Evolution of volume sensitivity during hemodialysis and ultrafiltration.

OBJECTIVE: This study aimed at assessing the evolution of cardiovascular characteristics during hemodialysis and ultrafiltration by a perturbation accurately defined in its magnitude and directly relevant to the problem of volume adjustment in stable hemodialysis patients. METHODS: Excess fluid volume was removed by constant ultrafiltration-rate as prescribed. Hemodynamic variables were continuously measured throughout treatments using non-invasive finger plethysmography. In addition to ongoing volume reduction by ultrafiltration (long-term perturbation), well-defined magnitudes of intravascular volume were transiently and reversibly sequestered (short-term perturbation) into the extracorporeal circulation at hourly intervals. Sensitivities of hemodynamic variables and of the baroreflex to the acute change in intravascular volume (volume sensitivities) were analyzed. RESULTS: Eight stable patients were assessed during two subsequent treatments. Treatments were accompanied by a decrease in cardiac output (p<0.05) and stroke volume (p<0.01), and by an increase in peripheral resistance (p<0.05) and diastolic pressure (p<0.05). Mean arterial pressure remained unchanged for the whole group but correlated with the change in total peripheral resistance in individual treatments (p<0.01). The average volume sensitivity of mean arterial pressure was 11.9±9.9 mmHg/L and increased (p<0.01) during treatments, while the average volume sensitivity of heart rate remained unchanged at -7.9±8.58 1/(min L). The corresponding volume sensitivity of the baroreflex was -0.81±1.5 1/(min mmHg) and remained unchanged for the whole group, but the change correlated with the change in mean arterial pressure in individual treatments (p<0.05). INTERPRETATION: The changes in arterial pressures during hemodialysis appear to relate to an unbalanced response of barocontrol mechanisms characterized by a compromised chronotropy and vascular over-reactivity.