Spinal anaesthesia has the advantage that produced nerve block by the injection of local anaesthetic into cerebrospinal fluid (CSF). However, the greatest challenge in spinal anaesthesia is to control the spread of local anaesthetic through the CSF to provide a block which is adequate for the proposed surgery without unnecessary extensive spread, and increased risk of complications.

The activity of the autonomic nervous system is of fundamental importance in the regulation of vital bodily functions. Unbalance of autonomic nerve system results in considerably disordered vital function. Clinically, this is of great significance, because if an anesthetic agent produces cause the sympathetic system to block, the effects can be serious in individual cases, particularly on the cardiovascular system. If complications are to be avoided, it is essential to assess the degree of block correctly. Clinical monitoring has a variety of applications, a particularly useful one being measurement of the sympathetic system during regional anesthesia, for which quantification of the blocking effect is a clinical necessity and the degree of block needs to be ascertained without delay. The autonomic nervous system (ANS) plays an important role in the regulation of hemodynamics during anesthesia. Analysis of beat-to-beat fluctuations of heart rate and blood pressure is a promising new approach to the clinical diagnosis and management of alterations in cardiovascular regulation. Continuous Wavelet Transform (CWT), which could overcome the limitation of the steady-state assumption in the classical spectral analysis, is believed as a reliable and robust method to access cardiorespiratory dynamics of the ANS and the investigators will applied during spinal anesthesia to evaluate the detail changes.

Inclusion Criteria:

1. patients scheduled for operation with spinal anesthesia

Exclusion Criteria:

1. recently treat with a sedative, beta-blocker, parasympatholytic, or opioid agent
2. emergency surgery
3. hypovolemia and hypothermia, arrhythmia, diabetes, or impairment of renal, hepatic, coagulation, cardiac, or respiratory function

The investigators conducted a prospective observational study on patients who were scheduled to undergo elective surgery and spinal anesthesia. The investigators excluded patients who had recently been treated with a sedative, beta-blocker, parasympatholytic, or opioid agent, patients who had undergone emergency surgery, those with hypovolemia and hypothermia, arrhythmia, diabetes, or impairment of renal, hepatic, coagulation, cardiac, or respiratory function