Effects of left bundle branch block on myocardial FDG PET in patients without significant coronary artery stenoses

Cardiac PET studies in patients with left bundle branch block (LBBB) are few, and the results are conflicting. In particular, even if a reduced uptake of FDG is reported, confirmation in a large group of patients and exact understanding of the underlying cause are lacking.

We selected 29 consecutive patients who had complete LBBB and no significant stenosis on coronary angiography scheduled for FDG and 13N-NH3 PET for myocardial viability evaluation at our center. Wall motion was evaluated using 2-dimensional echocardiography. Ten volunteers without coronary stenosis or LBBB served as a control group.

All LBBB patients had a reverse mismatch in the septum, defined as reduced uptake of FDG in comparison with 13N-NH3. The mismatch extended to the anterior and inferior walls in 17 patients. The mean (+/-SD) septal-to-lateral ratio was 0.57 +/- 0.11 for FDG (range, 0.28-0.76) and 0.99 +/- 0.12 for 13N-NH3 (range, 0.75-1.18), with P < 0.0001. In contrast, no significant differences in uptake were seen in the control group, which had a septal-to-lateral ratio of 0.95 +/- 0.13 for FDG (range, 0.78-1.15; P < 0.01 with respect to LBBB patients) and 0.94 +/- 0.11 (range, 0.85-1.20) for 13N-NH3.

Our study suggests that in LBBB patients without significant coronary stenosis, FDG uptake in the septum changes without a correlating change in perfusion. To avoid possible overestimation of necrosis, especially in the LAD territory, this phenomenon must be considered in evaluations of myocardial viability using FDG images.