Simulation study of the effects of hypovolaemia on cardiovascular response to orthostatic stress.

To investigate the role played by hypovolaemia in the mechanism of orthostatic intolerance, a mathematical model was developed. The model consisted of seven sub-models that describe: the redistribution of blood induced by lower body negative pressure (LBNP); filling of the left ventricle; contracting of the left ventricle; interaction between the left ventricle and peripheral circulation; and baroreflex regulation. The model was evaluated using experimental data. Using the model, computer simulations were performed to investigate the effects of hypovolaemia on the cardiovascular response to LBNP. The simulation results indicated that, first, when the blood loss is less than 5%, blood pressure can be maintained in the normal range by the baroreflex regulatory mechanism, even with high LBNP application; secondly, when the blood loss is between 15 and 20%, heart rate and blood pressure can be kept in the normal range if LBNP is not applied, but blood pressure falls sharply with LBNP application; and, thirdly, when the blood loss is 25%, the cardiovascular system is in an unstable state (heart rate: 116 beat min (-1), systolic blood pressure: 97 mmHg; diastolic blood pressure: 77 mmHg), even without any LBNP, and becomes more unstable with LBNP. The simulation results support the hypothesis that hypovolaemia is a cause of orthostatic intolerance.