Hemodynamic responses to head-up tilt in uremic patients.

The hemodynamic responses to 45 degrees C head-up tilt for 30 min were compared between a group of 13 normal healthy subjects, 16 patients with end-stage renal failure (Group 1) and 9 hypertensive patients (Group 2). In the normal subjects there was no change in systolic blood pressure (SBP), a significant increase in diastolic blood pressure (DBP) (9 +/- 1 mmHg, mean + SEM, p less than 0.01), heart rate (HR) (11 +/- 2 beats/min) and derived peripheral resistance (PR) (66 +/- 8%, p less than 0.001). Stroke volume (SV) and cardiac output (CO) fell significantly by 42 +/- 3% and 33 +/- 3%, respectively. The responses in Group 2 patients were similar to those of the controls. However in Group 1 patients, though there was a rise in heart rate (9 +/- 2, p less than 0.001), SBP fell significantly accompanied by no change in either DBP or PR. The decrease in SV was only 12 +/- 4% while there was no change in derived CO. The results suggest that there was a lack of vasoconstriction during postural stress in patients with endstage renal failure, which was probably due to a functional defect in the baroreceptor reflex. Concomitantly, the small decrease in SV suggests stiffness of the veins and/or active venoconstriction which may help to prevent a bigger fall in blood pressure.