Robert C. Dutton

Inhaled anesthetics and immobility: Mechanisms, mysteries, and minimum alveolar anesthetic concentration

Studies using molecular modeling, genetic engineering, neurophysiology/pharmacology, and whole animals have advanced our understanding of where and how inhaled anesthetics act to produce immobility (minimum alveolar anesthetic concentration; MAC) by actions on the spinal cord. Numerous ligand- and voltage-gated channels might plausibly mediate MAC, and specific animo acid sites in certain receptors present likely candidates for mediation. However, in vivo studies to date suggest that several channels or receptors may not be mediators (e.g., &ggr;-aminobutyric acid A, acetylcholine, potassium, 5-hydroxytryptamine-3, opioids, and &agr;2-adrenergic), whereas other receptors/channels (e.g., glycine, N-methyl-d-aspartate, and sodium) remain credible candidates.

Inhaled anesthetics have hyperalgesic effects at 0.1 minimum alveolar anesthetic concentration.

We investigated the hyperalgesic (antianalgesic) effect of the inhaled anesthetics isoflurane, halothane, nitrous oxide, and diethyl ether, or the nonimmobilizer 1,2-dichlorohexafluorocyclobutane at subanesthetic partial pressures (or, for the nonimmobilizer, subanesthetic partial pressures predicted from lipid solubility) in rats. Hyperalgesia was assessed as a decrease in the time to withdrawal of a rat hind paw exposed to heat. All four anesthetics, including nitrous oxide and diethyl ether, produced hyperalgesia at low partial pressures, with a maximal effect at 0.1 minimum alveolar anesthetic concentration (MAC) required to prevent response to movement in 50% of animals, and analgesia (an increased time to withdrawal of the hind paw) at 0.4 to 0.8 MAC. The nonimmobilizer had neither analgesic nor hyperalgesia effects. We propose that inhaled anesthetics with a higher MAC-Awake (the MAC-fraction that suppresses appropriate responsiveness to command), such as nitrous oxide and diethyl ether, can be used as analgesics because patients are conscious at higher anesthetic partial pressures, including those which have analgesic effects, whereas anesthetics with a lower MAC-Awake do not produce analgesic effects at concentrations that permit consciousness. Implications The inhaled anesthetics isoflurane, halothane, nitrous oxide, and diethyl ether produce antianalgesia at subanesthetic concentrations, with a maximal effect at approximately one-tenth the concentration required for anesthesia. This effect may enhance perception of pain when such small concentrations are reached during recovery from anesthesia.

Concentrations of Desflurane and Propofol That Suppress Response to Command in Humans

The anesthetic concentration just suppressing appropriate response to command (minimum alveolar anesthetic concentration awake [MAC-awake] for volatile anesthetics or plasma concentration to prevent a response in 50% of patients [Cp50]-awake for intravenous anesthetics) provides three important measures. First, along with pharmacokinetics, the ratio of the awakening concentration to the anesthetizing concentration (MAC-awake/MAC or Cp50-awake/Cp50) determines time to awakening. Second, a correlation between MAC-awake and the anesthetic concentration sufficient to prevent learning suggests MAC-awake provides a surrogate measure of amnestic potency. Third, population values for MAC-awake provide evidence for or against commonality in anesthetic mechanisms. We studied 22 male volunteers twice to determine both MAC-awake for desflurane (2.60% +/- 0.46%) and Cp50-awake for propofol (2.69 +/- 0.56 micro gram/mL). Awakening with desflurane occurs at a concentration closer to its anesthetizing concentration (36% of MAC) than propofol (18% of Cp50); that is, 1) desflurane requires less of a decrement in anesthetic concentration at the effect site for arousal; and 2) if MAC-awake (Cp50-awake) values reflect the concentrations providing amnesia, propofol is a more potent amnestic. Of interest, the dose response curves of desflurane and propofol were equivalently steep, a finding consistent with a common mechanism of action. In contrast, sensitivity of each volunteer to desflurane did not correlate with sensitivity to propofol (r (2) < 0.01, P = 0.98) arguing against a common mechanism. (Anesth Analg 1995;81:737-43)

The Concentration of Isoflurane Required to Suppress Learning Depends on the Type of Learning

Background Recent reports suggest that one type of learning, fear conditioning to context, requires more neural processing than a related type, fear conditioning to tone. To determine whether these types of learning were differentially affected by anesthesia, the authors applied isoflurane during the training phases of fear conditioning paradigms for freezing to context and freezing to tone. Methods The authors trained seven groups of eight rats to fear tone by administering a tone (conditioned stimulus) while breathing various concentrations of isoflurane from 0.00 to 0.75 minimum alveolar concentration (MAC; one concentration per group) separated by 0.12-MAC steps. On the succeeding day, and in the absence of isoflurane, the authors presented the tone (without shock) in a different context (different cage shape and odor) and measured the time each rat froze (became immobile). Six other groups of eight rats were trained to fear context by applying the shock in the absence of a tone but in the presence of environmental cues such as cage shape, texture, and odor. Fear to context was determined the succeeding day by returning the rat to the training cage (without shock) and measuring duration of freezing. Control groups (16 per group) received 0.75 MAC isoflurane but no foot shocks. Group scores were compared using analysis of variance, and the ED50 values for quantal responses of individual rats were calculated using logistic regression. Results Conditioning to context occurred at 0.00 and 0.13 MAC (P < 0.05 compared with unshocked control) but not 0.25 MAC; the ED50 was 0.25 ± 0.03 MAC (mean ± SEM). In contrast, conditioning to tone occurred at 0.48 MAC (P < 0.05) but not 0.62 MAC; the ED50 was 0.47 ± 0.02 MAC (P < 0.01 for the difference between ED50 values). Conclusions Suppression of fear conditioning to tone required approximately twice the isoflurane concentration that suppressed fear conditioning to context. Thus, the concentration of anesthetic required to suppress learning may depend on the neural substrates of learning. Our results suggest that isoflurane concentrations greater than 0.5 MAC may be needed to suppress both forms of fear conditioning.

Forty-hertz Midlatency Auditory Evoked Potential Activity Predicts Wakeful Response during Desflurane and Propofol Anesthesia in Volunteers

BACKGROUND Suppression of response to command commonly indicates unconsciousness and generally occurs at anesthetic concentrations that suppress or eliminate memory formation. The authors sought midlatency auditory evoked potential indices that successfully differentiated wakeful responsiveness and unconsciousness. METHODS The authors correlated midlatency auditory evoked potential indices with anesthetic concentrations permitting and suppressing response in 22 volunteers anesthetized twice (5 days apart), with desflurane or propofol. They applied stepwise increases of 0.5 vol% end-tidal desflurane or 0.5 microg/ml target plasma concentration of propofol to achieve sedation levels just bracketing wakeful response. Midlatency auditory evoked potentials were recorded, and wakeful response was tested by asking volunteers to squeeze the investigators hand. The authors measured latencies and amplitudes from raw waveforms and calculated indices from the frequency spectrum and the joint time-frequency spectrogram. They used prediction probability (PK) to rate midlatency auditory evoked potential indices and concentrations of end-tidal desflurane and arterial propofol for prediction of responsiveness. A PK value of 1.00 means perfect prediction and a PK of 0.50 means a correct prediction 50% of the time (e.g., by chance). RESULTS The approximately 40-Hz power of the frequency spectrum predicted wakefulness better than all latency or amplitude indices, although not all differences were statistically significant. The PK values for approximately 40-Hz power were 0.96 during both desflurane and propofol anesthesia, whereas the PK values for the best-performing latency and amplitude index, latency of the Nb wave, were 0.86 and 0.88 during desflurane and propofol (P = 0.10 for -40-Hz power compared with Nb latency), and for the next highest, latency of the Pb wave, were 0.82 and 0.84 (P < 0.05). The performance of the best combination of amplitude and latency variables was nearly equal to that of approximately 40-Hz power. The approximately 40-Hz power did not provide a significantly better prediction than anesthetic concentration; the PK values for concentrations of desflurane and propofol were 0.91 and 0.94. Changes of 40-Hz power values of 20% (during desflurane) and 16% (during propofol) were associated with a change in probability of nonresponsiveness from 50% to 95%. CONCLUSIONS The approximately 40-Hz power index and the best combination of amplitude and latency variables perform as well as predictors of response to command during desflurane and propofol anesthesia as the steady-state concentrations of these anesthetic agents. Because clinical conditions may limit measurement of steady-state anesthetic concentrations, or comparable estimates of cerebral concentration, the approximately 40-Hz power could offer advantages for predicting wakeful responsiveness.

Contrasting roles of the N-methyl-D-aspartate receptor in the production of immobilization by conventional and aromatic anesthetics.

We hypothesized that N-methyl-d-aspartate (NMDA) receptors mediate some or all of the capacity of inhaled anesthetics to prevent movement in the face of noxious stimulation, and that this capacity to prevent movement correlates directly with the in vitro capacity of such anesthetics to block the NMDA receptor. To test this hypothesis, we measured the effect of IV infusion of the NMDA blockers dizocilpine (MK-801) and (R)-4-(3-phosphonopropyl) piperazine-2-carboxylic acid (CPP) to decrease the MAC (the minimum alveolar concentration of anesthetic that prevents movement in 50% of subjects given a noxious stimulation) of 8 conventional anesthetics (cyclopropane, desflurane, enflurane, halothane, isoflurane, nitrous oxide, sevoflurane, and xenon) and 8 aromatic compounds (benzene, fluorobenzene, o-difluorobenzene, p-difluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene, pentafluorobenzene, and hexafluorobenzene) and, for comparison, etomidate. We postulated that MK-801 or CPP infusions would decrease MAC in inverse proportion to the in vitro capacity of these anesthetics to block the NMDA receptor. This notion proved correct for the aromatic inhaled anesthetics, but not for the conventional anesthetics. At the greatest infusion of MK-801 (32 &mgr;g · kg–1· min–1) the MACs of conventional anesthetics decreased by 59.4 ± 3.4% (mean ± sd) and at 8 &mgr;g · kg–1· min–1 by 45.5 ± 4.2%, a decrease not significantly different from a 51.4 ± 19.0% decrease produced in the EC50 for etomidate, an anesthetic that acts solely by enhancing &ggr;-amino butyric acid (GABA) receptors. We conclude that some aromatic anesthetics may produce immobility in the face of noxious stimulation by blocking the action of glutamate on NMDA receptors but that conventional inhaled anesthetics do not.

Isoflurane causes anterograde but not retrograde amnesia for pavlovian fear conditioning

BACKGROUND Production of retrograde amnesia by anesthetics would indicate that these drugs can disrupt mechanisms that stabilize memory. Such disruption would allow suppression of memory of previous untoward events. The authors examined whether isoflurane provides retrograde amnesia for classic (Pavlovian) fear conditioning. METHODS Rats were trained to fear tone by applying three (three-trial) or one (one-trial) tone-shock pairs while breathing various constant concentrations of isoflurane. Immediately after training, isoflurane administration was either discontinued, maintained unchanged, or rapidly increased to 1.0 minimum alveolar concentration for 1 h longer. Groups of rats were similarly trained to fear context while breathing isoflurane by applying shocks (without tones) in a distinctive environment. The next day, memory for the conditioned stimuli was determined by presenting the tone or context (without shock) and measuring the proportion of time each rat froze (appeared immobile). For each conditioning procedure, the effects of the three posttraining isoflurane treatments were compared. RESULTS Rapid increases in posttraining isoflurane administration did not suppress conditioned fear for any of the training procedures. In contrast, isoflurane administration during conditioning dose-dependently suppressed conditioning (P < 0.05). Training to tone was more resistant to the effects of isoflurane than training to context (P < 0.05), and the three-trial learning procedure was more was more resistant than the one-trial procedure (P < 0.05). CONCLUSIONS Isoflurane provided intense dose-dependent anterograde but not retrograde amnesia for classic fear conditioning. Isoflurane appears to disrupt memory processes that occur at or within a few minutes of the conditioning procedure.

Temporal Summation Governs Part of the Minimum Alveolar Concentration of Isoflurane Anesthesia

Background General anesthesia may delay the onset of movement in response to noxious stimulation. The authors hypothesized that the production of immobility could involve depression of time-related processes involved in the generation of movement. Methods The delays (latencies) between onset of tail clamp (n = 16) or 50-Hz continuous electrical stimulation (n = 8) and movement were measured in rats equilibrated at 0.1–0.2% increasing steps of isoflurane. In other rats (n = 8), the isoflurane concentrations just permitting and preventing movement (crossover concentrations) in response to trains of 0.5-ms 50-V square-wave pulses of interstimulus intervals of 10, 3, 1, 0.3, or 0.1 s during the step increases were measured. These measures were again made during administration of intravenous MK801, an N-methyl-d-aspartate receptor antagonist that can block temporal summation (n = 6). Temporal summation refers to the cumulative effect of repeated stimuli. Crossover concentrations to 10- and 0.1-s interstimulus interval pulses ranging in voltage from 0.25–50 V were also measured (n = 4). Results The increase in concentrations from 0.6 to nearly 1.0 minimum alveolar concentration progressively increased latency from less than 1 s to 58 s. Shortening the interstimulus interval (50 V) pulses from 10 to 0.1 s progressively increased crossover concentrations from 0.6 to 1.0 minimum alveolar concentration. In contrast, during MK801 administration shortening interstimulus intervals did not change crossover concentrations, producing a flat response to change in the interstimulus interval. Increasing the voltage of interstimulus interval pulses increased the crossover concentrations but did not change the response to change in interstimulus intervals for pulses greater than 1 V. Conclusions Increasing the duration or frequency (interstimulus interval) of stimulation increases the concentration of isoflurane required to suppress movement by a 0.4 minimum alveolar concentration MK801 blocks this effect, a finding consistent with temporal summation (which requires intact N-methyl-d-aspartate receptor activity) at concentrations of up to 1 minimum alveolar concentration isoflurane.

Long ascending propriospinal projections from lumbosacral to upper cervical spinal cord in the rat

The retrograde tracer cholera toxin beta-subunit (CTB) was used to trace long ascending propriospinal projections from neurons in the lumbosacral spinal cord to the upper cervical (C3) gray matter in adult male Sprague-Dawley rats. Following large 0.5 microl CTB injections restricted mainly to the upper cervical ventral horn (n=5), there were many lumbosacral CTB-positive neurons (14-17/section) in the intermediate gray and ventral horn (dorsal lamina VIII, medial VII extending into X) contralaterally, with fewer at corresponding ipsilateral locations. Labeled cells (4-8/section) were also observed in contralateral laminae IV-VI and the lateral spinal nucleus, with fewer ipsilaterally. Few labeled cells (<2/section) were observed in superficial laminae I-II. Smaller (0.15 microl) microinjections of CTB restricted to the upper cervical ventral gray matter labeled cells in contralateral laminae VII-VIII (approximately 6-9/section) with fewer ipsilaterally. There were relatively fewer (<2/section) in the intermediate dorsal horn and very few (<1/section) in lamina I. Larger (0.5 microl) CTB injections encompassing the C3 dorsal and ventral gray matter on one side labeled significantly more CTB-positive neurons (>6/section) in contralateral lamina I compared to ventral horn injections. These results suggest direct projections from ventromedially located neurons of lumbar and sacral segments to the contralateral ventral gray matter of upper cervical segments, as well as from neurons in the intermediate but not superficial dorsal horn. They further suggest that some lumbosacral superficial dorsal horn neurons project to the upper cervical dorsal horn. These propriospinal projections may be involved in coordinating head and neck movements during locomotion or stimulus-evoked motor responses.

The Differential Effects of Halothane and Isoflurane on Windup of Dorsal Horn Neurons Selected in Unanesthetized Decerebrated Rats

Halothane and isoflurane, in the peri-minimum alveolar anesthetic concentration (MAC) range, exert differential effects on spinal nociceptive neurons, whereby halothane further depresses their responses from 0.8 to 1.2 MAC, whereas isoflurane does not. We presently investigated if these anesthetics differentially affect windup, the progressive increase in neuronal responses to repetitive noxious stimuli, over a broad concentration range from 0 to 1.2 MAC. In decerebrated rats, single-unit recordings were made from dorsal horn neurons exhibiting windup to 20 1-Hz C-fiber strength electrical stimuli. Halothane and isoflurane (0, 0.4, 0.8, and 1.2 MAC) were tested in a counterbalanced crossover protocol. Increasing halothane and isoflurane from 0 to 1.2 MAC progressively suppressed the response to the first stimulus, as well as summed responses to all stimuli (to 34% ± 8% and 50% ± 8%, respectively; P < 0.05). Absolute windup (summed response minus 20× the first response) was suppressed by both anesthetics from 0 to 0.8 MAC, with further depression by halothane but not isoflurane at 1.2 MAC. Responses of neurons isolated at 0 MAC were partially, but never totally, depressed at 0.8 MAC. The dose-dependent suppression of windup is consistent with reduced temporal summation of pain. Further depression at 1.2 MAC halothane, but not isoflurane, suggests different sites of immobilizing action for these two anesthetics. Immobility seems to not be mediated by severe anesthetic depression of a subpopulation of nociceptive neurons.