BACKGROUND: The objective of this study was to determine whether pre-operative heart rate variability (HRV) estimated using the newly developed MemCalc method predicts hypotension after spinal anaesthesia (SA). METHOD: Fifty-two patients with American Society of Anesthesiologists physical status I or II, aged 38-84 years, scheduled to undergo elective transurethral surgery were investigated in this study. Ultra short-term entropy (UsEn) as a non-linear index of HRV was estimated before and after SA using the MemCalc method. The patients were then assigned to two groups (Group LO and HI) according to pre-operative UsEn. SA was performed by intrathecal injection of 17.5 mg of isobaric bupivacaine via interlumbar spaces L3-4 or L4-5. After SA, the haemodynamic fluctuations were recorded and compared between the two groups. RESULTS: SA significantly decreased LF/HF (a ratio of the low frequency component of HRV to that of the high frequency component) but did not affect UsEn. The percentage decrease in systolic blood pressure (SBP) after SA was 11.4 (9.9)% in group HI, whereas it was 17.5 (9.8)% in group LO (P < 0.05). Furthermore, the number of patients who developed hypotension was significantly higher in group LO. CONCLUSION: SA significantly decreased LF/HF without affecting UsEn. Patients with a low UsEn developed a greater decrease in SBP after SA. Furthermore, the incidence of hypotension after SA was higher in patients with a low UsEn.