Choosing a negative pressure ventilation pump: are there any important differences?

Five negative pressure ventilator pumps were assessed to determine whether there were any differences in performance which were likely to be of clinical importance. The pumps tested were the NEV-100 (Lifecare Inc.), the Negavent Respirator DA-1 (Dima Italia), the Thompson Maxivent (Puritan Bennett), the CCP-001 and Newmarket pumps (Si-Plan Electronics Research Ltd). A patient model was employed to investigate the relationship between the pressure waveform of each pump and the tidal volume generated, the stability of performance on a continuous 8 h test, and the response to a change in leak. On the continuous tests, the stability was better than +/- 1% for the inspiratory/expiratory (I:E) ratio and rate, and better than +/- 10% for the peak negative pressure for all of the pumps. The Negavent and NEV-100 pumps generated a square wave of pressure and for the same peak negative pressure produced a tidal volume up to 30% greater than the CCP-001 and the Newmarket pumps, which produced a half sine wave. The Maxivent produced an intermediate waveform, with a plateau at peak negative pressure but a curved decay back to atmospheric pressure. It is argued that if the pressure of the pumps producing a half sine wave is made more negative to compensate and produce an equivalent tidal volume, they are more likely to induce upper airway obstruction. The leak compensation of the NEV-100 and Negavent pumps was nearly complete but took several breaths to develop, the CCP-001 and Newmarket pumps had a very rapid compensation but this was less complete, whilst the Maxivent has no mechanism for leak compensation. From these results, it seems that in patients liable to upper airway obstruction the NEV-100 and Negavent pumps would be advantageous, whilst when rapidly changing leaks impair the efficiency of negative pressure ventilation the CCP-001 and Newmarket pumps would be preferable. The Maxivent pump seems to offer no advantages.