Effect of exercise-induced activation of sympathetic nerve activity on clearance of123I-MIBG from the myocardium

The effect of exercise on the cardiac kinetics of123I-MIBG was investigated in the present study.123I-MIBG was administered intravenously at rest in 6 healthy male volunteers, and anterior planar and SPECT images were obtained 15 minutes, and 2 and 4 hours after administration (protocol A). After 4 weeks,123I-MIBG was again administered intravenously at rest, and images were obtained 15 minutes later. After imaging, the subjects ran 10 km in approximately 1 hour, and anterior planar and SPECT images were obtained 2 and 4 hours after administration of123I-MIBG (protocol B). The heart-to-mediastinum uptake ratio (H/M) was calculated from each anterior planar image, and the mean123I-MIBG clearance from 15 minutes to 2 hours, and from 2 hours to 4 hours was calculated with a bull’s eye display. The H/M was much lower after exercise. The mean clearance rate between 15 minutes and 2 hours in protocol B was significantly higher than that between 2 hours and 4 hours, and that between 15 minutes and 2 hours in protocol A. It was concluded that the clearance rate of123I-MIBG may be a useful index of cardiac sympathetic nerve activity.