Theoretical Analysis of the Effect of Positioning on Hemodynamic Stability During Pregnancy

Abstract

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.

Theoretical Analysis of the Relative Impact of Obesity on Hemodynamic Stability During Acute Hemorrhagic Shock

“…Extrapolating from trauma in pregnancy, perhaps simple repositioning of the obese patient could decrease intraabdominal pressures and increase venous return, thereby improving cardiac output. However, unlike our previous study that examined the impact of a uniformly growing gravid uterus on hemorrhagic shock states, a condition of obesity is less homogenous and the influence on the intra-abdominal pressure less readily tracked with changes in size or weight ( 11 ). Therefore, a single weight point signifying the lower end of the definition of obesity (BMI 30) was chosen for the current analysis.…”

“…The mathematical package used in the current analysis was a homegrown product developed by one of the authors (Coleman) using C++ as the programming language. This model and methodology, which previously have been used in numerous studies, are intended to provide a more detailed understanding of the physiologic mechanisms involved in common clinical conditions ( 11 , 13 - 15 , 19 , 20 ). Further, several versions of this model have been previously demonstrated to accurately predict hemodynamic changes seen during hypotensive states ( 11 , 17 , 20 ).…”

“…This model and methodology, which previously have been used in numerous studies, are intended to provide a more detailed understanding of the physiologic mechanisms involved in common clinical conditions ( 11 , 13 - 15 , 19 , 20 ). Further, several versions of this model have been previously demonstrated to accurately predict hemodynamic changes seen during hypotensive states ( 11 , 17 , 20 ). The model has been specifically validated previously for hemorrhagic states in a published study in which humans were placed in a simulated shock state by a standard lower body negative pressure technique ( 17 ).…”

“…While there has been extensive validation and documentation of the basic computational model used in this study, to assure credibility in this context, the relevant elements of the model predictions for the obese patient were compared to experimental data from the literature. Although it is unrealistic to expect a generalized computational model to exactly reproduce the experimental results of select study participants, the observed results of these baseline simulations should accurately reflect the direction and degree of expected physiologic changes as set by standard methods used for validations of similar analyses ( 9 - 11 ). The target simulation experiment used in this validation differentiated the obese from nonobese subject while tracking the changes in venous pressures distal to the vena cava bifurcation.…”

“…Noting this compelling, though limited evidence, we sought to further examine the effects of obesity in hemorrhagic shock using a systems analysis approach. Computational modeling is a validated methodology for the evaluation and analysis of clinically relevant questions that might be feasibly difficult to answer ( 9 - 11 ). In trauma patients, particularly those with significant hemorrhage, therapeutic treatments are time sensitive, and fully evaluating all potential effects of obesity on cardiovascular physiology could be difficult and potentially complicate resuscitations without preliminary data.…”