Rebecca Lee Phillip and Tessa London
This chapter is a revision and update of that included in previous editions of the TSRA Review written by Michael D. Bolaños (2nd edition), Ramesh Singh (1st edition), and Ashok Babu (1st edition).
General Principles
Cardiopulmonary bypass (CPB) has revolutionized the way by which cardiovascular diseases and congenital anomalies of the heart are repaired. Its goals are directed towards the creation of a motionless and bloodless field in order to facilitate the surgical intervention required. The basic components of the CPB circuit are the pump, reservoir, oxygenator, arterial and venous cannulas, and tubing.
While the use of CPB has enabled surgeons to push the limits of surgical repair of the heart, its use is not without consequence. The use of CPB activates both intrinsic and extrinsic coagulation cascades and stimulates the production of inflammatory mediators as blood is exposed to the non-endothelialized circuit. Primary intrinsic plasma proteins involved in this are factors XI, XII, prekallikrein, and kininogen. This in and of itself may lead to coagulopathies and hemodynamic derangements. The interface at the level of the membrane oxygenator has the greatest effect on these blood components as the complement proteins begin to coat the membrane. Other complications may occur such as the precipitation of cold agglutinins in patients with these proteins during the cooling phase of CPB.
Cannulation
Venous Cannulation
To successfully begin and maintain CPB, both arterial and venous cannulation must be performed. Venous cannulation is typically achieved by the use of a single-stage, dual-stage, or triple stage cannulation. Bi-caval cannulation involves placement of a single-stage cannula in the SVC and IVC. Alternatively, a dual or triple-stage cannula can be placed in the right atrium such that the proximal opening of the cannula is localized in the RA and a distal opening in the IVC. Drainage into the reservoir from the venous system is largely a passive process and determined by the CVP, height differential of the pump and table, and resistance in the venous cannulae. Often, the application of vacuum is needed, particularly when femoral cannulation is used. The use of femoral cannulation is useful in scenarios such as reoperative surgery, emergent CPB, and other applications that do not require sternotomy. Complications arising from venous cannulation include inadequate drainage from the right heart, atrial arrhythmias, atrial or caval tears, air embolization, unexpected decannulation, and obstruction of the cavae from tourniquets or tying of improperly placed pursestring sutures.
Arterial Cannulation
Arterial cannulation strategies are largely dependent on the surgery to be performed. For most cardiac operations, distal ascending aortic cannulation is performed. However, cannulae may be placed in other major arterial vessels depending on the pathology encountered. During aortic cannulation, the aorta is palpated or epiaortic ultrasound is performed in order to find a clear, soft area devoid of atheroma. Typically, two purse-string sutures are placed and following an aortotomy, the cannula is inserted so that the bevel is directed towards the arch. Complications arising from aortic cannulation are largely due to malposition of the cannula and are often reflected in the pressure observed through the arterial line. Prevention of cannula malposition may be done by ensuring adequate exposure of the aortic adventitia, making an adequate-sized aortotomy, and picking a suitable location devoid of atheroma. The use of epiaortic ultrasound is the best method in determining this and can aid in identifying heavily calcified, porcelain aortas (1-4% of patients). High-velocity jets out of the cannula tip may cause a sand-blasting phenomenon potentially leading to iatrogenic dissections, atheroembolic events, and disruption of flow in nearby vessels. It is imperative that line pressures be tested by the perfusionist prior to the initiation of CPB. Aortic dissection occurs in approximately 0.1% of aortic cannulations and is commonly associated with severely calcified aortas. The first clues of aortic dissection include discoloration of the aorta beneath the adventitia, an increase in arterial line pressure, and a sharp reduction in return to the venous reservoir. Later signs include ischemic changes on EKG and a dissection flap on TEE. Should an aortic dissection be suspected, prompt discontinuation of CPB is required. The arterial cannula should be promptly transferred to a peripheral artery or an area of uninvolved aorta, making sure to be in the true lumen. Hypothermia protocols should be promptly initiated and the aorta should be patched, replaced, or directly sutured. The survival of an iatrogenic dissection identified intraoperatively approaches 85% versus postoperative identification nearing 50%. Other pitfalls that may be encountered include incomplete clamping of the aorta and failure to identify hemodynamically significant aortic insufficiency. Alternative cannulation strategies such as peripheral cannulation have their own complications including limb ischemia, compartment syndrome, and retrograde dissections.
Cardioprotection
After the initiation of CPB, the goals of executing a successful operation include adequate protection of the myocardium. This is achieved by the proper use of cardioplegia, maintaining electrochemical silence, and ensuring the heart remains decompressed. The goal of cardioplegia is to ensure rapid diastolic arrest of the heart by infusion of a highly concentrated potassium solution along with other plasma buffers. Cardioplegia may be crystalloid or have some component of blood in it. It may be given in an antegrade (via aortic root or directly down the coronary ostia) or retrograde (via coronary sinus). Adequate cardioplegia is needed to achieve electrochemical silence and preserve the myocardium. Particular attention should be given to proper cross-clamp placement. In patients with aortic insufficiency, high-grade left main or right coronary lesions, or distention, retrograde cardioplegia is useful, although its initiation is slower. It can also help in flushing the coronary arteries of air and emboli. Additionally, the use of antegrade cardioplegia may be insufficient to completely protect the right ventricle and retrograde cardioplegia can be helpful. Once cardioplegia has been initiated, the cardioprotective effects of hypothermia are often expressed using the Q10 rule, which says that for every 10°C drop in temperature, the metabolic rate decreases by 50%.
Cardiac Venting
The maintenance of a decompressed heart and a bloodless field is often established using various venting techniques that are aimed at preventing left heart distention. The over-distended left ventricle will not receive adequate cardioplegia and is at risk for ischemia. Blood can enter the left ventricle from incomplete drainage of the right heart, aortic insufficiency, Thebesian veins, bronchial arteries, and aortopulmonary collaterals returning blood through the pulmonary veins, or the presence of a PFO or PDA. Venting minimizes left ventricular distention and helps in the rewarming and de-airing process once CPB is weaned. Various venting strategies can be used but typical sites include direct venting from the aortic root (as a side port on the cardioplegia cannula), right superior pulmonary vein, main PA (there are no valves in the pulmonary circulation), or direct left atrial or left ventricular venting.
Conduction of Cardiopulmonary Bypass
After systemic heparinization and successful initiation of CPB, flows of approximately 2-2.5 L/min/m2 are maintained. Serial arterial blood gases are drawn and titration of the FiO2 and the flow of gas (sweep) is regulated by the perfusionist. Mean arterial pressures of 60 mmHg are desired in adults and supported by the administration of vasopressors or vasodilators. Systemic temperatures are also regulated by the perfusionist based on the procedure performed, the need for deep hypothermic circulatory arrest, and the temperatures requested by the surgeon.
Weaning and Separation of Cardiopulmonary Bypass
Separation from CPB is typically performed once the patient has been adequately rewarmed, is ventilating and oxygenating, has a perfusing rhythm, and the heart has been sufficiently deaired. Separation from CPB is a concerted effort between the surgeon, perfusionist, and anesthesiologist. The arterial flows are gradually decreased while simultaneously reducing the venous drainage, essentially filling the heart and allowing it to eject blood. It is useful for the surgeon to perform a checklist to ensure that the patient is ready to be safely weaned from the cardiopulmonary bypass circuit. The four key factors to remember are bleeding, beating, breathing, temperature.
Bleeding – There should be no active bleeding and adequate hemostasis must be achieved. All suture lines, anastomotic sites, and cannulation sites should be checked for bleeding.
Beating – The heart should be contracting well and have adequate ejection of blood, without becoming distended, and this can be assessed as the CPB pump flows are slowly weaned. The heart may need to be defibrillated at this time. The heart should be in an adequate perfusing rhythm. Pacing wires should be checked and appropriately sensing and capturing.
Breathing – The anesthesia team should be ventilating the patient and adequate ventilation needs to be confirmed between the surgeon and anesthesiologist.
Temperature – The patient should be rewarmed to normothermia (approximately 36°C)
Special Considerations
While the initiation, maintenance, and separation of CPB has been modified and perfected over the years, special attention must be given to common anomalies and pitfalls.
Airlock and air emboli
The termination of CPB is largely dependent on successful de-airing and the administration of protamine for heparin reversal. Incomplete deairing of the circuit at all stages of CPB can have devastating effects. While an arterial air embolus is significantly worse, the presence of an airlock or air within the venous line(s) should not be ignored. Common etiologies of air within the venous line(s) includes tears at the cannulation site or dislodged cannulae. If promptly identified, elevation of the venous lines, and replacement or repositioning of the venous cannulae may remedy the situation. If arising from a caval tear, repair must be performed in order to restore the bypass circuit. In contrast, an arterial air embolism carries a much larger morbidity and mortality. If identified, emergent de-airing strategies must be performed, including: cessation of bypass, clamping of the venous and arterial lines, steep Trendelenburg positioning, 100% oxygen, aortic root venting, retrograde cerebral perfusion/occlusion of carotids, de-airing of the bypass circuit, and addition of the necessary volume to the cardiopulmonary bypass reservoir. Once de-airing techniques have been performed, CPB should be re-instated, and the procedure should be completed. Some advocate for the administration of steroids and mannitol. Hyperbaric oxygen postoperatively has also been recommended where possible.
Protamine reactions
Derived from the sperm of salmon, protamine is a cationic protein used to neutralize the anionic heparin protein in a 1:1 ratio. After the discontinuation of CPB, a test dose of protamine is often administered to assess for reactions. Allergic responses to protamine are divided into three types of reactions: Type I (hypotension from rapid administration, mediated by histamine release), Type II (anaphylactic reaction, mediated by an antiprotamine IgE antibody), or Type III (acute pulmonary vasoconstriction leading to RV failure, mediated by complement activation and thromboxane A2). Prior exposure to protamine (protamine-zinc insulin) or vasectomized patients are at greatest risk for Type II reactions due to cross-reactivity. Management of all reactions typically involves discontinuation of the protamine infusion, steroids, and hemodynamic support. For type II reactions, chlorpheniramine is used and for Type III reactions, full dose heparin is administered, and CPB may need to be reinstituted.
Persistent left superior vena cava (PLSVC)
A PLSVC is present in up to 0.5% of the population and typically drains into the coronary sinus. Other variations include drainage directly into the LA. A high index of suspicion should be used when a small innominate vein is visualized in the setting of a large coronary sinus on echo. Its presence is clinically significant in the sense that retrograde cardioplegia will not function and venous drainage during bicaval cannulation may be insufficient. If an adequate-sized innominate vein is present (30% of patients), the PLSVC can be occluded during CPB. If the innominate vein is absent (40%) or small (33%), occlusion of the PLSVC may cause venous hypertension and possible cerebral edema. In these patients, direct cannulation of the PLSVC or via the coronary sinus should be performed instead of simple bicaval cannulation. In patients without a right SVC (approximately 20% of patients with PLSVC), the left SVC cannot be occluded.
Suggested Readings
- Abdelhadi Ismail, Szabolcs Miskolczi, Sunil Ohri. Southampton Reviews in Cardiothoracic Surgery Chapter Two: Cardiopulmonary Bypass. CTSNet.org. March 2019.
- BA Zwischenberger, JB Zwischenberger. Cardiopulmonary Bypass. STS Cardiothoracic Surgery E-Book. The Society of Thoracic Surgeons and Unbound Medicine, 2020. https://ebook.sts.org/sts/search?st=OSS&q=cardiopulmonary+bypass
- Bojar RM. Cardiopulmonary Bypass. Manual of Perioperative Care in Adult Cardiac Anesthesia, fifth edition, Wiley-Blackwell, 2011, pp 229-261.
- Hammon JW, Hines MH. Extracorporeal Circulation. Cardiac Surgery in the Adult, fifth edition, edited by L Cohn and D Adams, McGraw-Hill Education, 2018, pp 299-346.
