Myocardial protection-open heart surgery, Biology

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Myocardial Protection :  To a great extent, the result of cardiac surgery depends on how well the myocardium is protected during cardio pulmonary bypass. Temporary depression of myocardium (stunning) or myocardial necrosis resulting in low cardiac output may occur after bypass. Sub endocardial layer is particularly vulnerable to injury. A still heart, free of blood is essential for accurate intra cardiac repair. In the earlier era, fibrillatoiy arrest or ischaemic arrest by cross clamping the aorta was used to facilitate open-heart surgery. These days, diastolic arrest of the heart is achieved by administration of cold cardioplegic solution, proximal to aortic clamp. When there is aortic regurgitation, aorta is opened and direct coronary ostial cannulation is required for giving cardioplegia. When there are multiple blocks in coronary arteries, ante grade cardioplegia will not ensure uniform distribution of the solution. Such cases will need retrograde cardioplegia administered through a coronary sinus cannula. Special retrograde cardioplegia cannulae are available with balloons, which will fill up at the time of cardioplegia. The coronaiy sinus can be cannulated blindly through a purse string on the right atrium. The coronary sinus pressure should be kept below 30 mm of Hg at the time of cardioplegia. The cardioplegic solution could be cold asanguinous (clear) or with blood. The commonly used one is St. Thomas solution that essentially contains 20 .meq of potassium cooled down to 4°C. Cardioplegia is usually supplimented with topical cold saline or ice slush. Cardioplegia may be cold or warm. At times patients are operated at normothermia with warm blood cardioplegia. If the patient has recent

myocardial infarction or when the ejection fraction is low, it is good to give warm blood (hotshot) just before releasing aortic clamp, to reduce reperfusion injury. The cardioplegic solution is slightly hyper osmolar and buffered with sodium bicarbonate or THAM. Amino acids like Glutamate and Aspartate added to the solution reduces reperfusion injury. Adenosine adds to further protection. To reduce the injury due to oxygen free radicals, super oxide dismutase (SOD) and dimethyl sulphoxide (DMSO) can be added to the cardioplegic solution.


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