99mTc-MDP bone SPECT for the evaluation of sternal ischaemia following internal mammary artery dissection

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Abstract
Coronary artery bypass grafting (CABG) is one of the most frequently performed operations in the United States. The use of internal mammary artery (IMA) grafting has been identified as increasing the risk of sternal wound infections and mediastinitis. The purpose of our study was to prospectively evaluate the effect of different techniques of left internal mammary artery (LIMA) harvesting on sternal vascularity. Thirty-three patients undergoing primary coronary artery bypass grafting were studied. The patients were divided into groups that received a skeletonized IMA (group I, n = 11), a pedicled IMA (group II, n = 12), or a semiskeletonized IMA (group III, n = 10) graft. Each patient underwent a preoperative 99mTc-methylene diphosphonate bone scan using single photon emission computed tomography (SPECT). The ratio of the mean counts/pixel for each side of the sternum was obtained. Post-operatively, all patients had a repeat bone SPECT. Ratios of unilateral sternal uptakes were compared to the preoperative study. A univariable analysis of post-operative to pre-operative ratios revealed statistically significant reduction in vascularity to the left side of the sternum post-operatively in group II compared with groups I and III (0.68 0.12 vs 0.99 0.24 and 0.93 0.09; P<0.01). There was no difference between groups I and III (P = 1). Multivariable analysis revealed only the type of harvesting to be associated with post-operative reduction in left to right sternal activity ratio (P<0.02). Pairwise comparisons revealed that differences are due to pedicled type of harvesting (group II vs group I, P = 0.03; II vs III, P = 0.001; and I vs III, P = 0.115). A pedicled IMA graft causes acute post-operative sternal ischaemia. This does not occur when the IMA is skeletonized or semiskeletonized. Hence, it may be prudent to minimize dissection during mobilization of the IMA to decrease the likelihood of post-operative sternal complications.