Low magnesium enhances the pro-arrhythmic effect of low potassium in the hypertrophied rat heart but not in the normal rat heart.

OBJECTIVES

To investigate the influence of external magnesium on wall stress-induced arrhythmias and to establish whether early cardiac hypertrophy affects the sensitivity of the heart to these arrhythmias.

METHODS

We used a modified isolated working heart model to compare hypertrophied hearts from the spontaneously hypertensive rat (SHR) with hearts from normotensive control rats from the Wistar and Wistar-Kyoto strains. Young rats (aged 137.9 +/- 2.04 days mean +/- SEM) were used to ensure left ventricular hypertrophy uncomplicated by fibrosis and cardiac failure. Arrhythmias were induced by 20 s increases in ventricular afterload during which the electrocardiogram was recorded.

RESULTS

In control experiments SHR hearts showed a significantly greater arrhythmic response than did normotensive control rat hearts during perfusion with low-potassium (2.4 mmol/l) solutions. Concomitant low magnesium (0.4 mmol/l) perfusion significantly increased the arrhythmic response to 2.4 mmol/l potassium in SHR hearts but had no effect on arrhythmias in normotensive control rat hearts. With low magnesium, the number of ventricular premature beats was increased in SHR hearts compared with that in normotensive control rat hearts during perfusion with 4.2 and 2.4 mmol/l potassium. In addition, perfusion with low magnesium and 2.4 mmol/l potassium led to an increase in the complexity of arrhythmias, ventricular tachycardia occurring in nine of 10 SHR hearts but in only two of six normotensive control rat hearts.

CONCLUSIONS

There is a synergistic effect of low magnesium and low potassium on wall-stress-induced arrhythmia in the SHR heart. An analogous effect might be important in patients with hypertension and left ventricular hypertrophy, in whom systolic blood pressure is frequently labile, leading to wide fluctuations in ventricular wall stress; combined potassium and magnesium depletion is a common complication of antihypertensive treatment.