Electrophysiologic comparative study of procainamide and N-acetylprocainamide in anesthetized dogs: concentration-response relationships

Comparison of Acute Hemodynamic Effects of Lidocaine and Procainamide for Postoperative Ventricular Arrhythmias in Dogs

Heart rate and systolic, diastolic, and mean pressures were measured in two groups of dogs during treatment of postoperative ventricular arrhythmias either with intravenous (IV) 2% lidocaine hydrochloride or procainamide hydrochloride. Hemodynamic parameters were not significantly changed after IV administration of either drug. Additionally, changes in hemodynamic parameters for dogs treated with 2% lidocaine were not significantly different from those of dogs treated with procainamide. When dosed appropriately in the clinical setting, one bolus of IV procainamide was safe for the treatment of postoperative ventricular arrhythmias.

Classification and pharmacology of antiarrhythmic drugs

Despite the emergence of several forms of nonpharmacologic therapy for cardiac arrhythmias, antiarrhythmic drugs continue to play an important role in the management of patients with this common clinical problem. The key to the proper use of antiarrhythmic drugs is a thorough knowledge of their mode of action and pharmacology. The pharmacology of antiarrhythmic drugs is particularly important because patients with cardiac arrhythmias frequently have multiorgan disease, which may influence the metabolism and elimination of antiarrhythmic drugs. The accumulation of toxic amounts of these agents can lead to dire effects including, but not limited to, ventricular proarrhythmia and malignant bradycardia. The goals of pharmacologic therapy of cardiac arrhythmia are to provide the maximum benefit in terms of arrhythmia suppression while maintaining patient safety. To accomplish these goals, a knowledge of the pharmacology of several antiarrhythmic drugs is mandatory.

Application of computer-assisted radiotelemetry in the pharmacokinetic and pharmacodynamic modeling of procainamide and N-acetylprocainamide

The cardiovascular pharmacodynamics (PD) of procainamide and N-acetylprocainamide have not been well characterized in small rodents without the presence of anesthesia or restraint. This study was undertaken to examine the pharmacokinetics (PK) and PD relationship of procainamide and N-acetylprocainamide by use of electrocardiogram (ECG) telemetry in unrestrained rats. Male Sprague Dawley rats received the following treatments: vehicle, procainamide 50 and 100 mg/kg and N-acetylprocainamide 50 and 100 mg/kg via intraperitoneal (i.p.) administration. Blood samples were collected over 8 h and subsequently analyzed. PD measurements (PQ, QS, QR, QT, RR, and HR) were collected prior to dosing and over a 24 h period. Mean PK parameters after the 50 mg/kg dose were as follows: Cls/Fprocainamide = 86.42 mL min-1 kg-1, Cls/FN-acetylprocainamide = 36.62 mL min-1 kg-1, Vdprocainamide = 10.42 L/kg, and VdN-acetylprocainamide = 5.91 L/kg. The relationship between concentration (procainamide or N-acetylprocainamide) and effect (percent change QT interval) was best described by an Emax model for procainamide (EC50 = 445 ng/mL; Emax = 30.09%). These results approximate ECG changes noted in procainamide clinical studies, suggesting that telemetry can be used as a predictive tool of efficacy. Furthermore, the proposed PK-PD model describes the electrophysiological effects associated with procainamide.

Current status of class III antiarrhythmic drug therapy

Studies in animal models, as well as clinical experience with amiodarone and sotalol, suggest that action potential prolongation may be a useful antiarrhythmic mode of action. A number of agents that produce this class III effect are currently under development. The single greatest liability for further development of this group of drugs is the occasional, and apparently unpredictable, development of exaggerated QT prolongation and polymorphic ventricular tachycardia (torsades de pointes). Available data suggest that QT interval prolongation is not a good indicator of whether or not a class III antiarrhythmic will suppress a target arrhythmia; however, exaggerated QT prolongation is a predictor of torsades de pointes. Further studies to delineate the mechanism underlying the development of torsades de pointes might lead to safer and more effective antiarrhythmic drugs.

Pharmacology of the class III antiarrhythmic agent sematilide in patients with arrhythmias

Sematilide, a close structural analog of N-acetylprocainamide, prolongs cardiac action potentials in vitro, whereas it does not depress maximum action potential upstroke slope, a "class III" action. This report outlines an evaluation of the clinical pharmacologic actions of sematilide in 14 patients with chronic high-frequency nonsustained ventricular arrhythmias. In all, 36 intravenous infusions (range 0.15 to 1.5 mg/kg over 15 minutes) were administered in a dose-ranging, placebo-controlled study design. Sematilide prolonged rate-corrected QT (QTc) in a dose- and concentration-related fashion, did not alter PR or QRS, and slowed heart rate at high concentrations (greater than or equal to 2 micrograms/ml). The relations between dose and total area under the time-concentration curve, dose and peak plasma concentration, and peak plasma concentration and increase in QTc were linear (r = 0.66 to 0.92; p less than 0.001). QTc increases of approximately equal to 25% were seen at plasma concentrations of approximately equal to 2.0 micrograms/ml. The mean elimination half-life (+/- SD) was 3.6 +/- 0.8 hours, and most of a dose (77 +/- 13%) was recovered unchanged in the urine. Plasma concentrations greater than or equal to 0.8 micrograms/ml suppressed arrhythmias (5 patients) or aggravated them (3), including 1 patient who needed cardioversion for an episode of torsades de pointes (2.7 micrograms/ml). Thus, sematilide exerts class III actions in patients. Further studies to evaluate the role of this antiarrhythmic mode of action should be conducted at doses designed to limit QTc increases.

Promotion of ventricular tachycardia induction by procainamide in dogs with inducible ventricular fibrillation late after myocardial infarction

The influence of procainamide on inducible ventricular tachyarrhythmias was evaluated in 35 dogs with experimental myocardial infarction, and 9 normal dogs. Programmed stimulation was performed from the right ventricular apex via a percutaneously positioned electrode catheter, using up to five extrastimuli before and after intravenous administration of procainamide (15 mg/kg). Procainamide levels in postinfarct dogs were 8.5 +/- 0.7 micrograms/mL (range 5.3-13.6 micrograms/mL). Procainamide exerted its greatest effect in postinfarct dogs with reproducible baseline ventricular fibrillation. Six of nine dogs (P less than 0.05) with ventricular fibrillation had sustained monomorphic ventricular tachycardia (cycle length: 147 +/- 4 msec) induced after procainamide administration. This ventricular tachycardia required significantly more extrastimuli than baseline ventricular fibrillation (3 +/- 0.3 extrastimuli before vs 4 +/- 0.3 extrastimuli after procainamide). Procainamide never converted ventricular fibrillation to ventricular tachycardia in normal dogs. Procainamide had minimal effect on inducible ventricular tachycardia after myocardial infarction. Ventricular tachycardia induction was abolished in only 2 of 17 dogs despite significant prolongation of electrophysiological parameters. Ventricular tachycardia cycle length, and the number of extrastimuli required were unchanged by procainamide in this subgroup.

CONCLUSION

Ventricular tachycardia is insensitive to the antiarrhythmic properties of procainamide in this model. In contrast, procainamide is able to convert postinfarction ventricular fibrillation to ventricular tachycardia, presumably by promoting sustained, organized reentry. This previously undescribed action is an unusual form of proarrhythmic effect, and suggests that this drug should be used cautiously in patients after myocardial infarction.

Recent antiarrhythmic drugs

Clinical failure of antiarrhythmic drugs often occurs in practice. Therefore, there is a need for new, effective and long-acting drugs with a wide therapeutic range and a low level of toxicity. Most new class I compounds block the fast sodium ion inward current of myocardial cells. According to their effects on the recovery kinetics of the sodium ion channel, these drugs are classified into 3 groups: IA (intermediate--cibenzoline, pirmenol, hydroxy-3-S-dihydroquinidine, quinacainol); IB (fast--tocainide, moricizine); IC (slow--flecainide, encainide, propafenone, lorcainide, indecainide, recainam and penticainide). Class IC drugs greatly depress intracardiac conduction and are the most potent antiarrhythmic compounds able to suppress ventricular premature beats. However, it is doubtful that long-term suppression of ventricular arrhythmias will improve survival of the patients. Some new drugs have been developed belonging to other classes: class II, esmolol, a new ultrashort-acting beta blocker; class III, N-acetyl-procainamide and sotalol, which prolong duration of the action potential and increase ventricular refractoriness; class IV, the mixed sodium ion-calcium ion-potassium ion antagonist, bepridil. The pharmacologic properties and the clinical effects of these new antiarrhythmic drugs are reviewed. However, future therapeutic trends will depend on the results of large multicenter clinical secondary prevention trials such as the Cardiac Arrhythmia Suppression Trial. New antiarrhythmic drugs with original electrophysiologic profiles and minimal adverse effects must prove their ability not only to suppress arrhythmias but also to reduce sudden cardiac death rate.

Efficacy of procainamide on ventricular tachycardia: relation to prolongation of refractoriness and slowing of conduction

The effect of procainamide on intraventricular conduction and refractoriness, and the prevention of induction of ventricular tachycardia (VT) were studied in 29 patients who had remote myocardial infarction and inducible sustained monomorphic VT. AFter intravenous administration of 15 mg/kg procainamide, induction of VT was suppressed in seven (24%) patients (responders), while in 22 (76%) VT was still inducible (nonresponders). The percent change in paced QRS duration at a cycle length (CL) of 400 msec produced by procainamide was significantly less in responders than in nonresponders: 29.8 +/- 3.9% versus 38.9 +/- 10.8% (p = 0.0020). The percent change in the right ventricular effective refractory period (ERP) at CLs of 600 and 400 msec was significantly greater in responders than in nonresponders: 14.6 +/- 6.9% versus 7.9 +/- 7.3% (p = 0.0414) for ERP at a CL of 600 msec and 15.1 +/- 7.0% versus 8.0 +/- 7.4% (p = 0.0386) for ERP at a CL of 400 msec. Stepwise discriminant analysis showed that greater percent increase in ERP at a CL of 400 msec and lesser percent increase in paced QRS duration at a CL of 400 msec were significantly independent markers for the responders. These findings suggest that lesser slowing of conduction and greater prolongation of refractoriness by procainamide tend to abolish reentry within the reentrant circuit. Greater slowing of conduction and lesser prolongation of refractoriness tend to stabilize a reentrant circuit, and promote the continued induction of VT.

Oral N-acetylprocainamide compared to quinidine plus digoxin in the chronic suppression of atrial flutter in humans

Antiarrhythmic therapy for the suppression of atrial flutter has conventionally entailed the use of a class Ia agent such as quinidine or procainamide. However, atrial flutter often recurs despite the use of these conventional antiarrhythmic regimens. Experimental and clinical evidence suggests that the pharmacologic suppression of atrial flutter may depend on the prolongation of the atrial action potential duration and consequently the voltage-dependent refractoriness. Therefore, the efficacy and tolerance of the class III antiarrhythmic agent N-acetylprocainamide was compared to that of the conventional regimen of the class Ia agent quinidine combined with digoxin (to control ventricular response) in patients with a history of symptomatic sustained atrial flutter. The study was randomized but nonblinded, with a crossover to the alternate regimen if the first failed. Eighteen patients entered the study and were followed for up to 18 months. Of the 12 receiving N-acetylprocainamide (eight randomized and four crossovers), one (8%) failed therapy due to side effects, but none had atrial flutter. Of the 11 receiving quinidine and digoxin (10 randomized and one crossover), three (28%) had a recurrence of atrial flutter, two of whom also had intolerable side effects, and two more (18%) had side effects alone requiring withdrawal of therapy (total 46% failed). The probability of therapeutic success over time was greater (p less than 0.04) for N-acetylprocainamide than for quinidine and digoxin. The data suggest that N-acetylprocainamide may be more effective and better tolerated than the conventional regimen of quinidine plus digoxin. Therefore, large-scale blinded studies of the efficacy of N-acetylprocainamide in the suppression of atrial flutter may be warranted.

The electrophysiological effects of alpha-chloralose anesthesia in the intact dog: (1) alone and (2) in combination with verapamil

The electrophysiological effects of alpha-chloralose anesthesia were determined in 13 chronically instrumented dogs and compared to baseline electrophysiological parameters in the conscious state. Alpha-chloralose anesthesia (75 mg/kg of a 4% solution in polyethylene glycol (PEG) delayed conduction and prolonged refractoriness of the AV node: (1) the P-R interval increased from 108 +/- 14 msec (mean +/- SD) in the conscious state to 125 +/- 23 msec (P less than 0.02); (2) the A-H from 98 +/- 12 msec to 108 +/- 16 msec (P less than 0.04); (3) the AV nodal effective refractory period from 136 +/- 16 to 153 +/- 29 msec (P = .05) and the AV nodal functional refractory period from 232 +/- 58 to 247 +/- 46 msec (P = 0.07); and (4) the AV block cycle length from 228 +/- 54 msec to 248 +/- 43 msec (P less than 0.04). Chloralose anesthesia also increased the ventricular refractory period from 139 +/- 13 msec to 161 +/- 22 msec (P less than .03) and the QTc interval from 273 +/- 22 to 306 +/- 32 msec (P less than 0.0002). To determine whether these effects on AV nodal conduction would influence experimental results, responses to verapamil were studied in the conscious state and during chloralose anesthesia. During chloralose anesthesia, (1) no relationship was detected between the sinus cycle length and verapamil concentrations; (2) a greater increment in AV conduction time was seen for a given verapamil concentration; and (3) AV block occurred at verapamil concentrations associated with 1:1 conduction in the conscious state. We conclude that chloralose anesthesia has significant electrophysiological effects and that these effects must be taken into consideration during the interpretation of experiments performed in animals during chloralose anesthesia.

Rapid assessment of rate and antiarrhythmic drug effect on the myocardium using asymmetric biphasic pulse stimulation

An asymmetric biphasic pulse which stimulates the heart and neutralizes the poststimulation polarization at the electrode-myocardial interface permitting the recording of the evoked endocardial response (EER) up to approximately 1 ms poststimulation with the same electrode used for stimulation is described. Using this mode of cardiac stimulation in 20 dogs the effects on the EER of increasing heart rate and antiarrhythmic drugs, procainamide (PA) and N-acetylprocainamide (NAPA), were studied. EERs were recorded during bipolar and unipolar pacing rates of 120, 150, and 200/min before and during a five step PA or NAPA infusion which resulted in progressively increasing PA and NAPA plasma concentrations (Cps), 1.7-32.5 mg/l for PA and 8.1-116.1 mg/l for NAPA. The effects of progressively increasing heart rates were: The T wave amplitude and QS area increased with increases in rate; during pacing at 120, 150, and 200/min, the T wave amplitudes were 7.6 +/- 2.3, 8.2 +/- 2.1, and 9.8 +/- 2.5 mV and the QS areas were 905 +/- 204, 995 +/- 199, and 1101 +/- 231 mVms. The QT interval and QST area decreased with increases in rate; during pacing at 120, 150, and 200/min, the QT intervals were 265 +/- 61, 249 +/- 57, and 226 +/- 52 ms and the QST areas were 288 +/- 198, 221 +/- 154, and 154 +/- 52 mVms. The effects of the antiarrhythmic drugs, PA and NAPA, on the EER were: PA prolonged both the QS duration and QT interval at low Cp (type Ia antiarrhythmic drug property); at a therapeutic PA Cp of 15.0 +/- 0.2 mg/l and a heart rate of 120/min the percent increase of the QS duration was 12 +/- 4% (P = 0.001) and that of the QT interval was 20 +/- 6% (P less than 0.001). The prolongation of the QS duration by PA was rate dependent, the faster the rate the greater the prolongation. NAPA prolonged the QT interval at low Cp, while the QS duration was not significantly effected at low or therapeutic Cps (type III antiarrhythmic drug property); at a therapeutic NAPA Cp of 15.9 +/- 1.6 mg/l and a heart rate of 120/min the percent increase of the QS duration was 1 +/- 1% (NS) and that of the QT interval was 13 +/- 9% (P = 0.018). Our results show that the use of an asymmetric biphasic pulse allows for pacing and recording of an EER, QS and T waves, with a single electrode.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the electrocardiogram in determining electrophysiologic end points of drug therapy

Electrocardiographic monitoring can be a useful adjunct to antiarrhythmic therapy, since the electrocardiogram is a simple indicator of net cardiac drug effect, irrespective of factors such as pharmacokinetic variability, drug-metabolite interactions or intraindividual variability in drug sensitivity. Changes associated with antiarrhythmic drug therapy include markers of sodium channel block, such as increased QRS or sinus-ectopic coupling intervals and increased QT interval, a marker of action potential prolongation. Electrocardiographic changes can serve 3 purposes: They can correlate with arrhythmia suppression, they may be a guide to impending drug toxicity, and they can indicate the presence of an antiarrhythmic drug at some electrophysiologically active site in the heart. This latter indication may be used as an assessment of compliance, as a clue to drug-drug interactions that may lower antiarrhythmic drug concentrations or preparatory to electrophysiologic testing when it is desirable to avoid testing patients who have no demonstrable drug effect. Drug-induced changes in the microelectrophysiologic environment may sometimes fail to express themselves on the surface electrocardiogram. Overall, however, the electrocardiogram is an inexpensive, readily available tool to monitor net antiarrhythmic drug effects on the heart. Monitoring of the electrocardiogram should, therefore, be an integral part of managing antiarrhythmic drug therapy in patients with arrhythmias.

Role of plasma concentration monitoring in the evaluation of response to antiarrhythmic drugs

Plasma concentration monitoring of antiarrhythmic agents is valuable, but it is often misused or overemphasized in therapeutic decision-making. There are strict requirements for its appropriate use that are often not met--for both the newer and even the conventional antiarrhythmic drugs. For maximum value, there must be a reliable, accurate relation between the plasma drug concentration and drug action, a relation closer than that between dosage and drug action. The time of sample collection is important--most guidelines are based on "trough" plasma concentrations measured after steady-state equilibrium has been achieved. The use of an accurate, sensitive and specific assay is crucial to the value of plasma concentration monitoring guidelines. However, for agents having active metabolites, monitoring the concentration of only the parent drug can be misleading and limits (but does not necessarily eliminate) the value of plasma concentration monitoring guidelines for these agents. Plasma concentration monitoring of most antiarrhythmic agents is of value for certain specific purposes: to determine compliance to antiarrhythmic therapy, to detect and analyze possible drug interactions, to assess the benefit to risk ratio for increasing the dose of a particular antiarrhythmic agent, to maintain a stable drug effect in the presence of a patient's changing clinical condition and, to a limited extent, to assess the role of an agent in causing an adverse drug reaction. The importance of understanding the assay methods currently in use, as well as how plasma concentration monitoring of individual antiarrhythmic agents is affected by the presence of active metabolites, optical isomers differing in their activity and variations in protein binding, is essential in interpreting data obtained from plasma concentration monitoring.

Effects of N-acetylprocainamide on experimental atrial flutter and atrial electrophysiologic properties in conscious dogs with sterile pericarditis: comparison with the effects of quinidine

N-acetylprocainamide (NAPA) is said to have class III antiarrhythmic drug properties. The effects of NAPA (25 mg/kg intravenously) on sustained, stable, reentrant atrial flutter induced in 12 conscious dogs using a sterile pericarditis model were studied and compared with the effects of quinidine (5 mg/kg intravenously) given on a different day in 10 of the same 12 dogs. The effects of these drugs on atrial excitability, the atrial effective refractory period and intraatrial conduction time measured during rapid atrial pacing performed during sinus rhythm were also compared. The mean NAPA and quinidine serum levels were 17.7 and 7.1 micrograms/ml, respectively. Both NAPA and quinidine immediately prolonged the atrial flutter cycle length in all dogs, from 118 +/- 15 to 141 +/- 18 ms and from 119 +/- 17 to 153 +/- 21 ms, respectively (both p less than 0.001), and then terminated atrial flutter in 11 of the 12 NAPA studies and in 6 of the 10 quinidine studies. Neither drug affected atrial excitability. Both NAPA and quinidine increased the atrial effective refractory period significantly, from 138 +/- 17 to 168 +/- 20 ms (p less than 0.001) and from 136 +/- 14 to 148 +/- 16 ms (p less than 0.01), respectively. NAPA did not change intraatrial conduction time measured during atrial pacing at 150 beats/min, but during atrial pacing at 300 beats/min, it prolonged it from 51 +/- 9 to 54 +/- 10 ms (p less than 0.05), and at 400 beats/min, from 52 +/- 10 to 64 +/- 13 ms (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Procainamide in the dog: antiarrhythmic plasma concentrations after intravenous administration

Procainamide hydrochloride was administered to ouabain-intoxicated dogs to determine an antiarrhythmic plasma concentration of procainamide. Ventricular arrhythmias were produced in dogs following intravenous injections of ouabain. After a sustained ventricular tachycardia was achieved, procainamide was administered and plasma samples collected for assay. Plasma procainamide was assayed by fluorescence polarization immunoassay. Procainamide was administered at increasingly higher constant rate infusions in order to achieve intermittent, steady-state plasma concentrations. Infusion rates were calculated on the basis of previous pharmacokinetic information. All six dogs that received procainamide converted to a normal sinus cardiac rhythm after attaining a mean plasma concentration of 33.8 micrograms/ml with a range of 48.5 micrograms/ml-25.0 micrograms/ml. It was observed that the computer-generated prediction of plasma concentrations based upon previous pharmacokinetic data produced an underestimate of the actual plasma concentrations. These data may suggest that plasma concentrations of procainamide for controlling some cardiac arrhythmias in dogs may be higher than plasma concentrations cited for human patients.

QT prolongation and arrhythmia suppression

It seems possible to draw some overall conclusions from the data on antiarrhythmic drug-induced QT prolongation and its role in drug effects. At one end of the spectrum are patients with highly exaggerated QT responses during therapy, most often in the setting of hypokalemia and long cycle lengths (post-ectopic pauses, bradycardia). These patients may be at high risk for development of arrhythmias during therapy. It should also be remembered that the presence of hypokalemia may render antiarrhythmic agents less effective in general. On the other hand, modest QT prolongation in the course of therapy with an antiarrhythmic drug may well be a marker of reduction of dispersion of action potential durations or refractory periods and hence represent an antiarrhythmic effect. The clinical actions of these drugs in patients with arrhythmias strongly suggest that this is the case. New agents with the ability to reduce dispersion of repolarization or of refractoriness without inducing arrhythmias may well become the agents of choice for the treatment of serious cardiac rhythm disturbances.

N-acetylprocainamide kinetics during intravenous infusions and subsequent oral doses in patients with coronary artery disease and ventricular arrhythmias

The kinetics of N-acetylprocainamide (NAPA) were studied in 5 patients (all men, mean age = 62) with coronary artery disease and ventricular arrhythmias during loading infusions of 0.22-0.45 mg/kg/min, prolonged (19-48 hrs) intravenous infusions 2.5-5.2 mg/min, and in 4 of the patients, during subsequent oral doses 1.5-3 g every 8 hrs. Serum, concentrations of NAPA were determined by high-performance liquid chromatography. The individual concentration-time profiles could, with one exception, be described by a two-compartment, open, kinetic model with apparent first-order elimination. The kinetic variables were: initial distribution volume (Vc) 0.20 +/- 0.11 l/kg (mean +/- SD); steady-state distribution volume (Vss) 1.58 +/- 0.55 l/kg; distributional clearance (Cle) 133 +/- 23 ml/(kg X hr); absorption rate constant (Ka) 0.354 +/- 0.173 hr-1; and fraction of dose reaching systemic circulation (F) 1.00 +/- 0.14. The data for one patient who had received increasing oral dosages of 1.5, 2, 2.5 and 3 g every 8 hours resulted in systematic underprediction of observed concentrations at the two highest oral dosing rates. This suggests the possibility of some degree of nonlinearity or time-dependent change in the kinetic behavior of NAPA. Only low concentrations of procainamide, less than 1 mg/L, were found at the end of the infusions.

Antiarrhythmic therapy: clinical pharmacology update

Currently available antiarrhythmic agents are limited by side effects and the potential for increasing arrhythmias. In addition, drug interactions, altered disposition of drug as a result of changes in protein binding or concomitant disease processes, active metabolites, and poorly defined therapeutic ranges with great interpatient variability are some of the factors which complicate therapy. An awareness of the possible contribution of these factors in the use of antiarrhythmics is invaluable in both the choice of agent and the establishment of an optimum benefit-to-risk ratio for the patient.

Changes in ventricular refractoriness after an extrastimulus: effects of prematurity, cycle length and procainamide

This study was performed to determine the ability of extrastimuli to change ventricular refractoriness. We prospectively evaluated the effects of stimulus prematurity and paced cycle length (PCL) in 30 patients and the effect of procainamide in 8 patients on changes in the ventricular effective refractory period (ERP) after a right ventricular extrastimulus (S2). An S2 was introduced at preselected coupling intervals at a PCL (S1-S1) of 600 and 400 ms. At each S1-S2 interval, a second extrastimulus (S3) was introduced in 5-ms decrements and the ERP of S2 measured. The decrease in the ERP after an S2 was directly related to prematurity and most of the shortening occurred over a narrow range of S1-S2 intervals. At a PCL of 600 ms, the ERP of S2 at S1-S2 intervals less than or equal to 400 ms was significantly shorter than the ERP of S1 (maximal shortening 23%). At a PCL of 400 ms, the ERP of S2 at S1-S2 intervals less than or equal to 350 ms was significantly shorter than the ERP of S1 (maximal shortening 25%). The ERP of S2 at the shortest S1-S2 interval was greater with a PCL of 600 ms than with 400 ms (200 +/- 31 versus 180 +/- 26 ms, p less than 0.001). However, the total shortening in ERP (ERPS1 - ERPS2 at shortest S1-S2 interval) was similar at both PCLs (55 +/- 14 versus 59 +/- 13 ms). Procainamide significantly prolonged the ERP of S2 at each S1-S2 interval.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Electrophysiologic properties and antiarrhythmic mechanisms of intravenous N-acetylprocainamide in patients with ventricular dysrhythmias

To define electrophysiologic properties and antiarrhythmic mechanisms of N-acetylprocainamide (NAPA), we studied 16 patients with symptomatic ventricular dysrhythmias. Electrophysiologic studies were performed before and after intravenous infusion of NAPA at 20 mg/kg over 20 minutes, achieving plasma concentrations of 24 +/- 3.2 to 35.5 +/- 4.5 micrograms/ml. NAPA did not significantly change sinus cycle length or atrioventricular (AV) conduction times (PA, AH, HV, and QRS), but it lengthened the QTc interval (p less than 0.001) during sinus rhythm. Programmed atrial stimulation revealed that NAPA had no discernible effects on AV nodal conduction; however, it exerted depressive effects on the His-Purkinje system in 9 of 16 patients. In 7 of 16 patients who manifested frequent ventricular premature beats (VPBs), NAPA abolished VPBs in only three of them; NAPA induced progressive prolongation of the premature coupling interval before complete abolition of VPBs. In 8 of 16 patients who had inducible repetitive ventricular response (RVR) because of reentry within the His-Purkinje system, NAPA narrowed or abolished the RVR zone in 3 patients and slowed the RVR rate with widening of the RVR zone in the remaining 5 patients. In 2 of 16 patients with slow ventricular tachycardia (VT), NAPA had no antiarrhythmic effects. By contrast, in the other 2 of 16 patients in whom sustained VT could be reproducibly elicited with programmed ventricular stimulation, NAPA slowed the rate of VT and suppressed VT inducibility. We conclude that electrophysiologic properties of NAPA are slightly different from those of procainamide and that NAPA is not uniformly effective for suppressing ventricular dysrhythmias, but its antiarrhythmic mechanisms are similar to those of procainamide.

The electropharmacology of acute drug testing using pacemakers

Acute drug testing in patients is useful to select prophylactic treatment for life-threatening or intractable tachycardias. This is generally done by induction of tachycardias with pacing. Acute studies that depend on temporary insertion of pacing electrodes do not determine efficacy in the same sense as longer term clinical drug trials because of the biased population referred for testing with pacemakers. However, the pharmacologic activity of compounds can be tested in terms of electrical functions such as conductivity and refractoriness not merely of the heart in general, but also of the arrhythmogenic focus. Such data can be directly applied to patients with similar arrhythmias, obviating the confusion often caused by interspecies and disease differences.

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.

Selection of an antiarrhythmic drug for a sudden-death-prevention trial

Several therapeutic approaches have sought to prevent the occurrence of sudden cardiac death. Many sudden deaths are presumed to be due to an arrhythmia, but the drugs best known for antiarrhythmic activity, the local anesthetic agents, have not been suitable prophylactic agents because of their toxicities and other undesirable pharmacologic characteristics. Several new drugs in this class have been synthesized and are currently being tested for antiarrhythmic activity in clinical trials. One of them may prove to be worthy of a large-scale clinical trial to determine whether chronic arrhythmia suppression reduces the risk of sudden death. The characteristics of the "ideal" antiarrhythmic agent are discussed, and a brief summary of the drugs currently being tested in the United States is presented. The discussion of each drug emphasizes the characteristics that might make it suitable--or unsuitable--for use in a sudden-death trial. Many agents being tested are clearly not satisfactory for such a trial. However, they may be prototypes for an ideal drug or combination of drugs that might yet be developed.

Long-term antiarrhythmic therapy with acetylprocainamide

Nineteen patients whose arrhythmias were initially suppressed with acetylprocainamide underwent long-term treatment with this drug. Eleven patients were still taking the drug at the end of 12 months. Drug withdrawal with substitution of a placebo caused an increase in ventricular premature beats. Thus, suppression of ventricular premature beats persisted for 1 year. The eight withdrawals from the study were due to death during the year (n = 6) or recurrence of arrhythmias. The deaths occurred in patients who were in New York Heart Association functional class II (one patient), III (three patients) and IV (two patients). Ventricular performance, assessed from systolic time intervals, improved with drug therapy and declined during drug withdrawal. Symptomatic effects were common, with seven patients requiring a reduction in dosage or discontinuation of therapy. Three patients treated for 3 years continued to show drug suppression of ventricular premature beats compared with the level during placebo substitution. Small amounts of procainamide were present in all patients because of in vivo deacetylation of acetylprocainamide. Many patients with good initial responses to this drug had recurrent arrhythmias during long-term therapy. For this reason, the usefulness of acetylprocainamide as an antiarrhythmic drug appears to be limited.

Electrophysiologic effects of N-acetylprocainamide in human beings

The electrophysiologic properties of N-acetylprocainamide (NAPA) were studied in 10 patients undergoing cardiac catheterization. Each patient received two successive intravenous infusions: one loading infusion over 15 minutes and one maintenance infusion at a slower rate for 30 minutes. Eight patients received 10.5 mg/kg body weight and two received larger doses (16 and 21 mg/kg, respectively). NAPA plasma concentration was measured at 5 minute intervals from 0 to 25 minutes, and then at 15 and 30 minutes of the second infusion. Mean blood pressure and electrophysiologic data obtained by programmed stimulation were recorded before drug administration and at 15 and 30 minutes of the infusion when the concentration of NAPA was nearly constant in each patient (range 12 to 35 microgram/ml). NAPA decreased blood pressure (p less than 0.005), increased corrected Q-T interval (p less than 0.01) and increased the atrial and ventricular effective refractory periods from 267 +/- 40 to 307 +/- 41 ms (p less than 0.01) and from 278 +/- 37 to 301 +/- 32.8 ms (p less than 0.05), respectively. NAPA did not significantly change sinus cycle length or sinus nodal recovery time, conduction intervals (A-H, H-V, P-R, QRS), atrioventricular nodal functional refractory period or nodal Wenckebach cycle length. The patient receiving the largest dose experienced mild nausea when the plasma concentration was above 35 microgram/ml. These data show that the electrophysiology of NAPA in human beings is different from that reported for procainamide. At the plasma concentrations studied NAPA increases atrial and ventricular refractory periods without increasing cardiac conduction times

Clinical pharmacology and antiarrhythmic efficacy of N-acetylprocainamide

Eleven patients with chronic ventricular arrhythmias took part in a study of N-acetylprocainamide (NAPA), the major metabolite of procainamide, in order to characterize further NAPA's clinical pharmacology and antiarrhythmic action. The frequency of ventricular arrhythmia on 24 hour ambulatory electrocardiographic recordings was comparable on recordings obtained in a prestudy screening, during treatment with placebo before administration of NAPA and after treatment with NAPA. The initial dosage of NAPA was 500 mg every 8 hours, which was increased by 500 mg increments every few days until 90 percent suppression of arrhythmia or intolerable adverse effects occurred. Only two patients achieved 90 percent suppression of ventricular ectopic complexes. The mean plasma concentration associated with 90 percent suppression of arrhythmia in these two patients ws 12.6 and 32.3 mg/ml, respectively. One of these two patients was unable to continue long-term therapy with NAPA because of a rash. Other adverse effects included gastrointestinal symptoms in seven patients with visual symptoms in four patients at plasma concentratons as low as 6.9 mg/ml. NAPA obeyed linear pharmacokinetics over the range of dosages studied (500 to 2,500 mg every 8 hours) and had a half-life of 10.7 +/- 1.98 hours (mean +/- standard deviation). There was no change in the P-R or QRS intervals and there was a dose-dependent prolongation of the Q-Tc interval. It is concluded that in this patient group, NAPA suppressed chronic ventricular ectopic complexes without adverse effects in only a minority of patients.

Antiarrhythmic efficacy, pharmacokinetics and safety of N-acetylprocainamide in human subjects: comparison with procainamide

The antiarrhythmic efficacy and pharmacokinetics of N-acetylprocainamide (NAPA), the major metabolite of procainamide, were investigated in 23 patients with chronic, high frequency ventricular ectopic depolarizations. An extensive trial design incorporated the approaches of (1) generation of dose-response relations, (2) randomized crossover, and (3) prolonged electrocardiographic monitoring. Seven patients with reproducible suppression of arrhythmias (70 percent or greater reduction in frequency) were thus identified. The mean plasma concentration of acecainide associated with efficacy was 14.3 micrograms/ml (range 9.4 to 19.5) and with side effects (primarily gastrointestinal) was 22.5 micrograms/ml (10.6 to 37.9). The antiarrhythmic response to procainamide did not predict response to acecainide; this finding implies that estimates of the antiarrhythmic contribution of acecainide concentrations achieved during long-term procainamide therapy are unlikely to be meaningful in a given person. The mean half-life of elimination after a single 500 mg dose of acecainide was 7.5 hours; this had prolonged significantly (p < 0.05) to 10.3 hours after higher dosages. No variable examined (including acetylator phenotype) was found to be a predictor of responsiveness to acecainide. Outpatient therapy (2 to 20 months) was not associated with the development of antinculear antibodies or the lupus syndrome; one patient's procainamide-induced arthritis resolved during therapy. Acecainide, unlike procainamide, is an agent whose pharmacokinetics allow long-term therapy on a practical schedule. It is effective in a subset of patients with ventricular arrhythmias yet appears much less likely to induce the lupus syndrome seen with the parent compound.