Premedication and Haemodynamics After Spinal Anesthesia

Blood pressure drop following spinal anesthesia is connected with sympathetic/parasympathetic activity which may be determinated by Heart Rate Variability (HRV) assessment. Sympathetic predomination expressed as LF/HF ratio above 2.5 is strongly connected with deeper blood pressure fall. As drugs given for premedication may have impact on HRV variables, the investigators would like to determine if pharmacological premedication may modify hemodynamic changes following spinal blockade. Two drugs will be compared - midazolam which is known to lead to increase in LF/HF ratio and morphine - opioid which provokes opposite effect.