Imposed work and oxygen delivery during spontaneous breathing with adult disposable manual ventilators.

BACKGROUND

Manual ventilators (resuscitators) are used primarily to ventilate the lungs of patients lacking spontaneous ventilatory effort. However, in many settings patients are allowed to breathe through the manual ventilator. Although many aspects of manual ventilator function have been studied, very little has been reported on the use of manual ventilators during spontaneous breathing. The purpose of this study was to evaluate inspiratory and expiratory imposed work of breathing and oxygen delivery during spontaneous breathing through disposable manual ventilators.

METHODS

Simulated spontaneous breathing was established with a two-chambered test lung, with one chamber serving as the test chamber and the other as the driving chamber. Imposed work of breathing was evaluated with decelerating inspiratory flow at a rate of 20 breaths/min at tidal volume (VT) 0.25 1 and peak flow 40 l/min, at VT 0.5 l and peak flow 80 l/min, and VT 0.81 and peak flow 120 l/min. Flow (integrated to volume) and pressure were measured between the manual ventilator and test lung, and inspiratory and expiratory imposed work of breathing were calculated by integration of the volume-pressure curve. Oxygen concentration was measured with an oxygen analyzer placed between the manual ventilator and the test lung at 20 breaths/min, VT 0.5 l, and flow 45 l/min. An oxygen flow of 15 l/min was added to the device for all evaluations. Two of the manual ventilators had built-in positive end-expiratory pressure valves, and imposed work was evaluated at 10 cmH2O with these.

RESULTS

There were significant differences in imposed work between inspiration and expiration (P < 0.001) and among the three levels of ventilatory demand (P < 0.001). For each ventilatory demand, there was a significant difference in work between manual ventilator brands for inspiratory work and expiratory work (P < 0.001). At a VT of 0.5 l and peak flow of 80 l/min, the pooled inspiratory imposed work for all manual ventilators was 0.44 +/- 0.12 J/l, and the pooled expiratory imposed work was 0.29 +/- 0.05 J/l. With 10 cmH2O positive end-expiratory pressure, the inspiratory imposed work was very high (> 1 J/l). Four of the devices were unable to deliver more than 0.85 oxygen concentration at the spontaneous ventilatory pattern evaluated.

CONCLUSIONS

Adult disposable manual ventilators produce a substantial imposed work of spontaneous breathing, which is increased with the addition of positive end-expiratory pressure. With some manual ventilators, a high oxygen concentration may not be delivered during spontaneous breathing. We recommend that patients not be allowed to spontaneously breathe through disposable manual ventilators.