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VECURONIUM BROMIDE (vecuronium bromide for injection) Clinical Pharmacology

CLINICAL PHARMACOLOGY

Vecuronium for injection is a nondepolarizing neuromuscular blocking agent possessing all of the characteristic pharmacological actions of this class of drugs (curariform). It acts by competing for cholinergic receptors at the motor end-plate. The antagonism to acetylcholine is inhibited and neuromuscular block is reversed by acetylcholinesterase inhibitors such as neostigmine, edrophonium, and pyridostigmine. Vecuronium is about 1/3 more potent than pancuronium; the duration of neuromuscular blockade produced by vecuronium bromide is shorter than that of pancuronium at initially equipotent doses. The time to onset of paralysis decreases and the duration of maximum effect increases with increasing vecuronium bromide doses. The use of a peripheral nerve stimulator is recommended in assessing the degree of muscular relaxation with all neuromuscular blocking drugs. The ED90 (dose required to produce 90% suppression of the muscle twitch response with balanced anesthesia) has averaged 0.057 mg/kg (0.049 to 0.062 mg/kg in various studies). An initial vecuronium bromide dose of 0.08 to 0.1 mg/kg generally produces first depression of twitch in approximately 1 minute, good or excellent intubation conditions within 2.5 to 3 minutes, and maximum neuromuscular blockade within 3 to 5 minutes of injection in most patients.  

Under balanced anesthesia, the time to recovery to 25% of control (clinical duration) is approximately 25 to 40 minutes after injection and recovery is usually 95% complete approximately 45-65 minutes after injection of intubating dose. The neuromuscular blocking action of vecuronium is slightly enhanced in the presence of potent inhalation anesthetics. If vecuronium is first administered more than 5 minutes after the start of the inhalation of enflurane, isoflurane, or halothane, or when steady state has been achieved, the intubating dose of vecuronium bromide may be decreased by approximately 15% (see DOSAGE AND ADMINISTRATION). Prior administration of succinylcholine may enhance the neuromuscular blocking effect of vecuronium and its duration of action. With succinylcholine as the intubating agent, initial doses of 0.04 to 0.06 mg/kg of vecuronium will produce complete neuromuscular block with clinical duration of action of 25-30 minutes. If succinylcholine is used prior to vecuronium, the administration of vecuronium should be delayed until the patient starts recovering from succinylcholine-induced neuromuscular blockade. The effect of prior use of other nondepolarizing neuromuscular blocking agents on the activity of vecuronium has not been studied (see PRECAUTIONS, Drug Interactions).

Repeated administration of maintenance doses of vecuronium has little or no cumulative effect on the duration of neuromuscular blockade. Therefore, repeat doses can be administered at relatively regular intervals with predictable results. After an initial dose of 0.08 to 0.1 mg/kg under balanced anesthesia, the first maintenance dose (suggested maintenance dose is 0.01 to 0.015 mg/kg) is generally required within 25 to 40 minutes; subsequent maintenance doses, if required, may be administered at approximately 12 to 15 minute intervals. Halothane anesthesia increases the clinical duration of the maintenance dose only slightly. Under enflurane a maintenance dose of 0.01 mg/kg is approximately equal to 0.015 mg/kg dose under balanced anesthesia.

The recovery index (time from 25% to 75% recovery) is approximately 15-25 minutes under balanced or halothane anesthesia. When recovery from vecuronium neuromuscular blocking effect begins, it proceeds more rapidly than recovery from pancuronium. Once spontaneous recovery has started, the neuromuscular block produced by vecuronium is readily reversed with various anticholinesterase agents, e.g., pyridostigmine, neostigmine, or edrophonium in conjunction with an anticholinergic agent such as atropine or glycopyrrolate. Rapid recovery is a finding consistent with vecuronium short elimination half-life, although there have been occasional reports of prolonged neuromuscular blockade in patients in the intensive care unit (see PRECAUTIONS).

The administration of clinical doses of vecuronium bromide is not characterized by laboratory or clinical signs of chemically mediated histamine release. This does not preclude the possibility of rare hypersensitivity reactions (see ADVERSE REACTIONS).

Pharmacokinetics

At clinical doses of 0.04 to 0.1 mg/kg, 60-80% of vecuronium bromide is usually bound to plasma protein. The distribution half-life following a single intravenous dose (range 0.025 to 0.28 mg/kg) is approximately 4 minutes. Elimination half-life over this sample dosage range is approximately 65-75 minutes in healthy surgical patients and in renal failure patients undergoing transplant surgery.

In late pregnancy, elimination half-life may be shortened to approximately 35-40 minutes. The volume of distribution at steady state is approximately 300 to 400 mL/kg; systemic rate of clearance is approximately 3 to 4.5 mL/minute/kg. In man, urine recovery of vecuronium varies from 3-35% within 24 hours. Data derived from patients requiring insertion of a T-tube in the common bile duct suggests that 25-50% of a total intravenous dose of vecuronium may be excreted in bile within 42 hours. Only unchanged vecuronium has been detected in human plasma following use during surgery. In addition, one metabolite 3-desacetyl vecuronium has been rarely detected in human plasma following prolonged clinical use in the I.C.U. (see PRECAUTIONS, Long-term Use in I.C.U.). The 3-desacetyl vecuronium metabolite has been recovered in the urine of some patients in quantities that account for up to 10% of injected dose; 3-desacetyl vecuronium has also been recovered by T-tube in some patients accounting for up to 25% of the injected dose.

This metabolite has been judged by animal screening (dogs and cats) to have 50% or more of the potency of vecuronium; equipotent doses are of approximately the same duration as vecuronium in dogs and cats. Biliary excretion accounts for about half the dose of vecuronium bromide within 7 hours in the anesthetized rat. Circulatory bypass of the liver (cat preparation) prolongs recovery from vecuronium. Limited data derived from patients with cirrhosis or cholestasis suggests that some measurements of recovery may be doubled in such patients. In patients with renal failure, measurements of recovery do not differ significantly from similar measurements in healthy patients.

Studies involving routine hemodynamic monitoring in good risk surgical patients reveal that the administration of vecuronium bromide in doses up to three times that needed to produce clinical relaxation (0.15 mg/kg) did not produce clinically significant changes in systolic, diastolic or mean arterial pressure. The heart rate, under similar monitoring, remained unchanged in some studies and was lowered by a mean of up to 8% in other studies. A large dose of 0.28 mg/kg administered during a period of no stimulation, while patients were being prepared for coronary artery bypass grafting was not associated with alterations in rate-pressure-product or pulmonary capillary wedge pressure. Systemic vascular resistance was lowered slightly and cardiac output was increased insignificantly. (The drug has not been studied in patients with hemodynamic dysfunction secondary to cardiac valvular disease). Limited clinical experience with use of vecuronium during the surgery for pheochromocytoma has shown that administration of this drug is not associated with changes in blood pressure or heart rate.

Unlike other nondepolarizing skeletal muscle relaxants, vecuronium has no clinically significant effects on hemodynamic parameters. Vecuronium will not counteract those hemodynamic changes or known side effects produced by or associated with anesthetic agents, other drugs or various other factors known to alter hemodynamics.

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