Can inflammatory pain prevent the development of acute tolerance to alfentanil?

Constant pain could, in principle, counteract mobilization of antianalgesia systems and prevent the development of acute tolerance to the analgesic effects of opioids. We sought to determine whether a tonic nociceptive input caused by inflammation inhibits the development of acute tolerance to alfentanil. The inflammation was induced by injection of carrageenan into the rat hind paw. A threshold of motor response to increasing pressure on the paw was used to determine analgesia. Alfentanil was administered IV with an infusion algorithm designed to maintain a constant plasma level of opioid for 4 h. The degree of acute tolerance was determined on the basis of decline in the level of analgesia. The continuous decline of the analgesic effect from its peak at 30 min to the end of the 4-h infusion period was profound, despite the constant-rate infusion of alfentanil. The degrees of decline were very similar in rats with and without carrageenan-induced inflammation (from 242 +/- 31 to 154 +/- 20 g, P < 0.0001; and from 242 +/- 33 to 148 +/- 14 g, P < 0.0001, respectively). The results suggest that inflammatory nociceptive input does not prevent the development of acute tolerance to opioid-induced analgesia measured as an increased reaction threshold to painful pressure. We conclude that acute tolerance to the analgesic effect of opioids is profound and develops very rapidly, even in the presence of constant nociceptive input.

IMPLICATIONS

We examined whether inflammatory pain can prevent the rapid decline in analgesic effectiveness (acute tolerance) of alfentanil during its IV infusion. We found that acute tolerance to the analgesic effect of alfentanil, in the presence of constant pain caused by inflammation, develops as rapidly as without it.

The effect of ketamine on opioid-induced acute tolerance: can it explain reduction of opioid consumption with ketamine-opioid analgesic combinations?

Ketamine administered intraoperatively in very small doses reduces postoperative opioid consumption. We suggest that this effect is the result of attenuation of acute tolerance to the analgesic effect of opioids. We sought to demonstrate that acute tolerance induced by alfentanil infusion can be attenuated by a dose of ketamine that is too small to produce a direct antinociceptive effect. The experiments were conducted in rats with the use of an infusion algorithm designed to maintain a constant plasma level of the opioid for 4 h. The degree of acute tolerance was determined on the basis of decline in the level of analgesia measured with a tail compression test. Ketamine (10 mg/kg) did not change the baseline pain threshold and did not increase the peak of alfentanil-induced analgesia. At the same time, it attenuated the development of acute tolerance to analgesia during alfentanil infusion and suppressed rebound hyperalgesia observed the day after the infusion. These effects were similar to those observed with dizocilpine (0.1 mg/kg). The development of acute tolerance to analgesia induced by the infusion of an opioid can be attenuated by ketamine administered in doses that are not large enough to provide a direct antinociceptive effect. Therefore, ketamine has the potential to reduce opioid consumption even in subanalgesic doses.

IMPLICATIONS

Ketamine attenuated the development of acute tolerance to analgesia during alfentanil infusion and suppressed rebound hyperalgesia observed the day after the infusion.

Acute tolerance to continuously infused alfentanil: the role of cholecystokinin and N-methyl-D-aspartate-nitric oxide systems

To test the role of cholecystokinin (CCK) and N-methyl-D-aspartate-nitric oxide (NMDA-NO) systems in the development of acute tolerance to analgesia during alfentanil IV infusion, we conducted experiments in rats with the use of an infusion algorithm designed to maintain a constant plasma level of the opioid for 4 h. The degree of acute tolerance was determined on the basis of decline in the level of analgesia measured with a tail compression test. CCK(B) receptor antagonists (proglumide, CI-988, and L-365,260) and NMDA-NO cascade inhibitors (dizocilpine and NO synthase inhibitor) were administered before the start of alfentanil infusion. Use of 30 mg/kg proglumide, 10 mg/kg CI-988, and 1 mg/kg L-365,260 attenuated acute tolerance at 1 h of alfentanil infusion by approximately 60%, 55%, and 70%, respectively, and by the end of 4-h infusion by 50%, 50%, and 25%, respectively. Use of 0.1 mg/kg dizocilpine and 10 mg/kg N(G)-nitro-L-arginine methyl ester attenuated acute tolerance at 1 h of alfentanil infusion by approximately 65% and 65% and by the end of 4-h infusion by 30% and 0%, respectively. Comparison of the results with CCK(B) receptor antagonists and inhibitors of NMDA-NO cascade demonstrates that both groups of drugs provide more or less similar degrees of attenuation of acute tolerance to the antinociceptive effect of alfentanil, and none of these drugs completely prevents tolerance development.

IMPLICATIONS

The mechanism of acute tolerance to the analgesic effect of alfentanil depends on participation of multiple systems of adaptation that include cholecystokinin(B) receptors and N-methyl-D-aspartic acid-nitric oxide cascade. Drugs that inhibit function of these systems attenuate tolerance development.

Hyperalgesia caused by nerve transection: long-lasting block prevents early hyperalgesia in the receptive field of the surviving nerve

The aim of our study was to test the hypothesis that a long-lasting N-butyl tetracaine nerve block (>2 wk) would be much more effective in the prevention of hyperalgesia caused by nerve transection than the short-lasting lidocaine block. The study was performed with the use of the saphenous nerve section model in rats. The saphenous nerve was exposed and injected with saline, lidocaine (37 mM), or N-butyl tetracaine (37 mM). Ten minutes later, the nerve was transected in some of the rats. The development of mechanical hyperalgesia (pressure threshold) of the hindpaw was assessed during a 5-wk period. In rats with saphenous nerve transection without nerve block (saline injection), 3 h after the transection, the pressure threshold decreased by approximately 30% (from 175+/-11 g to 122+/-23 g, P < 0.0001); the threshold increased somewhat the next day, then it remained stable for 2 wk, with a slow process of recovery afterward. N-butyl tetracaine block without nerve transection caused a slow-developing decrease in the pressure threshold with the first statistically significant change at the sixth day. The comparison of the preventive effects of lidocaine and N-butyl tetracaine blocks on early hyperalgesia caused by nerve transection demonstrated that both lidocaine and N-butyl tetracaine prevented hyperalgesia 3 h after the transection. However, the protective effect of lidocaine disappeared the next day. In contrast, N-butyl tetracaine prevented early hyperalgesia for almost a week. The slow-developing late hyperalgesia caused by long-lasting nerve block makes it impossible to study the protective effect of such a block on late hyperalgesia caused by axotomy. As far as early hyperalgesia is concerned, the preventive effect of the N-butyl tetracaine was much longer than that of lidocaine and continued for approximately 1 wk.

IMPLICATIONS

A long-lasting nerve block can prevent early hyperalgesia caused by nerve transection.

Isobolographic analysis of propofol-thiopental hypnotic interaction in surgical patients

Drugs acting via the same mechanism interact additively, whereas a supraadditive effect can result from an interaction of drugs with different mechanisms of action. Hypnotic midazolam-propofol and midazolam-thiopental interactions are supraadditive. In contrast to midazolam, the mechanisms of actions of propofol and thiopental are quite similar. The aim of this study was to test the hypothesis that similarity in the mechanisms of action of propofol and thiopental results in the additive hypnotic interaction. We studied the hypnotic effects of thiopental, propofol, and their combinations in 150 unpremedicated patients in a randomized, double-blind fashion. The ability to open eyes on command was used as an end point. Dose-response curves for the drugs given separately and in combinations at three different dose ratios between the drugs were determined by using a probit procedure, and the 50% effective dose values were compared by using isobolographic and algebraic (fractional) analysis. The hypnotic propofol-thiopental combination was additive with all dose ratios between components of the combination. The absence of propofol-thiopental synergy, as demonstrated with midazolam-thiopental or propofol-midazolam combinations, suggests that the mechanisms underlying the hypnotic effects of propofol and thiopental, in contrast to the above combinations with midazolam, are very similar and could be identical.

IMPLICATIONS

The propofol-thiopental hypnotic interaction is additive.

The peripheral effect of fentanyl on postoperative pain

The clinical value of the analgesic effect of opioids administered peripherally (except for intraarticular administration) has not been clearly demonstrated. The aim of this study was to test the hypothesis that fentanyl, added to a local anesthetic for wound infiltration, can enhance postoperative analgesia via a peripheral mechanism. Patients with inguinal herniorrhaphy performed under spinal anesthesia were randomly assigned to one of two groups (n = 10 each). At the end of surgery, the wound was infiltrated with 10 mL of lidocaine 0.5% and fentanyl 0.001% (10 microg) in one group; in the other group, the wound was infiltrated with 10 mL of lidocaine 0.5% alone (and fentanyl 10 microg IM contralaterally). The following variables were determined in a double-blind manner: the duration of anesthesia (response to a von Frey filament), the duration of analgesia (time to mild postoperative pain), postoperative meperidine consumption, intensity visual analog scale of spontaneous and movement-associated pain 24 h after surgery, and wound pain threshold 24 h after surgery (pressure algometry). The addition of fentanyl for wound infiltration enhanced the duration of anesthesia (130+/-37 vs 197+/-27 min; P < 0.001) and decreased the intensity of spontaneous (50+/-17 vs 19+/-18 mm; P < 0.002) and movement-associated (56+/-15 vs 26+/-21 mm; P < 0.002) pain 24 h postoperatively. Differences between groups for other variables were not statistically significant. Fentanyl added to a local anesthetic for wound infiltration after spinal anesthesia can enhance postoperative analgesia by a peripheral mechanism.

IMPLICATIONS

Fentanyl can enhance analgesia by a peripheral mechanism. Added to a local anesthetic for wound infiltration, it may be of benefit for the relief of postoperative pain.

Rapid development of tolerance to analgesia during remifentanil infusion in humans

Studies in experimental animals have demonstrated a rapidly developing acute tolerance to the analgesic effect of opioids administered by continuous i.v. infusion. The aim of the present study was to determine whether acute tolerance plays an important role in the analgesic effect of remifentanil provided by i.v. infusion to humans. The analgesic effect of remifentanil, infused at a constant rate of 0.1 microg x kg(-1) x min(-1) for 4 h, was evaluated by measuring pain tolerance with thermal (2 degrees C water) and mechanical (pressure) noxious stimulations in 13 paid volunteers. The constant-rate infusion of remifentanil resulted in a threefold increase in pain tolerance with both tests. After reaching its maximum in 60-90 min, the analgesic effect of remifentanil began to decline despite the constant-rate infusion, and after 3 h of infusion, it was only one fourth of the peak value. A comparative rate in the development of acute tolerance measured in terms of time to 50% recovery during infusion was 129 +/- 27 min (mean +/- SD) with the cold water test and 138 +/- 39 min with the pressure test. We conclude that the development of tolerance should be included in the calculations for target-controlled infusions.

IMPLICATIONS

Our study shows that tolerance to analgesia during remifentanil infusion is profound and develops very rapidly. The administration of opioids during anesthesia based on target-controlled infusions should include corrections for the development of tolerance.

Effect of subarachnoid bupivacaine block on anesthetic requirements for thiopental in rats

BACKGROUND

Subarachnoid bupivacaine blockade has been reported to reduce thiopental and midazolam hypnotic requirements in patients. The purpose of this study was to examine if local anesthetically induced lumbar intrathecal blockade would reduce thiopental requirements for blockade of motor responses to noxious and nonnoxious stimuli in rats.

METHODS

After intrathecal and external jugular catheter placement, rats were assigned randomly to two groups in a crossover design study, with each rat to receive either 10 microl of 0.75% bupivacaine or 10 microl of normal saline intrathecally. The doses of intravenously administered thiopental required to ablate the eyelid reflex, to block the withdrawal reflex of a front limb digit, and to block the corneal reflex were compared. In two separate groups of animals, hemodynamic parameters and concentrations of thiopental in the brain were compared between intrathecally administered bupivacaine and saline.

RESULTS

The thiopental dose required to block the described responses was decreased with intrathecally administered bupivacaine versus intrathecally administered saline from (mean +/- SD) 40 +/- 5 to 24 +/- 4 mg/kg (P < 0.001) for the eyelid reflex, from 51 +/- 6 to 29 +/- 6 mg/kg (P < 0.005) for front limb withdrawal, and from 67 +/- 8 to 46 +/- 8 mg/kg (P < 0.01) for the corneal reflex. The concentration of thiopental in the brain at the time of corneal reflex blockade for the group given bupivacaine was significantly lower than in the group given saline (24.1 vs. 35.8 microg/g, P = 0.02).

CONCLUSION

This study demonstrates that lumbar intrathecally administered local anesthetic blockade decreases anesthetic requirements for thiopental for a spectrum of end points tested. This effect is due neither to altered pharmacokinetics nor to a direct action of the local anesthetic on the brain; rather, it is most likely due to decreased afferent input.

Effect of prolonged nerve block on inflammatory hyperalgesia in rats: prevention of late hyperalgesia

BACKGROUND

Recent evidence suggests that the duration of the nociceptive block may be an important factor in determining the effect of the block on injury-induced hyperalgesia after block resolution. The authors examined whether a tonicaine nerve block lasting for 12 to 16 h could prevent late inflammatory hyperalgesia.

METHODS

Inflammatory hyperalgesia was induced by injection of carrageenan into the rat paw. A threshold of motor response to increasing pressure was determined for the injected paw, contralateral paw, and tail The development of edema of the paw and an increase in paw temperature also were determined. The block was achieved by simultaneous percutaneous injections of tonicaine (a new long-acting anesthetic agent) or lidocaine at the sciatic nerve (greater trochanter level) and the saphenous nerve (midthigh level).

RESULTS

Carrageenan without nerve block caused a profound primary (injected paw) and secondary (contralateral paw and tail) hyperalgesia that lasted for 3-5 days. Tonicaine nerve block administered before carrageenan completely prevented primary and secondary hyperalgesia. Tonicaine block administered 5 h after carrageenan injection reversed secondary hyperalgesia and prevented the development of late (> or = 24 h) primary and secondary hyperalgesia. Edema and temperature of the paw were not significantly affected by the nerve block administered before or after carrageenan.

CONCLUSIONS

A prolonged nerve block (12-16 h) can prevent the development of long-lasting (3-5 days) inflammatory hyperalgesia. Prevention of late hyperalgesia can be provided not only by the preinjury block but also by the postinjury block administered when hyperalgesia is already well established.

Pharmacokinetic nature of tachyphylaxis to lidocaine: peripheral nerve blocks and infiltration anesthesia in rats

Tachyphylaxis to peripheral neural blockade was determined with repeated injections of a constant dose of lidocaine in three experimental models: sciatic nerve block, produced by intraneural or extraneural injections, and infiltration anesthesia. A decrease in the duration of the subsequent blocks was used as the index of tachyphylaxis development. The anesthetic content in the nerve or skin was determined using radiolabeled lidocaine. Repeated injections of a constant dose of lidocaine resulted in a marked decrease in the duration of the blocks. Accelerated decline in lidocaine content of nerve or skin was observed with repeated blocks. Our data show that tachyphylaxis rapidly develops with both sciatic nerve blocks and infiltration anesthesia. The data also suggest that the mechanism is largely pharmacokinetic in nature.

Anesthetic interactions following bolus injections

The article presents two main principles of anesthetic interactions: (1) In contrast to the interactions of inhalational anesthetics (when deviations from additivity are debatable), profound synergistic and antagonistic anesthetic interactions between different classes of intravenous drugs are possible due to the difference in the mechanism of their action, (2) A combination of intravenous anesthetics may provide different outcomes (addition, synergism, or antagonism) regarding different components of anesthesia (eg, unconsciousness or movement to noxious stimulation) because the components, even if induced by one anesthetic drug, have different underlying mechanisms.

Effect of midazolam on development of acute tolerance to alfentanil: the role of pharmacokinetic interactions

This study in rats was performed to explore whether the inhibitory effect of midazolam on the development of acute tolerance to the analgesic effect of alfentanil is due to pharmacokinetic mechanisms. Analgesia was determined with tail-compression and hot-plate tests. Alfentanil and midazolam concentrations in plasma and the brain were measured using a radioimmunoassay and chromatographic technique, respectively. After the 4-h period of alfentanil administration (155 microg x kg(-1) x h(-1) after a 50-microg/kg bolus), when acute tolerance had already developed, midazolam (2-mg/kg bolus) enhanced the alfentanil-induced analgesia by 120% (P < 0.001) with the tail-compression test and 76% (P < 0.01) with the hot-plate test. At the height of midazolam-induced enhancement of the analgesic effect of alfentanil, the measurements of the alfentanil and midazolam plasma and brain concentrations did not demonstrate any significant changes in the drugs' concentrations. The results confirm that midazolam attenuates the development of acute tolerance to the analgesic effect of alfentanil and indicate that this interaction is not pharmacokinetic.

Time course characteristics of acute tolerance development to continuously infused alfentanil in rats

The time course of the development of acute tolerance to alfentanil was characterized in rat experiments using various algorithms of continuous infusion of the drug. To maintain analgesia at a constant level (tail compression test), alfentanil use was steadily increased: Time to 50% increase in alfentanil use was 255 +/- 98 min with analgesia maintained at a level of 50% increase in the pressure threshold, and 203 +/- 116 min with analgesia maintained at a level of 200% increase. With the use of the alfentanil infusion regimen designed to rapidly achieve and maintain the constant alfentanil plasma concentration (120 ng/mL), the rate of development of acute tolerance, measured in terms of time to 50% recovery during continuing infusion, was 81 +/- 26 min with the compression test and 88 +/- 38 min with the hot-plate test, but 374 +/- 46 min with the rotarod test (P < 0.0001). Thus we demonstrated rapid development of acute tolerance to continuously infused alfentanil with different methods of tolerance assessment. The results also indicate that the time course of tolerance development does not depend on the type of noxious stimulation (mechanical versus thermal) if the response is organized predominantly supraspinally, and that acute tolerance to the direct effect of alfentanil on motor functions does not mask the rapid development of tolerance to its analgesic effect.

Ketamine enhances local anesthetic and analgesic effects of bupivacaine by peripheral mechanism: a study in postoperative patients

Patients with unilateral (n = 14) and bilateral (n = 4) herniorrhaphy participated in this study. With bilateral herniorrhaphy, at the end of the surgery, the wound was infiltrated with a solution of bupivacaine 0.5% and ketamine 0.3% on one side and a solution of bupivacaine 0.5% only, on the other. With unilateral herniorrhaphy, the patients were randomly assigned to one of two groups (n = 7). One group at the end of the surgery received the infiltration with a solution of bupivacaine 0.5% and ketamine 0.3%, the other group received the infiltration with a solution of bupivacaine 0.5% only. The duration of the local anesthetic (response to a von Frey filament) and postoperative analgesic (time to mild spontaneous pain) effects of the infiltrations, as well as wound pain threshold 24 h after surgery (pressure algometry), were determined. In patient with unilateral herniorrhaphy, the addition of ketamine for wound infiltration enhanced the duration of infiltration anesthesia (206 +/- 76 versus 343 +/- 108 min, P < 0.02) and analgesia (240 +/- 45 versus 420 +/- 151 min, P < 0.03). Similar enhancement of the local anesthetic effect was observed in patients with bilateral herniorrhaphy. The increase in pain threshold to pressure on the wound with the addition of ketamine was evident in bilateral herniorrhaphy patients and also with a combination of bilateral and unilateral results (1.39 +/- 0.40 versus 2.35 +/- 0.92 kg, P < 0.02). In the group of five volunteers, the subcutaneous infiltration with 0.3% ketamine produced a local anesthetic effect lasting only 10-20 min. The results indicate that ketamine acting via a peripheral mechanism can profoundly enhance anesthetic and analgesic actions of a local anesthetic administered for infiltration anesthesia.

Alfentanil dose-response relationships for relief of postoperative pain

The aim of this study was to characterize within-patient alfentanil dose-response curves for the relief of spontaneous postoperative pain and to test the closeness of relationships 1) between pain intensity and alfentanil analgesic requirements, and 2) between alfentanil requirements for analgesic and nonanalgesic (sedative and miotic) effects. The effects of alfentanil were studied in 23 patients after elective abdominal surgery. During a 40- to 60-min testing session, the patient received two intravenous (i.v.) injections of saline (placebo) and up to six 3-micrograms/kg increments of alfentanil at 5-min intervals. The patient rated the pain intensity with a visual analog scale, and in a subgroup of 15 patients the blinded observer rated patients' sedation and measured pupil diameter. Spontaneous postoperative pain was completely relieved in all patients with cumulative doses of alfentanil ranging from 6 to 8 micrograms/kg. The within-patient alfentanil dose-analgesic response curves were primarily quantal in nature: a precipitous decrease in pain intensity (representing > or = 2/3 of the total effect) after the injection of only one of the increments of alfentanil was observed in 15 of 23 patients. When the analgesic effect of alfentanil was presented as the collective response of a group of individuals, the quantal nature of the response was concealed by the wide interindividual variability of the response. No statistically significant relationship was noted between predrug pain intensity and the cumulative dose of alfentanil necessary to produce pain relief. A strong correlation was found between interpatient variabilities in the analgesic and sedative effects of alfentanil (r = 0.75, P < 0.002). At the same time, the relationship between alfentanil requirements for pain relief and that for pupil constriction did not demonstrate any significant correlation. The results suggest that, in a population of patients with postoperative pain, the intensity of spontaneous pain cannot be the primary factor determining the dose of alfentanil necessary for its relief.

Effect of epidural bupivacaine block on midazolam hypnotic requirements

BACKGROUND AND OBJECTIVES

The aim of the study was to determine the effect of epidural bupivacaine block on midazolam hypnotic requirements.

METHODS

Sixty unpremedicated, ASA physical status I or II male patients, aged 45-65 years and scheduled for elective lower abdominal, pelvic, or lower limb surgery participated in this randomized, double-blind, placebo-controlled study. The study population was composed of three groups: Before administration of midazolam, patients in the first group received an intramuscular injection of 15 mL saline (M), those in the second group an intramuscular injection of 15 mL 0.5% bupivacaine (MIB), and those in the third group an epidural injection of 15 mL 0.5% bupivacaine at the L3-L4 level (MEB). Loss of the ability to respond to verbal command was used as an endpoint for the effect of midazolam injected intravenously in predetermined doses (five patients per dose) 30 minutes after a bupivacaine (or saline) injection. Midazolam dose-response curves were obtained by probit analysis.

RESULTS

The midazolam median effective dose values for the hypnotic effect were 0.20 mg/kg (95% confidence limit, 0.10-0.27 mg/kg) in the M group; 0.10 mg/kg (0.06-0.22 mg/kg) in the MIB group; and 0.04 mg/kg (0.03-0.07 mg/kg) in the MEB group. The differences between the midazolam median effective dose values had high levels of significance: P < .00001 for MEB versus M, P < .002 for MEB versus MIB, and P < .01 for MIB versus M.

CONCLUSIONS

Epidural bupivacaine block profoundly decreases midazolam hypnotic requirements. The most likely explanation for this effect is the reduction in afferent input induced by the block.

Subarachnoid bupivacaine blockade decreases midazolam and thiopental hypnotic requirements

STUDY OBJECTIVE

To test the hypothesis that subarachnoid bupivacaine blockade decreases hypnotic requirements for thiopental sodium and midazolam.

DESIGN

Randomized, double-blind, placebo-controlled study.

SETTING

Teaching hospital.

PATIENTS

53 nonpremedicated ASA physical status I and II adult male patients scheduled for elective lower abdominal, pelvic, or lower limb surgery.

INTERVENTIONS

Intravenous injections of midazolam or thiopental were administered with or without subarachnoid bupivacaine blockade (12.5 mg) at the L3-L4 level. Thiopental or midazolam hypnotic requirements were determined using loss of ability to open eyes in response to verbal command as an endpoint. The thiopental requirements were determined by titration; the midazolam requirements were determined from dose-response curves obtained with bolus injections of predetermined doses of the drug.

MEASUREMENTS AND MAIN RESULTS

Subarachnoid bupivacaine blockade decreased the hypnotic dose of thiopental from 3.40 +/- 0.68 mg/kg (mean +/- SD) with a dose range of 2.3 to 4.5 mg/kg (intramuscular saline) to 2.17 +/- 0.48 mg/kg with a dose range of 1.3 to 2.8 mg/kg (p < 0.005 for the difference). The ED50 value of midazolam decreased with the bupivacaine blockade, from 0.23 mg/kg (95% confidence limits: 0.08 to 0.38 mg/kg) to 0.06 mg/kg (0.01 to 0.14 mg/kg), with p < 0.0001 for the difference.

CONCLUSION

Subarachoid bupivacaine blockade decreases hypnotic requirements for both thiopental and midazolam. The results suggest that the reduction in hypnotic requirements is due to the decrease in afferent input induced by spinal anesthesia.

Preemptive effect of fentanyl and ketamine on postoperative pain and wound hyperalgesia

The aim of this study was to test the hypothesis that the induction and maintenance of anesthesia with the use of fentanyl or ketamine reduces postoperative pain and wound hyperalgesia beyond the period when these effects can be explained by the direct analgesic action of these drugs. Twenty-seven patients scheduled for elective hysterectomy were investigated in a double-blind, randomized study. Patients were divided into three groups. In the fentanyl group, anesthesia was induced with fentanyl 5 micrograms/kg combined with thiopental 3 mg/kg and maintained with isoflurane and fentanyl 0.02 microgram.kg-1.min-1. In the ketamine group, anesthesia was induced with ketamine 2 mg/kg in combination with thiopental 3 mg/kg and maintained with isoflurane and ketamine 20 micrograms.kg-1.min-1. In the control group, anesthesia was induced with thiopental 5 mg/kg and maintained with isoflurane only. Patients in all three groups received identical postoperative pain treatment. The intensity of spontaneous incisional pain and movement-associated pain was measured with a visual analog self-rating method. The surgical wound hyperalgesia was assessed by measuring pain threshold to pressure on the wound by using an algometer, and also by measuring the intensity of pain to suprathreshold pressure on the wound with the visual analog self-rating method. Forty-eight hours after surgery, the pain threshold was 0.90 +/- 0.06 kg in controls, 1.69 +/- 0.19 kg (P < 0.001) in the fentanyl group, and 1.49 +/- 0.15 kg (P < 0.01) in the ketamine group.(ABSTRACT TRUNCATED AT 250 WORDS)

Triple anesthetic combination: propofol-midazolam-alfentanil

The hypnotic effects of propofol, midazolam, alfentanil, and their binary and triple combinations, were studied in 130 unpremedicated patients in a randomized, double-blind fashion. The ability to open eyes on verbal command was used as an end-point. Dose-response curves for the three drugs given separately and in combination were determined with a probit procedure and the ED50 values were compared with an isobolographic analysis. The ratios of a single-drug fractional dose (ED50 = 1.0) to a combined fractional dose (in fractions of single-drug ED50 values) indicating the degree of superadditivity (synergism) were: 1.4 (P < 0.05) for propofol-alfentanil, 1.8 (P < 0.0005) for midazolam-propofol, 2.8 (P < 0.0001) for midazolam-alfentanil, and 2.6 (P < 0.0001) for propofol-midazolam-alfentanil. The results indicate that the propofol-midazolam-alfentanil interaction produces a profound hypnotic synergism which is not significantly different from that of the binary midazolam-alfentanil combination.

Metoclopramide decreases thiopental hypnotic requirements

We compared the effect of metoclopramide (MCA) with droperidol (DPD) on thiopental hypnotic requirements in 96 unpremedicated female patients. The study was randomized, double-blind, and placebo-controlled. The response to the verbal command was used as an end-point of anesthesia. Two methods of determination of thiopental hypnotic requirements were used: titration (infusion of thiopental at a rate of 0.5 mg.kg-1 x min-1) until the end-point was reached and construction of the dose-response curve based on bolus injections of predetermined doses of thiopental (with the use of probit analysis). The ED50 value of thiopental determined by probit analysis was reduced after the 0.2 mg/kg MCA administration by 44% (P < 0.0001). The thiopental hypnotic requirements obtained with the titration method were as follows: 5.3 +/- 0.3 mg/kg in control, 4.5 +/- 0.2 mg/kg (delta 14%, P < 0.03) with 0.1 mg/kg MCA, 3.2 +/- 0.2 mg/kg (delta 39%, P < 0.0001) with 0.2 mg/kg MCA, and 2.9 +/- 0.2 mg/kg (delta 45%, P < 0.0001) with 0.4 mg/kg MCA. DPD decreased thiopental hypnotic requirements almost to the same degree as MCA, with the ceiling effect observed at doses of 0.01 and 0.02 mg/kg (delta 44%, P < 0.0001). The results indicate that MCA causes a profound decrease in thiopental hypnotic requirements. The similarity between the thiopental sparing effects of MCA and DPD suggests that the blockade of D2 receptors is the main mechanism underlying this effect.

Barbiturate-benzodiazepine interactions at the gamma-aminobutyric acidA receptor in rat cerebral cortical synaptoneurosomes

Combinations of benzodiazepines (midazolam and diazepam) with barbiturates (pentobarbital and phenobarbital) exhibit synergistic (supra-additive) hypnotic interactions in rats. Because both benzodiazepines and barbiturates interact with the gamma-aminobutyric acidA (GABAA) receptor complex, we have tested the hypothesis that these supra-additive hypnotic interactions are due to a synergistic effect on Cl- conductance subsequent to binding at allosterically coupled sites on the GABAA receptor ionophore complex. Equilibrium binding and 36Cl- flux measurements were performed under nearly identical conditions using rat brain cerebrocortical synaptoneurosomes. The benzodiazepines and barbiturates alone both allosterically enhance binding of [3H]muscimol to comparable, but modest, extents (range = 18%-32% enhancement). Isobolographic analysis reveals that combinations of benzodiazepines and barbiturates do in fact produce a synergistic enhancement of [3H]muscimol binding. Paradoxically, this effect is not translated into a synergistic enhancement of muscimol-stimulated 36Cl- flux. Because the positively cooperative interactions between benzodiazepines and barbiturates, as demonstrated both behaviorally and by binding measurements, are not reflected in enhanced Cl- conductance, the mechanistic basis for hypnotic synergism may involve other non-GABAergic components.

Pentobarbital-morphine anesthetic interactions in terms of intensity of noxious stimulation required for arousal

BACKGROUND

Previous reports suggest that the outcome (synergism, antagonism, summation) of opioid-barbiturate interactions may depend on the depth of anesthesia. One aim of the present study was to determine whether pentobarbital, alone and in combination with morphine, blocks awakening caused by noxious stimulation in a dose-related manner: the more intense the noxious stimulation, the more pentobarbital is required to suppress the response. A second aim of the study was to determine whether the pentobarbital-morphine anesthetic interaction depends on the depth of anesthesia measured in terms of intensity of noxious stimulation required for behavioral arousal (recovery of the righting reflex).

METHODS

Experiments were performed on rats, with the measure of anesthetic effect being suppression of the righting reflex. The noxious stimulus was pressure on the tail at four levels of intensity: 0.0, 0.25, 2.5, and 3.3 kg, generated with an Analgesy-Meter. Pentobarbital and morphine were injected intravenously via chronically implanted catheters. Dose-response curves for pentobarbital given alone and in combination with morphine were determined (by probit analysis) separately for each of the pressure levels.

RESULTS

Pentobarbital, alone and in combination with morphine, blocked awakening caused by noxious stimulation of different intensities in a dose-related fashion so that more anesthetic was required to block awakening with more intense stimulation. The pentobarbital ED50 values were: 12.0, 19.5, 22.7, and 24.3 mg/kg for 0.0, 0.25, 2.5, and 3.3 kg pressure, respectively. The addition of morphine (1 mg/kg) reduced the pentobarbital ED50 values for 0.0, 0.25, and 2.5 kg pressure by 34% (P < 0.0001), 39% (P < 0.0001), and 21% (P < 0.005), respectively. No change was seen in the pentobarbital ED50 value at the maximal (3.3 kg) pressure level.

CONCLUSIONS

The results suggest that the depth of anesthesia can be measured in terms of intensity of noxious stimulation required for arousal and that the outcome of barbiturate-opioid anesthetic interaction depends on the depth of anesthesia.

Does midazolam inhibit the development of acute tolerance to the analgesic effect of alfentanil?

Alfentanil-midazolam analgesic interactions were studied in rats with continuous infusions or bolus injections of the drugs. Analgesia was determined by measuring the threshold of motor response to noxious pressure. The continuous constant-rate infusion of alfentanil demonstrated that after an initial peak, the analgesia profoundly declined due to the development of acute tolerance. When alfentanil (250 micrograms.kg-1.h-1) was given together with midazolam (3 mg.kg-1.h-1), the decline in the analgesic effect of alfentanil was attenuated. Following the 4 h period of the constant-rate (250 micrograms.kg-1.h-1) infusion of alfentanil, when acute tolerance was already developed, midazolam (3 mg.kg-1) given as a bolus injection enhanced the alfentanil-induced anesthesia. At the same time, when alfentanil was given as a bolus injection (30 micrograms.kg-1) with or without midazolam (3 mg.kg-1) also by bolus injection, no changes were seen to indicate an enhancement of the analgesic effect of alfentanil by midazolam. The results suggest that midazolam attenuates the development of acute tolerance to the analgesic effect of alfentanil.

Tonsillectomy and adenoidectomy pain reduction by local bupivacaine infiltration in children

Experimental data in humans and animals suggest that during surgery, pain impulses enter the CNS creating a hyperexcitable state in spite of general anesthesia. In a prospective double blind study, pain levels in 22 children undergoing tonsillectomy and adenoidectomy under general anesthesia were compared from day of surgery to Day 10. Patients received pre-incisional infiltration with either bupivacaine (A) or saline (B). Subjective pain was assessed by visual analog scale, and objective pain by deglutition time (100 ml). Subjective constant pain was less (P < 0.05) in group (A) on Day 1: 16 (A) vs. 59 (B) and Day 5: 4 (A) vs. 45 (B). We conclude that local nerve blockade by bupivacaine reduces short- and long-term pain in children undergoing tonsillectomy and adenoidectomy in the presence of general anesthesia.

Locomotor activity after recovery from hypnosis: midazolam-morphine versus midazolam

This study was performed to test the hypothesis that sedation after recovery from pharmacologic hypnosis is less pronounced if hypnosis is induced with a midazolam-morphine combination compared with midazolam administered alone. Loss of the righting reflex was used as an index for the hypnotic effect and reduction of locomotor activity as an index for the sedative effect. One group of rats received midazolam (20 mg/kg i.v) and another group an equipotent (in relation to the hypnotic ef.fect) combination of midazolam (4 mg/kg i.v.) and morphine (1.3 mg/kg i.v.). The duration of loss of the righting reflex in the midazolam and midazolam-morphine groups was 30 +/- 3 and 28 +/- 2 min, respectively (mean +/- SE). The difference between the groups in locomotor activity after recovery from hypnosis was very pronounced. The locomotor activity in the midazolam-morphine group at 1 and 2 h was seven and five times greater, respectively, than in the midazolam group (P < 0.005). The profound difference in locomotor activity for the two treatment groups was explained on the basis of the difference in the outcomes of midazolam-morphine interactions with regard to hypnosis (synergism) and sedation (summation). When the animals recovered from hypnosis, the synergism of the drug interaction ceased to be a contributing factor.

Anaesthetic interaction between thiopentone and ketamine

The anaesthetic effect of thiopentone, ketamine and their combinations were studied in 150 patients of ASA Grades I and II. The ability to open eyes on command and purposeful motor response to noxious pressure on the trapezius muscle were used as endpoints of anaesthesia. Dose-response curves for thiopentone, ketamine and their combinations were determined with a probit procedure and compared with isobolographic analysis. The interaction between thiopentone and ketamine was found to be additive for both endpoints of anaesthesia.

Magnitude of acute tolerance to opioids is not related to their potency

It was suggested that for a given analgesic effect, more potent opioids may produce smaller degrees of tolerance than those with lower analgesic potency. The use of opioids with high analgesic potency to reduce the rate of tolerance development would be an important therapeutic consideration. This study tested the hypothesis that the degree of acute tolerance to the analgesic effect of opioids is inversely related to their potency. In the experiments on rats, the analgesic effects of morphine, alfentanil, and sufentanil given by a continuous 8-h infusion at a constant rate, were determined by measuring the threshold of motor response to noxious pressure on the tail. The comparative degree of acute tolerance was determined on the basis of the decline in the level of analgesia at the end of the infusion period. Morphine 4 mg.kg-1.h-1, alfentanil 0.45 mg.kg-1.h-1, and sufentanil 0.0085 mg.kg-1.h-1 caused approximately similar increases in the pain threshold. The peak of analgesia could not be maintained; it declined by 74 +/- 6% (P less than 0.0001) with morphine, 86 +/- 6% (P less than 0.0001) with alfentanil, and 92 +/- 2% (P less than 0.0001) with sufentanil. The results indicate that the infusion of alfentanil and sufentanil, which differ from morphine by higher analgesic potency (by 10-fold and more than 100-fold, respectively), results in a decline in the degree of analgesia during infusion similar to that of morphine. These data reject the hypothesis that the magnitude of acute tolerance to the analgesic action of opioid drugs following their systemic administration is inversely related to their potency.

Acute tolerance to the hypnotic effect of morphine in rats

To demonstrate the development of acute tolerance to the hypnotic effects of morphine, loss and recovery of the righting reflex with a constant-rate morphine infusion was studied in rats. In one group of animals, brain and serum concentrations of morphine were detected (radioimmunoassay) at the time of loss of the righting reflex, and in another group, at the time of the reflex recovery. The morphine infusion at a constant rate of 14 mg.kg-1.h-1 caused a loss of the righting reflex in all animals that was achieved by 2.5 h. However, this level of response could not be maintained, and at 5 h it began to decline. All animals recovered the righting reflex by the ninth hour, despite the continuing morphine infusion. The morphine brain and serum levels at the times of loss and recovery of the righting reflex were not different. The results suggest a development of acute tolerance to the hypnotic effect of morphine, which is determined primarily by pharmacodynamic mechanisms.

Midazolam potentiates thiopental sodium anesthetic induction in patients

STUDY OBJECTIVE

To test the hypothesis that midazolam potentiates thiopental sodium-induced unconsciousness.

DESIGN

Randomized, double-blind study.

SETTING

A university medical center.

PATIENTS

Fifty nonpremedicated ASA physical status I and II adult patients scheduled for eye surgery.

INTERVENTIONS

Intravenous (IV) injections of thiopental sodium in doses ranging from 1.0 mg/kg to 4 mg/kg with or without the addition of midazolam 0.02 mg/kg.

MEASUREMENTS AND MAIN RESULTS

Inability to open eyes on command was used as an end point of anesthesia and the dose-response curves were determined using a probit procedure. A dose of 0.02 mg/kg, which constitutes less than one-tenth of the hypnotic ED50 value for midazolam, potentiated thiopental sodium anesthesia. The thiopental sodium ED50 value was decreased from 2.4 mg/kg to 1.6 mg/kg (p less than 0.001). Midazolam also reduced individual variability in the response to thiopental sodium. As a result, the thiopental sodium dose that reliably induced any nonpremedicated patient decreased from 6 mg/kg (ED99 of 5.57 mg/kg) to 2.5 mg/kg (ED99 of 2.37 mg/kg).

CONCLUSIONS

A subhypnotic dose of midazolam potentiates thiopental sodium-induced unconsciousness. This effect suggests the possibility that midazolam enhances barbiturate binding.

Effect of alfentanil on hypnotic and antinociceptive components of thiopental sodium anesthesia

STUDY OBJECTIVE

To determine the effects of alfentanil on the hypnotic and antinociceptive components of thiopental sodium anesthesia.

DESIGN

Randomized double-blind study.

SETTING

Inpatients at a university-affiliated county hospital.

PATIENTS

Eighty unpremedicated ASA physical status I or II female patients aged 18 to 60 years, with a weight range of 50 to 90 kg.

INTERVENTIONS

Intravenous injection of thiopental sodium in doses ranging from 1.0 mg/kg to 6.0 mg/kg with or without the addition of alfentanil, 0.01 mg/kg.

MEASUREMENTS AND MAIN RESULTS

Dose-response curves were determined for the following three endpoints of anesthesia: eye opening in the response to voice command; eye opening in the response to noxious stimulation induced by pressure on the trapezius muscle; and purposeful movement in the response to the same type of noxious stimulation. The addition of alfentanil, 0.01 mg/kg, markedly decreased thiopental ED50 values for all three endpoints: from 2.6 mg/kg to 1.9 mg/kg (p less than 0.02) for eye opening to voice command, from 3.2 mg/kg to 1.9 mg/kg (p less than 0.0005) for eye opening to noxious pressure, and from 4.2 mg/kg to 2.4 mg/kg (p less than 0.0001) for purposeful movement to noxious pressure. The alfentanil-induced increase in thiopental potency for the antinociceptive effect was greater than that for the hypnotic effect (75% vs 36%, p = 0.02).

CONCLUSIONS

Alfentanil strengthened both the hypnotic and antinociceptive components of thiopental anesthesia, although to a different degree: the antinociceptive component more so than the hypnotic, possibly because each component of anesthesia has different underlying mechanisms.

Sedation in the ICU

The effect of midazolam on the induction dose-response curve for alfentanil was studied in non-premedicated ASA physical status I or II patients. The response to the verbal command was used as an end point of anaesthesia. Dose-response curves for midazolam, alfentanil, and their combination were determined with a probit procedure, and compared with algebraic (fractional) analysis of drug interaction. Interaction between midazolam and alfentanil was found to be synergistic (supra-additive). The results suggest that the use of this combination is advantageous not only because it helps to achieve different anaesthetic goals with specific drugs (a benzodiazepine for unconsciousness and an opioid for blockade of the responses to noxious stimulation), but also because its components are complementary for unconsciousness.

Morphine and fentanyl anesthetic interactions with diazepam: relative antagonism in rats

The anesthetic effects of morphine-diazepam and fentanyl-diazepam combinations as characterized by abolition of the movement response to noxious stimulation were studied in rats to test the hypothesis of antagonistic interactions between the components of these combinations. Noxious pressure on the tail was used to induce the response. Dose-effect curves were constructed for the drugs given alone and in combination. With the use of probit procedure ED50 values for single drugs and their combinations were determined, and the interactions were analyzed with algebraic (fractional) and isobolographic methods. It was found that both morphine and fentanyl have a less than additive (antagonistic) interaction with diazepam. In combination the sum of fractional doses was higher than a single-drug fractional dose, 1.67 versus 1.00 (P less than 0.05) for morphine-diazepam and 1.61 versus 1.00 (P less than 0.05) for fentanyl-diazepam. The observed antagonism is a relative one that does not increase the requirement for one agent upon the addition of another agent.

Sedative and hypnotic midazolam-morphine interactions in rats

The midazolam-morphine interactions in relation to the sedative effect and in relation to the hypnotic effect were studied in rats. Two series of experiments (sedative and hypnotic) were performed. In the sedative series, doses that inhibited locomotor activity to 10% or more of the control level were determined when the agents were given singly or in combination. Dose-response curves were determined with a probit procedure. The ED50 values of both agents and their combination were compared with algebraic (fractional) and isobolographic analyses in one subseries of experiments. The effect of a small fixed dose of morphine (1/10 of ED50 value for the sedative effect) on the slope of the sedative dose-response curve for midazolam was determined in the other subseries. In the hypnotic series of experiments, doses (ED50) that blocked the righting reflex with drugs given separately and in combination were determined by a probit procedure and, as in the sedative series, compared with algebraic (fractional) and isobolographic analyses. Sedative interaction between midazolam and morphine was found to have a tendency for synergism (interaction coefficient of 1.56, P greater than 0.05) with decreased individual variability in the sedative response to the combination. Hypnotic midazolam-morphine interaction was highly synergistic with the interaction coefficient of 3.70 (P less than 0.0001). A difference in the outcomes of midazolam-morphine interaction regarding sedation and hypnosis suggests that underlying mechanisms for these two effects are different; therefore, they should not be regarded as only increasing depths of the same action.

Alfentanil potentiates midazolam-induced unconsciousness in subanalgesic doses

The effects of alfentanil on the midazolam dose-response curve for hypnosis was studied with response to the verbal command as an end point in 95 patients. The analgesic effect of alfentanil was studied by measuring the threshold for pain caused by pressure on the trapezius muscle with the use of a dolorimeter in 21 patients. The study was randomized, double-blind, and performed on the unpremedicated patients with ASA physical status I or II. Alfentanil was found to reduce the midazolam ED50 value for the induction of anesthesia in a dose-dependent fashion. The smallest dose of alfentanil (3 micrograms/kg) that caused a marked shift of the midazolam dose-response curve to the left along the dose axis (from the ED50 of 270 micrograms/kg to the ED50 of 142 micrograms/kg, P less than 0.0005) represents approximately 2% of the alfentanil ED50 for induction of unconsciousness (130 micrograms/kg). Alfentanil (10 micrograms/kg) caused only a tendency for increase in the pain threshold, whereas a dose of 15 micrograms/kg significantly increased the pain threshold by 37% (P less than 0.05). The results demonstrate that alfentanil potentiates the hypnotic effect of midazolam in very small doses. The high potency of alfentanil in this respect, as compared to its analgesic potency, suggests a very specific mechanism of alfentanil-midazolam hypnotic interaction, one that most likely is based on a functional relationship between the GABA receptor-benzodiazepine receptor system and the opioid receptor system in mediation of hypnosis.

Postoperative pain after inguinal herniorrhaphy with different types of anesthesia

In a randomized, double-blind study, postoperative pain was assessed in 36 patients undergoing inguinal herniorrhaphy with three types of anesthesia: general (thiopental-nitrous oxide-halothane); general with the addition of local (infiltration of the abdominal wall with 0.25% bupivacaine along the line of the proposed incision); and spinal (0.5% bupivacaine). The severity of constant incisional pain, movement-associated incisional pain, and pain upon pressure applied to the surgical wound using an algometer was assessed with a visual analogue self-rating method at 24 h, 48 h, and 10 days after surgery. The addition of local anesthesia significantly decreased the intensity of all types of postoperative pain. This effect was especially evident with constant incisional pain that disappeared almost completely 24 h after surgery. With pain caused by pressure on the site of the surgical incision, the pain score difference between general and general plus local anesthesia was obvious even 10 days after the surgery (with 0.4-kg/cm2 pressure, the pain scores were 16 +/- 3 vs 2 +/- 1, P less than 0.01). The difference in postoperative pain scores between spinal and general anesthesia groups indicated that spinal anesthesia also decreases the pain intensity. However, this decrease is less pronounced than that seen with the addition of local anesthesia: movement-associated pain scores 24 h after surgery were 72 +/- 5 in the general anesthesia group, 40 +/- 6 in the spinal anesthesia group, and 16 +/- 3 in the general plus local anesthesia group (with P less than 0.002 between the groups).(ABSTRACT TRUNCATED AT 250 WORDS)