Does exogenous magnesium suppress myocardial irritability and tachyarrhythmias in the nondigitalized patient?

Atrial fibrillation after coronary artery bypass surgery: predictors and the role of MgSO4 replacement

BACKGROUND

Supraventricular arrhythmias continue to complicate the postoperative course of patients undergoing myocardial revascularization. The aim of the study was to identify factors associated with atrial fibrillation (AF) and to determine the efficacy of postoperative magnesium sulphate (MgSO4) replacement on the incidence of AF after coronary artery bypass grafting (CABG) operation.

METHODS

Fifty patients undergoing CABG were studied prospectively. Consenting patients with good left ventricular function and without any documented arrhythmias were randomly divided into two groups of 25 patients each in a double-blind fashion. The clinical characteristics of both groups were similar. In the study group, 200 mEq MgSO4 was given for the first 5 postoperative days, in the control group, placebo was given instead of MgSO4.

RESULTS

Five (20%) patients in the control group and one (4%) patient in the MgSO4 group experienced AF. There was no significant relationship between the development of AF and the following variables: age; sex; diabetes mellitus; hypertension; previous myocardial infarction; smoking; extension of coronary artery disease; aortic cross-clamp time; number of grafts; cardiopulmonary bypass time; postoperative pericarditis; and anemia.

CONCLUSION

The use of MgSO4 in early postoperative period is effective in reducing the incidence of AF after CABG in patients with good ventricular function.

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.

Magnesium and the heart: antiarrhythmic therapy with magnesium

Magnesium is an essential transmembrane and intracellular modulator of the electrical activity of cardiac cells. This review provides an up-to-date consideration of the cellular and clinical electrophysiological role of magnesium. This ubiquitous element seems to be important from both the theoretical and clinical point of view, because magnesium salts (MgSO4, MgCl2) administered intravenously are particularly effective in those arrhythmias in which the mechanism involves early or delayed after depolarization-induced triggered activity. The authors share the view that I.V. magnesium is the drug of choice in "torsade de pointes" ventricular tachycardia accompanying acquired long QT/QTU syndrome. It is complementary therapeutic agent in digitalis-induced tachycardias. Further studies are needed to elucidate magnesium's mode of action and efficacy in other types of clinical tachyarrhythmias.

Clinical and electrophysiologic effects of magnesium sulfate on paroxysmal supraventricular tachycardia and comparison with adenosine triphosphate

Electrophysiologic studies have shown that intravenous magnesium sulfate prolongs atrioventricular (AV) nodal conduction and refractoriness and thus could play a role in the management of patients with paroxysmal AV reentrant supraventricular tachycardia (SVT). The present study evaluates the clinical and electrophysiologic effects of intravenous magnesium sulfate in patients with SVT and compares them with those of adenosine triphosphate (ATP), one of the most potent drugs in the treatment of this arrhythmia. Patients with inducible sustained SVT were treated with ATP (10 or 20 mg) and magnesium sulfate (2 g over 15 seconds) during electrophysiologic study. If the tachycardia failed to terminate by the sixth minute, an additional 2 g dose of magnesium was given. ATP (10 or 20 mg) was significantly better than magnesium for terminating induced tachycardias (14 of 14 vs 6 of 14, p less than 0.0001). Arrhythmia termination with ATP was due to anterograde AV nodal blockade in all but 1 patient who developed retrograde block over an accessory pathway with decremental conduction. Arrhythmia termination by magnesium was due to retrograde block over an accessory pathway in 3 patients (including the patient with accessory pathway exhibiting decremental conduction), anterograde AV nodal conduction block in 2 patients and premature ventricular complexes in 1 patient. During induced tachycardias, only AH intervals were prolonged by ATP, whereas magnesium significantly prolonged AH and QRS intervals. Short-lasting side effects (chest pain, flushing, nausea) occurred after both drugs were administered but were more severe after magnesium.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Intravenous magnesium is reported to be effective in the treatment of ventricular arrhythmias associated with hypomagnesemia, digitalis toxicity, or prolongation of the QT interval. In most previous reports, magnesium was added to conventional antiarrhythmic drugs that had failed. There are few data on the antiarrhythmic efficacy of magnesium as monotherapy in patients without these associated abnormalities. Ten patients with life-threatening ventricular arrhythmia and inducible ventricular tachyarrhythmia by programmed electrophysiologic testing were treated with intravenous magnesium. Following magnesium infusion, all patients still had inducible ventricular tachyarrhythmia. Moreover, magnesium therapy was not associated with significant changes in ventricular refractory period or in the morphology, cycle length, or hemodynamic response to induced ventricular tachycardia. These data suggest that intravenous magnesium has no significant electrophysiologic or antiarrhythmic effects in patients with life-threatening ventricular arrhythmia and inducible ventricular tachyarrhythmia.

Resuscitation from prolonged cardiac arrest with high-dose intravenous magnesium sulfate

We present evidence of resuscitation from prolonged (70-min) cardiac arrest, temporally associated with administration of 8 g intravenous (IV) magnesium sulfate (MgSO4). A patient undergoing liposuction surgery developed bradycardia and a fall in oxygen tension after reversal of general anesthesia with physostigmine. The electrocardiogram (ECG) rhythm degenerated to ventricular asystole, which was refractory to standard therapy, including multiple boluses of epinephrine, atropine, wide-open dopamine, and attempts at right heart pacing. External cardiopulmonary resuscitation (CPR) was continuously maintained with the patient intubated on 100% oxygen. Multiple electric countershocks (x7) and lidocaine were also administered when ventricular tachycardia/ventricular fibrillation (VT/VF) occurred, but without clinical success. Approximately one hour into the resuscitation, after all of the above occurred, 8 g IV MgSO4 was given and countershock repeated. Whereas the 7 previous countershocks had resulted in unsuccessful conversion of VT/VF to a pulseless rhythm (EMD), the 8th countershock (applied immediately after two 4 g boluses of IV MgSO4) resulted in a stable pulse and normal sinus rhythm developing within 4 minutes. The patient recovered without neurologic deficit.

Prophylaxis of atrial fibrillation with magnesium sulfate after coronary artery bypass grafting

Ninety-nine consecutive consenting patients were prospectively entered into a randomized, double-blind, placebo-controlled trial to determine the efficacy of postoperative magnesium therapy on the incidence of cardiac arrhythmias after elective coronary artery bypass grafting. No patient had documented or suspected arrhythmias preoperatively. Forty-nine patients received 178 mEq of magnesium given over the first 4 postoperative days, and 50 patients received only placebo. The clinical characteristics of both groups were similar. The preoperative mean serum magnesium concentration was similar in both study (1.90 mEq/L) and placebo (1.90 mEq/L) groups. The mean postoperative serum magnesium concentration in study patients was significantly elevated over postoperative days 1 through 4 when compared with preoperative levels (p less than 0.001). The postoperative mean serum magnesium concentration in control patients declined and remained significantly depressed through postoperative day 3 (p less than 0.001), but increased to preoperative levels by postoperative day 4. The mean serum magnesium concentration was significantly greater in the study patients as compared with the control patients over postoperative days 1 through 4 (p less than 0.001). Although there was no significant difference between groups with respect to episodes of ventricular arrhythmias, there was a significant decrease in the number of episodes of atrial fibrillation in the group receiving magnesium therapy (p less than 0.02). There were no recognized adverse effects of magnesium therapy. Prophylactic magnesium administration seems to lessen the incidence and severity of atrial fibrillation after coronary artery bypass grafting.

The anesthesiologist's response to a low-output state after cardiopulmonary bypass: etiologies and remedies

Artifacts or mechanical problems may cause data which suggest poor myocardial performance during emergence from cardiopulmonary bypass (CPB). Transducer and monitoring equipment malfunctions, damping of the arterial blood pressure tracing, effects of drugs, hypercarbia, inordinately high intrathoracic pressure, cardiac tamponade, and others are all possible culprits. It is important to have a systematic plan for evaluating and interpreting the signs and data that are evident. Causes of hypotension after CPB include low hematocrit, hypercarbia, sympathetic inhibition, vasodilator action, anaphylaxis or anaphylactoid reactions, protamine reactions, and impaired myocardial performance. Impaired myocardial performance can be attributable to rate and rhythm disturbances, inadequate ventricular preload, inappropriately elevated right and left ventricular afterload, and decreased myocardial contractility. Common causes of hypoxemia include a malfunctioning ventilator system, pulmonary problems such as atelectasis and shunt, anemia, and inordinately high utilization of oxygen.

Mechanical and electrical complications of acute myocardial infarction

Although much of the current enthusiasm in the management of acute myocardial infarction is related to revascularization strategies, mechanical and electrical complications continue to pose a major threat to recovery in some patients. Some of the major complications of acute myocardial infarction are cardiogenic shock, rupture of the free wall and pseudoaneurysm, rupture of the ventricular septum, acute mitral regurgitation, right ventricular myocardial infarction, infarct expansion or extension, pericarditis and tamponade, peri-infarction hypertension, and tachyarrhythmias and bradyarrhythmias. For each of these complications, general guidelines for diagnosis and management are offered. Early, aggressive, and judicious treatment of these complications may substantially decrease the morbidity and mortality associated with acute myocardial infarction.

Depressant effect of magnesium on early afterdepolarizations and triggered activity induced by cesium, quinidine, and 4-aminopyridine in canine cardiac Purkinje fibers

Magnesium chloride has been shown to terminate torsades de pointes in some patients with the acquired long QT syndrome. The mechanism for this effect is unknown. Recently early afterdepolarizations (EADs) and triggered activity (TA) have been proposed as causes of torsades de pointes. The purpose of the present study was to examine whether magnesium suppressed EADs that were initiated in vitro by different agents and if so its mechanism of action. TA arising from EADs was induced by quinidine (1 to 4 mumol/L, n = 5) at high temperature (38.5 to 40 degrees C), cesium chloride (5 to 12 mmol/L, n = 6), and 4-aminopyridine (1.5 to 5 mmol/L, n = 7) in canine cardiac Purkinje fibers superfused with modified Tyrode's solution (KCI = 2.7 mmol/L). MgCl2 (2 to 7 mmol/L) reversibly abolished TA and suppressed EADs. Tetrodotoxin (TTX; 1 to 5 mumol/L) also abolished TA elicited by 4-aminopyridine (n = 6). We then examined the effects of MgCl2, TTX, and verapamil on depolarization-induced automaticity by means of a single sucrose gap technique to gain insight into the mechanism of action of magnesium. MgCl2 (5 mmol/L) abolished automaticity arising from membrane potentials more negative than -70 mV and prolonged the spontaneous cycle length at less negative membrane potentials. The effects of TTX (1 to 5 mumol/L) resembled those of MgCl2. Verapamil (1 mumol/L) prolonged the cycle length of the initial automatic response at high levels of membrane potential and progressively reduced the amplitude of the subsequent automatic potentials. It abolished automaticity arising from less negative membrane potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of the infusion of magnesium sulfate during atrial pacing on ECG intervals, serum electrolytes, and blood pressure

Magnesium salts have been used for decades for the empiric treatment of arrhythmias, particularly torsades de pointes, associated with long QT syndrome. The mechanism underlying this antiarrhythmic effect is still not clear. Therefore the effect of intravenous MgSO4 on serum electrolytes, blood pressure, and ECG variables was evaluated in nine patients with sick sinus syndrome, equipped with a DDD pulse generator, programmed in the atrial asynchronous mode. A total dose of 10 gm MgSO4 was given intravenously over 6 hours at a constant rate. Blood pressure and serum electrolytes were determined before (t0), 3 hours after (t3), and at the end of the magnesium infusion (t6). ECG variables were measured at t0, t3, and t6 at different pacing frequencies (60, 80, and 100 beats/min). Serum magnesium levels rose significantly from 0.88 mmol.l-1 at t0 to 1.91 mmol.l-1 at t6 (p less than 0.05). Magnesium infusion did not affect blood pressure, pulse rate, PR or QRS or QT interval. Increasing the pacing frequency resulted in a statistically significant QT shortening at each serum magnesium level. We conclude that intravenous magnesium administration does not influence the QT interval. Increasing atrial pacing rate shortens the QT interval and this QT shortening is not affected by magnesium. Sustained serum magnesium levels between 1.5 and 2 mmol.l-1 are hemodynamically well tolerated and do not give rise to the development of higher degree atrioventricular block.

Hemodynamic effects of intravenously administered magnesium on patients with ischemic heart disease

Central hemodynamic parameters were registered by right-side heart catheterization before and after intravenous administration of 12 mmol magnesium chloride (MgCl) in 15 patients with chronic ischemic heart disease and heart failure, New York Heart Association classes II and III. Serum magnesium concentrations increased from 0.76 +/- 0.03 (mean +/- SD) to 1.54 +/- 0.05 mmol/l, which resulted in a reduction in mean arterial as well as pulmonary artery pressure by 10% (p less than 0.0001) and 7% (p less than 0.05), respectively. This reduction was caused by a marked decrease in systemic as well as pulmonary vascular resistance (from 1323 +/- 205 to 1132 +/- 158 dyn.s/cm5, p less than 0.001 and from 156 +/- 73 to 133 +/- 72 dyn.s/cm5 (p less than 0.05). Heart rate, cardiac index, stroke volume index, and stroke work index increased slightly, although these differences did not reach statistical significance. Right and left ventricular filling pressures were not influenced, which indicates that the dilatory effect of magnesium, at the dosages used in the present study, is pronounced only at the arterial side of the vascular bed. The observed hemodynamic effects of the magnesium infusions may be beneficial in the setting of an acute myocardial infarction by reducing left ventricular afterload, which, together with the antiarrhythmic effect of magnesium may contribute to the positive effect of magnesium infusions on mortality in patients with acute myocardial infarction.

Calcium, magnesium and phosphate during and after hypothermic cardiopulmonary bypass without temperature correction of acid base status

The plasma concentration variations of calcium, magnesium and phosphate were studied in ten patients during and after hypothermic cardiopulmonary bypass (CPB) without temperature correction of acid base status. During the study, pH remained stable, but all the other studied components varied significantly (P less than 0.001). At the start of CPB, the mean ionized calcium concentration increased 25%, and magnesium and phosphate decreased 29% and 40%, respectively, from their control values. At the end of blood cooling, ionized calcium was still 11% above its initial value, magnesium 50% above, and phosphate 39% below. Before weaning from CPB, ionized calcium remained 10% above its initial level, magnesium 41% above, and phosphate 26% below. After CPB, the different divalent ions returned to their initial levels within 1 h for ionized calcium, 6 h for phosphate and 9 h for magnesium. One day post-CPB, ionized calcium was at its start level, magnesium 13% lower, and phosphate 36% higher. During cardiac surgery, the acid base regulation without temperature correction (so-called "alpha stat mode") avoided the appearance of carbon dioxide acidosis. There were widespread disturbances of the divalent ions concentrations, due principally to the different fluids used during CPB, pump priming fluids and cardioplegic solution.

Myocardial contractility and performance capacity after magnesium infusions in young healthy persons: a double-blind, placebo-controlled, cross-over study

To evaluate the effect of intravenous magnesium (Mg) treatment on the inotropic state of the heart and maximal work capacity, 9 healthy volunteers were entered in a double-blind, placebo-controlled, cross-over study. Separated by an interval of three weeks, the volunteers were tested twice, each time randomly allocated to receive either an intravenous injection of 10 mmol magnesium chloride dissolved in 100 ml isotonic sodium chloride or placebo of isotonic sodium chloride only. Before and after each infusion myocardial inotropism was evaluated by echocardiography. Mitral-septal distance (MSA) was used as a measure for ejection fraction. On each test day an ergometer bicycle exercise test was performed, and maximal work capacity was calculated. Magnesium treatment reduced the MSA (from 4.2 to 2.9 mm, p = 0.07), while no difference was found after placebo treatment. Likewise, a tendency toward increasing fractional shortening after magnesium treatment was detected, although this difference was not statistically significant (p = 0.1). No difference in maximal work capacity between the magnesium and placebo periods was found. Serum magnesium concentrations and placebo periods was found. Serum magnesium concentrations rose significantly after the infusions (from 0.82 to 1.38 mmol/l, p less than 0.001). It is concluded that intravenous magnesium does not exert a negative inotropic effect on the myocardium as previously stated. On the contrary, we found a tendency toward a positive inotropic effect. However, the observed differences are of borderline statistical significance and a more extended study, employing invasive measurements of cardiac inotropism appears to be necessary.