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Ann Thorac Surg 1995;60:82-83
© 1995 The Society of Thoracic Surgeons
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Introduction |
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 Top Introduction |
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DR BRADLEY S. ALLEN (Chicago, IL): This is a very elegant model that you have used to look at cardioplegic distribution in human hearts. I have two questions. First, did you count the number of microspheres that were trapped in the myocardium itself? In other words, did you account for all of the injected microspheres to make sure that your calculations of flow were correct?
Second, you started out by questioning whether retrograde cardioplegic infusion provides adequate nutritive properties to the right ventricle in human hearts. To investigate this you first gave retrograde cardioplegia into the coronary sinus and measured the amount of blood flow exiting into the ventricular cavity and repeated this using microspheres. Because the amount of microspheres reaching the ventricular cavity was less than the amount of blood, this suggests that retrograde cardioplegia reached the ventricular cavity either through venovenous collaterals or, alternatively, by first traversing capillaries and then exiting into the ventricular cavity via thebesian veins. If some of this flow does indeed traverse capillaries, then I agree that it would supply nutritive flow. However, to ascertain whether this nutritive flow provided adequate right ventricular perfusion, you must examine the right and left ventricles separately to determine the quantity of microspheres trapped in each. Therefore, did you do this, and what were your results?
When we investigated retrograde cardioplegic flow in human hearts (using contrast echography and metabolic measurements) we demonstrated that nutritive flow was not adequate to supply right ventricular demands. That is, most of the flow reaching the right ventricle was secondary to shunting through noncapillary, nonnutritive, venovenous collateral and then into the ventricle via thebesian veins. Conversely, most of the flow to the left ventricle traversed capillaries exiting either into the ventricular cavity or out the coronary ostium, thereby providing nutritive flow.
DR STEVEN R. GUNDRY (Loma Linda, CA): I congratulate Dr Ardehali and associates in coming up with this very nice study; it is long overdue. Of course, it should be remembered that the purpose of research is to ``re-search,'' and I think it wise to note that one of our former Society presidents, Vincent Gott, pointed out this exact same finding in 1957, clearly showing the difference between dog hearts and human hearts in the distribution of retrograde perfusion. In fact, whenever I am questioned about the difference between dogs and humans, I always refer to Dr Gott's classic article on the subject, and it is nice to find out 40 years later that you can confirm results from 1957.
Be that as it may, I think your findings confirm what most people working in the clinical arena have found, and that is, the nutritive properties in the human heart of retrograde cardioplegia are totally different than in the dog heart.
I would point out that there is tremendous variability between the distribution of retrograde cardioplegia through thebesian veins in your explanted hearts, some with as low as 0.11 trapping. This may explain why in the clinical arena, a flow of, say, 125 mL/min in one human heart is perfectly adequate to meet metabolic needs, whereas in a second human heart a flow of 300 mL/min may be needed to meet the metabolic needs of that heart depending on how much actual capillary flow or nutrient flow is present. Your data support the concept that you need to have very high flows or some form of monitoring of warm continuous retrograde cardioplegia to make sure that in any particular heart you are providing nutrient flow to the heart.
DR LUDWIG K. VON SEGESSER (Zurich, Switzerland): I also congratulate you on this excellent work. My question is with regard to the pathology of the hearts that you have explanted: one could imagine that in patients with coronary artery disease the flow through the coronary arteries is less and therefore more shunting through the thebesian veins can be expected. How would you evaluate this situation?
DR SIDNEY LEVITSKY (Boston, MA): I am going to ask one additional question. You have performed a careful but somewhat artificial research experimental protocol in that, as I understand it, the catheter tip is positioned at the origin of the coronary sinus. However, most of us who use a retrograde catheter advance it in as far as it will go and then either inflate or use a self-inflating balloon. So in reality we are perfusing only the veins in the distal coronary sinus, which is the basis of my question. Have you perfused only the veins in the distal coronary sinus and demonstrated that there were interconnections between the veins throughout the entire heart so that the right heart is perfused as well as the left heart?
DR JOHN E. MAYER, JR (Boston, MA): It seems to me that we have used the term nutritive flow and yet we do not really have any indices of nutritional delivery. Do you have any data on oxygen saturations in the effluent or lactate concentrations or utilization of any other substrates?
DR ARDEHALI: I thank all the discussants for their comments.
Let me start with the questions brought up by Dr Allen. Regarding the regional distribution of retrograde cardioplegia, we have shown that the capillary perfusion to the right ventricle is barely adequate to meet its theoretic metabolic requirements. To determine if all the infused microspheres are accounted for, the tubing and cannula were examined for bound microspheres in 4 explanted hearts. Approximately 6% of infused microspheres were recovered in the tubing system and the catheter. The rest of the microspheres were accounted for in the myocardium and the ventricular chambers effluent.
Regarding the differences between the right and the left ventricular effluent in the human heart, we did notice a difference. The effluent from the left ventricle had a higher rate of extraction. The extraction from the left ventricular effluent was approximately 50%, whereas that of the right ventricle was approximately 34%. Therefore, it appears that the left ventricular effluent has traversed more capillary beds and probably has contributed more to the metabolic homeostasis of the myocardium.
I thank Dr Gundry for his comments. Similar to what was noted by Dr Gott in 1957, the findings of this study confirm that the human heart is anatomically different from the canine heart. Thus, one should be very cautious in extrapolating data obtained in canine models to human hearts. I concur with you regarding the high flow rates, which I think is a prerequisite for successful myocardial protection in selected cases in the setting of retrograde administration.
On pathologic examination, these explanted human hearts demonstrated evidence of interstitial fibrosis. It is not known how the interstitial fibrosis affects the distribution of perfusate through the capillaries. This limitation of our model should be kept in mind when extrapolating these findings to clinical situations.
Doctor Levitsky brought up the importance of coronary osteal occlusion and appropriately questioned if that changes the distribution of retrograde cardioplegia. We have compared the placement of a pursestring around the coronary sinus versus a balloon catheter advanced into the coronary sinus, and we have measured the distribution of the microspheres under those two settings. The placement of a pursestring around the coronary sinus appears to improve the capillary flow to the right ventricle and enhance cardioplegic solution delivery.
Finally, Dr Mayer questioned the definition of nutritive flow. I think this is an important and valid question that, unfortunately, I cannot provide a convincing answer to. Entrapment of microspheres measuring 15 ± 5 µm in diameter has been used conventionally as an estimate of capillary or nutritive flow. We have employed the same principle. We do not have any metabolic data such as oxygen saturation in the effluent solution in these explanted hearts.
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