Is nonionic isotonic iohexol the contrast agent of choice for quantitative myocardial videodensitometry?

All currently used contrast media in coronary angiography induce a considerable hyperemic response interfering with the interpretation of circulation times derived from myocardial time-density curves. Aim of this study therefore, was to find a contrast agent with minimal hyperemic response. For this purpose 2, 4 and 6 ml of the nonionic isotonic low iodinated contrast agent iohexol (Omnipaque 140) and 6 ml of a similarly low iodinated but still hypertonic solution of the ionic diatrizoate (Urographin 30%) were administered into the left coronary artery of 8 anesthetized instrumented dogs. Heart rate was held constant by atrial pacing and left ventricular pressure, left ventricular dP/dt and mean and phasic coronary blood flow were recorded. To test the hypothesis that the hyperemic response to nonionic contrast media is partly due to an increase in inotropic state mediated by Ca++ion influx, all measurements were repeated 30 minutes after intracoronary administration of 0.5 mg verapamil. For iohexol the increase in coronary blood flow was small but significant: 12 +/- 7%, 25 +/- 11% and 38 +/- 16% for the 2, 4 and 6 ml administrations, respectively (mean +/- s.d.; p less than 0.01). For the diluted diatrizoate the increase in coronary blood flow was 65 +/- 23%. Increases for currently used contrast agents are on the order of 200-300%. After verapamil, the hyperemic response to iohexol decreased significantly to 9 +/- 5%, 20 +/- 8% and 29 +/- 12% for the 2, 4 and 6 ml administrations, respectively (p less than 0.01). The reaction to diatrizoate was not affected by verapamil. Moreover, there was a significant positive correlation between the increase in coronary blood flow and left ventricular dP/dt max under all conditions for all but one dog. We conclude that the isotonic, low iodinated nonionic contrast agent iohexol has only a moderate influence on coronary blood flow, which can be further attenuated by verapamil. By this approach, a more reliable assessment of circulation times from myocardial time-density curves obtained by digital subtraction angiography and videodensitometry becomes possible.