Theoretical analysis of the effect of positioning on hemodynamic stability during pregnancy.

OBJECTIVES: A left lateral tilt of 15° has been advocated during trauma resuscitation of near-term pregnant patients to avoid the potential for hemodynamic compromise caused by aortocaval compression in the supine position. This recommendation is supported by limited objective evidence, and an experimental determination of the optimal tilt required would be very difficult to accomplish logistically. A derivation of the Guyton/Coleman/Summers computer model of cardiovascular physiology was used to analyze the theoretically expected hemodynamic responses to varying degrees of lateral tilt for a normal pregnancy and during a simulated hemorrhagic shock. METHODS: Computer simulation studies were used to predict the degree of left lateral tilt required to restore hemodynamic normalcy during the final 20 weeks of gestation. The analytic procedure involved recreating the clinical conditions for a virtual subject through a simulated reenactment of the clinical transfer of a pregnant patient from a lateral to a supine positioning. An analysis of model validity in the context of this particular clinical condition found the model predictions to be within 5% to 12% of experimental results. RESULTS: During the simulated lateral to supine position transfer, the virtual patient with Class I hemorrhage had a 7% greater fall in cardiac output and a 17% greater fall in mean arterial pressure (MAP) than the corresponding nonhemorrhagic patient. The model suggests that 15° of tilt will result in hemodynamic normalization only up to 26 weeks of gestation. In addition, 13% greater tilt is required to achieve hemodynamic normalcy in the hemorrhaged term pregnant patient. CONCLUSIONS: Current trauma guidelines suggest that the pregnant trauma patient be placed in a 15° left lateral tilt position to prevent aortocaval compression. A computer simulation study suggests that this tilt may be inadequate to offload the vena cava and normalize the circulation.