Iodophenylpentadecanoic acid-myocardial blood flow relationship during maximal exercise with coronary occlusion

Imaging ¹²³I-labeled iodophenylpentadecanoic acid (IPPA) uptake and clearance from the myocardium following exercise has been advocated as a means of detecting myocardial ischemia because fatty acid deposition is enhanced and clearance prolonged in regions of low flow. However, normal regional myocardial blood flows are markedly heterogeneous, and it is not known how this heterogeneity affects regional metabolism or substrate uptake and thus image interpretation. In five instrumented dogs running at near maximal workload on a treadmill, ¹³¹I-labeled IPPA and 15-micron ⁴⁶Sc microspheres were injected into the left atrium after 30 sec of circumflex coronary artery occlusion. Microsphere and IPPA activity were determined in 250 mapped pieces of myocardium of ~ 400 mg. Myocardial blood flows (from microspheres) ranged from 0.05 to 7.6 ml/min/g. Deposition of IPPA was proportional to regional flows (r = 0.83) with an average retention of 25%. The mean endocardial-epicardial ratio for IPPA (0.90 ± 0.43) was similar to that for microspheres (0.94 ± 0.47; p = 0.08). Thus, initial IPPA deposition during treadmill exercise increases in proportion to regional myocardial blood flow over a range of flows from very low to five times normal.