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Vol. 84, Issue 4, 1278-1288, April 1998
Departments of 1 Physiology and Biophysics, 2 Medicine, and 3 Anesthesiology, University of Washington, Seattle 98195; 4 The Mountain-Whisper-Light Statistical Consulting, Seattle, Washington 98122; and 5 Crew Technology Division, Armstrong Laboratory, Air Force Medical Center, Brooks Air Force Base, Texas 78235
This study was undertaken to assess the
influence of gravity on the distribution of pulmonary blood flow (PBF)
using increased inertial force as a perturbation. PBF was studied in
unanesthetized swine exposed to
Gx (dorsal-to-ventral
direction, prone position), where G is the magnitude of the force of
gravity at the surface of the Earth, on the Armstrong Laboratory
Centrifuge at Brooks Air Force Base. PBF was measured using 15-µm
fluorescent microspheres, a method with markedly enhanced spatial
resolution. Each animal was exposed randomly to 1, 2, and
3 Gx. Pulmonary vascular
pressures, cardiac output, heart rate, arterial blood gases, and PBF
distribution were measured at each G level. Heterogeneity of PBF
distribution as measured by the coefficient of variation of PBF
distribution increased from 0.38 ± 0.05 to 0.55 ± 0.11 to
0.72 ± 0.16 at 1, 2, and 3
Gx, respectively. At 1
Gx, PBF was greatest in the
ventral and cranial and lowest in the dorsal and caudal regions of the
lung. With increased Gx,
this gradient was augmented in both directions. Extrapolation of these
values to 0 G predicts a slight dorsal (nondependent) region dominance
of PBF and a coefficient of variation of 0.22 in microgravity. Analysis
of variance revealed that a fixed component (vascular structure)
accounted for 81% and nonstructure components (including gravity)
accounted for the remaining 19% of the PBF variance across the entire
experiment (all 3 gravitational levels). The results are inconsistent
with the predictions of the zone model.
fluorescent microspheres; cardiac output; pulmonary gas exchange; centrifuge; acceleration; gravity
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