Drug concentrations antiarrhythmics

Somani P. Basic and clinical pharmacology of amiodarone relationship of antiarrhythmic effects, dose and drug concentrations to intracellular inclusion bodies. J Chn Pharmacol 1989 29(5) 405-12. 

Procainamide (Pwnestyl, Procan SR) is a derivative of the local anesthetic agent procaine. Procainamide has a longer half-life, does not cause CNS toxicity at therapeutic plasma concentrations, and is effective orally. Procainamide is a particularly useful antiarrhythmic drug, effective in the treatment of supraventricular, ventricular, and digitalis-induced arrhythmias. 

A 68-year-old female has AF, which is treated with an antiarrhythmic agent that blocks Na" channels. On a recent office visit, she complained of recurrent attacks of feeling faint and of experiencing an episode of loss of consciousness. An EKG showed marked prolongation of the QT interval. Plasma concentration of the drug was in the therapeutic range. 

Local anesthetic action, also known as "membrane-stabilizing" action, is a prominent effect of several 3 blockers (Table 10-2). This action is the result of typical local anesthetic blockade of sodium channels (see Chapter 26) and can be demonstrated experimentally in isolated neurons, heart muscle, and skeletal muscle membrane. However, it is unlikely that this effect is important after systemic administration of these drugs, since the concentration in plasma usually achieved by these routes is too low for the anesthetic effects to be evident. These membrane-stabilizing 3 blockers are not used topically on the eye, where local anesthesia of the cornea would be highly undesirable. Sotalol is a nonselective 3-receptor antagonist that lacks local anesthetic action but has marked class III antiarrhythmic effects, reflecting potassium channel blockade (see Chapter 14). 

Antiarrhythmic therapy carries with it a number of risks. In some cases, the risk of an adverse reaction is clearly related to high dosages or plasma concentrations. Examples include lidocaine-induced tremor or quinidine-induced cinchonism. In other cases, adverse reactions are unrelated to high plasma concentrations (eg, procainamide-induced agranulocytosis). For many serious adverse reactions to antiarrhythmic drugs, the combination of drug therapy and the underlying heart disease appears important. 

In small doses, local anesthetics can depress posttetanic potentiation via a prejunctional neural effect. In large doses, local anesthetics can block neuromuscular transmission. With higher doses, local anesthetics block acetylcholine-induced muscle contractions as a result of blockade of the nicotinic receptor ion channels. Experimentally, similar effects can be demonstrated with sodium channel-blocking antiarrhythmic drugs such as quinidine. However, at the doses used for cardiac arrhythmias, this interaction is of little or no clinical significance. Higher concentrations of bupivacaine (0.75%) have been associated with cardiac arrhythmias independent of the muscle relaxant used. 

Use of loading dose is important, especially for those drugs in which it is desirable to immediately or rapidly reach the steady-state plasma concentration (Css) (e.g., anticoagulant, antiepileptic, antiarrhythmic, and antimicrobial therapy). 

The cardiac arrhythmias are life-threatening, so the patient must be closely monitored, with facilities available for possible resuscitation. Drugs such as quinidine and procainamide are contraindicated, but lidocaine, propranolol, or phenytoin has been used safely and effectively. The arterial blood gas levels, pH, and electrolyte concentrations should be monitored so that metabolic acidosis or hypokalemia can be identified that would further aggravate the arrhythmias. Electrical pacing may be required if the antiarrhythmic drugs fail. Hyperpyrexia is treated by cooling. Seizures may be managed by intravenous doses of diazepam. 

The only other anesthetic to cause serious toxicity for which a metabolic drug interaction has been reasonably well characterized is the local anesthetic and antiarrhythmic agent lidocaine. Amiodarone decreased lidocaine systemic clearance in a patient (primarily by inhibition of CYP3A4 N-dealkylation of lidocaine) and yielded concentrations of lidocaine that led to seizures (78,79). 

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