INTRAOPERATIVE MYOCARDIAL-ISCHEMIA DETECTION WITH LASER-INDUCED FLUORESCENCE

Myocardial ischemia can be detected at the mitochondrial level by meas uring shifts in nicotinamide adenine dinucleotide and its reduced form . Using a pulsed nitrogen laser and an optical multichannel analyzer, we monitored myocardial metabolism by measuring laser-induced nicotina mide adenine dinucleotide (reduced form) fluorescence in a large anima l model of acute ischemia. Eight opened-chest sheep underwent occlusio n of branches of the left anterior descending coronary artery, establi shing a 15% infarct of the left ventricle. For the simulation of the c linical scenario, after 60 minutes of occlusion, the animals were supp orted by cardiopulmonary bypass, the aorta was crossclamped, and cold crystalloid cardioplegic solution was administered. The occlusion was removed after 10 minutes, and two additional doses of cardioplegic sol ution were delivered at 10-minute intervals. The aortic crossclamp was released, and a 30-minute period of reperfusion on bypass ensued. The hearts were then weaned off bypass and allowed to recover. Laser-indu ced fluorescence was measured inside, outside, and along the border of the infarct. Baseline measurements were made before occlusion, immedi ately after occlusion, and then at 5, 10, and 20 minutes after occlusi on. The results show that immediately after occlusion there is a 200% +/- 30% (mean +/- standard deviation ) increase in laser-induced fluor escence in the infarct zone, a 110% +/- 30% increase along the border, and no significant change in the area outside the infarct, The fluore scence in the infarct reaches a plateau in 5 minutes at 270% +/- 30%, whereas along the border it reaches a peak near end ischemia of 110% /- 40%. With the first dose of cardioplegic solution, fluorescence inc reases outside the infarct and decreases inside the infarct and along the border to 120% +/- 30 %, where it remains for all areas until the aortic crossclamp is removed. Fluorescence then drops to 70% +/- 20% a nd finally returns to baseline after 5 minutes of recovery. Ah of thes e shifts in laser-induced fluorescence were statistically significant (p < 0.01). The changes noted with doses of cardioplegic solution refl ect the hypothermic and hyperkalemic effects on the myocardium. Laser- induced fluorescence provides a sensitive and specific method of monit oring myocardial ischemia during the operation. it also provides insta ntaneous feedback of metabolic changes that may be useful in evaluatin g the effects of different cardioplegic regimens and in monitoring rep erfusion injury.