The purpose of this study was to evaluate the autonomic response to standard haemodialysis and the changes associated with the onset of intradialytic hypotension in 12 normotensive patients with uraemia. Power spectra of R–R interval and of blood pressure fluctuations were obtained during a standard dialysis session and estimated in the low-frequency (LF, 30–150 ;mHz) and high-frequency (HF, 150–400 ;mHz) range. The absolute power of the LF component of blood pressure variations and the LF/HF ratio of R–R interval were assumed as indexes of sympathetic activity. Standard haemodialysis induced hypotension in six patients (unstable) while a minor pressure decline was present in the other six (stable). Normalized blood volume before dialysis and percentage volume reduction were similar in the two groups. Tachycardia in response to pressure and volume decrease was more pronounced in stable than in unstable patients, as evidenced by a higher slope of the relation between R–R interval and systolic blood pressure (7.9 versus 0.9 ;ms/mmHg, P< 0.01). Sympathetic tone was enhanced during early dialysis in all patients (+2±1 for R–R LF/HF ratio, +2.4±0.6 ;mmHg2 and +7.2±2 ;mmHg2 for absolute LF power of diastolic and of systolic blood pressure respectively, P< 0.05), compared with baseline predialysis values. During late dialysis, unstable patients showed an impairment of sympathetic activation which preceded hypotension and was maximal during the crisis (-2.9±1.4 for R–R LF/HF ratio, -2.7±1.4 ;mmHg2 and -8.6±4.0 ;mmHg2 for absolute LF power of diastolic and of systolic blood pressure respectively, P< 0.05). On the contrary, stable patients showed constantly elevated indexes (+3.7±1.4 for R–R LF/HF ratio, +5.9±2.7 ;mmHg2 and +13.3±6.2 ;mmHg2 for LF of diastolic and of systolic blood pressure, P< 0.05). Values returned to predialysis levels after the end of the dialysis session in all patients. We conclude that standard haemodialysis activates a marked and reversible sympathetic response in both stable and unstable uraemic patients. However, in unstable patients, such activation is impaired in late dialysis, therefore contributing to the onset of the hypotensive crisis.

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