Physiological inverse relationship between heart rate and thallium-201 lung uptake, clearance and lung/myocardial uptake ratio.

Increased lung uptake of thallium-201 during exercise has been suggested to indicate left ventricular dysfunction. The normal lung thallium-201 uptake, clearance and lung/myocardial uptake ratio were studied prospectively in 7 healthy men. Initial and delayed (3 h) images were quantitated by computer from regions of interest over the heart and lungs, after three bicycle exercise tests: to maximal, 80% and 60% of predicted maximal heart rate. The initial lung uptake was lowest at peak heart rate, and increased linearly to 121% and 140% of this at 80% and 60% of predicted maximal heart rate. The three delayed lung images were identical. The myocardial activity was highest at maximal exercise, and decreased linearly with heart rate, resulting in a net fall in the sum of initial lung and myocardial activity to 93% and 86% at the two lowest exercise levels. The mean initial lung/myocardial uptake ratio was 0.44, 0.67, and 1.00 from highest to lowest heart rate, and the lung thallium-201 clearance 9%, 21% and 33%. These results suggest that the initial lung thallium-201 uptake, clearance and lung/myocardial uptake ratio are normally inversely related to cardiac output. When used as indicators of left ventricular dysfunction, interpretation should be cautious in patients performing submaximal exercise tests. The lung thallium prognostic indicators seem less specific than previously indicated.