Chinese J Physiol
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The effect of left lung atelectasis on the regional distribution of blood flow (Q), ventilation (V(A)) and gas exchange on the right lung ventilated with 100% O2 was studied in anesthetized dogs in the lateral decubitus posture. Q and V(A) were measured in 1.7 ml lung volume pieces using injected and aerosolized fluorescent microspheres, respectively. Hypoxic pulmonary vasoconstriction (HPV) in the atelectatic lung shifted flow to the ventilated lung. ⋯ Left lung atelectasis caused a compensatory increase in the ventilated lung FRC that was smaller in the right (RLD) than left (LLD) lateral posture, the effect of lung compression by the atelectatic lung and mediastinal contents in the RLD posture. The O2 deficit measured by (A-a)DO2 increased with left lung atelectasis and was exacerbated in the LLD posture by 10 cm H2O PEEP, a result of increased shunt caused by a shift in Q from the ventilated to the atelectatic lung. The PEEP-induced O2 deficit was eliminated with inversion to the RLD posture.
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Previous studies in anesthetized humans positioned in the left lateral decubitus (LLD) posture have shown that unilateral positive end-expiratory pressure (PEEP) to the dependent lung produce a more even ventilation distribution and improves gas exchange. Unilateral PEEP to the dependent lung may offer special advantages during LLD surgery by reducing the alveolar-to-arterial oxygen pressure difference {(A-a)PO2 or venous admixture} in patients with thoracic trauma or unilateral lung injury. We measured the effects of unilateral PEEP on regional distribution of blood flow (Q) and ventilation (V(A)) using fluorescent microspheres in pentobarbital anesthetized and air ventilation dogs in left lateral decubitus posture with synchronous lung inflation. ⋯ Bilateral PEEP disproportionately increased FRC in the right lung but again produced no significant changes in venous admixture or V(A)/Q distribution. We conclude that the reduced dependent lung blood flow observed without PEEP occurs secondary to a reduction in lung volume. When tidal volume is maintained, unilateral PEEP increases dependent lung volume with little effect of perfusion distribution maintaining gas exchange.