Spatial distribution of ventilation and perfusion in the lateral decubitus posture
Pulmonary blood flow distribution is affected by many factors, including gravity, hypoxic pulmonary vasoconstriction (HPV), lung volume and positive end-expiratory pressure (PEEP). In the lateral posture, the effects of posture, bilateral PEEP, differential ventilation with unilateral PEEP and left lung atelectasis on regional ventilation (V̇A) and perfusion (Q̇) in the lateral decubitus posture were studied. Q̇ and V̇A were measured using 15-mum i.v. injected and 1-mum aerosolized fluorescent microspheres and analyzed in ∼1.7 cm3 lung volume pieces. Multiple linear regression analysis was used to evaluate 3-dimensional spatial gradients of Q̇, V̇A, V̇A/Q̇ and regional PO2 (PRO2).In the left lateral decubitus (LLD) posture, a gravity-dependent vertical gradient in Q̇ was observed in conjunction with a reduced blood flow and PRO2 to the dependent left lung. This observation was consistent with HPV occurring in the dependent left lung. Either a change from the LLD to the RLD posture or 10 cm H2O PEEP increased local V̇ A/Q̇ and PRO2 in the left lung and minimized any role of hypoxia. Lung distortion caused by the mediastinum was a major factor that determines regional blood flow and ventilation. In this respect, the smaller left lung was the most susceptible to impaired gas exchange in the LLD posture.With 100% O2 ventilation, the A-aDO2 observed with conventional ventilation doubled with differential ventilation, with no change in blood flow. Unilateral PEEP increased dependent left lung FRC with no change in either blood flow or A-aDO2. Bilateral PEEP produced no change in A-aDO2 with an increased dependent blood flow. Lower dependent blood flow and A-aDO2 in the LLD posture during differential ventilation with unilateral dependent PEEP was not due primarily to airway closure, lung volume distortion or V̇A/Q̇ inequality.HPV in the atelectatic lung shifted flow to the ventilated lung. The increased flow in the ventilated lung ensured adequate gas exchange, compensating for the shunt contributed by the atelectatic lung. A-aDO2 increased with left lung atelectasis and was exacerbated in the LLD posture by 10 cm H2O PEEP, a result of increased Q̇ to the atelectatic lung. This PEEP-induced O2 deficit was eliminated with inversion to the RLD posture.