Influence of the chest wall on regional intrapleural pressure during acceleration (+Gz) stress

The purpose of this study was to determine the influence of altered chest wall compliance, chest wall shape, and G-suit abdominal bladder inflation on regional intrapleural pressure (Ppl) during +Gz stress. Experiments were conducted in two species (9 dogs, 10 pigs) so that data relating to a spectrum of chest wall characteristics could be obtained. Anesthetized animals were exposed to +Gz levels ranging from 1 to + 5 Gz. Regional Ppl was monitored at 2-4 sites during spontaneous breathing with and without G-suit abdominal bladder inflation and after removal of active chest wall muscular tone by sacrificing the animal. In the dog, Ppl became more negative in the upper and middle thoracic regions with +Gz stress (-1.63 and -1.66 cm H2O.G-1, respectively), but, in the lower thorax, Ppl increased slightly (0.05 cm H2O.G-1). When active muscular tone was removed, the rate of Ppl change increased in the upper thorax (-2.55 cm H2O.G-1), but not in more dependent regions. In the pig, Ppl became more negative with +Gz stress (upper = -3.3; middle = -1.99; lower = -2.16 cm H2O.G-1), and without active muscular tone, the rate of Ppl change tended to increase in all regions (upper = -4.3; middle = -3.47; lower = -2.97 cm H2O.G-1). With G-suit application in both species, Ppl increased toward or exceeded control values at all levels. Results indicate that chest wall characteristics play a significant role in determining regional Ppl during +Gz stress.