THE INFLUENCE OF CHANGES IN HAND TEMPERATURE ON THE INDIRECTLY EVOKED ELECTROMYOGRAM OF THE FIRST DORSAL INTEROSSEOUS MUSCLE

The evoked EMG response commonly decreases in amplitude during the first few minutes of anaesthesia. The purpose of this study was to determine if a relationship exists between changes in hand temperature, which are known to occur with induction of anaesthesia, and drift in the EMG signal. The indirectly evoked response of the 1st dorsal interosseous muscle was measured using a Datex(TM) Relaxograph(R) in 15 patients undergoing elective surgery. The test arm was wrapped in towels in order to minimize heat loss. Core body temperature, hand temperature, and T-1 were recorded at two minute intervals for the next 30 min. Patients then received a bolus of mivacurium 0.08 mg . kg(-1) and additional doses were given as needed. Complete recovery was defined as a TOF ratio >0.90. Regression analysis plotting Delta temperature against Delta T-1 was performed for each individual. The slope of the regression line for the relationship between Delta degrees C and Delta T-1 was then used to calculate a correction factor (CF) which might be used to ''fine tune'' the last measured T-1. The initial hand temperature averaged 30.8 +/- 1.4 degrees C and this increased by 4.1 +/- 1.2 degrees C over the next 30 min. During this period T-1 decreased by 24.8 +/- 5.9% or -6.05%/degrees C. The final mean T-1 value at the end of anaesthesia (uncorrected) was 70.6 +/- 7% of control. The average corrected T-1 value was 94.7 +/- 8.5% (range, 83-111%). It is concluded that there was a correlation between Delta degrees C and Delta T-1 during the first 30 min of anaesthesia (r(2) = 0.77, P < 0.0001). However, in 5 of 15 individuals it was not possible to ''temperature correct'' the final T-1 value to within +/- 10% of control. Hence, while changes in muscle temperature probably play a major role in the T-1 drift seen with the Datex monitor, other factors remain to be identified.