Parenteral magnesium sulfate versus amiodarone in the therapy of atrial tachyarrhythmias: a prospective, randomized study

The therapeutic use of magnesium in anesthesiology, intensive care and emergency medicine: a review

PURPOSE

To review current knowledge concerning the use of magnesium in anesthesiology, intensive care and emergency medicine.

METHODS

References were obtained from Medline(R) (1995 to 2002). All categories of articles (clinical trials, reviews, or meta-analyses) on this topic were selected. The key words used were magnesium, anesthesia, analgesia, emergency medicine, intensive care, surgery, physiology, pharmacology, eclampsia, pheochromocytoma, asthma, and acute myocardial infarction.

PRINCIPLE FINDINGS

Hypomagnesemia is frequent postoperatively and in the intensive care and needs to be detected and corrected to prevent increased morbidity and mortality. Magnesium reduces catecholamine release and thus allows better control of adrenergic response during intubation or pheochromocytoma surgery. It also decreases the frequency of postoperative rhythm disorders in cardiac surgery as well as convulsive seizures in preeclampsia and their recurrence in eclampsia. The use of adjuvant magnesium during perioperative analgesia may be beneficial for its antagonist effects on N-methyl-D-aspartate receptors. The precise role of magnesium in the treatment of asthmatic attacks and myocardial infarction in emergency conditions needs to be determined.

CONCLUSIONS

Magnesium has many known indications in anesthesiology and intensive care, and others have been suggested by recent publications. Because of its interactions with drugs used in anesthesia, anesthesiologists and intensive care specialists need to have a clear understanding of the role of this important cation.

Amiodarone for pharmacological cardioversion of recent-onset atrial fibrillation

The efficacy and safety of amiodarone for pharmacological cardioversion of recent-onset atrial fibrillation was examined by reviewing the trials on the subject identified through a comprehensive literature search. Amiodarone has been used both intravenously (i.v.) and orally for the pharmacological cardioversion of recent-onset atrial fibrillation. Intravenous amiodarone has been used as a bolus only or as a bolus followed by a continuous i.v. infusion until conversion or up to 24 h. The dose of i.v. bolus given ranged from 3 to 7 mg/kg body weight and that of infusion from 900 to 3000 mg/day. The efficacy reported is 34-69% with the bolus only regimens, and 55-95% with the bolus followed by infusion regimens. Only the higher dose (>1500 mg/day) amiodarone is superior to placebo in converting recent-onset atrial fibrillation to sinus rhythm. The highest 24-h conversion rates have been reported with the i.v. regimen of 125 mg/h until conversion or a maximum of 3 g and the oral regimen of 25-30 mg/kg body weight administered as a single loading-dose (>90% and >85%, respectively). Most of the conversions occur after 6-8 h of the initiation of therapy. Predictors of successful conversion are shorter duration of atrial fibrillation, smaller left atrial size, and higher amiodarone dose. Amiodarone is not superior to the other antiarrhythmic drugs conventionally used for the pharmacological cardioversion of recent-onset atrial fibrillation but is relatively safe in patients with structural heart disease and in those with depressed left ventricle function. Therefore, amiodarone could be used particularly in patients with structural heart disease and in those with left ventricular systolic dysfunction as the use of class IC drugs, propafenone and flecainide, for cardioversion of atrial fibrillation is contraindicated in such patients.

Advanced life support drugs: do they really work?

Basic life support and rapid defibrillation for ventricular fibrillation or pulseless ventricular tachycardia are the only two interventions that have been shown unequivocally to improve survival after cardiac arrest. Several drugs are advocated to treat cardiac arrest, but despite very encouraging animal data, no drug has been reliably proven to increase survival to hospital discharge after cardiac arrest. This review focuses on recent experimental and clinical data concerning the use of vasopressin, amiodarone, magnesium, and fibrinolytics during advanced life support (ALS). Animal data indicate that, in comparison with epinephrine (adrenaline), vasopressin produces better vital organ blood flow during cardiopulmonary resuscitation (CPR). These apparent advantages have yet to be converted into improved survival in large-scale trials of cardiac arrest in humans. Data from two prospective, randomized trials suggest that amiodarone may improve short-term survival after out-of-hospital ventricular fibrillation cardiac arrest. On the basis of anecdotal data, magnesium is recommended therapy for torsades de pointes and for shock-resistant ventricular fibrillation associated with hypomagnesemia. In the past, CPR has been a contraindication to giving fibrinolytics, but several studies have demonstrated the relative safety of fibrinolysis during and after CPR. Fibrinolytics are likely to be beneficial when cardiac arrest is associated with plaque rupture and fresh coronary thrombus or massive pulmonary embolism. Fibrinolysis may also improve cerebral microcirculatory perfusion once a spontaneous circulation has been restored. A planned, prospective, randomized trial may help to define the role of fibrinolysis during out-of-hospital CPR.

Magnesium as part of balanced general anaesthesia with propofol, remifentanil and mivacurium: a double-blind, randomized prospective study in 50 patients

BACKGROUND AND OBJECTIVE

To test the hypothesis that magnesium sulphate reduces the amount of remifentanil needed for general anaesthesia in combination with propofol and mivacurium, we studied 50 patients undergoing elective pars plana vitrectomy in a double-blind, randomized prospective fashion.

METHODS

Magnesium sulphate (50 mg kg(-1) body weight) or placebo (equal volume of NaCl) was given slowly intravenously after induction of anaesthesia with propofol 1-2 mg kg(-1). Anaesthesia was maintained with propofol (using electroencephalographic control), mivacurium (according to train-of-four monitoring of neuromuscular blockade) and remifentanil (according to heart rate and arterial pressure).

RESULTS

We observed a significant reduction in remifentanil consumption from 0.14 to 0.09 microg kg(-1) min(-1) (P < 0.01). Mivacurium consumption was also markedly reduced from 0.01 to 0.008 mg kg(-1) min(-1) (P < 0.01), whereas propofol consumption remained unchanged. There was a trend towards lower postoperative pain scores, less pain medication requirements in 24 h after surgery and less postoperative nausea and vomiting in the magnesium group but not statistically significant. No side-effects were observed.

CONCLUSION

We can recommend the use of magnesium sulphate as a safe and cost-effective supplement to a general anaesthetic regimen with propofol, remifentanil and mivacurium, although we cannot clearly distinguish between a mechanism as a (co)analgesic agent at the NMDA-receptor site or its properties as a sympatholytic. The effect of a single bolus dose of 50 mg kg(-1) on induction lasts for about 2 h. For longer cases, either a continuous infusion or repeated bolus doses might be necessary.

Weathering the storm: beta-blockade and the potential for disaster in severe hyperthyroidism

Some patients with advanced thyrotoxicosis have occult cardiac dysfunction. The use of long-acting beta-blockers, traditional in the management of thyrotoxicosis, may have disastrous consequences in this setting. The following report documents the cardiovascular collapse and asystolic arrest of a patient with hyperthyroidism when treated with sotolol. Though the patient was successfully resuscitated, the long duration of action of sotolol necessitated prolonged inotropic and vasopressor support. A shorter acting beta-blocker, such as esmolol, theoretically may be a safer option.

Pharmacologic conversion of atrial fibrillation: a systematic review of available evidence

This report reviews the efficacy of currently available antiarrhythmic agents for conversion of atrial fibrilation (AF) to normal sinus rhythm (NSR). A systematic search of literature in the English language was done on computerized databases, such as MEDLINE, EMBASE, and Current Contents, in reference lists, by manual searching, and in contact with expert informants. Published studies involving humans that described the use of antiarrhythmic therapy for conversion of AF to NSR were considered and only studies that examined the use of agents currently available in the United States were included. Studies exclusively describing antiarrhythmic therapy for conversion of postsurgical AF were excluded. The methodology and results of each trial were assessed and attempts were made to acquire additional information from investigators when needed. Assessment of methodological quality was incorporated into a levels-of-evidence scheme. Eighty-eight trials were included, of which 34 (39%) included a placebo group (level I data). We found in recent-onset AF of less than 7 days, intravenous (i.v.) procainamide, high-dose i.v. or high-dose combination i.v. and oral amiodarone, oral quinidine, oral flecainide, oral propafenone, and high-dose oral amiodarone are more effective than placebo for converting AF to NSR. In recent-onset AF of less than 90 days, i.v. ibutilide is more effective than placebo and i.v. procainamide. In chronic AF, oral dofetilide converts AF to NSR within 72 hours, and oral propafenone and amiodarone are effective after 30 days of therapy. We conclude than for conversion of recent-onset AF of less than 7 days, procainamide may be considered a preferred i.v. agent and propafenone a preferred oral agent. For conversion of recent-onset AF of longer duration (less than 90 days), i.v. ibutilide may be considered a preferred agent. For patients with chronic AF and left ventricular dysfunction, direct current cardioversion is the preferred conversion method. Larger, well-designed randomized controlled trials with clinically important endpoints in specific populations of AF patients are needed.

Intravenous magnesium sulfate versus diltiazem in paroxysmal atrial fibrillation

BACKGROUND

Drugs currently available for the acute treatment of paroxysmal atrial fibrillation have significant limitations. We assessed the safety and effectiveness of intravenous magnesium sulfate versus diltiazem therapy in patients with prolonged episodes of paroxysmal atrial fibrillation.

METHODS

In a prospective randomized trial, 46 symptomatic patients presenting with paroxysmal atrial fibrillation were given intravenous magnesium sulfate (n=23) or diltiazem (n=23) therapy. Primary outcome measures were effects on ventricular rate control and proportion of patients restored to sinus rhythm at 6 h after initiation of treatment.

RESULTS

There were no differences in baseline characteristics between the two groups. Both forms of treatment were well tolerated, with no adverse clinical events. Both drugs had similar efficacy in reducing the ventricular rate at the first hour of treatment (P<0.05) with a tendency toward a further decrease during infusion times of 2 (P<0.01), 3, 4, 5 and 6 h, respectively (P<0.001). However, at the end of the 6-h treatment period, restoration of sinus rhythm was observed in a significantly higher proportion of patients in the magnesium group compared with the diltiazem group [13 of 23 patients, (57%), versus five of 23 patients, (22%), P=0.03].

CONCLUSIONS

Magnesium sulfate favorably affects rate control and seems to promote the conversion of long lasting episodes of paroxysmal atrial fibrillation to sinus rhythm, representing a safe, reliable and cost-effective alternative treatment strategy to diltiazem.

Hypomagnesemic disorders

Adequate magnesium stores are vitally important for life. Critically ill patients will almost always have diminished levels of circulating magnesium, and this predisposes them to a variety of adverse effects, some life threatening. The causes of hypomagnesemia are many and varied, but in the critically ill, losses from the kidneys, often secondary to medications and from the gastrointestinal (GI) tract, predominate. The measurement of magnesium is not straightforward, although many clinicians are now switching to the use of ionized magnesium from ion selective electrodes. The use of supplemental magnesium in acute flares of asthma has some support in medical literature, especially for those patients with severe disease who fail traditional therapy. Magnesium holds the preeminent position in the treatment of pre-eclampsia and eclampsia in the minds of most obstetricians, who have decades of experience showing it to be both effective and safe. Magnesium is clearly useful for certain types of ventricular tachycardia, and probably assists in the treatment of several types of supraventricular tachycardia. Its role in acute myocardial ischemia is less certain, although there is no benefit once reperfusion therapy has already been carried out. Finally, the role of magnesium in the treatment of acute cerebral insults is an exciting area of active investigation with initial studies suggesting much promise.

The Management of Atrial Fibrillation in the ICU

Atrial tachyarrhythmias are the most frequent arrhythmias occurring in ICU patients, being particularly common in patients with cardiovascular and respiratory failure. Unlike ambulatory patients in whom atrial fibrillation/flutter (AF) is likely to be short lived, in the critically ill these arrhythmias are unlikely to resolve until the underlying disease process has improved. Urgent cardioversion is indicated for hemodynamic instability. Treatment in hemodynamically stable patients includes correction of treatable precipitating factors, control of the ventricular response rate, conversion to sinus rhythm, and prophylaxis against thromboembolic events in those patients who remain in AF. Diltiazem is the preferred agent for rate control, while procainamide and amiodarone are generally considered to be the antiarrhythmic agents of choice.

Effect of intravenous magnesium on heart rate and heart rate variability in patients with chronic atrial fibrillation

The present double-blind, placebo-controlled study investigated the effects of intravenous magnesium on heart rate and rate variability in 30 patients with chronic atrial fibrillation. During standardized conditions, intraindividual variation in heart rate and rate variability was low in patients with chronic atrial fibrillation and magnesium had no effect on heart rate or rate variability.

[Clinico-electrophysiologic effects of magnesium, especially in supraventricular tachycardia]

Clinical electrophysiological effects of magnesium (Mg2+) are known for more than 60 years. Mg2+ is a cation to be found ubiquitously in the human body and is involved in more than 300 different enzymatic reactions. However, so far this ion has not been established as a standard therapeutic tool for the treatment of supraventricular tachyarrhythmia. This may be explained by the inconsistent efficacy of Mg2+, partly in relationship to a given plasma Mg(2+)-concentration, partly caused by the uncertainty regarding the dosage and injection rate or the unawareness of the clinical effects of the cation. Mg2+ influences myocardial metabolism by its effects on contractility and electrical activity. Both effects are closely linked. About 12% of cardiac Mg2+ is found in the mitochondria and 2 to 3% in the myofibrils. A large portion is incorporated in adenosin mono-, di- and triphosphate. Mg2+ affects intracellular calcium by inhibiting the influx of calcium into the myocyte through sarcolemmal channels, by modulation of cyclic AMP and by competing with calcium for binding to a single high affinity site on actin. Mg2+ has been linked to a naturally occurring calcium channel blocker. Furthermore Mg2+ blocks the outward current through some potassium channels resulting in an inward rectification of these channels. This suggests that internal magnesium functions as a potassium channel-blocking agent. Early afterdepolarizations are oscillations in the membrane potential and lead to triggered activity and therefore are the electrophysiological substrate of "torsade de pointes" type of ventricular flutter. Mg2+ is able to inhibit both early afterdepolarizations and tachyarrhythmias. Additionally Mg2+ interferes with the sodium-potassium-ATPase system by stabilizing the transmembrane gradient of both cations. Mg2+ deficiency alters this balance and leads to increased neuromuscular excitability. Digitalis is able to block the sodium-potassium-ATPase system, which can be cancelled by Mg2+. Thus the first clinical reports of the therapeutic use of Mg2+ refer to digitalis-induced atrial arrhythmia and ventricular ectopy which could be converted to sinus-rhythm or suppressed by the intravenous application of Mg2+ in 1935. Some years later, the first successful termination of paroxysmal supraventricular and ventricular tachycardia following application of 1.5 to 3 g of Mg2+ was published. But only in the late eighties, systematic studies of the electrophysiological effects of Mg2+ were performed and clinical use was first tested in random fashion in the nineties. Summarizing studies in older patients with different heart diseases and young healthy volunteers the most pronounced and clinically important effect seems to be related to the modulation of the AV node function. The prolongation of the PR interval by 7 to 12% without changing significantly heart rate, QRS duration and QT duration, can be considered a consistent and reproducible effect of Mg2+. In electrophysiological studies a prolongation of the AH interval by 8 to 18%, of the Wenckebach cycle length by up to 20% and of the refractory period of the AV node by 6 to 20% is usually observed, but no change of the retrograde conduction, or the HV interval can be found. Furthermore sinus node recovery time increases by 10% and sinuatrial conduction time by up to 25%. There is no significant effect on intraventricular conduction and atrial and ventricular refractory period. Additionally no significant effect on the anterograde and retrograde refractory period of accessory pathways could be measured; however in some cases (up to 40%) an anterograde block in the accessory pathway may be observed after intravenous Mg(2+)-injection. For the treatment of paroxysmal atrioventricular tachycardia like AV-nodal reentrant tachycardia or orthodromic atrioventricular reentrant tachycardia in WPW syndrome, Mg2+ has been applied in a limited number of recent prospective but uncontrolled studies. Recently, an