Residency · Residency · Cardiothoracic Surgery
Myocardial Protection Strategies
Introduction
Myocardial protection during cardiac surgery aims to minimize ischemic injury to the heart while the aorta is cross-clamped and the heart is arrested. The strategy encompasses cardioplegia delivery, hypothermia, and metabolic optimization. Despite decades of clinical use, the ideal myocardial protection strategy remains debated, and the surgeon must understand the principles underlying each approach to tailor protection to the clinical scenario.
Principles of Myocardial Injury During Aortic Cross-Clamping
Ischemia-Reperfusion Injury
Aortic cross-clamping eliminates coronary perfusion, initiating ischemia. ATP depletion leads to intracellular calcium accumulation, cellular swelling, and acidosis. Reperfusion upon cross-clamp removal paradoxically worsens injury through reactive oxygen species generation, complement activation, and endothelial dysfunction. The goal of myocardial protection is to reduce metabolic demand during ischemia and attenuate reperfusion injury.
Determinants of Myocardial Oxygen Demand
Electromechanical activity is the most important determinant: the arrested, quiescent heart consumes 90% less oxygen than the beating heart. Temperature plays a significant role, as hypothermia reduces metabolic rate by approximately 7% per degree Celsius. Wall tension contributes as well, with decompressed ventricles having lower oxygen demand. Basal cellular metabolism continues even in the arrested heart but is further reduced by hypothermia.
Cardioplegia Solutions
Crystalloid Cardioplegia
St. Thomas' solution is a high-potassium crystalloid solution (16-20 mEq/L K+) that induces diastolic arrest by depolarizing the cell membrane. Its advantages include simplicity, low cost, and excellent visibility in the operative field. Disadvantages include hemodilution, limited oxygen-carrying capacity, and shorter duration of protection requiring re-dosing every 20-30 minutes.
Blood Cardioplegia
Blood cardioplegia uses a mixture of blood and crystalloid additive, typically at a 4:1 blood-to-crystalloid ratio. The Buckberg protocol involves warm induction, cold maintenance, and warm reperfusion (the "hot shot"). Advantages include oxygen-carrying capacity, natural buffering, free radical scavenging, and reduced hemodilution. Blood cardioplegia provides superior myocardial protection in high-risk patients with severe LV dysfunction or acute ischemia.
del Nido Cardioplegia
Originally developed for pediatric cardiac surgery, del Nido cardioplegia is increasingly adopted in adult practice. It provides prolonged arrest from a single dose lasting up to 60-90 minutes without re-dosing. The solution contains lidocaine (a sodium channel blocker), magnesium (a calcium antagonist), and mannitol (a free radical scavenger) in a 4:1 blood-to-crystalloid mixture with lower potassium concentration. The single-dose convenience reduces operative interruptions, and growing evidence supports equivalence or superiority to traditional blood cardioplegia in adults.
Custodiol (HTK Solution)
Custodiol is a histidine-tryptophan-ketoglutarate solution of the intracellular type. A single large-volume dose (20 mL/kg) provides protection lasting 90-180 minutes. It is commonly used in transplant procurement and complex reconstructive surgery. While it offers prolonged single-dose protection, the large volume causes significant hemodilution.
| Solution | Type | Composition | Arrest Duration | Re-dosing | Key Advantages | Key Disadvantages |
|---|---|---|---|---|---|---|
| St. Thomas' (Crystalloid) | Extracellular crystalloid | High K+ (16-20 mEq/L) | 20-30 min | Every 20-30 min | Simple, low cost, clear field | Hemodilution, no O2 carrying capacity |
| Blood Cardioplegia (Buckberg) | 4:1 blood:crystalloid | Modified blood with K+ additive | 20-30 min | Every 15-20 min | O2 carrying, buffering, free radical scavenging | Complex delivery, field obscuration |
| del Nido | 4:1 blood:crystalloid | Lidocaine, Mg2+, mannitol, lower K+ | 60-90 min | Single dose | Single-dose convenience, fewer interruptions | Limited long-term adult data |
| Custodiol (HTK) | Intracellular crystalloid | Histidine-tryptophan-ketoglutarate | 90-180 min | Single dose (20 mL/kg) | Prolonged protection, single dose | Significant hemodilution from large volume |
Delivery Methods
Antegrade Delivery
Antegrade cardioplegia is administered via the aortic root (between the cross-clamp and the aortic valve) or directly into the coronary ostia, providing physiologic distribution through the coronary arteries. Its limitation is that it may not adequately protect myocardium distal to severe coronary stenoses. Delivery is pressure-limited (typically 150-200 mmHg root pressure) to avoid endothelial injury.
Retrograde Delivery
Retrograde cardioplegia is administered via a coronary sinus catheter positioned through the right atrium, perfusing the myocardium in a retrograde fashion through the venous system. It protects myocardium distal to coronary occlusions and is useful during aortic valve surgery when root delivery is impractical. However, it provides poor perfusion of the right ventricle (since most RV venous drainage bypasses the coronary sinus) and incomplete distribution overall.
Combined Antegrade-Retrograde
Alternating or simultaneous antegrade and retrograde delivery provides the most complete myocardial distribution. This approach is particularly valuable in patients with severe coronary artery disease, LV hypertrophy, or prolonged cross-clamp times.
Direct Ostial Cannulation
Direct ostial cannulation is used during aortic valve or root surgery when the aortic root is opened. Selective cannulation of the left and right coronary ostia provides antegrade delivery, though it requires periodic re-dosing and interrupts the surgical procedure.
| Delivery Method | Route | Best For | Limitations |
|---|---|---|---|
| Antegrade | Aortic root or coronary ostia | Standard cases; physiologic distribution | Poor perfusion distal to coronary stenoses |
| Retrograde | Coronary sinus catheter | Severe CAD; aortic valve surgery | Poor RV perfusion; incomplete distribution |
| Combined antegrade-retrograde | Alternating or simultaneous | Severe CAD, LVH, long cross-clamp | More complex setup |
| Direct ostial cannulation | Selective coronary cannulation | Aortic root/valve surgery | Periodic interruption of surgery |
Temperature Strategies
Cold Cardioplegia (4-8 degrees C)
Cold cardioplegia is the standard approach, combining chemical arrest with hypothermic metabolic suppression. Topical cooling with ice slush or cold saline in the pericardium augments protection. Re-dosing is required every 20-30 minutes for crystalloid solutions, with longer intervals for blood cardioplegia.
Warm Cardioplegia
Continuous warm blood cardioplegia at 37 degrees C maintains aerobic metabolism during arrest and avoids hypothermic injury and rewarming time. However, it requires continuous delivery, as any interruption causes normothermic ischemia with higher risk. It is used by some surgeons for CABG surgery.
Tepid Cardioplegia (27-30 degrees C)
Tepid cardioplegia is an intermediate approach that balances metabolic suppression with enzymatic function, reducing the risk of hypothermic injury while maintaining adequate protection.
Warm Induction and Terminal Warm Reperfusion (Hot Shot)
Warm induction uses initial warm blood cardioplegia to arrest the heart in a metabolically favorable state before cooling. The hot shot delivers warm blood cardioplegia just before cross-clamp removal, providing substrate for aerobic metabolism, washing out metabolic waste, and reducing reperfusion injury. This approach is particularly beneficial in patients with acute ischemia or severe LV dysfunction.
Special Situations
Severe Left Ventricular Hypertrophy
Increased myocardial mass requires higher cardioplegia volumes and more frequent re-dosing. Combined antegrade-retrograde delivery is essential for uniform distribution, and there is a higher risk of subendocardial ischemia, warranting consideration of continuous or near-continuous perfusion strategies.
Acute Myocardial Ischemia
Substrate-enriched warm blood cardioplegia induction with glutamate and aspartate may improve recovery. Controlled reperfusion with modified reperfusate reduces injury, and excessive crystalloid hemodilution should be avoided.
Minimally Invasive and Robotic Surgery
Peripheral cannulation with endoaortic balloon occlusion replaces direct aortic cross-clamping. Antegrade cardioplegia is delivered through the endoaortic balloon catheter, and retrograde delivery uses a percutaneous coronary sinus catheter.
Key Clinical Pearls
Del Nido cardioplegia has revolutionized adult cardiac surgery by providing single-dose protection for up to 90 minutes, reducing cross-clamp interruptions. Combined antegrade-retrograde delivery provides the most uniform myocardial protection, especially in patients with coronary disease or LV hypertrophy. The terminal warm reperfusion "hot shot" reduces reperfusion injury and should be considered in all cases with prolonged cross-clamp times. Topical hypothermia augments cardioplegic protection, but care must be taken to avoid phrenic nerve injury from ice application. The adequacy of myocardial protection is reflected in post-bypass ventricular function, and persistent regional wall motion abnormalities suggest inadequate distribution.
References
- Matte GS, del Nido PJ. History and use of del Nido cardioplegia solution at Boston Children's Hospital. J Extra Corpor Technol. 2012;44(3):98-103.
- Buckberg GD, Athanasuleas CL. Cardioplegia: solutions or strategies? Cardiovasc Res. 1998;38(3):609-614.
- Viana FF, Shi WY, Hayward PA, et al. Custodiol versus blood cardioplegia in complex cardiac operations. J Thorac Cardiovasc Surg. 2013;146(6):1268-1275.
- Ad N, Holmes SD, Massimiano PS, et al. The use of del Nido cardioplegia in adult cardiac surgery: a prospective randomized trial. J Thorac Cardiovasc Surg. 2018;155(3):1011-1018.