Measurement of myocardial fatty acid metabolism: kinetics of iodine-123 heptadecanoic acid in normal dog hearts.

To define the potential of iodine-123 heptadecanoic acid (IHA) for the noninvasive assessment of myocardial fatty acid metabolism with gamma camera imaging, the influence of myocardial oxygen consumption (MVO2) and blood flow (MBF) on extraction and half-times of IHA were investigated in dogs. Following IHA injection into the left circumflex coronary artery, extraction fraction and half-times were derived from the peak and slope of the IHA time activity curve, which consisted of a vascular, early, and late phase. Single-pass extraction fraction of IHA averaged 0.53 +/- 0.11 s.d. at control and was not influenced by MVO2 and MBF. The half-time of the early phase (T = 9.3 min +/- 2.8 s.d. in controls) as well as the ratio between the size of the early and late phase increased with MVO2 (r = 0.82, r = 0.87, respectively). Thus, early phase intracellular turnover of IHA increased, yet clearance of 123I activity was slowed by augmented cardiac work. Preliminary data of HPLC and electrophoretic analysis of myocardial arterial and venous blood samples over time indicate that the early phase is characterized by a decreasing washout of IHA and a relative increase of radioiodine washout. The half-time of the late phase (T = 245 min +/- 156 s.d. at control) was not related to MVO2 and MBF. In conclusion, myocardial fatty acid metabolism cannot be measured from the half-time of the early phase but might be analyzed from the ratio between the size of the early and late phase when using IHA.