Assessment of sensory block in epidural anaesthesia by electric stimulation

The onset of sensory block in lumbar epidural anesthesia was investigated in 26 patients, aged 18 to 84 years, employing the loss of discrimination to cold and pinprick, as well as by determining threshold electric stimulation (threshold intensities). A standard dose of 2% mepivacaine with adrenaline, 5 micrograms.ml-1, (0.1 ml per cm body height) was given and the patients' ability to discriminate stimuli within dermatomes T8, T10, T12, L2, L4 and S1 was investigated at five min intervals for 30 min after injection. From the results of the study it is concluded that i) The interval to peak analgesic efficacy of the anaesthetic solution used is < 30 min when assessments are based on the patients' ability to discriminate cold or pinprick but > 30 min when determinations of threshold intensities are employed. ii) Cold discrimination is lost earlier than discrimination to pinprick and at lower threshold intensities. iii) Threshold intensities describe the time course of onset of sensory block more precisely than results of testing by cold or pinprick. iv) The onset of sensory block was found to be positively correlated to the age of patients in the following respects: a) Threshold intensities during early onset in all investigated dermatomes except L2. b) Intensity of block in T8, T10, and S1 at the end of the study period. c) Time to loss of discrimination to cold and pinprick in T12, L2 and S1, and d) Threshold intensities at loss of discrimination to cold and pinprick. We propose that determinations of threshold intensities offer distinct advantages over conventional testing by cold and pinprick discrimination, especially when detailed analyses of the sensory blocking effects of local anaesthetic drugs are being investigated.