Physostigmine antagonizes morphine-induced respiratory depression but not analgesia in dogs and rabbits

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

Effects of cholinesterase inhibitors and serotonin-1A receptor agonists on morphine-induced ventilatory depression and antinociception in rats

Ventilatory depression is a serious side-effect of opioid analgesics. Naloxone, an antagonist of opioid receptors, eliminates not only ventilatory depression but also analgesic effect of opioids. Pharmacological dissociation of adverse reactions from the main action is important clinically and basically. Cholinergic and serotonergic mechanisms are suggested to counteract the opioid-induced ventilatory disturbances, but their influence on analgesia is still controversial. The present study evaluated the effects of cholinesterase inhibitors and serotonin-1A (5-HT1A) receptor agonists on morphine (1.0mg/kg, i.v.)-induced ventilatory depression and analgesia in rats. In anesthetized animals, spontaneous ventilation and hind leg withdrawal reflexes against nociceptive thermal stimuli were measured simultaneously. Physostigmine (0.1 and 0.2mg/kg, i.v.) and donepezil (0.5 and 1.0mg/kg, i.v.) relieved the morphine-induced ventilatory depression and enhanced its antinociception. On the other hand, (±)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.03 and 0.1mg/kg, i.v.) and buspirone (0.1 and 0.3mg/kg, i.v.) did not influence antinociception of morphine while they restored the decreased ventilation. In unanesthetized animals, hypercapnic ventilatory response was measured by using whole-body plethysmography. Physostigmine (0.3mg/kg, i.p.), donepezil (1.0mg/kg, i.p.), 8-OH-DPAT (0.3mg/kg, i.p.) and buspirone (3.0mg/kg, i.p.) all recovered the morphine (10mg/kg, i.p.)-induced depression of hypercapnic ventilatory response. The present study suggests that activation of cholinergic or serotonergic (5-HT1A) mechanisms may be a useful therapeutic approach for morphine-induced ventilatory depression without loss of its analgesic action.

Excitatory and inhibitory effects of opioid agonists on respiratory motor output produced by isolated brainstems from adult turtles (Trachemys)

To determine how central opioid receptor activation alters turtle breathing, respiratory-related hypoglossal (XII) motor bursts were recorded from isolated adult turtle brainstems during 60 min bath applications of agonists for delta- (DOR), kappa- (KOR), or nociceptin/orphanin (NOR) receptors. DADLE (DOR agonist) abolished XII burst frequency at 0.3-0.5 microM. DPDPE (DOR agonist) increased frequency by 40-44% at 0.01-0.1 microM and decreased frequency by 88+/-8% at 1.0 microM. U-50488 and U-59693 (KOR agonists) decreased frequency by 65-68% at 100 and 50 microM, respectively. Orphanin (NOR agonist) decreased frequency by 31-51% at 1.0-2.0 microM during the first 30 min period. Orphanin (0.5 and 2.0 microM) increased bursts/episode. Although morphine (10 microM) abolished frequency in nearly all brainstems, subsequent co-application of phenylephrine (alpha(1)-adrenergic agonist, 20-100 microM) with morphine restored activity to 16-78% of baseline frequency. Thus, DOR, KOR, and NOR activation regulates frequency and NOR activation regulates episodicity, while alpha(1)-adrenergic receptor activation reverses opioid-induced respiratory depression in turtles.

Retracted: Do patients profit from physostigmine in recovery from desflurane anaesthesia?

BACKGROUND

Physostigmine is the drug of choice in the central anticholinergic syndrome, but has also been used in post-operative mental derangement secondary to sedatives and volatile anaesthetics. The aim of this double-blind, randomized, prospective study was to determine whether physostigmine alters recovery after desflurane anaesthesia.

METHODS

One hundred patients undergoing urologic or surgical procedures were enrolled to receive either NaCl 0.9% (n = 50) or 2 mg of physostigmine (n = 50) at the end of general anaesthesia with propofol, fentanyl, cisatracurium and desflurane. Times to extubation, stating name, birthday and place of residence, and obeying commands such as eye opening and hand squeezing were noted. Haemodynamics, Aldrete and pain scores, the analgesic requirements, and any adverse side-effects were documented until the 1st post-operative day.

RESULTS

Demographic, peri-operative data including duration of anaesthesia, surgery and postanaesthetic care unit (PACU) stay, and consumption of anaesthetics were comparable in both groups. No significant difference between the groups was found for extubation time or other emergence parameters. Patients undergoing anaesthesia >150 min showed after receiving physostigmine significantly (P < 0.05) faster spontaneous breathing (2.6 +/- 3.1 vs. placebo 5.0 +/- 4.2 min) and extubation time (6.2 +/- 3.7 vs. placebo 8.8 +/- 5.0 min). Women showed significantly shorter extubation times (5.5 +/- 3.4 min) and eye opening (5.5 +/- 2.6 min) with physostigmine than placebo (7.7 +/- 4.5 and 7.8 +/- 4.0 min). The incidence of post-operative nausea and vomiting (PONV) was significantly higher after physostigmine than placebo, whereas shivering occurred more often after placebo.

CONCLUSION

Physostigmine does not alter desflurane-based anaesthesia compared with placebo. An option is to use physostigmine in patients with a duration of anaesthesia >150 min who profit in earlier return to spontaneous breathing and shorter extubation time.

Anticholinergic Syndrome After Anesthesia: A Case Report and Review

Anticholinergic syndrome may present with a wide variety of signs and symptoms. Central manifestations range from excitatory symptoms including delirium and agitation to central nervous system depression, stupor and coma. Anticholinergic syndrome was once a common phenomenon after general anesthesia because of the frequent administration of the anticholinergic agents atropine and scopolamine. Now that these agents are rarely administered, anesthesia-related anticholinergic syndrome is currently infrequently reported. Still, many prescription and over the counter medications as well as many anesthetic agents possess anticholinergic activity, and this diagnosis should be considered in patients with altered mental status following general anesthesia. We report a case of prolonged somnolence following general anesthesia for an MRI. A rapid improvement of mental status with physostigmine confirmed the diagnosis of anticholinergic syndrome. This case is unique in that anticholinergic syndrome-related respiratory depression was promptly reversed with physostigmine.

Effects of opioid dependence and tobacco use on ventilatory response to progressive hypercapnia

Respiratory depression is a serious medical risk of opioid use. Most opioid abusers also smoke cigarettes, perhaps further compromising breathing. Differences in ventilatory response to nonhypoxic hypercapnia were studied in healthy volunteers with limited substance use (LU), tobacco smokers (SM), and opioid-dependent, methadone-maintained smokers (OD). The last two groups had similar current cigarette use and all groups were similar in gender and body mass index. Because previous data suggest that SM are sensitive to hypoxia but not hypercapnia, it was predicted that only the OD group would exhibit decreased carbon dioxide (CO(2)) sensitivity. All subjects rebreathed CO(2) during three identical sessions (four trials per session). Fractional end-tidal (Fet) CO(2) levels during repeated 4-min exposures to progressive hypercapnia (6% to 10%) were similar across groups. Ventilatory response (breathing rate, tidal volume and minute volume) linearly increased with FetCO(2) concentration and did not differ significantly across sessions. Relative to the LU and SM groups (which did not significantly differ), the CO(2)-minute volume and CO(2)-breathing rate functions were significantly shifted rightward (decrease in intercept but not slope) for OD subjects. These data are consistent with the hypothesis that chronic opioid exposure and/or short-term methadone maintenance (but not tobacco or nicotine use) produces a specific decrease in CO(2) sensitivity, primarily through an inhibitory effect on respiratory frequency.

Donepezil in the treatment of opioid-induced sedation: report of six cases

Donepezil, an oral acetylcholinesterase inhibitor approved for the treatment of Alzheimer's disease, was given to 6 cancer pain patients having sedation related to the analgesic use of opioids. Each patient was taking more than 200 mg of oral morphine equivalents per day, and several were receiving complex analgesic regimens consisting of multiple adjuvant medications. Sedation improved at least moderately in 5 of the patients and mildly in 1 after they began taking donepezil. Patients reported a decrease in episodes of spontaneous sleeping during the day, fewer myoclonic twitches, improved daily function and greater social interaction. Several also reported improved sleep at night. Analgesia was not compromised by the use of donepezil, and in some cases it appeared improved. Donepezil may be a valuable alternative to psychostimulants in the treatment of opioid-induced sedation. A prospective controlled trial comparing the treatment effects of psychostimulants and donepezil on patients having opioid-induced sedation is underway.

Reversal of orphenadrine-induced ventricular tachycardia with physostigmine

A 3-year-old boy developed confusion, generalized tonic-clonic seizures, and sustained ventricular tachycardia following ingestion of an unknown quantity of orphenadrine (Norflex). Although refractory to precordial thump, synchronous cardioversion, and lidocaine, the ventricular tachycardia was reversed by intravenous administration of the tertiary acetylcholinesterase inhibitor physostigmine. We discuss the underlying physiology and manifestations of anticholinergic overdose, the specific manifestations of orphenadrine overdose, and the current recommendations regarding the utilization and toxicity of physostigmine in the treatment of anticholinergic syndromes and orphenadrine intoxication.

[Central anticholinergic syndrome during postoperative period]

The central anticholinergic syndrome (CAS) includes central signs (somnolence, confusion, amnesia, agitation, hallucinations, dysarthria, ataxia, delirium, stupor, coma) and peripheral signs (dry mouth, dry skin, tachycardia, visual disturbances and difficulty in micturition). It occurs when central cholinergic sites are occupied by specific drugs and also as a result of an insufficient release of acetylcholine. The CAS can be caused by atropine sulphate, hyoscine (scopolamine), promethazine, benzodiazepines, opioids, halothane, influrane, ketamine. The incidence of CAS during the postoperative period depends on choice and dose of anaesthetic agents, type of surgery, patient's condition and diagnostic criteria. It is close to 10% following general anaesthesia and 4% following regional anaesthesia with sedation. The differential diagnosis of CAS includes an overdose of anaesthetic drugs or an alteration in pharmacokinetics, altered hydratation, electrolyte or acid-base state, hypoglycaemia, hypoxia, hypercapnia, hypocapnia, hyperthermia, hypothermia, hormonal disorders, neurological damage resulting from surgery, embolism, haemorrhage or trauma. The diagnosis of CAS is often determined by a process of exclusion and not actually made until a positive therapeutic response to physostigmine, a centrally active anticholinesterase agent has taken place.

Reversal of postoperative somnolence using a two-rate infusion of physostigmine

In order to antagonize immediate postoperative somnolence, 24 surgical patients were given a two-rate infusion of physostigmine, aiming at a constant plasma concentration in the range of 1 to 10 ng/ml. Plasma concentrations of physostigmine were determined during infusion and after infusion and the effects of physostigmine on analgesia and postoperative sedation, and its side effects were monitored throughout. On the 1st postoperative day some of the patients (n = 8) were given 5 mg physostigmine orally, after which plasma concentrations as well as effects were measured. Steady-state concentrations were generally lower than predicted. Clearance varied between 10 and 85 ml/min x kg with a mean of 40.8 +/- 21.0 ml/min x kg. Oral bioavailability was 25.3 +/- 11.1%. Physostigmine administered as an intravenous infusion antagonized immediate postoperative somnolence in 21 out of 24 patients. Effects were poorly correlated with the established steady-state concentration of physostigmine. The patients' experience of postoperative pain relief was mostly satisfactory and the side effects of physostigmine infusion were generally limited. The effects of physostigmine in the immediate postoperative period seemed dependent on the dose as well as on the time which had elapsed since administration of anticholinergic drugs. After oral physostigmine administration the following morning, the majority of patients experienced side effects such as nausea and abdominal pain. In conclusion, physostigmine given as infusion antagonizes postoperative somnolence. However, the arousal effect was considered not better than that resulting from a bolus dose of the drug, although the infusion regimen allowed a prolonged clinical effect duration.

Physostigmine in the treatment of intrathecal baclofen overdose. Report of three cases

The expanding use of intrathecal baclofen for spasticity has raised a concern about the treatment of overdose in these patients, since no specific baclofen antagonist is available. Since physostigmine has been reported to reverse the respiratory depression and somnolence due to opiates, the drug was tried for the treatment of baclofen overdose. In three cases, intravenous physostigmine (2 mg) completely reversed the respiratory depression and coma caused by boluses of 80 to 800 micrograms of lumbar intrathecal baclofen. Physostigmine, although not a specific antagonist, should provide increased safety for patients receiving intrathecal baclofen.

Effect of parasympathetic blockade on ventilatory and cardiac depression induced by opioids

We have previously shown that delta-opioid agonists decrease ventilation and heart rate. Because of these results and the known interactions between opioid and acetylcholine metabolism, we hypothesized that opioids induce cardiorespiratory changes via the parasympathetic nervous system. To test this hypothesis, we administered atropine sulfate (systemically) at maximal effect of D-Ala-D-Leu-enkephalin (DADLE; a preferential delta-opioid agonist), injected intracisternally, and examined its effect on cardiorespiratory function. All experiments were performed on chronically instrumented and conscious adult dogs. Mean instantaneous minute ventilation or VT/TTOT decreased and PaCO2 increased after DADLE; atropine had little effect on these changes. Naloxone, even in small doses, reversed opioid effects on VT/TTOT and PaCO2. Atropine, however, reversed the DADLE-induced depression in cardiac rate. In doses that reversed this cardiac depression, atropine had no effect on cardiorespiratory function at rest, i.e., with no prior administration of DADLE. We conclude that DADLE decreases heart rate by increasing parasympathetic activity to the heart and induces hypoventilation by a different mechanism. We speculate that the opioid-induced ventilatory depression is due to either direct opioid action on central respiratory regulation or parasympathetic non-muscarinic or non-cholinergic mediating mechanisms.

Reversal of sedation and respiratory depression after anaesthesia by the combined use of physostigmine and naloxone in neurosurgical patients

A clinical trial of the combination of naloxone in a low dose (1-1.5 micrograms X kg-1 body weight) with physostigmine (0.5-1.0 mg i.v.) was made to elucidate whether this combination could reverse postanaesthetic overdosing in neurosurgical patients without increasing postoperative pain. The investigation was made following previous findings that physostigmine has analgesic properties in addition to its systemic antisedative and anticholinergic effects as well as a stimulatory effect on morphine-depressed ventilation. Altogether 198 neurosurgical patients were investigated. The results showed that postanaesthetic over-sedation can be safely treated by a combination of naloxone and physostigmine in the dosages named above, resulting in the rapid reversal of sedation, where opiates, neuroleptics and benzodiazepines have been used. In contrast, this combination has very little effect on sedation following the administration of agents such as halothane and isoflurane. In the great majority of patients (95%), the treatment resulted in excellent analgesia during the first postoperative hour. The incidence of nausea and vomiting was increased somewhat by this treatment, but these side-effects could be minimized by decreasing the rate of drug administration. Physostigmine is contra-indicated in patients having symptoms and signs similar to those of Parkinson's disease, and the dose of physostigmine should also be reduced to 0.5 mg i.v. in all patients over the age of 65.

A double-blind trial of the analgesic properties of physostigmine in postoperative patients

A double-blind clinical trial of the analgesic and antisedative effects of physostigmine was carried out on surgical patients (n = 60) during the first hours postoperatively. Pethidine and placebo were included for comparison in the double-blind study. The degree of pain and sedation was estimated when the patient demanded analgesics and immediately before the administration of the test drug. The dosage administered i.v. was: physostigmine salicylate 2 mg, placebo = saline, or pethidine chloride 50 mg. After this, the same parameters were recorded at regular intervals. In addition, ventilatory rate, pulse rate, systolic blood pressure and side effects, if any, were noted. The results showed that physostigmine caused analgesia that was of the same magnitude as pethidine during the first 15 min, after which it decreased to the level of the placebo at 30 min. An antisedative or arousal effect was recorded over a somewhat longer time period; after this, there was no difference between placebo and physostigmine. In contrast to pethidine, physostigmine caused no decrease in the ventilatory rate. The pulse rate and systolic blood pressure did not change in any of the groups. Although the durations of the analgesic and antisedative effects of physostigmine were short, the use of this drug may well be preferable to the use of e.g. naloxone when immediate alertness of the patient is wanted without causing an increase in postoperative pain.

Pharmacokinetics of physostigmine after intravenous, intramuscular and subcutaneous administration in surgical patients

The pharmacokinetics of physostigmine after intravenous, intramuscular or subcutaneous administration as well as its arousal effect after anaesthesia have been studied in surgical patients in the early postoperative period. After intravenous administration physostigmine had a very rapid plasma elimination with a plasma clearance ranging from 47 to 163 l/h with a mean +/- s.d. of 92.5 +/- 37.7 l/h. The volume of distribution was 46.5 +/- 19.2 l, while distribution and plasma elimination half-lives were 2.3 and 22 min, respectively. A fraction of the dose was probably hydrolyzed in blood since its blood elimination half-life in vitro was approximately 190 min. After both intramuscular and subcutaneous administration the systemic availability was almost complete, the plasma terminal half-lives only being somewhat longer than after intravenous administration. Plasma clearance, volume of distribution and elimination half-life of physostigmine were not correlated to age or body weight of the patients. The rapid plasma clearance of physostigmine resulted in a short duration of antisedative effect. After administration of 1 mg physostigmine salicylate i.v., drug-induced sedation was rapidly reversed with a duration of 30-60 min. The duration of action was similar after intramuscular injection but onset was delayed by 20-30 min. It was concluded that a plasma concentration of 3-5 ng/ml of physostigmine should be exceeded if an adequate analeptic effect is to be achieved, meaning that 2 mg of physostigmine had to be administered subcutaneously in order to achieve a satisfactory reversal of sedation. The short duration of action may hamper the use of physostigmine as an agent for reversal of drug-induced sedation and anticholinergic effects after surgery.

Opposite effects of the delta- and mu-opioid receptor agonists on ventilation in conscious adult dogs

We studied ventilation and ventilatory pattern in adult unanesthetized dogs after intracisternal administration of morphiceptin analogue (MA) (Tyr-Pro-NMePhe-D-Pro-NH2) and morphine sulfate (MS) which are mu-receptor opioid agonists and after D-Ala-D-Leu-enkephalin (DADLE), a preferential delta-receptor opioid agonist. DADLE induced a prolongation in expiratory time, Te, and a reduction in instantaneous minute ventilation, Vt/Ttot, which lasted for about 2 h and was dose dependent. In contrast, MA and MS induced a striking decrease in Te and tidal volume with a net increase in Vt/Ttot. Both MA and DADLE increased the number of sighs per unit time while morphine did not. Naloxone increased Vt/Ttot when used after both DADLE or MA but larger doses were required for an observable effect after DADLE than after MA. These data suggest that in the unanesthetized dog: (1) the effect of intracisternal opioids on ventilation and ventilatory pattern is not uniform and (2) the opioid mu-receptor subsystem may involve different neuronal pathways from those of the opioid delta-receptor subsystem to modulate breathing.

The relevance of cholinergic transmission at the spinal level to opiate effectiveness

Rats chronically implanted with intrathecal catheters displayed a dose-dependent increase in the hot-place and tail-flick response latencies following the injection of morphine or nicomorphine into the subarachnoid space through the indwelling catheter. Naloxone inhibited the antinociceptive effect of both opiate drugs, but the inhibition of nicomorphine-induced antinociception was incomplete. To evaluate the importance of cholinergic mechanisms in opiate effectiveness, the interactions with atropine or physostigmine were evaluated. Atropine reduced the effects of morphine and abolished the effects of nicomorphine at the doses used. Physostigmine markedly potentiated morphine effectiveness, but had a negligible effect on nicomorphine effectiveness. It is proposed that these differences relate to a specific cholinergic mechanism involved in antinociception after intrathecal nicomorphine. The data indicate that at a spinal level cholinergic mechanisms are relevant to opiate effectiveness.