Pulmonary arterial shunting in man during forward acceleration

Abstract

The arterial blood gases were determined during forward acceleration 90∘ to the acceleration vector at 6 g and 8 g breathing room air and at 8 g breathing 100% oxygen. Arterial saturation fell to 84% at 6 gand 75% at 8 g. Prebreathing O2 for 15 min prior to acceleration with continued inhalation during the acceleration plateau only partially corrected the undersaturation to 86% at 8 g. Recovery was not complete in 3 min unless O g therapy was used. Whole blood carbon dioxide content was depressed at 6 g and 8 g on room air, but this was corrected by O g inhalation. However, during the recovery period while breathing oxygen the carbon dioxide content was depressed. pH was reduced and pCO g elevated slightly during each acceleration period. Since cardiac output and alveolar ventilation have been reported to be essentially unaltered during forward acceleration at these magnitudes, the observed effects must represent substantial alterations in the individual ventilation to blood flow ratios throughout the lung, with approximately 50% of the cardiac output shunted through totally nonventilated areas at 8 g. There also must be some inadequately perfused or nonperfused peripheral areas, as evidenced by the fall in CO g content and pH and the accumulation of a substantial O g debt previously reported during acceleration. Note: With the Technical Assistance of Alice M. Caton and Justin L. Taylor, Jr. Submitted on June 12, 1961

Cardiovascular Adjustments to Gravitational Stress

“…acceleration arterial oxygen saturation at rest can decline to as low as 75% at 8 +G. (312) . The increase in accelerative forces increases alveolar size in superior lung regions and decreases it in the dependent regions, causing a greater ventilation/perfusion mismatch (57, 131, 339).…”

Spacecraft launch and re-entry: Effects of simulated +Gx acceleration on cardiorespiratory parameters

“…These shunts are likely to magnify the inequalities of the ventilation-perfusion ratio (V/Q); leading to increase in perfusion but poor ventilation in the dependent parts of the lungs causing marked reduction of SpO 2. [19][20][21][22][23] Mean SpO2 recorded post-run (96.6 ± 2.3%) did not reach normality indicating that recovery was incomplete. However, mean SpO2 recorded during recovery (98.7 ± 0.59%) was statistically different from post +Gx run (P < 0.05).…”

Oh G: The x, y and z of human physiological responses to acceleration

“…Overall, Gx has a reputation for being more ‘tolerable’ than Gz, although this naturally depends on the magnitude, rate of onset and duration of exposure. Trained centrifuge participants have tolerated 3 Gx for 30 min (Serrador et al., 2001), 8 Gx for 3 min (Steiner & Mueller, 1961), 12 Gx for 30 s, and 15 Gx for 10 s (Gillies, 1965; Torphy et al., 1966). As a basic guide, from the authors’ experience, 2 Gx is comfortable, although it is hard to raise a leg, at 4 Gx moving and breathing feel noticeably more difficult and it is hard to raise an arm, and 6 Gx is uncomfortable, with substantial resistance to breathing and difficulty in moving.…”