低镁在心血管病里的重要影响 hypomagnesemia in cardiovasculardisease

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Significance of hypomagnesemia in cardiovascular disease



INTRODUCTION —
Mild hypomagnesemia is a common electrolyte abnormality, particularly in the elderly who may have increased urinary magnesium losses due to diuretic therapy or interstitial renal disease. Whether this abnormality should be treated or prevented with prophylactic magnesium administration is unclear. The major concern is whether mild magnesium depletion predisposes to cardiac arrhythmias .
There are conflicting data as to whether this occurs in otherwise healthy subjects. A report on over 3000 patients from the Framingham Heart Study suggests that how arrhythmia is defined is an important determinant .
No association with hypomagnesemia was noted for more than 10 ventricular premature complexes (VPCs) per hour or for repetitive VPCs. There was, however, an increased risk of complex or frequent (≥30/h) PVCs with reductions in the plasma magnesium concentration of 0.16 meq/L (0.2 mg/dL or 0.08 mmol/L) or more.


Thus, attempts have been made to identify those patients who might be at risk. The data suggest that this might occur when hypomagnesemia occurs in the setting of an acute ischemic event, congestive heart failure, torsades de pointes, after cardiopulmonary bypass, or in the acutely ill patient in the intensive care unit.
The mechanisms underlying the possible association between hypomagnesemia and arrhythmias is at present unknown. Arrhythmias could be due to concurrent hypokalemia, hypomagnesemia itself, or both .
Magnesium regulates several cardiac ion channels, including the calcium channel and outward potassium currents through the delayed rectifier . Lowering the cytosolic magnesium concentration in magnesium depletion will markedly increase these outward currents, shortening the action potential and increasing susceptibility to arrhythmias.

ACUTE ISCHEMIC HEART DISEASE — Patients with acute myocardial infarction who have mild hypomagnesemia appear to have a two to three-fold increase in the frequency of ventricular arrhythmias in the first 24 hours when compared to those with normal plasma magnesium levels . Uncontrolled studies suggest that the administration of intravenous magnesium at this time can reduce the frequency of potentially fatal ventricular arrhythmias .
A relationship has also been found between the plasma magnesium concentration and ventricular arrhythmias occurring in the second or third week after myocardial infarction. In one study, for example, the mean plasma magnesium concentration was 1.83 mg/dL (0.76 mmol/L) in patients with no abnormal rhythms, 1.68 mg/dL (0.7 mmol/L) in those with multifocal ventricular premature complexes, and 1.55 mg/dL (0.65 mmol/L) in those with unsustained ventricular tachycardia . Thirteen patients with complex arrhythmias and hypomagnesemia received intravenous magnesium over 24 hours; a normal rhythm was restored in ten.
A related issue is the possible protective effect of doubling the normal plasma magnesium concentration in patients with suspected acute myocardial infarction. The main rationale behind this regimen is that mild hypermagnesemia protects the myocardium from ischemia-reperfusion injury, at least in part by promoting restoration of high energy phosphates. Some studies have suggested that magnesium may be beneficial if given before reperfusion with thrombolysis or angioplasty.

HEART FAILURE — An increased incidence of hypomagnesemia has been found repeatedly in patients with heart failure and may be due in part to diuretic therapy . A role for magnesium depletion in sudden death has been suggested but not proven. In a prospective study involving over 1000 patients with class III or IV heart failure, for example, no correlation was found between hypomagnesemia at the beginning of the study and survival at a median follow-up of six months . However, measurements were not made later in the study and all patients were receiving digoxin, diuretics, and an angiotensin converting enzyme inhibitor, any or all of which may have altered magnesium balance during the course of the study。


TORSADES DE POINTES — Hypomagnesemia increases the risk of a unique form of polymorphic ventricular tachycardia called torsades de pointes. The risk is particularly increased in patients taking class IA or class III antiarrhythmic drugs.


The American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiac Care include a recommendation that magnesium sulfate be added for the management of torsades de pointes, severe hypomagnesemia, or refractory ventricular fibrillation . Treatment is aimed at accelerating the heart rate and/or shortening the QT interval. Intravenous magnesium is now regarded as the treatment of choice even when hypomagnesemia is not present .

CARDIOPULMONARY BYPASS — Hypomagnesemia may both develop during cardiopulmonary bypass and predispose to arrhythmias . The causes of hypomagnesemia in this setting are unclear. Possible contributing factors include hemodilution, increased fractional excretion by the kidney, chelation by free fatty acids and/or citrate, and enhanced cellular uptake induced by increasing circulating levels of catecholamines.
One prospective study, for example, evaluated 101 patients: 18 percent had hypomagnesemia before induction as compared to 71 percent following cessation of bypass . The hypomagnesemic patients had a significantly higher frequency of atrial dysrhythmias and an increased requirement for prolonged mechanical ventilatory support. A separate study suggested that two grams of magnesium sulfate given after surgery reduced the level of ventricular ectopy .

These findings were corroborated in a prospective study of 100 patents who were randomized to either placebo or two grams of magnesium chloride intravenously after the termination of cardiopulmonary bypass . Normomagnesemic patients had significantly fewer postoperative supraventricular and ventricular dysrhythmias, higher indices of cardiac performance, and a less frequent requirement for prolonged mechanical ventilatory support than patients with low plasma magnesium levels。

INTENSIVE CARE UNIT — Hypomagnesemia is extremely common in patients in the intensive care unit and is frequently associated with hypokalemia and hypocalcemia 。
In one study, for example, hypomagnesemia present on admission to the intensive care unit was associated with a mortality rate approximately twice that of comparably ill normomagnesemic patients . It has not been shown, however, that treatment with magnesium supplementation would improve the outcome。


CORONARY HEART DISEASE — Two large prospective epidemiologic studies have examined the relationship between the serum magnesium concentration and the subsequent development of coronary heart disease (CHD) . Both suggest that a low serum magnesium is a risk factor for coronary disease.
One study, for example, examined and followed a cohort of 15,792 subjects aged 45 to 64 years old over a four to seven year period as part of the Atherosclerosis Risk in Communities (ARIC) study . The relative risk of CHD across quartiles of serum magnesium in women was 1.0 (in the lowest quartile), 0.92, 0.48 and 0.44. The data in men showed a similar trend but was less striking and did not achieve statistical significance. Both men and women who developed CHD had lower mean baseline serum magnesium concentration than the disease-free controls. How a low serum magnesium might predispose to CHD is not known. However, it may be related to its effects on endothelial function. Support for this comes from one study of 50 patients with coronary heart disease which found that oral magnesium therapy (30 mmol/day for 30 days) improved endothelial function and exercise tolerance compared to placebo 。

转自"与时并进"2009年五月.

brightwith

看不懂，楼主是不是可以用中文翻译一下。

shenxiu2

看不懂，楼主是不是可以用中文翻译一下。
brightwith 发表于 2009-12-16 17:18

                               
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对不起,我懂的中文不足以翻译这篇文章呢.只好期待其他热心战友了.

大意是说低镁会引起心率失常,尤其是心室率. 适当补充镁可以改善病人的生存机会.包括心肌梗阻,心脏衰竭,体外循环后,ICU 病患,和TORSADES DE POINTES 的病人.