier significantly increases the absolute humidity. Use of a heated wire circuit reduces the amount of water accumulating in the circuit. Further work is needed to determine the optimum humidity during NPPV.

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