Quantitation of depth of thiopental anesthesia in the rat

Background: In contrast to that of inhalational anesthetics, quantitation of anesthetic depth for,intravenous agents has not been well defined. In this study, using rodents, the relationship between the constant plasma thioperital concentrations and the clinical response to multiple nociceptive stimuli were investigated characterizing the anesthetic state from light sedation to deep anesthesia and correlated to the degree of electroencephalogram (EEG) drug effect. Methods: Thirty rats were instrumented with chronically implanted EEG electrodes, arterial and venous catheters. A computer-driven infusion pump was used to rapidly attain and then maintain constant, target plasma thiopental concentrations ranging from 7 io 100 mu g/ml. Three different target plasma thiopental concentrations were achieved in each rat. Electroencephalographic effects were monitored with aperiodic waveform analysis. The following nociceptive stimuli were applied: (1) unprovoked righting reflex, (2) provoked lighting reflex, (3) noise stimulus, (4) tail clamping with an alligator clip, (5) constant tail pressure with an analgesia-meter, (6) corneal reflex, and (7) tracheal intubation. For, tail clamping, tail pressure, and intubation, either purposeful extremity movement or abdominal muscle contraction response was noted to be present or absent. The clinical responses (present dr absent) were modeled using logistic regression to estimate the CP50, the plasma thiopental concentration with a 50% probability of no response. Results:The following mean Cp(50), values (95% confidence interval) were obtained: unprovoked righting reflex, 15.9 (15.1-16.6) mu g/ml; provoked righting reflex, 21.4 (20.2-22.7)mu g/ml; noise stimuli, 31.3 (29.7-33.0) mu g/ml; tail clamp and limb movement, 38.3 (36.1-40.4) mu g/ml, tail pressure and limb movement, 39.2 (37.1-41.3) mu g/ml; tail pressure and abdominal muscle contraction, 52.5 (50.0-55) mu g/ml; tail clamping and abdominal muscle contraction, 56.1 (50.0-56.2) mu g/ml; corneal reflex, 60.0 (56.6-63.4) mu g/ml; and limb movement or muscle abdominal contraction response to intubation, 67.7 (59.2-76.1) mu g/ml. At an EEG-effect of 9.1 and 2.2 waves/s, there was a 50% chance of limb movement response to tan clamping add tracheal intubation, respectively. There was a poor relationship between the plasma thiopental concentration and the percent increase of either heart rite or mean arterial blood pressure after applying either tail pressure or tail clamp stimuli. Conclusions: A range of nociceptive stimuli and their observed clinical responses can be used to quantitate thiopental anesthetic depth, ranging from light sedation to deep anesthesia (isoelectric EEG and unresponsive to intubation) in the rodent. Clinical response can be mapped to surrogate EEG measures.