Effects of intermittent negative pressure ventilation on effective ventilation in normal awake subjects

Previous studies have shown that an increase in inspiratory pressure during nasal intermittent positive pressure ventilation (IPPV) does not result in increased effective minute ventilation (E) due to glottic interference.

To test the consequences of increases in negative pressure ventilation (NPV) on V(E).

Eight healthy awake subjects underwent NPV delivered by an iron lung. First, NPV was started at a respirator frequency (f) of 15 cycles per minute with an inspiratory negative pressure (INP) of - 15 cm H(2)O (F15-P15). Then, f was increased to 20 cycles per minute and INP was kept at - 15 cm H(2)O. Next, f was kept at 20 cycles per minute and INP was reduced to - 30 cm H(2)O (F20-P30). Finally, f was decreased to 15 cycles per minute and INP was kept at - 30 cm H(2)O. At each step and for each breath, effective tidal volume (VT), V(E), and end-tidal carbon dioxide pressure were measured. In three subjects, the glottis width was assessed using fiberoptic bronchoscopy.

From spontaneous breathing to the first step of NPV (F15-P15), we observed an inhibition of the phasic inspiratory diaphragmatic electromyogram concomitant to a significant increase in V(E) (p < 0.0005). For the group as a whole, the increase in mechanical ventilation (from F15-P15 to F20-P30) resulted in significant increases in VT and V(E) leading to hypocapnia (p < 0.0005). Moreover, the glottis width did not decrease with the increase in mechanical ventilation.

We conclude that in normal awake subjects, NPV allowed a significant increase in V(E). These results differ from those previously obtained with nasal IPPV in which the glottic width interferes with the delivered mechanical ventilation.