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Experimental evidence of regional myocardial ischemia during beating heart coronary bypass: prevention with temporary intraluminal shunts.

BACKGROUND
Our center has been performing beating heart coronary artery bypass grafting with a temporary intraluminal shunt since 1983. Based on our clinical observations of more than 846 surgical cases, we believe that a temporary intraluminal shunt (TIS) greatly reduces the risk of the patient developing regional myocardial ischemia during clamping of the coronary artery. To seek evidence in support of our clinical observations, we evaluated the effects of coronary clamping with and without TIS in a porcine experimental model.


METHODS
We compared 2 groups of healthy Landrace pigs that underwent the same period of coronary occlusion but differed only in whether a TIS was used. The shunt device was a straight flow-through silicone tube that has been described in detail in previous publications. Ischemic changes during the test period were detected via analysis of monophasic action potential (MAP) recordings. MAPs were recorded with the contact electrode technique, which has been shown to be specific for ischemia. In group I (no shunt) animals (n = 25), MAPs were monitored during a single 15-minute occlusion of the left anterior descending (LAD) coronary artery without any form of distal perfusion. In group II (shunted) animals (n = 15), MAPs were sampled over the same intervals after the LAD was snared and opened and the TIS was introduced within the first 2 minutes. Infarct analysis using biochemical end points (serum lactate dehydrogenase [LDH] and creatine phosphokinase-myocardial band [CPK-MB]) was performed with standard serologic assays.


RESULTS
Confirming the presence of regional ischemia in group I (no shunt) were significant changes from baseline in measurements of mean action potential duration, upstroke velocity (dV/dt), and total MAP area (millivolts milliseconds). The presence of ischemia in group I was also confirmed by significant elevations in serum LDH and CPK-MB levels. Furthermore, the use of lidocaine was greater in group I (no shunt) animals than in group II (shunted) animals because of the greater frequency of ventricular arrhythmias in group I (P =.001). Six animals (24%) in group I and no animals in group II developed ventricular fibrillation during the 15 minutes of occlusion (P =.046). Ischemic changes in the MAP were found only prior to shunt insertion in Group II animals, and the MAP then promptly returned to normal a few minutes after TIS flow was established. Statistical analysis revealed significant differences between group I and group II in MAP duration, dV/dt, total area, lidocaine requirements, incidence of ventricular fibrillation, and serum LDH levels.


CONCLUSIONS
There has been controversy about the relative effectiveness of temporary intraluminal shunting for reducing the risk of regional myocardial ischemia during beating heart coronary artery bypass grafting. At least in this porcine model, we could demonstrate a positive effect of shunting, which parallels our clinical experience using TIS in hundreds of patients for the past 2 decades. In the animal model, we demonstrated preservation of the MAP, as well as a reduction in both the incidence of ventricular arrhythmias and the serum levels of ischemic by-products, when temporary intraluminal shunting was used. It is our conclusion that intraluminal shunts do protect the vulnerable myocardium from regional ischemia during the period of temporary coronary occlusion necessary for construction of a bypass graft on the beating heart. Temporary intraluminal shunting is a costeffective adjunct that can increase safety and reliability in offpump coronary artery bypass grafting.