High continuous positive airway pressure level induces ventilation/perfusion mismatch in the prone position

Gas exchange in patients with adult respiratory distress syndrome is influenced by posture. The combined effect of continuous positive airway pressure and posture has not been investigated. We studied the effect of normal spontaneous breathing, and that of continuous positive airway pressure, on ventilation/perfusion distributions in healthy volunteers while they were in supine and prone positions.

Nuclear medicine department in a university hospital.

Experimental study.

Sixteen healthy volunteers.

In the supine or prone position, the subjects inhaled a technetium-labeled aerosol (technetium-99m diethylenetriamine pentaacetic acid) through a tight-fitting mask. Single photon emission computed tomography images of the lungs were obtained. The subjects then received an intravenous injection of technetium-99m-labeled macroaggregates of albumin, and an identical single photon emission computed tomography imaging was performed. In the group that received continuous positive airway pressure, an end-expiratory pressure of 10 cm H2O was applied during both inhalation and injection.

During spontaneous breathing, ventilation/perfusion distribution assessed by regression analysis was uniform (i.e., not significantly different from zero) both in supine and prone positions, with a slope of -1.5 +/- 3.5%/cm supine and 1.5 +/- 3.5%/cm prone. During continuous positive airway pressure breathing in the supine position, ventilation/perfusion had a slope of -3.4 +/- 2.4 compared with 8.3 +/- 1.1%/cm in the prone position according to analysis of spatial resolution.

There was a less favorable ventilation/perfusion ratio in the prone position when the subjects were exposed to continuous positive airway pressure of 10 cm H2O.