Beneficial effect of magnesium sulfate in acute myocardial infarction

Investigation of the effects of intravenous magnesium sulphate on cardiac rhythm in acute myocardial infarction

OBJECTIVE

To examine the effect of doubling serum magnesium concentration on the incidence of arrhythmias in patients with suspected acute myocardial infarction.

DESIGN

Randomised double blind clinical trial.

SETTING

Coronary care unit of a teaching hospital.

PATIENTS

Clinical data were collected on 2316 randomised patients with suspected acute myocardial infarction. Holter monitoring was performed in a subgroup of 70 patients and analysed in 48 patients in whom acute myocardial infarction was confirmed.

INTERVENTIONS

By random allocation, patients received either an intravenous loading dose of 8 mmol magnesium sulphate over five minutes plus 65 mmol over the next 24 hours, or equal volumes of saline.

MAIN OUTCOME MEASURES

(a) Clinically documented arrhythmias; (b) use of antiarrhythmic treatments, cardioversion, and insertion of a pacemaker; (c) incidence of all abnormal rhythms during Holter monitoring.

RESULTS

In the main trial the incidence of rhythm disturbance while in the coronary care unit (expressed as the odds ratio (OR) for magnesium: placebo and its 95% confidence interval) was not significantly different between treatment groups for ventricular fibrillation (OR 0.74; 0.46 to 1.20), ventricular tachycardia (OR 0.87; 0.63 to 1.20), supraventricular tachycardia (OR 0.69; 0.38 to 1.26), atrial fibrillation (OR 0.92; 0.69 to 1.23), or heart block of any degree (OR 1.17; 0.83 to 1.65). Sinus bradycardia was significantly more common in the magnesium group (OR 1.38; 1.03 to 1.85; p = 0.02). These findings were corroborated by the use of treatments for rhythm disturbance and the data from Holter monitoring.

CONCLUSION

The regimen of intravenous magnesium sulphate used here had no significant effect on arrhythmia in acute myocardial infarction. The reduction in mortality that has been shown with this form of treatment is not attributable to suppression of life threatening rhythm disturbances.

Magnesium and its therapeutic uses: a review

Magnesium has been reported as an effective medical therapy in an expanding array of conditions. Evidence investigating magnesium's use is presented, with a number of studies suggesting it should be seriously considered in such conditions as ischemic heart disease, cardiac arrhythmias, and asthma. Magnesium balance and metabolism are briefly reviewed, and then various hypotheses are presented that may explain magnesium's physiologic mechanisms of action, most likely involving calcium and potassium flux across cellular membranes in smooth muscle. In a number of the conditions to be discussed, it has been uncertain whether magnesium administration serves the purpose of merely correcting an underlying deficiency state or of utilizing a specific pharmacologic effect of magnesium. Magnesium deficiency is a relatively common condition, and predisposing factors as well as recent methods for assessing total body stores of magnesium are discussed. Physicians should be familiar with the numerous conditions and therapeutics that are risk factors for an underlying magnesium deficiency and in which empiric magnesium replacement should be considered. Guidelines for administration of parenteral magnesium are presented with specific focus on the low risk of adverse effects, as suggested by the large and rapid dosing regimens used in many of the clinical studies discussed here.

Interrelationship of magnesium and estrogen in cardiovascular and bone disorders, eclampsia, migraine and premenstrual syndrome

The anticonvulsive and antihypertensive values of magnesium (Mg) in eclampsia, and its antiarrhythmic applications in a variety of cardiac diseases, have caused Mg to be considered only for parenteral administration by many physicians. In contrast, nutritionists have long recognized Mg as an essential nutrient, because severe deficiencies elicit neuromuscular manifestations similar to those justifying its use in eclampsia. More recently, this element has been used to favorably influence latent tetany with and without thrombotic complications, to delay preterm birth, to influence premenstrual syndrome, and to ameliorate migraine headaches. Most of these disorders exclusively or largely afflict women. The lesions of arteries and heart caused by experimental Mg deficiency have been well documented and may contribute to human cardiovascular disease. Estrogen's enhancement of Mg utilization and uptake by soft tissues and bone may explain resistance of young women to heart disease and osteoporosis, as well as increased prevalence of these diseases when estrogen secretion ceases. However, estrogen-induced shifts of Mg can be deleterious when estrogen levels are high and Mg intake is suboptimal. The resultant lowering of blood Mg can increase the Ca/Mg ratio, thus favoring coagulation. With Ca supplementation in the face of commonly low Mg intake, risk of thrombosis increases.

Ventricular arrhythmias late after myocardial infarction are related to hypomagnesemia and magnesium loss: preliminary trial of corrective therapy

It has been well established that in acute myocardial infarction (MI) many patients display low serum magnesium (Mg). This is associated with complex ventricular arrhythmias. The question arises whether predischarge arrhythmias occurring late after MI might also be related to Mg imbalance. In 118 patients subjected to heart rhythm 24 h Holter monitoring in the second or third week after MI, we investigated (1) the relationship between serum Mg, urinary Mg loss, and ventricular arrhythmias, and (2) the effect of Mg supplementation on heart rhythm disturbances. In patients with undisturbed rhythm or monomorphic ventricular ectopic beats (VEB) (Lown 0-2; n = 84), mean serum Mg level (mg% +/- SD) was 1.83 +/- 0.21, whereas in patients with multifocal VEB, pairs, or nonsustained ventricular tachycardia (VT) (Lown 3-4; n = 34) serum Mg was decreased to 1.68 +/- 0.27 (p < 0.01). Serum Mg normal range in our laboratory is 1.7-2.6 mg%. The lowest serum Mg reaching 1.55 +/- 0.27 was found in nonsustained VT (Lown 4 b) subgroup (n = 14). Urinary Mg loss measured in 81 patients was more pronounced in those with Lown 3-4 arrhythmias (n = 26) than with Lown 0-2 (n = 55). The daily values were 73 +/- 22 and 54.4 +/- 26 mg, respectively (p < 0.001). Thirteen patients with complex arrhythmias and low serum Mg received Mg supplementation (MgSO4, 8 g in 500 ml 5% glucose intravenously during 24 h). This resulted in restoration of almost undisturbed rhythm in 10 subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionised magnesium and calcium in plasma from healthy volunteers and patients undergoing cardiopulmonary bypass

OBJECTIVES

To measure the concentration of ionised magnesium, [Mg2+], and ionised calcium [Ca2+], in plasma from healthy volunteers and patients undergoing cardiopulmonary bypass (CPB). These measurements were carried out because there have been few reliable measurements of these values in healthy volunteers and no direct measurements in this patient group.

PATIENTS AND METHODS

Dip cast ion selective electrodes were used to measure Mg2+, Ca2+, and H+ in plasma at 37 degrees C. These values were correlated with total metal concentrations, [Mg] and [Ca], plasma sodium [Na], and albumin concentrations found by standard techniques. Blood samples were taken from the patient group immediately before and after CPB and a further sample 24 hours later.

RESULTS

In healthy volunteers the [Mg] was 0.86 (0.12) mM and [Mg2+], was 0.48 (0.06) mM, and the corresponding value for [Ca] was 2.34 (0.06) and for [Ca2+] 1.01 (0.13) mM. Values for [Mg], [Ca], and [Ca2+] have been reported by others and those quoted here are similar. In the CPB group the preoperative [Mg] was lower than the normal group but did not alter one hour after CPB and was slightly raised after 24 hours. The [Mg2+], however, was significantly reduced after 24 hours. Both [Ca] and [Ca2+] were slightly reduced after 24 hours but when adjusted for plasma albumin concentrations they were unchanged over this period.

CONCLUSIONS

The most important finding is that around 24 hours after CPB the plasma [Mg2+] is significantly reduced, with no change to the total [Mg]. Corresponding changes to [Ca] and [Ca2+] were much smaller. This supposes the presence of an Mg2+ binding ligand of unknown origin in the plasma that may contribute to the cardiac arrhythmias that occur in some patients at this time after CPB.

Effects of magnesium infusion on thrombolytic and non-thrombolytic treated patients with acute myocardial infarction

A total of 109 consecutive patients were included in a double blind, randomized trial of the effect of intravenous magnesium sulfate in acute myocardial infarction. Of these 63% received intravenous fibrinolytic therapy. Twenty four-hour Holter monitoring of heart rhythm was performed during the initial hospital stay. A significant reduction in total cardiac mortality in hospital and during the 9 months follow-up was found in the magnesium treated non-thrombolytic group (P < 0.05). Within this subgroup development of heart failure was decreased (P < 0.01). No effect of magnesium infusion on ventricular arrhythmias was demonstrated, instead we found a greater proportion of patients with short runs of ventricular tachycardias in the magnesium treated non-thrombolytic group (P < 0.05), which may represent an increase in spontaneous reperfusion.

CONCLUSION

these results indicates that magnesium infusion may have a beneficial effect on mortality in patients with acute myocardial infarction not receiving thrombolytic therapy, but opposes the view that the benefit is related to an antiarrhythmic effect. No additional effect of magnesium to ongoing fibrinolytic therapy could be demonstrated regarding mortality, reinfarction and heart failure.

The use of antiarrhythmics in advanced cardiac life support

Antiarrhythmic agents have been used to treat malignant ventricular arrhythmias in the setting of acute myocardial ischemia with proven efficacy for many years. Thus, it has been presumed that these agents would be efficacious for the treatment of cardiac arrest. Unfortunately, hard data supporting this contention are unavailable to date. Furthermore, some of the experimental data in this area are conflicting, especially regarding the relative effects of lidocaine and bretylium. Thus, little definitive can be said based on experimental information. In two randomized patient studies, lidocaine and bretylium performed comparably. Because of the frequent use of lidocaine and thus the familiarity of most health care professionals with its use, it makes educational sense to utilize lidocaine as the antiarrhythmic drug of first choice during the cardiac arrest sequence. Recent data suggesting that amiodarone may be efficacious in patients with recurrent arrhythmias require additional confirmation. Although antiarrhythmic agents have been shown to be effective in the treatment of malignant arrhythmias in patients with acute myocardial infarction, their use prophylactically for patients with suspected infarction (advocated in the past) has recently undergone reevaluation. It is now clear that despite a reduction in ventricular fibrillation, overall mortality may be increased. This may be because the prophylactic treatment of patients with suspected infarction includes a large number of patients not at risk for ventricular fibrillation who still may be at risk for drug toxicity. Thus, prophylactic administration of lidocaine to all patients with suspected acute myocardial infarction can no longer be recommended. There are inadequate data upon which to base a recommendation concerning the use of lidocaine in patients receiving thrombolytic therapy. The group most likely to benefit from lidocaine are patients with ST segment elevation who present early after the onset of acute myocardial infarction. The use of lidocaine in this group requires additional study. At present, despite enthusiasm for the prophylactic use of magnesium for the treatment of arrhythmias, data are inadequate to support its routine administration. However, given the importance of magnesium and potassium levels in the genesis of malignant arrhythmias, their levels in plasma should be assessed, and abnormalities should be promptly corrected. The potential uses of antiarrhythmic agents during advanced cardiac life support span a remarkably diverse number of applications. For the purpose of this review, only the use of these agents during CPR and during the early hours of acute or suspected acute myocardial infarction will be considered.

Efficacy of intravenous magnesium in acute myocardial infarction in reducing arrhythmias and mortality. Meta-analysis of magnesium in acute myocardial infarction

BACKGROUND

To ascertain the effect of the intravenous administration of magnesium in acute myocardial infarction on the frequency of arrhythmias and mortality, a meta-analysis of randomized controlled trials was performed.

METHODS AND RESULTS

The study included 930 patients with acute myocardial infarction admitted to primary referral hospitals. Administration of magnesium in acute myocardial infarction was associated with a 49% reduction in ventricular tachycardia and fibrillation. The incidence of cardiac arrest was reduced by 58%. The frequency of supraventricular tachycardias was also lower. Overall, there was a 54% reduction in mortality.

CONCLUSIONS

Intravenous magnesium is a safe and effective method of reducing the frequency of arrhythmias and mortality in acute myocardial infarction.

Intravenous magnesium sulphate in suspected acute myocardial infarction: results of the second Leicester Intravenous Magnesium Intervention Trial (LIMIT-2)

The cardiovascular actions of the magnesium ion at pharmacological concentrations include coronary and systemic vasodilatation, platelet inhibition, and antiarrhythmic effects. Magnesium has also been reported to protect myocardial tissue in experimental models of ischaemia and reperfusion. Several small clinical trials in suspected acute myocardial infarction have suggested that early mortality can be reduced by intravenous infusion of magnesium salts in the acute phase, but none has been of sufficient size to be conclusive. We therefore conducted a randomised, double blind, placebo controlled study in 2316 patients with suspected acute myocardial infarction who received either intravenous magnesium sulphate (8 mmol over 5 min followed by 65 mmol over 24 h) or physiological saline. The primary outcome measure was 28-day mortality, which was ascertained in 99.3% of patients. The groups were well balanced for prognostic factors. By intention-to-treat analysis mortality from all causes was 7.8% in the magnesium group and 10.3% in the placebo group (2p = 0.04), a relative reduction of 24% (95% confidence interval 1-43%). Within the coronary care unit the incidence of left ventricular failure was reduced by 25% (7-39%) in the magnesium group (2p = 0.009). There was no significant difference between the groups in the incidence of heart block or the use of antiarrhythmic drugs, direct-current cardioversion, or temporary pacing. Myocardial infarction was confirmed in 65% of each group, with closely similar rises in cardiac enzymes. The side-effects of magnesium treatment were transient flushing, related to speed of injection of the loading dose, and an increased incidence of sinus bradycardia (2p = 0.02). Exploratory subgroup analyses of 28-day mortality did not indicate any effect modification by thrombolysis or aspirin, or by previous treatment with beta blockers, calcium antagonists, or diuretics. Intravenous magnesium sulphate is a simple, safe, and widely applicable treatment. Its efficacy in reducing early mortality of myocardial infarction is comparable to, but independent of, that of thrombolytic or antiplatelet therapy.

Intravenous magnesium for acute asthma: failure to decrease emergency treatment duration or need for hospitalization

STUDY OBJECTIVE

To evaluate the efficacy of routine early administration of i.v. magnesium to patients with acute asthma.

DESIGN

Prospective, randomized clinical trial.

SETTING

Urban teaching hospital emergency department.

TYPE OF PARTICIPANTS

One hundred twenty consecutive patients aged 18 to 65 years with acute asthma unresponsive to a single albuterol treatment.

INTERVENTIONS

All patients received oxygen, 125 mg i.v. methylprednisolone, and hourly albuterol inhalation therapy. The study group also received 2 g i.v. magnesium sulfate infused over 20 minutes.

MEASUREMENTS AND MAIN RESULTS

Demographic and clinical characteristics were similar in both groups. Hospitalization was necessary in 13 of 58 patients who received magnesium (22%; 95% confidence intervals [CI], 13% to 32%) and 11 of 62 control patients (17%; 95% CI 10% to 26%; P = .523). Duration of ED treatment in discharged patients was 224 +/- 75 minutes in the magnesium group (95% CI, 208 to 240 minutes) and 228 +/- 90 minutes in the control group (95% CI, 209 to 247 minutes, P = .832). In addition, changes in peak expiratory flow were not statistically different.

CONCLUSION

Routine early administration of IV magnesium in acute asthma does not alter treatment outcome.

Effects of intravenous magnesium in suspected acute myocardial infarction: overview of randomised trials

OBJECTIVE

To investigate the effect of intravenous magnesium on mortality in suspected acute myocardial infarction.

DESIGN

Systematic overview of all available randomised trials in which patients were allocated to receive either intravenous magnesium or otherwise similar treatment without magnesium.

SETTING

Coronary care units of several hospitals.

PATIENTS

1301 patients in seven randomised trials.

MAIN OUTCOME MEASURE

Short term mortality.

RESULTS

Considering the seven trials collectively there were 25 (3.8%) deaths among 657 patients allocated to receive magnesium and 53 (8.2%) deaths among 644 patients allocated control, generally during hospital follow up. This represents a 55% reduction in the odds of death (p less than 0.001) with 95% confidence intervals ranging from about one third to about two thirds. 70 of 648 patients allocated magnesium compared with 109 of 641 controls had serious ventricular arrhythmias, suggesting that magnesium reduces the incidence, though the definition varied among trials. Other adverse effects were rare in the limited number of patients for whom this data were available.

CONCLUSION

Despite the limited number of patients randomised this overview suggests that intravenous magnesium therapy may reduce mortality in patients with acute myocardial infarction. Further large scale trials to confirm (or refute) these findings are desirable.

Inhibition by magnesium of calcium inward current in heart ventricular muscle

It was investigated whether the negative inotropic effect of magnesium in mammalian heart ventricular muscle is due to inhibition of the calcium inward current. Whole-cell voltage-clamp experiments were carried out with isolated guinea-pig heart cells. The sodium inward current was inactivated by a conditioning pre-pulse or by addition of tetrodotoxin. Magnesium concentration dependently and reversibly diminished the calcium inward current (by about 45% after an increase in magnesium concentration, from 1.2 to 9.6 mM). The decrease was mainly due to diminution of the maximally available calcium inward current but was additionally due to a shift of the current-voltage relationship to more positive potentials. The crucial dependence of the inhibition of the inward current on the pre-pulse potential was demonstrated. Conditioning depolarization to potentials more negative than -40 mV led to an increase of an inward current by magnesium. This was probably the consequence of incomplete inactivation of the sodium current and the shift of its current-voltage relationship by the divalent cation.