Relative contribution of gravity to pulmonary perfusion heterogeneity

We designed a series of experiments and analyses to quantify the contribution of gravity to pulmonary perfusion heterogeneity. Regional pulmonary perfusion was measured in five anesthetized and ventilated dogs in both supine and prone positions by use of radiolabeled microspheres injected during apnea at functional residual capacity. Measurements of flow were repeated in each position, and the sequence of positions was prospectively designed to nullify any effect of order. The lungs of each animal were excised, perfused with saline until clear, dried at an inflation pressure of 25 cmH2O, and cut into 1.9-cm3 pieces. Each piece was weighed and the radioactivity determined in a scintillation counter. Measurement errors were minimized by excluding lung pieces that had greater than 25% airway and weighed less than 10 mg or greater than 60 mg. Weight-normalized flows in each position and repetition were determined for each lung piece. An analysis of variance model was used to identify the percentage of variation in regional flow that was due to position (supine vs. prone), to random error and time (measurement and repetition), and to structure, where structure was defined as the component of flow that remained constant across position and replication. The contributions of position, error/time, and structure to the total variability of flow across the five dogs were 7.8 +/- 0.6, 8.4 +/- 8.3, and 83.8 +/- 8.4%, (SD), respectively. Because the contribution of position represents the additive effect of gravity between two opposite positions, the contribution of gravity to perfusion heterogeneity in one position may be as little as 4%.(ABSTRACT TRUNCATED AT 250 WORDS)