Modeling of the Cardiovascular System During Hemodialysis

OBJECTIVE: Development of a cardiovascular model for the simulation of the hemodynamic state of patients during hemodialysis treatment. It shall include the autonomic mechanisms involved in cardiovascular regulation during dialysis-induced hypovolemia. PROBLEM: The imbalance between fluid withdrawal by ultrafiltration and vascular refilling from the interstitial space during dialysis tretament causes a decrease in total blood volume. Consequences are a reduction in central venous pressure, in venous return, in stroke volume and finally in blood pressure. A drop in blood pressure stimulates a complex pattern of autonomic compensatory mechanisms. In cases of a large drop or insufficient (pathological) conditions the mechanisms may not reach appropriate compensation. Additional changes in blood composition (e. g. hematocrit, electrolytes) may contribute to the failure in compensation. PRESENT STATE: The cardiovascular system is described as a serial arrangement of 4 pulsatile cardiac and 6 vascular compartments representing the systemic and pulmonary circulation. The efferent autonomic reflexes are considered (1) by a sigmoidal transfer function between blood pressure and efferent nervous activity, and (2) by using a low-pass transfer function for the efferent signals into efferent controls (e.g. chronotry, inotropy, constriction of peripheral vessels). Blood viscosity (hematocrit) and electrolyte concentration (e. g. Ca++) are properly considered.