Comparative electrophysiology of lorcainide and norlorcainide in the dog

Electrophysiologic effects of intravenous lorcainide and its major metabolite, norlorcainide, were examined in 18 anesthetized dogs, using intracardiac electrophysiologic measurements and programmed stimulation. Lorcainide and norlorcainide were studied separately, using six dogs for each experiment. Each compound was administered by a series of 5 one-h graded infusions each involving a loading dose over 15 min followed by a 45-min maintenance infusion. Plasma concentrations ranged from 94 +/- 34 to 1345 +/- 471 ng/ml for lorcainide and 81 +/- 22 to 1344 +/- 458 ng/ml for norlorcainide. Six additional dogs were studied, using a combination of a constant lorcainide infusion and four progressively increasing doses of norlorcainide. Both lorcainide and norlorcainide caused concentration-dependent prolongation of PR interval, QRS duration, AH and HV intervals, atrial and ventricular effective refractory periods, and the atrioventricular nodal functional refractory period. For each electrophysiologic parameter, plots of percent change as a function of log plasma concentration were nearly superimposable for lorcainide and norlorcainide. Plots for the combined treatment group using the total plasma concentration of lorcainide and norlorcainide were similar to those for each compound alone. We conclude that in dogs norlorcainide has considerable electrophysiologic activity, and is approximately equieffective and equipotent when compared with lorcainide, and that the electrophysiologic effects of the two compounds appear additive.

Propafenone disposition kinetics in cardiac arrhythmia

Propafenone disposition kinetics were studied after intravenous and oral doses in patients with ventricular arrhythmias. Plasma concentration-time data were fit to a two-compartment model for all but one patient, whose data required fitting to a three-compartment model. The model-independent calculated values of clearance, steady-state volume of distribution, and terminal t1/2 were 11.2 +/- 4.8 ml/min/kg, 3.6 +/- 2.1 l/kg, and 5.0 +/- 3.6 hr. After 5 days on oral propafenone, elimination t1/2 was 6.2 +/- 3.3 hr. The longer t1/2s and the estimates of steady-state bioavailability above 100% suggests that clearance decreases during chronic oral dosing. Considerable intersubject variability was noted in all disposition parameters.

Myocardial uptake of encainide and its two major metabolites, O-demethyl encainide and 3-methoxy-O-demethyl encainide, in the dog

Encainide is a new antiarrhythmic agent which is currently undergoing clinical evaluation. Two metabolites, O-demethyl encainide (ODE) and 3-methoxy-O-demethyl encainide (MODE), have been identified. We have investigated the myocardial accumulation of these three compounds in an anesthetized open-chested dog model. We also considered the degree of plasma protein binding and the oil/water partitioning characteristics of these three compounds to see if they explained differences in myocardial accumulation. Each compound was administered by intravenous infusion to a group of five dogs. Blood and myocardial biopsy samplings were carried out under steady-state conditions. The myocardial/plasma concentration ratios for encainide, ODE, and MODE were 8.4, 5.4, and 4.8, respectively. The ratios were compared with a completely randomized analysis of variance followed by multiple comparisons. The myocardial/plasma concentration ratio for encainide was significantly greater (p less than .05) than the ratios of the metabolites; however, the difference between ODE and MODE was not significant. Myocardial uptake of encainide, ODE, and MODE was quite rapid, and the myocardial concentration-time course of each compound followed its time course in plasma closely. Encainide and its two metabolites are only moderately bound to plasma proteins. The mean (+/- SD) percent bound for encainide, ODE, and MODE are 49 +/- 10, 55 +/- 19, 44 +/- 18, respectively. The whole blood/plasma concentration ratios are 1.04 +/- 0.16, 1.08 +/- 0.23, and 1.06 +/- 0.08 for encainide, ODE, and MODE, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacodynamics of intravenous amiodarone in the dog

We have investigated the relationship between the myocardial disposition kinetics of amiodarone and the time course of its electrophysiologic effects in an open-chested anesthetized dog model. We observed that the maximal changes in both sinus cycle length and Wenkebach cycle length occurred about 30 min following a single intravenous dose of amiodarone or at the same time as the occurrence of the maximal myocardial concentration. The myocardial concentrations of amiodarone more closely reflect the electrophysiologic effects of amiodarone in the dog than do plasma concentrations. This difference is observable during the first 30 min following intravenous administration owing to the disequilibrium between plasma and myocardial disposition characteristics. Interanimal variability of electrophysiologic response is not accounted for by differences in myocardial concentrations.

Clinical pharmacokinetics of amiodarone

Amiodarone is an iodinated benzofuran derivative with recognised antiarrhythmic activity in man. As yet, its pharmacokinetic behaviour has not been satisfactorily characterised. Specific and sensitive high-pressure liquid chromatographic methods have become available only recently and this partly explains the scarcity of pharmacokinetic data on the drug. Available evidence suggests that absorption of amiodarone following oral administration is erratic and unpredictable; oral bioavailability ranges from 22 to 86%. The drug is eliminated largely by metabolism; less than 1% of the dose is excreted unchanged in the urine. Biliary excretion may have a role in the overall elimination of the drug. Desethyl-amiodarone is the only metabolite positively identified in the plasma of patients receiving treatment with amiodarone; no data are available on its possible pharmacological activity. Since it is a highly lipophilic drug, amiodarone is extensively distributed into tissues. Adipose tissue and skeletal muscle accumulate large amounts of the drug during long term treatment. Myocardium/plasma ratios of amiodarone are high both in man and in animals; peak concentrations in the myocardium are reached within half an hour after administration of an intravenous bolus to dogs. Placental transfer of amiodarone has been demonstrated in humans, while its blood profile is not modified by dialysis treatment. In vitro protein binding of amiodarone has been reported to be 96.3 +/- 0.6%. The plasma half-life of amiodarone after single-dose administration has been reported to be in the range of 3.2 to 79.7 hours. However, after withdrawal of long term amiodarone treatment the half-life is as long as 100 days. Total body clearance ranges from 0.10 to 0.77 L/min after single-dose intravenous administration, and the apparent volume of distribution ranges between 0.9 and 148 L/kg. Amiodarone disposition kinetics in patients with cardiac arrhythmias are not different from those in healthy volunteers. However, the possible effects of liver and cardiac failure on the drug's kinetics have not been studied. Amiodarone potentiates the anticoagulant effect of warfarin, probably by inhibition of its metabolism. Increases of steady-state concentrations of digoxin, together with the appearance of signs of digitalis toxicity, have been reported when amiodarone was given to patients receiving long term treatment with digoxin. Amiodarone has also been shown to interact with other antiarrhythmic agents such as quinidine and procainamide. The time of onset of action of amiodarone after a single intravenous dose ranges between 1 and 30 minutes and its duration of effect between 1 and 3 hours.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacodynamics of the initiation of antiarrhythmic therapy with lorcainide

Lorcainide is an antiarrhythmic drug with unusual pharmacokinetics and an active metabolite, norlorcainide, which complicate oral drug loading. In order to characterize the accumulation of lorcainide and norlorcainide and to define the onset of antiarrhythmic action during lorcainide loading, 9 patients with frequent ventricular ectopic beats were studied. During lorcainide loading with 100 mg orally twice daily, frequent ambulatory electrocardiographic recordings were monitored and blood samples for drug concentrations were determined. There was a 10-fold range of intersubject variation in plasma concentrations. Despite a half-life of only 8.9 +/- 2.3 hours, lorcainide did not reach steady state until after 4.5 days of therapy. Norlorcainide had a half-life of 26.5 +/- 7.2 hours and was estimated to come to steady state after 7 to 10 days. There was considerable intersubject variation in time of onset of antiarrhythmic response (2 to more than 4.5 days) and a 4- to 5-fold range of intersubject variation in threshold therapeutic plasma concentration (lorcainide 40 to 200 ng/ml, norlorcainide 80 to 300 ng/ml). These observations suggest that lorcainide should be started at low doses and the dose should not be increased more frequently than once a week.

Clinical efficacy and electrophysiology of oral propafenone for ventricular tachycardia

Sixteen patients with ventricular tachycardia (VT) or nonfatal cardiac arrest were treated with propafenone (P), 900 mg/day. Electrophysiologic studies were performed before and during therapy with P. All patients had inducible sustained VT at the baseline study. During P therapy, VT was not inducible in 1 patient, was unsustained in 1 and was harder to induce in 2 patients. P increased the cycle length of VT from 307 +/- 67 to 382 +/- 107 ms. Five patients began outpatient therapy with P, including 2 in whom VT was slowed to less than 125 beats/min. Two are arrhythmia-free during follow-up of 2 and 8 months. P significantly increased intraatrial conduction time (from 44 +/- 12 to 72 +/- 22 ms), AH interval (from 115 +/- 36 to 152 +/- 45 ms), HV interval (from 55 +/- 18 to 92 +/- 42 ms), QRS duration (from 140 +/- 36 to 180 +/- 48 ms) and QT interval (from 402 +/- 30 to 459 +/- 60 ms). P increased atrial (from 247 +/- 36 to 288 +/- 38 ms) and ventricular (from 249 +/- 20 to 277 +/- 32 ms) effective refractory periods, Sinus cycle length did not change, but the corrected sinus node recovery time increased (from 162 +/- 85 to 821 +/- 1,607 ms). P aggravated arrhythmias in 4 patients. The plasma P concentration, measured either at the time of electrophysiologic studies of when therapy was discontinued, was 753 +/- 428 ng/ml. P suppressed ventricular ectopic beats in 33% and increased them in 1 patient. P has antiarrhythmic activity against VT similar to that of other antiarrhythmic drugs and has potential for serious adverse effects in some patients.

Verapamil decreases MAC for halothane in dogs

Verapamil hydrochloride is a calcium entry blocking drug that is being prescribed with increasing frequency for cardiovascular disorders in the perioperative setting. Verapamil's calcium channel blocking effect is not selective, because it also exerts activity on the sodium channel. Because of the well-described effects of sodium channel blockers on anesthetic requirements, the authors studied the MAC for halothane in dogs before and after a therapeutic dose of verapamil 0.5 mg . kg-1. There was a 25% reduction in halothane MAC from 0.97-0.72% (P less than 0.01) when a therapeutic plasma level of verapamil (64 ng . ml-1) was present. Anesthetic requirements for halothane are reduced by dl-verapamil possibly on the basis of its local anesthetic-like sodium channel blocking properties. Adjustments in anesthetic dosage may be necessary in patients receiving verapamil.

Plasma level monitoring of antiarrhythmic drugs

It is widely accepted that the effects (both cardiac and extracardiac) of antiarrhythmic drugs are modulated by their concentration at some unidentified active site, and that the drug concentrations in the systemic circulation and at these active sites are in equilibrium. Thus, antiarrhythmic drug effects can be related directly to systemic plasma concentrations, and an optimal plasma concentration can be identified at which satisfactory arrhythmia suppression can be achieved in the absence of intolerable adverse effects. This optimal concentration is influenced by several factors that give rise to significant interpatient variability. These factors include serum protein binding, active metabolites, intrinsic responsiveness and myocardial accumulation. Although plasma concentration guidelines have been suggested for most antiarrhythmic drugs, they are generally not statistically derived and, with the exception of procainamide, are extrapolated from small patient samples. They generally represent the experience of an investigator or group of investigators treating a small homogeneous patient population. Interpretation of plasma concentrations of antiarrhythmic drugs also requires consideration of pharmacokinetic factors. Plasma drug levels are only useful when dosing history and timing of the blood sample, relative to drug administration, are considered. Despite several limitations, plasma concentration monitoring of antiarrhythmic drugs can be helpful if evaluated with an understanding of the pharmacokinetic properties of the drug being measured, the clinical status of the patient and an appreciation of the factors that may influence the relation between the measured level and resultant clinical response.

Clinical pharmacology of propafenone

We determined the efficacy, pharmacokinetics, and plasma concentration-response relationships of propafenone, a promising new antiarrhythmic drug. Thirteen patients with frequent and complex ventricular premature beats were studied after receiving four increasing doses, during drug washout and during a randomized double-blind placebo-controlled trial, to evaluate the optimal dose in each patient. A nonlinear relationship was found between propafenone dose and steady-state mean concentration with a 10-fold increase in drug concentration as dose increased threefold from 300 to 900 mg/day. There was great intersubject variability in elimination half-life (mean 6 hr, range 2.4 to 11.8), steady-state mean concentration on 900 mg/day of propafenone (mean 1008 ng/ml, range 482 to 1812), and "therapeutic" plasma concentration (mean 588 ng/ml, range 64 to 1044). The interaction of these three parameters in individual patients determined the duration of the antiarrhythmic action of propafenone during washout (mean 11.5 hr, range 4 to 22). There was a greater than 90% reduction of ventricular premature beats in 10 subjects during dose ranging and in seven during double-blind crossover. Side effects requiring discontinuation of the drug occurred in three patients and included apparent worsening of arrhythmias in two. We conclude that propafenone effectively suppresses ventricular arrhythmias and that nonlinear drug accumulation and intersubject variability in elimination of half-life, steady-state mean plasma concentration, and therapeutic concentration indicate a need for individual therapy.

Comparison of the electrophysiologic effects of intravenous and oral lorcainide in patients with recurrent ventricular tachycardia

The electrophysiologic effects of intravenous lorcainide (2.2 mg/kg) in 10 patients were compared with the electrophysiologic effects of oral lorcainide (mean dose 400 mg/day for 8 days) in 11 patients, all with recurrent ventricular tachycardia that could be induced with programmed stimulation. Intravenous and oral lorcainide resulted in similar prolongation of the QRS, QT, and HV intervals, but only oral lorcainide resulted in prolongation of the AH interval and atrial and ventricular effective refractory periods. After both oral and intravenous lorcainide, ventricular tachycardia could still be induced, but the arrhythmia was slower and better tolerated hemodynamically. The mean plasma lorcainide level during a maintenance intravenous infusion was 1254 +/- 662 ng/ml compared with a lorcainide level of 562 +/- 41 ng/ml and a norlorcainide level of 1212 +/- 653 ng/ml after oral dosing. No norlorcainide was detected in plasma after intravenous lorcainide. These data suggest that the short-term electrophysiologic effects of intravenous lorcainide may be different from those of short-term therapy with the oral drug. These differences should be considered during short-term studies of lorcainide.

Possible contribution of encainide metabolites to the long-term antiarrhythmic efficacy of encainide

To establish long-term efficacy and the relation between drug plasma concentration and antiarrhythmic response, 12 patients with encainide-responsive frequent complex ventricular ectopic activity underwent 1 year of therapy with encainide. Twenty-four hour ambulatory electrocardiograms were obtained at baseline and every 2 months. Drug withdrawal with concomitant plasma sampling and electrocardiographic monitoring was performed at 6 and 12 months. Average group premature ventricular contraction (PVC) suppression during the year was 97 to 99%, with nearly total suppression of pairs and salvos. The most common adverse effects were transient visual disturbances and dizziness or lightheadedness. During a dose interval (6 to 12 hours) the concentration of encainide metabolites exceeded that of encainide by several-fold. The median time of arrhythmia return after drug withdrawal was 12 to 14 hours. At the time of arrhythmia return encainide was generally no longer detectable but the average concentration of O-demethylencainide and 3 methoxy-O-demethylencainide was 72 +/- 49 and 172 +/- 74 ng/ml, respectively. It is concluded that encainide therapy is extremely effective for continuous long-term suppression of complex ventricular arrhythmias and its metabolites contribute significantly to its antiarrhythmic action during chronic oral therapy.

Myocardial disposition of amiodarone in the dog

The time course of myocardial uptake and disposition of amiodarone was studied after both acute i.v. and chronic oral administration. In addition, the myocardial disposition of a metabolite, N-desethylamiodarone, was studied after chronic oral amiodarone administration. After i.v. administration, the plasma concentrations of amiodarone fell rapidly; however, peak myocardial concentrations were not observed until 10 to 30 min after administration. Amiodarone was highly concentrated in the myocardium; the average (+/- S.D.) myocardial/plasma concentration ratio between 2 and 6 hr after administration was 89 +/- 32. Although there was significant interanimal variability, there was relative consistency over time in the ratio for each dog during this time period. Although no metabolite (N-desethylamiodarone) was detected in the plasma after the single i.v. dose, it was present in both plasma and myocardial samples after chronic oral therapy. Mean steady-state plasma concentrations of amiodarone and N-desethylamiodarone ranged from 0.62 to 1.63 micrograms/ml and 0.19 to 0.43 micrograms/ml, respectively. These studies show that the myocardial disposition kinetics of amiodarone are different from other drugs studied and both amiodarone and its N-desethyl metabolite accumulate extensively in the myocardium.

Calcium antagonists. Pharmacokinetic properties

An understanding of the pharmacokinetics of the calcium antagonists (slow-channel blocking drugs) is essential in order to design appropriate dosage regimens which will provide optimum therapeutic efficacy with these agents. This review summarises and evaluates the current state of knowledge of the absorption and disposition characteristics of the 3 most extensively used calcium antagonists in cardiovascular therapeutics: verapamil, diltiazem and nifedipine. While an extensive literature regarding the kinetics of verapamil exists, reports dealing with diltiazem and nifedipine are limited. This is, in part, due to difficulties in developing simple, specific and sensitive analytical procedures. All 3 drugs undergo extensive metabolism in the liver. Metabolites of verapamil (norverapamil) and diltiazem (desacetyldiltiazem) accumulate in the plasma of patients and have been shown to produce some effects similar to those of their parent compounds. The bioavailability of diltiazem and nifedipine has not been well studied, and no investigations of the absolute bioavailability of these compounds have been reported. However, the bioavailability of verapamil has been studied extensively; about 22% of an orally administered dose of verapamil is systemically available. Bioavailability is increased when liver function is impaired, such as in patients with hepatic cirrhosis. The high first-pass extraction of verapamil has been suggested to be stereoselective, with preferential elimination of the (-) isomer. The plasma concentration-time curves of verapamil and diltiazem have been studied following oral administration. The elimination half-lives of verapamil and diltiazem are about 8 and 5 hours, respectively. All 3 drugs are highly protein-bound in the plasma. Several other drugs have the ability to displace verapamil from plasma protein binding sites, but the clinical significance of this interaction is doubtful. Other drug interactions have been investigated with these agents. Verapamil causes digoxin plasma levels to rise during concomitant administration, but no drugs have been shown to alter the disposition of verapamil. Diazepam affects the plasma levels of diltiazem leading to a decrease. The mechanism of this interaction has not been reported, but an effect on bioavailability has been suggested. Age has been shown to be a factor in the disposition of both diltiazem and verapamil. Older patients tend to have lower clearances of these 2 drugs than do younger patients. It has also been shown that hepatic cirrhosis leads to a decreased clearance of verapamil. Plasma level monitoring may be helpful for adjusting doses of both verapamil and diltiazem, despite the absence of a definition of therapeutic plasma concentrations. These agents all have low, and highly variable, systemic availability, and plasma concentrations cannot be predicted after oral administration.

Lorcainide disposition kinetics in arrhythmia patients

Lorcainide disposition kinetics were studied after intravenous and oral administration to patients with ventricular arrhythmias. After intravenous doses ranging from 100 to 200 mg, blood samples were drawn and plasma was analyzed for lorcainide concentration by high-pressure liquid chromatography. A three-compartment model was used to fit the data. The model-independent calculated values for clearance, steady-state volume of distribution, and terminal half-life were 14.4 +/- 3.28 ml/min/kg, 6.33 +/- 2.23 l/kg, and 7.8 +/- 2.2 hr. After nine doses of oral lorcainide (100 mg every 12 hr) blood samples were drawn and analyzed for lorcainide and its active metabolite, norlorcainide. The lorcainide and norlorcainide half-lifes were 9.6 +/- 2.8 and 26.8 +/- 8.2 hr. Mean steady-state level of norlorcainide was 2.2 +/- 0.9 times the level of lorcainide. The data suggest that the clearance of lorcainide decreases with time during long-term dosing.

Prolongation of verapamil elimination kinetics during chronic oral administration

The elimination of verapamil and its n-demethylated metabolite, norverapamil, was studied in nine patients with chronic atrial fibrillation after the first oral verapamil dose and during chronic oral verapamil administration. Significant increases (p less than 0.01) were seen in the elimination half-lives (t 1/2's) of both verapamil (6.4 +/- 3.5 to 12 +/- 5 hours, mean +/- SD) and norverapamil (10.3 +/- 6 to 16.5 +/- 7 hours) during chronic oral verapamil administration. These pharmacokinetic observations have important clinical implications for the rational long-term administration of this agent. Once steady-state serum concentrations have been achieved during chronic verapamil administration, verapamil doses should be given at less frequent intervals or in smaller doses in order to produce the desired serum concentration and therapeutic response and to minimize unwanted or toxic drug effects.

Clinical pharmacokinetics of N-acetylprocainamide

Since N-acetylprocainamide was identified in the urine of patients receiving procainamide, this compound has been studied both as a metabolite of procainamide and as a separate antiarrhythmic agent. N-acetylprocainamide absorption following oral administration is more than 8-% complete. 59 to 89% of N-acetylprocainamide is excreted unchanged in the urine in subjects with normal renal function. Deacetylation of N-acetylprocainamide to procainamide is a minor route of N-acetylprocainamide elimination. The half-life of N-acetylprocainamide in patients with normal renal function has been reported to vary between 4.3 and 15.1 hours. Total body clearance (mean +/- SD) of N-acetylprocainamide in patients with normal renal function has been reported to range from 2.08 +/- 0.36 ml/min/kg to 3.28 +/- 0.52 ml/min/kg. There is a linear relationship between N-acetylprocainamide clearance and creatinine clearance. The half-life of N-acetylprocainamide in functionally anephric patients may be as long as 42 hours; however, it can be effectively cleared from plasma by haemodialysis. N-acetylprocainamide is 10% protein-bound. There is an age-related decline in N-acetylprocainamide clearance, mostly due to the decrease in creatinine clearance that occurs with ageing. In the neonate, the half-life of acetylprocainamide is prolonged. Several therapeutic trials carried out to assess the effectiveness of N-acetylprocainamide in suppressing chronic ventricular premature beats have now been reported. If there is a therapeutic response to N-acetylprocainamide it will probably occur at a plasma concentration between 15 and 25 micrograms/ml. A high degree of overlap has been reported between the concentration range associated with arrhythmic suppression and the range of concentrations where intolerable side effects begin to occur. No severe cardiac toxicity has been reported with oral therapy despite plasma concentrations as high as 40 micrograms/ml. However, hypotension has been reported in association with a rapid intravenous bolus of N-acetylprocainamide. A maximum intravenous infusion rate of 50 mg/min has been recommended. N-acetylprocainamide in patients receiving procainamide; however, N-acetylprocainamide concentrations remain below the therapeutic range in patients with normal renal function. In patients with renal failure receiving procainamide, N-acetylprocainamide concentrations rise dramatically. The dose of N-acetylprocainamide must be adjusted in patients with renal insufficiency, and it should be used more cautiously in the very old and very young. N-acetylprocainamide plasma concentration monitoring would be valuable clinically in patients with renal insufficiency receiving either N-acetylprocainamide or procainamide, and in the very young and the aged.

Metabolite cumulation during long-term oral encainide administration

Cumulation of encainide and its major metabolites, O-demethylencainide (ODE), 3-methoxy-ODE (MODE), and N-demethylencainide (NDE) was examined in patients with frequent complex ventricular ectopy. After 6 mo on encainide patients were admitted to Stanford University Hospital and the drug was discontinued for 24 hr. During this time blood samples were drawn to characterize the cumulation and disposition of the drug and metabolites. The mean steady-state concentrations of encainide, ODE, and MODE were 56.3, 214.6, and 184.6 ng/ml after doses ranging from 100 to 250 mg/day. The concentration ratios of ODE/encainide and MODE/encainide were 5.02 +/- 2.61 and 5.15 +/- 4.13. NDE was detected in the plasma of only one patient. Elimination half lifes of encainide and ODE were 1.16 +/- 0.5 and 11.41 +/- 9.58 hr. MODE disappeared slowly and at 24 hr the plasma concentration was still 59.8 +/- 39.9% of its mean steady-state concentration. Our data indicate that the metabolites of encainide cumulate in the plasma of patients on long-term oral therapy and must be considered when evaluating its clinical efficacy.

Myocardial disposition and cardiac pharmacodynamics of verapamil in the dog

The disposition of verapamil was studied in anesthetized open-chested dogs following administration of intravenous doses of 0.5 mg/kg. The plasma and myocardial verapamil concentration-time data were fit to a three-compartment model to describe the disposition kinetics. The distribution equilibrium between myocardium and plasma was achieved rapidly and the concentration of verapamil decayed in parallel in these two tissues. The partition coefficient which describes the time averaged myocardial/plasma concentration ratio was 6.21 +/- 2.38. Examination of the relationship between the plasma and myocardial concentrations and the time course of the effect of verapamil, as defined as PR interval prolongation, revealed a hysteresis effect in some dogs. Despite this hysteresis, there was a linear relationship between plasma and myocardial concentrations of verapamil and the degree of prolongation of the PR interval. The results of this study indicate that the concentration in the plasma is in equilibrium with the myocardium and changes in plasma concentration are indicative of parallel changes in myocardial levels. The effect of verapamil on the atrioventricular node is related to the concentration in the myocardium and plasma, but there is substantial interanimal variability in the sensitivity to verapamil.

Clinical pharmacology and antiarrhythmic efficacy of encainide in patients with chronic ventricular arrhythmias

We determined the pharmacokinetics, efficacy and therapeutic plasma concentration of encainide, a new antiarrhythmic drug that affects His-Purkinje conduction but not ventricular refractoriness. Nine patients with frequent and complex premature ventricular complexes were studied in a 3-day double-blind protocol. Each day, each patient received 75 mg of i.v. or oral encainide or placebo. Frequent blood samples for encainide plasma concentration determination and continuous ambulatory ECGs were obtained. There was a marked intersubject variation in bioavailability (mean 42 +/- 24%, range 7.4-82%), clearance (13.2 +/- 5.6 ml/min/kg, range 3.75-22.1 ml/min/kg) and half-life (3.4 +/- 1.7 hours i.v., 2.5 +/- 0.8 hours oral). Eight of nine patients had more than 90% suppression of premature ventricular complexes for 3-36 hours. Minimal antiarrhythmic plasma concentration was higher (39 +/- 54 ng/ml, range 3.5-170 ng/ml) after i.v. dosing than after oral dosing (14 +/- 16 ng/ml, range 1.5-48 ng/ml), suggesting an active metabolite after oral dosing in many patients. Minimal side effects were seen despite high peak plasma concentrations (range 794-1556 ng/ml i.v., 36-495 ng/ml oral). The minimal ratio of toxic to therapeutic plasma concentration ranged from 4.3-326 (median 23) after oral dosing. Antiarrhythmic action was associated with an 11-44% widening of the QRS complex that was not associated with other adverse effects. We conclude that encainide effectively suppresses ventricular arrhythmias. Despite a variable bioavailability, high clearance and short half-life, its wide ratio of toxic to therapeutic concentration and probable active metabolite permit a long duration of action, which should allow a reasonable dose schedule in most patients during chronic oral dosing.