Effects of contrast media on the conducting system of the heart. Mechanism of action and identification of toxic component.

Infusion of progressive volumes of meglumine sodium diatrizoate (Renografin 76) into the arteries to the sinoatrial and atrioventricular nodes in anesthetized dogs produced a dose rate-dependent decrease in heart rate and an increase in "PR interval." At high dose rates, Renografin 76 caused sinus arrest and complete heart block in 50% and 70% of the animals, respectively. Saline and meglumine solutions (0.939 M) equiosmolar with Renografin 76 produced similar effects. Infusions of metrizamide (equal in iodine content with Renografin 76), dextrose solution (0.939 M), and autologous blood each had substantially less effects. Contrast media have a direct inhibitory action on sinoatrial automaticity and atrioventricular conductivity which is dependent on ionic strength, not iodine content.