LIDOCAINE USE IN A PATIENT WITH OLD MYOCARDIAL INFARCTION
Case presented by:
A 60-year old man presents to the emergency department with a 3-hour history of palpitations. He has a history of 2 myocardial infarctions and is currently receiving appropriate drug therapy for symptoms of chronic congestive heart failure (CHF). His ECG shows ventricular tachycardia (VT) at 140/min, and lidocaine is started.
Question No. 1: What is the optimal initial dosage scheme?
A.A large, rapid intravenous (IV) 100 mg bolus.
B.100 mg bolus over 2 to 3 minutes.
C.Infusion of 3 mg/min (the usual maintenance dose).
D.Infusion of 20 mg/min.
The creation of coronary care units in the 1960s led to the observation that ventricular fibrillation (VF) was frequently preceded by ventricular ectopic beats. As a result, the electrophysiology community spent considerable effort testing various strategies to suppress ventricular ectopy, in the hope of reducing the incidence of VF. Among these strategies was the use of “prophylactic” lidocaine in the coronary care unit. Although this strategy has been abandoned because it does not reduce the incidence of VF (and may increase mortality due to bradyarrhythmias), the extensive use of the drug led to intensive characterization of its pharmacokinetics and an understanding of the way in which disease alters fundamental determinants of plasma concentration and effect. These principles apply to therapy with most drugs, whether administered oral or IV.
To achieve an immediate therapeutic effect, a loading dose is administered, but to maintain that effect, a chronic dose regimen (in this case a maintenance IV infusion) is required. Lidocaine pharmacokinetics can be described by parameters such as volume of distribution or clearance. Volumes of distribution include a central volume into which the drug distributes immediately upon bolus administration (often equivalent to plasma volume) and a total volume of distribution, which may be much larger than plasma and may even exceed total body volume. The usual loading dose of lidocaine in a normal adult subject is 3 to 4 mg/kg. Since administering a large dose such as this can lead to very high drug concentrations, the load is usually administered as a series of boluses over 10 to 20 minutes. Disease states can have major impacts on drug disposition. CHF leads to a decrease in both clearance and volumes of distribution of lidocaine. The decrease in volume of distribution mandates a decrease in load doses; otherwise, a usual loading dose will lead to excessive early plasma concentrations and toxicity.
A.It must be very short (5–10 minutes).
B.It is 4 to 5 times 5 minutes or 20 to 25 minutes.
C.No inference about elimination can be made.
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