Changes in the T-wave amplitude of ECG during isoflurane anaesthesia

R/T-wave amplitude ratio of electrocardiogram (ECG), heart rate (HR) and systolic arterial pressure (SAP) were recorded in 15 patients awake, at 1 minimal alveolar concentration of isoflurane before and during surgery, and in deep anaesthesia (electroencephalogram burst suppression) during surgery. R/T-wave amplitude ratio and HR were sensitive to both surgery and changes in the level of isoflurane anaesthesia; induction of anaesthesia, skin incision and the rapid increase in the concentration of isoflurane all significantly decreased the T-wave amplitude, without influence on the R-wave. Changes in the T-wave amplitude correlated directly to HR. SAP increased at skin incision and decreased when the anaesthesia was deepened. The authors conclude that the R/T-wave amplitude ratio of ECG provides a reliable method for monitoring the sympathetic tone during isoflurane anaesthesia.