MATHEMATICAL INVESTIGATION OF SOME PHYSIOLOGICAL FACTORS INVOLVED IN HEMODIALYSIS HYPOTENSION

A previously developed mathematical model is used to investigate the r ole of some hemodynamic, regulatory, and osmotic factors in the develo pment of symptomatic hypotension during hemodialysis. Sensitivity anal ysis of the model parameters suggests that a decrease in atrial pressu re, with a consequent fall in cardiac output (Frank-Starling mechanism ), is the primary hemodynamic perturbation induced by ultrafiltration. Also, during the first hours of a hemodialysis session, the sympathet ic mechanism working on systemic resistance is the main factor respons ible for arterial pressure maintenance, and the physiological response is probably characterized by a prevalence of the cardiopulmonary over the arterial baroreflex control. During this period, a decrease in pl asma osmolarity, caused mainly by urea removal, may contribute to the reduction of vascular refilling. During the last hours of the session, the arterial pressure level is also significantly affected by other f actors that influence vascular refilling and mean circulatory filling pressure (systemic compliance; action of feedback mechanisms working o n venous unstressed volume; plasma oncotic pressure; and, especially, capillary wall permeability and interstitial space elastance). Simulat ion of hemodialysis with different modalities emphasizes the importanc e of avoiding high ultrafiltration rates and of maintaining the sodium concentration in the dialysate close to the sodium concentration of t he extracellular fluid to limit the risk of symptomatic hypotension. H igher values of sodium in the dialysate are, however, associated with poor sodium removal from the extracellular pool with risks of interdia lytic morbidity. Ln the future, the model may be used to optimize the ultrafiltration rate and sodium profile in the dialysate according to individual patient prescriptions.