Pressure support ventilation and biphasic positive airway pressure improve oxygenation by redistribution of pulmonary blood flow.

BACKGROUND: Spontaneous breathing (SB) activity may improve gas exchange during mechanical ventilation mainly by the recruitment of previously collapsed regions. Pressure support ventilation (PSV) and biphasic positive airway pressure (BIPAP) are frequently used modes of SB, but little is known about the mechanisms of improvement of lung function during these modes of assisted mechanical ventilation. We evaluated the mechanisms behind the improvement of gas exchange with PSV and BIPAP. METHODS: Five pigs (25-29.3 kg) were mechanically ventilated in supine position, and acute lung injury (ALI) was induced by surfactant depletion. After stabilization, BIPAP was initiated with lower continuous positive airway pressure equal to 5 cm H2O and the higher continuous positive airway pressure titrated to achieve a tidal volume between 6 and 8 mL/kg. The depth of anesthesia was reduced, and when SB represented > or = 20% of total minute ventilation, PSV and BIPAP + SB were each performed for 1 h (random sequence). Whole chest helical computed tomography was performed during end-expiratory pauses and functional variables were obtained. Pulmonary blood flow (PBF) was marked with IV administered fluorescent microspheres, and spatial cluster analysis was used to determine the effects of each ventilatory mode on the distribution of PBF. RESULTS: ALI led to impairment of lung function and increase of poorly and nonaerated areas in dependent lung regions (P < 0.05). PSV and BIPAP + SB similarly improved oxygenation and reduced venous admixture compared with controlled mechanical ventilation (P < 0.05). Despite that, a significant increase of nonaerated areas in dependent regions with a concomitant decrease of normally aerated areas was observed during SB. In five of six lung clusters, redistribution of PBF from dependent to nondependent, better aerated lung regions were observed during PSV and BIPAP + SB. CONCLUSIONS: In this model of ALI, the improvements of oxygenation and venous admixture obtained during assisted mechanical ventilation with PSV and BIPAP + SB were explained by the redistribution of PBF toward nondependent lung regions rather than recruitment of dependent zones.

Noisy pressure support ventilation: a pilot study on a new assisted ventilation mode in experimental lung injury.

OBJECTIVE: To describe and evaluate the effects of the new noisy pressure support ventilation (noisy PSV) on lung physiologic variables. DESIGN: Crossover design with four modes of mechanical ventilation. SETTING: Experimental research facility of a university hospital. SUBJECTS: A total of 12 pigs weighing 25.0-36.5 kg. INTERVENTIONS: Animals were anesthetized, the trachea was intubated, and lungs were ventilated with a mechanical ventilator (volume-controlled mode). Acute lung injury was then induced by surfactant depletion. Biphasic intermittent airway pressure/airway pressure release ventilation (BIPAP/APRV) was initiated, and anesthesia depth was decreased to allow spontaneous breathing. After that, each animal was ventilated with four different modes of assisted mechanical ventilation (1 hr each, Latin squares sequence): 1) PSV, 2) PSV combined with intermittent sighs (PSV + Sighs), 3) BIPAP/APRV + spontaneous breathing, and 4) noisy PSV with random variation of pressure support (normal distribution). The mean level of pressure support was set identical in all PSV forms. MEASUREMENTS AND MAIN RESULTS: We found that noisy PSV increased tidal volume variability compared with PSV and PSV + Sighs (19% vs. 5% and 7%, respectively, p < .05) independently from the inspiratory effort; improved oxygenation and reduced venous admixture but did not affect the amount of nonaerated lung tissue as compared with other assisted ventilation modes; reduced mean airway pressure at comparable minute ventilation; redistributed pulmonary blood flow toward nondependent lung regions similar to other PSV forms, whereas BIPAP/APRV + spontaneous breathing did not; and reduced the inspiratory effort and cardiac output in comparison with BIPAP/APRV + spontaneous breathing. CONCLUSIONS: In the surfactant depletion model of acute lung injury, the new noisy PSV increased the variability of the respiratory pattern and improved oxygenation by a redistribution of perfusion toward the ventilated nondependent lung regions with simultaneous lower mean airway pressure, comparable minute ventilation, and no increase in the inspiratory effort or cardiac output.