Abstract
We compared the efficacy of three devices delivering assisted non-invasive ventilation with different working mechanisms, during room air breathing and during CO2-induced hyperventilation. In seven healthy volunteers, breathing pattern, respiratory muscle activity and comfort were assessed: during unassisted spontaneous breathing through a mouth-piece (SB); during assisted breathing with a device delivering inspiratory pressure support (IPS); and with two devices delivering intermittent positive pressure breathing (IPPB), the Monaghan 505 (IPPB1), and the CPU 1 ventilator (IPPB2). All three devices were set at 10 cmH2O of maximal pressure. During room air breathing, the work of breathing expressed as power, was significantly greater with the two IPPB devices than with the two other modes (IPPB1 and IPPB2 7.3 +/- 5.2 and 7.2 +/- 6.2 J.min-1, respectively, versus SB and IPS 2.4 +/- 0.7 and 2.3 +/- 3.3 J.min-1, respectively). The difference did not reach the statistical significance for the pressure-time product (PTP). Discomfort was also greater during the IPPB modes. During CO2-induced hyperventilation, considerable differences in power of breathing were found between the two IPPB devices and the other two modes. The PTP was also much higher with IPPB. Transdiaphragmatic pressure was significantly smaller during IPS than during the three other modes (IPS 18 +/- 2.6 cmH2O versus SB 22 +/- 2.6, IPPB1 32 +/- 5.2, and IPPB2: 28 +/- 5.2). Maximal discomfort was observed during the IPPB modes and was correlated with the magnitude of transdiaphragmatic pressure (r = 0.60). Despite similarities in their operational principles, IPS and IPPB had very different effects on respiratory muscle activity in healthy non-intubated subjects. IPPB machines not only failed to reduce patient's effort but also induced a significant level of extra work by comparison to spontaneous ventilation at ambient pressure. Great caution is, therefore, needed in the use of patient-triggered devices for non-intubated patients with acute respiratory failure.
