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Cardiovascular Research 2001 50(2):345-353; doi:10.1016/S0008-6363(01)00259-0
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Copyright © 2001, European Society of Cardiology

Phase 2 prolongation, in the absence of instability and triangulation, antagonizes class III proarrhythmia

Luc M Hondeghema,*, Karl Dujardinb and Fred De Clerckc

aDepartment of Pharmacology, K.U.L., Leuven B-3000, Belgium
bDepartment of Cardiology, H. Hart Ziekenhuis, 8800 Roeselare, Belgium
cDepartment of Cardiovascular Safety Pharmacology, Janssen Research Foundation, B-2340 Beerse, Belgium

* Corresponding author. Present address: H.P.C. nv, Westlaan 85, B-8400 Oostende, Belgium. Tel.: +32-59-510-047; fax: +32-59-510-048 luchondeghem{at}yahoo.com

Received 8 November 2000; accepted 30 January 2001


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objective: To evaluate whether prolongation of the plateau of the action potential duration, in the absence of instability and triangulation, can reverse the proarrhythmia elicited by a class III antiarrhythmic agent. Methods: The effects of almokalant, erythromycin and their combination, on cardiac electrophysiological parameters (action potential duration (APD), instability, triangulation and ectopics) were evaluated in isolated hearts from female albino rabbits. In this study, proarrhythmia was estimated quantitatively by number of ectopic beats. Results: Erythromycin lengthened the APD primarily by a prolongation of the plateau, while having only minor effects upon phase 3 repolarization. The prolongation did not induce much instability, triangulation or reverse use dependence and, as expected, erythromycin did not induce significant proarrhythmia. Almokalant also lengthened APD, but it did not lengthen the plateau; instead, it prolonged phase 3 repolarization. The prolongation markedly triangulated the action potential, elicited much instability and marked reverse use dependence. This combination of effects induced very marked proarrhythmia. When almokalant and erythromycin were combined, their effects upon APD appeared additive: both the plateau and the repolarization phase were prolonged. However, the larger prolongation of APD did not lead to more proarrhythmia; this suggests that a prolongation of APD is not proarrhythmic per se. On the contrary, proarrhythmia as a function of APD prolongation was reduced in the presence of erythromycin (P<0.05). Conclusion: Instability plus triangulation consistently lead to serious proarrhythmia especially when combined with reverse use dependence, but prolongation of APD in itself is not necessarily proarrhythmic. In fact, APD prolongation in the absence of instability and triangulation can be antiarrhythmic.

KEYWORDS APD, action potential duration; APDx, APD to x% repolarization, e.g. APD60 is the APD to 60% repolarization; EAD, early afterdepolarization; TdP, torsade de pointes; Triangulation, difference between APD30 and APD90 prolongation, the action potential assumes a more triangular shape. The APD0 to APD30 is used as a quantitative index of the plateau or phase 2, while the APD30 to APD90 is used as a quantitative index of phase 3.


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
Class III antiarrhythmic effects, i.e. by prolongation of action potential duration (APD) [1], are frequently associated with proarrhythmia, early afterdepolarizations (EAD) and torsade de pointes (TdP) [2]. Therefore, whenever a drug induces a QT prolongation, clinicians as well as regulatory bodies reflexly become concerned about the potential proarrhythmic effect of the agent [3]. However, we reported that a prolongation of APD could be antiarrhythmic, provided that it is not accompanied by instability and triangulation of the action potential or reverse use dependence [4]. The most rigorous test of this thesis consists of combining a class III proarrhythmic agent (e.g. almokalant [5,6]) with another one that lengthens the APD without instability, triangulation or reverse use dependence. If prolongation of APD by itself is proarrhythmic, then the proarrhythmic action of almokalant should be augmented. If instead prolongation of APD is antiarrhythmic, then the proarrhythmic effect of almokalant should be attenuated.

At present, no agent prolonging APD without instability, triangulation or reverse use dependence is commercially available. Amiodarone may approach such a profile of activity [7]. However, its complex pharmacokinetics renders it less practical to use in an acute in vitro test. In therapeutic concentrations erythromycin prolongs the QT interval by ~35 ms, but is very rarely associated with TdP [8,9]. Therefore, we speculated that erythromycin might prolong the APD, with little instability, triangulation or reverse use dependence; if so, this agent could be suitable to test the above almokalant combination hypothesis.

We therefore characterized the electrophysiological effects of erythromycin upon APD, instability, triangulation and reverse use dependence in isolated rabbit hearts, singly as well as in combination with almokalant.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
Cardiac electrophysiological properties were measured using SCREENIT [10]. Briefly, experiments were done in 30 hearts from 2.5-kg albino female rabbits and perfused in the Langendorff mode. Female donors were used because they are more sensitive to proarrhythmic effects of class III agents [11]. The investigation conforms with the Guide for Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996). Perfusion occurred at a constant pressure of 80 cm water (NaCl 118, KCl 4, NaHCO3 22, MgCl2 1.1, NaH2PO4 0.4, CaCl2 1.8, dextrose 5, pyruvate 2 and creatine 0.038 mM; equilibrated with 95%O2~5%CO2; pH 7.4 and 34°C). The His bundle was sectioned and two stimulating electrodes were sutured in proximity of the distal His bundle. A recording electrode was advanced to the left ventricular subendocardium of the septum. An epicardial recording electrode and reference electrode were positioned on the left ventricular epicardium. The reference electrode was grounded and perfused at ~1 ml/min with isotonic KCl, enriched with 1.8 mM CaCl2 [10]. The heart was paced at 1.5 times threshold stimulation current. If automaticity and escape cycle length were >1000 ms, threshold stimulation current <300 µA, coronary perfusion >17 ml/min, ectopic rate <8 beats/min and the cardiac activation time <60 ms, then the preparation was stimulated until instability (determined by the BES method, described below) of the last 20 trains became less than 10 ms. Preparations (n = 5) that did not achieve these criteria were rejected before the study started.

2.1 Experimental procedures
The experiment consisted of brief (executed every minute) and large protocols (executed at 10, 60, 70 and 120 min in the experiment) (Fig. 1). The brief protocol started with a readjustment of the stimulation current; then the action potentials of a 10-beat train at 300 ms and a 30-beat train at 1000 ms were saved to disk. In the large protocol, stimulation current was adjusted; automaticity, escape cycle lengths, conduction times and APDs for cycle lengths at 2000, 1500, 1000, 750, 500, 300 and 250 ms were determined.


Figure 1
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  		Fig. 1 Schematic diagram of the experimental sequence. In the diagram each ‘.’ represents a brief protocol, and every tenth brief protocol is indicated by a ‘,’ for clarity, while the large protocols at minutes 10, 60, 70 and 120, are represented by ‘!’. The bar at the bottom represents the time during which erythromycin was infused at 100 µM in two of the three series of experiments. The staircase at the top represents the infusion of almokalant at 0.01, 0.03, 0.1, 0.3 and 1 µM for 10 min each in two of the three series of experiments.

 
Following a 10-min control perfusion, in two groups a 110-min erythromycin (100 µM) perfusion was started, while the third was perfused with control. Starting at minute 70, in the control and one erythromycin group, almokalant was added for 10 min each at 0.01, 0.03, 0.1, 0.3 and 1 µM. Almokalant was obtained from AstraZeneca and erythromycin from Sigma.

2.2 Electrophysiological determinations
Monophasic action potentials were digitized at 1 kHz (12 bits). Conduction data were sampled at 10 kHz (each channel) and saved to a disk.

2.2.1 APD duration
APD10–APD90 were measured from the midpoint of the upstroke until 10, 20...90% repolarization.

2.2.2 Triangulation
Triangulation is defined as the repolarization time from APD30 to APD90.

2.2.3 Reverse use dependence
Reverse use dependence was measured as the difference between the APD60 of the first ten and the last 20 action potentials of a 30-pulse train.

2.2.4 Ectopics
If the upstroke of an action potential did not occur within 80 ms following the stimulus, it was considered to be an ectopic. However, only action potential amplitudes exceeding at least 50% of the average upstroke in a train were considered valid. The number of ectopics per minute was reported as the average during the last 3 min of any 10-min drug exposure. This algorithm underestimated the true ectopic incidence for several reasons: closely coupled ectopics having too small an amplitude were not counted; when ectopics became too frequent (TdP, VT) the experiment could not be continued; sometimes ectopics only developed transiently and did not occur during the last 3 min of drug exposure. Nevertheless, the numbers should give an indication of the presence, absence and degree of proarrhythmia. The ectopics induced by class III agents are not the usual premature beats, but mostly EAD induced ectopics or resulting from runs of TdP.

2.2.5 Instability of APD
Instability of APD was tested using a non-parametric test. In order to minimize the bias induced by a few exceptionally long or short APDs, the best easy systematic [12] was used to estimate the APD60: basically, steady state action potentials were sorted according to their APD60 and by linear interpolation, the median, the upper and the lower 25% values were computed. An instability index was obtained by computing the difference between the upper and lower quartile estimates in milliseconds. For the experimental trains in each drug concentration, the last 20 action potentials for the final 3 min of drug perfusion were used, i.e. 60 action potentials in total.

2.3 Statistical analysis
Data are presented as mean±standard error (S.E.), unless explicitly stated otherwise. Where data did not appear to be normally distributed, comparisons between groups were done using non-parametric tests [12]. Significant differences between the data were checked with the Friedman test [13]. If a significant difference was found, then the means that were statistically significant were identified using the Wilcoxon signed-rank test [13]. Differences with P<0.05 were considered significant. Slopes through the origin were estimated using a least squares estimate and tested for significant difference using a t-test [13].


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Effects upon the monophasic action potential
3.1.1 Control
In control the APD60 was 209±8 ms, which is in the normal range for the adult female rabbit [4]. In Fig. 2 (top left panel), the average normalized (n = 10) action potentials at 10 and 60 min in control conditions are superimposed. Over this 50-min control perfusion period the action potential configuration changed very little. Typically, as was also the case in these experiments, there was a small shortening manifested primarily as a faster repolarization rate during phase 3 of the action potential. APD60 declined by 11.1±5.1 ms over the time course of the present control experiment (Table 1), and although relatively small, this shortening became significant after 40 min. It is important to remember that, during perfusion with control APD tends to shorten, making any prolongation contrary to the drug free experimental APD change.


Figure 2
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  		Fig. 2 Action potentials in control conditions and during perfusion with erythromycin, almokalant and the combination of erythromycin plus almokalant. In the figure the normalized monophasic action potential amplitudes are recorded from the septal electrode and plotted as a function of time. Each vertical line represents 100 ms. The action potentials represent the average effects (n = 10). The top panels are recorded at 10 and 60 min into the experiment during control (left) and erythromycin 100 µM (right). The bottom panels are recorded at 70, 110 (0.3 µM) and 120 min (1 µM) into the experiment. Almokalant singly (left) and in the combination of erythromycin plus almokalant (right).

 

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  		Table 1 Electrophysiological changes during control conditionsa

 
3.1.2 Erythromycin
Perfusion with erythromycin (100 µM) prolonged APD. This prolongation occurred primarily in the plateau range (Fig. 2, top right panel), while there was little slowing of repolarization (phase 3). The prolongation of APD by erythromycin was quite marked: 56.8±17.8 ms (Table 2). Moreover, the change was already significant after 10-min perfusion with the compound. The prolongation of APD continued to increase to reach a quasi-steady state after a 50-min perfusion period. Note also that while in control there was a significant shortening of triangulation: this was not present in erythromycin. In the group that was perfused with erythromycin alone, erythromycin (100 µM) also significantly prolonged the APD during the 1st hour of perfusion (43.2±13.4 ms; P<0.02; n = 10), but during an additional 50 min of perfusion there was no significant further increase (15.4±11.9 ms; P = 0.4).


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  		Table 2 Electrophysiological changes with erythromycina

 
3.1.3 Almokalant
Almokalant is a prototype class III agent [5,6] and thus acts primarily by prolongation of the action potential (Fig. 2, bottom left panel). The plateau phase of the action potential is changed relatively little by the compound, while the primary mechanism for lengthening is a concentration dependent slowing of phase 3 repolarization rate. Such slowing of phase 3 repolarization renders the action potential more triangular. Almokalant 1 µM increased APD30–90 from less than 100 ms to over 300 ms (Fig. 2, bottom left panel).

3.1.4 Erythromycin and almokalant
When almokalant (0.01–1 µM) was added to the continuous perfusion of erythromycin (100 µM), the action potential duration was further prolonged (Fig. 2, bottom right panel). This increase of APD60 was concentration dependent and became significant at 0.1 µM, where it reached 42.9±22.6 ms. At 0.3 µM, it reached 72.9±37 ms and became 135±49 ms at 1 µM. However, relative to erythromycin singly, the plateau phase remained virtually unchanged by addition of almokalant, while slowing of phase 3 repolarization was the primary mechanism for prolongation of the APD. Thus, also in combination with erythromycin, almokalant again induced triangulation.

Thus, the four panels of Fig. 2 clearly demonstrate that erythromycin primarily lengthens phase 2 (plateau), while almokalant primarily prolongs phase 3 (repolarization). The combination of the two compounds yields a prolongation of both phase 2 and phase 3.

3.2 Reverse use dependence
Reverse use dependence refers to the fact that many class III agents prolong the action potential primarily at slow heart rates, but prolong it to a lesser extend at fast heart rates [7]. In the present study we did not examine reverse use dependence extensively at different cycle lengths for all concentrations of the compounds under scrutiny. Nevertheless, we can compare the reverse use dependence of these two agents. Indeed, in control conditions APD60 shortened only by 4 ms, when the cycle length was reduced from 750 to 300 ms (Fig. 3, top panel). With 100 µM erythromycin APD60 shortened by 32 ms (middle panel) and with almokalant 1 µM, it shortened by 72 ms (bottom panel). From Fig. 3 it is clear that these differences would even be more marked for APD90.


Figure 3
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  		Fig. 3 Action potentials at cycle lengths of 300 and 750 ms in control conditions and with erythromycin and almokalant. In the figure the normalized monophasic action potential amplitude recorded from the septal electrode is plotted as a function of time. Each vertical line represents 100 ms. The two superimposed action potentials are the average effects (n = 10) in control (top), erythromycin 100 µM (middle) and almokalant (bottom). The action potentials were obtained at the end of a pulse train at a cycle length of 300 and 750 ms.

 
When reverse use dependence was assessed by calculating the shortening of APD60 during the first ten beats relative to the last 20 beats in the same pulse train, APD60 lengthened by 0.3±1.2 ms with erythromycin (100 µM), i.e. there was little or no reverse use dependence. By contrast, a similar calculation showed that almokalant (0.3 µM) induced 21.4±53-ms reverse use dependence.

3.3 Effects upon instability
3.3.1 Control
Instability of the action potential is a most sensitive predictor of proarrhythmia. Indeed, it frequently precedes the proarrhythmia and occurs at markedly lower concentrations (one to two orders of magnitude) than a prolongation of APD [4]. In control, the instability was 7.4±1.0 ms, which is similar to that observed previously [4]; likewise, there were no significant increases of instability during the control perfusion (Table 1).

3.3.2 Erythromycin
Perfusion with erythromycin at 100 µM also did not yield relevant changes in instability (Table 2). At 30 min into erythromycin the increase of instability became just statistically significant, but the increase remained very small (2.6±1.2 ms). During continued perfusion with erythromycin, the instability declined again. During 50 more minutes of perfusion, instability increased slightly (8.7±4.7 ms; P>0.05).

3.3.3 Almokalant
During perfusion with almokalant, we observed marked and concentration dependent increase of instability: with 0.03 µM almokalant, instability increased by 4.8±2.2 ms. This significant effect increased further with incrementing concentrations of the agent (Table 3), to reach a maximal extent of 71±35.7 ms (P<0.05). It is important to note that instability reached significance at a concentration of 0.03 µM, which is well below that where the APD60 was prolonged significantly (0.3 µM).


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  		Table 3 Electrophysiological changes with almokalanta

 
3.3.4 Erythromycin and almokalant
Also in the presence of 100 µM erythromycin, almokalant increased the instability (Table 4). Although not reaching statistical significance until the almokalant concentration became 1 µM, the extent of the instability was nevertheless very marked. Such large increases never occurred in control or erythromycin.


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  		Table 4 Electrophysiological changes with erythromycin plus almokalanta

 
3.4 Effects upon triangulation
3.4.1 Control
The basal APD30–APD90 was 80±4.7 ms, which is similar to that observed in a larger series [4]. During control perfusion triangulation was reduced by 15.3±8.5 ms; this small reduction was significant at 40 and 50 min of perfusion (Table 1).

3.4.2 Erythromycin
Erythromycin did not cause a significant increase of triangulation (Table 2) and it declined by 19±24 ms during an additional 50 min of perfusion. Thus, erythromycin lengthened the APD60 while causing little or no triangulation.

3.4.3 Almokalant
In contrast, almokalant induced a marked triangulation (Table 3); P<0.05 from 0.1 µM where it increased by 23.5±17.7 ms. Furthermore, this triangulation increased in a concentration dependent fashion: at 0.3 µM it amounted to 65.6±28 ms and at 1 µM it reached 116.5±29 ms.

3.4.4 Erythromycin and almokalant
Upon addition of almokalant to erythromycin, the triangulation of the action potential reached significance at 1 µM only, where it increased by 86.2±67.6 ms (Table 4).

3.4.5 Conclusion
The prolongation of APD by almokalant is thus very different from that of erythromycin: (a) with erythromycin there was no triangulation, little instability and a moderate amount of reverse use dependence; (b) almokalant induced a marked instability, a prominent triangulation and a pronounced reverse use dependence. It will thus be of interest to compare the proarrhythmia of erythromycin, almokalant and their combination.

3.5 Proarrhythmia
3.5.1 Control
During the control perfusion period, there was no significant change in the low incidence of arrhythmias per minute (Table 1).

3.5.2 Erythromycin
Erythromycin did not induce significant changes in proarrhythmia either (Table 2). Even after an additional 50 min of perfusion, there was no significant proarrhythmia (0.5±0.6 beats/min).

3.5.3 Amokalant
Almokalant was markedly proarrhythmic (Table 3) from low concentrations on (0.3 µM, where the increase reached 48.1±30.7 beats/min). At 1 µM the incidence of almokalant induced proarrhythmia further increased to 60.3±37 beats/min (P<0.05). In every single experiment, almokalant induced instability, which was associated with triangulation. In all ten experiments this led to EADs and TdPs.

3.5.4 Erythromycin and almokalant
Relative to baseline the number of ectopics per minute also increased during perfusion of almokalant in the presence of erythromycin (Table 4). However, with the combination of both compounds, the changes were smaller than with almokalant singly and they did not reach statistical significance. In eight of ten experiments, there was marked instability and in seven out of the ten there was serious triangulation. Although these are very high incidences, one should note that it is no longer 100% as in the presence of almokalant alone. More remarkable is that the three preparations that no longer combined instability and triangulation also developed no EADs or TdPs. Thus, erythromycin tended to reduce the proarrhythmic effect of almokalant.

3.6 Proarrhythmia as a function of APD prolongation
The prolongation of APD by almokalant is much larger than that achieved by erythromycin. Therefore, the lower proarrhythmic effect of erythromycin might be due to the smaller prolongation of APD it produces, when compared with almokalant. It is therefore mandatory to compare the effects of almokalant in the presence and absence of erythromycin as a function of their prolongation of APD (Fig. 4). In the top panel, the data are plotted for instability. For almokalant the slope was 0.5 (P = 0.0001), i.e. almokalant exhibits a very significant increase of instability as a function of prolongation of APD (we certainly do not imply a causal relationship). For erythromycin combined with almokalant the slope was 0.6 (P = 0.0004). Thus, clearly erythromycin could not reduce the induction of instability by almokalant.


Figure 4
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  		Fig. 4 Instability, triangulation and induced arrhythmias as a function of prolongation of APD. The least squares error for instability (top), triangulation (middle) and proarrhythmia (bottom) are computed for almokalant in control and in the presence of erythromycin. For details see Results section.

 
In the middle panel, the data for triangulation are investigated. Almokalant had a slope of 0.82 (P<0.0001), i.e. almokalant markedly and significantly induces triangulation as a function of prolongation of APD (again no causal relation is implied). Although this slope (0.59) was slightly smaller in the presence of the combination of erythromycin plus almokalant, it remained highly significant (P = 0.004).

Most importantly, for proarrhythmia almokalant had a slope of 0.46, which was highly significant (P = 0.002). However, when combining erythromycin with almokalant, the slope declined to 0.23. Since this reduction of the slope was significant (P<0.05) it must be concluded that erythromycin significantly reduced the proarrhythmic effect of almokalant.


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
The present results show that erythromycin prolongs the APD by lengthening the plateau of the cardiac action potential without inducing much instability or triangulation and relatively little reverse use dependence. Almokalant also prolongs the APD, but here the plateau is not prolonged. Instead, repolarization is slowed so that the action potential becomes more triangular. In addition this agent also induces much instability and marked reverse use dependence. The combination of erythromycin and almokalant leads to both a prolongation of the plateau and of phase 3; it is also characterized by an increased instability. However, for any degree of prolongation of APD, the proarrhythmia of the combination is smaller than that of almokalant alone. We therefore conclude that addition of erythromycin attenuates the proarrhythmia of almokalant. Thus, prolongation of APD is antiarrhythmic.

4.1 Erythromycin
Our results confirm that erythromycin lengthens the cardiac APD or QT-interval [8,14,15]. In the present experiments, the lengthening of the APD by 100 µM erythromycin was ~60 ms; it was primarily due to a prolongation of the plateau, with little or no slowing of phase 3 repolarization and it did not induce much instability or triangulation and relatively little reverse use dependence. Agents that prolong the APD without instability, triangulation or reverse use dependence are not expected to be proarrhythmic but antiarrhythmic [4]. In the present experiments, we observed no EADs or TdPs during the perfusion with erythromycin at 100 µM. Therapeutic concentrations of erythromycin induce less prolongation of QT (~35 ms) [8] and the incidence of proarrhythmia is extremely low [9,16]. In the rare instances of clinical proarrhythmia, there are nearly always predisposing factors [9,16].

Although the prolongation of the APD by 100 µM erythromycin was generally fine, it was not totally devoid of problems: there was some induction of instability and reverse use dependence. We did not observe EADs at 100 µM, but we are aware that, when higher concentrations are used, EADs and TdPs can occur both in vitro and in vivo [15,16]. At these higher concentrations erythromycin blocks iKr, which contributes importantly to slowing of phase 3 repolarization. Marked slowing of repolarization also started to appear in one of the present experiments. The lengthening of plateau without much slowing of repolarization must result from an effect upon a plateau current, either an augmentation of an inward current or a reduction of an outward current, but based on the present study we can only speculate and cannot differentiate between these possibilities.

4.2 Almokalant
Almokalant concentration dependently prolonged the APD. However with this compound there was no prolongation of the plateau. Instead, all of the prolongation was due to slowing of phase 3 repolarization or triangulation. Also, almokalant induced concentration dependently instability and had marked reverse use dependence. Instability, triangulation and reverse use dependence form a powerful trio for the prediction of proarrhythmia [4]. This was confirmed by our present experiments and others [5,17], as well as by clinical observations [18].

4.3 Erythromycin plus almokalant
The effects of erythromycin and almokalant on APD were superimposed and additive: the plateau was lengthened and the repolarization was slowed. In addition, induction of instability and triangulation, as a function of APD prolongation, were not markedly modified (Fig. 4), but the development of proarrhythmia by almokalant singly was attenuated by the addition of erythromycin to almokalant. This suggests that APD prolongation by itself is not necessarily proarrhythmic; otherwise addition of erythromycin would have increased the proarrhythmia induced by almokalant alone. On the contrary, at each action potential duration, the frequency of arrhythmias was significantly reduced. Thus APD prolongation induced by an agent without instability or triangulation (as erythromycin) protects against the proarrhythmic effect of an agent that induces instability and triangulation (as almokalant).

4.4 Triangulated or squared prolongation of the APD
Triangulation is potentially proarrhythmic for at least four reasons: slower transition through the calcium and sodium window currents may lead to reactivation of these inward currents [19]; prolonged slower reactivation of the sodium current lengthens the window of slowed conduction, which in turn may promote re-entry [20]; temporal dispersion of repolarization may promote re-excitation by current flow [21], especially when the potassium currents are reduced, so that the membrane is less well maintained near the potassium equilibrium potential. For these reasons it is probably better not to interfere with repolarizing currents that flow during phase 3. In other words, a class III antiarrhythmic effect is expected to be more safely accomplished by lengthening the plateau of the action potential: a more positive and longer plateau may indirectly activate more repolarization current and lead to a faster, more robust and better synchronized repolarization.

Finally, if prolongation of APD, associated with triangulation, instability and reverse use dependence is proarrhythmic not only in the isolated rabbit heart, then this trio may also be readily recognizable in the clinic: the QT prolongation would be associated with T-wave broadening, instability (recognition of the latter may require high resolution sampling techniques) and the prolongation of the QT interval would be most marked at slow heart rates. Whether the presence of this proarrhythmic trio might also forewarn impending problems in patients will require additional investigations.

Time for primary review 27 days.


   Acknowledgements
 
The authors wish to thank Betty Beck and Bruno Hespel for their invaluable assistance in the laboratories and Dr G. Duker from AstraZeneca for providing the almokalant.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 

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