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Electrophysiologic effects of amiodarone: Experimental and clinical observation relative to serum and tissue drug concentrations
Electrophysiologic effects of amiodarone: Experimental and clinical observation relative serum and tissue drug concentrations
to
Oral amiodarone is a potent antiarrhythmic agent with a slow onset of action. Its electrophysiologic properties following chronic administration are well known, but its acute electrophysiologic actions are poorly defined. The objectives of the present study were to correlate the electrophysiologic actions of intravenous amiodarone in humans with the acute and chronic effects of the drug relative to plasma and tissue concentrations of the drug. In humans (n = lo), 5 mg/kg intravenous amiodarone (serum concentration 6.50 + 3.34 fig/ml at 10 minutes; 2.13 rf 0.71 Ag/ml at 20 minutes, n = 7) increased the AH interval by 16.4% (p < ODOS), the antegrade effective refractory period (ERP) of the atrioventricular (AV) node by 14.4% (p < 0.025) and the functional refractory period (FRP) of the AV node by 15.5% (p < 0.005). The ERP or FRP of the atrium of the right ventricle was not significantly changed; there was no effect on the HV interval or the QT and R-R intervals of the ECG. In rabbits (n = 11) given 10 mg/kg intravenous amiodarone (mean * SD serum concentration 0.49 F 0.17 Ag/ml; mean myocardial concentration 7.0 +- 1.9 Ag/gm, n = 3), there were no significant effects on the ECG intervals. In isolated rabbit sinoatrial (SA) node, atria, and AV node (three preparations) superfused with 5 X lo-‘M amiodarone (3.41 Ag/ml), there was no effect on the action potential duration (APD) or other parameters of the transmembrane potential. Rabbits chronically pretreated with amiodarone (20 mg/kg intraperitoneally) for 3 weeks had serum drug concentrations of 0.98 i 0.52 Ag/ml (n = 4) and myocardial levels of 11.52 t 7.2 Ag/gm at 3 weeks and 0.50 f 0.18 pg/ml and 14.8 ? 6.4 Ag/gm, respectively, at 6 weeks. Compared to the values in control series, the spontaneous cycle length of the SA node was prolonged by 24% (p < 0.05) at 3 weeks and 35.5% (p < 0.01) at 6 weeks. The Vmax was affected in none of the tissues but APD was significantly lengthened in all. In atria the APD was increased by 27.6% (p < 0.01) at 3 weeks and 32.8% (p < 0.01) at 6 weeks; in ventricular muscle the corresponding values were 11.0% (p < 0.05) and 25.3% (p < 0.01). Our clinical and experimental data indicate major differences between the chronic and acute electrophysiologic effects of amiodarone, differences which are relevant to the interpretation of the antiarrhythmic actions of the drug following intravenous and oral administration. (AM HEART J 108:890, 1984.)
Nobuo Ikeda, M.D., Koonlawee Nademanee, M.D., Ramaswamy Bramah N. Singh, M.D., D. Phil. Los Angeles, Calif.
Amiodarone hydrochloride has recently emerged as a powerful antiarrhythmic drug for the treatment of a wide variety of cardiac arrhythmias.‘.lo When the drug is given by mouth, the onset of its antiarrhythmic action is slow over a period of days2,4-10and the peak action is often not evident for weeks.4v5*7Such an effect can be correlated with significant increases From the Department of Cardiology, Veterans Center, and the Department of Medicine, UCLA
Administration Medical School of Medicine.
Supported by grants from the Medical Research Service of the Veterans Administration, and the Group Investigator Award of the American Heart Association, the Greater Los Angeles Affiliate. Received
for publication
Reprint requests: Bramah VA Medical Center West Angeles, CA 90073.
890
Feb.
1, 1984;
accepted
March
N. Singh, M.D., Cardiology Los Angeles, Wilshire and
2, 1984. Section Sawtelle
(691/111E), Blvd., Los
Kannan,
Ph.D., and
in the effective refractory period (ERP) of atria, ventricles, atrioventricular (AV) node, His-Purkinje tissues, as well as the bypass tracts.5r6r11,12 The reason for the delay in the onset of antiarrhythmic action of the compound remains essentially unknown. However, in recent years, the use of the intravenous formulation of the drug-containing Tween 80 as diluent-has suggested that the compound might exert certain antiarrhythmic effects.13-18The electrophysiologic correlates of such effects, which have been variable from center to center 14-16, lg,2o are poorly defined. The purpose of the present study was twofold. First, it was to determine the electrophysiologic effects of single intravenous doses of commercially available amiodarone hydrochloride in patients undergoing electrophysiologic
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studies for the evaluation of cardiac arrhythmias. Second, it was to correlate these findings with the alterations in the various parameters of the transmembrane action potentials in isolated rabbit heart induced by the superfusion with homologous plasma containing amiodarone relative to the changes produced by the drug in chronically pretreated anhmals. METHODS Clinical studies. Clinical studieswere undertaken in 10 patients, who underwent electrophysiologicstudiesfor the evaluation of refractory recurrent cardiac arrhythmias. They were all men; meanagewas56 (range 49 to 74) years. Eight had recalcitrant ventricular arrhythmias and two had supraventricular tachyarrhythmias. Studies were performed in the electrophysiologic laboratory in the postabsorptive state without premeditation. Electrode catheters were inserted percutaneously or by cut-down technique and positioned at multiple cardiac sites under fluoroscopic control. The recording sites included high right atrium (RA), His bundle (HB), right ventricular (RV) apex, and coronary sinus. Standard quadripolar electrode catheters with 1 cm interelectrode distance (USCI, Billerica, Mass.) were used for stimulation and recording from specific sites. The HB electrogram was obtained by a tripolar catheter. Surface ECG leads I, aV,, and V, were displayed simultaneously with those of the intracardiac signalsand were recorded on a VR-12 (Electronics for Medicine) recorder at paper speed of 100 mm/set. Cardiac stimulation was performed with a programmable constant-current stimulator (Medtronic 5235), which delivered rectangular pulsesat 2 msecduration at twice the diastolic threshold. Intracardiac electrogramswere filtered at 30 to 500 Hz. The RA, RV, and AV nodal refractory periods were determined by the extrastimulus method.5The AH interval was measured from the initial rapid deflection of the low RA electrogram to the initial deflection of HB depolarization (normal range 60 to 120 msec).The HV interval was measuredfrom the initial His deflection to the earliest onset of ventricular activation (normal range 35 to 55 msec).The ERP of the ventricle was defined as the longest S, S, interval during ventricular pacing with ventricular depolarization at which S, failed to produce ventricular depolarization. Amiodarone hydrochloride was given at a dose of 5 mg/kg intravenously via a peripheral vein over 2 minutes after control parameters had been obtained. Ten to 20 minutes after the completion of the injection theseparameters were again recorded under identical conditions, and whenever possibleblood was withdrawn for the determination of serum concentrations of amiodarone and its principal metabolite, desethylmetabolite. Experimental studies. Since amiodaronehydrochloride is not soluble in physiologic media,21,22 to determine its electrophysiologic effects in isolated preparations by superfusion techniques, the following method was used. Amiodarone (68 mg) was first dissolved in 10 ml of 50%
Electrophysiologic
effects
of
amiodarone
891
ethanol solution (10 mM solution). One milliliter of the stock solution was slowly added to 9 ml of plasmaof the rabbit killed (1 mM solution). Five milliliters of 1 mM solution was then dissolved in 1 L of Tyrode solution to make the final concentration of 5 X lo-” M amiodarone. The procedure for obtaining preparations wasthe sameas that (see below) used in the studies to determine the effects of the drug given chronically before the removal of the atria. In the seriesinvolving acute experiments, rabbits were not pretreated with the drug. The control data in this serieswere obtained when the preparation wassuperfused with Tyrode solution containing 0.5 ml of 50% ethanol and 4.5 ml of plasma (of the killed rabbit)/1 L of Tyrode solution. The effects of amiodarone were examined after 60 minutes of superfusion with the drug. In another seriesof rabbits, the effects of the chronic pretreatment with amiodarone on the intracellularly recorded action potentials were determined. Rabbits (weighing 1.5 or 3.5 kg) were treated with amiodarone (20 mg/kg, intraperitoneally) 5 days/wk for 3 weeks (n = 5) and 6 weeks(n = 6), respectively. A control group (n = 5) was injected with distilled water, 5 dayslwk for 3 weeks, instead of amiodaronesolution. After each treatment, the rabbits were anesthetized with sodium pentobarbital (30 mg/kg, intravenously), and the hearts were rapidly removed and dissected in oxygenated Tyrode solution. The following preparations were obtained from each heart: (1) sinoatrial (SA) node preparation (2 x 3 mm) from the middle part of SA node region, (2) AV junctional preparation including RA, interatrial septum, AV node, and HB, and (3) papillary musclefrom the right ventricle. These preparations were mounted in a tissue bath (10 ml in volume) and superfused with Tyrode solution (15 ml/min) at 37’ f 0.5” C. The composition of Tyrode solution was as follows (mM): NaCl 130, KC1 4.0, CaCl, 1.8, MgSO, 0.5, NaH, PO, 1.8, NaHCO, 18.0, and dextrose 5.5. Tyrode solution wasbubbled with mixed gascontaining 95% 0, and 5 C02, and its pH was maintained at 7.42 2 0.02. SA node preparations were allowed to beat spontaneously while the AV junctional preparations and papillary muscles were electrically stimulated through bipolar electrodes at 2.5 Hz and 1 Hz, respectively. Rectangular pulsesof twice the threshold voltage (1 msec in duration) were delivered by Grass S88 stimulator with isolated output. Action potentials were recorded through glass microelectrodes filled with 3M KC1 (tip resistance = 10 to 30 megaohms).Signalswere amplified with microelectrode amplifier (Mentor N-950) with capacity compensation, and were displayed on an oscilloscope (Tektronics R564B) and photographed on Polaroid film. Maximum upstroke velocity of phase0 of action potentials (dV/d&,,,,) was obtained by electronic differentiation. After an equilibration period of 60 minutes, action potentials were recorded in the standard manner. Ouantitation of amiodarone in serum and myocardial tissues. Serum concentrations of amiodaronewere deter-
mined by high-pressureliquid chromatography according to the procedure of Flanagan et a1.23 This method has the capability to quantitate amiodaroneas well asits deseth-
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Table
I. Electrophysiologic effects of intravenously administered amiodarone in patients with cardiac arrhythmias
American
Parameters
n
Abbreviations: significant.
ERP
= effective
refractory
period;
Baseline
9 9
Heart rate (bpm) QT, interval (set) AH interval (msec) HV interval (msec) Cycle length at which AV Wenckebach developed (msec) ERP RV (msec) FRP A (msec) ERP A (msec) FRP A (msec) Anterograde ERP AV node (msec) AV node (msec)
70.1 0.40
10
lir 13.4 + 0.04
10
107.0 53.0
t I
10
428.4 253.0
+ 50.5 ztz22.8 2 30.3 + 25.4 + 32.9 _t 40.7 t 39.7
10
276.0 270.0
10 10
318.0 347.1
9 FRP
10 to 12 min after Fj mglkg intravenous amiodarone
438.3
= functional
49.2 14.2
refractory
yl metabolite (desethylamiodarone) and has been previously validated in our laboratory.24 For determining amiodarone levels in heart muscle tissue from rabbits, the muscletissue(left ventricle or left atrium, 300mg net weight) washomogenizedin a Polytron (Brinkman Instruments, Westbury, N. Y.) with 2 ml deionized distilled water, and amiodarone was extracted from an aliquot (250 ~1)of the fresh homogenateimmediately for high-pressureliquid chromatography, since levels of amiodaronedecreaseon storing the homogenatein cold and are unreliable. There was no significant difference in amiodarone concentrations between the atria1 muscleand the ventricular musclepreparations, and only data from the ventricles are reported here. Statistical analysis. The mean data are presented with k standard deviations of n observations from patients or animals. Statistical comparisonswere made with a t test for paired and unpaired data and data for multiple groups by the analysis of variance. A probability value of 10.05 was consideredstatistically significant. RESULTS Electrophysiologic tered amiodarone
effects in patients.
of intravenously
adminis-
The mean data from 10 patients, who were given 5 mg/kg of the drug, are summarized in Table I. The compound had no significant effect on the spontaneous cycle length or on the QT, interval; similarly, the ERP of the atria and ventricles was not affected and infranodal AV conduction was not significantly lengthened. The major effect of intravenous amiodarone was on the AV node: the AI-I interval was increased (+16.4% p < 0.005) as was the longest cycle length for AV Wenckebach periodicity (+19.7% ; p < 0.005) following RA pacing. Increases in the functional (+X.5% ; p < 0.005) as well as the anterograde (+14.4% ; p < 0.025) ERPs of the AV node were also
k
9.9 k 0.03
NS NS
125.5 54.5
f i
52.8 12.6
p < 0.005 NS
513.0 250.0
61.2 22.1
p < 0.006 NS NS NS NS p < 0.025 p < 0.005
320.5 397.1
+ 55.6
506.7
-t 48.9
274.0 287.0
period;
Significance of difference from baseline
68.3 0.39
f i i t i
A = atrial;
27.6 31.6 36.1
RV = right
1984
Heart Journal
ventricle;
AV = atrioventricular;
NS = not
significant. The electrophysiologic measurements were made between 10 and 20 minutes after drug injection; at these times, the amiodarone serum concentrations were 6.50 + 3.34 pg/ml at 10 minutes and 2.13 f 0.71 pg/ml at 20 minutes (n = 7). Desethylamiodarone was not detectable following intravenous injections of amiodarone. Effects of intravenous amiodarone on ECG intervals in anesthetized rabbits relative to serum and tissue drug concentrations. In 11 rabbits, amiodarone adminis-
tered intravenously, 10 mg/kg, produced serum concentrations of 2.56 f 1.22 pg/ml at 10 minutes (n = 8) and 0.49 * 0.17 pg/ml (mean + SD) at 60 minutes after drug injection; ECGs recorded in six of these rabbits at these times showed no significant effect on the QRS duration or the QT, interval. The PR interval was 71 f 7 msec before the drug and 69 -t 3 msec after the drug injection, a difference that was not significant. When the animals were killed, the level of the ventricular tissue concentration of amiodarone was 7.0 + 1.9 pg/gm (n = 3). Effects of superfusion of isolated rabbit by homologous rabbit plasma containing
Three preparations
heart tissues amiodarone.
each of rabbit SA node, AV
node, and atria1 tissue were superfused with 5 x 10e6 M (3.41 pg/ml) amiodarone at the highest concentration that was readily soluble in homologous plasma with ethanol. Despite over 60 minutes of continuous superfusion, the drug had no significant effect on the action potential amplitude, maximal rate of rise of phase 0, phase 4 depolarization (in the SA node and spontaneously beating fibers), and in the 50% (APD& and 90% (APD,,) action potential durations. Representative control recordings and those following superfusion with amiodarone are shown in Fig. 1.
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effects of amiodarone
893
AMIODARONE 5~10-~M 0
SA NODE CELL
I -60
mv
I 10
vlsec
ATRIAL FIBER
AV NODE
1. Effects of acute superfusionof isolatedrabbit heart musclepreparations with homologousplasma containing amiodarone,5 x 10e6M.The upper traces of eachpanel showzero potential; the middle panel, the transmembrane action potential; the lower traces represent the rate of rise of the action potential obtained by electronic differentiation. Note that cycle length of the SA node firing frequency is not significantly affected, neither are the various parametersof the transmembranepotentials from atria and ventricles, but small lengthening of the APD,, is noted.
Fig.
Electrophysiologic bits pretreated with
effects of amiodarone in the rabamiodarone. When rabbits were
pretreated with amiodarone (20 mg/kg intraperitoneally) for 3 weeks, the serum drug concentration was 0.98 +- 0.52 pg/ml (n = 4); the corresponding tissue level in the ventricular myocardium was 11.52 + 7.2 wg/gm tissue. In rabbits treated for 6 weeks (n = 4), the serum levels were 0.50 +- 0.18 pg/ml with the tissue levels being 14.8 + 6.4 pg/gm. Neither the serum drug levels nor the tissue levels were significantly different after 3 weeks vs 6 weeks of drug administration. The mean data showing the electrophysiologic effects of amiodarone on the various parameters of the transmembrane action potential in the SA node and AV node after 3 and 6 weeks of pretreatment compared to those in a control series are shown in Table II; the corresponding changes found in the
atrial and ventricular muscle are presented in Table III. Representative records from all the tissues studied are reproduced in Fig. 2. After pretreatment with amiodarone, the cycle length of the spontaneously beating SA node was prolonged by 24% 03 < 0.05) at 3 weeks and by 35.4% (p < 0.01) at 6 weeks. The change was due to the depression in the slope of the spontaneous diastolic depolarization (Fig. 2); increases in the overall repolarization in the SA nodal potentials were not quantifiable. In the AV node, amiodarone had no effect except on the APD,, (at 6 weeks) and in the APD, (at 3 and 6 weeks); significant lengthening was induced by amiodarone (Table I). In atrial and ventricular muscle, significant changes induced by amiodarone pretreatment were evident only with respect to repolarization (Fig. 2). After 3 weeks of pretreatment, the APD, was 16.6% (p < 0.05)
October,
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et al.
American
Heart
1984 Journal
II. Effects of chronic administration of amiodaroneon the electrophysiologic properties of rabbit SA and AV nodesexamined in vitro __Table
APA (m vi SA nodal cells Control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) AV nodal cells Control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) 6 wk
OS (mV)
MDP (mV)
dVldt,, (Vlsec)
APD, (msec)
APD, (msec)
16.2 + 7.8 77.5 +- 4.1
11.9 t 1.4 12.5 a 2.7
64.3 + 6.8 65.0 + 4.6
10.4 k 2.3 10.8 k 1.8
-
75.8 rk 4.9
11.5 t 2.9
64.3 f 3.1
10.1 t 2.2
78.3 + 8.3 77.7 + 5.3
11.7 f 3.3 12.5 + 2.5
66.6 t 4.1 65.2 f 2.3
17.4 -+ 5.5 17.1 -+ 6.3
c54.7 t 6.5 66.1 + 2.4
81.6 ‘- 5.5 92.0 f 3.0*
79.3 +- 3.5
12.3 k 3.1
67.1 f 4.1
16.8 t 5.3
67.3 f 4.1
96.2 + 5.2t
SPCL (msec)
343.1 425.5
+ 29.0 t 29.4*
464.7
k 353 -
Abbreviations: APA = action potential amplitude; OS = overshoot potential; MDP = maximal diastolic potential; APD,, and APD, = action potential duration at 50% and 905 repolarization times: SPCL = spontaneous cycle length; statistical significance of difference from control: *p < 0.05: tp < 0.01.
Ill. Effects of chronic treatment with amiodarone on the electrophysiologic parameters of the action potentials recorded from rabbit atria and ventricular muscle in vitro Table
dV/dt,,,
APD, (msec)
APD, (msec)
dz 24.9
45.7 + 3.6
83.3 YIZ 5.6
218.6
f 23.2
53.3 +- 3.8’
106.4
83.0 i 4.1
221.7
i
27.1
60.8 + 3.7t
110.6 t 8.8t
18.4 k 2.3
84.8 + 3.1
206.0
t 14.6
92.5 2 5.4
132.6 + 7.6
+ 8.6
17.9 t 1.8
84.6 + 5.2
198.8
f 17.1
105.6
2 5.5*
147.2
1 6.5*
104.8 * 4.9
18.6 t 2.7
86.2 + 4.7
200.0
+ 19.5
121.6 + 5.9t
166.1
+ 8.2t
APA
(mV)
OS (mV)
RP (mV)
Atria1 muscle control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) 6 wk
102.7
t 5.9
18.2 it 2.3
84.6 + 2.5
227.5
99.8 -+ 4.2
17.0 t 2.1
82.8 f 3.5
100.2
? 6.5
17.2 z!z 3.1
Ventricular muscle control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) 6 wk
103.2
i- 7.5
102.5
(Vlsec)
k 7.8’
Abbreviations: APA = action potential amplitude; OS = overshoot potential; RP = resting membrane potential: APD,, and APD, = action potential duration at 50”; and 90% repolarization times; n = in parentheses indicates the numbers of animals (rather then number of impalements) from which data were obtained; Statistical significance of difference from control; 'p < 0.05; tp < 0.01.
longer in atria and after 6 weeks it was 33.0%
(p < 0.01) longer than in the control series; the corresponding increases in APDW were 27.6% at 3 weeks @ < 0.01) and 32.8% at 6 weeks (p < 0.01). Similar increases in the APD,, and ADP, (14.2% and 11.0% at 3 weeks, p < 0.05; and 31.5% and 25.3% at 6 weeks, p < 0.01) were also found in ventricular muscle of rabbits pretreated with amiodarone (Table III; Fig. 2). DISCUSSION
The data obtained in this study show that amiodarone (suspended in the commercially available diluent, Tween 80), given intravenously in single bolus injections in humans, has electrophysiologic
properties which differ strikingly from those of the chronically administered drug.6, 6,l1 For example, there were no significant changes in the ERP in the atria or ventricle or in the QT, interval of the surface ECG or the cycle length during sinus rhythm. It is now known25s 26that chronically administered amiodarone consistently lengthens the QT, interval12* 25,26 and the ERP in all myocardial tissues (atria, ventricle, AV node, His-Purkinje system, and the accessory tracts) in which this parameter has been determined.5*6r 11*12.25-z7In contrast, our data, which extend preliminary observations of others,15p16,ls demonstrated that intravenous amiodarone had the main effect on the AV node, characterized by the prolongation of the intranodal conduction time (repre-
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AMIODARONE
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AMIODARONE
SA NODE CELL
ATRIAL FIBER
Fig. 2. Changesin action potentials in myocardial tissuesobtained from the hearts of rabbits chronically administered amiodaroneintraperitoneally (20 mg/kg) for 3 weeksand for 6 weeks,and compared to those from untreated animals. Amiodarone slows the spontaneous firing frequency of the sinus node by retarding phase4 depolarization, and it delays repolarization in the atria, ventricle, and the AV node but without affecting the maximal rate of depolarization. The records shown are representative onesfrom a typical control animal, one treated for 3 weeks,and another treated for 6 weeks.
sented by the AH interval) with the associated lengthening of the FRPs and ERPs at this level of impulse transmission. Chronic
electrophysiologic
effects
of amiodarone.
The differences in the electrophysiologic actions of amiodarone given intravenously as single bolus injections and administered chronically by mouth are clearly relevant to the interpretation of the drug effects in the acute conversion or control of cardiac arrhythmias versus its overall prophylactic efficacy in preventing their recurrences. There is now substantial experience that, when given in an appropri-
ate chronic oral dosage regimen, amiodarone has a high degree of potency in controlling most cardiac dysrhythmias.‘.l2 The previously reported data of Singh and Vaughan Williams,21s22 as well as our newer animal findings reported herein, indicate that the time course of attainment of such salutary effects closely parallels the development of the lengthening of the APD,, in tissues in which the
arrhythmias arise. For example, the consistent prolongation of the APD, (and thus by inference the ERP) was found by us in the atrial, ventricular, and AV nodal transmembrane potentials in rabbits pretreated with amiodarone with a small reduction in the maximal rate of depolarization.2* These changes in rabbits, corresponding to the gradual development of the QT, interval prolongation and other well-defined electrophysiologic effects in humans given oral amiodarone, were evident at tissue/serum concentrations of 30 to 151 with immeasurably low levels of desethylamiodarone. Thus, in the rabbit it is unlikely that the slow onset of the electrophysiologic and antiarrhythmic effects is accountable by the development of a pharmacologically active metabolite. However, this possibility in humans is not discounted either by previously published reports or by our own current data. In the current study, data on the electrophysiologic effects of chronic amiodarone administration in humans are
896
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American
not presented. However, such data have been reported by us elsewhere,5 as well as by others.6,“*‘2. 27-2sIt is in close agreement with animal data reported here, being characterized by significant lengthening of the ERP in all tissues in association with the marked prolongation in the AH interval but with only a small change in the HV interval.2s-30 The fact that the HV interval does increase is consistent with the reports that amiodarone may reduce dV/dt,,,,, to a small extent, as demonstrated by standard microelectrode studies2vZ2 as well as by voltage clamp studies, which have revealed a depressant effect of the drug on inactivated sodium channels.31 Electrophysiologic tered amiodarone.
effects
of intravenously
adminis-
The data reported here are particularly germane to the issue of whether a significant difference exists between the antiarrhythmic effects of intravenously vs orally administered drug. If the lengthening of the APD were the basis for the observed potency for the prophylactic control of arrhythmias by amiodarone, then one might expect the intravenous bolus injections of the compound to exhibit a considerably lower order of antiarrhythmic effectiveness. Our data and those of Wellens et a1.3o have shown that single bolus injections of amiodarone have a variable but generally insignificant influence on atria1 or bypass tract refractoriness, there being no effect on the ERP of the ventricle. These observations are consistent with our finding that in intact rabbits as well as in humans intravenous amiodarone had no effect on the QT, interval of the EGG, and it did not affect the spontaneous cycle length. Furthermore, these overall data are in line with our in vitro studies in which we found that maximal concentrations of amiodarone soluble in homologous rabbit plasma superfusate had no effect on phase 4 depolarization in sinus node cells or on the various parameters of the transmembrane potentials in the atrial and AV nodal tissues removed from untreated rabbits. It must be emphasized, however, that our data on SA node cells are at variance with those of Goupil and Lenfant,32 who found a depressant effect on phase 4 depolarization with a lengthening of the SA action potential induced by amiodarone suspended in physiologic media. One possible explanation for the discrepancy is that in their study an excessively prolonged exposure to amiodarone in the tissue both may have been undertaken; in ours, the equilibration period was 60 minutes and duration of drug exposure may be a crucial determinant of the onset of amiodarone action. We have not been able to directly dissolve amiodarone hydrochloride in
October, 1984 Heart Journal
Tyrode solution used in our laboratory for in vitro electrophysiologic studies. It should also be indicated that the concentrations used by Goupil and Lenfant32 were much higher. It is also noteworthy that the transient increases in the QT, interval following intravenous amiodarone, noted by Holt et al.,18 remain unexplained. However, the differences between the electrophysiologic effects of intravenously administered amiodarone and those noted during chronic oral therapy with the drug are not accountable in terms of low plasma drug concentrations after single intravenous injections. After intravenous injections, plasma amiodarone levels often rose to concentrations above 5 to 12 ug/ml in the current study; during chronic therapy levels of 1 to 3 ug/ml have been found. 23,24Furthermore, the relatively weak to no effect on the ERP of the atria and ventricles following intravenous amiodarone injections does not appear to be related to low myocardial tissue concentrations. For example, in our rabbit experiments, in which single intravenous injections of amiodarone were given, the tissue levels of amiodarone and tissue/serum drug ratios were comparable to those found during chronic steady-state drug administration. The unexpected finding in this present study, as well as in those of Brugada et al.,” Waleffe et al.,‘” and Touboul et al.,‘s* 2ois the marked and significant lengthening of the ERP and FRP of the AV node induced by intravenous amiodarone. The precise mechanism of such an effect is as yet unexplained, but it is clearly a property that accounts for the termination of paroxysmal reentrant supraventricular tachycardias and of the slowing of the ventricular response in atrial flutter and fibrillation1”-‘6; it may also lead to the occasional cases of conversion of atria1 flutter and fibrillation that follow the intravenous administration of amiodarone resulting from improved hemodynamic function due to the slowed ventricular response. It is possible, however, that changes in the ERP of the atria,13-16 while not being nearly as striking as those during chronic drug administration,5 may also contribute in this regard. It is nevertheless unlikely that the modest variable increase in the atria1 ERP is due to the lengthening of the APD. Amiodarone has intrinsic noncompetitive antisympathetic3” as well as coronary dilator actions, properties which may also influence its overall antiarrhythmic effects by “indirect” means. Such actions may constitute the basis for the variable effects of the intravenous drug on the electrophysiologic properties of the atria and bypass tracts30 and for the reports that the intravenous bolus of the
Volume Number
108 4, Part 1
drug may exert a variable effect in the suppression of premature ventricular contractions and for the acute termination of ventricular tachycardia.‘“-‘* However, it is unlikely that the antiadrenergic action of the intravenous drug is the sole basis for the observed changes effected by amiodarone on the AV node. The possibility that such a change is due to the lengthening of the APD appears to be negated by our in vitro experimental studies, although a complete concentration-response relationship was not examined. The results of these studies also do not lend credence to the suggestion that the lengthening of the AV conduction induced by intravenous amiodarone may be due in part to calcium antagonism in the AV node.31 Finally, the possibility that the commercial diluent (Tween 80) itself might exert significant electrophysiologic effects and contribute to the overall actions of the intravenous formulation of amiodarone is not excluded by our data. In this regard, it is noteworthy that a recent report suggests that Tween 80 has distinct hemodynamic effects in anesthetized dogs.35 Clinical implications. The therapeutic and pharmacologic implications of our experimental and clinical observations on the electrophysiologic actions of amiodarone merit emphasis. A significant difference has been demonstrated between the chronic vs the acute effects of the drug. When administered chronically in rabbits, the APD lengthened in atria, AV node, and the ventricles, consistent with the known wide spectrum of antiarrhythmnic effects in humans relative to the increases in the ERP in all tissues. In contrast, the in vitro superfusion studies in animal preparations and the intravenous single bolus injections of amiodarone in humans demonstrated minimal and variable electrophysiologic effects except in the AV node. They suggest that the antiarrhythmic activity of the acute intravenously administered drug is likely to differ from that of the chronically administered compound. The data are in accord with the general observation regarding the slow onset of action and attainment of peak effect of amiodarone. The precise reason for the observed delay in the onset of action at present remains unclear. It may be due to the slow formation of active drug metabolites; alternatively, the slow onset and the gradual development of electrophysiologic action may be related to mechanisms involving interference with myocardial tissue receptors requiring a finite duration of time for the full effect to be achieved.
Electrophysiologic
staff of the cardiac catheterization in the electrophysiologic studies.
897
laboratory for their assistance
REFERENCES
1. Rosenbaum MB, Chiale PA, Ryba D, Elizari MV: Control of tachyarrhythmias associated with Wolff-Parkinson-White syndrome by amiodarone hydrochloride. Am J Cardiol 34:215, 2.
3. 4.
5.
6.
7.
a.
9.
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Il.
12. 13.
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18.
1974.
Rosenbaum MB, Chiale PA, Halpern MS: Clinical efficacy of amiodarone as an antiarrhythmic agent. Am J Cardiol 38:934, 1976. Leak D, Eydt JN: Control of refractory cardiac arrhythmias with amiodarone. Arch Int Med 139:425, 1979. Nademanee K, Hendrickson JA, Cannom DS, Godreyer BN, Singh BN: Control of refractory life-threatening ventricular arrhythmias by amiodarone. AM HEART J 101:759, 1981. Nademanee K, Hendrickson JA, Kannan R, Singh BN: Electrophysiologic effects of amiodarone in patients with life-threatening ventricular arrhythmias; modification of induced ventricular tachycardia versus suppression of spontaneously occurring tachyarrhythmias. AM HEART J 103:950, 1982. Heger JJ, Prystowsky E, Jackman WM, Nacarelli GV, Warfel KA, Rinkenberger RL, Zipe DP: Amiodarone: Clinical efficacy and electrophysiology during long-term therapy for recurrent ventricular tachycardia or ventricular fibrillation. N Engl J Med 306:539, 1981. Nademanee K, Singh BN, Hendrickson JA, Reed AW, Melmed S, Hershman JM: Pharmacokinetic significance of serum reverse T, levels during amiodarone treatment: a potential method for monitoring chronic drug therapy. Circulation 66:202, 1982. Kaski JC, Girotti LA, Messuti, Rubitzky B, Rosenbaum MB: Long-term management of sustained recurrent symptomatic ventricular tachycardia with amiodarone. Circulation 64:273, 1981. Lubbe WF, Mercer CJ, Roche AH, Lowe JB: Amiodarone in long-term treatment of refractory cardiac tachyarrhythmias. NZ J Med 93:31, 1981. Podrid PJ, Lown B: Amiodarone therapy in symptomatic, sustained refractory atrial, and ventricular tachyarrhythmias. AM HEART J 101:374, 1981. Rowland E, Krikler DM: Electrophysiological assessment of amiodarone in treatment of resistant supraventricular arrhythmias. Br Heart J 44:82, 1980. Rasmussen V, Berning J: Effect of amiodarone in the WolffParkinson-White syndrome. Acta Med Stand 205:31, 1979. Benaim R, Denizeau JP, Melon J, Domengie B, Kolsky H, Chanelle M. Chiche P: Les effects antiarrhvthmiaue de l’am>odarone injectable. Arch Ma1 Coeur 69:51>, 1976. Touboul P, Heurta F, Porte J, Delahaye JP: Bases electrophysiologiques de l’action antiarrhythmique de l’amiodarone chez l’homme. Arch Ma1 Coeur 69:845, 1976. Waleffe A, Bruninx P, Kulbertus H: Effects of amiodarone studied by programmed electrical stimulation of the heart in patients with paroxysmal re-entrant supraventricular tachycardia. J Electrocardiol 11:253, 1978. Brugada P, Roy D, Barr FW, Heddle B, Dassen WR, Wellens HJJ: Electrophysiological effects of intravenous amiodarone (abstr). Am J Cardiol 49:1044, 1982. Hoogenhuyze DV, Burg PVD, deWilde A, Remme WJ, Kraus HX: Acute effects of intravenous amiodarone in patients with complex ventricular dysrhythmias (abstr.). Am J Cardiol 49:1001, 1982. Holt P, Curry P, Way B, Holt D: Intravenous amiodarone: An effective antiarrhvthmic agent (abstr.1. Am J Cardiol 49:1001,
19.
We are indebted to Alma Gump and Lawrence Kimble for secretarial help, to Medical Media for photography, and to the
effects of amiodarone
1982.
-
Touboul P, Porte J, Heurta F, Delahaye JP: Electrophysiological effects of amiodarone in man (abstrl. Am J Cardiol 353173.
1975.
October,
898
Ikeda
et al.
20. Touboul P, Porte J, Huerta F, Delahaye JP: Effects of amiodarone hydrochloride given intra-atrially in patients with Wolff-Parkinson-White syndrome (abstrl. Circulation 52 (Suppl 11):250, 1975. 21. Singh BN: A study of the pharmacological actions of certain drugs and hormones with a particular reference to cardiac muscle. D. Phil. Thesis, University of Oxford, England, 1971. 22. Singh BN, Vaughan Williams EM: The effect of amiodarone, a new anti-angina1 drug, on cardiac muscle. Br ,J Pharmacol 39:657, 1970. 23. Flanagan RJ, Storey GCA, Holt DW: Rapid high performance liquid chromatographic method for the measurement of amiodarone in blood plasma or serum at the concentrations attained during therapy. J Chromatogr 187:391, 1980. 24. Kannan R, Nademanee K, Hendrickson JA, Rostami HJ, Singh BN: Amiodarone kinetics after oral doses. Clin Pharmacol Ther 31:438, 1982. 25. Singh BN, Collett JT, Chew CYC: New perpectives in the pharmacologic therapy of cardiac arrhythmias. Prog Cardiovast Dis 22:243, 1980. 26. Pritchard DA, Singh BN, Hurley PJ: Effects of amiodarone on thyroid function in patients with ischemic heart disease. Br Heart J 37:856, 1975. 27. Wellens HJJ, Lie KI, Bar FB, Wesdrop JC, Dohmen HJ, Duren DR, Durrer D: Effects of amiodarone in the WolffParkinson-White syndrome. Am J Cardiol 38:189, 1976.
American
1984
Heart Journal
28. Hamer AW, Finerman WB, Peter T, Mandel WJ: Disparity between the clinical and electrophysiologic effects of amiodarone in the treatment of recurrent ventricular tachyarrhythmias. AM HEART J 102:992, 1981. 29. Saksena S, Rothbart ST, Cape110 G: Chronic effects of amiodarone in patients with refractory ventricular tachycardia. Int J Cardiol 3:339. 1983. 30. Wellens HJJ. Brugada P. Abdollah H, Dassen WR: A comparison of the electrophysiologic effects of intravenous and oral amiodarone in the same patient. Circulation 69:120, 1984. 31. Mason ,JW, Hondeghem LM, Katzung BG: Amiodarone blocks inactivated sodium channels. Pflugers Arch 396:79, 1983. 32. Goupil N, Lenfant J: The effects of amiodarone on the sinus node activity of the rabbit heart. Eur J Pharmacol 39:23, 1976. 33. Charlier R: Cardiac actions in the dog of a new antagonist 01 adrenergic excitation which does not produce competitive blockade of adrenoceptor. Br J Pharmacol 39:668, 1970. 34. Gloor HO, Urthaler F, James TN: The immediate electrophysiologic effects of amiodarone on the canine sinus node and AV junctional region (abstr). Am .J Cardiol 49:981, 1982. X5. Gough WB, Zeiler RH, Barreca P, El-Sherif N: Hypotensive action of commercial intravenous amiodarone and Polysorbate 80 in dogs. J Cardiovasc Pharmacol 4:375, 1982.
to
Oral amiodarone is a potent antiarrhythmic agent with a slow onset of action. Its electrophysiologic properties following chronic administration are well known, but its acute electrophysiologic actions are poorly defined. The objectives of the present study were to correlate the electrophysiologic actions of intravenous amiodarone in humans with the acute and chronic effects of the drug relative to plasma and tissue concentrations of the drug. In humans (n = lo), 5 mg/kg intravenous amiodarone (serum concentration 6.50 + 3.34 fig/ml at 10 minutes; 2.13 rf 0.71 Ag/ml at 20 minutes, n = 7) increased the AH interval by 16.4% (p < ODOS), the antegrade effective refractory period (ERP) of the atrioventricular (AV) node by 14.4% (p < 0.025) and the functional refractory period (FRP) of the AV node by 15.5% (p < 0.005). The ERP or FRP of the atrium of the right ventricle was not significantly changed; there was no effect on the HV interval or the QT and R-R intervals of the ECG. In rabbits (n = 11) given 10 mg/kg intravenous amiodarone (mean * SD serum concentration 0.49 F 0.17 Ag/ml; mean myocardial concentration 7.0 +- 1.9 Ag/gm, n = 3), there were no significant effects on the ECG intervals. In isolated rabbit sinoatrial (SA) node, atria, and AV node (three preparations) superfused with 5 X lo-‘M amiodarone (3.41 Ag/ml), there was no effect on the action potential duration (APD) or other parameters of the transmembrane potential. Rabbits chronically pretreated with amiodarone (20 mg/kg intraperitoneally) for 3 weeks had serum drug concentrations of 0.98 i 0.52 Ag/ml (n = 4) and myocardial levels of 11.52 t 7.2 Ag/gm at 3 weeks and 0.50 f 0.18 pg/ml and 14.8 ? 6.4 Ag/gm, respectively, at 6 weeks. Compared to the values in control series, the spontaneous cycle length of the SA node was prolonged by 24% (p < 0.05) at 3 weeks and 35.5% (p < 0.01) at 6 weeks. The Vmax was affected in none of the tissues but APD was significantly lengthened in all. In atria the APD was increased by 27.6% (p < 0.01) at 3 weeks and 32.8% (p < 0.01) at 6 weeks; in ventricular muscle the corresponding values were 11.0% (p < 0.05) and 25.3% (p < 0.01). Our clinical and experimental data indicate major differences between the chronic and acute electrophysiologic effects of amiodarone, differences which are relevant to the interpretation of the antiarrhythmic actions of the drug following intravenous and oral administration. (AM HEART J 108:890, 1984.)
Nobuo Ikeda, M.D., Koonlawee Nademanee, M.D., Ramaswamy Bramah N. Singh, M.D., D. Phil. Los Angeles, Calif.
Amiodarone hydrochloride has recently emerged as a powerful antiarrhythmic drug for the treatment of a wide variety of cardiac arrhythmias.‘.lo When the drug is given by mouth, the onset of its antiarrhythmic action is slow over a period of days2,4-10and the peak action is often not evident for weeks.4v5*7Such an effect can be correlated with significant increases From the Department of Cardiology, Veterans Center, and the Department of Medicine, UCLA
Administration Medical School of Medicine.
Supported by grants from the Medical Research Service of the Veterans Administration, and the Group Investigator Award of the American Heart Association, the Greater Los Angeles Affiliate. Received
for publication
Reprint requests: Bramah VA Medical Center West Angeles, CA 90073.
890
Feb.
1, 1984;
accepted
March
N. Singh, M.D., Cardiology Los Angeles, Wilshire and
2, 1984. Section Sawtelle
(691/111E), Blvd., Los
Kannan,
Ph.D., and
in the effective refractory period (ERP) of atria, ventricles, atrioventricular (AV) node, His-Purkinje tissues, as well as the bypass tracts.5r6r11,12 The reason for the delay in the onset of antiarrhythmic action of the compound remains essentially unknown. However, in recent years, the use of the intravenous formulation of the drug-containing Tween 80 as diluent-has suggested that the compound might exert certain antiarrhythmic effects.13-18The electrophysiologic correlates of such effects, which have been variable from center to center 14-16, lg,2o are poorly defined. The purpose of the present study was twofold. First, it was to determine the electrophysiologic effects of single intravenous doses of commercially available amiodarone hydrochloride in patients undergoing electrophysiologic
Volume
108
Number
4, Part 1
studies for the evaluation of cardiac arrhythmias. Second, it was to correlate these findings with the alterations in the various parameters of the transmembrane action potentials in isolated rabbit heart induced by the superfusion with homologous plasma containing amiodarone relative to the changes produced by the drug in chronically pretreated anhmals. METHODS Clinical studies. Clinical studieswere undertaken in 10 patients, who underwent electrophysiologicstudiesfor the evaluation of refractory recurrent cardiac arrhythmias. They were all men; meanagewas56 (range 49 to 74) years. Eight had recalcitrant ventricular arrhythmias and two had supraventricular tachyarrhythmias. Studies were performed in the electrophysiologic laboratory in the postabsorptive state without premeditation. Electrode catheters were inserted percutaneously or by cut-down technique and positioned at multiple cardiac sites under fluoroscopic control. The recording sites included high right atrium (RA), His bundle (HB), right ventricular (RV) apex, and coronary sinus. Standard quadripolar electrode catheters with 1 cm interelectrode distance (USCI, Billerica, Mass.) were used for stimulation and recording from specific sites. The HB electrogram was obtained by a tripolar catheter. Surface ECG leads I, aV,, and V, were displayed simultaneously with those of the intracardiac signalsand were recorded on a VR-12 (Electronics for Medicine) recorder at paper speed of 100 mm/set. Cardiac stimulation was performed with a programmable constant-current stimulator (Medtronic 5235), which delivered rectangular pulsesat 2 msecduration at twice the diastolic threshold. Intracardiac electrogramswere filtered at 30 to 500 Hz. The RA, RV, and AV nodal refractory periods were determined by the extrastimulus method.5The AH interval was measured from the initial rapid deflection of the low RA electrogram to the initial deflection of HB depolarization (normal range 60 to 120 msec).The HV interval was measuredfrom the initial His deflection to the earliest onset of ventricular activation (normal range 35 to 55 msec).The ERP of the ventricle was defined as the longest S, S, interval during ventricular pacing with ventricular depolarization at which S, failed to produce ventricular depolarization. Amiodarone hydrochloride was given at a dose of 5 mg/kg intravenously via a peripheral vein over 2 minutes after control parameters had been obtained. Ten to 20 minutes after the completion of the injection theseparameters were again recorded under identical conditions, and whenever possibleblood was withdrawn for the determination of serum concentrations of amiodarone and its principal metabolite, desethylmetabolite. Experimental studies. Since amiodaronehydrochloride is not soluble in physiologic media,21,22 to determine its electrophysiologic effects in isolated preparations by superfusion techniques, the following method was used. Amiodarone (68 mg) was first dissolved in 10 ml of 50%
Electrophysiologic
effects
of
amiodarone
891
ethanol solution (10 mM solution). One milliliter of the stock solution was slowly added to 9 ml of plasmaof the rabbit killed (1 mM solution). Five milliliters of 1 mM solution was then dissolved in 1 L of Tyrode solution to make the final concentration of 5 X lo-” M amiodarone. The procedure for obtaining preparations wasthe sameas that (see below) used in the studies to determine the effects of the drug given chronically before the removal of the atria. In the seriesinvolving acute experiments, rabbits were not pretreated with the drug. The control data in this serieswere obtained when the preparation wassuperfused with Tyrode solution containing 0.5 ml of 50% ethanol and 4.5 ml of plasma (of the killed rabbit)/1 L of Tyrode solution. The effects of amiodarone were examined after 60 minutes of superfusion with the drug. In another seriesof rabbits, the effects of the chronic pretreatment with amiodarone on the intracellularly recorded action potentials were determined. Rabbits (weighing 1.5 or 3.5 kg) were treated with amiodarone (20 mg/kg, intraperitoneally) 5 days/wk for 3 weeks (n = 5) and 6 weeks(n = 6), respectively. A control group (n = 5) was injected with distilled water, 5 dayslwk for 3 weeks, instead of amiodaronesolution. After each treatment, the rabbits were anesthetized with sodium pentobarbital (30 mg/kg, intravenously), and the hearts were rapidly removed and dissected in oxygenated Tyrode solution. The following preparations were obtained from each heart: (1) sinoatrial (SA) node preparation (2 x 3 mm) from the middle part of SA node region, (2) AV junctional preparation including RA, interatrial septum, AV node, and HB, and (3) papillary musclefrom the right ventricle. These preparations were mounted in a tissue bath (10 ml in volume) and superfused with Tyrode solution (15 ml/min) at 37’ f 0.5” C. The composition of Tyrode solution was as follows (mM): NaCl 130, KC1 4.0, CaCl, 1.8, MgSO, 0.5, NaH, PO, 1.8, NaHCO, 18.0, and dextrose 5.5. Tyrode solution wasbubbled with mixed gascontaining 95% 0, and 5 C02, and its pH was maintained at 7.42 2 0.02. SA node preparations were allowed to beat spontaneously while the AV junctional preparations and papillary muscles were electrically stimulated through bipolar electrodes at 2.5 Hz and 1 Hz, respectively. Rectangular pulsesof twice the threshold voltage (1 msec in duration) were delivered by Grass S88 stimulator with isolated output. Action potentials were recorded through glass microelectrodes filled with 3M KC1 (tip resistance = 10 to 30 megaohms).Signalswere amplified with microelectrode amplifier (Mentor N-950) with capacity compensation, and were displayed on an oscilloscope (Tektronics R564B) and photographed on Polaroid film. Maximum upstroke velocity of phase0 of action potentials (dV/d&,,,,) was obtained by electronic differentiation. After an equilibration period of 60 minutes, action potentials were recorded in the standard manner. Ouantitation of amiodarone in serum and myocardial tissues. Serum concentrations of amiodaronewere deter-
mined by high-pressureliquid chromatography according to the procedure of Flanagan et a1.23 This method has the capability to quantitate amiodaroneas well asits deseth-
October,
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et al.
Table
I. Electrophysiologic effects of intravenously administered amiodarone in patients with cardiac arrhythmias
American
Parameters
n
Abbreviations: significant.
ERP
= effective
refractory
period;
Baseline
9 9
Heart rate (bpm) QT, interval (set) AH interval (msec) HV interval (msec) Cycle length at which AV Wenckebach developed (msec) ERP RV (msec) FRP A (msec) ERP A (msec) FRP A (msec) Anterograde ERP AV node (msec) AV node (msec)
70.1 0.40
10
lir 13.4 + 0.04
10
107.0 53.0
t I
10
428.4 253.0
+ 50.5 ztz22.8 2 30.3 + 25.4 + 32.9 _t 40.7 t 39.7
10
276.0 270.0
10 10
318.0 347.1
9 FRP
10 to 12 min after Fj mglkg intravenous amiodarone
438.3
= functional
49.2 14.2
refractory
yl metabolite (desethylamiodarone) and has been previously validated in our laboratory.24 For determining amiodarone levels in heart muscle tissue from rabbits, the muscletissue(left ventricle or left atrium, 300mg net weight) washomogenizedin a Polytron (Brinkman Instruments, Westbury, N. Y.) with 2 ml deionized distilled water, and amiodarone was extracted from an aliquot (250 ~1)of the fresh homogenateimmediately for high-pressureliquid chromatography, since levels of amiodaronedecreaseon storing the homogenatein cold and are unreliable. There was no significant difference in amiodarone concentrations between the atria1 muscleand the ventricular musclepreparations, and only data from the ventricles are reported here. Statistical analysis. The mean data are presented with k standard deviations of n observations from patients or animals. Statistical comparisonswere made with a t test for paired and unpaired data and data for multiple groups by the analysis of variance. A probability value of 10.05 was consideredstatistically significant. RESULTS Electrophysiologic tered amiodarone
effects in patients.
of intravenously
adminis-
The mean data from 10 patients, who were given 5 mg/kg of the drug, are summarized in Table I. The compound had no significant effect on the spontaneous cycle length or on the QT, interval; similarly, the ERP of the atria and ventricles was not affected and infranodal AV conduction was not significantly lengthened. The major effect of intravenous amiodarone was on the AV node: the AI-I interval was increased (+16.4% p < 0.005) as was the longest cycle length for AV Wenckebach periodicity (+19.7% ; p < 0.005) following RA pacing. Increases in the functional (+X.5% ; p < 0.005) as well as the anterograde (+14.4% ; p < 0.025) ERPs of the AV node were also
k
9.9 k 0.03
NS NS
125.5 54.5
f i
52.8 12.6
p < 0.005 NS
513.0 250.0
61.2 22.1
p < 0.006 NS NS NS NS p < 0.025 p < 0.005
320.5 397.1
+ 55.6
506.7
-t 48.9
274.0 287.0
period;
Significance of difference from baseline
68.3 0.39
f i i t i
A = atrial;
27.6 31.6 36.1
RV = right
1984
Heart Journal
ventricle;
AV = atrioventricular;
NS = not
significant. The electrophysiologic measurements were made between 10 and 20 minutes after drug injection; at these times, the amiodarone serum concentrations were 6.50 + 3.34 pg/ml at 10 minutes and 2.13 f 0.71 pg/ml at 20 minutes (n = 7). Desethylamiodarone was not detectable following intravenous injections of amiodarone. Effects of intravenous amiodarone on ECG intervals in anesthetized rabbits relative to serum and tissue drug concentrations. In 11 rabbits, amiodarone adminis-
tered intravenously, 10 mg/kg, produced serum concentrations of 2.56 f 1.22 pg/ml at 10 minutes (n = 8) and 0.49 * 0.17 pg/ml (mean + SD) at 60 minutes after drug injection; ECGs recorded in six of these rabbits at these times showed no significant effect on the QRS duration or the QT, interval. The PR interval was 71 f 7 msec before the drug and 69 -t 3 msec after the drug injection, a difference that was not significant. When the animals were killed, the level of the ventricular tissue concentration of amiodarone was 7.0 + 1.9 pg/gm (n = 3). Effects of superfusion of isolated rabbit by homologous rabbit plasma containing
Three preparations
heart tissues amiodarone.
each of rabbit SA node, AV
node, and atria1 tissue were superfused with 5 x 10e6 M (3.41 pg/ml) amiodarone at the highest concentration that was readily soluble in homologous plasma with ethanol. Despite over 60 minutes of continuous superfusion, the drug had no significant effect on the action potential amplitude, maximal rate of rise of phase 0, phase 4 depolarization (in the SA node and spontaneously beating fibers), and in the 50% (APD& and 90% (APD,,) action potential durations. Representative control recordings and those following superfusion with amiodarone are shown in Fig. 1.
Volume Number
108 4, Part 1
Electrophysiologic
CONTROL
effects of amiodarone
893
AMIODARONE 5~10-~M 0
SA NODE CELL
I -60
mv
I 10
vlsec
ATRIAL FIBER
AV NODE
1. Effects of acute superfusionof isolatedrabbit heart musclepreparations with homologousplasma containing amiodarone,5 x 10e6M.The upper traces of eachpanel showzero potential; the middle panel, the transmembrane action potential; the lower traces represent the rate of rise of the action potential obtained by electronic differentiation. Note that cycle length of the SA node firing frequency is not significantly affected, neither are the various parametersof the transmembranepotentials from atria and ventricles, but small lengthening of the APD,, is noted.
Fig.
Electrophysiologic bits pretreated with
effects of amiodarone in the rabamiodarone. When rabbits were
pretreated with amiodarone (20 mg/kg intraperitoneally) for 3 weeks, the serum drug concentration was 0.98 +- 0.52 pg/ml (n = 4); the corresponding tissue level in the ventricular myocardium was 11.52 + 7.2 wg/gm tissue. In rabbits treated for 6 weeks (n = 4), the serum levels were 0.50 +- 0.18 pg/ml with the tissue levels being 14.8 + 6.4 pg/gm. Neither the serum drug levels nor the tissue levels were significantly different after 3 weeks vs 6 weeks of drug administration. The mean data showing the electrophysiologic effects of amiodarone on the various parameters of the transmembrane action potential in the SA node and AV node after 3 and 6 weeks of pretreatment compared to those in a control series are shown in Table II; the corresponding changes found in the
atrial and ventricular muscle are presented in Table III. Representative records from all the tissues studied are reproduced in Fig. 2. After pretreatment with amiodarone, the cycle length of the spontaneously beating SA node was prolonged by 24% 03 < 0.05) at 3 weeks and by 35.4% (p < 0.01) at 6 weeks. The change was due to the depression in the slope of the spontaneous diastolic depolarization (Fig. 2); increases in the overall repolarization in the SA nodal potentials were not quantifiable. In the AV node, amiodarone had no effect except on the APD,, (at 6 weeks) and in the APD, (at 3 and 6 weeks); significant lengthening was induced by amiodarone (Table I). In atrial and ventricular muscle, significant changes induced by amiodarone pretreatment were evident only with respect to repolarization (Fig. 2). After 3 weeks of pretreatment, the APD, was 16.6% (p < 0.05)
October,
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Ikeda
et al.
American
Heart
1984 Journal
II. Effects of chronic administration of amiodaroneon the electrophysiologic properties of rabbit SA and AV nodesexamined in vitro __Table
APA (m vi SA nodal cells Control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) AV nodal cells Control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) 6 wk
OS (mV)
MDP (mV)
dVldt,, (Vlsec)
APD, (msec)
APD, (msec)
16.2 + 7.8 77.5 +- 4.1
11.9 t 1.4 12.5 a 2.7
64.3 + 6.8 65.0 + 4.6
10.4 k 2.3 10.8 k 1.8
-
75.8 rk 4.9
11.5 t 2.9
64.3 f 3.1
10.1 t 2.2
78.3 + 8.3 77.7 + 5.3
11.7 f 3.3 12.5 + 2.5
66.6 t 4.1 65.2 f 2.3
17.4 -+ 5.5 17.1 -+ 6.3
c54.7 t 6.5 66.1 + 2.4
81.6 ‘- 5.5 92.0 f 3.0*
79.3 +- 3.5
12.3 k 3.1
67.1 f 4.1
16.8 t 5.3
67.3 f 4.1
96.2 + 5.2t
SPCL (msec)
343.1 425.5
+ 29.0 t 29.4*
464.7
k 353 -
Abbreviations: APA = action potential amplitude; OS = overshoot potential; MDP = maximal diastolic potential; APD,, and APD, = action potential duration at 50% and 905 repolarization times: SPCL = spontaneous cycle length; statistical significance of difference from control: *p < 0.05: tp < 0.01.
Ill. Effects of chronic treatment with amiodarone on the electrophysiologic parameters of the action potentials recorded from rabbit atria and ventricular muscle in vitro Table
dV/dt,,,
APD, (msec)
APD, (msec)
dz 24.9
45.7 + 3.6
83.3 YIZ 5.6
218.6
f 23.2
53.3 +- 3.8’
106.4
83.0 i 4.1
221.7
i
27.1
60.8 + 3.7t
110.6 t 8.8t
18.4 k 2.3
84.8 + 3.1
206.0
t 14.6
92.5 2 5.4
132.6 + 7.6
+ 8.6
17.9 t 1.8
84.6 + 5.2
198.8
f 17.1
105.6
2 5.5*
147.2
1 6.5*
104.8 * 4.9
18.6 t 2.7
86.2 + 4.7
200.0
+ 19.5
121.6 + 5.9t
166.1
+ 8.2t
APA
(mV)
OS (mV)
RP (mV)
Atria1 muscle control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) 6 wk
102.7
t 5.9
18.2 it 2.3
84.6 + 2.5
227.5
99.8 -+ 4.2
17.0 t 2.1
82.8 f 3.5
100.2
? 6.5
17.2 z!z 3.1
Ventricular muscle control (n = 5) Amiodarone (n = 5) 3 wk Amiodarone (n = 6) 6 wk
103.2
i- 7.5
102.5
(Vlsec)
k 7.8’
Abbreviations: APA = action potential amplitude; OS = overshoot potential; RP = resting membrane potential: APD,, and APD, = action potential duration at 50”; and 90% repolarization times; n = in parentheses indicates the numbers of animals (rather then number of impalements) from which data were obtained; Statistical significance of difference from control; 'p < 0.05; tp < 0.01.
longer in atria and after 6 weeks it was 33.0%
(p < 0.01) longer than in the control series; the corresponding increases in APDW were 27.6% at 3 weeks @ < 0.01) and 32.8% at 6 weeks (p < 0.01). Similar increases in the APD,, and ADP, (14.2% and 11.0% at 3 weeks, p < 0.05; and 31.5% and 25.3% at 6 weeks, p < 0.01) were also found in ventricular muscle of rabbits pretreated with amiodarone (Table III; Fig. 2). DISCUSSION
The data obtained in this study show that amiodarone (suspended in the commercially available diluent, Tween 80), given intravenously in single bolus injections in humans, has electrophysiologic
properties which differ strikingly from those of the chronically administered drug.6, 6,l1 For example, there were no significant changes in the ERP in the atria or ventricle or in the QT, interval of the surface ECG or the cycle length during sinus rhythm. It is now known25s 26that chronically administered amiodarone consistently lengthens the QT, interval12* 25,26 and the ERP in all myocardial tissues (atria, ventricle, AV node, His-Purkinje system, and the accessory tracts) in which this parameter has been determined.5*6r 11*12.25-z7In contrast, our data, which extend preliminary observations of others,15p16,ls demonstrated that intravenous amiodarone had the main effect on the AV node, characterized by the prolongation of the intranodal conduction time (repre-
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AMIODARONE
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AMIODARONE
SA NODE CELL
ATRIAL FIBER
Fig. 2. Changesin action potentials in myocardial tissuesobtained from the hearts of rabbits chronically administered amiodaroneintraperitoneally (20 mg/kg) for 3 weeksand for 6 weeks,and compared to those from untreated animals. Amiodarone slows the spontaneous firing frequency of the sinus node by retarding phase4 depolarization, and it delays repolarization in the atria, ventricle, and the AV node but without affecting the maximal rate of depolarization. The records shown are representative onesfrom a typical control animal, one treated for 3 weeks,and another treated for 6 weeks.
sented by the AH interval) with the associated lengthening of the FRPs and ERPs at this level of impulse transmission. Chronic
electrophysiologic
effects
of amiodarone.
The differences in the electrophysiologic actions of amiodarone given intravenously as single bolus injections and administered chronically by mouth are clearly relevant to the interpretation of the drug effects in the acute conversion or control of cardiac arrhythmias versus its overall prophylactic efficacy in preventing their recurrences. There is now substantial experience that, when given in an appropri-
ate chronic oral dosage regimen, amiodarone has a high degree of potency in controlling most cardiac dysrhythmias.‘.l2 The previously reported data of Singh and Vaughan Williams,21s22 as well as our newer animal findings reported herein, indicate that the time course of attainment of such salutary effects closely parallels the development of the lengthening of the APD,, in tissues in which the
arrhythmias arise. For example, the consistent prolongation of the APD, (and thus by inference the ERP) was found by us in the atrial, ventricular, and AV nodal transmembrane potentials in rabbits pretreated with amiodarone with a small reduction in the maximal rate of depolarization.2* These changes in rabbits, corresponding to the gradual development of the QT, interval prolongation and other well-defined electrophysiologic effects in humans given oral amiodarone, were evident at tissue/serum concentrations of 30 to 151 with immeasurably low levels of desethylamiodarone. Thus, in the rabbit it is unlikely that the slow onset of the electrophysiologic and antiarrhythmic effects is accountable by the development of a pharmacologically active metabolite. However, this possibility in humans is not discounted either by previously published reports or by our own current data. In the current study, data on the electrophysiologic effects of chronic amiodarone administration in humans are
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not presented. However, such data have been reported by us elsewhere,5 as well as by others.6,“*‘2. 27-2sIt is in close agreement with animal data reported here, being characterized by significant lengthening of the ERP in all tissues in association with the marked prolongation in the AH interval but with only a small change in the HV interval.2s-30 The fact that the HV interval does increase is consistent with the reports that amiodarone may reduce dV/dt,,,,, to a small extent, as demonstrated by standard microelectrode studies2vZ2 as well as by voltage clamp studies, which have revealed a depressant effect of the drug on inactivated sodium channels.31 Electrophysiologic tered amiodarone.
effects
of intravenously
adminis-
The data reported here are particularly germane to the issue of whether a significant difference exists between the antiarrhythmic effects of intravenously vs orally administered drug. If the lengthening of the APD were the basis for the observed potency for the prophylactic control of arrhythmias by amiodarone, then one might expect the intravenous bolus injections of the compound to exhibit a considerably lower order of antiarrhythmic effectiveness. Our data and those of Wellens et a1.3o have shown that single bolus injections of amiodarone have a variable but generally insignificant influence on atria1 or bypass tract refractoriness, there being no effect on the ERP of the ventricle. These observations are consistent with our finding that in intact rabbits as well as in humans intravenous amiodarone had no effect on the QT, interval of the EGG, and it did not affect the spontaneous cycle length. Furthermore, these overall data are in line with our in vitro studies in which we found that maximal concentrations of amiodarone soluble in homologous rabbit plasma superfusate had no effect on phase 4 depolarization in sinus node cells or on the various parameters of the transmembrane potentials in the atrial and AV nodal tissues removed from untreated rabbits. It must be emphasized, however, that our data on SA node cells are at variance with those of Goupil and Lenfant,32 who found a depressant effect on phase 4 depolarization with a lengthening of the SA action potential induced by amiodarone suspended in physiologic media. One possible explanation for the discrepancy is that in their study an excessively prolonged exposure to amiodarone in the tissue both may have been undertaken; in ours, the equilibration period was 60 minutes and duration of drug exposure may be a crucial determinant of the onset of amiodarone action. We have not been able to directly dissolve amiodarone hydrochloride in
October, 1984 Heart Journal
Tyrode solution used in our laboratory for in vitro electrophysiologic studies. It should also be indicated that the concentrations used by Goupil and Lenfant32 were much higher. It is also noteworthy that the transient increases in the QT, interval following intravenous amiodarone, noted by Holt et al.,18 remain unexplained. However, the differences between the electrophysiologic effects of intravenously administered amiodarone and those noted during chronic oral therapy with the drug are not accountable in terms of low plasma drug concentrations after single intravenous injections. After intravenous injections, plasma amiodarone levels often rose to concentrations above 5 to 12 ug/ml in the current study; during chronic therapy levels of 1 to 3 ug/ml have been found. 23,24Furthermore, the relatively weak to no effect on the ERP of the atria and ventricles following intravenous amiodarone injections does not appear to be related to low myocardial tissue concentrations. For example, in our rabbit experiments, in which single intravenous injections of amiodarone were given, the tissue levels of amiodarone and tissue/serum drug ratios were comparable to those found during chronic steady-state drug administration. The unexpected finding in this present study, as well as in those of Brugada et al.,” Waleffe et al.,‘” and Touboul et al.,‘s* 2ois the marked and significant lengthening of the ERP and FRP of the AV node induced by intravenous amiodarone. The precise mechanism of such an effect is as yet unexplained, but it is clearly a property that accounts for the termination of paroxysmal reentrant supraventricular tachycardias and of the slowing of the ventricular response in atrial flutter and fibrillation1”-‘6; it may also lead to the occasional cases of conversion of atria1 flutter and fibrillation that follow the intravenous administration of amiodarone resulting from improved hemodynamic function due to the slowed ventricular response. It is possible, however, that changes in the ERP of the atria,13-16 while not being nearly as striking as those during chronic drug administration,5 may also contribute in this regard. It is nevertheless unlikely that the modest variable increase in the atria1 ERP is due to the lengthening of the APD. Amiodarone has intrinsic noncompetitive antisympathetic3” as well as coronary dilator actions, properties which may also influence its overall antiarrhythmic effects by “indirect” means. Such actions may constitute the basis for the variable effects of the intravenous drug on the electrophysiologic properties of the atria and bypass tracts30 and for the reports that the intravenous bolus of the
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drug may exert a variable effect in the suppression of premature ventricular contractions and for the acute termination of ventricular tachycardia.‘“-‘* However, it is unlikely that the antiadrenergic action of the intravenous drug is the sole basis for the observed changes effected by amiodarone on the AV node. The possibility that such a change is due to the lengthening of the APD appears to be negated by our in vitro experimental studies, although a complete concentration-response relationship was not examined. The results of these studies also do not lend credence to the suggestion that the lengthening of the AV conduction induced by intravenous amiodarone may be due in part to calcium antagonism in the AV node.31 Finally, the possibility that the commercial diluent (Tween 80) itself might exert significant electrophysiologic effects and contribute to the overall actions of the intravenous formulation of amiodarone is not excluded by our data. In this regard, it is noteworthy that a recent report suggests that Tween 80 has distinct hemodynamic effects in anesthetized dogs.35 Clinical implications. The therapeutic and pharmacologic implications of our experimental and clinical observations on the electrophysiologic actions of amiodarone merit emphasis. A significant difference has been demonstrated between the chronic vs the acute effects of the drug. When administered chronically in rabbits, the APD lengthened in atria, AV node, and the ventricles, consistent with the known wide spectrum of antiarrhythmnic effects in humans relative to the increases in the ERP in all tissues. In contrast, the in vitro superfusion studies in animal preparations and the intravenous single bolus injections of amiodarone in humans demonstrated minimal and variable electrophysiologic effects except in the AV node. They suggest that the antiarrhythmic activity of the acute intravenously administered drug is likely to differ from that of the chronically administered compound. The data are in accord with the general observation regarding the slow onset of action and attainment of peak effect of amiodarone. The precise reason for the observed delay in the onset of action at present remains unclear. It may be due to the slow formation of active drug metabolites; alternatively, the slow onset and the gradual development of electrophysiologic action may be related to mechanisms involving interference with myocardial tissue receptors requiring a finite duration of time for the full effect to be achieved.
Electrophysiologic
staff of the cardiac catheterization in the electrophysiologic studies.
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laboratory for their assistance
REFERENCES
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effects of amiodarone
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Heart Journal
28. Hamer AW, Finerman WB, Peter T, Mandel WJ: Disparity between the clinical and electrophysiologic effects of amiodarone in the treatment of recurrent ventricular tachyarrhythmias. AM HEART J 102:992, 1981. 29. Saksena S, Rothbart ST, Cape110 G: Chronic effects of amiodarone in patients with refractory ventricular tachycardia. Int J Cardiol 3:339. 1983. 30. Wellens HJJ. Brugada P. Abdollah H, Dassen WR: A comparison of the electrophysiologic effects of intravenous and oral amiodarone in the same patient. Circulation 69:120, 1984. 31. Mason ,JW, Hondeghem LM, Katzung BG: Amiodarone blocks inactivated sodium channels. Pflugers Arch 396:79, 1983. 32. Goupil N, Lenfant J: The effects of amiodarone on the sinus node activity of the rabbit heart. Eur J Pharmacol 39:23, 1976. 33. Charlier R: Cardiac actions in the dog of a new antagonist 01 adrenergic excitation which does not produce competitive blockade of adrenoceptor. Br J Pharmacol 39:668, 1970. 34. Gloor HO, Urthaler F, James TN: The immediate electrophysiologic effects of amiodarone on the canine sinus node and AV junctional region (abstr). Am .J Cardiol 49:981, 1982. X5. Gough WB, Zeiler RH, Barreca P, El-Sherif N: Hypotensive action of commercial intravenous amiodarone and Polysorbate 80 in dogs. J Cardiovasc Pharmacol 4:375, 1982.