Quantitative correlation of plasma and myocardial quinidine concentrations with biochemical and electrocardiographic changes

Effects of quinidine on repolarization in canine epicardium, midmyocardium, and endocardium: I. In vitro study

BACKGROUND

The antiarrhythmic action of quinidine is associated with a slowing of conduction and prolongation of repolarization. The latter effect has no consistent correlation with quinidine actions on action potential duration (APD) in isolated tissue experiments. To enhance our understanding of the mechanisms of quinidine action, we studied its effect on APD in canine epicardial, midmyocardial, and endocardial tissues.

METHODS AND RESULTS

Standard microelectrode techniques were used to study the effects of quinidine 2.5 to 20 micromol/L on APD in ventricular epicardial, endocardial, and transmural (M-cell) slabs at cycle lengths (CLs) from 300 to 4000 ms. Qualitatively different time courses of actions and concentration- and rate-dependent effects were seen in M cells compared with the others. In endocardium and epicardium, quinidine induced monotonic and concentration-dependent APD prolongation at all CLs. In contrast, the effects of quinidine in M cells varied from prolongation to shortening, depending on duration of superfusion, concentration, and CL. Experiments with E4031 and TTX suggested that in M cells, quinidine-induced APD lengthening was attributable to block of delayed rectifier potassium current and APD shortening was due to inhibition of TTX-sensitive steady-state sodium current.

CONCLUSIONS

In vitro, there is a significant difference of quinidine effects in M cells versus epicardial and endocardial cells that appears to reflect differences in the contributions of specific ion channels to the APD at the three sites. The differences may influence the actions of quinidine on repolarization of the heart in situ and determine both the proarrhythmic and antiarrhythmic actions of the drug.

Treatment of atrial fibrillation in horses: new perspectives

Forty-one horses were treated for atrial fibrillation (AF) with 22 mg/kg quinidine sulfate via nasogastric tube every 2 hours until conversion to sinus rhythm, a cumulative dose of 88 to 132 mg/kg had been administered in 2-hour increments, or the horse had adverse or toxic effects from the drug. Treatment intervals were prolonged to every 6 hours if conversion had not occurred. Digoxin was administered before treatment if the horse had a fractional shortening < or = 27% (3 horses), was prone to tachycardia (resting heart rate > or = 60 beats/min) (1 horse), or had a previous history of sustained tachycardia of over 100 beats/min during prior conversion (3 horses). Digoxin was administered during day 1 of quinidine sulfate treatment if the horse developed a sustained tachycardia of over 100 beats/min during treatment (11 horses) or on day 2 if conversion had not occurred (7 horses). Plasma quinidine concentrations within 1 hour of conversion of AF to sinus rhythm ranged from 1.7 to 7.5 micrograms/mL (mean, 4.05 +/- 1.6) and ranged from 1.7 to 4.7 micrograms/mL in 97% of horses. Most horses (92%) with plasma quinidine concentrations > 5 micrograms/mL exhibited an adverse or toxic effect of quinidine sulfate (clinical or electrocardiographic). There was no statistical association between plasma quinidine concentrations and sustained tachycardia (> 100 beats/min), diarrhea, or colic. Ataxia and upper respiratory tract stridor were significantly associated with plasma quinidine concentrations. In most instances (98%) conversion did not occur while toxic or adverse effects of quinidine sulfate were present or when plasma quinidine concentrations were > 5 micrograms/mL.

Quinidine: an update on therapeutics, pharmacokinetics and serum concentration monitoring

After 70 years of regular use, quinidine remains one of the most useful drugs in the management of both supraventricular and ventricular arrhythmias. Quinidine sulfate, from conventional dosage forms, is absorbed rapidly and reaches peak levels in 1 hour. Sustained-release quinidine sulfate and gluconate are absorbed more slowly and, thus, may provide a more satisfactory response with less fluctuation in the peak and trough at quinidine concentrations. The half-life and absorption characteristics of sustained-release quinidine suggest that an 8- to 12-hour dosage regimen will produce smooth, sustained quinidine levels. Whereas quinidine and digitalis have been used concomitantly for years, dramatic elevations in blood digoxin levels have been observed. When quinidine is added to the regimen of a patient taking digitalis, these increased digoxin levels may be responsible for some of the adverse effects previously attributed to quinidine administration and can be reduced by adjustment of the digoxin dosage. Drug monitoring of serum quinidine concentrations as well as peak levels is essential to assess patient compliance and to determine that therapeutic levels are maintained. This is especially important because of the drug's relatively narrow therapeutic index and the changes in clearance that can result from drug interactions and renal and hepatic disturbances. A personal computer program is available to carry out pharmacokinetic calculations and assist in determining appropriate doses for individual patients.

Intravenous quinidine: relations among concentration, tachyarrhythmia suppression and electrophysiologic actions with inducible sustained ventricular tachycardia

A computer simulation was used to devise quinidine sulfate infusions to produce pseudo-steady-state concentrations in the low (8 microM/liter) and high (14 microM/liter) therapeutic ranges, avoiding high peak concentrations. Using this infusion, efficacy and electrophysiologic actions of quinidine sulfate were assessed in 21 patients with sustained inducible ventricular tachycardia (VT) when concentrations were 12.6 +/- 11 microM/liter (mean +/- standard deviation) and 18 +/- 9 microM/liter. Although mean concentrations approximated target levels, there was substantial individual variation. A reciprocal linear relation (r = 0.8, p less than 0.01) was noted between resultant serum concentrations and drug-free ejection fraction (EF). Transient hypotension occurred early in 3 patients, 2 of whom had a normal left ventricular (LV) EF. No hemodynamic compromise was seen in patients with LVEFs of less than 30%. Induced VT was suppressed in 5 patients at low concentrations and in an additional 4 at high concentrations (total 9 of 21, 42%). Concentration-dependent changes in the ventricular effective refractory period of the beat induced by S3 paralleled antiarrhythmic efficacy. Independent of response or lack of response to intravenous quinidine, 17 patients received gradually increasing oral quinidine dosages adjusted to reproduce plasma levels that had been effective during intravenous administration, or to maximal well-tolerated dosage (if side effects occurred). VT was still inducible during oral treatment in 4 of 5 patients in whom VT had been suppressed during the intravenous infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum and myocardial kinetics of amiodarone and its deethyl metabolite after intravenous administration in rabbits

The serum kinetics of amiodarone and its major metabolite the deethyl analogue were studied in rabbits after intravenous administration. The elimination of the drug and the metabolite from serum occurred as a biexponential function. Both compounds exhibited a rapid distribution phase (6.5 and 4.4 min, respectively) and had elimination half-lives of 136 and 235 min, respectively. There was a rapid uptake of both drugs by the myocardium, with maximal concentrations at 5 and 15 min. The myocardial concentrations were higher than the respective serum concentrations and declined with time. There was a wide scatter in myocardium-serum ratios, which ranged from 1 to 11 for amiodarone and 12 to 29 for the metabolite. Neither the drug nor the metabolite produced significant changes in the surface electrocardiogram after intravenous administration. These data suggest that accumulation of the metabolite does not account for the slow onset of action of amiodarone.

Effect of quinidine on the digoxin receptor in vitro

To investigate the basis for a clinically important digitalis-quinidine interaction that is characterized by increases in serums digoxin concentrations when quinidine is administered to digoxin-treated patients, we have studied in vitro the interaction of quinidine with the digoxin receptor. Evidence has been obtained that quinidine is capable of decreasing the affinity for digoxin of cardiac glycoside receptor sites on purified Na,K-ATPase and on intact human erythrocyte membranes. As others have shown, quinidine is capable of inhibiting Na,K-ATPase activity, and evidence has been obtained in the current study that, while quinidine can reduce the affinity of the enzyme for digoxin, it is also capable of acting together with digoxin in inhibiting enzyme activity to a degree greater than the inhibitory effect of digoxin alone. The concentrations of digoxin and quinidine used in this study were considerably greater than their therapeutic serum concentrations. Nevertheless, these observations are consistent with the hypothesis that the increases in serum digoxin concentrations and the decreases in volumes of digoxin distribution observed clinically when quinidine is administered to digoxin-treated patients may reflect, at least in part, a decrease in the affinity of tissue receptors for digoxin. The possibility must also be considered that enhanced cardiac effects of digoxin may occur clinically as the result of an augmentation, by quinidine, of digoxin effects, which more than compensates for the modest reduction in digoxin binding.

The effect of quinidine and its metabolites on the electrocardiogram and systolic time intervals: concentration--effect relationships

1 A combined pharmacokinetic and pharmacodynamic model has been used to analyze the relationship between electrocardiographic (ECG) and systolic time intervals (STI) and changes in plasma concentration of quinidine after oral and i.v. doses in ten normal subjects. 2 The major effects of quinidine were on cardiac repolarization. Contrary to previous descriptions, we found no important change in the U wave, but the T wave was split into two peaks. The amplitude of these two peaks (T and T') was reduced, and the QT' peak and QT intervals were prolonged. The QT peak interval and systolic intervals did not change appreciably. There were small increases in the PQ and QRS intervals. 3 The effect of quinidine on the QT interval could be explained by a linear pharmacodynamic model. The equilibration between plasma and effect site had a half-time of 8 min. The slope of the pharmacodynamic model was 20.3 ms . mg 1(-1) after i.v. dosing and 33.5 ms . mg 1(-1) after oral dosing. 4 The difference in effect model slopes suggests pharmacologically active metabolites of quinidine are formed during absorption from the gut. 5 The total effect of a single oral dose of quinidine appears to be the same as the same dose given intravenously, even though only 70% of the oral dose reaches the systemic circulation as quinidine.

Comparative clinical evaluation of two salts of quinidine using 24-hour Holter monitor recordings

In 13 patients admitted to a coronary care unit, antiarrhythmic efficacy and safety of quinidine gluconate in a nonmatrix formulation were compared to placebo and quinidine sulfate. Both quinidine preparations were effective in reducing the number and severity of ventricular and supraventricular extrasystoles when compared to placebo. Except for a few cases in which quinidine gluconate was significantly more effective, both quinidine preparations appear to have similar clinical antiarrhythmic efficacy and duration of action. A 24-hour Holter ECG monitoring device and electrocardioscanner were utilized in this research. No major unexpected adverse effects were observed.

Adverse reactions to quinidine in hospitalized patients: findings based on data from the Boston Collaborative Drug Surveillance Program

This study systematically analyzed the incidence and variety of adverse reactions to quinidine sulfate in 652 consecutively monitored hospitalized patients. Fourteen percent of the patients sustained adverse reactions of sufficient severity to warrant discontinuation of the drug. These reactions include GI intolerance, cardiac arrhythmias, fever with and without associated hepatic dysfunction or leukopenia, cinchonism, and hemolytic anemia. Cardiac arrhythmias were infrequent and, when present, generally occurred within 3 days of institution of quinidine therapy. There were no quinidine-related fatalities in this study group.

The pharmacokinetics and organ distribution of ajmaline and quindine in the mouse

After i.v. infusion into mice (lasting 10 s) the time courses of ajmaline and quinidine concentrations in blood, heart, lung, liver, and brain were studied. The drugs were assayed by a spectrofluorophotometric procedure. Blood concentration data obtained were fitted graphically and calculations were performed in accordance with an open two compartment model. Blood kinetic data were very similar for both alkaloids. A rapid distribution phase with a t0.5alpha of 3.0 min for ajmaline and 2.5 min for quindine was followed by a disposition phase with a t0.5beta of 16 min for ajmaline and 20 min for quinidine. High tissue accumulation of both alkaloids was found in lung, liver, and heart and this is also reflected by the volume of distribution Vdbeta, which was 136 ml for ajmaline and 116 ml for quinidine (body weight of the mice = 31 g). With equilibrium dialysis a 62% binding of ajmaline and a 77% binding of quinidine to mouse blood constituents was found. Both drugs were highly metabolized since only 5% of a given dose was excreted unchanged in the urine.