Autonomic and cardiovascular effects of neuromuscular blockade antagonism in the dog

The effects of acetylcholinesterase inhibitors on morbidity after general anesthesia and surgery

Non-depolarizing neuromuscular blocking agents are used during general anesthesia to facilitate intubation and optimize surgical conditions. When patients leave the operating room after surgery, postoperative residual neuromuscular block occurs frequently, increasing vulnerability to respiratory complications such as hypoxemia and unplanned postoperative mechanical ventilation. To restore neuromuscular transmission and skeletal muscle strength, anesthesiologists typically administer peripherally acting acetylcholinesterase inhibitors such as neostigmine. However, neostigmine's desirable effects have a narrow therapeutic range. Even at recommended dose (15-50 μg/kg), neostigmine induces nicotinic (upper airway muscle weakness leading to dysphagia and upper airway obstruction, and decreased maximum inspiratory airflow) and muscarinic (blurred vision, bronchial constriction, abdominal cramping and nausea) side effects. Recent data have questioned as to whether neostigmine reversal of neuromuscular blockade improves relevant patient outcomes such as postoperative respiratory and perioperative cardiovascular complications. A central strategy to avoid side effects of neuromuscular blocking agents is their judicious use based on quantitative monitoring of neuromuscular transmission using repetitive peripheral nerve stimulation (train-of-four ratio). Peripherally acting acetylcholinesterase inhibitors such as neostigmine should then only be administered when indicated and dosed based on results of the train-of-four ratio.

Effects of rocuronium bromide on globe position and respiratory function in isoflurane-anesthetized dogs: a comparison between three different dosages

OBJECTIVE

To evaluate the effect on globe position and respiration of three dosages of intravenous rocuronium in isoflurane-anesthetized dogs.

ANIMAL STUDIED

Thirty-two dogs anesthetized for ophthalmic procedures.

PROCEDURES

The dogs were divided into four groups, each of eight animals (G1-G4). G1, G2, G3 received 0.075, 0.05, 0.03 mg/kg of IV rocuronium, respectively; G4 received 0.9% NaCl IV; all the treatments were administered when an end-tidal isoflurane of 1.1-1.2% was reached. Anesthesia was obtained with dexmedetomidine (2.5 mcg/kg IV), methadone (0.1 mg/kg IV), propofol (2 mg/kg IV), and isoflurane in oxygen. Neuromuscular function was assessed with acceleromyography by stimulation of the peroneal nerve using the train-of-four (ToF) and the ToF ratio (ToFR). Monitoring of cardiovascular and respiratory functions was performed. Changes in globe position were recorded.

RESULTS

All three dosages of rocuronium produced centralization of the globe. Duration was 24.3 ± 4.2, 23.4 ± 3.6, and 8.7 ± 2.8 min, for G1, G2, and G3, respectively. The control group did not show globe centralization. No significant differences were found among the four groups in cardiovascular and respiratory parameters. Minute volume and ToFR were significantly lower in G1 compared with baseline values.

CONCLUSIONS

All doses of rocuronium resulted in globe centralization. The higher dose provoked a transient respiratory depression and some degree of skeletal muscular blockade detectable with ToFR. No alterations in respiratory activity were present when 0.05 mg/kg was used. The 0.03 mg/kg dosage could be useful for very short ophthalmic procedures.

Clinical implications of sugammadex

Sugammadex is a cyclodextrin molecule that encapsulates and inactivates rocuronium and vecuronium. As a result, any degree of neuromuscular block produced by rocuronium or vecuronium can be rapidly and completely reversed without autonomic effects. Because sugammadex is optimised for reversing rocuronium it is most likely to be used in conjunction with this drug. Sugammadex will allow deep levels of block to be maintained until the very end of surgery, and will allow block to be reversed at any time after rocuronium administration, even just a few minutes. The recommended dose-range is 2-16 mg.kg-1 (ascender), depending on the level of block. The availability of sugammadex reversal may increase the use of rocuronium, and decrease the use of suxamethonium and benzylisoquinoline neuromuscular blocking drugs. In addition, it will certainly increase pharmacy costs, which may be offset by faster recovery and discharge from the post-anesthesia recovery unit. Sugammadex may also change monitoring practices in that post-tetanic count will be required to quantify deep block, and quantitative monitoring of recovery may be driven by cost concerns in order to allow the use of the smallest dose of sugammadex that gives a satisfactory train-of-four ratio. Alternatively, monitoring may essentially be abandoned since a large dose of sugammadex will reliably reverse any degree of rocuronium-induced block. The ultimate clinical utility of sugammadex will be clear only after large-scale clinical use.

Clinical limitations of acetylcholinesterase antagonists

Administration of a nondepolarizing neuromuscular blocking drug (muscle relaxant) is a standard practice in many anesthetic scenarios. These muscle relaxants work by competitive antagonism of the neurotransmitter acetylcholine at nicotinic cholinergic receptors within the neuromuscular junction of skeletal muscle (Martyn et al; Neuromuscular physiology and pharmacology, anesthesia. Edited by RD Miller, Philadelphia, Churchill Livingstone, 2000). At the conclusion of the procedure for which the muscle relaxant was administrated, there will be a greater or lesser degree of residual muscle weakness. It is usually necessary to pharmacologically reverse this residual weakness to restore full function to the patient's muscles. Indeed, failure to reverse neuromuscular block is associated with increased perioperative morbidity and mortality (Arbous et al; Impact of anesthesia management characteristics on severe morbidity and mortality. Anesthesiology 2005;102:257-268; quiz 491-2). Pharmacologic reversal currently relies on the administration of an anticholinesterase drug, which decreases the metabolism of acetylcholine at the neuromuscular junction and allows its concentration to increase and hopefully overcome the effect of the muscle relaxant. This approach to reversal has significant limitations; the mechanism of reversal is indirect, the efficacy is limited and unpredictable, and undesirable autonomic responses occur. This review will address these limitations.

Cardiovascular and autonomic nervous effects of edrophonium and atropine combinations during neuromuscular blockade antagonism in sheep

OBJECTIVE

To study heart rate (HR), arterial blood pressure (BP) and autonomic nervous (AN) effects of edrophonium-atropine combinations during neuromuscular blockade (NMB) antagonism in sheep.

EXPERIMENTAL DESIGN

Randomized, prospective and experimental study.

ANIMALS

Seventy-eight Scottish blackface ewes; mean age: 4.5 years; mean body mass: 54 kg.

METHODS

After induction with IV etomidate (0.5 mg kg(-1)) and midazolam (0.5 mg kg(-1)), anaesthesia was maintained with halothane and NMB produced with atracurium or mivacurium. In the first study (n = 53), the electrocardiographic (ECG), HR, BP and AN effects of low (40 microg kg(-1)) and high (80 microg kg(-1)) atropine doses combined with either of two edrophonium doses (0.5 or 1.0 mg kg(-1)) were investigated. These variables were also measured in a second study when edrophonium (1.0 mg kg(-1)) was administered 5 minutes before atropine (80 microg kg(-1)) and vice versa. Data were analysed using one-way within-subjects and repeated measures anova.

RESULTS

In the first study, all combinations reversed NMB but significantly (p < 0.001) increased HR and BP within 2 minutes without arrhythmias. In the second study, edrophonium (1.0 mg kg(-1)) significantly increased HR and BP, saliva flow (n = 1) and lung sounds (n = 3) and caused ECG changes (n = 1). Cardiovascular changes were partially reversed by atropine (80 microg kg(-1)) administered 5 minutes later. Administered first, atropine (80 microg kg(-1)) significantly decreased HR and BP effects which were fully (HR) and partially (BP) reversed by edrophonium (1 mg kg(-1)) administered 5 minutes later.

CONCLUSION AND CLINICAL RELEVANCE

The cardiovascular effects of edrophonium and atropine were opposite to those reported in humans and dogs. Edrophonium (0.5 mg kg(-1)) and atropine (80 microg kg(-1)) caused the mildest HR changes without ECG and noncardiac AN disturbances, and is recommended for the antagonism of NMB in sheep.