Work of breathing, inspiratory flow response, and expiratory resistance during continuous positive airway pressure with the ventilators EVITA-2, EVITA-4 and SV 300.

OBJECTIVE

To analyze work of breathing (WOB) imposed by the respirators EVITA-2, EVITA-4 (Drägerwerk, Lübeck, Germany) and SV 300 (Siemens-Elema, Sweden) as well as inspiratory flow response and expiratory flow resistance during continuous positive airway pressure (CPAP).

DESIGN

Five study conditions on a lung model (CPAP at 0, 5, and 10 mbar, CPAP 5 mbar plus pressure support 2 mbar with both EVITA models, and CPAP 5 mbar with decreasing levels of flow and pressure trigger sensitivity with the SV 300) and three randomized study conditions in nine patients recovering from open heart surgery (condition A: EVITA-2, CPAP 5 mbar; condition B: SV 300, CPAP 5 mbar, flow trigger; condition C: SV 300, pressure trigger-4 mbar).

SETTING

University hospital intensive care unit and laboratory of pulmonary physiology.

MEASUREMENTS AND RESULTS

At each study condition we measured WOB, pressure-time product (PTP), WOB and PTP imposed (WOBimposed and PTPimposed), tidal volume, minute ventilation, respiratory rate, inspiratory trigger time, trigger pressure, trigger PTP, duration of inspiration, mean and peak inspiratory flow, and the delay from the onset of inspiration to peak inspiratory flow. Since the SV 300 automatically generates an additional pressure support of 2 cm H2O PTP, WOB, WOBimposed, and PTPimposed were higher with the EVITA-2 and EVITA-4 regardless of the trigger sensitivity set on the SV 300. The difference was neutralized with both types of EVITA ventilator by adding 2 mbar of pressure support during CPAP in order to achieve comparable conditions. Inspiratory flow response was faster with both EVITA models, expiratory flow resistance was higher with the SV 300. Decrements of trigger sensitivity with the SV 300 accelerated the flow response.

CONCLUSIONS

Under similar conditions, no difference in WOBimposed was observed, although inspiratory flow response and expiratory flow resistance differed substantially between the three ventilators tested. Trigger sensitivity plays a minor role in determining PTP and WOB but has major influence on flow.