Determinants of effective ventilation during nasal intermittent positive pressure ventilation.

Our aim was to verify in healthy subjects submitted to nasal intermittent positive pressure ventilation (nIPPV) with a volumetric ventilator on controlled mode, whether changes in ventilator settings (delivered tidal volume (VT), respiratory frequency (fR) and inspiratory flow (V'I) could influence effective minute ventilation (V'E), thus allowing identification of the settings resulting in the highest V'E during nIPPV. We then compared these experimentally obtained "best" settings to those obtained retrospectively in a group of patients submitted to long-term nIPPV for clinical reasons. We studied 10 healthy subjects awake and asleep, and 33 patients with restrictive ventilatory disorders. Changes in delivered V'I (for a constant delivered VT and fR) led to significant changes in V'E. V'E was significantly higher when a given delivered V'E was obtained using higher fR and lower VT than when it was obtained using lower delivered fR and higher VT. Increases in fR generally resulted in increases in V'E. The "best" settings derived from these results were: VT: 13 mL.kg-1 of body weight; fR: 20 breaths.min-1 and V'I: 0.56-0.85 L.s-1. The corresponding average values found in the patient group were: delivered VT: 14 mL.kg-1; fR: 23 breaths.min-1 and delivered V'I: 0.51 L.s-1. Changes in minute ventilation resulting from modifications in ventilator settings can be attributed to the glottic response to mechanical influences. This leads to "ideal" settings quite different from the standard ones in intubated patients. Values derived from nasal intermittent positive pressure ventilation in healthy subjects seem to apply to patients submitted to long-term nasal intermittent positive pressure ventilation.