Attenuation of the ischaemia-induced fall of electrical ventricular fibrillation threshold by a calcium antagonist, diltiazem

A novel mechanism for electrical currents inducing ventricular fibrillation: The three-fold way to fibrillation

It has been long recognized that there are 2 methods for inducing VF (ventricular fibrillation) with electrical currents‥ These are: (1) delivering a high-charge shock into the cardiac T-wave, and (2) delivering lower level currents for 1-5 seconds. Present electrical safety standards are based on this understanding. We present new data showing a 3(rd) mechanism of inducing VF which involves the steps of delivering sufficient current to cause high-rate cardiac capture, causing cardiac output collapse, leading to ischemia, for sufficiently long duration, which then lowers the VFT (VF threshold) to the level of the current, which finally results in VF. This requires about 40% of the normal VF-induction current but requires a duration of minutes instead of seconds for the VF to be induced. Anesthetized and ventilated swine (n=6) had current delivered from a probe tip 10 mm from the epicardium sufficient to cause hypotensive capture but not directly induce VF within 5 s. After a median time of 90 s, VF was induced. This 3(rd) mechanism of VF induction should be studied further and considered for electrical safety standards and is relevant to long-duration TASER Electronic Control Device applications.

Magnesium--a profibrillatory or antifibrillatory drug depending on plasma concentration, heart rate and myocardial perfusion

BACKGROUND

The opinions on the efficacy of magnesium as an antiarrhythmic drug vary considerably. The action of magnesium on vulnerability to fibrillation was therefore investigated in anaesthetized, open-chest pigs under different conditions as regards plasma concentration, heart rate and myocardial perfusion.

METHODS

Vulnerability to fibrillation was assessed by electrical fibrillation threshold (EFT), measured with 100-ms duration diastolic impulses. These stimuli were delivered to the heart normally perfused, at a usual (90 and 120 beats/min) or accelerated (180 beats/min) rate. Vulnerability to fibrillation was also assessed at the high rate (180 beats/min) in the heart made ischaemic by complete occlusion of the left anterior descending coronary artery near its origin. EFT was then measured at the end of occlusion periods which were of increasing duration (30, 60, 90, 120 s). Monophasic action potential (MAP) duration, intraventricular conduction time, left ventricular dP/dt max (LVdP/dt max) and mean blood pressure were concurrently measured.

RESULTS

In the absence of ischaemia, 5 mumol.kg-1.min-1 magnesium i.v. infusion, which raised plasma concentration to 1.78 +/- 0.14 mmol/L, lowered EFT, measured at the rate of 116 beats/min, from 14.0 +/- 1.1 to 6.8 +/- 1.0 mA (P < 0.001), without significant variation of the other parameters. Administered as previously or in a markedly higher dose (400 mumol.kg-1 loading dose and 10 mumol.kg-1.min-1 infusion) which raised plasma concentration up to 4.84 +/- 0.52 mmol/L, magnesium significantly influenced neither EFT nor MAP duration, reduced by the high rate (180 beats/min) to 6.2-6.7 mA and 212-220 ms respectively. Under the same conditions, at the same 180 beats/min rate, ischaemia brings about a fall of EFT, from 6.9 down to nearly 0 mA, with occurrence of fibrillation, in approximately 120 s. Magnesium failed to slow this fall and to delay the onset of fibrillation. In contrast, within the minutes following the end of occlusion, magnesium increased EFT to a great extent (from 7.1 +/- 0.4 to 13.5 +/- 0.7 mA, P < 0.001), with a significant prolongation of MAP duration (212 +/- 6 to 234 +/- 8 ms, P < 0.01).

CONCLUSION

Magnesium may develop profibrillatory or antifibrillatory effects depending on plasma concentration, heart rate and myocardial perfusion.

Ischaemia-induced loss or reversal of the effects of the class I antiarrhythmic drugs on vulnerability to fibrillation

1. In the last decade, a number of clinical observations have questioned the efficacy of certain class I antiarrhythmic drugs against ischaemia-induced ventricular fibrillation. The effects of three drugs of this class, disopyramide (Ia), lignocaine (Ib) and flecainide (Ic) on the vulnerability to fibrillation during experimental ischaemia were investigated. 2. The study was carried out in anaesthetized, open-chest pigs (n = 8 for each of the drugs, in addition to the control group, n = 6). Vulnerability to fibrillation was evaluated by measuring electrical fibrillation threshold (EFT) by means of stepwise increased intensity of wide (100 ms) diastolic impulses applied to the ischaemic tissue at a 180 beats min-1 rate. Monophasic action potential (MAP) duration and conduction time in the ischaemic region were also measured. 3. EFT determinations were performed before and during periods of ischaemia induced by complete occlusion of the left anterior descending coronary artery near its origin. Ischaemic periods of increasing duration (30, 60, 90, 120, 150 s) were induced to determine the electrophysiological changes, of EFT especially, leading to fibrillation. 4. In the absence of ischaemia, all three drugs, administered by intravenous route (1 mg kg-1 plus 0.04 mg kg-1 min-1) increased EFT to a similar extent (from approximately 7 to 10 mA), despite a 25% prolongation of conduction time. 5. During ischaemia, none of the drugs prevented the fall in EFT towards 0 mA, resulting in spontaneous fibrillation. After 30 s of ischaemia, they no longer had any capacity for raising EFT and, after 60, 90 and 120 s of ischaemia, the decrease in EFT was exacerbated. This accelerated reduction in EFT shortened the time to onset of fibrillation (after 120 s of ischaemia, 62.5% of fibrillations with flecainide instead of 12.5 under control conditions, 75% instead of 25 with lignocaine and 50% instead of 25 with disopyramide). The reduction in MAP duration due to ischaemia was also significantly accelerated (at 60 s, 178 +/- 5 ms instead of 192 +/- 4 with flecainide, 175 +/- 3 ms instead of 194 +/- 5 with lignocaine and 180 +/- 5 ms instead of 196 +/- 3 with disopyramide) and the slowing of conduction was made worse (prolongation of conduction time by 70% instead of 50). 6. In conclusion, the antifibrillatory properties normally manifested by these drugs are first suppressed, then inverted by ischaemia, depending on oxygen debt varying with severity and duration of ischaemia.

Delay by a calcium antagonist, amlodipine, of the onset of primary ventricular fibrillation in myocardial ischemia

Calcium antagonists have been reported to counteract the increase by ischemia of vulnerability to ventricular fibrillation. This ability might be especially of interest in the prevention of sudden death subsequent to a major, but transitory, inadequacy between myocardial oxygen requirements and available coronary blood flow produced by exercise, emotion, etc., because death is then not related to irreversible damage of myocardial fibers. This study has been undertaken to examine the protective effect of a calcium antagonist on an animal model of this type of ischemia. This model used complete, but transient occlusion of the left anterior descending coronary artery near its origin during pacing at a constant high rate (180 beats/min) in anesthetized, open-chest pigs, most often resulting in fibrillation within 1-2 minutes after a progressive fall of the electrical fibrillation threshold. Amlodipine was the preferred calcium antagonist for this study because it is only moderately negatively inotropic. The results of the preventive administration of amlodipine was assessed by the time to onset of fibrillation. Amlodipine 0.30 mg/kg prolonged this time by 50-100% (p < 0.05) without appreciable impairment of blood pressure or myocardial contractility. Concurrently, amlodipine delayed the shortening of the monophasic action potential duration, the lengthening of conduction time, and the alterations of ST segments and T waves linked to ischemic depolarization. Consequently, when given experimentally before the occurrence of major, but transitory ischemia, amlodipine protected against fibrillation. Similarly, in clinical settings it ought to delay sudden death that may occur as a result of a major but transitory inadequacy between myocardial oxygen requirements and available coronary blood flow.

Change of a beneficial effect into an untoward effect by ischaemia: effect of quinidine-like drugs on vulnerability to ventricular fibrillation

The effects of three quinidine-like drugs, disopyramide, lidocaine and flecainide were investigated in anaesthetized, open-chest pigs on vulnerability to ventricular fibrillation under normal conditions and under myocardial ischaemia conditions. Vulnerability to fibrillation was evaluated by electrical ventricular fibrillation threshold (VFT), measured with 100 ms duration diastolic impulses the intensity of which was increased by steps of 1.0 or 0.5 mA. Impulses were delivered at the rate of 180 beats · min(-1). The ventricles were subjected to pacing at the same rate before the VFT determination, particularly throughout periods of ischaemia of increasing duration (30, 60, 90, 120, 150 s), separated by appropriate intervals for reproducibility of the results. Monophasic action potential (MAP) duration and conduction time were monitored in the ischaemic area under pacing. Ischaemia was obtained by complete occlusion of the left anterior descending coronary artery near its origin. The three drugs were i.v. administered in clinical dose range (1.00 mg · kg(-1) plus 0.04 mg · kg(-1) · min(-1)). In the absence of ischaemia, they increased almost equally VFT (from about 7 to 10 mA), despite 25% prolongation of conduction time. But, none of them was able to impede the increasingly marked fall of VFT caused by ischaemia: at 30 s, they had already lost any capacity for raising VFT and, beyond this time, they even aggravated its fall which led to spontaneous fibrillation when VFT approached 0 mA. The faster fall of VFT shortened time to onset of fibrillation (20 24 fibrillations for the three drugs at 150 s as against 12 24 in control period), the ischaemia-induced reduction of MAP duration (by 20%) being also hastened and slowing of conduction enhanced, given the addition of the depressant effects of ischaemia and drugs on conduction. Consequently, the antifibrillatory properties normally manifested by the studied drugs are first suppressed, then inverted by ischaemia.

Cardiac implications of amlodipine-dantrolene combinations

PURPOSE

Cardiac disorders, cardiac arrest and ventricular fibrillation in the most severe cases, have been observed after the administration of dantrolene to patients treated by verapamil for coronary artery disease. This study was designed to examine the interaction of dantrolene with amlodipine, a dihydropyridine.

METHODS

In 12 anaesthetized, open-chest pigs, the effects of the interaction have been studied on heart rate, atrioventricular conduction, monophasic action potential duration, intraventricular conduction time, left ventricular dP/dt max and mean blood pressure. The study was performed with normal coronary circulation and ischaemia of a large area of the left ventricule, obtained by complete occlusion of the left anterior descending coronary artery near its origin, under pacing at a constant high rate, 180 beats.min-1. The drugs were injected iv, amlodipine 0.4 mg.kg-1 first and dantrolene 3.0 mg.kg-1 20 min later in six animals and the order was reversed in the other animals.

RESULTS

Sinus rate and atrioventricular conduction were not affected by amlodipine, but were slowed by dantrolene added (145 +/- 9 to 131 +/- 7 beats.min-1, P < 0.01 and 150 +/- 15 to 180 +/- 20 msec, P < 0.01). In contrast, amlodipine or amlodipine plus dantrolene did not change MAP duration or conduction time in the normal heart. Similarly, they did not alter the maximal variations due to ischaemia, but delayed them, while prolonging the time to onset of fibrillation (111 +/- 8 to 343 +/- 33 sec. P < 0.001 with amlodipine alone, 289 +/- 11 to 323 +/- 16 sec, P < 0.05 with dantrolene). Left ventricular dP/dt max was lowered from 1670 +/- 86 to 1532 +/- 50 mmHg.sec-1 (P < 0.001) and mean blood pressure from 79 +/- 4 to 70 +/- 3 mmHg (P < 0.01) by amlodipine, but dantrolene did not enhance and even counteracted these effects. Finally, potassium plasma concentration did not increase above 5.1 +/- 0.2 mmol.L-1 under the dual influence of amlodipine and dantrolene.

CONCLUSION

In usual clinical doses, dantrolene may be safely administered concurrently with amlodipine.

Bupivacaine hastens the ischemia-induced decrease of the electrical ventricular fibrillation threshold

Myocardial ischemia sensitizes the cardiotoxic effects of bupivacaine, especially the propensity to ventricular fibrillation. To investigate this sensitization and to elucidate its mechanism, the influence of bupivacaine alone, or associated with ischemia, was studied on electrical fibrillation threshold in anesthetized, open chest pigs. Determination of fibrillation threshold was performed with impulses of 100 ms duration at the rate of 180 bpm, in the absence of ischemia and at the end of increasing periods of ischemia (30, 60, 120, 180 s) obtained by complete occlusion of the left anterior descending coronary artery close to its origin. The effect of bupivacaine (1.00 mg/kg initial dose plus 0.04 mg.kg-1.min-1 over 25 min) was compared to the control in the same animals. This effect corresponded to 1.4-1.8 micrograms/mL plasma concentrations likely to be observed in humans after regional anesthesia. Bupivacaine significantly increased the fibrillation threshold before coronary occlusion from approximately 7.0 to 9.5 mA. In contrast, during ischemia the fibrillation threshold was shifted to the left and down, with a hastening of spontaneous fibrillation. Recording of monophasic action potentials in the ischemic area revealed that conduction time was prolonged by more than 100% under the combined influence of ischemia and bupivacaine, whereas the major enhancement of excitability due to ischemia was not attenuated by bupivacine. Therefore, bupivacaine should be used with caution in the condition of ischemia, especially if heart rate is rapid. In the present experiments, tachycardia is another factor in the enhancement of bupivacaine effects on conduction.