Remifentanil (RMFNT) is a very short active opioid, used for analgesia during general anaesthesia and for analgesic and sedative effect in intensive care units (ICU) patients. Registration for anesthesia includes bolus dose and continuous infusion, in ICU only infusion regimen is allowed. Pharmacokinetic/pharmacodynamic parameters of RMFNT results in rapid onset and offset of clinical effect, which makes this drug almost ideal in many situations. Unfortunately, its vagomimetic influence on cardiac activity may result in decrease of heart rate. It may be hypothesized that patients with parasympathetic predominance may be prone to more intense parasympathomimetic effect of this opioid. An optimal method for assessment of autonomic nervous system activity and assessment of influence of RMFNT on that activity is Heart Rate Variability (HRV) analysis. Parasympathetic predominance is expressed as high frequency (HF) power and HF/(LF+HF) (LF-low frequency) ratio in frequency domain and Root Mean Square of the Successive Difference (RMSSD), the number of pairs of successive NNs that differ by more than 50 ms (NN50), the proportion of NN50 divided by total number of NNs (pNN50) in time domain. The aim of this study is to verify the hypothesis that patients with predominance of parasympathetic activity are more subject to vagomimetic effect of RMFNT. HRV based on 5 minutes electrocardiogram (ECG) recorded before and after bolus dose of RMFNT will be analyzed. The occurrence of heart rate decrease will be than compared between patients with sympathetic and parasympathetic predominance measured prior to drug injection.