Magnesium and the heart: antiarrhythmic therapy with magnesium

Nitric oxide, endothelium-derived hyperpolarizing factor, and smooth muscle-dependent mechanisms contribute to magnesium-dependent vascular relaxation in mouse arteries

AIM

Magnesium (Mg2+ ) is a vasorelaxant. The underlying physiological mechanisms driving this vasorelaxation remain unclear. Studies were designed to test the hypothesis that multiple signaling pathways including nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) in endothelial cells as well as Ca2+ antagonization and TRPM7 channels in vascular smooth muscle cells mediate Mg2+ -dependent vessel relaxation.

METHODS

To uncover these mechanisms, force development was measured ex vivo in aorta rings from mice using isometric wire myography. Concentration responses to Mg2+ were studied in intact and endothelium-denuded aortas. Key findings were confirmed in second-order mesenteric resistance arteries perfused ex vivo using pressure myography. Effects of Mg2+ on NO formation were measured in Chinese Hamster Ovary (CHO) cells, isolated mesenteric vessels, and mouse urine.

RESULTS

Mg2+ caused a significant concentration-dependent relaxation of aorta rings. This relaxation was attenuated significantly in endothelium-denuded aortas. The endothelium-dependent portion was inhibited by NO and cGMP blockade but not by cyclooxygenase inhibition. Mg2+ stimulated local NO formation in CHO cells and isolated mesenteric vessels without changing urinary NOx levels. High extracellular Mg2+ augmented acetylcholine-induced relaxation. SKCa and IKCa channel blockers apamin and TRAM34 inhibited Mg2+ -dependent relaxation. The endothelium-independent relaxation in aorta rings was inhibited by high extracellular Ca2+ . Combined blockade of NO, SKCa , and IKCa channels significantly reduced Mg2+ -dependent dilatation in mesenteric resistance vessels.

CONCLUSIONS

In mouse conductance and resistance arteries Mg2+ -induced relaxation is contributed by endothelial NO formation, EDHF pathways, antagonism of Ca2+ in smooth muscle cells, and additional unidentified mechanisms.

Circulating magnesium as a potential risk stratification tool for sudden cardiac death: a systematic review

BACKGROUND

Sudden cardiac death (SCD) is one of the main causes of cardiovascular mortality and accounts for 15-20% of deaths worldwide. The current stratification strategy using depressed left ventricular ejection fraction is insufficient to stratify the risk of SCD, especially in the general population. In recent years, there has been increasing evidence showing the antiarrhythmic properties of magnesium. In this systematic review, the authors aim to determine circulating magnesium as a potential risk stratification tool for SCD.

METHODS

This systematic review was based on Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and was conducted in July 2021 with sources from Google Scholar, PubMed, Science Direct, EBSCO Medline, and ProQuest.

RESULTS

A total of six studies were included in this review. Three studies conducted in the general population consistently showed lower risk of SCD in populations with high circulating magnesium. There was no association between circulating magnesium level and risk of SCD in intensive cardiac care unit (ICCU) patients, whilst the results were conflicting in congestive heart failure (CHF) patients.

CONCLUSION

High circulating magnesium might have the potential to be utilized as a risk stratification tool for SCD, especially in the general population. However, further study is needed to support this evidence.

Refractory Torsades de Pointes Due to Dofetilide Overdose

Dofetilide, a class III antiarrhythmic, is widely used in the treatment of cardiac arrhythmias. Antiarrhythmic drugs can have a long duration of action that prolongs the QT interval. This causes bradycardia that predisposes to R-on-T phenomenon subsequently leading to torsades de pointes (TdP). This necessitates constant monitoring to prevent or treat ventricular arrhythmias or bradycardia associated with cardiac medications. Although extremely rare, dofetilide overdose has been described in the literature. However, no evidence found in the current literature required prolonged intervention after the initial acute stabilization, leading to scarcity of data for treatment of ongoing dofetilide overdose. We present the case of an intentional dofetilide overdose in a 61-year-old Caucasian woman with a history of congestive heart failure, atrial fibrillation, stage IIIb chronic kidney disease, diabetes mellitus type II, hypothyroidism, morbid obesity, and hypertension that required extensive interventions for refractory TdP that lasted 4 days. Therapeutic as well as excess dosage of dofetilide can lead to TdP, which is usually controlled by decreasing the dose or terminating drug administration. If the arrhythmia is not resolved, guidelines recommend management with activated charcoal if ingestion is within 15 minutes, followed by administration of 2 g IV (intravenous) magnesium and addressing the electrolyte imbalance. However, if the arrhythmia is persistent due to ongoing dofetilide toxicity, isoproterenol is given as a bridge to overdrive pacing and dopamine is used as an alternative to isoproterenol.

Mg2+ restriction downregulates NCC through NEDD4-2 and prevents its activation by hypokalemia

Hypomagnesemia is associated with reduced kidney function and life-threatening complications and sustains hypokalemia. The distal convoluted tubule (DCT) determines final urinary Mg²⁺ excretion and, via activity of the Na⁺-Cl^- cotransporter (NCC), also plays a key role in K⁺ homeostasis by metering Na⁺ delivery to distal segments. Little is known about the mechanisms by which plasma Mg²⁺ concentration regulates NCC activity and how low-plasma Mg²⁺ concentration and K⁺ concentration interact to modulate NCC activity. To address this, we performed dietary manipulation studies in mice. Compared with normal diet, abundances of total NCC and phosphorylated NCC (pNCC) were lower after short-term (3 days) or long-term (14 days) dietary Mg²⁺ restriction. Altered NCC activation is unlikely to play a role, since we also observed lower total NCC abundance in mice lacking the two NCC-activating kinases, STE20/SPS-1-related proline/alanine-rich kinase and oxidative stress response kinase-1, after Mg²⁺ restriction. The E3 ubiquitin-protein ligase NEDD4-2 regulates NCC abundance during dietary NaCl loading or K⁺ restriction. Mg²⁺ restriction did not lower total NCC abundance in inducible nephron-specific neuronal precursor cell developmentally downregulated 4-2 (NEDD4-2) knockout mice. Total NCC and pNCC abundances were similar after short-term Mg²⁺ or combined Mg²⁺-K⁺ restriction but were dramatically lower compared with a low-K⁺ diet. Therefore, sustained NCC downregulation may serve a mechanism that enhances distal Na⁺ delivery during states of hypomagnesemia, maintaining hypokalemia. Similar results were obtained with long-term Mg²⁺-K⁺ restriction, but, surprisingly, NCC was not activated after long-term K⁺ restriction despite lower plasma K⁺ concentration, suggesting significant differences in distal tubule adaptation to acute or chronic K⁺ restriction.

Hypokalemia-Induced Arrhythmias and Heart Failure: New Insights and Implications for Therapy

Routine use of diuretics and neurohumoral activation make hypokalemia (serum K⁺ < 3. 5 mM) a prevalent electrolyte disorder among heart failure patients, contributing to the increased risk of ventricular arrhythmias and sudden cardiac death in heart failure. Recent experimental studies have suggested that hypokalemia-induced arrhythmias are initiated by the reduced activity of the Na⁺/K⁺-ATPase (NKA), subsequently leading to Ca²⁺ overload, Ca²⁺/Calmodulin-dependent kinase II (CaMKII) activation, and development of afterdepolarizations. In this article, we review the current mechanistic evidence of hypokalemia-induced triggered arrhythmias and discuss how molecular changes in heart failure might lower the threshold for these arrhythmias. Finally, we discuss how recent insights into hypokalemia-induced arrhythmias could have potential implications for future antiarrhythmic treatment strategies.

Dynamic monitoring of transmembrane potential changes: a study of ion channels using an electrical double layer-gated FET biosensor

In this research, we have designed, fabricated and characterized an electrical double layer (EDL)-gated AlGaN/GaN high electron mobility transistor (HEMT) biosensor array to study the transmembrane potential changes of cells. The sensor array platform is designed to detect and count circulating tumor cells (CTCs) of colorectal cancer (CRC) and investigate cellular bioelectric signals. Using the EDL FET biosensor platform, cellular responses can be studied in physiological salt concentrations, thereby eliminating complex automation. Upon investigation, we discovered that our sensor response follows the transmembrane potential changes of captured cells. Our whole cell sensor platform can be used to monitor the dynamic changes in the membrane potential of cells. The effects of continuously changing electrolyte ion concentrations and ion channel blocking using cadmium are investigated. This methodology has the potential to be used as an electrophysiological probe for studying ion channel gating and the interaction of biomolecules in cells. The sensor can also be a point-of-care diagnostic tool for rapid screening of diseases.

Does Intravenous Magnesium Benefit Patients of Cardiac Arrest? a Meta-Analysis

Objective

Evidence shows that magnesium has anti-arrhythmic roles. However, clinical trials suggest that there are conflicting outcomes of cardiac arrest patients given magnesium. The study aimed to examine the effects of magnesium sulfate therapy on cardiac arrest patients using a meta-analysis.

Methods

We searched the MEDLINE (1966-July 2011), OVID (1980-July 2011), EMBASE (1980-July 2011), Chinese bio-medical literature & retrieval system (1978-July 2011), Chinese medical current contents (1995-July 2011), and Chinese medical academic conference (1994-July 2011). Studies were included if (1) the study design was a randomised controlled trial (RCT); (2) the study population included cardiac arrest patients who received cardiopulmonary resuscitation and either intravenous magnesium or placebo (0.9% normal saline); (3) the study provided data on return of spontaneous circulation (ROSC) and survival to hospital discharge. Relative risk (RR) and 95% confidence interval (CI) were used to pool the effect.

Results

The study included six RCTs with a total of 531 participants from cardiac arrest. The pooled data demonstrated that ROSC and hospital discharge rates in patients treated with magnesium were 35.5% (95% CI=29.7%-41.3%) and 10.4% (95% CI=6.7%-14.1%) compared with placebo (ROSC rate: 31.3%, 95% CI=25.8%-36.8%; hospital discharge rate: 8.5%, 95% CI=5.2%-11.8%). Administration of intravenous magnesium sulfate could not significantly increase ROSC (RR=1.14, 95% CI=0.91-1.43, P=0.26) and survival to hospital discharge (RR=1.22, 95% CI=0.74-2.02, P=0.43) compared with placebo.

Conclusions

Administration of magnesium sulfate does not improve ROSC and survival to hospital discharge in patients of cardiac arrest.

Basic Cardiac Electrophysiology and Common Drug-induced Arrhythmias

Drugs can be a double-edged sword, providing the benefit of symptom alleviation and disease modification but potentially causing harm from adverse cardiac arrhythmic events. Proarrhythmia is the ability of a drug to cause an arrhythmia, the number one reason for drugs to be withdrawn from the patient. Drug-induced arrhythmias are defined as the production of de novo arrhythmias or aggravation of existing arrhythmias, as a result of previous or concomitant pharmacologic treatment. This review summarizes normal cardiac cell and tissue functioning and provides an overview of drugs that effect cardiac repolarization and the adverse effects of commonly administered antiarrhythmics.

Altered biometal homeostasis is associated with CLN6 mRNA loss in mouse neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses, the most common fatal childhood neurodegenerative illnesses, share many features with more prevalent neurodegenerative diseases. Neuronal ceroid lipofuscinoses are caused by mutations in CLN genes. CLN6 encodes a transmembrane endoplasmic reticulum protein with no known function. We characterized the behavioural phenotype of spontaneous mutant mice modeling CLN6 disease, and demonstrate progressive motor and visual decline and reduced lifespan in these mice, consistent with symptoms observed in neuronal ceroid lipofuscinosis patients. Alterations to biometal homeostasis are known to play a critical role in pathology in Alzheimer's, Parkinson's, Huntington's and motor neuron diseases. We have previously shown accumulation of the biometals, zinc, copper, manganese and cobalt, in CLN6 Merino and South Hampshire sheep at the age of symptom onset. Here we determine the physiological and disease-associated expression of CLN6, demonstrating regional CLN6 transcript loss, and concurrent accumulation of the same biometals in the CNS and the heart of presymptomatic CLN6 mice. Furthermore, increased expression of the ER/Golgi-localized cation transporter protein, Zip7, was detected in cerebellar Purkinje cells and whole brain fractions. Purkinje cells not only control motor function, an early symptomatic change in the CLN6 mice, but also display prominent neuropathological changes in mouse models and patients with different forms of neuronal ceroid lipofuscinoses. Whole brain fractionation analysis revealed biometal accumulation in fractions expressing markers for ER, Golgi, endosomes and lysosomes of CLN6 brains. These data are consistent with a link between CLN6 expression and biometal homeostasis in CLN6 disease, and provide further support for altered cation transporter regulation as a key factor in neurodegeneration.

Cardiac effects of magnesium sulfate pretreatment on acute dichlorvos-induced organophosphate poisoning: an experimental study in rats

Although atropine and oximes are traditionally used in the management of organophosphate poisoning, investigations have been directed to finding additional therapeutic approaches. Thus, the aim of this study was to evaluate the cardiac effects of magnesium sulfate pretreatment on dichlorvos intoxication in rats. Rats were randomly divided into three groups as control, dichlorvos, and magnesium sulfate groups. After 6 h of dichlorvos or corn oil (as a vehicle) injection, venous blood samples were collected, and cardiac tissue samples were obtained. Biochemical analyses were performed to measure some parameters on serum and cardiac tissue. Immunohistochemical analyses of apoptosis and inducible nitric oxide (NO) synthase showed no change in cardiac tissue. Serum cholinesterase levels were markedly depressed with dichlorvos, and further suppressed markedly with magnesium sulfate pretreatment. Although we have demonstrated that serum NO levels in dichlorvos and magnesium sulfate groups were lower than the control group, cardiac tissue NO levels in magnesium sulfate group were higher than the other two groups. Mortality was not significantly affected with magnesium sulfate pretreatment. Uncertainty still persists on the right strategies for the treatment of organophosphate acute poisoning; however, it was concluded that our results do not suggest that magnesium sulfate therapy is beneficial in the management of acute dichlorvos-induced organophosphate poisoning, and also further studies are required.

Pharmacological management of atrial fibrillation following cardiac surgery

Atrial fibrillation (AF) is the most common complication following coronary artery bypass graft surgery (CABG). Post-CABG AF occurs most commonly on the second postoperative day and declines in incidence thereafter. A number of risk factors have been found to be associated with a higher frequency of post-CABG AF. These risk factors include advanced age, a prior history of AF, hypertension, and heart failure. Postoperative complications--including low cardiac output, use of an intra-aortic balloon pump, pneumonia, and prolonged mechanical ventilation--are also associated with higher rates of post-CABG AF. Post-CABG AF increases the risk of stroke, and the length and cost of hospitalization. Prophylactic administration of conventional beta-adrenoceptor antagonists (beta-blockers) or sotalol produces a consistent and significant reduction in the incidence of post-CABG AF; however, results with prophylactic amiodarone or magnesium are less consistent. Termination of post-CABG AF, once it occurs, can be accomplished with a number of antiarrhythmic agents. Ibutilide has been the most widely studied agent for this indication. Sotalol is not indicated for cardioversion of AF and has not been studied in the post-CABG setting. Electrical cardioversion and biatrial pacing have also been used to terminate post-CABG AF. Ventricular rate is best controlled with beta-blockers and calcium channel antagonists. Esmolol has a rapid onset of action and is easily titrated to effect. Digoxin can control the ventricular rate, but has a slow onset of action. There are limited data available to guide decisions regarding the optimal management of post-CABG AF.

Magnesium prophylaxis for arrhythmias after cardiac surgery: a meta-analysis of randomized controlled trials

BACKGROUND

Magnesium supplementation may reduce the incidence of arrhythmias, which often occur after cardiac surgery; however, recent findings of the effectiveness of magnesium prophylaxis have yielded discrepant results.

METHODS

We searched electronic databases for randomized controlled trials of magnesium for the prevention of arrhythmias after cardiac surgery. The primary outcomes comprised the incidence of supraventricular and ventricular arrhythmias, and the secondary outcomes comprised serum magnesium concentration, length of hospital stay, myocardial infarction, and mortality. Effect sizes were estimated using a random-effects model.

RESULTS

Seventeen trials (n=2069 patients) met the inclusion criteria. Pooled serum magnesium concentration at 24 hours after surgery in the treatment group was significantly higher than that in the control group (weighted mean difference=0.45 mmol/L [1.1 mg/dL]; 95% confidence interval [CI]: 0.30 to 0.59 mmol/L [0.7 to 1.4 mg/dL]; P <0.001). Magnesium supplementation reduced the risk of supraventricular arrhythmias (relative risk [RR]=0.77; 95% CI: 0.63 to 0.93; P=0.002) and ventricular arrhythmias (RR = 0.52; 95% CI: 0.31 to 0.87; P <0.0001), but had no effect on the length of hospital stay (weighted mean difference=-0.28 days; 95% CI: -0.70 to 1.27 days; P=0.48), the incidence of perioperative myocardial infarction (RR=1.03; 95% CI: 0.52 to 2.05; P = 0.99), or mortality (RR=0.97; 95% CI: 0.43 to 2.20; P=0.94).

CONCLUSION

Administration of prophylactic magnesium reduced the risk of supraventricular arrhythmias after cardiac surgery by 23% (atrial fibrillation by 29%) and of ventricular arrhythmias by 48%. Supplementation had no notable benefit with respect to length of hospitalization, incidence of myocardial infarction, or mortality.

Novel therapeutics for treatment of long-QT syndrome and torsade de pointes

Long-QT syndrome is a clinically and genetically heterogeneous syndrome characterized by lengthening of the QT interval and increased dispersion of the ventricular repolarization on surface electrocardiogram and a propensity to malignant ventricular arrhythmias, torsade de pointes and ventricular fibrillation, which may lead to sudden cardiac death. Long-QT syndrome mostly affects adolescents and young adults with structurally and functionally normal hearts and is caused by aberrations in potassium and sodium ion channels. Standard therapies for long-QT syndrome include correction of the underlying cause, alleviation of the precipitating factors, magnesium sulfate, isoproterenol, antiadrenergic therapy (beta-adrenergic receptor blockers, left cervicothoracic sympathectomy), cardiac pacing, and implantable cardioverter defibrillator. The potential therapies include sodium channel blockers (mexiletine, flecainide, lidocaine, pentisomide, phenytoin), potassium, potassium channel activators (nicorandil, pinacidil, cromakalim), alpha-adrenergic receptor blockers, calcium channel blockers, atropine, and protein kinase inhibitors. The purpose of this review is to outline the established therapies and update the recent advances and potential future strategies in the treatment of long-QT syndrome and torsade de pointes.

Cytosolic Ca2+ triggers early afterdepolarizations and Torsade de Pointes in rabbit hearts with type 2 long QT syndrome

The role of intracellular Ca2+ (Ca2+i) in triggering early afterdepolarizations (EADs), the origins of EADs and the mechanisms underlying Torsade de Pointes (TdP) were investigated in a model of long QT syndrome (Type 2). Perfused rabbit hearts were stained with RH327 and Rhod-2/AM to simultaneously map membrane potential (V(m)) and Ca2+i with two photodiode arrays. The I(Kr) blocker E4031 (0.5 microM) together with 50 % reduction of [K+]o and [Mg2+]o elicited long action potentials (APs), V(m) oscillations on AP plateaux (EADs) then ventricular tachycardia (VT). Cryoablation of both ventricular chambers eliminated Purkinje fibres as sources of EADs. E4031 prolonged APs (0.28 to 2.3 s), reversed repolarization sequences (baseapex) and enhanced repolarization gradients (30 to 230 ms, n = 12) indicating a heterogeneous distribution of I(Kr). At low [K+]o and [Mg2+]o, E4031 elicited spontaneous Ca2+iand V(m) spikes or EADs (3.5 +/- 1.9 Hz) during the AP plateau (n = 6). EADs fired 'out-of-phase' from several sites, propagated, collided then evolved to TdP. Phase maps (Ca2+ivs. V(m)) had counterclockwise trajectories shaped like a 'boomerang' during an AP and like ellipses during EADs, with V(m) preceding Ca2+iby 9.2 +/- 1.4 (n = 6) and 7.2 +/- 0.6 ms (n = 5/6), respectively. After cryoablation, EADs from surviving epicardium (~1 mm) fired at the same frequency (3.4 +/- 0.35 Hz, n = 6) as controls. At the origins of EADs, Ca2+ipreceded V(m) and phase maps traced clockwise ellipses. Away from EAD origins, V(m) coincided with or preceded Ca2+i. In conclusion, overload elicits EADs originating from either ventricular or Purkinje fibres and 'out-of-phase' EAD activity from multiple sites generates TdP, evident in pseudo-ECGs.

Hormone-stimulated Mg(2+) accumulation into rat hepatocytes: a pathway for rapid Mg(2+) and Ca(2+) redistribution

Many diseases such as cardiac arrhythmia, diabetes, and chronic alcoholism are associated with a marked decrease of plasma and parenchymal Mg(2+), and Mg(2+) administration is routinely used therapeutically. This study uses isolated rat hepatocytes to ascertain if and under which conditions increases in extracellular Mg(2+) result in an increase in intracellular Mg(2+). In the absence of stimulation, changing extracellular Mg(2+) had no effect on total cellular Mg(2+) content. By contrast, carbachol or vasopressin administration promoted an accumulation of Mg(2+) that increased cellular Mg(2+) content by 13.2 and 11.8%, respectively, and stimulated Mg(2+) uptake was unaffected by the absence of extracellular Ca(2+). Mg(2+) efflux resulting from stimulation of alpha- or beta-adrenergic receptors operated with a Mg(2+):Ca(2+) exchange ratio of 1. These data indicate that cellular Mg(2+) uptake can occur rapidly and in large amounts, through a process distinct from Mg(2+) release, but operating only upon specific hormonal stimulation.

A randomised trial to investigate the efficacy of magnesium sulphate for refractory ventricular fibrillation

BACKGROUND

Ventricular fibrillation (VF) remains the most salvageable rhythm in patients suffering a cardiopulmonary arrest (CA). However, outcome remains poor if there is no response to initial defibrillation. Some evidence suggests that intravenous magnesium may prove to be an effective antiarrhythmic agent in such circumstances.

STUDY HYPOTHESIS

Intravenous magnesium sulphate given early in the resuscitation phase for patients in refractory VF (VF after 3 DC shocks) or recurring VF will significantly improve their outcome, defined as a return of spontaneous circulation (ROSC) and discharge from hospital alive.

DESIGN

A randomised, double blind, placebo controlled trial. Pre-defined primary and secondary endpoints were ROSC at the scene or in accident and emergency (A&E) and discharge from hospital alive respectively. SETTING, PARTICIPANTS, AND INTERVENTION: Patients in CA with refractory or recurrent VF treated in the prehospital phase by the county emergency medical services and/or in the A&E department. One hundred and five patients with refractory VF were recruited over a 15 month period and randomised to receive either 2-4 g of magnesium sulphate or placebo intravenously.

RESULTS

Fifty two patients received magnesium treatment and 53 received placebo. The two groups were matched for most parameters including sex, response time for arrival at scene and airway interventions. There were no significant differences between magnesium and placebo for ROSC at the scene or A&E (17% v 13%). The 4% difference had 95% confidence intervals (CI) ranging from -10% to +18%. For patients being alive to discharge from hospital (4% v 2%) the difference was 2% (95% CI -7% to +11%). After adjustment for potential confounding variables (age, witnessed arrest, bystander cardiopulmonary resuscitation and system response time), the odds ratio (95% CI) for ROSC in patients treated with magnesium as compared with placebo was 1.69 (0.54 to 5.30).

CONCLUSION

Intravenous magnesium given early in patients suffering CA with refractory or recurrent VF did not significantly improve the proportion with a ROSC or who were discharged from hospital alive.

Intravenous magnesium sulfate for acute termination of sustained monomorphic ventricular tachycardia associated with coronary artery disease

The efficacy of intravenous magnesium in terminating sustained monomorphic ventricular tachycardia was examined in this study. This therapy was found to be ineffective in aborting monomorphic ventricular tachycardia induced in the electrophysiology laboratory.

Effects of magnesium sulfate on electrical ventricular defibrillation of dogs

Magnesium ion infusion has been reported for treatment of hypomagnesemia often associated with myocardial infarction and with surgeries involving cardiopulmonary bypass (CPB). Magnesium infusion before CPB has been reported to adversely affect the ability to defibrillate after CPB. However, there are also reports of magnesium ion infusion facilitating defibrillation of refractory ventricular fibrillation. This study evaluated the isolated effect of magnesium ion infusion on the shock intensity requirements for electrical defibrillation. The electric current required to defibrillate with 50% success (the ED50) was estimated in five mongrel dogs at baseline and again after each of four magnesium sulfate (80 mg/kg) infusions. The total serum magnesium level increased from 2.32 +/- 0.08 mg/dL (mean +/- SD) to 7.92 +/- 0.80 mg/dL. The mean estimated ED50 decreased from 12.8 +/- 2.9 A at baseline, to 11.1 +/- 0.8 A after the fourth infusion (P < .05), decreasing the delivered energy by 25%. Magnesium sulfate infusion was associated with a significant decrease in the electrical requirements for defibrillation. Key words: magnesium, electrical ventricular defibrillation.

Ionic mechanisms of ischemia-related ventricular arrhythmias

The aim of this review is the utmost simplification of the cellular electrophysiologic background of ischemia-related arrhythmias. In the acute and subacute phase of myocardial infarction, arrhythmias can be caused by an abnormal impulse generation, abnormal automaticity or triggered activity caused by early or delayed afterdepolarizations (EAD and DAD), or by abnormalities of impulse conduction (i.e., reentry). This paper addresses therapeutic intervention aimed at preventing the depolarization of "pathologic" slow fibers, counteracting the inward calcium (Ca) influx that takes place through the L-type channels (Ca antagonists), or hyperpolarizing the diastolic membrane action potential, increasing potassium (K) efflux (K-channel openers) in arrhythmias generated by an abnormal automaticity (ectopic tachycardias or accelerated idioventricular rhythms). If the cause enhanced impulse generation is related to triggered activity, and since both EAD and DAD are dependent on calcium currents that can appear during a delayed repolarization, the therapeutic options are to shorten the repolarization phase through K-channel openers or Ca antagonists, or to suppress the inward currents directly responsible for the afterdepolarization with Ca blockers. Magnesium seems to represent a reasonable choice, as it is able to shorten the action potential duration and to function as a Ca antagonist. Abnormalities of impulse conduction (re-entry) account for the remainder of arrhythmias that occur in the acute and subacute phase of ischemia and for most dysrhythmias that develop during the chronic phase. Reentrant circuits due to ischemia are usually Na channel-dependent. Drug choice will depend on the length of the excitable gap: in case of a short gap (ventricular fibrillation, polymorphic ventricular tachycardia, etc.), the refractory period has been identified as the most vulnerable parameter, and therefore a correct therapeutic approach will be based on drugs able to prolong the effective refractory period (K-channel blockers, such as class III antiarrhythmic drugs); on the other hand, for those arrhythmias characterized by a long excitable gap (most of the monomorphic ventricular tachycardias), the most appropriate therapeutic intervention consists of depressing ventricular excit-ability and conduction by use of sodium-channel blockers such as mexiletine and lidocaine. Compared with other class I antiarrhythmic agents, these drugs minimally affect refractoriness and exhibit a use-dependent effect and a voltage dependent action (i.e., more pronounced on the ischemic tissue because of its partial depolarization).

ISIS 4: clinical controversy regarding magnesium infusion, thrombolytic therapy, and acute myocardial infarction

Magnesium (Mg) infusions over 24 hours were given to patients with suspected acute myocardial infarction (AMI) at least 2 hours after thrombolysis. Patients showed no benefit and even some increased risk in contrast to reduction in mortality obtained by Mg therapy in smaller trials. Results of all of the studies were pooled and statistically analyzed, according to a fixed-effects model that is inappropriate for studies of different protocols. The panel concluded that further study of Mg in AMI is not needed. This conclusion has been questioned.

Antiarrhythmic effects of magnesium sulphate. Report of three cases

A wide variety of antiarrhythmic agents is used in treatment of both supraventricular and ventricular arrhythmias. Magnesium sulphate has previously been used mainly in the treatment of torsade de pointe arrhythmias but several reports show that this agent may be used in the treatment of arrhythmias of different aetiology. We describe 3 patients who exhibited arrhythmias affecting haemodynamic performance. Case #1 had a subarachnoid haemorrhage and developed a supraventricular tachycardia. In case #2, ventricular tachycardia appeared during the postoperative course after abdominal surgery. Case #3 experienced critical heart failure due to dilated cardiomyopathy and had an irregular heart rhythm with multiple ectopic beats. In all three cases the administration of intravenous magnesium sulphate was successful in treating the arrhythmias. Magnesium sulphate is an antiarrhythmic agent that is effective mainly in treatment of ventricular arrhythmias. The drug can also be employed as second-line treatment of supraventricular arrhythmias.