Influence of peak exercise heart rate on normal thallium-201 myocardial clearance

Measurement of myocardial clearance rates between initial and delayed images is a major justification for adding computer quantification to the interpretation of exercise ²⁰¹Tl images. To clarify the range of normal thallium clearance and its relationship to the level of exercise achieved, exercise thallium images in 89 normal subjects were analyzed: 45 asymptomatic subjects with <1% probability of coronary artery disease (CAD) (Group I), and 44 patients with chest pain found to have significant CAD on angiography (Group II). Mean initial regional thallium uptake was similar in two groups, but myocardial thallium clearance (mean ± 1 s.d.) was slower in Group II, expressed as a longer half-life in the myocardium (8.2 ± 7.6 hr compared with 3.4 ± 0.7 hr p<0.001). Analysis of variance using ten clinical and exercise variables as covariates showed that the slower clearance in Group II was related to a lower peak exercise heart rate (HR) (154 ± 27 compared with 183 ± 11, respectively, p<0.001). By linear regression analysis, a decrease in peak HR of 1 beat/min was associated with a slower thallium clearance (longer half-life) of 0.05 hr. Using this formula, the clearance value in each patient was then corrected for peak exercise heart rate by decreasing measured clearance by 0.05 hr multiplied by the amount peak exercise heart rate which was below 183 (the mean value in Group I). There were no differences in the 'corrected' clearance between the two groups. We conclude that thallium myocardial clearance after exercise is related in part to factors other than the presence of CAD, being slower when peak exercise HR is lower. Therefore, thallium clearance rates alone uncorrected for peak exercise heart rate should be used with caution when diagnosing CAD.