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Electrocardiographic changes induced by ionic contrast media can cause complications during coronary angiography. A conduction delay through various parts of the heart is one factor in the genesis of asystole or ventricular fibrillation. Hypaque-76 (H76) and Renografin-76 (R76) are nearly identical ionic contrast media except that R76 binds more calcium than H76 because of the presence of sodium citrate and EDTA in R76. To determine whether the calcium binding additives in ionic contrast media contribute to the cardiac conduction abnormalities, we examined conduction time through the atrioventricular (AV) nodal tissue (via bipolar His bundle electrograms) and through the distal part of the conduction system (recording the QRS complex from the ECG) during coronary angiography. We injected 10 mL of H76 and R76 in 19 closed chest dogs in a blinded, randomized fashion during coronary angiography. The effects of H76 and R76 on heart rate, AH interval, HV interval, V interval and PR interval, and QRS complex duration were recorded. In 14 nonatrial pacing dogs, compared with H76, R76 produced a greater increase in the AV interval (32.9 ± 6 milliseconds vs 12.4 ± 2 milliseconds, P<.01) and the PR interval (29.6 ± 6 milliseconds vs 11.9 ± 4milliseconds, P<.02). Additionally, the heart rate decreased 13.9 ± 3.5 beats/minute from control with R76 compared with a decrease of 4.2 ± 2.6 beats/minute from control with H76 (P<.05). There was no significant difference between the prolongation of the HV interval and V interval, or QRS complex duration generated by R76 and H76. Similar results were observed in five other atrial pacing dogs, except for the change in the heart rate. Thus, R76 produces significantly greater sino-atrial or AV node abnormalities than H76 but does not produce ventricular conduction delay. Calcium binding additives in R76 have detrimental effects on the cardiac conducting system.