Regional pulmonary blood flow during rest, tilt, and exercise in unanesthetized dogs

We assessed the heterogeneity of regional pulmonary blood flow (PBF(r)), using radioactive microspheres in five unanesthetized dogs standing at rest (Rest), standing at a 45° upward tilt (Tilt), and during moderate treadmill exercise (Exer). The excised lungs were cut into 1-cm³ pieces along transverse, horizontal, and longitudinal planes. Mean PBF(r) increased from 23.3 ml · min^-1 · g^-1 at Rest to 57.4 ml · min^-1 · g^-1 during Exer, but the relative dispersions were not statistically different between states (47.3-51.9%). A small but significant gravity-dependent gradient in PBF(r) of ≤4.7%/cm (r² ≤ 0.118) as well as a PBF(r) decreasing radial gradient from the lung midpoint of ≤7.2%/cm (r² ≤ 0.108) were present in all states. PBF(r) at Rest was highly correlated with those at Tilt (r² = 0.773) and Exer (r² = 0.888), and a variable PBF, gradient of ≤2.5%/cm from base to apex was observed. Fractal dimensions calculated using relative dispersion as a function of aggregated sample size were not significantly different between states and were 1.132 (r² = 0.987) at Rest, 1.121 (r² = 0.973) at Tilt, and 1.149 (r² = 0.986) during Exer. Thus, gravity and centripetal gradients consistently accounted for a maximal difference of only about twofold in PBF, and <11% of overall PBF(r) heterogeneity in 1-cm³ samples. Recursive anatomic branching of pulmonary arteries and local mechanical factors apparently account for most of the blood flow heterogeneity in small pieces of lung.