Influence of Thoracic Paravertebral Block on Cardiac Repolarization

After obtaining written informed consent, 60 women above 18 years of age scheduled to elective breast surgery in combined regional and general anaesthesia will be enrolled to study, divided in two groups depending of the side of operation. Participants will undergo the ultrasound-guided paravertebral block with single injection of 0.5% ropivacaine (0.3 ml·kg-1, not exceeding 30 ml, based on ideal body weight in obese patients) without adjuvants. Before injection (T0) and after confirmation of sufficient sensory block area covering 1st to 4th thoracic dermatomes (T1), 12-lead electrocardiogram (ECG) will be recorded using a Holter device. First examination of sensory block distribution will be performed 6 minutes after injection, with subsequent examinations if needed - every minute up to 15 minutes. No sedative premedication, general anaesthesia induction or additional medications (except of neutral saline maintaining intravenous access) will be administered until completions of ECG recording. Both T0 and T1 electrocardiogram will be preceded by rest in supine position for at least 5 minutes, without any medical procedures other than monitoring. Cases with baseline QT or corrected QT longer than 440 milliseconds (ms) will be excluded from study. The QT interval and TDR (measured as time in milliseconds between ECG T wave peak \[Tpeak\] and end \[Tend\]) values obtained from baseline and post-blockade electrocardiograms will be statistically analyzed and compared between study groups. The minimum sample size was calculated as 26 cases per study group, assuming a change in corrected QT of 20 milliseconds from the baseline value, with a significance level of p=0.05 and a test power of 0.9. Sample size was expanded to 30 cases per group, due to anticipated exclusions at the stage of intervention or data analysis.