Influence of respiratory behavior on ventilation, respiratory work and intrinsic PEEP during noninvasive nasal pressure support ventilation in normal subjects.

BACKGROUND: In clinical practice, patients have different inspiratory behaviors during noninvasive pressure support ventilation (PSV): some breathe quietly, others actively help PSV by an additional effort, and others even resist the inspiratory pressure of PSV. OBJECTIVE: What is the influence of patient collaboration (inspiratory behavior) on the efficiency of PSV? METHODS: We ventilated 10 normal subjects with nasal PSV (inspiratory/expiratory: 10/0 and 15/5 cm H(2)O) and measured their flow and volume with a pneumotachograph and their esophageal and gastric pressures during three different respiratory voluntary behaviors: relaxed inspiration, active inspiratory work and resisted inspiration. RESULTS: When compared with relaxed inspiration with 10/0 cm H(2)O PSV: (1) an active inspiratory effort increased tidal volume (from 789 +/- 356 to 1,046 +/- 586 ml; p = 0.006), minute ventilation (from 10.40 +/- 4.45 to 15.77 +/- 7.69 liters/min; p < 0.001), transdiaphragmatic work per cycle (from 0.55 +/- 0.33 to 1.72 +/- 1.40 J/cycle; p = 0.002) and inspiratory work per cycle (from 0.14 +/- 0.20 to 1.26 +/- 1.01 J/cycle; p = 0.003); intrinsic positive end-expiratory pressure (PEEP(i)) increased from 1.23 +/- 1.02 to 3.17 +/- 2.30 cm H(2)O; p = 0.002); (2) a resisted inspiration decreased tidal volume (to 457 +/- 230 ml; p = 0.007), minute ventilation (to 6.93 +/- 3.04 liters/min; p = 0.028) along with a decrease in transdiaphragmatic work but no change in PEEP(i). Data obtained during a bilevel PSV of 15/5 cm H(2)O were similar to those obtained with the 10/0 cm H(2)O settings. CONCLUSIONS: Active inspiratory effort increases ventilation during PSV at the expense of an increased breathing work and PEEP(i). Resisted inspiration inversely decreases inspiratory work and ventilation with no air trapping. These differences between inspiratory behaviors could affect the expected beneficial effects of PSV in acutely ill patients