Introduction:
Myocardial protection denotes the strategies that aim to preserve myocardial function while providing a bloodless field during cardiac surgeries. Various concepts were developed to provide a bloodless field during open heart surgeries because a bloodless operating field with direct vision is required to correct or cure cardiac conditions during surgery. Myocardial protection provides a bloodless operating field during cardiac surgery to facilitate the precision of the surgery and prevent iatrogenic injuries and reperfusion injuries.
What Is Myocardial Protection?
Myocardial protection refers to the procedures that protect myocardial function while providing a bloodless operating field during cardiac surgery. Initially, two concepts such as inflow obstruction and controlled cross circulation were followed during open-heart surgeries. However, these techniques had disadvantages, and it was found that a bloodless field can be best achieved by arresting the heart. Cardiac arresting helps in achieving a bloodless field and also avoids the risk of air embolism.
How Is Myocardial Protection Achieved?
Initially, a cardioplegic solution was developed to achieve the arrest. A high potassium concentration solution was directly administered into the coronary arteries to arrest the heart. It was found that the extracellular fluid around the myocyte (cells of the heart) gets displaced by the cardioplegic solution and traps the myocyte in a resting phase. However, the cardioplegic solution caused abnormal heart failure, necrosis, and intractable fibrillation. Later, it was discovered that cardioplegic solutions cause serious complications along with their protective effects. Further, researchers developed new concepts to achieve myocardial protection. It includes the following techniques in the order of their evolution:
-
Cardioplegic and Non-cardioplegic Techniques:
Initially, a cardioplegic solution such as 2.5 % potassium citrate was used to achieve a bloodless field after decompressing the heart using a heart-lung machine. The cardioplegic solution was infused into the aortic root until a full arrest occurred. The arrest can be maintained for 3o minutes, after which redosing is essential. However, myocardial necrosis, poor contractility, and intractable fibrillation developed in all cases, and it was discontinued.
Later, non-cardioplegic techniques, such as perfused beating heart using continuous antegrade or retrograde coronary perfusion for aortic valve surgeries, cooled heart technique for acquired cardiac disease surgery, and ischemic arrest with intermittent aortic occlusion were developed.
-
Extracellular and Intracellular Crystalloid Cardioplegia:
Researchers continued developing the cardioplegic solution as non-cardioplegic techniques are unsuitable for high-risk and prolonged surgeries. A solution containing sodium concentration was administered to achieve the arrest. This solution is equivalent to the intracellular fluid in the myocyte. Later, a solution with sodium concentration resembling the extracellular fluid concentration was developed and delivered using cold antegrade.
-
Blood and Crystalloid Cardioplegia:
Several maneuvers, including adequate perfusion pressures, limiting hemodilution, venting, and decreasing hypothermia, were proposed to enhance subendocardial flow and reduce the occurrence of ventricular fibrillation. It was recognized that the blood cardioplegic solution resuscitates the heart, and blood was substituted for the cardioplegic crystalloid portion, thereby improving the microvascular flow. Various types of research showed that blood allows better myocardium recovery. It also acts as a scavenger for oxygen-free radicals and reduces the occurrence of hemodilution and myocardial edema.
-
Warm and Cold Cardioplegia:
Initially, blood cardioplegia was administered at a normal temperature of 36 degrees Celsius. Later, studies were performed on delivering warm and cold blood cardioplegia. It was proved that oxygen consumption in the arrested heart is minimal at lower temperatures and almost negligible below 20 degrees Celsius. Later, it was discovered that cold blood cardioplegia better restores myocardial functions, especially after prolonged arrest, compared to warm blood cardioplegia.
-
Continuous and Intermittent Cardioplegia:
Long cross-clamping times are required for complex surgeries. Scientists discovered techniques to overcome the problems associated with long cross-clamp times. It was found that instead of protecting the by reducing the mechanical arrest and hypothermia, it was protected through continuous coronary perfusion with normothermic cardioplegic solution. The concept was to increase supply rather than decrease demand.
-
Antegrade and Retrograde Cardioplegia:
Cardioplegia solutions are administered through an antegrade route based on the anatomy. The scientists also tried the retrograde route via the coronary sinus. Though it has various disadvantages, it favors conditions like critical coronary stenosis, heavy calcifications occluding coronary ostia in aortic surgeries, and aortic regurgitation in non-aortic surgeries or if continuous cardioplegia is required.
-
Diluted Cardioplegia and Microplegia:
The dilution followed in the blood cardioplegia solution is four parts of blood to one part of the cardioplegia solution. Hemodilution reduces rouleaux formation and improves microvascular flow. Scientists proposed that limiting the additives and administering them directly to the bloodstream has various benefits, including improved hemoglobin concentrations and oxygen delivery.
How Is Cardioplegia Delivered?
The myocardial protection system is a part of the cardiopulmonary bypass circuit. Cardioplegic solutions can be delivered through different routes. It can be administered as a crystalloid via a simple infusion set. Blood cardioplegia is delivered via a dedicated pump present on the cardiopulmonary bypass machine. It controls the infusion rate, temperature, and ratios of blood and cardioplegia.
What Are the Indications for Myocardial Protection?
Myocardial protection is indicated in open-heart surgeries such as mitral valve repair and aortic valve replacement to access and assess the pathologies in the chambers of the heart.
What Are the Contraindications for Myocardial Protection?
Myocardial protection procedures such as hypothermia are contraindicated in cryoglobulinemia because cooling the blood results in intravascular clumping. Also, cross-clamping is contraindicated in the porcelain aorta due to the risk of stroke and aortic dissection.
Conclusion:
Myocardial protection is essential during cardiac surgeries that require a cardiopulmonary bypass. It forms a significant part of the cardiopulmonary bypass circuit and is crucial for providing a bloodless field operating field that also preserves cardiac function. It is employed in open-heart surgeries to access the chambers of the heart and assess the pathologies. The main aim of this system is to achieve a direct vision of the heart during surgeries while preserving cardiac function. Research is being carried out to discover more advanced and challenging techniques concerning myocardial protection during cardiac surgeries.
