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.
Carvalho
Alysson R
AR
Clinic of Anesthesiology and Intensive Care Medicine, University Clinic Carl Gustav Carus, Dresden, Germany.
Spieth
Peter M
PM
Pelosi
Paolo
P
Beda
Alessandro
A
Lopes
Agnaldo J
AJ
Neykova
Boriana
B
Heller
Axel R
AR
Koch
Thea
T
Gama de Abreu
Marcelo
M
D055371:Acute Lung Injury; D000818:Animals; D016000:Cluster Analysis; D005260:Female; D005456:Fluorescent Dyes / Q000737:chemistry; D008168:Lung / Q000098:blood supply*; D008863:Microspheres; D010100:Oxygen / Q000378:metabolism*; D011175:Positive-Pressure Respiration / Q000379:methods*; D011312:Pressure; D012119:Respiration; D015656:Respiratory Mechanics; D013552:Swine; D013990:Tidal Volume; D014057:Tomography, X-Ray Computed / Q000379:methods
D005456:Fluorescent Dyes; D010100:Oxygen