THE TIME COURSE OF A POSTURAL CHANGE ON THE PULMONARY BLOOD FLOW DISTRIBUTION

Abstract

It has been well established that a postural change causes an alteration in the pulmonary blood flow distribution. However, the precise time course of this phenomenon has not been defined. To clarify this time relationship, 6 adult mongrel dogs were studied, using radioactive microspheres, pulmonary arteriography and computed tomography (CT). Radioactive microspheres were used to determine the ratio of blood flow between the lower and upper pulmonary lobes after 30 minutes in the upright position, and 1 minute, 3 minutes and 5 minutes after placing the dogs in the supine position. The change of this ratio with time was analyzed. Pulmonary arteriograms were performed after 30 minutes in the upright position, and also 5 minutes, 15 minutes, 30 minutes and 60 minutes after a change to the supine position The ratio of pulmonary artery diamaters in the lower to those in the upper lung zone, measured at reproducible points, was calculated on each arteriogram. The time-dependent change of this ratio was then examined. Serial CT scans at the base of the lung were obtained up to 60 minutes after a change from the supine to the prone position and then after returning the dogs to the supine position. The mean attenuation values in the anterior, middle and posterior lung zones were analyzed as a function of time. A significant change of the flow ratio in the microsphere study occurred within 1 minute after postural change, without significant change beyond that time. This rapid change was confirmed by pulmonary arteriography which showed a significant change in the ratio of the lower to the upper lung zone pulmonary artery diameters, and by the CT study which identified a significant change of the mean attenuation values in the anterior and posterior lung zones. We conclude that an alteration in the pulmonary blood flow distribution occurs almost immediately-within 1 minute-after a postural change.