ELECTROCARDIOGRAPHIC CHANGES AND CONCENTRATION OF MAGNESIUM IN SERUM FOLLOWING INTRAVENOUS INJECTION OF MAGNESIUM SALTS

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Abstract
Isotonic magnesium sulfate was given to unanesthetized dogs by continuous intraven. inj. Frequent electrocardiograms and blood samples were taken for analysis of the magnesium in the serum. The heart rate immediately increased as the conc. of Mg in the serum rose from 2 to 5 mEq. per liter. This was soon followed by a bradycardia. Intracardiac conduction was depressed as seen from (1) progressive increase in P-R interval beginning at 5 to 10 mEq. magnesium per liter of serum and continuing till death, (2) occasional S-A and A-V block of various grades, occurring at levels greater than 15 mEq. per liter, (3) widening of QRS complex indicating an increase in intraventricular conduction time, beginning at 5 to 10 mEq. per liter. Natural respiration disappeared at 17 to 27 mEq. per liter. Occasionally cardiac arrest occurred simultaneously, but never preceded respiratory arrest; more often under artificial respiration the heart continued to beat until a much higher conc. of Mg was attained. Cardiac arrest appeared over a wide range, 17 to 66 mEq. per liter, but most frequently at 27 to 44 mEq. per liter. Normal systole was maintained till the end. Inj. of Mg in cats produced a sequence of changes similar to that observed in dogs.