Importance of gravity in determining the distribution of pulmonary blood flow

We aimed to assess the influence of lateral decubitus postures and positive end-expiratory pressure (PEEP) on the regional distribution of ventilation and perfusion. We measured regional ventilation ((V) over dot A) and regional blood flow ((Q) over dot)in six anesthetized, mechanically ventilated dogs in the left (LLD) and right lateral decubitus (RLD) postures with and without 10 cmH(2)O PEEP. (Q) over dot was measured by use of intravenously injected 15-mum fluorescent microspheres, and (V) over dot A was measured by aerosolized 1-mum fluorescent microspheres. Fluorescence was analyzed in lung pieces similar to1.7 cm(3) in volume. Multiple linear regression analysis was used to evaluate three-dimensional spatial gradients of (Q) over dot, (V) over dot A, the ratio (V) over dot A/(Q) over dot, and regional PO2 (Pr-O2) in both lungs. In the LLD posture, a gravity-dependent vertical gradient in (Q) over dot was observed in both lungs in conjunction with a reduced blood flow and Pr-O2 to the dependent left lung. Change from the LLD to the RLD or 10 cmH(2)O PEEP increased local (V) over dot A/ and Pro(2) in the left lung and minimized any role of hypoxia. The greatest reduction in individual lung volume occurred to the left lung in the LLD posture. We conclude that lung distortion caused by the weight of the heart and abdomen is greater in the LLD posture and influences both (Q) over dot and (V) over dot A, and ultimately gas exchange. In this respect, the smaller left lung was the most susceptible to impaired gas exchange in the LLD posture.