Differential effects of halothane and isoflurane on lumbar dorsal horn neuronal windup and excitability  J.M. Cuellar, R.C. Dutton, J.F. Antognini, E. Carstens  British Journal of Anaesthesia  Volume 94, Issue 5, Pages 617-625 (May 2005) DOI: 10.1093/bja/aei107 Copyright © 2005 British Journal of Anaesthesia Terms and Conditions

Fig 1 Representative example of responses to electrical stimulation (1 Hz; 0.5 ms; 3×C-fibre threshold) of a wide-dynamic range neurone, and the effects of increasing isoflurane (a) and halothane (b) concentration from 0.8 to 1.2 MAC. Shown are raw tracings of the 1 s after the 1st, 4th, 8th, 12th and 16th (of 20) stimuli applied to the ipsilateral hindpaw while recording from a lumbar dorsal horn neurone during 1.1% (0.8 MAC) and 1.7% (1.2 MAC) isoflurane (a) and 0.7% (0.8 MAC) and 1.1% (1.2 MAC) halothane (b). All four recordings were from the same neurone. Note the reduction in the number of spikes evoked by the 1st stimulus at the higher isoflurane concentration, but the persistence of a progressive increase in the response to subsequent stimuli (windup). Increasing from 0.8 to 1.2 MAC halothane also reduced the number of spikes evoked by the 1st stimulus, and although windup occurs, this is less when compared with the lower halothane concentration. The reduction in windup is most significant when comparing 1.2 MAC isoflurane (a) with 1.2 MAC halothane (b). British Journal of Anaesthesia 2005 94, 617-625DOI: (10.1093/bja/aei107) Copyright © 2005 British Journal of Anaesthesia Terms and Conditions

Fig 2 Mean and standard error of responses to electrical stimulation (1 Hz; 0.5 ms; 3×C-fibre threshold) during isoflurane (left panels) and halothane (right panels) anaesthesia (n=18). Responses during halothane and isoflurane were recorded from the same neurones in a paired crossover design (see Methods). (a) Mean responses for the A-fibre latency range (0–100 ms after the stimulus) recorded during 1.1% (0.8 MAC) and 1.7% (1.2 MAC) isoflurane (left panel) and 0.7% (0.8 MAC) and 1.1% (1.2 MAC) halothane (right panel). The 0.8 and 1.2 MAC values for isoflurane were significantly different from the values at 0.8 and 1.2 MAC halothane (P<0.01). (b) Format as in (a) for responses in the C-fibre latency range (100–400 ms after the stimulus). #Response to initial (1st) stimulus was significantly different from 1.2 MAC value, P<0.05. (c) Format as in (a) for responses in the afterdischarge (AD) latency range (400–1000 ms after stimulus). (d) Format as in (a) for responses in the C-fibre plus afterdischarge latency range (100–1000 ms after stimulus). *Total number of action potentials (area under curve) was significantly different from 1.2 MAC value, P<0.05. British Journal of Anaesthesia 2005 94, 617-625DOI: (10.1093/bja/aei107) Copyright © 2005 British Journal of Anaesthesia Terms and Conditions

Fig 3 Mean and standard error of responses to electrical stimulation (1 Hz; 0.5 ms; 3×C-fibre threshold) during approximately equivalent MAC fractions of isoflurane vs halothane anaesthesia (n=18). Responses during halothane and isoflurane were recorded from the same neurones in a paired cross-over design (see Methods). (a) Mean responses during the A-fibre latency (0–100 ms after the stimulus) range recorded during 0.7% halothane and 1.1% isoflurane (≈0.8 MAC; left panel) and 1.1% halothane and 1.7% isoflurane (≈1.2 MAC; right panel). (b) Format as in (a) for responses in the C-fibre (100–400 ms after the stimulus) latency range. (c) Format as in (a) for responses in the afterdischarge (AD; 400–1000 ms after stimulus) latency range. (d) Format as in (a) for responses in the C-fibre plus afterdischarge latency range (100–1000 ms after stimulus). Absolute windup calculated as the total train response minus 20× input, where input equals the number of action potentials evoked by the first stimulus. #P<0.05, response to initial stimulus was significantly different between isoflurane and halothane; *P<0.05, total number of action potentials (area under curve) was significantly different between isoflurane and halothane. British Journal of Anaesthesia 2005 94, 617-625DOI: (10.1093/bja/aei107) Copyright © 2005 British Journal of Anaesthesia Terms and Conditions

Fig 4 Mean and standard error of spontaneous firing recorded during the 30 s before electrical stimulation in the presence of two different concentrations of halothane and isoflurane in the same population of spinal dorsal horn neurones (n=18). *P<0.05, 1.2 MAC different from 0.8 MAC; #P<0.05, halothane different from isoflurane. British Journal of Anaesthesia 2005 94, 617-625DOI: (10.1093/bja/aei107) Copyright © 2005 British Journal of Anaesthesia Terms and Conditions