Monitoring depth of anesthesia

Devices which monitor some aspect of anesthetic drug effects have evolved in the past few years into imperfect, but very useful, clinical tools. With appropriate respect for their limitations these monitors can be used to reduce anesthetic drug utilization and turnover time. The intriguing hypothesis that such monitors will reduce the risk of intraoperative awareness is currently under test.

Inhaled nonimmobilizers do not alter the middle latency auditory-evoked response of rats

General anesthetics cause surgical immobility and oblivion (unconsciousness and amnesia). A class of compounds known as "nonimmobilizers" were predicted to be anesthetic, based on their physiochemical properties, but found to cause only amnesia. In humans, cerebrocortical electrical activity after auditory stimulation is depressed by concentrations of anesthetics which impair auditory recall. We sought to use these evoked responses to characterize the effects of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N) and conventional inhaled anesthetics on early sensory processing in rats. Unrestrained rats with chronically implanted epidural silver screw electrodes were put into a chamber. On separate days, the same population of rats were exposed to isoflurane, desflurane, nitrous oxide, or 2N, each at several subminimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus concentrations. After equilibration at each concentration, auditory-evoked responses were obtained. The behavioral state (activity and righting reflex) and electroencephalogram were also examined. 2N did not significantly change the middle latency auditory-evoked response, whereas the anesthetics all slowed conduction and depressed amplitude in a dose-dependent fashion. 2N neither depressed the righting reflex, nor induced epileptiform activity.

IMPLICATIONS

Although the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N) suppresses learning, we find that 2N does not depress middle latency auditory-evoked responses. This suggests that 2N may suppress learning by depressing transmission through rostral subcortical structures, such as the amygdala, rather than by acting on the brainstem or neocortical structures.

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 investigator's 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.

Ethanol directly depresses AMPA and NMDA glutamate currents in spinal cord motor neurons independent of actions on GABAA or glycine receptors

Ethanol is a general anesthetic agent as defined by abolition of movement in response to noxious stimulation. This anesthetic endpoint is due to spinal anesthetic actions. This study was designed to test the hypothesis that ethanol acts directly on motor neurons to inhibit excitatory synaptic transmission at glutamate receptors. Whole cell recordings were made in visually identified motor neurons in spinal cord slices from 14- to 23-day-old rats. Currents were evoked by stimulating a dorsal root fragment or by brief pulses of glutamate. Ethanol at general anesthetic concentrations (50-200 mM) depressed both responses. Ethanol also depressed glutamate-evoked responses in the presence of tetrodotoxin (300 nM), showing that its actions are postsynaptic. Block of inhibitory gamma-aminobutyric acidA and glycine receptors by bicuculline (50 microM) and strychnine (5 microM), respectively, did not significantly reduce the effects of ethanol on glutamate currents. Ethanol also depressed glutamate-evoked currents when the inhibitory receptors were blocked and either D, L-2-amino-5-phosphonopentanoic acid (40 microM) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (10 microM) were applied to block N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors, respectively. The results show that ethanol exerts direct depressant effects on both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate glutamate currents in motor neurons. Enhancement of gamma-aminobutyric acidA and glycine inhibition is not required for this effect. Direct depression of glutamatergic excitatory transmission by a postsynaptic action on motor neurons thus may contribute to general anesthesia as defined by immobility in response to a noxious stimulus.

Bispectral EEG index during nitrous oxide administration

BACKGROUND

Nitrous oxide (N2O) is a commonly used sedative for painful diagnostic procedures and dental work. The authors sought to characterize the effects of N2O on quantitative electroencephalographic (EEG) variables including the bispectral index (BIS), a quantitative parameter developed to correlate with the level of sedation induced by a variety of agents.

METHODS

Healthy young adult volunteers (n = 13) were given a randomized sequence of N2O/O2 combinations via face mask. Five concentrations of N2O (10, 20, 30, 40, and 50% atm) were administered for 15 min (20 min for the first step). EEG was recorded from bilateral frontal poles continuously. At the end of each exposure, level of sedation was assessed using primarily the Observer Assessment of Alertness/Sedation (OAA/S) scale.

RESULTS

One subject withdrew from the study because of emesis at 50% N2O. N2O (50%) increased theta, beta, 40-50 Hz, and 70-110 Hz band powers. BIS and spectral edge frequency during 50% N2O/O2 did not differ significantly from baseline values. Abrupt decreases from higher to lower concentrations frequently evoked a profound, transient slowing of activity. No significant change in OAA/S was detected during the study.

CONCLUSIONS

Although the spectral content of the EEG changed during N2O administration, reflecting some pharmacologic effect, the subjects remained cooperative and responsive throughout, and therefore N2O can only be considered a weak sedative at the tested concentrations. Despite changes in the lower and higher frequency ranges of EEG activity, the BIS did not change, which is consistent with its design objective as a specific measure of hypnosis.

A multicenter study of bispectral electroencephalogram analysis for monitoring anesthetic effect

Bispectral analysis (BIS) of the electroencephalogram (EEG) has been shown in retrospective studies to predict whether patients will move in response to skin incision. This prospective multicenter study was designed to evaluate the real-time utility of BIS in predicting movement response to skin incision using a variety of general anesthetic techniques. Three hundred patients from seven study sites received an anesthetic regimen expected to give an approximately 50% movement response at skin incision. EEG was continuously recorded via an Aspect B-500 monitor and BIS was calculated in real time from bilateral frontocentral channels displayed on the monitor. Half of the patients were randomized to a treatment group in which anesthetic drug doses were increased to produce a lower BIS. In the control group, BIS was recorded, but no action taken on the data displayed. A determination of movement in response to skin incision was made in the 2 min succeeding incision. Retrospective pharmacodynamic modeling was performed using STANPUMP to estimate effect-site concentrations of intravenously administered anesthetics. BIS values were significantly higher in the control group (66 +/- 19) versus the BIS-guided group, in which additional anesthesia was administered to produce a lower BIS (51 +/- 19). The movement response rate was significantly higher in the control group at 43% compared with 13% in the BIS-guided group, but response rates were low at sites which used larger doses of opioids. Logistic regression analysis showed that BIS, estimated opioid effect-site concentrations, and heart rate (in that order) were the best predictors of movement at skin incision. This study demonstrates that dosing anesthetic drugs to lower BIS values achieves a lower probability of movement in response to surgical stimulation. BIS is a significant predictor of patient response to incision, but the utility of the BIS depends on the anesthetic technique being used. When drugs such as propofol or isoflurane are used as the primary anesthetic, changes in BIS correlate with the probability of response to skin incision. When opioid analgesics are used, the correlation to patient movement becomes much less significant, so that patients with apparently "light" EEG profiles may not move or otherwise respond to incision. Therefore, the adjunctive use of opioid analgesics confounds the use of BIS as a measure of anesthetic adequacy when movement response to skin incision is used as the primary end point.

Volatile anesthetics depress spinal motor neurons

BACKGROUND

Depression of spinal alpha-motor neurons apparently plays a role in the surgical immobility induced by isoflurane. Using the noninvasive technique of F-wave analysis, the authors tested the hypothesis that depressed motor neuron excitability is an effect common to other clinically relevant inhaled anesthetics.

METHODS

The authors measured F-wave amplitude in rats anesthetized with desflurane, enflurane, halothane, or sevoflurane. Each animal received one anesthetic at five equipotent anesthetic concentrations (0.6, 0.8, 1.2, and 1.6 minimum alveolar concentration [MAC] and 0.8 MAC with 65% N2O). F waves were detected as late potentials in electromyographic responses evoked in the intrinsic muscles of the hind paw after monopolar stimulation of the ipsilateral posterior tibial nerve.

RESULTS

All tested inhaled anesthetics depressed F-wave amplitude but not M-wave (orthodromic, early muscle activation) amplitude, and increased M-F latency in a dose-dependent manner. At 1.0 MAC, the estimated F/M ratio was 70 +/- 13% SD of that at baseline (0.6 MAC). Nitrous oxide added to 0.8 MAC of the potent vapors depressed F/M ratio by 63 +/- 17%.

CONCLUSIONS

All anesthetics tested appeared to depress the excitability of spinal motor neurons. This effect may contribute to surgical immobility, and its magnitude is comparable at equipotent concentrations of agents. The authors hypothesize that this effect is due to hyperpolarization, although, currently, there is insufficient information to discriminate between pre- and postsynaptic mechanisms.

Nitrous oxide depresses spinal F waves in rats

BACKGROUND

Evoked, recurrent electromyographic activity (F waves) reflect alpha-motor neuron excitability. Based on observations that other inhaled anesthetics do so, we hypothesized that nitrous oxide, alone or in combination with isoflurane, would depress F-wave activity and correlate with depression of movement response to tail clamp or electric stimulation.

METHODS

In study 1, the authors examined the effect of nitrous oxide in combination with isoflurane in 13 normocapnic Sprague-Dawley rats anesthetized with 1.0% isoflurane (0.7 minimum alveolar concentration) in oxygen. The tibial nerve was stimulated at the popliteal fossa, and evoked electromyographic activity [M (direct neuromuscular junctional response) and F waves] were recorded from ipsilateral foot muscles. The effect of the addition of 30% or 70% nitrous oxide was measured. F-wave amplitude/M-wave amplitude ratio (F/M) was determined from each stimulus-electromyographic response pair. F/M vs. movement response to 60-s tail clamp was assessed after each recording session. F-wave amplitude/M-wave amplitude ratio at adjacent doses that permitted and prevented movement were compared. In study 2, the authors examined the effect of (hyperbaric) nitrous oxide as the sole anesthetic agent on F waves. In 11 rats anesthetized with isoflurane, stimulation and recording electrodes were placed as described above, with additional electrodes for stimulation placed in the tail. Rats were placed in a pressure chamber pressurized with nitrous oxide/oxygen to 3.4 atm. Thirty m were allowed for isoflurane washout. Electromyographic activity was evoked and recorded at 1.0, 1.6, 2.2 and 2.7 atm N2O (random order). Movement in response to 60 s of 15 V, 50-Hz tail stimulation was evaluated after each recording session.

RESULTS

Nitrous oxide with or without isoflurane produced a dose-dependent decrease in F/M. By interpolation of this data, the authors found that 2 atm N2O alone, or 44% N2O added to 1.0% isoflurane at 1.0 atm, produced 1.0 minimum alveolar concentration anesthesia. At the deepest level of isoflurane/ nitrous oxide that permitted movement, mean F/M was 20.6 +/- 17.5%; at the lowest concentration that blocked movement, rats had a mean F/M of 13.7 +/- 13.9% (P = 0.01). At the minimal hyperbaric nitrous oxide blocking movement, rats had a mean F/M of 3.7 +/- 2.9%, whereas the F/M at the highest nitrous oxide dose that permitted movement was 4.4 +/- 2.7% (P < 0.04).

CONCLUSIONS

Because nitrous oxide depressed F-wave but not M-wave activity, the data suggest a central (spinal) rather than neuromuscular junctional site of action of this agent. The direct correlation between nitrous oxide dose, F-wave amplitude depression, and surgical immobility suggests the possibility of using F-wave activity to predict the likelihood of anesthetic-induced immobility. However, the mechanism of action of nitrous oxide may differ from that of the potent inhaled agents.

Preoperative valproate administration does not increase blood loss during temporal lobectomy

Surgical treatment is increasingly used for patients with medically re fractory seizures. Valproate (VPA) is an effective, widely used anticonvulsant in this patient population, but believed by some researchers to increase surgical bleeding because of quantitative thrombocytopenia and functional defects in platelet aggregation. Because we have observed no clinical evidence that perioperative administration of VPA increases blood loss or complications related to postoperative bleeding in patients undergoing temporal lobectomy at our institution, we sought to test this hypothesis. We made a retrospective review of the medical records of all patients who underwent epilepsy surgery at the University of California, San Francisco Medical Center, from September 1986 through January 1993. Patients who had a temporal lobectomy and whose medical records documented preoperative platelet counts and pre- and postoperative hematocrit and hemoglobin values were included. We excluded patients who had cranial surgery before temporal lobectomy and those with intracranial neoplasms or vascular malformations. Patients were divided into two groups: those who received VPA in the immediate preoperative period and those who had not received VPA recently. We compared the estimated surgical blood loss and the estimated change in red blood cell (RBC) volume between groups by unpaired t tests. The charts of 87 consecutive patients qualified for inclusion in the study. Patients in the VPA group had relative (but not absolute) thrombocytopenia preoperatively (235 +/- 64 vs. 277 +/- 69 k in the No-VPA group). There were no differences in the estimated blood loss, RBC volume, or in the incidence of postoperative transfusion. VPA apparently does not increase complications of hemostasis during therapeutic surgical resections for epilepsy. Therefore, we do not recommend routinely discontinuing VPA before craniotomy.

Halothane selectively inhibits bradykinin-induced synovial plasma extravasation

BACKGROUND

In vitro studies demonstrate that halothane, but not isoflurane, inhibits bradykinin-induced calcium currents and prostacyclin release in cultured endothelial cells. Because bradykinin is an important endogenous mediator of inflammation, we assessed the effects of halothane, isoflurane, and pentobarbital on plasma extravasation, a component of tissue inflammation induced by bradykinin, in rats.

METHODS

We anesthetized 23 rats with halothane (0.8 or 1.3 minimum alveolar concentration [MAC]), isoflurane (1.3 MAC), or pentobarbital (total of 85 mg/kg intraperitoneally). Their tracheas were intubated and their lungs mechanically ventilated. After intravenous administration of Evans blue dye, we perfused normal saline followed by bradykinin or platelet-activating factor, another inflammatory mediator, intraarticularly via needles placed in the knee joint. We collected perfusate and estimated extravasation by measuring dye in the perfusate using spectrophotometry.

RESULTS

Bradykinin increased plasma extravasation eight- to ninefold above baseline in both pentobarbital- and isoflurane-anesthetized rats. In contrast, bradykinin-induced plasma extravasation at 0.8 MAC and 1.3 MAC of halothane was approximately 40% (P < 0.01) and 15% (P < 0.001), respectively, of that in pentobarbital- and isoflurane-anesthetized rats. Baseline plasma extravasation was lower in rats anesthetized with either concentration of halothane compared with pentobarbital or isoflurane (all P < 0.001). Platelet-activating factor-induced plasma extravasation was similar for all anesthetics.

CONCLUSION

Halothane, but not isoflurane or pentobarbital, inhibits both baseline and bradykinin-induced peripheral plasma extravasation, demonstrating that volatile anesthetics differentially modulate this important component of inflammation.

Anesthetic depression of spinal motor neurons may contribute to lack of movement in response to noxious stimuli

BACKGROUND

Previous studies suggest that anesthetics produce immobility by an action on the spinal cord. We postulated that immobility results from a depression of alpha-motor neuron excitability in vivo, and that this depression would be reflected in a depression of recurrent, (F)-wave activity.

METHODS

The lungs of 15 normocapnic, normothermic, normotensive rats were mechanically ventilated with 0.5, 0.8, 1.2, and 1.6 MAC isoflurane, in random sequence, with at least 30 min of equilibration at each step. In addition, at 1.2 MAC, inspired carbon dioxide was altered to create hypercapnia and hypocapnia. The sizes of the orthodromic (M) wave and F wave were measured in ten sequential trials as the activity in the intrinsic muscles of the ipsilateral foot evoked by stimulation of the tibial nerve.

RESULTS

M-wave amplitude did not change. F-wave amplitude did not decrease between 0.5 and 0.8 MAC but decreased 50% between 0.8 and 1.2 MAC (P < 0.001) and 60% between 1.2 and 1.6 MAC (P < 0.05). Hypocapnia (17 mmHg) increased F-wave amplitude by 15%, and hypercapnia (73 mmHg) reduced it by 60% compared with normocapnia at 1.2 MAC (31 mmHg) (P < 0.0001).

CONCLUSIONS

Anesthetics may cause and moderate hypercapnia may contribute to surgical immobility by depressing excitability of alpha-motor neurons. Monitoring F waves may indicate the adequacy of this aspect of anesthesia and may detect states in which spontaneous or nocifensive movements might occur.

The electroencephalogram does not predict depth of isoflurane anesthesia

BACKGROUND

The power spectrum of the electroencephalogram (EEG) may be analyzed to provide quantitative measures of EEG activity (e.g., spectral edge, which defines the highest EEG frequency at which significant activity is found). The current study tested the hypothesis that spectral edge and similar measures distinguish different functional depths of anesthesia in humans.

METHODS

Three groups were studied. Group 1 consisted of 34 surgical patients (ASA physical status 1 or 2) who received 0.6, 1.0 and 1.4 MAC isoflurane anesthesia. A subgroup (group 2) of group 1 was tested during 1.0 MAC isoflurane anesthesia at surgical incision. Group 3 consisted of 16 volunteers who listened to an audiotape while receiving 0.15, 0.3, and 0.45 MAC isoflurane or 0.3, 0.45, and 0.6 MAC nitrous oxide in oxygen. The audiotape contained information designed to test implicit and explicit memory formation. We tested the ability of six EEG parameters (spectral-edge, 95th percentile power frequency, median power, and zero crossing frequencies and total power in the alpha- [8-13 Hz] and delta- [< 4 Hz] power ranges) to predict movement after surgical incision, purposeful response to command, or memory of information presented during anesthetic administration.

RESULTS

Isoflurane decreased EEG activity in group 1 in a dose-related fashion. The 55% of group 2 who made purposeful movements in response to incision did not differ in their EEG from nonresponders (e.g., spectral edge 19.8 +/- 3.1 vs. 19.3 +/- 2.6 Hz, mean +/- SD). In group 3, memory of the information presented did not correlate with values of any EEG parameter. Response to verbal command was associated with lower anesthetic concentrations and with smaller alpha- and delta-band power (298 +/- 66 vs. 401 +/- 80 watts; and 75 +/- 20 vs. 121 +/- 49 watts, mean +/- SD), but there was no difference in values for other parameters.

CONCLUSIONS

We conclude that our EEG measures do not predict depth of anesthesia as defined by the response to surgical incision, the response to verbal command or the development of memory.

Anesthetic potency is not altered after hypothermic spinal cord transection in rats

BACKGROUND

In essence, the clinical goal of general anesthesia is to produce a state of unresponsiveness and amnesia. These endpoints are commonly achieved with drugs like isoflurane, but the sites and mechanisms by which these specific endpoints are achieved remain unknown. Blocking the somatic motor response to painful stimuli is widely used as an indicator of anesthetic adequacy, and the concentration of anesthetic agent (minimum alveolar concentration [MAC]) required to achieve this unresponsiveness is the benchmark of anesthetic potency. Recent work has demonstrated that precollicular decerebration does not alter MAC in rats, suggesting that the forebrain is not a major site of action of isoflurane in blocking motor responses. The brain stem contains systems that modulate pain processing in the spinal cord. The current study was undertaken to assess the relative roles of the brain stem and spinal cord as sites of anesthetic action in blocking somatic responsiveness.

METHODS

In seven rats, anesthesia was induced and maintained with isoflurane in oxygen. MAC was determined by observing the response to tail clamp and fore- and hind limb toe pinch at three times: after intubation, after cervical laminectomy, and after staged hypothermic spinal cord transection.

RESULTS

MAC determined by tail clamp did not change during the protocol (1.28 +/- 0.08% [mean +/- standard deviation] baseline vs. 1.25 +/- 0.18% postlaminectomy vs. 1.03 +/- 0.40% posttransection). In one animal, the MAC value decreased from a prelesion value of 1.2% to 0.25%, accounting for most of the variance in the postlesion mean; the MAC value as determined by withdrawal to rear paw pinch was unchanged from its prelesion value in this animal. The MAC values as determined by toe pinch in all animals remained unchanged after spinal transection of the lesion both rostrally and caudally.

CONCLUSIONS

Somatic motor responsiveness and its sensitivity to isoflurane appeared to be unaltered despite acute loss of descending cortical and bulbar controls. This observation suggests that the site of anesthetic inhibition of motor response may be in the spinal cord.

Central nervous system effects of intrathecal muscle relaxants in rats

When given for a sufficient time and dose intravenously, neuromuscular blocking drugs eventually can enter the cerebrospinal fluid (CSF). To study the potential pharmacologic consequences of neuromuscular blocking drugs in the CSF, a model was developed in the rat by using an intrathecal infusion of these drugs. A cannula was stereotaxically implanted in a lateral cerebral ventricle of anesthetized male Sprague-Dawley rats (250-300 g). Several days later, the effects of an intraventricular infusion (5 microL/min) of atracurium (0.804 mumol/mL), pancuronium (0.172 mumol/mL), and vecuronium (21.978 mumol/mL) were studied in unanesthetized rats. These rats (n = 6 in each group) exhibited dose-dependent hyperexcitability, during drug infusion, with seizures occurring at threshold doses of (mean), 0.12, 0.26, and 0.065 +/- 0.010 and 3.32 mumol/kg of atracurium, pancuronium, and vecuronium, respectively. The neuromuscular ED50 (intravenous dose required to produce a 50% depression of twitch tension) in rats determined by other investigators are 0.408, 0.115, and 0.352 mumol/kg for atracurium, pancuronium, and vecuronium, respectively. Therefore, seizure threshold doses were not related to the potencies of these drugs as neuromuscular blocking drugs. Based on these data, central nervous system effects were studied over the subseizure dose range approximating 1/100, 1/10, and 1/5 of the cumulative dose causing seizures for each drug (n = 5 for each dose). At 1/100 of seizure dose, decreased locomotor activity and piloerection occurred. At 1/10 to 1/5 of seizure dose, agitation, shivering, splayed limbs, and whole body shaking resulted.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthetic potency (MAC) is independent of forebrain structures in the rat

BACKGROUND

The ability of general anesthetics to suppress somatomotor responses to surgical incision and other noxious stimuli is of particular clinical relevance. When the blockade is due to inhaled agents, this effect can be quantified as the minimum alveolar concentration (MAC), i.e., that concentration that blocks movement evoked by a noxious stimulus (ED50).

METHODS

To identify the neural structures that subtend this somatomotor response, we anesthetized 14 rats with isoflurane in oxygen and performed bilateral parietal-temporal craniotomies. In each rat, MAC was repeatedly tested using tail-clamping and Dixon's up-down concentration technique. After determination of baseline MAC, seven rats underwent aspiration decerebration, after which MAC was repeatedly measured.

RESULTS

In the control group (N = 7), MAC (mean +/- SD) remained constant at 1.30 +/- 0.25% for more than 6 h. In the seven rats that underwent aspiration decerebration, baseline MAC was 1.26 +/- 0.14%. These seven rats with histologically validated precollicular decerebration demonstrated no change in MAC relative to control rats, as much as 11 h after decerebration (P = 0.14).

CONCLUSIONS

These findings suggest that the anesthetic-induced unresponsiveness to noxious stimuli measured by MAC testing does not depend on cortical or forebrain structures in the rat.

Effects of fentanyl versus sufentanil in equianesthetic doses on middle cerebral artery blood flow velocity

BACKGROUND

Sufentanil has been reported to increase cerebral blood flow in comparison with fentanyl. However, because of the use of animal models, supraclinical doses and/or background anesthetic agents, the clinical applicability of these studies remains difficult to assess. Therefore, transcranial Doppler ultrasonography was used to determine the cerebral hemodynamic effects of equianesthetic doses of fentanyl and sufentanil on middle cerebral artery (MCA) blood flow velocity in patients without intracranial pathologic conditions.

METHODS

Twenty-four unpremedicated American Society of Anesthesiologists physical status 1 and 2 patients undergoing elective nonintracranial neurosurgery were assigned randomly to receive equipotent blinded infusions of either sufentanil (15 micrograms/min) or fentanyl (150 micrograms/min) for anesthetic induction during spontaneous ventilation of 100% oxygen. Normocapnia, as measured by infrared capnography, was maintained by manually assisting ventilation, as necessary. The cerebral opioid effect was quantified using the spectral edge frequency parameter. The infusion was continued until either 1) spectral edge frequency decreased below 10 Hz or 2) 150 micrograms of sufentanil or 1,500 micrograms of fentanyl was infused, whichever occurred first. On average, the patients received 1.7 +/- 0.55 micrograms/kg or 16 +/- 4 micrograms/kg of sufentanil or fentanyl, respectively. The right MCA mean, peak systolic, and peak diastolic velocities and pulsatility index were measured continuously by transcranial Doppler ultrasonography.

RESULTS

The mean arterial pressure decreased slightly in both groups, but only in the fentanyl group were the changes significant. The MCA velocity increased by approximately 25% in both groups. However, the relative changes in MCA velocity were not different between groups. The pulsatility indexes were unchanged in both groups.

CONCLUSIONS

These data suggest that, at clinically relevant doses in the absence of other drugs, cerebral blood flow velocity is increased by both fentanyl and sufentanil. Furthermore, there appears to be no significant differences in the cerebral hemodynamic profiles of the two drugs, as assessed by transcranial Doppler ultrasonography.

No correlation between quantitative electroencephalographic measurements and movement response to noxious stimuli during isoflurane anesthesia in rats

A meaningful use of the electroencephalogram (EEG) for monitoring depth of anesthesia has proven elusive. Although changes in the EEG with changing anesthetic dose or concentration have been noted for 60 yr, it has been difficult to demonstrate reliable, quantitative correlation between the EEG and other physiologic measures of anesthetic depth. We attempted to correlate several quantitative EEG measurements in rats, including average amplitude, spectral edge frequency, and burst suppression ratio, with the movement response to supramaximal noxious stimulation. We anesthetized 21 Sprague-Dawley rats with isoflurane 1.5% and allowed them to breathe spontaneously. After equilibration, EEG was recorded for off-line analysis; then a noxious stimulation was delivered with a tail clamp and the somatic response noted. Isoflurane concentration was adjusted up and down, and the EEG and movement response to tail clamp were assessed at each level until the minimum alveolar concentration was determined in each rat. We found no EEG dose response to increasing inspired concentrations of isoflurane, except for an increasing degree of burst suppression. We found no difference in any parameter between rats that responded and those that did not respond to stimuli at a given concentration of isoflurane. Finally, we found that the presence of burst suppression did not predict lack of response.

Correlated, simultaneous, multiple-wavelength optical monitoring in vivo of localized cerebrocortical NADH and brain microvessel hemoglobin oxygen saturation

Current forms of brain monitoring, such as electroencephalography (EEG), have had limited clinical utility. The EEG records spontaneous cerebrocortical activity and thus is an indirect indicator of metabolic demand and, to a lesser extent, an indicator of mismatch of supply versus demand. Ischemia modulates EEG activity in ways that can usually be detected, but EEG patterns can be similarly modulated by many other factors, including temperature and pharmacologic manipulation. This in vivo study in physiologically monitored animals evaluated the use of correlated optical spectroscopy, performed with an instrument having a fiberoptic light-guide bundle in contact with the cerebral cortex, for the simultaneous monitoring of cerebrovascular oxygen availability and intracellular oxygen delivery. A highly specific monitor of cerebral intracellular oxygen supply, the cerebrocortical intramitochondrial NADH redox state, was monitored in vivo with a fluorescence technique. Absorption spectroscopy was used concurrently to monitor hemoglobin content (blood volume) and oxygen saturation in the microcirculation. Correlated changes in optical signals from cerebrocortical NADH and hemoglobin were studied in a swine model (n = 7) of nitrogen hypoxia. Measurements were made at four wavelengths with a time-division, multiplexed fluorometer/reflectometer. Because the NADH fluorescence signal at 450 nm is affected by local changes in blood volume, a "corrected" fluorescence signal is usually calculated. In previous studies, where only two wave lengths have been measured, attempts at correction were based on reflectance at the excitation wavelength (366 nm). We compared estimators of changes in microcirculatory blood volume using reflection at two wavelengths: 366 nm and 585 nm, the wavelengths for maximum and isobestic absorption. The results of the studies were as follows: (1) during transient hypoxia, NADH and local hemoglobin saturation signals changed in concert with arterial pulse oximetry, with changes in NADH lagging behind changes in saturation by an average of 5.3 seconds; (2) after hypocapnic ventilation to a mean PaCO2 of 20.2 +/- 0.8 mm Hg, NADH increased by 11.5 +/- 8.7% (as compared with maximal change during anoxia), local hemoglobin saturation decreased by 7.7 +/- 6.4%, and local blood volume decreased by 12.5 +/- 13%, while arterial SpO2 was unchanged; (3) our two measures of local blood volume were closely correlated during carbon dioxide perturbations, but poorly correlated during hypoxic perturbation; and (4) NADH fluorescence provided a more rapid, sensitive indicator of oxygen deprivation than did the EEG. During transient hypoxia, EEG changes occurred 57.4 +/- 10.4 seconds after the onset of decline in local hemoglobin saturation, after NADH had completed 50% of its maximal increase.

Desflurane does not produce hepatic or renal injury in human volunteers

We examined the potential toxicity of desflurane in 13 young 25.0 +/- 2.3 (mean +/- SD) yr-old men, given 7.35 +/- 0.81 MAC-hours of desflurane anesthesia. Hepatic and renal function tests, serum electrolytes, and standard urine and hematologic tests were performed before, during, and after anesthesia. No toxicity was found. There were no changes in tests of hepatocellular integrity (plasma alanine transferase activity), synthetic function (serum albumin, prothrombin time, partial thromboplastin time), or renal function (serum creatinine concentration, blood urea nitrogen concentration). Decreases in red blood cell count, hematocrit, and blood hemoglobin concentration during and immediately after anesthesia were attributed to blood sampling and infusion of intravenous electrolyte solution. These values returned by 4 days after anesthesia to values not different from those before anesthesia. Increased white blood cell counts and blood glucose concentrations noted during anesthesia with other inhaled anesthetics were also seen in these volunteers. Desflurane appears to have no greater toxicity than currently used inhaled anesthetics and, because of its lesser metabolism, may have lesser or not toxicity.

Does desflurane modify circulatory responses to stimulation in humans?

We asked if desflurane with or without nitrous oxide at 0.83, 1.24, and 1.66 MAC prevented cardiovascular responses to stimulation. We measured cardiac output, heart rate, systemic arterial blood pressure, central venous pressure, pulmonary arterial blood pressure, and systemic vascular resistance in six healthy male volunteers before (control) and at 0, 1, 2, 4, and 6 min after tetanic electrical stimulation (50, 100, and 200 Hz) of the ulnar nerve. At 0.83 and 1.24 MAC, cardiac output, mean systemic arterial blood pressure, heart rate, and pulmonary arterial blood pressure increased. Peak changes averaged 13%-20% and most frequently occurred 0-2 min after stimulation (P less than 0.05) with return to control values at 4-6 min (except for pulmonary arterial blood pressure). At 1.66 MAC, heart rate and systemic blood pressure responses were attenuated, but this level of anesthesia had equivocal effects on the cardiac output and pulmonary blood pressure responses. The addition of nitrous oxide attenuated the peak response of heart rate and cardiac output but not the peak response of mean systemic arterial blood pressure. In summary, 0.83 and 1.24 MAC desflurane did not abolish cardiovascular responses to stimulation, but 1.66 MAC attenuated the responses.

Cardiovascular actions of desflurane in normocarbic volunteers

The cardiovascular actions of three concentrations of desflurane (formerly I-653), a new inhalation anesthetic, were examined in 12 unmedicated normocapnic, normothermic male volunteers. We compared the effects of 0.83, 1.24, and 1.66 MAC desflurane with measurements obtained while the same men were conscious. Desflurane caused a dose-dependent increase in right-heart filling pressure and a decrease in systemic vascular resistance and mean systemic arterial blood pressure. As measured by echocardiography, left ventricular end-diastolic area did not change except for a small increase at 1.66 MAC desflurane, and systolic wall stress was less at all concentrations of desflurane than during the conscious state. Desflurane did not change cardiac index or left ventricular ejection fraction. Heart rate did not change at 0.83 MAC, but progressively increased with deeper desflurane anesthesia. Stroke volume index was less at all concentrations of desflurane than while the men were conscious, but desflurane did not alter the velocity of ventricular circumferential fiber shortening. Mixed venous blood PO2 and oxyhemoglobin saturation were higher during all concentrations of desflurane anesthesia than during the conscious state. No volunteer developed a metabolic acidosis. We conclude that desflurane with controlled ventilation and constant PaCO2 causes cardiovascular depression, as indicated by the increased cardiac filling pressure and decreased stroke volume index and by no change in the velocity of circumferential fiber shortening in the presence of decreased systolic wall stress. However, cardiac output is well maintained, and heart rate does not increase at light levels of anesthesia. The cardiovascular actions of 0.83 and 1.66 MAC desflurane were also reexamined in 6 of the 12 men during the seventh hour of anesthesia. Prolonged desflurane anesthesia resulted in lesser cardiovascular depression than was evidenced during the first 90 min. The measures of cardiac filling (central venous pressure and left ventricular end-diastolic cross-sectional area) did not differ between the early and late periods of anesthesia. Systemic vascular resistance decreased further during the late period, but systolic wall stress did not differ between the two time periods. During the seventh hour of desflurane anesthesia, heart rate and cardiac index were higher at both anesthetic concentrations than during the first 90 min of anesthesia. Left ventricular ejection fraction and velocity of fiber shortening did not change with duration of desflurane anesthesia. Oxygen consumption, oxygen transport, the ratio of the two, mixed venous PO2, and mixed venous oxyhemoglobin saturation (SO2) increased late in the anesthetic in comparison with the first 90 min.

Cardiovascular actions of desflurane with and without nitrous oxide during spontaneous ventilation in humans

We investigated the cardiovascular actions of desflurane (formerly I-653) during spontaneous ventilation. We gave 0.8-0.9, 1.2-1.3, and 1.6-1.7 MAC desflurane in oxygen (n = 6) and in 60% nitrous oxide, balance oxygen (n = 6) to unmedicated healthy male volunteers. Both anesthetic regimens decreased ventilation, increased partial pressure of arterial carbon dioxide, and produced similar cardiovascular changes. In comparison with values obtained when the volunteers were conscious, desflurane anesthesia with spontaneous ventilation decreased systemic vascular resistance and mean arterial blood pressure. Cardiac index, heart rate, stroke volume index, and central venous blood pressure increased. Left ventricular ejection fraction increased at 0.83 MAC desflurane in oxygen, and otherwise did not differ from the conscious value. The velocity of ventricular circumferential fiber shortening, estimated by echocardiography, increased with desflurane in oxygen but did not change with desflurane in nitrous oxide. Oxygen consumption increased during desflurane and oxygen anesthesia, but not when nitrous oxide plus oxygen was the background gas. Desflurane increased oxygen transport, the ratio of oxygen transport to oxygen consumption, mixed venous partial pressure of oxygen, and oxyhemoglobin saturation. The cardiovascular changes with desflurane during spontaneous ventilation differ from those during controlled ventilation. With both background gases, spontaneous ventilation, in comparison with controlled ventilation, increased cardiac index, stroke volume, central venous pressure, left ventricular ejection fraction, velocity of circumferential fiber shortening, oxygen transport, and the ratio of oxygen transport to oxygen consumption but did not change mean arterial blood pressure except at 1.66 MAC desflurane in oxygen (when it was higher with spontaneous than with controlled ventilation).

Visceral losses of desflurane, isoflurane, and halothane in swine

Percutaneous loss of inhaled anesthetics is small relative to their uptake. The minor nature of this loss results in part from the substantial barrier to diffusion posed by the skin. Pleural and peritoneal surfaces pose less effective barriers because diffusion distances are smaller than in the skin. Accordingly, we measured visceral loss to air of desflurane, isoflurane, and halothane from pleural and peritoneal surfaces in five juvenile swine. Pleural and peritoneal losses per percent end-tidal anesthetic correlated directly with the solubility of the anesthetic in blood or tissues. The total pleural losses for the first 30 min of anesthetic administration were desflurane, 1.22 +/- 0.22 mL (mean +/- standard deviation for the 30-min period); isoflurane, 2.34 +/- 0.52 mL; and halothane, 4.69 +/- 0.98 mL; respective peritoneal losses were 0.64 +/- 0.12 mL, 1.23 +/- 0.25 mL, and 2.69 +/- 0.57 mL. Pleural loss per unit time did not change with increasing duration of anesthesia, whereas peritoneal loss increased for all anesthetics. These visceral losses are greater than total percutaneous losses in humans given these anesthetics for the same period of time, but the loss of anesthetic by either route is too small to affect measurements of anesthetic kinetics or recovery.

Fluoride metabolites after prolonged exposure of volunteers and patients to desflurane

We examined the metabolism of desflurane in 13 healthy volunteers given 7.35 +/- 0.81 MAC-hours (mean +/- SD) of desflurane and 26 surgical patients given 3.08 +/- 1.84 MAC-hours (mean +/- SD). Markers of desflurane metabolism included fluoride ion measured via an ion-specific electrode, nonvolatile organic fluoride measured after sodium fusion of urine samples, and trifluoroacetic acid determined by a gas chromatographic-mass spectrometric method. In both volunteer and patient groups, postanesthesia serum fluoride ion concentrations did not differ from background fluoride ion concentrations. Similarly, postanesthesia urinary excretion of fluoride ion and organic fluoride in volunteers was comparable to preanesthesia excretion rates. However, small but significant levels of trifluoroacetic acid were found in both serum and urine from volunteers after exposure to desflurane. A peak serum concentration of 0.38 +/- 0.17 mumol/L of trifluoroacetic acid and a peak urinary excretion rate of 0.169 +/- 0.107 mumol/h were detected in volunteers at 24 h after desflurane exposure. Although these increases in trifluoroacetic acid after exposure to desflurane were statistically significant, they are approximately 10-fold less than levels seen after exposure to isoflurane. Thus, desflurane strongly resists biodegradation, but a small amount is metabolized in humans.

Hemodynamic effects of desflurane/nitrous oxide anesthesia in volunteers

We determined the cardiovascular effects of 0.91, 1.34, and 1.74 MAC of desflurane/nitrous oxide anesthesia (60% inspired nitrous oxide contributed 0.5 MAC at each level) in 12 healthy, normocapnic male volunteers. Desflurane/nitrous oxide anesthesia decreased systemic blood pressures, cardiac index, stroke volume index, systemic vascular resistance, and left ventricular stroke work index, and increased pulmonary arterial pressures and central venous pressure in a dose-dependent fashion, while heart rate was 10%-12% and mixed venous oxygen tension was 2-4 mm Hg higher at all MAC levels than at baseline (awake). Desflurane/nitrous oxide anesthesia modestly increased left ventricular end-diastolic cross-sectional area (preload) and decreased velocity of left ventricular circumferential fiber shortening, systolic wall stress (afterload), and area ejection fraction; this combination of changes indicates myocardial depression. At approximately comparable MAC levels, heart rate was lower and systemic blood pressures, central venous pressure, left ventricular stroke work index, and systemic vascular resistance usually were significantly higher during anesthesia with desflurane and nitrous oxide than during desflurane anesthesia alone (same volunteers, data collected in crossover design). After 7 h of anesthesia, regardless of the background gas, somewhat less cardiovascular depression and/or modest stimulation was apparent: cardiac index, area ejection fraction, and velocity of left ventricular circumferential fiber shortening recovered to or toward awake values, whereas heart rate was further increased. Evidence of circulatory insufficiency did not develop in any volunteers during the study. Segmental left ventricular function was normal at baseline, and no segmental wall-motion abnormalities, ST-segment change, or dysrhythmias developed.(ABSTRACT TRUNCATED AT 250 WORDS)

No EEG evidence of acute tolerance to desflurane in swine

Desflurane is a potent inhaled anesthetic associated with a dose-dependent depression of cortical electrical activity. Recently, it has been suggested that the burst suppression pattern seen in dogs given moderately high doses (2.0 MAC) of desflurane may spontaneously subside. This observation suggests the development of acute tolerance to at least some of the anesthetic effects of this drug. No other volatile anesthetic has been found to produce acute tolerance. We attempted to replicate these findings in domestic swine. Five juvenile swine (25-30 kg) were anesthetized with desflurane in oxygen and during normocapnia were exposed to two doses of desflurane sufficient to induce burst suppression (1.5 and 1.7 MAC) for 35 min at each dose, with a period of EEG recovery (0.6 MAC) before, between (in 3 of 5 animals), and after the high doses. Frontoparietal EEG was continuously recorded and the burst suppression ratio continuously calculated. Suppression was more complete at 1.7 MAC than at 1.5 MAC (98.24 +/- 1.75 vs. 90.80 +/- 3.05%, respectively, mean +/- standard deviation). The degree of burst suppression activity did not change over time at either 1.5 (P greater than 0.33) or 1.7 MAC desflurane (P greater than 0.41). There was no EEG evidence of tolerance to desflurane anesthesia in swine.

Depression of ventilation by desflurane in humans

We studied the ventilatory effects of desflurane (formerly I-653) with and without N2O in healthy male volunteers. After insertion of venous and arterial (radial and pulmonary) catheters, baseline measurements of tidal volume (VT), respiratory rate (RR), ventilatory response to CO2, and arterial and mixed venous blood gases were made. Subjects were randomly assigned to receive either desflurane with O2 (n = 6) or with O2 and 60% N2O (n = 6). Anesthesia was induced by inhalation of desflurane followed by tracheal intubation without muscle relaxants. In each volunteer, at end-tidal concentrations totaling 0.83, 1.24, and 1.66 MAC, we repeated measurements of VT, RR, response to CO2, and arterial and mixed venous blood gases. As depth of anesthesia increased, VT significantly (P less than 0.05) decreased from 363 +/- 22 ml awake to 76 +/- 22 ml at 1.66 MAC without N2O and from 473 +/- 70 ml awake to 128 +/- 6 ml at 1.66 MAC with N2O (mean +/- SE). Similarly, RR increased from 15 +/- 0.5 breaths per min awake to 32 +/- 2 breaths per min at 1.66 MAC without N2O and from 14 +/- 0.5 breaths per min awake to 40 +/- 3 breaths per min at 1.66 MAC with N2O. Desflurane without N2O depressed the ventilatory response to CO2 to 45 +/- 9, 31 +/- 5, and 11 +/- 4% of the awake values at 0.83, 1.24, and 1.66 MAC, respectively. With N2O, values were 52 +/- 14, 23 +/- 5, and 26 +/- 9% of the awake value at 0.83, 1.24, and 1.66 MAC, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect on outcome of prolonged exposure of patients to nitrous oxide

Prolonged (several days or repeated) exposure to nitrous oxide (N2O) can cause injury or death. To assess whether relatively prolonged anesthesia with N2O in normal patients might similarly cause untoward effects, we investigated whether the addition of N2O to isoflurane anesthesia caused injury to patients having surgical resection of acoustic neuroma lasting approximately 10 h. Twenty-six patients undergoing surgical resection of acoustic neuroma were randomly assigned to a regimen that included or excluded N2O (50%-60%) during isoflurane anesthesia plus intravenous adjuvants. On average, slightly less isoflurane (0.24%) was used during anesthesia with N2O. We measured standard clinical variables (blood pressure, heart rate), oxygen saturation, neurologic status, pain, and the incidence and type of morbid outcomes. Exposure to N2O did not increase the incidence of morbid outcomes (including hepatic injury, infection, or hypoxemia), prolong hospitalization, or increase common postoperative complaints such as nausea, vomiting, coughing, or headache. Patients anesthetized with either regimen were equally satisfied with their anesthetic.

Nitrous oxide and epinephrine-induced arrhythmias

We asked whether the sympathomimetic effect of nitrous oxide (N2O) predisposed patients receiving N2O to arrhythmias in response to epinephrine administration. We also asked whether aging contributed to the development of arrhythmias, with or without N2O. One hundred patients having transsphenoidal hypophysectomy were randomly assigned to receive anesthesia including (n = 49) or excluding (n = 51) N2O. All patients were given an injection of epinephrine 1:200,000, with 0.5% lidocaine to produce hemostasis. Using intermittent 12-lead and continuous lead II electrocardiography, we determined the incidence of premature ventricular contraction, isorhythmic atrioventricular (AV) dissociation, and changes in T-wave morphology. Patients given N2O had a significantly higher incidence of isorhythmic AV dissociation (61.2% vs 41.2%). A trend toward a higher incidence of multiple premature ventricular contractions (16.3% vs 7.8%) was not statistically significant. Both anesthetic groups had a high incidence of postoperative changes in T-wave morphology (46.9% in the N2O group vs 50.9% in the group not given N2O). Aging alone did not affect the incidence of ventricular ectopic beats, isorhythmic AV dissociation, or changes in electrocardiographic morphology, but correlated with the development of ventricular ectopy during N2O anesthesia. We conclude that the use of N2O correlated with a higher incidence of isorhythmic AV dissociation in response to injection of epinephrine with lidocaine.

Hepatocellular integrity in swine after prolonged desflurane (I-653) and isoflurane anesthesia: evaluation of plasma alanine aminotransferase activity

Desflurane, formerly known as I-653 (CF2H-O-CFH-CF3), is a new inhalation anesthetic derived by fluorine substitution for the alpha-ethyl chlorine of isoflurane (CF2H-O-CClH-CF3). The lower solubility and increased stability of desflurane provided by the C-F bond lessen biotransformation to potentially hepatotoxic metabolites. Repeated administration of desflurane to rats, with or without induced hepatic enzymes, does not result in evidence of hepatic injury. In the recent study we extended the tests for liver cell injury to another species, the pig. Our test included prolonged exposure to desflurane or isoflurane, both in the absence and presence of commonly used adjuvants. We measured plasma alanine aminotransferase activity in eight young female swine anesthetized in random order with desflurane (0.8-1.6 MAC) and isoflurane (0.7-1.4 MAC), for a total dose of about 5.5 MAC-hours of each anesthetic, 3-8 days apart. Plasma alanine aminotransferase activities remained in the normal range, and were not significantly greater over baseline values in samples drawn immediately after, 4 h after, or 3-8 days after (mean +/- SD, 6.1 +/- 2.1) the administration of either anesthetic was discontinued after the first study with either desflurane or isoflurane. Five additional pigs were given a mean total dose of 9.7 MAC-hours of desflurane or isoflurane in conjunction with succinylcholine, N2O, fentanyl, naloxone, atracurium, thiopental, edrophonium, and atropine. No changes in plasma alanine aminotransferase activity were detected in blood samples drawn at termination of the anesthesia, 24 h later, and 4-7 days later (mean +/- SD, 5.8 +/- 1.3).(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular actions of common anesthetic adjuvants during desflurane (I-653) and isoflurane anesthesia in swine

To determine the cardiovascular actions of drugs commonly combined with inhalation anesthetics, we administered one drug from each of several classes of adjuvants to seven swine already anesthetized with equipotent concentrations (1.2 MAC) of desflurane, formerly I-653, a new inhaled anesthetic, or isoflurane. Succinylcholine (1 and 2 mg/kg), atracurium (0.6 mg/kg), and atropine (5 micrograms/kg) plus edrophonium (5 mg/kg) had no cardiovascular effects. Fentanyl was given in amounts that decreased MAC for the inhaled anesthetics by 25%-35%. A dose of 50 micrograms/kg IV had no cardiovascular effects during either anesthetic, whereas 100 micrograms/kg IV modestly increased systemic vascular resistance without changing other variables. Naloxone (100 micrograms/kg IV) during infusion of fentanyl decreased systemic vascular resistance and increased cardiac output during both desflurane and isoflurane anesthesia, increased heart rate during only isoflurane anesthesia, and did not affect mean arterial blood pressure during either anesthetic. Thiopental (2.5 and 5.0 mg/kg IV) decreased mean aortic blood pressure, cardiac output, stroke volume, and systemic vascular resistance during both anesthetics without altering heart rate or left- or right-sided cardiac filling pressures. The addition of 60% nitrous oxide caused no cardiovascular changes during desflurane anesthesia, but increased systemic vascular resistance and decreased cardiac output and stroke volume during isoflurane without altering heart rate or cardiac preload. We conclude that the usual clinical doses of adjuvants commonly administered during anesthesia have no untoward cardiovascular actions during 1.2 MAC desflurane or isoflurane anesthesia in swine.

The effect of halothane, isoflurane, or sufentanil on the hypertensive response to cerebellar retraction during posterior fossa surgery

The blood pressure (BP) response to cerebellar retraction during microvascular decompression of the fifth cranial nerve was investigated in 26 ASA physical status 2 or 3 patients with trigeminal neuralgia. One surgeon performed all operations. To determine the effect of three anesthetic techniques on the BP response, patients were randomly assigned to receive halothane, isoflurane, or sufentanil in sufficient doses with 60% nitrous oxide to achieve a precerebellar retraction systolic BP that was 10-20% below the average ward systolic BP (as per standard clinical practice). The resultant doses were halothane 1.65 +/- 0.27 (mean +/- SD) MAC, isoflurane 1.56 +/- 0.17 MAC (P greater than 0.05), and sufentanil 2.7 micrograms/kg (MAC values include 0.6 MAC contribution from 60% nitrous oxide). In all patients BP increased during the cerebellar retractor placement period compared with the preretractor placement period (P less than 0.05). The peak increase in systolic BP in response to cerebellar retraction was 17 +/- 6 mmHg for halothane, 38 +/- 20 mmHg for isoflurane, and 26 +/- 19 mmHg for sufentanil. The difference between halothane and isoflurane was significant (P less than 0.05). Mean and diastolic BP showed similar significant differences. The authors conclude that halothane attenuates the hypertensive response to cerebellar retraction more than isoflurane when administered in approximately 1.6 MAC concentrations (MAC value includes contribution from nitrous oxide).

Cardiovascular safety and actions of high concentrations of I-653 and isoflurane in swine

The ratio of lethal-to-anesthetic concentration can be used to define the margin of safety of an inhaled anesthetic. In mechanically ventilated swine the fatal concentration of I-653, a new inhaled anesthetic, was 23.9 +/- 0.06% (mean +/- SE), and of isoflurane, 6.22 +/- 0.23%. The ratio of fatal anesthetic concentration-to-MAC for I-653 (2.45 +/- 0.11) was less than that determined for isoflurane (3.02 +/- 0.13; P less than 0.01) but relatively greater than that reported previously for other inhaled anesthetics. As with other inhaled anesthetics, the concentration of I-653 causing cardiovascular collapse exceeds that producing apnea, making cardiovascular collapse during spontaneous ventilation unlikely. Mean aortic blood pressure and cardiac output decreased as linear functions of anesthetic concentration. Values for these variables for isoflurane were greater than those for I-653 at concentrations exceeding 1.5 MAC. Heart rate, blood lactate concentration, and base-deficit did not change with anesthetic depth. Mixed venous PO2, mixed venous oxyhemoglobin saturation, and the ratio of oxygen transport to oxygen consumption remained at or above values in conscious swine but decreased similarly with both anesthetics when anesthetic concentration increased to within 0.5 MAC of the fatal concentration. Thus, the latter three variables, reflecting the fraction of delivered oxygen that is consumed, and "mean" tissue PO2 appear to be useful indices of anesthetic concentrations approaching those producing cardiovascular collapse.(ABSTRACT TRUNCATED AT 250 WORDS)

Epinephrine-induced premature ventricular contractions and changes in arterial blood pressure and heart rate during I-653, isoflurane, and halothane anesthesia in swine

I653 is a new inhalation anesthetic having especially desirable recovery characteristics because of its very low blood and tissue solubility. Investigations of its cardiovascular and electroencephalographic effects have revealed actions similar to those of isoflurane. However, these studies did not evaluate the potential of I653 to predispose the heart to epinephrine-induced arrhythmias. In this investigation, we studied eight domestic swine to compare the effects of I653 with those of other anesthetics on the cardiac arrhythmogenic actions of intravenously infused epinephrine. I653, isoflurane, and halothane each were given, on separate days, at 0.7-0.8 and at 1.1-1.2 MAC. The rate of infusion of epinephrine needed to produce premature ventricular contractions (PVCs) when the animals were anesthetized with I653 (6.9 +/- 0.7 and 6.6 +/- 0.9 micrograms.kg-1.min-1 at 0.8 and 1.2 MAC) did not differ from that during isoflurane anesthesia (5.7 +/- 1.1 and 6.0 +/- 1.0 micrograms.kg-1.min-1 at 0.7 and 1.1 MAC), but was greater than that required during halothane anesthesia (1.3 +/- 0.2 and 1.1 +/- 0.3 micrograms.kg-1.min-1 at 0.7 and 1.1 MAC). Similar mean arterial blood pressures and heart rates resulted from like infusions of epinephrine during I653 and isoflurane anesthesia. PVCs occurred at lesser infusion rates of epinephrine and at lower mean arterial blood pressures and heart rates with halothane than with I653 or isoflurane. Anesthetic concentration, over the range studied, did not alter the infusion rate of epinephrine required to produce arrhythmias with any anesthetic. The authors conclude that I-653 and isoflurane have similar properties with respect to epinephrine-induced arrhythmias and increases in heart rate and arterial blood pressure.

Metabolism of I-653 and isoflurane in swine

Fluoride ion concentrations were measured in plasma samples taken from chronically instrumented domestic swine before, immediately after, and 4 hours after exposure to either I-653 or isoflurane. Each anesthetic was administered at concentrations between 0.7 and 1.6 MAC and the total dose of anesthetic given was approximately 5.5 MAC hours for each agent. Plasma fluoride ion concentrations immediately after and 4 hours after exposure to isoflurane were approximately three times greater than values obtained in awake swine before anesthesia. In contrast, swine given I-653 had no detectable elevation in plasma fluoride concentration immediately after anesthesia, but a 17% (P less than 0.05) increase in plasma fluoride ion concentration 4 hours after anesthesia. These results imply that I-653 is metabolized less than is isoflurane in swine.