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Ann Thorac Surg 2001;72:S2226
© 2001 The Society of Thoracic Surgeons
a Division of Cardiovascular Surgery, The Toronto Hospital, EN 14-215, 200 Elizabeth Street, Toronto, ON M5G 2C4, Canada
e-mail: rweisel{at}uhn.on.ca
Coronary sinus measurements are extremely valuable in that they give you a great deal of information about what is happening with the myocardium. About 20 years ago, we performed a study in which we obtained postoperative coronary angiograms on 200 consecutive patients undergoing coronary bypass surgery. Hoping to gain a better understanding of why complications occurred, I carefully reviewed every patient who had a postoperative complication, particularly patients who developed new Q-wave infarcts or low-output syndrome. Reviewing the postoperative angiograms, I could identify the problem causing the complication in 50% of these patients. For example, a diagonal branch that was open preoperatively would be occluded postoperatively; a distal LAD would not be adequately perfused postoperatively because of a technical problem with a newly constructed graft to the LAD, etc. In 50% of the patients with complications, I could not identify any problem on the postoperative angiogram that would account for the complication. The complications in these patients could be due to any of a variety of technical and nontechnical factors such as the method of myocardial protection, the nature of native distal coronary perfusion, etc. It is often difficult to identify a problem intraoperatively and often more difficult to do something about it. For example, problems can occur because of poor distal vessels that one can do very little about. On the other hand, there are situations in which problems are identified intraoperatively and corrected immediately, thus avoiding postoperative complications. A valve occluding the coronary ostium can be resutured to avoid this occlusion, and a new regional wall motion abnormality can be reversed by grafting the coronary vessel subtending that region.
The value of the coronary sinus measurements is that they allow for an assessment of the adequacy of the myocardial protection technique. Unfortunately a number of factors prevent these measurements from providing this assessment in real time, during the course of the operation. First, there is the wide variability in coronary blood flow during the early postoperative period, which is also influenced by the degree of myocardial protection technique. When the protection is good, there may be overperfusion early during the cross-clamp period. When the protection is poor there will also be overperfusion with very high blood flows and, therefore, the coronary sinus concentrations of ischemic metabolites may be very low despite the fact that their production could be substantial. Second, there is the limitation that Dr Crittenden alluded to, which related to the heterogeneity in regional ischemia in these patients. After release of the aortic clamp, myocardial perfusion may be heterogeneous; regions that were adequately protected during the period of aortic occlusion usually show good perfusion, whereas regions that were inadequately protected may exhibit poor perfusion. Third, the heart is both extracting lactate and producing it at the same time; thus, coronary sinus lactate levels reflect the net effects of both extraction and production. Any lactate measured in the coronary sinus is in excess of what the heart is using for lactate substrate utilization. As such, it may underestimate the degree of myocardial ischemia present.
One can obtain real-time measurements of coronary sinus oxygen content and pH by placing the respective sensor/electrode in the coronary sinus. Unfortunately, these measurements are too sensitive; it is difficult to know whether the changes observed in these variables warrant an intervention or whether they would normalize spontaneously over time.
The major value that we found in measuring coronary sinus metabolites is the provision of endpoints to conduct randomized studies that compare alternate techniques of myocardial protection. The randomization process corrects for the variation in technical and other factors that influence coronary sinus metabolites, allowing the residual difference to reflect the difference between the myocardial protection techniques being investigated. We have used this approach to evaluate the comparative efficacy of a variety of intraoperative management techniques in improving the protection of the ischemic heart. Improving current myocardial protection techniques may not be critical in the routine, low-risk patient undergoing cardiac surgery. Almost any myocardial protection technique will yield good results in this group of patients. Improving our current myocardial protection techniques is critical, though, in the high-risk patient undergoing cardiac surgery. But as we have already heard during this symposium, we have mortality rates in the range of 20% to 30% in some high-risk subgroups of patients undergoing cardiac surgery. These are the subgroups that would benefit from new alternative methods of protection. Evaluation of newer myocardial protection techniques by periodic sampling of coronary sinus metabolites needs to be performed in a low-risk patient population first. Once determined to be efficacious in this group of patients, new myocardial protection techniques can then be applied to high-risk patients with the hope of improving their outcomes.
In summary, coronary sinus measurements are not helpful in real time in the operating room because of several limitations, including their extensive sensitivity. They are, however, very helpful in the assessment and identification of newer intraoperative myocardial management techniques, which could improve the outcomes of our high-risk patient populations.
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