How Cardioplegic Solutions Stop and Protect the Heart

A cardioplegic solution is a specially formulated fluid injected into the coronary arteries or veins during open-heart surgery to deliberately stop the heart. The word itself comes from the Greek for “heart paralysis,” and that is exactly what it does: it creates a still, quiet surgical field so the surgeon can work on valves, coronary arteries, or other cardiac structures without the heart contracting. High-potassium solutions were first introduced for this purpose in the 1950s, and over the following decades nearly every aspect of the approach has been refined, from the chemical recipe to the temperature to the route of delivery.

Why the Heart Needs to Be Stopped

Operating on a beating heart is possible in some situations, but most complex cardiac procedures demand a motionless field. Repairing a valve, patching a septal defect, or replacing a section of the aorta requires precision down to millimeters. A beating heart moves too much and pushes blood through the surgical site. To get around this, the patient is placed on a heart-lung machine (cardiopulmonary bypass), which takes over the job of circulating blood and exchanging oxygen. With the machine doing the work, the surgeon can clamp the aorta and infuse cardioplegic solution to stop the heart entirely.

The challenge is that a stopped heart is also an oxygen-deprived heart. Every minute the aorta stays clamped, heart muscle cells are at risk of ischemic damage. The entire science of cardioplegia revolves around minimizing that damage while keeping the heart still long enough to complete the operation.

How Potassium Stops the Heart

Most cardioplegic solutions work by flooding heart muscle cells with potassium. Under normal conditions, the inside of a cardiac cell has a high concentration of potassium and a low concentration of sodium, creating an electrical gradient that the cell uses to fire and contract. When a solution loaded with extra potassium washes over the heart, it collapses that gradient. The cell membrane depolarizes and the heart loses its ability to generate the rhythmic electrical impulses that drive contraction. The result is rapid, complete arrest.

The trouble is that high potassium is not a benign parking brake. Over more than four decades of use, potassium-based cardioplegia has been linked to ionic imbalances across cell membranes, coronary artery spasm, conduction disturbances, and a form of post-operative weakness called contractile stunning.¹ Early formulations in the 1950s used very high potassium concentrations and were soon recognized as harmful. Since then, surgeons have lowered the potassium dose, changed the carrier fluid, added protective drugs, and adjusted the temperature, all to blunt those side effects while still getting a still heart.²

An Alternative Approach to Arrest

Not everyone is satisfied with potassium-based arrest. A fundamentally different strategy uses a combination of adenosine and lidocaine. Adenosine activates specific receptors on heart muscle cells that hyperpolarize them, meaning the cell’s resting electrical charge is pushed even further from the threshold needed to fire. Lidocaine, meanwhile, blocks the fast sodium channels that initiate each heartbeat.³ Together, they produce arrest without the ionic disruption that comes from dumping potassium across every cell membrane. This “polarized arrest” concept has gained attention in both surgical and transplantation research, though potassium-based solutions remain the standard in most operating rooms worldwide.

Major Cardioplegic Formulations

Dozens of recipes exist, but a few dominate clinical practice. St. Thomas solution, named after the London hospital where it was developed, is a crystalloid (clear, non-blood-based) solution with potassium, magnesium, and procaine. It has been used since the 1970s and remains one of the most common formulations globally. Buckberg solution takes a different approach by mixing the cardioplegic chemicals with the patient’s own blood, delivering oxygen and nutrients along with the arrest-inducing agents.

Del Nido cardioplegia, originally designed for pediatric surgery at Boston Children’s Hospital, has crossed over into adult cardiac surgery with growing enthusiasm. It uses a base of Plasma-Lyte mixed with the patient’s blood in a ratio of roughly four parts crystalloid to one part blood, plus potassium, magnesium, mannitol, sodium bicarbonate, and lidocaine. A distinctive advantage is that a single dose can keep the heart arrested for up to 90 minutes, whereas many traditional solutions require re-dosing every 15 to 20 minutes. That means fewer interruptions during a complex operation.

A systematic review and meta-analysis comparing del Nido to St. Thomas solution found that del Nido was associated with roughly 20 fewer minutes of aortic cross-clamp time in adult surgery and significantly lower rates of defibrillation in both adults and children.⁴ Another meta-analysis confirmed that del Nido can serve as a viable alternative, offering similar postoperative outcomes along with shorter bypass times and lower defibrillation rates.⁵

A prospective randomized trial in adults found that patients who received del Nido cardioplegia returned to a spontaneous heartbeat at higher rates after surgery (about 98% versus 82% for the conventional group) and showed lower troponin levels at 24 hours, suggesting less heart muscle injury, though some of these comparisons were on the edge of statistical significance.⁶ The findings are encouraging, though del Nido was not originally validated for adult hearts with pre-existing disease, which keeps some surgeons cautious about adopting it for every case.

Blood Versus Crystalloid

One of the longest-running debates in cardioplegia is whether to mix the arrest chemicals into the patient’s own blood or deliver them in a clear crystalloid solution. Blood-based cardioplegia carries oxygen, buffers acid, and contains natural free-radical scavengers. A meta-analysis of randomized clinical trials found that blood cardioplegia provides superior heart muscle protection compared with crystalloid, including lower rates of low cardiac output syndrome and smaller early rises in markers of heart muscle damage, though rates of heart attack and death were similar between the two approaches.⁷

In practice, the differences are often modest. A study comparing warm blood cardioplegia to cold crystalloid cardioplegia during coronary bypass surgery found that markers of heart muscle injury, rates of perioperative heart attack, stroke, kidney injury, and length of hospital stay were all similar between the two groups.⁸ The study did note that warm blood cardioplegia required a shorter interval between re-doses to maintain adequate protection. Many surgical teams end up choosing based on institutional tradition and the specific operation rather than on a universal rule.

Cold, Warm, or Somewhere in Between

Temperature is another variable surgeons manipulate. Cooling the heart slows its metabolic rate, which means it burns through its energy stores more slowly during the clamped, oxygen-deprived period. Cold cardioplegia (typically around 4°C) has been standard for decades. Warm cardioplegia (around 37°C, or body temperature) gained popularity on the theory that it avoids the cellular damage cold can cause and allows ongoing aerobic metabolism if the solution carries enough oxygen.

In practice, both strategies perform well for most operations. One area where temperature may make a meaningful difference is in hearts with thickened walls, a condition common in aortic valve disease. A study comparing cold crystalloid to warm blood cardioplegia in aortic valve replacement found that cold cardioplegia produced lower levels of cardiac enzymes at each measurement point, with the biggest difference appearing immediately after surgery. The authors suggested that in the presence of significant left ventricular hypertrophy, cold cardioplegia may offer better protection.⁹ The clinical outcomes (survival, complications, hospital stay) were still similar, so the enzyme differences did not clearly translate into worse recovery for the warm group. Still, many surgeons favor cold solutions for operations expected to involve long cross-clamp times or thickened heart muscle.

How Cardioplegic Solution Gets to the Heart

There are three main delivery routes. Antegrade delivery sends the solution forward through the aortic root, following the normal path of blood into the coronary arteries. Retrograde delivery threads a catheter into the coronary sinus (a large vein on the back of the heart) and pushes the solution backward through the venous system. A combined antegrade-retrograde technique uses both routes, either simultaneously or alternating between doses.

Antegrade delivery is simpler and tends to cool the heart more effectively in straightforward cases. A thermography study found that at the end of a second dose, hearts receiving antegrade cardioplegia reached a surface temperature about 2°C lower than hearts receiving retrograde delivery.¹⁰ But antegrade delivery has a weakness: if a coronary artery is severely blocked, the solution cannot reach the muscle downstream of the blockage. Retrograde delivery bypasses that problem by approaching from the venous side.

The combined technique was developed to get the best of both worlds. In patients with blockages in the left anterior descending artery, retrograde delivery cooled the septum to roughly 12°C, while antegrade delivery alone only reached about 16°C. The combined approach also proved useful in redo operations (where scar tissue makes antegrade delivery unpredictable), during aortic and mitral valve procedures, and in cases of unexpected aortic valve leakage that would let antegrade solution escape.¹¹

Continuous versus intermittent delivery is another consideration. In an animal study simulating a blocked coronary artery, continuous antegrade delivery did not improve acid washout, temperature, or heart function compared to intermittent dosing every 20 minutes. Both approaches resulted in similar tissue acidity and similar degrees of wall motion abnormality in the area beyond the blockage.¹² The finding reinforces a recurring theme: when a coronary artery is blocked, antegrade delivery alone has limits regardless of how often you infuse it.

Monitoring the Arrested Heart

How do surgeons know the cardioplegia is working well enough? The traditional approach relies on a peripheral electrocardiogram, myocardial temperature probes, and visual inspection to confirm the heart is not moving. Research has shown these tools can be insufficient. One study documented persistent electrical activity inside the heart wall even when the surface appeared still and the ECG showed a flat line. That silent electrical activity persisted for a significant period before visible contractions returned, meaning the heart was not as “arrested” as it appeared.¹³ Continuous monitoring of intramural electrical activity appears more reliable, though it is not yet standard practice everywhere.

Researchers have also used intramyocardial pH probes, which track the acidity inside the heart muscle in real time. Because an ischemic heart produces acid as it burns through fuel without oxygen, a dropping pH signals inadequate protection. One study found that pH during hypothermic arrest reliably reflected the degree of oxygenation in the cardioplegic solution and predicted how well the heart would recover afterward.¹⁴ More recently, fluorescence imaging using indocyanine green dye has been tested in animal models as a way to visualize cardioplegia distribution across the heart surface in real time, with imaging defects clearly highlighting areas of poor delivery.¹⁵

Reperfusion Injury and the Paradox of Restarting

The heart must eventually be restarted, and that moment carries its own dangers. When oxygenated blood floods back into tissue that has been oxygen-deprived, it can trigger a wave of damage sometimes worse than the ischemia itself. This reperfusion injury involves the rapid generation of oxygen free radicals, an inflammatory reaction in the coronary microvasculature, calcium overload inside cells, and dysfunction of the inner lining of blood vessels.¹⁶ The clinical manifestations range from temporary “stunning” (the heart contracts weakly for hours or days before recovering) to arrhythmias, low blood pressure, and in severe cases, heart failure requiring mechanical support.

The potential for this type of damage exists not only when the aortic clamp is removed at the end of surgery but also with each re-dose of cardioplegia during the operation, since every infusion briefly reperfuses the heart muscle.¹⁷ The severity of ischemia during the clamped period sets the stage for how bad the reperfusion injury will be, making effective cardioplegia delivery during arrest a critical factor in limiting the damage at the end.

At the microvascular level, crystalloid cardioplegia followed by reperfusion has been shown to impair the ability of tiny arterioles to relax in response to normal signals. Adding an enzyme that scavenges superoxide free radicals to the cardioplegic solution markedly preserved these endothelium-dependent responses, implicating free radicals generated during high-potassium exposure and reperfusion as key culprits in the vessel damage.¹⁸

Pediatric Hearts and Age-Dependent Vulnerability

Children are not small adults when it comes to cardioplegic protection. A study examining metabolic stress during cold-crystalloid cardioplegia found that infants experienced greater reperfusion injury and worse clinical outcomes than older children, with both factors tied to how long the heart was clamped. Cyanosis (the low-oxygen state present in certain congenital heart defects) did not worsen outcomes in infants but significantly worsened reperfusion injury and clinical results in older cyanotic children compared to their acyanotic counterparts.¹⁹ These findings have pushed pediatric cardiac surgeons to tailor their cardioplegic strategies more carefully to patient age and underlying defect, and del Nido’s original pediatric design reflects that awareness.

There are also rare but serious events that underscore how dangerous these solutions can be outside their intended space. A case report described the accidental systemic administration of a full liter of cardioplegic solution to a child during surgery. The result was refractory low blood pressure, severe dilutional anemia, dangerously high potassium and magnesium levels, and a profound metabolic acidosis.²⁰ The episode highlights that these solutions are designed to be confined to the heart’s own vessels, not to enter the general circulation in large volumes.

Cardioplegia in Heart Transplantation

Cardioplegic solutions play a different role in transplantation. Once a donor heart is removed, it must be preserved until it can be implanted in the recipient, a window that can stretch to several hours. The initial arrest of the donor heart typically uses a cold cardioplegic flush, and the organ is then stored on ice or, increasingly, on a machine perfusion device that continuously circulates a preservation solution through the coronary arteries.

The composition of that preservation solution matters. Research comparing different formulations for machine perfusion found that donor hearts could be supported over extended storage intervals with ongoing aerobic metabolism and minimal lactic acid buildup. Adding an oncotic agent (a substance that holds water inside blood vessels rather than letting it leak into tissue) was important for limiting myocardial swelling, with a specific machine-perfusion solution showing superiority in reducing edema development during storage.²¹ Edema stiffens the heart and impairs function after transplantation, so keeping it in check is a practical priority during the preservation window.

Operating Without Cardioplegic Arrest

Not every cardiac operation uses cardioplegia. Off-pump coronary artery bypass (OPCAB) is performed on a beating heart, using mechanical stabilizers to hold a small area still while the surgeon sews grafts. On-pump beating-heart surgery is a hybrid: the patient goes on the heart-lung machine for hemodynamic support, but the heart is not clamped or arrested. These strategies avoid ischemia-reperfusion injury altogether.

A network meta-analysis comparing on-pump cardioplegic arrest, off-pump, and on-pump beating-heart approaches for patients with acute coronary syndromes found that off-pump surgery had the highest probability of the lowest 30-day mortality, with a subgroup analysis of patients with acute heart attacks showing a 72% reduction in the likelihood of dying within 30 days after off-pump surgery.²² These results come with wide confidence intervals, meaning the uncertainty is real, but they illustrate why the field has not abandoned alternatives to cardioplegic arrest. Off-pump approaches have their own drawbacks, including technical difficulty and concerns about long-term graft patency, so cardioplegic arrest remains the dominant method for complex operations.

Where the Research Is Heading

Researchers continue to look for additives and strategies that reduce reperfusion injury beyond what current solutions achieve. Recent work has focused on adjuvant components added to existing cardioplegic solutions, including antioxidants, anti-inflammatory agents, and metabolic substrates that help heart cells recover more quickly once blood flow resumes.²³ Polarized arrest strategies using adenosine-lidocaine combinations remain an active area of investigation, as do normothermic perfusion techniques for transplant organ preservation that aim to keep the donor heart metabolically active rather than chilling it into hibernation. The field is incremental rather than revolutionary at the moment, but the accumulation of small improvements in solution design, delivery monitoring, and reperfusion management continues to push the safety margins of heart surgery outward.