Cardiopulmonary Bypass (CPB)
Cardiopulmonary bypass (CPB), also known as extracorporeal circulation, is a life-supporting technique that temporarily replaces the heart and lung functions during open-heart surgery. Using devices such as blood pumps and oxygenators, CPB facilitates blood circulation and gas exchange. The fundamental principle involves diverting venous blood from the body to an external oxygenator, where it is oxygenated, carbon dioxide is removed, temperature is regulated, and filtration occurs. The blood is then pumped back into the arterial system to ensure adequate perfusion of tissues and organs, maintaining their basic metabolic and functional needs. CPB allows for a bloodless or minimal-blood surgical field in open-heart procedures and is also utilized in other applications such as severe respiratory failure, complex endotracheal intubation, and liver transplant support.
Basic Components of CPB
The CPB system primarily consists of a heart-lung machine and its accessories, which include blood pumps (artificial heart), oxygenators (artificial lung), heat exchangers, blood reservoirs, filters, and arterial and venous cannulas.
Blood Pump
The blood pump replaces the heart's blood ejection function and includes roller pumps and centrifugal pumps.
Roller Pumps
These use rotating pump heads to compress a flexible tube in an alternating manner, driving unidirectional blood flow. The diameter of the tubing determines the blood flow per revolution, while adjusting the rotation speed controls the flow rate per minute.
Centrifugal Pumps
These utilize a drive motor and magnetic couplings to spin multilayered rotating cones or impellers inside the pump, generating centrifugal force to drive unidirectional blood flow. Advantages include reduced blood component damage, making them suitable for prolonged CPB or ventricular support.
Membrane Oxygenator
The membrane oxygenator is made of biocompatible, semi-permeable materials that enable gas exchange through diffusion. Oxygen is infused and carbon dioxide is removed without direct contact between blood and gas, minimizing blood damage and the formation of microemboli. Membrane oxygenators are further classified as:
- Hollow Fiber Membrane Oxygenators: These use thin-walled polypropylene hollow fibers for gas exchange and are commonly used in standard open-heart surgeries.
- Non-Porous Silicone Membrane Oxygenators: These achieve gas exchange through diffusion and are highly biocompatible with minimal plasma leakage and limited blood damage, making them suitable for long-duration extracorporeal membrane oxygenation (ECMO).
Heat Exchanger
This is a device that regulates blood temperature using circulating water and thermally conductive thin metal isolation plates.
Filter
Filters are composed of polymer mesh materials with a pore size of 20–40 μm and are placed in the arterial blood pathways. They effectively remove microemboli, thrombi, fat emboli, and small tissue fragments during CPB.
Other Components
Additional equipment includes vascular cannulas, connecting tubing, blood reservoirs, and monitoring systems.
CPB Implementation
Preparation for CPB
Individualized CPB plans are formulated based on the patient’s condition and the type of surgery. Blood dilution is often required during CPB, typically achieved with moderate dilution to a hematocrit level of 22%–25%. Crystalloids, colloids, albumin, plasma, or red blood cells may be chosen as priming solutions depending on the patient’s condition.
Establishment of CPB
Heparin (300–350 U/kg) is administered via central venous injection to maintain an activated clotting time (ACT) of ≥480–600 seconds. Cannulas are sequentially inserted into the ascending aorta and superior and inferior venae cavae, then connected to the primed CPB circuit.
Hypothermia During CPB
Depending on the surgical requirements, hypothermia techniques may be employed during CPB. Cooling ranges are typically categorized as follows:
- Mild hypothermia (32–35°C)
- Moderate hypothermia (26–31°C)
- Deep hypothermia (20–25°C)
- Profound hypothermia (15–19°C)
Mild-to-moderate hypothermia is commonly used, while deep hypothermia is generally reserved for circulatory arrest procedures.
CPB Circulation
The perfusion flow rate of the heart-lung machine is calculated based on the patient’s body weight or body surface area. For adults, the normothermic perfusion rate is 60–80 ml/(kg·min). In younger patients with higher basal metabolic rates, flow rates are correspondingly higher. For example, in children weighing 10–15 kg, perfusion rates may range from 100–150 ml/(kg·min), while in children under 10 kg, rates can reach 150–200 ml/(kg·min). During the phases from CPB initiation to aortic cross-clamping and from aortic unclamping to CPB cessation, both the heart’s ejection and the pump’s output simultaneously contribute to aortic blood flow. This dual circulation method facilitates temperature regulation and aids cardiac functional recovery.
CPB Ultrafiltration
Ultrafiltration removes water from the blood using transmembrane hydrostatic pressure differences while retaining high-molecular-weight substances (e.g., proteins) and blood cells. This process concentrates the blood, reduces inflammatory mediators, and alleviates tissue edema.
Monitoring During CPB
Routine monitoring includes mean arterial pressure, maintained at 50–70 mmHg, and central venous pressure, which helps evaluate blood volume. Monitoring of blood pump pressure reflects resistance at the aortic cannula end. Additional monitoring involves activated clotting time, nasopharyngeal and blood temperatures, perfusion flow rates, urine output and color, blood gas analysis, and blood electrolytes.
Termination of Cardiopulmonary Bypass
CPB is discontinued under conditions where the heart is adequately filled, myocardial contraction is strong, mean arterial pressure sits at 60–80 mmHg, nasopharyngeal temperature measures between 36–37°C, and hemoglobin concentration is ≥80 g/L for adults, ≥90 g/L for children, and ≥110 g/L for infants. Electrocardiography should be stable, and blood gas analysis and electrolytes should be within normal ranges. Following termination of CPB, protamine sulfate is intravenously administered to neutralize heparin, with a protamine-to-heparin ratio of 1.5:1.
Pathophysiology of Cardiopulmonary Bypass
CPB is inherently a non-physiological process essentially resembling a state of shock, characterized by impaired tissue microcirculatory perfusion. Common pathophysiological manifestations include metabolic acidosis, hypokalemia, blood cell destruction, thrombocytopenia, and the production of inflammatory mediators. Prolonged CPB durations exacerbate organ dysfunction, potentially leading to failure of the liver, kidneys, lungs, brain, and gastrointestinal system.
Extracorporeal Membrane Oxygenation (ECMO) and Extracorporeal Life Support (ECLS)
Extracorporeal membrane oxygenation (ECMO) is a form of extracorporeal life support (ECLS) that uses a system comprising an artificial heart-lung machine and a closed circuit to replace heart and lung functions over an extended period. Its primary role is to provide time for treatment and recovery in cases of cardiac or pulmonary dysfunction.
Myocardial Protection
During CPB-based open-heart surgeries, coronary blood flow is interrupted to create a bloodless surgical field. This interruption can result in myocardial ischemia, hypoxia, and ischemia-reperfusion injury (IRI). Measures taken to mitigate these effects are collectively known as myocardial protection.
Under ischemic and hypoxic conditions, the myocardium relies on minimal energy derived from anaerobic glycolysis due to impaired oxidative metabolism. This leads to dysfunction of the myocardial cell membrane, disruption of intracellular electrolyte dynamics, and massive calcium influx into cells, resulting in sustained myocardial contraction. Severe cases involve the release of intracellular enzymes and myocardial cell death. Upon reoxygenation and reperfusion, myocardial damage is further exacerbated, manifesting as impaired oxygen utilization, depletion of high-energy phosphates, myocardial edema, and reduced compliance. These phenomena define ischemia-reperfusion injury.
The primary mechanisms of ischemia-reperfusion injury involve energy depletion, calcium overload, and oxidative damage caused by free radicals. Myocardial protection measures focus on preserving and replenishing high-energy phosphates, minimizing their consumption and loss, preventing intracellular calcium overload, and eliminating the toxic effects of oxygen free radicals. The critical aspect of myocardial protection is the prevention of high-energy phosphate depletion.
Composition of Cardioplegic Solution
Cardioplegic solutions are a vital component of myocardial protection. Based on their ionic composition, these solutions are divided into "extracellular type" and "intracellular type."
Extracellular Cardioplegic Solution
Crystalloid Cardioplegic Solution: Represented by St. Thomas' Hospital solution.
Modified St. Thomas' Solution with Blood: A mixture of oxygenated blood from the CPB circuit and high-potassium crystalloid solution at a ratio of 4:1, maintaining a potassium concentration of 20–24 mmol/L.
Intracellular Cardioplegic Solution
HTK Solution (Histidine-Tryptophan-Ketoglutarate Solution): Composed of low sodium, slightly elevated potassium, and histidine as a buffering agent. This solution provides buffering against intracellular acidosis over a wide temperature range (5–35°C) and offers myocardial protection for up to 2 hours after a single infusion. Sequential perfusion using cold crystalloid cardioplegia and HTK solution can protect a donor heart during cold ischemia for 6–8 hours.
Key Principles for Cardioplegic Solution Composition
High potassium levels induce rapid diastolic arrest, preventing electromechanical activity and reducing energy consumption.
Lowered myocardial temperature reduces metabolic rates and preserves energy reserves. Commonly, 4°C cardioplegic solution is used, with ice slush applied for adults and ice water applied locally to the pericardial cavity in children.
Provision of energy substrates maintains the energy supply required during ischemia and reperfusion phases.
Additional properties include alkalinity (pH 7.6–8.0), hypertonicity [320–380 mOsm/(kg·H2O)], and membrane stabilizers (lidocaine or procainamide) to create a favorable metabolic environment, preserve cell structure, and maintain cell membrane proton pump function.
Methods of Cardioplegic Solution Administration
There are two main methods for administering cardioplegic solution:
- Antegrade Perfusion: Delivered via the ascending aorta or coronary ostia, this technique is the most widely used in clinical practice.
- Retrograde Perfusion: Administered via the coronary sinus, this method is suitable for patients where antegrade perfusion is difficult or where severe coronary artery stenosis or occlusion exists.
Antegrade and retrograde perfusion techniques can be combined depending on the clinical scenario.