Myocardial sympathetic innervation in the athlete's sinus bradycardia: Is there selective inferior myocardial wall denervation?

Background. Sinus bradycardia in trained athletes is predominantly a manife station of increased vagal tone, but it is not known whether an alteration in the cardiac sympathetic system can contribute to blunted chronotropic re sponse, This study assessed the integrity of the; sympathetic system in tra ined athletes with sinus bradycardia by means of the iodine-123-metaiodoben zylguanidine (I-123-MIBG) procedure. Methods and Results, Fourteen athletes with sinus bradycardia and 8 athlete s with a normal heart rate were explored by means of planar and single phot on emission computed tomography MIBG studies, The heart/mediastinum ratio, regional myocardial distribution, and percent of regional myocardial MIBG u ptake were evaluated. The heart/mediastinum ratio in athletes with sinus br adycardia was 1.87 +/- 0.10, and in athletes with a normal heart rate, the heart/mediastinum ratio was 1.86 +/- 0.16 (P = not significant), In athlete s with sinus bradycardia, the regional distribution of MIBG showed an infer ior and apical uptake defect in 8 athletes, an inferior, apical, and septal defect in 3 athletes, an inferior defect in 1 athlete, and normal distribu tion in 2 athletes (14%). In athletes with a normal heart rate, the regiona l distribution of MIBG showed an apical uptake defect in 3 athletes and nor mal distribution in 5 athletes (63%), The percent of regional MIBG uptake i n the inferior region was significantly reduced in athletes with sinus brad ycardia (44% +/- 13% vs 72% +/- 11%, P < .01). Conclusion. These results show severely reduced myocardial MIBG distributio n in the inferior region in athletes with sinus bradycardia, suggesting sel ective inferior myocardial wall sympathetic denervation, which may be relat ed to increased vagal tone.