Electrophysiologic and antiarrhythmic effects of magnesium in patients with inducible ventricular tachyarrhythmia

Spironolactone Reduced Arrhythmia and Maintained Magnesium Homeostasis in Patients With Congestive Heart Failure

BACKGROUND

Patients with congestive heart failure (CHF) often have increased aldosterone activity that leads to hypomagnesemia. Hypomagnesemia can induce arrhythmias, an important cause of death in patients with CHF. We determined whether the aldosterone receptor antagonist spironolactone improved magnesium homeostasis and reduced arrhythmias in patients with CHF.

METHODS AND RESULTS

We randomized 116 consecutive patients with CHF into placebo control group (n = 58) and spironolactone group (20 mg daily, n = 58) in addition to conventional therapy. Plasma magnesium concentration (PMC), erythrocyte magnesium concentration (EMC), and erythrocyte magnesium efflux were not different between the 2 groups of patients before treatment. Compared with control patients, patients treated with spironolactone for 6 months had increased PMC and EMC and decreased erythrocyte magnesium efflux. Patients on spironolactone therapy also had a marked decrease of 24-hour mean heart rate, ventricular and atrial premature beats, and the risk of atrial fibrillation/flutter. Pooled data from the 116 patients showed that patients with a higher EMC or a lower sodium-dependent erythrocyte magnesium efflux had a slower heart rate, fewer ventricular premature beats, and a lower risk of atrial fibrillation/flutter.

CONCLUSIONS

Our results suggest that reducing cellular magnesium efflux and loss may contribute to the spironolactone-reduced arrhythmias in patients with CHF.

Acute effects of intravenous magnesium on ventricular refractoriness and monophasic action potential duration in humans

The objective of this study was to examine the antiarrhythmic mechanisms of magnesium (Mg). The 12 patients who were selected for the study were given 12 mmol Mg sulphate followed by an infusion of 8 mmol/h. Ventricular effective refractory period (VERP). monophasic action potential duration at 50% and 90% repolarization during spontaneous rhythm (MAPD50, MAPD90) and atrial pacing at a cycle length of 600 ms (MAPD(50)600, MAPD(90)600) were measured before and after the intervention. Plasma magnesium concentration rose from 0.83 +/- 0.05 to 1.85 +/- 0.24 mmol/l (p < 0.001). Shortening of MAPD(50) (250 +/- 45 vs 240 +/- 46 ms; p < 0.05), MAPD(90) (294 +/- 42 vs 283 +/- 41 ms; p < 0.05) and shortening of sinus cycle length (SCL) (783 +/- 153 vs 742 +/- 160 ms; p < 0.01) were detected. During controlled cycle length, magnesium decreased MAPD(50)600 (230 +/- 28 vs 221 +/- 28 ms; p < 0.01), MAPD(90)600 (274 +/- 25 vs 261 +/- 29 ms; p < 0.01) and VERP (247 +/- 25 vs 241 +/- 21 ms: p < 0.05). QRS duration, QT interval and blood pressure remained unchanged. The change in SCL correlated with the change in MAPD(50) (r = 0.58; p < 0.05) and MAPD(90) (r = 0.55; p = 0.06 ) but not with MAPD(50)600, MAPD(90)600 or VERP. Magnesium acutely shortens ventricular monophasic action potential duration and refractoriness. This effect is partly mediated by mechanisms that increase heart rate. The reductions might bring salutary effects in arrhythmias evoked by prolonged repolarization.

Randomised trial of magnesium in in-hospital cardiac arrest. Duke Internal Medicine Housestaff

BACKGROUND

The apparent benefit of magnesium in acute myocardial infarction, and the persistently poor outcome after cardiac arrest, have led to use of magnesium in cardiopulmonary resuscitation. Because few data on its use in cardiac arrest were available, we undertook a randomised placebo-controlled trial (MAGIC trial).

METHODS

Patients treated for cardiac arrest by the Duke Hospital code team were randomly assigned intravenous magnesium (2 g [8 mmoles] bolus, followed by 8 g [32 mmoles] over 24 h; 76 patients) or placebo (80 patients). Only patients in intensive care or general wards were eligible; those whose cardiac arrest occurred in emergency, operating, or recovery rooms were excluded. The primary endpoint was return of spontaneous circulation, defined as attainment of any measurable blood pressure or palpable pulse for at least 1 h after cardiac arrest. The secondary endpoints were survival to 24 h, survival to hospital discharge, and neurological outcome. Analysis was by intention to treat.

FINDINGS

There were no significant differences between the magnesium and placebo groups in the proportion with return of spontaneous circulation (41 [54%] vs 48 [60%], p = 0.44), survival to 24 h (33 [43%] vs 40 [50%], p = 0.41), survival to hospital discharge (16 [21%] vs 17 [21%], p = 0.98), or Glasgow coma score (median 15 in both).

INTERPRETATION

Empirical magnesium supplementation did not improve the rate of successful resuscitation, survival to 24 h, or survival to hospital discharge overall or in any subpopulation of patients with in-hospital cardiac arrest.

[Effect of magnesium on sustained ventricular tachycardia]

Intravenous application of magnesium was suggested for the management of persistent ventricular tachycardia. In patients with torsade de pointes tachycardia the injection of magnesium controlled the life threatening arrhythmias reliably, thus magnesium became the treatment of choice in this setting. The results in patients with persistent monomorphic ventricular tachycardia are controversial. In one study, ventricular tachycardias could be controlled in 8 of 10 patients by 2,000 mg magnesium sulfate. In a randomized trial with 43 patients, termination of the ventricular tachycardia could be accomplished in 6 of 20 patients under the influence of 4,000 mg magnesium sulfate, whereas 3 of 24 patients exhibited termination of the tachycardia following placebo; the difference did not reach statistical significance. In our study 4 patients with torsade de pointes tachycardia could be controlled by the application of magnesium glutamate (2 x 1,000 mg intravenously). Of 25 patients with persistent monomorphic ventricular tachycardia, the arrhythmia ended in 8 patients under the influence of magnesium. No significant change of the RR intervals and the QRS duration during ventricular tachycardia could be demonstrated following magnesium injection. Cardiac index during ventricular tachycardia increased from 2.0 +/- 0.6 l/min x m2 to 2.5 +/- 0.1 l/min x m2 (p < 0.05). In a further investigation the dose of magnesium was increased to 2 x 9 mmol. To detect possible interactions with antiarrhythmic agent, 10 patients under chronic antiarrhythmic therapy with class-III-agents were compared with 10 patients without such treatment. Plasma levels of magnesium increased from 0.79 +/- 0.1 mmol/l to 1.87 +/- 0.5 mmol/l. In 5 of the 20 patients ventricular tachycardia ended under the influence of magnesium: 2 patients were on chronic antiarrhythmic agents, whereas 3 had no chronic therapy. There were no significant differences in the RR intervals and the duration of the monophasic action potentials during the ventricular tachycardia under the influence of magnesium. These data indicate that the bolus therapy of magnesium controls persistent monomorphic ventricular tachycardia in a minority of patients only. Therefore, magnesium injection cannot be recommended for treatment of monomorphic ventricular tachycardia in the emergency setting. On the other hand, magnesium application can be considered the therapy of choice for patients with torsade de pointes tachycardia.

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.

Effect of acute magnesium administration on the frequency of ventricular arrhythmia in patients with heart failure

BACKGROUND

There is a high incidence of ventricular arrhythmia and sudden death in patients with heart failure. Unfortunately, currently available antiarrhythmic agents have only limited efficacy and may result in proarrhythmia and hemodynamic deterioration in these patients.

METHODS AND RESULTS

We studied the acute effect of intravenous magnesium chloride on the frequency and severity of ventricular arrhythmia in 30 patients with symptomatic heart failure using a double-blind, placebo-controlled crossover design. The left ventricular ejection fraction was 23.0 +/- 8.0% (mean +/- SD). No patient had a history of symptomatic ventricular arrhythmia or was receiving antiarrhythmic agents, calcium channel antagonists, or beta-blockers. Patients were randomized to receive placebo (5% dextrose [D5W] in water alone) or magnesium chloride in D5W given as a bolus of 0.3 mEq/kg over 10 minutes followed by a maintenance infusion of 0.08 mEq/kg per hour for 24 hours. The magnesium concentrations 30 minutes and 24 hours after the bolus were 3.6 +/- 0.1 and 4.2 +/- 0.1 mg/dL, respectively. There was no significant change in serum potassium concentration during magnesium administration. Blinded analysis revealed that administration of intravenous magnesium chloride, compared with placebo, significantly decreased total ventricular ectopy per hour (mean +/- SEM, 70 +/- 26 versus 149 +/- 64, P < .001), couplets per day (23 +/- 11 versus 94 +/- 59, P = .007), and episodes of ventricular tachycardia per day (0.8 +/- 0.2 versus 2.6 +/- 1.0, P = .051).

CONCLUSIONS

Intravenous magnesium chloride administration reduces the frequency of ventricular arrhythmia in patients with symptomatic heart failure.