Propafenone

Propafenone in a usual dose produces severe side-effects: the impact of genetically determined metabolic status on drug therapy

We report the case of an elderly lady presenting with dizziness, a head injury resulting from a fall and bradycardia. Propafenone 150 mg t.i.d. had been prescribed for atrial fibrillation with tachyarrhythmia, induced by hyperthyroidism, 18 months earlier. A toxic concentration of parent propafenone, and no 5-hydroxy metabolite, was detected in a plasma sample. Symptoms disappeared after the discontinuation of propafenone. The poor metaboliser (PM) phenotype of sparteine/debrisoquine was assumed and subsequently confirmed by phenotyping (sparteine test) and genotyping (allele-specific polymerase chain reaction). The PM phenotype is common in European populations, with a prevalence of about 7%. If drugs with narrow therapeutic ranges undergo genetically polymorphic metabolism, toxicity may arise even with recommended doses. Individualization of doses is required to avoid adverse effects.

Influence of renal function on the steady-state pharmacokinetics of the antiarrhythmic propafenone and its phase I and phase II metabolites

The aim of this study was to investigate the disposition of propafenone and its Phase I and II metabolites in relation to kidney function under steady-state conditions. The mechanism of the renal handling of propafenone glucuronides (filtration, secretion) was also examined. Racemic (R/S) propafenone was administered to 7 young volunteers, to 5 older patients with a normal glomerular filtration rate and to 4 patients with chronic renal failure. No difference was found in the plasma concentrations of propafenone and 5-hydroxypropafenone between the three groups. The propafenone glucuronide (PPFG) concentration was elevated in the older compared to the younger subjects (S-PPFG: 544 vs. 222 nmol.ml-1.mol-1; R-PPFG: 576 vs. 304 nmol.ml-1.mol-1). Although Glomerular filtration rate did not differ, the renal clearance of propafenone glucuronides was reduced in the former group, which could be attributed to their impaired renal secretion. A dramatic increase in propafenone glucuronide concentration was observed in the patients with renal failure (S-PPFG: 2783 nmol.ml-1.mol-1; R-PPFG: 7340 nmol.ml-1.mol-1). In summary, the disposition of propafenone and of its active metabolite 5-hydroxypropafenone was not affected by kidney dysfunction, indicating that no dose adjustment is necessary in patients with renal failure. The accumulation of drug glucuronides in older patients with apparently normal kidney function should be taken into account as a possible factor modifying drug therapy.

Voltage- and time-dependent inhibitory effects on rat aortic and porcine coronary artery contraction induced by propafenone and quinidine

1. Class I antiarrhythmic drugs (e.g. Na+ channel blockers) such as propafenone and quinidine also inhibit voltage-gated Ca2+ and K+ channels. In the present paper the voltage- and time-dependent inhibitory effects of propafenone and quinidine were studied on depolarization-induced vascular contractions and 45Ca2+ uptake in isolated endothelium denuded rat aorta and pig left descending coronary artery. 2. Quinidine and propafenone (10(-7) M -5 x 10(-5) M) produced a concentration-dependent relaxation of the contractions induced by 80 mM KCl. Propafenone was significantly more potent (P < 0.05) than quinidine in both rat aorta and pig coronary arteries but both drugs more potent (P < 0.05) in relaxing rat aorta than pig coronary arteries. In rat aortic rings, the relaxant effects of propafenone were unaffected by pretreatment with the Na+ channel blocker, tetrodotoxin. 3. The degree of inhibition produced after prolonged exposure (40 min) to propafenone and quinidine differed as the time of depolarization with 80 mM KCl was increased. Quinidine (3 x 10(-6) M, 10(-5) M and 3 x 10(-5) M) not only produced an inhibition at the very early stage of contraction, but also a time-dependent inhibition was observed. In contrast, propafenone (10(-6) M, 3 x 10(-6) M and 10(-5) M) produced a more marked concentration-dependent early block but only a mild time-dependent inhibition.4. The voltage-dependence of propafenone- and quinidine-induced inhibition, was studied in rat aorta and coronary arteries which had been incubated in 5 or 40mM KCl Ca2+-free solution and then contracted by changing the bath solution to 100 mM KCI and 2 mM CaCl2 solution. The inhibitor effects of quinidine were significantly enhanced (P <0.05) when the preparations were preincubated in 40 mMKCl (depolarizing) solution. In contrast, the effects of propafenone were quite similar in 5 or in 40 mMKCI solution.5. Quinidine, 10-5 M, produced a greater inhibition (P<0.05) of 100 mM KCl-stimulated 45Ca2+ uptake in aortic rings preincubated in depolarizing as compared to normal solution. In contrast, the inhibition produced by 3 x 10-6 M propafenone was similar in aortic rings incubated in 5 or 40 mM KCl solution.6.It is concluded that both quinidine and propafenone inhibited vascular smooth muscle contraction which could be attributed to reduced Ca2+ entry. The voltage- and time-dependent inhibitory effects of quinidine may reflect an increased binding of the drug to Ca2+ channels at depolarized potentials.

Regular ventricular rhythms before conversion of recent onset atrial fibrillation to sinus rhythm

The incidence of fast atrial tachycardias with regular ventricular rhythm was assessed in a population of 243 patients with recent onset (< 72 hours) atrial fibrillation (AF), without heart failure, randomly treated with single loading oral dose of propafenone (600 mg), flecainide (300 mg), digoxin (1 mg), or placebo for acute conversion to sinus rhythm (SR). Fast atrial arrhythmias developed in 14 (6%) patients: 6/92 treated with propafenone, 3/34 treated with flecainide, 1/25 treated with digoxin, and 4/92 who received placebo (P = NS). Heart rate > 175 beats/min with 1:1 AV conduction ensued in 4 cases: 2 treated with flecainide and 2 treated with placebo; in the other cases 2:1 AV conduction was observed. Widening of QRS during regular tachycardia was observed in 4 patients; 3 who received propafenone and 1 who received flecainide. Conversion to SR within 4 hours was achieved in 55/92 (60%) patients treated with propafenone, 20/34 (59%) patients treated with flecainide, 7/25 (28%) patients treated with digoxin, and 19/92 (20%) treated with placebo (P < 0.001 propafenone vs placebo and flecainide vs placebo; P < 0.05 propafenone vs digoxin and flecainide vs digoxin). Periods of regular tachycardia are expected in recent onset AF and may not necessarily represent a proarrhythmic effect of Class 1C drugs, rather than mark the transition from AF to SR. Class 1C agents are probably responsible for widening of the QRS complex seen during these tachycardias. Propafenone and flecainide appear equally effective in converting recent onset AF.

New antiarrhythmic drugs in pediatric use: propafenone

Propafenone hydrochloride, a class 1C antiarrhythmic agent, combines sodium channel-blocking effects with beta-blocking capacities and a weak calcium antagonism. The drug exerts marked electrophysiologic effects on accessory atrioventricular pathways. In patients with atrioventricular nodal reentry tachycardia, propafenone is able to block conduction in the fast conducting pathway. In addition, propafenone is very effective in young patients with supraventricular tachycardia based on enhanced abnormal automaticity. In pediatric patients, left ventricular performance remains unimpaired. Proarrhythmic events have been noted in children only occasionally. In accordance with the electrophysiologic profile, intravenous and oral propafenone is an effective agent for treatment of supraventricular tachycardia based on a reentry mechanism and due to abnormal automaticity (i.e., supraventricular tachycardia based on an accessory atrioventricular pathway, atrioventricular nodal reentry tachycardia, junctional ectopic tachycardia, and atrial ectopic tachycardia). In children with ventricular dysrhythmias, efficacy seems to be related to the underlying cardiac diagnosis. Propafenone is well tolerated in the majority of young patients. Incidence of proarrhythmic events seems to be lower with propafenone than with other class 1C agents. However, the risk of these serious adverse events should be taken into account when therapy with propafenone is considered, particularly in patients with structural heart disease.

Propafenone overdose

Propafenone is an infrequently used class IC antiarrhythmic drug. We report our experience with a patient who overdosed with propafenone and developed coma, seizures, bradycardia, hypotension, and conduction delay. The clinical manifestations and management of this patient are discussed in light of the known pharmacology of propafenone and compared with the limited number of cases that appear in the literature.

Sixty-six episodes of ventricular fibrillation within four hours: electrical stability restored by propafenone

Ventricular fibrillation is a complication in patients hospitalized for acute myocardial infarction. We report the case of an unusually high number of episodes of ventricular fibrillation (66 episodes) that occurred within a period of 4 h in a patient with recent anterior myocardial infarction. In patients with acute or recent myocardial infarction and recurrent episodes of ventricular fibrillation, intravenous lidocaine is the antiarrhythmic medication of choice. In our patient, recurrences of ventricular fibrillation were not prevented either by lidocaine, or amiodarone. The rate of recurrence of ventricular fibrillation, however, decreased dramatically, twice, in the minutes following two separate boluses of propafenone, and the arrhythmia did not recur when continuous infusion of propafenone was started. We cannot exclude that were the combined antiarrhythmic effects of lidocaine, amiodarone and propafenone that led to the final control of the arrhythmia.

Propafenone and disopyramide enhance post-ischemic contractile and metabolic recovery of perfused hearts

The effects of sodium channel blockers, propafenone and disopyramide, on post-ischemic contractile dysfunction of perfused rat hearts were examined. Isolated hearts were subjected to 35 min ischemia, followed by 60 min reperfusion with and without administration of either drug during 3 min of pre-ischemia. Ischemia/reperfusion induced complete cardiac dysfunction, rise in left ventricular end-diastolic pressure, increase in perfusion pressure, accumulation of Na+ and Ca2+ and loss of K+ and Mg2+, and release of creatine kinase and purine nucleosides and bases from the heart. These observations suggest that ischemia/reperfusion in the current study induces cardiac cell necrosis or an increase in cell membrane permeability to ions, substrates and macromolecules. Treatment of perfused hearts with either propafenone at concentrations ranging from 5 to 70 microM, or disopyramide at concentrations of 100 microM or higher resulted in a pronounced contractile recovery of the heart, associated with suppression of reperfusion-induced tissue ion alteration and inhibition of reperfusion-induced release of creatine kinase and purine nucleosides and bases. Ischemic insult itself caused tissue Na+ accumulation and K+ loss without any change in tissue Ca2+ and Mg2+. The alterations in the electrolytes were attenuated by treatment with either agent. The results suggest that prevention of ischemia- and reperfusion-induced ionic disturbance of cardiac cells by propafenone and disopyramide plays a role in the improvement of post-ischemic contractile dysfunction.

Effect of sotalol on heart rate variability assessed by Holter monitoring in patients with ventricular arrhythmias

Reduced vagal activity has been demonstrated to be associated with an increased risk of sudden death. Assessing the heart rate variability as a measure of the autonomic control of the heart has been established as a useful tool for the risk stratification of patients after myocardial infarction. In the current study, heart rate variability assessed by time- and frequency-domain measures was determined from Holter recordings before and during treatment with sotalol in 28 patients with chronic ventricular arrhythmias. The heart rate variability at baseline was independent of the presence or absence of spontaneous arrhythmias and of left ventricular function. Therapy with sotalol produced a significant improvement over control values in indices of parasympathetic tone (root mean square of the difference in successive RR intervals, proportion of adjacent RR intervals different by > 50 msec, high-frequency power spectrum). This improvement was not related to drug-induced changes in the mean heart rate or the suppression of ventricular ectopic activity. These effects on heart rate variability may contribute significantly to the overall efficacy profile of sotalol.

Propafenone-induced liver injury: report of a case and review of the literature

A case of an acute cholestatic syndrome associated with use of the antiarrhythmic drug propafenone is reported here. The close time relationship between the administration of the drug and the acute onset of the liver damage, the histological findings, and the reappearance of biochemical signs of liver dysfunction upon rechallenge with the same medication strongly suggest that propafenone was involved in the pathogenesis of this syndrome. Although rare, hepatotoxicity from this widely used antiarrhythmic medication should be kept in mind in the differential diagnosis of sudden cholestatic syndrome of obscure origin.

Effects of propafenone on calcium current in guinea-pig ventricular myocytes

1. The modulation of L-type voltage-sensitive calcium channels in guinea-pig isolated ventricular myocytes by propafenone was examined by the whole cell voltage-clamp technique. 2. Propafenone, 10(-7) -5 x 10(-5) M, produced a concentration-dependent inhibition of Ca current (ICa) without any significant change in the current-voltage relation. Half-blocking concentration (IC50) of propafenone for the peak ICa at +10 mV was 5 x 10(-6) M. 3. The voltage-dependence of ICa inactivation was shifted in the hyperpolarizing direction in the presence of 5 x 10(-6) M propafenone. 4. A frequency-dependent relative block by propafenone was observed after repetitive depolarizing test pulses at a frequency of 0.5 and 1 Hz. The recovery of ICa from inactivation was prolonged by propafenone and the reactivation exhibited an additional exponential component attributed to the slow release from drug block of Ca channels. 5. These results suggest that propafenone, at therapeutic concentrations exhibits Ca channel blocking properties that may be involved in its antiarrhythmic mechanism.

Beta-blocking effect of propafenone based on spectral analysis of heart rate variability

RR variability was analyzed in 15 patients with ventricular arrhythmias to evaluate whether the antiarrhythmic action of propafenone is associated with alteration of neural control mechanisms. Before drug administration, spectral analysis of RR variability was characterized by 2 major components at low and high frequency, which are considered to reflect sympathetic and parasympathetic modulation of the heart period. After propafenone (600 to 900 mg/day), there was a marked reduction in RR variance (826 +/- 184 to 412 +/- 77 ms2; p < 0.05), although the mean RR interval was unchanged. The drug significantly reduced the low-frequency component (52 +/- 6 to 28 +/- 4 nu) and augmented the high-frequency component (39 +/- 6 to 55 +/- 5 nu). As a result, the low-/high-frequency ratio (an index of sympathovagal balance) decreased from 2.0 +/- 0.4 to 0.6 +/- 0.1. A positive correlation between serum levels and drug-induced changes in the low-frequency component was also observed. Furthermore, the increase in the low-frequency component induced by tilt (53 +/- 5 to 79 +/- 3 nu) was markedly attenuated after drug administration (27 +/- 5 to 54 +/- 7 nu). Thus, propafenone administration is associated with changes in spectral components that are consistent with a beta-blocking effect of the drug.

Short-term and long-term treatment with propafenone: determinants of arrhythmia suppression, persistence of efficacy, arrhythmogenesis, and side effects in patients with symptoms

OBJECTIVE

To assess the clinical criteria predicting the short and long-term efficacy of propafenone, an agent with class IC antiarrhythmic activity and a broad pharmacological profile.

DESIGNS

Prospective study of propafenone at doses of 450 to 900 mg/day during a six week dose titration period (including a placebo phase with two separate 24 Holter recordings). Responders to treatment were followed for one year.

PATIENTS

One hundred patients with frequent ventricular arrhythmias (greater than 30 extrasystoles/h) of Lown class III and IVA/B and without evidence of myocardial infarction within the past six months.

ANALYSIS

Multivariate regression analysis of spontaneous arrhythmia variability and of different clinical variables to determine the short and long-term efficacy and safety of propafenone.

MEASUREMENTS AND MAIN RESULTS

Propafenone 450 mg/day was effective in 30/100 patients (30%), and at 600 mg/day another 14 responded. The efficacy of propafenone correlated with a low spontaneous arrhythmia variability and, as shown by multivariate analysis, with a lower patient age (p less than 0.05). When the dose was increased to 900 mg/day a further six (12%) patients responded. However, with increasing doses of propafenone, the one year probability of effective treatment decreased from 86% (450 mg/day) to 67% (600 mg/day) and to 44% (900 mg/day). After restudying the patients at three, six, and 12 months and after dose adjustment in 11/44 patients (25%), 31 patients (70%) remained responders. Loss of permanent antiarrhythmic efficacy was best predicted by the initial dose that achieved a response. No patient died suddenly or had arrhythmogenic effects during Holter monitoring. Side effects occurred in 36% of patients but these rarely limited long-term treatment.

CONCLUSIONS

A younger age, low spontaneous arrhythmia variability, and particularly a low titration dose were the best predictors of the short and long term efficacy of propafenone. All other responders should have repeated Holter recordings during the first year of treatment.

Influence of dietary fat on the pharmacodynamics of propafenone in isolated, perfused rabbit hearts

BACKGROUND

The fatty acid composition of the phospholipids in sarcolemma may significantly influence cell membrane functions. We evaluated the effects of dietary fat on the pharmacodynamics of the antiarrhythmic drug propafenone in isolated, perfused rabbit hearts.

METHODS AND RESULTS

Three groups of weanling rabbits (n = 9 each group) were fed diets of 10% wt/wt lard, fish oil, or safflower oil for 40 days. Differences in electrophysiological variables were assessed at baseline and during propafenone perfusion. Myocardial concentration-effect relations were determined by plotting electrophysiological effects versus coronary sinus propafenone concentrations. The linoleic acid content of isolated sarcolemma was higher in the safflower group (33.4 +/- 11.4%) than in the lard (13.4 +/- 2.3%, p less than 0.01) and fish oil (8.5 +/- 1.4%, p less than 0.01) groups, whereas the omega-3 fatty acid content was higher in the fish oil group (p less than 0.01). During propafenone perfusion, greater changes in ventricular conduction time were observed in the lard group (22 +/- 11 msec) than in the safflower oil group (10 +/- 7 msec, p less than 0.05), whereas changes in ventricular conduction time in the fish oil group (16 +/- 7 msec) were intermediate between the lard and safflower oil groups. The slopes of the linear myocardial concentration-effect relations describing changes in QRS duration were steeper in the lard group (0.22 +/- 0.07 msec/micrograms/ml) than in the safflower oil group (0.13 +/- 0.04 msec/micrograms/ml, p less than 0.01) but not in the fish oil group (0.17 +/- 0.08 msec/microgram/ml, p = NS). Strength-interval curves were similar at baseline in all three groups. During propafenone perfusion, the threshold current was increased significantly at long coupling intervals (250-380 msec) in the lard group (1.8 +/- 1.0 mA) compared with the safflower oil group (0.8 +/- 0.6 mA, p less than 0.05) but not compared with the fish oil group (1.2 +/- 0.6 mA, p = NS).

CONCLUSIONS

Dietary fat significantly alters the fatty acid composition of the phospholipids in sarcolemma. Propafenone effects on ventricular conduction time and ventricular excitability are significantly influenced by the type of dietary fat.

Identification of propafenone metaboliser phenotype from plasma and urine excretion data

The aim of the study was to validate a test based on analyses of urine to identify the propafenone metaboliser phenotype during routine chronic therapy. Twenty seven patients chronically treated with propafenone were studied. A debrisoquine test was performed in 10. Propafenone and its metabolites in plasma and urine were measured by HPLC. Propafenone, 5-hydroxypropafenone and N-depropylpropafenone concentrations in plasma were 1.09, 0.182 and 0.101 ng.ml-1, respectively. Total recovery of the administered dose in urine was 30.7%. Two patients were identified as PM, based on the result of the debrisoquine test (log D/4OHD of 1.26 and 1.36). This finding was confirmed by the propafenone metabolic ratio in urine, but the plasma data did not permit clearcut separation of the phenotypes. Propafenone/5-hydroxypropafenone in plasma was not a good predictor of metabolizer phenotype. Although the number of patients who completed all three tests was limited, it is concluded that analysis of propafenone/5-hydroxypropafenone in urine collected between two consecutive doses at steady-state is more practical than the debrisoquine test and more specific than determining the propafenone/5-hydroxypropafenone ratio in plasma, for identification of the propafenone metaboliser phenotypes.

Acute effects of intravenous propafenone on the internal ventricular defibrillation threshold in the anesthetized dog

In 10 treated and 2 control dogs, the short-term effects of intravenous propafenone (2 mg/kg/10 minutes, followed by 1 mg/min [n = 2] or 25 micrograms/kg/min [n = 8]) on the internal ventricular defibrillation energy requirements (DER) were investigated. Multiple stored energy levels were randomly tested and the percent successful defibrillation was plotted against the stored energy, and the raw data were fit by logistic regression. The energy at 50% (E50) and 80% (E80) defibrillation success increased after propafenone by a mean of 75% (8.4 +/- 2.4 to 14.7 +/- 5.9 joules, p less than or equal to 0.05) and 59% (11.1 +/- 3.5 to 17.6 +/- 6.7 joules, p less than or equal to 0.05), respectively. Plasma propafenone levels ranged from 778 to 2554 ng/ml (1495 +/- 592 ng/ml) at the beginning to 833 to 2193 ng/ml (1297 +/- 389 ng/ml) at the end of the defibrillation trials. Two dogs served as controls and received Ringer's solution instead of propafenone and showed the temporal stability of the preparation. In conclusion, intravenous propafenone increases the internal ventricular DER in this canine model. This may have important clinical implications in patients with automatic implantable cardioverter-defibrillators (AICDs) receiving concomitant antiarrhythmic drug therapy and in patients undergoing therapy with intravenous propafenone.

Efficacy and safety of intravenous and oral propafenone in pediatric cardiac dysrhythmias

Propafenone was administered to 58 patients with a mean age of 3.2 years (range 0.1 to 16). Mean intravenous dose was 1.2 mg/kg body weight (range 0.3 to 1.5 mg). The final mean oral maintenance dose was 308 mg/m2 body surface area (range 200 to 600 mg/m2, 16.8 mg/kg body weight). After intravenous application, propafenone was effective in 21 of 36 patients; atrial flutter was converted in 1 of 5 patients, and reentry supraventricular tachycardia was controlled in 15 of 25 patients. Propafenone was partially or completely effective in 3 of 4 patients with chaotic atrial tachycardia. Junctional ectopic tachycardia was suppressed in 2 infants. Thirty-seven patients had oral treatment with a mean follow-up of 2.2 years. Propafenone was effective in 33 of 37 patients (89%); atrial flutter was controlled in 2 patients, ventricular ectopy was suppressed in 1 of 2 patients. In reentry supraventricular tachycardia, propafenone was effective in 25 of 28 patients. Chaotic atrial tachycardia (n = 3) and junctional ectopic tachycardia (n = 2) were controlled after successful intravenous therapy. Systemic side effects were rare. Two patients developed a proarrhythmic effect, and 1 patient with ventricular ectopy after repair of tetralogy of Fallot died suddenly during propafenone maintenance therapy.

Effects of propafenone on 45Ca movements and contractile responses in vascular smooth muscle

1. In rat isolated aorta the class Ic antiarrhythmic drug, propafenone, dose-dependently inhibited the contractile responses induced by high K (80 mM) and noradrenaline (NA, 10(-5) M), the IC50s being 2.5 +/- 0.7 x 10(-6) M and 8.7 +/- 0.8 x 10(-6) M, respectively. These inhibitory actions were also observed with propafenone added after the induced contractions. 2. Contractile responses induced by addition of Ca to 0 Ca high-K solution were also inhibited by propafenone (IC50 = 2.5 +/- 0.8 x 10(-6) M). Moreover, propafenone inhibited the contractile responses elicited by NA in strips incubated in 0 Ca (IC50 = 1.9 +/- 0.9 x 10(-6) M). 3. Propafenone also inhibited (IC50 = 1.2 +/- 0.4 x 10(-5) M) the development of spontaneous mechanical activity in portal vein segments. 4. Propafenone, 5 x 10(-6) M and 10(-5) M, inhibited 45Ca uptake stimulated by high K or NA without altering 45Ca uptake in resting strips. 5. These results indicated that in rat isolated aortae and portal veins propafenone inhibited Ca entry through voltage-operated channels and NA-induced Ca uptake as well as Ca release from intracellular stores. As a consequence it would reduce the concentration of intracellular free Ca available at the contractile apparatus for vascular smooth muscle contraction.

The role of genetically determined polymorphic drug metabolism in the beta-blockade produced by propafenone

Propranolol and the sodium-channel-blocking antiarrhythmic agent propafenone share structural features. Although propafenone's beta-blocking actions are readily demonstrable in vitro, clinically significant beta-blockade occurs inconsistently in vivo. In this study, we tested the hypothesis that genetically determined variations in the biotransformation of propafenone to its 5-hydroxy metabolite account for variations in the drug's beta-blocking action. We assessed beta-blockade by measuring the reduction in tachycardia produced by boluses of isoproterenol and treadmill exercise in 14 normal subjects during treatment with placebo and with 150, 225, and 300 mg of propafenone every eight hours for five days each. Nine subjects (with the extensive-metabolizer phenotype) metabolized most of the propafenone to 5-hydroxy propafenone, and five (with the poor-metabolizer phenotype) did not produce this metabolite. At the lower dosages, beta-blockade was present in both groups but was significantly greater in the subjects with poor metabolism, in whom deficient 5-hydroxylation was associated with higher plasma propafenone levels. At the highest dose, a similar degree of beta-blockade was observed in the two groups. Propafenone also had a higher affinity for beta 2 receptors in vitro than either of its major metabolites. We conclude that the degree of beta-blockade during propafenone therapy reflects genetically determined variations in the metabolism of the parent drug, which is necessary for beta-blockade, and that this action of propafenone is considerably enhanced in patients with deficient 5-hydroxylation of propafenone.