Hemodynamics of graded water immersion in the baboon: +Gz protection potential

Fluid-filled anti-G suits depend on external fluid counterpressure to maintain near-normal hemodynamics during +Gz (head-to-foot inertial force). However, the relationship between external fluid level, hemodynamic events, and +Gz tolerance has not been determined.

Seven anesthetized male baboons (avg. wt. = 27.2 kg) were catheterized for measurement of left ventricular pressure (LVP), right ventricular pressure (RVP), aortic BP (ABP), central venous pressure (CVP), pulmonary artery pressure, cardiac output, and esophageal pressure. The animals were seated in a restraint chair and control data were collected, followed by six discrete steps of water immersion (WI): 1) knee; 2) hip; 3) xiphoid; 4) mid-chest; 5) neck; and 6) return to the xiphoid (data check). Each level was maintained until hemodynamic stabilization. Rectal temperature averaged 33.5 degrees C; water bath temperature was maintained at 34 degrees C.

There was a significant (p < 0.05) increase in most hemodynamic parameters with WI to the xiphoid, mid-chest, and neck. With WI to the neck, CVP, LV, and RV end diastolic pressures increased by 8.7 mmHg, 19.6 mmHg, and 8.0 mmHg, respectively, suggesting a major passive increase in thoracic and cardiac blood volume, whereas mean ABP (MABP) increased by 18.6 mmHg and total peripheral resistance significantly decreased. Heart rate did not change significantly.

The increase in MABP with WI to the neck would provide slightly more than +1 Gz relaxed protection. Additional +Gz would require pressure breathing to overcome the increased external hydrostatic pressure on the thorax. Water-filled anti-G suits filled to the xiphoid cannot provide adequate relaxed +Gz protection for current high performance aircraft without supplemental factors such as additional pressure in the suit or positive pressure breathing.