R/T amplitude ratio of electrocardiogram ECG), heart rate (HR) and sys
tolic arterial pressure (SAP) were recorded in 15 patients awake, at 1
minimal alveolar concentration of isoflurane before and during surger
y, and in deep anaesthesia (electroencephalogram burst suppression) du
ring surgery. R/T-wave amplitude ratio and HR were sensitive to both s
urgery and changes in the level of isoflurane anaesthesia; induction o
f anaesthesia. skin incision and the rapid increase in the concentrati
on of isoflurane all significantly decreased the T-wave amplitude, wit
hout influence on the R-wave. Changes in the T-wave amplitude correlat
ed directly to HR. SAP increased at skin incision and decreased when t
he anaesthesia was deepened. The authors conclude that the R/T-wave am
plitude ratio of ECG provides a reliable method for monitoring the sym
pathetic tone during isoflurane anaesthesia.