HUMAN DOSE-RESPONSE RELATIONSHIP FOR DECOMPRESSION AND ENDOGENOUS BUBBLE FORMATION

The dose-response relationship for decompression magnitude and venous gas emboli (VGE) formation in humans was examined. Pressure exposures of 138, 150, and 164 kPa (12, 16, and 20.5 ft of seawater gauge pressure) were conducted in an underwater habitat for 48 h. The 111 human male volunteer subjects then ascended directly to the surface in <5 min and were monitored for VGE with a continuous-wave Doppler ultrasound device over the precordium or the subclavian veins at regular intervals for a 24-h period. No signs or symptoms consistent with decompression sickness occurred. However, a large incidence of VGE detection was noted. These data were combined with those from our previously reported experiments at higher pressures, and the data were fit to a Hill dose-response equation with nonlinear least-squares or maximum likelihood routines. Highly significant fits of precordial VGE incidences were obtained with the Hill equation (saturation depth pressure at which there is a 50% probability of detectable VGE [D(VGE)50] = 150 ± 1.2 kPa). Subclavian monitoring increased the sensitivity of VGE detection and resulted in a leftward shift [D(VGE)50 = 135 ± 2 kPa] of the best-fit curve. We conclude that the reduction in pressure necessary to produce bubbles in humans is much less than was previously thought; 50% of humans can be expected to generate endogenous bubbles after decompression from a steady-state pressure exposure of only 135 kPa (11 ft of seawater). This may have significant implications for decompression schedule formulation and for altitude exposures that are currently considered benign. These results also imply that endogenous bubbles arise from preexisting gas collections.