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Cardiovascular Research 1998 37(3):646-655; doi:10.1016/S0008-6363(97)00304-0
© 1998 by European Society of Cardiology
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Copyright © 1998, European Society of Cardiology

Efficacy of a β-adrenergic receptor antagonist, propranolol, in preventing ischaemic ventricular fibrillation: dependence on heart rate and ischaemia duration

Jean F Aupetita, Dominique Frassatib, Bernard Bui-Xuanb, Marc Freyszb, Georges Fauconb and Quadiri Timourb,*

aDepartment of Cardiology, St. Joseph—St. Luc Hospital, 9, Professór Grignard Street, 69365 Lyon Cedex 08, France
bDepartment of Medical Pharmacology, Claude Bernard University, 8, Rockefeller Avenue, 69373 Lyon Cedex 08, France

* Corresponding author.

Received 21 April 1997; accepted 15 October 1997


   Abstract
Top
 Abstract
1 Introduction
 2 Material and methods
 3 Results
 4 Discussion
 References
 
Objectives: To investigate the prevention of ventricular fibrillation with a β-adrenergic receptor (β-AR) antagonist in anaesthetized, open-chest pigs in a model of ischaemia, intended to reproduce what happens either in anginal attack or in the first hour of infarction. Methods: Ventricular fibrillation threshold (VFT) was determined with trains of diastolic stimuli of 100 ms duration delivered by a subepicardial electrode inserted in the area subjected to ischaemia. Ischaemia was obtained by the complete occlusion of the left anterior descending coronary artery, either near its origin during brief but increasing periods (30, 60, 90, 120, 150, 180, 240, 300 s), or half-way from its origin for a much longer time (more than 60 min). Results: During transient proximal occlusion and isoprenaline infusion (0.25 µg/kg/min), propranolol (50 µg/kg plus 2 µg/kg/min) attenuated both tachycardia and the fall in VFT to 0 mA. The shortening of MAP duration accompanying depolarization of the fibres was concurrently slowed down, and time to fibrillation prolonged (122±15 to 262±14 s, p<0.001). In the absence of isoprenaline infusion, propranolol exerted similar effects, but to a lesser degree, in proportion to heart rate dependent on sympathetic activity. In contrast, it became unable to raise VFT before and during ischaemia, when heart rate was kept constant by pacing. After persistent midportion occlusion, significant differences in VFT were found only at the 5th min, depending on whether heart rate was accelerated by isoprenaline (0.8±0.2 mA), left normal (1.8±0.3 mA) or slowed down by propranolol (1.6±0.3 mA). Later on, especially after 15 and 25 min of ischaemia, VFT, which was below 1.0 mA, did not appear to be influenced by the activation or blockade of β-ARs: spontaneous fibrillations were observed in the same number in this period with or without the administration of propranolol. Beyond 30 min after occlusion, the rise in VFT, subsequent to the first irreversible cell damage, also occurred in the same way. Conclusions: The prevention of ischaemic ventricular fibrillation by a β-AR antagonist, judged from VFT, is easily checked experimentally when ischaemia is only transitory, especially if sympathetic activity is high. The maintenance of VFT at a relatively high level is essentially related to the depressant effect on the sinus rate. The same animal model does not give support to an effective protection in the first hour of infarction. However, the control of heart rate may also be beneficial in these circumstances by attenuating systemic haemodynamic disorders.

KEYWORDS Pig; β-adrenergic receptor antagonists; Propranolol; β-adrenergic receptors; Ventricular fibrillation; Myocardial ischaemia


   1 Introduction
Top
 Abstract
 1 Introduction
2 Material and methods
 3 Results
 4 Discussion
 References
 
The incidence of sudden cardiac death related to myocardial ischaemia has long been shown to be greatly diminished, 40 to 50% [1–6], by β-adrenergic receptor (β-AR) antagonists. This is a ‘class effect’ since there seems to be little difference in the results obtained with the various drugs of the same class, propranolol, alprenolol, timolol, acebutolol, metoprolol, etc. [7]. These clinical data could be confirmed by the blockade of cardiac β-ARs in animals [7–10], but not in all experimental models of ischaemia. Although they always tend to adjust cardiac metabolism to reduced coronary blood flow, β-AR antagonists failed to decrease the rhythm disorders preceding fibrillation, attenuate the fall in ventricular fibrillation threshold due to ischaemia and delay fibrillation produced by a given coronary occlusion in certain models [11–13].

The objective of this study was to elucidate the cause of these discrepancies. This cause might lie in the method used for the assessment of vulnerability to fibrillation, and the type of ischaemia. Vulnerability was essentially judged from electrical threshold for ventricular fibrillation, measured with wide impulses delivered in diastolic period, according to a method whose validity has previously been tested on antiarrhythmic drugs [14, 15], calcium channel inhibitors [16, 17], and even a β-AR agonist, isoprenaline [18]. Two types of ischaemia were explored. The first was ischaemia secondary to the transient occlusion of the left anterior descending coronary artery near its origin. This transient ischaemia was explored because it results in spontaneous fibrillation within 2 or 3 min in a manner resembling an anginal attack where the imbalance between oxygen demand and oxygen supply is generally brief. The second type of ischaemia, induced by occluding the left anterior descending coronary artery at its midpoint, was maintained for more than one h without reperfusion in order to correspond approximately with the first hour of infarction. In both cases, the determinations of ventricular fibrillation threshold were performed under various conditions of heart rate, since the action of isoprenaline had previously appeared very different, even opposite in its direction, depending on these conditions [18].


   2 Material and methods
Top
 Abstract
 1 Introduction
 2 Material and methods
3 Results
 4 Discussion
 References
 
2.1 Animal preparation
The experiments were conducted in 50 young domestic pigs of either sex weighing 22–32 kg. After the protocol was approved by the Animal Care Committee, the animals were premedicated with 1.25 mg/kg droperidol, intramuscularly injected one h before starting the experiment. They were anaesthetized with 0.05 mg/kg flunitrazepam and 80 mg/kg chloralose injected via the marginal ear vein. They were ventilated through a tracheotomy tube by means of a PR2 respirator (Bennett, Santa Monica, CA, USA). An air-oxygen mixture was thus delivered, initially in the respective proportions of 40 and 60%. Such proportions were adjusted later depending on the content in O2 and CO2 of the arterial blood, sampled at regular intervals to keep the former between 16–18 vol/100 and the latter between 54–58. Core temperature, measured by an electronic oesophageal thermometer, was similarly corrected to be maintained between 38–39°C using an infrared heater placed at a variable distance from the animals.

The pigs were laid on their right side and a wide left thoracotomy performed with resection of the 4th and 5th ribs. The heart was exposed and the pericardium was opened.

An arterial pressure line was established through a catheter inserted in the left carotid artery and connected to a Narcotrace 80 polygraph (Narco-Biosystem, Houston, TX, USA). Concurrently with mean arterial blood pressure, the force of cardiac contractions was monitored continuously. Left ventricular dP/dt max (LVdP/dt max) was electronically derived by a physio-differentiator (Hugo Sachs, Freiburg im Brisgau, Germany) from intraventricular pressure signal. This signal was obtained through a catheter passed down to the left ventricle from the right carotid artery and connected to the polygraph.

2.2 Determination of ventricular fibrillation threshold (VFT)
VFT was determined at the end of increasing duration ischaemia periods in group A (Fig. 1), as well as in the course of a lasting ischaemia in group B (see below). It was determined according to a technique previously described [14–18], using trains of 8 wide diastolic stimuli synchronized with respect to the R waves of the electrocardiogram in lead II. The duration of these stimuli (100 ms instead of 2) was far greater than that sufficient to pacing. Their intensity was gradually increased by steps of 1.0 or 0.5 mA until VFT was attained (where VFT is the lowest intensity required to trigger fibrillation). Their frequency, peculiar to each group or subgroup of animals, is indicated below. It was, however, in all cases higher than that of the impulses originating from the sinus node to pace the heart. The stimuli were delivered by an S1 stimulator (Hugo Sachs, Freiburg im Brisgau, Germany) and transmitted with a bipolar electrode, the tip of which had been introduced into the subepicardial layer of the left ventricle wall, near the centre of the area which could be subjected to ischaemia.


Figure 1
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  		Fig. 1 Timing of the coronary occlusions in the subgroup A1. The occlusions of increasing duration are separated from each other by intervals increasing in proportion to the preceding ischaemia duration. Similar protocol in the subgroups A2 and A3, propranolol being administered straight off and not after the assessment of the effects of isoprenaline alone. Occl, occlusions. VFT, ventricular fibrillation threshold. MAPD, monophasic action potential duration. CT, conduction time.

 
As soon as fibrillation was triggered, defibrillation was ensured by a 250–360 J current shock applied to the thoracic wall. The shock was provided by a D 802 defibrillator (Siemens, Erlangen, Germany). Sinus rate was generally restored by a single shock after the shortest ischaemia periods, whereas several shocks were often necessary after more prolonged ischaemia periods (group A) or a lasting ischaemia (group B). Defibrillations were carried out after withdrawal of the coronary occlusion in group A, and without withdrawal of this occlusion in group B. In group A, the shocks were shown to have no influence on VFT and electrophysiological parameters, when defibrillations were separated by sufficient intervals to allow the disappearance of the disorders secondary to ischaemia (Fig. 1). In group B, the persistent interruption of blood flow impaired electrophysiological properties of the myocardial fibres to such an extent that the impairment due to repeated shocks, if any, was negligible.

2.3 Electrophysiological recordings
Electrical activity of the ventricular fibres was registered locally through a Catronic ORX electrode 6F (Plastimed, Saint-Leu-La-Forêt, France), advanced within the subepicardial layer of the left ventricle wall like the electrode used for determination of VFT, not far from the centre of the ischaemic zone. The recording electrode was connected to a special lead of a Mingograf 34 electrocardiograph (Siemens-Elema, Solna, Sweden). Monophasic action potential (MAP) was registered as often as necessary and, in the intervals, observed on an E-2089 oscilloscope (Siemens, Erlangen, Germany). Given the correlation between membrane polarization of the cardiac fibres and MAP duration [19], this duration, measured at 90% repolarization, provided experimental evidence for variations of polarization linked to the variations of heart rate, when variable, and to ischaemia, when heart rate was kept constant by pacing.

The pacing associated with the MAP recording made it possible to measure intraventricular conduction time – time elapsing between the spike of stimulation and the steep upstroke of MAP.

Finally, an electrocardiogram was recorded in standard limb leads on the Mingograf 34 electrocardiograph for control of sinus rate variations and rhythm disorder identification.

2.4 Ischaemias
2.4.1 Transient proximal occlusion of the left anterior descending coronary artery
In the animals of a first group (group A), the artery was dissected free near its origin and a snare looped around it in preparation for occlusion. The snare, when not tightened, did not interfere with blood circulation in the artery. In contrast, the artery could be easily compressed against the tip of a glass tube by tightening the snare whose two extremities had previously been passed into the glass tube. The artery could thus be completely occluded without damage at the appointed time and maintained occluded over brief periods.

However, the duration of the occlusion periods was gradually increased (30, 60, 90, 120, 150 s, etc.) to follow the decline of VFT in the course of ischaemia from control level to the occurrence of spontaneous fibrillations [14–18]. To allow the electrical and biochemical impairments resulting from ischaemia to disappear before the next ischaemia, ischaemia periods were separated from each other by intervals which were proportional to the preceding ischaemia duration, from 2.5 min for the shortest to 20 min for the longest (Fig. 1). The tightness of the occlusion was visually controlled by cyanosis which involved a large area of the left ventricle from the artery to the central line of the anterior ventricular wall.

In all the animals of this group, the ventricles were paced through the electrode used for determination of VFT, for a few seconds immediately before this determination, carried out at the end of each ischaemia period (in addition to a control period of ischaemia absence). This pacing, performed with 2 ms duration and 0.5 mA intensity pulses, allowed MAP duration, intraventricular conduction time and haemodynamic parameters to be measured at the same rate as VFT. Electrophysiological and haemodynamic parameters as well as VFT were therefore determined at a rate slightly higher than sinus rate to pace the heart, the sinus rate being accelerated by isoprenaline in subgroup Al, as indicated below, or not in subgroup A2. The determination was performed at the rate of the pacing to which the ventricles were already subjected to keep constant the rate of their beats in subgroup A3.

2.4.2 Persistent midportion occlusion of the left anterior descending coronary artery
In a second group of animals (group B), the artery was dissected in its midportion. As it was easier to reach this portion of the artery and the occlusion was unique in the experiment, the artery was occluded more simply than in the animals of group A by means of a vascular clamp. The occlusion was maintained for 60 min and more. Test and control values were then compared in two similar series of animals, since the ischaemia duration was too long for the recovery of normal state and, thereby, for the use of the treated animals as their own control. In both series, VFT was measured 5 min before and 5, 15, 25, 40 and 60 min after starting occlusion. Haemodynamic parameters were recorded under pacing at the same rate as VFT measurement, as in group A, within the last seconds before VFT determination. MAP and conduction time were not taken into account because of a lack of steadiness which made interpretation difficult. Moreover, the ventricles were defibrillated without discontinuing the coronary occlusion.

In the interval of the determinations, ventricular beats remained at the sinus rate since the desired conditions were close to the clinical settings of infarction. In all cases, VFT was measured at a rate hardly higher than the sinus rate, just to obtain the ventricular response, consequently at a rate varying with the effects concurrently exerted on this rate. There was no effect in the control series, a considerable acceleration by isoprenaline in subgroup B1a of the test series, a slight acceleration in subgroup B1b receiving isoprenaline and propranolol simultaneously, and a slowing down in subgroup B2 given propranolol alone.

2.5 Experimental protocol
2.5.1 Group A (n=18). Effects of a β-AR antagonist, propranolol, on the fall in VFT induced by brief proximal occlusion of the left anterior descending coronary artery
For all the animals in the group, control ischaemia periods of increasing duration (30, 60, 90, 120, 150 s) were observed to monitor the gradual decline of VFT, from the normal level to the onset of spontaneous fibrillation [14–18]. Propranolol (hydrochloride) was administered at a 50 µg/kg loading dose and a 2 µg/kg/min infusion to act on cardiac β-ARs, without directly acting on sodium channel. The effects of this administration were studied under three conditions.

1. Previous acceleration of the rate of ventricular beats governed by the sinus node under the influence of isoprenaline (hydrochloride), infused in a 0.25 µg/kg/min dose, for the consequences of β-AR blockade might be expected to be more obvious in the case of a full activation (subgroup A1, n=6).
2. Normal rate, relatively high in the pig, particularly after thorax opening (subgroup A2, n=6).
3. Maintenance by ventricular pacing of a constant rate at a level just above control sinus rate (subgroup A3, n=6).

2.5.2 Group B (n=32). Effects of a β-AR antagonist, propranolol, on the fall in VFT in the first hour of a lasting midportion occlusion of the left anterior descending coronary artery
In a control series (n=8), the decline observed in VFT following the lower coronary occlusion was, as expected, slower than that observed in group A since the area deprived of blood flow, revealed by cyanosis, was smaller. Therefore, VFT was measured at 5, 15 and 25 min. The effects of propranolol were then studied under two conditions.

1. Enhancement of cardiac sympathetic activity, reproduced by infusion of isoprenaline. The values obtained during the administration of isoprenaline alone in subgroup B1a (n=8) and of isoprenaline associated with propranolol in subgroup B1b (n=8) were compared to those of the control series.
2. Normal or slightly enhanced cardiac sympathetic activity. The values obtained during the administration of propranolol alone (subgroup B2, n=8) were compared to those of the control series.

Propranolol and isoprenaline were administered, as in group A, in an infusion of 0.25 µg/kg/min for isoprenaline, in a loading dose of 50 µg/kg and an infusion of 2 µg/kg/min for propranolol. In both cases, clinically relevant doses were chosen to provide a maximal effect.

2.6 Statistics
A multiple measures analysis of variance (ANOVA) was first carried out. When a significant difference was apparent, test values were compared to reference values according to Dunnett's test. Reference values in group A were those obtained in the same animal before drug administration for the same time point (no occlusion, occlusion of 30, 60, 90 s, etc.), in group B those obtained in the control series of animals for the same time point (before occlusion, occlusion of 5, 15, 25 min, etc.).

Results are expressed as arithmetic means±SEM and differences are considered as significant when p<0.05.


   3 Results
Top
 Abstract
 1 Introduction
 2 Material and methods
 3 Results
4 Discussion
 References
 
3.1 β-AR antagonists and fall in VFT in transient myocardial ischaemia (group A)
3.1.1 Under submaximal activation of cardiac β-ARs, with large variations in heart rate (subgroup A1)
In the absence of ischaemia, isoprenaline infusion was accompanied by a decrease in VFT from 12.2±0.7 to 6.8±0.8 mA (p<0.001) (Fig. 2). In fact, this decrease depended on the acceleration of ventricular beats (102±6 to 174±8 beats/min, p<0.001), since a similar acceleration by pacing lowers VFT to the same degree [18, 20]. Bradycardia due to propranolol, from 174±8 to 112±6 beats/min (p<0.001), was followed by the return of VFT to 11.6±0.8 mA (NS versus control). Conceivably, isoprenaline hastened the gradual decline of VFT induced by ischaemia periods of increasing duration (Fig. 2), and propranolol brought it back to normal. Therefore, the time to onset of spontaneous fibrillation, subsequent to the fall in VFT to 0 mA [14–17], was shortened by isoprenaline and restored by propranolol. Spontaneous fibrillation needed 271±12 s of ischaemia to occur under control conditions, 122±15 s (p<0.001) only after administration of isoprenaline alone and 262±14 s (NS versus control) after administration of both isoprenaline and propranolol.


Figure 2
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  		Fig. 2 Effects, at a rate varying with the action exerted on the sinus node, of isoprenaline alone (0.25 µg/kg/min) and isoprenaline (0.25 µg/kg/min) associated with propranolol (50 µg/kg plus 2 µg/kg/min) on ventricular fibrillation threshold (VFT) lowered by no-flow ischaemia periods of increasing duration, obtained by proximal occlusion of the left anterior descending coronary artery. No columnn for isoprenaline alone from 150 s and no column for control and isoprenaline plus propranolol at 300 s because VFT had already fallen to 0 mA. Subgroup A1, n=6. Means±SEM.

 
Moreover, isoprenaline reduced MAP duration before coronary occlusions (310±8 to 192±12 ms, p<0.001) and shortened the time necessary for its ischaemia-induced fall to critical values (165–180 ms) resulting in fibrillation. This effect, like the effect on VFT, was antagonized by propranolol which prolonged MAP duration to 296±7 ms (p<0.001) prior to ischaemia. Again, the variations of heart rate played a primary role in the alterations of MAP by the β-AR agonist and antagonist drugs. The lengthening of intraventricular conduction time by ischaemia (30±4 to 54±5 ms, p<0.01) was less influenced than the shortening of MAP duration by isoprenaline and propranolol.

As for haemodynamic parameters, the decrease in LVdP/dt max secondary to ischaemia was notably enhanced by isoprenaline (Table 1), compromising ventricular filling via tachycardia and lowering mean arterial blood pressure, from 88±4 to 76±3 mmHg (p<0.05), because of its vasodilator properties. Propranolol then not only limited hypotension, but also the decrease in LVdP/dt max (Table 1), by restoring a better filling of the ventricles together with sinus rate. Coronary blood flow was not measured in these experiments, since the complete occlusion of the left anterior descending coronary artery implied a near 100% reduction, at least in the centre of the ischaemic area. Moreover, earlier measurements by radionuclide-labelled microspheres [21–23]had confirmed this degree of the ischaemia and shown that the drug-induced variations could not exceed 10% of the residual flow under these circumstances.


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  		Table 1 Effects, at a rate varying with the action exerted on the sinus node, of isoprenaline alone (0.25 µg/kg/min) and isoprenaline (0.25 µg/kg/min) associated with propranolol (50 µg/kg plus 2 µg/kg/min) on left ventricular dP/dtmax, at the end of ischaemia periods of increasing duration, obtained by proximal occlusion of the left anterior descending coronary artery

 
3.1.2 Under experimental conditions of normal heart rate (subgroup A2)
The slowing down of sinus rate produced by blockade of β-ARs was, as expected, less marked, from 110±8 to 86±6 beats/min (p<0.05), and, in the absence of ischaemia, VFT underwent only a slight rise from 11.6±0.5 to 13.3±0.6 mA (p<0.05) (Fig. 3). However, the fall in VFT appeared to be significantly restricted by propranolol in several of the successive ischaemia periods, so that time to fibrillation was appreciably prolonged (338±16 s instead of 268±10, p<0.05). MAP duration, lengthened by bradycardia, from 276±10 to 316±14 ms (p<0.05), also required more time to fall to the low values (165–180 ms) observed at the time of spontaneous fibrillation.


Figure 3
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  		Fig. 3 Effects, under experimental conditions of normal heart rate, of propranolol (50 µg/kg plus 2 µg/kg/min) on ventricular fibrillation threshold (VFT) lowered by no-flow ischaemia periods of increasing duration, obtained by proximal occlusion of the left anterior descending coronary artery. No column for control at 300 s because VFT had already fallen to 0 mA. Subgroup A2, n=6. Means±SEM.

 
The beneficial effect of propranolol on haemodynamics was again significant. After a brief transitory decrease, LVdP/dt max rose to some degree with the development of bradycardia, and mainly diminished to a lesser extent with ischaemia (1860±58 to 1752±44 mmHg/s, NS, instead of 1626±54, p<0.05). Hypotension recorded during ischaemia was also less pronounced. The drop was 80±4 to 74±2 mmHg (NS), instead of 80±4 to 64±4 (p<0.05).

3.1.3 Under pacing at a constant rate (subgroup A3)
When ventricular pacing was performed at the same rate under control and test conditions, chosen in each animal as close as possible to its sinus rate of the start of experiment (120 beats/min for instance for sinus rate of 106), VFT was no longer increased by propranolol. It was even slightly decreased (11.0±0.5 to 9.4±0.4, p<0.05) in the absence of ischaemia and its fall due to ischaemia was not significantly attenuated by propranolol (Fig. 4). Therefore, time to fibrillation did not differ in the absence or presence of propranolol. Although MAP duration was significantly lengthened by propranolol in the absence of ischaemia (264±14 to 286±12 ms, p<0.05) and during the shortest ischaemia periods (218±8 to 240±10 ms, p<0.05 at 60 s), it ended by diminishing almost identically, whether propranolol was administered or not (Fig. 5).


Figure 4
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  		Fig. 4 Effects, under pacing at a constant rate, not far from sinus rate, of propranolol (50 µg/kg plus 2 µg/kg/min) on ventricular fibrillation threshold (VFT) lowered by no-flow ischaemia periods of increasing duration obtained by proximal occlusion of the left anterior descending coronary artery. No colomn for propranolol at 180 s because VFT had already fallen to 0 mA. Subgroup A3, n=6. Means±SEM.

 

Figure 5
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  		Fig. 5 Effects, under pacing at a constant rate, not far from sinus rate, of propranolol (50 µg/kg plus 2 µg/kg/min) on monophasic action potential (MAP) duration reduced by no-flow ischaemia periods of increasing duration, obtained by proximal occlusion of the left anterior descending coronary artery. Subgroup A3, n=6. Means±SEM.

 
LVdP/dt max, already adversely affected by pacing (1770±62 to 1710±56 mmHg/s, NS), was further reduced by propranolol (1620±52 mmHg/s, p<0.05). Moreover, propranolol tended to worsen the fall in LVdP/dt max (1534±52 mmHg/s, p<0.05, instead of 1666±58, NS) as well as the drop in mean blood pressure (78±4 to 64±3 mmHg, p<0.05, instead of 71±3, NS) attributable to ischaemia.

3.2 β-AR antagonists and fall in VFT in persistent myocardial ischaemia (group B)
3.2.1 Under submaximal activation of cardiac β-ARs, with large variations in heart rate (subgroup B1)
The decline in VFT secondary to coronary occlusion was much slower in the control series of group B than in the control ischaemia periods of group A. This is consistent with the level of the occlusion and the size of the ischaemic area. VFT, which was 12.8±0.4 mA in the min preceding occlusion, given the relatively low heart rate (108±6 beats/min), was markedly diminished 5 min after the occlusion (1.8±0.3 mA, p<0.001). However, no fibrillation was then observed. On the contrary, after 15 and 25 min of occlusion, VFT was further lower, below 1.0 mA (Fig. 6), therefore bordering on the values previously seen to immediately precede fibrillation. Effectively, most spontaneous fibrillations occurred in this period, whereas such fibrillations were no longer observed beyond the 30th min following the start of occlusion. From then on, VFT tended to return to far higher values which were attained at the 40th (16.2±0.9 mA) and 60th min (22.2±1.4 mA) (Fig. 6). In fact, this return certainly reflected only hypoexcitability secondary to the first cell injury.


Figure 6
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  		Fig. 6 Effects, at a rate varying with the action exerted on the sinus node, of isoprenaline alone (0.25 µg/kg/min) (subgroup B1a, n=8) and isoprenaline (0.25 µg/kg/min) associated with propranolol (50 µg/kg plus 2 µg/kg/min) (subgroup B1b, n=8) on ventricular fibrillation threshold (VFT) in the first hour of a lasting midportion occlusion of the left anterior descending coronary artery. Means±SEM.

 
The effects of isoprenaline and propranolol before coronary occlusion were similar to those reported for animals in subgroup A1. VFT fell to 6.2±0.8 mA (p<0.001) because of tachycardia (184±10 beats/min) with isoprenaline alone in subgroup B1a, whereas it remained near control (11.4±0.5 mA, NS) together with heart rate 122±8 beats/min, NS) when propranolol was added to isoprenaline in subgroup B1b (Fig. 6). After 5 min of occlusion, there was still a significant difference between the values obtained in the two subgroups, 0.8±0.2 in subgroup B1a (p<0.05 versus control) and 1.6±0.3 mA (NS versus control) in subgroup B1b. In contrast, at 15 and 25 min of occlusion, VFT was between 1.0 and 0 mA in the two subgroups, as in the control series. Such values implied the likelihood of spontaneous fibrillations and, indeed, at least one fibrillation occurred spontaneously in all the animals, sometimes two, possibly three, with no obvious relationship between this number and administration of the drug. Time to occurrence of fibrillations was also the same with isoprenaline alone and isoprenaline associated with propranolol. In both cases as well as in the control series, the majority of fibrillations occurred from the 15th to 25th min of the occlusion. Similarly, spontaneous fibrillations became infrequent beyond 25 min in the three series, control, B1a and B1b, when the measurement of VFT through the electrode at the centre of the ischaemic zone showed it to be high (even higher than it was before occlusion), above 15 mA at 40 min of occlusion and 19 mA at 60. The time required for the appearance of hypoexcitability and its extent were apparently independent of the drug administered.

3.2.2 Under experimental conditions of normal heart rate (subgroup B2)
Before coronary occlusion, VFT was raised only from 12.8±0.6 to 14.6±0.7 mA (p<0.05) and heart rate lowered only from 98±6 to 78±5 beats/min (p<0.05) by propranolol administered in the absence of activation of cardiac β-ARs or in the presence of a moderate endogenous activation. However, the influence of bradycardia or of the prevention of some degree of tachycardia secondary to coronary occlusion (84±5 versus 111±7 beats/min, p<0.01) was still appreciable on VFT at 5 min (3.3±0.4 versus 1.8±0.3 mA, p<0.01). In contrast, the difference ceased to be significant at 15 and 25 min. During this period, there was no difference in either the number or the time to occurrence of spontaneous fibrillations between the animals of control series and of subgroup B2. The opposite alteration of VFT related to the first cell damage was also similar.


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Material and methods
 3 Results
 4 Discussion
References
 
4.1 β-AR antagonists and vulnerability to fibrillation under non-ischaemic conditions
4.1.1 Overall effects on the heart
Since VFT measured as indicated above, i.e. with wide stimuli delivered in diastolic period, was inversely correlated to vulnerability to fibrillation according to earlier investigations [14–18], tachycardia induced by isoprenaline predisposed to fibrillation, as VFT fell by several mA when the rate of impulses used for its determination was increased to the same extent as sinus rate, greatly accelerated by isoprenaline. Conversely, it returned to values near its previous ones when the effects of isoprenaline on sinus rate were antagonized by propranolol which enabled lower pacing rate. Increasing heart rate is known to reduce per se the electrical current intensity necessary for fibrillation [13, 18, 20]so that the risk of fibrillation existing in any ventricular tachycardia is enhanced by the high rate of discharge of the ventricular pacemakers. The mechanism does not seem to lie in the defect of adjustment of coronary circulation to metabolic stimulation when there is no obstacle on the coronary vessels, but rather in the changes introduced by tachycardia in ion transmembrane exchanges. Increase in the number of systoles per unit time enhances the passive ion fluxes [24], while the shortening of diastolic periods counteracts active ion transfers [25]. The result is calcium overload in the myocardial fibres [26]and potassium accumulation in the extracellular space [27]. Therefore, tachycardia is a depolarizing factor, as is ischaemia [12, 28], but to a lesser degree [18]. It enhances myocardial excitability, since this property varies in inverse ratio to the difference between resting membrane potential and threshold potential for depolarization [29]. Conversely, bradycardia produced by propranolol diminishes excitability, raises VFT, and tends to prevent fibrillation.

In fact, the alteration of VFT, like that of heart rate, due to propranolol is large only when sympathetic activity is enhanced by muscular work, hypotension, pain, etc. Apart from this case, sinus rate is moderately slowed and VFT modestly influenced.

4.1.2 Effects at a constant ventricular rate
If heart rate is maintained unchanged by ventricular pacing at the same rate under control and test conditions, not only VFT is not raised by propranolol, but it is even somewhat lowered. This is consistent with the clear rise in VFT due to isoprenaline under the same conditions [18, 30, 31]. These variations may first depend on haemodynamic effects. Isoprenaline increases and propranolol decreases cardiac output and coronary blood flow as a result of their inotropic properties. Nevertheless, the essential determinant of the drop in VFT attributable to the direct influence of propranolol on the ventricles is certainly the depression of ion pump mechanisms secondary to the restriction in cAMP production [32]. In these circumstances, propranolol tends to reduce active ion transfers and, thereby, resting membrane potential. Given the relationship of excitability with the difference between resting membrane potential and threshold potential for depolarization [29], as mentioned above, propranolol then enhances myocardial excitability.

4.2 β-AR antagonists and vulnerability to fibrillation under ischaemic conditions
4.2.1 In brief ischaemic episodes
The outcome of propranolol administration is further complicated by deprivation of oxygen and nutritive supplies, mainly in no-flow ischaemia, like that of these experiments. By virtue of competitive antagonism, propranolol opposes the fall in VFT related to tachycardia produced by isoprenaline [18], or to a relatively high rate maintained by some degree of cardiac sympathetic tone. It should be even more active in the presence than in the absence of ischaemia, since the additional energy expenditure implied by tachycardia results in a rapid exhaustion of cardiac energy stores in the latter case. Propranolol then delays this exhaustion, and the use of β-AR antagonists to protect the myocardium against ischaemia is essentially founded on this fact. Nevertheless, β-AR antagonists only prevent the worsening of VFT fall due to the acceleration of cardiac contractions. They do not attenuate the fall in VFT dependent on ischaemia itself contrary to calcium channel inhibitors [16, 17], since they bring back VFT, lowered by isoprenaline, towards control values but never higher values. Moreover, at a rate experimentally kept constant, propranolol left VFT similar to control. Anyhow, the influence of heart rate on polarization is unable to balance the depolarizing influence of ischaemia, as bradycardia can slow down the fall in VFT and, thereby, delay the occurrence of spontaneous fibrillation, but cannot prevent the VFT fall down to 0 mA with fibrillation. However, β-AR antagonists may prevent sudden death in a brief ischaemic episode such as anginal attack [8, 10, 19]. The inadequacy between myocardial oxygen need, increased by tachycardia, and oxygen supply is often brief enough to disappear before VFT has fallen to 0 mA. If time to occurrence of fibrillation is appreciably prolonged, the risk of sudden death is minimized.

4.2.2 In the first hour of coronary obstruction
The occlusion of the left anterior descending coronary artery halfway from its origin has been reported to cause a fall in VFT slower than the proximal occlusion, but leading to one spontaneous fibrillation at least, in all the animals. This fibrillation may remain unique or be followed by one or even two similar fibrillations. All the spontaneous fibrillations were recorded between 14 and 26 min after occlusion, while VFT was below 1.0 mA. Although ventricular beats were markedly slowed down by propranolol under isoprenaline infusion and moderately slowed down even with normal heart rate, the total number of these fibrillations as well as VFT values obtained at 15 and 25 min of occlusion were not statistically different in the control series and treated group. Some benefit on VFT appeared to be derived from propranolol at 5 min of the occlusion only.

Consequently, contrary to the observations made in the model of transient ischaemia, the model of persistent ischaemia did not demonstrate that the control of heart rate by propranolol was of any real interest in ischaemia such as that of infarction. The failure of bradycardia in preventing fibrillation in these circumstances is, in fact, conceivable. Bradycardia is less potent to maintain high VFT than ischaemia to lower it, since VFT remains far from 0 mA (about 3.0 mA) with the fastest heart rates, whereas it falls to 0 mA with ischaemia [18]. Therefore, a slope down persists in the time course of VFT, even though bradycardia is pronounced, until VFT reaches 0 mA if time is sufficient. β-AR antagonists may succeed in preventing sudden death of anginal attacks because the time needed by the fall in VFT to 0 mA, prolonged by bradycardia, becomes longer than the time necessary for the attenuation of ischaemia. On the contrary, in the case of a steady ischaemia for several hours, fibrillation will occur, with some delay only. In fact, secondarily, bradycardia also loses its influence on VFT in proportion as the increasing oxygen debt reduces the activity of the ion pump mechanisms because the enhancement of active ion transfers by the prolongation of diastolic periods, underlying the rise in VFT by bradycardia [25], is only possible on condition that ion pumps work. Accordingly, these experiments do not give support to an effective protection against ventricular fibrillation by β-AR antagonists in the first hour of coronary obstruction. This is in accordance with and accounts for the negative results which were previously obtained in animal models of ischaemia based on a permanent ligation of a large coronary artery [11–13].

In contrast, the conclusion drawn from such experimental studies is at variance with the clinical observations of reduction of mortality by β-AR antagonists in patients with acute myocardial infarction [1–6]. More accurately, a linear relationship has even been established between this reduction of mortality and the slowing down of heart rate by propranolol [6, 33], as the more the heart rate was lowered the greater the reduction of mortality. The beneficial effect of bradycardia is then probably indirect. As it improves the ventricular filling, bradycardia ensures a better perfusion of the organs, particularly of the heart itself. The tendency to circulatory collapse and shock linked to too high a heart rate may then be counteracted and a possible positive feed-back process avoided, such a process being accompanied by a fall in VFT [9, 34].

In conclusion, β-AR antagonists appeared to be effective in preventing ventricular fibrillation in the experimental model of anginal attack by depressing the sinus node activity. Thus, they decrease cardiac nutritive needs and, mainly, counteract depolarization of the ventricular fibres. On the contrary, their efficacy might not be evidenced in the model of the first hour of infarction. Nevertheless, some protective action against fibrillation is not then excluded, as the control of too rapid a heart rate limits hypotension and the resulting defect of perfusion likely to worsen the fall in VFT.

Time for primary review 31 days.


   Acknowledgements
 
This study was supported by a grant from the Ministère de l'Education Nationale, de l'Enseignement et de la Recherche, EA 1896. We are grateful to Mrs. Sue Million for her linguistic assistance.


   References
Top
 Abstract
 1 Introduction
 2 Material and methods
 3 Results
 4 Discussion
 References
 

  1. Wilhelmsen C., Wilhelmsen L., Vedin J.A. Reduction of sudden death after myocardial infarction by treatment with alprenolol. Lancet (1974) 2:1157–1161.[CrossRef][Web of Science][Medline]
  2. Beta-blocker Heart Attack Trial (BHAT) Research Group. A randomized trial of propranolol in patients with acute myocardial infarction. I. Mortality results. J Am Med Assoc (1982) 247:1707–1714.[Abstract/Free Full Text]
  3. Capone R., Friedman L., Byington R. The effect of propranolol on mortality in patients following acute myocardial infarction with complex ventricular arrhythmias (for the BHAT Study Group). Circulation (1983) 68(Suppl. III):294–302.[Abstract/Free Full Text]
  4. ISIS-1 Collaborative Group. Randomized trial of intravenous atenolol among 16027 cases of suspected myocardial infarction. Lancet (1986) 2:57–65.[Medline]
  5. Puddu P.E., Jouve R., Langlet F. Prevention of postischaemic ventricular fibrillation by long-term beta adrenoceptor blockade with acebutolol in the anaesthetised dog. Cardiovasc Res (1986) 20:721–726.[Abstract/Free Full Text]
  6. Rehnqvist N. Prevention of sudden death by beta-blockade. Cardiovasc Drugs Ther (1990) 4:675–680.[CrossRef][Web of Science][Medline]
  7. Anderson J.L., Rodier H.E., Green L.S. Comparative effects of beta-adrenergic blocking drugs on experimental ventricular fibrillation threshold. Am J Cardiol (1983) 51:1196–1202.[CrossRef][Web of Science][Medline]
  8. Bergey J.L., Wendt R.L., Nocella K., McCallum J.D. Acute coronary artery occlusion-reperfusion arrhythmias in pigs: antiarrhythmic and antifibrillatory evaluation of verapamil, nifedipine, prenylamine and propranolol. Eur J Pharmacol (1984) 97:95–103.[CrossRef][Web of Science][Medline]
  9. Nanas J.N., Kralios A. Relative effects of propranolol and timolol on ventricular fibrillation threshold and hemodynamics in the dog. Am Heart J (1986) 111:1094–1100.[CrossRef][Web of Science][Medline]
  10. Muller C.A., Opie L.H., Peisach M., Pineda C.A., Hamm C.W., Thandroyen F.T. Critical role of dose in protection by propranolol against ventricular fibrillation in a pig model of acute myocardial ischemia. J Cardiovasc Pharmacol (1989) 13:192–197.[Web of Science][Medline]
  11. Verdouw P.D., van Bremen R.H., Verkeste C.M., van der Giessen W.J. Antiarrhythmic action and protection of ischaemic myocardium after beta-blockade with bevantolol. Eur J Pharmacol (1985) 111:377–380.[CrossRef][Web of Science][Medline]
  12. Knopf H., McDonald F.M., Bischoff A., Hirche Hj., Addicks K. Effect of propranolol on early postischemia arrhythmias and noradrenaline and potassium release of ischemic myocardium in anesthetized pigs. J Cardiovasc Pharmacol (1988) 12(Suppl. I):S41–S47.
  13. Aupetit J.F., Timour Q., Loufoua-Moundanga J., Omar S., Chevrel G., Faucon G. Vulnerability to ventricular fibrillation related to ischaemia. Comparison of the acute effects of β-blockers and calcium antagonists. Arch Int Pharmacodyn (1994) 327:25–39.[Medline]
  14. Aupetit J.F., Timour Q., Loufoua-Moundanga J., Barral-Cadière L., Lopez M., Freysz M., Faucon G. Profibrillatory effects of lidocaine in the acutely ischemic porcine heart. J Cardiovasc Pharmacol (1995) 25:810–816.[Web of Science][Medline]
  15. Aupetit J.F., Freysz M., Faucon G., Loufoua-Moundanga J., Timour Q. Change of a beneficial effect into an untoward effect by ischaemia: effect of quinidine-like drugs on vulnerability to ventricular fibrillation. Environm Tox Pharmacol (1996) 2:1–7.[CrossRef]
  16. Aupetit J.F., Timour Q., Chevrel G., Loufoua-Moundanga J., Omar S., Faucon G. Attenuation of the ischaemia-induced fall of electrical ventricular fibrillation threshold by a calcium antagonist, diltiazem. Naunyn-Schmiedeberg's Arch Pharmacol (1993) 348:509–514.[Web of Science][Medline]
  17. Bui-Xuan B., Aupetit J.F., Freysz M., Loufoua-Moundanga J., Faucon G., Timour Q. Disappearance with ischaemic depolarization of the antifibrillatory activity in a sodium channel blocker and appearance in a calcium channel blocker. Arch Int Pharmacodyn (1996) 331:246–262.[Medline]
  18. Bui-Xuan B., Aupetit J.F., Freysz M., Loufoua-Moundanga J., Faucon G., Timour Q. Cardiac β-adrenoceptor activation and ventricular fibrillation under normal and ischaemic conditions. Cardiovasc Res (1996) 32:1056–1063.[Abstract/Free Full Text]
  19. Janse M.J., Wit A.L. Electrophysiological mechanisms of ventricular arrhythmias resulting from myocardial ischemia and infarction. Physiol Rev (1989) 69:1049–1169.[Free Full Text]
  20. James R.G.G., Arnold J.M.O., Allen J.D., Pantridge J.F., Shanks R.G. The effects of heart rate, myocardial ischemia and vagal stimulation on the threshold for ventricular fibrillation. Circulation (1977) 55:311–317.[Abstract/Free Full Text]
  21. Clusin W.T., Buchbinder M., Ellis A.K., Kernoff R.S., Giacomini J.C., Harrison D.C. Reduction of ischemic depolarization by the calcium channel blocker diltiazem. Circ Res (1984) 54:10–20.[Abstract/Free Full Text]
  22. Kloner R.A., Braunwald E. Effects of calcium antagonists on infarcting myocardium. Am J Cardiol (1987) 59:84B–94B.[CrossRef][Medline]
  23. Timour Q., Bui-Xuan B., Faucon G., Aupetit J.F. Delay by a calcium antagonist, amlodipine, of the onset of primary ventricular fibrillation in myocardial ischemia. Cardiovasc Drugs Ther (1996) 10:447–454.[CrossRef][Web of Science][Medline]
  24. Boyett M.R., Jewell B.R. Analysis of the effects of changes in rate and rhythm upon electrical activity of the heart. Prog Biophys Mol Biol (1980) 36:1–52.[Web of Science][Medline]
  25. Lang J., Timour Chah Q., Charve P., Aupetit J.F., El Chebly M., Faucon G. Role of calcium in the rate-dependent depression of atrioventricular nodal conduction in the dog heart under vagal influence. Int J Cardiol (1987) 15:65–76.[CrossRef][Web of Science][Medline]
  26. Franz M.R., Schaefer J., Schöttler W., Seed W.A., Noble M.M. Electrical and mechanical restitution of the human heart at different rates of stimulation. Circ Res (1983) 53:815–822.[Abstract/Free Full Text]
  27. Boyett M.R., Fedida D. A simple model of ion concentration changes in the heart during repetitive activity. J Physiol (Lond.) (1983) 342:52P–53P.
  28. Hill J.L., Gettes L.S. Effect of acute coronary artery occlusion on local myocardial extracellular K+ activity in swine. Circulation (1980) 61:768–778.[Abstract/Free Full Text]
  29. Lyons C.J., Burgess M.J., Abildskov J.A. Effects of acute hyperkalemia on cardiac excitability. Am Heart J (1977) 94:755–763.[CrossRef][Web of Science][Medline]
  30. Vassalle M., Barnabei O. Norepinephrine and potassium fluxes in cardiac Purkinje fibers. Arch ges Physiol (1971) 322:287–298.[CrossRef]
  31. Skinner J.E., Lie J.T., Entman H.L. Modification of ventricular fibrillation latency following coronary artery occlusion in the conscious pig. The effect of psychological stress and beta-adrenergic blockade. Circulation (1975) 51:656–667.[Abstract/Free Full Text]
  32. Sutherland E.W., Robison G.A. The role of cyclic-3'5' AMP in response to catecholamines and other hormones. Pharmacol Rev (1966) 18:145–161.[Free Full Text]
  33. Kjekshus J.K. Importance of heart rate in determining beta-blocker efficacy in acute and long-term myocardial infarction intervention trials. Am J Cardiol (1986) 57:43F–49F.[CrossRef][Medline]
  34. Kralios A.C., Tsagaris T.J., Kuida H. Effect of coronary vasodilation on ventricular fibrillation threshold of vasodilators and perfusion conditions. Am Heart J (1983) 105:580–590.[CrossRef][Web of Science][Medline]

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