• Cardiopulmonary bypass (CPB) machines temporarily take over the function of the heart and lungs. It is often referred to as a "heart-lung-machine."
  • It redirects venous blood return away from the heart to the bypass machine where it adds oxygen and removes carbon dioxide. It returns the blood to the systemic circulation via a large artery.
  • Nearly all blood flow through the heart and the lungs stops.
  • Machines are operated by allied health professionals known as perfusionists.
• Surgical procedures in which CPB are used:
  • Coronary artery bypass surgery
  • Cardiac valve repair and/or replacement
  • Repair of large septal defects
  • Repair and/or palliation of congenital heart defects
  • Repair of some large aneurysm (cerebral aneurysm, aortic aneurysm)
  • Pulmonary thrombectomy
  • Heroic measures when oxygenation or perfusion of organs cannot be maintained by other means (e.g., local anesthetic systemic toxicity, anterior mediastinal mass).

• Basic circuit of the CPB machine:
  • Venous cannulae are placed in the right atrium and/or in the inferior or superior vena cava to receive and redirect the patient's blood away from the heart and lungs into a venous reservoir.
  • Oxygenator: From the venous reservoir, the blood passes through an oxygenator where oxygen is added (membrane oxygenators are less traumatic than bubble oxygenators).
  • Heat exchanger: The blood then passes through a heat exchanger that functions to cool or warm the patient to the desired temperature.
  • Main pump: either a roller head pump or a centrifugal pump.
    • Roller pump: The pump console is made of several rotating motor-driven pumps that peristaltically "massage" the tubing. This action gently propels the blood through the tubing.
    • Centrifugal pump: By altering the speed of revolution of the pump head, blood flow is produced by centrifugal force. This type of pumping action is considered by many to be superior to the action of the roller pump because it is thought to produce less blood damage.
  • Arterial cannula: Placed in the ascending aorta; the main pump returns the blood through an arterial cannula to the patient's systemic circulation.
  • Arterial line filter: Is incorporated in the CPB circuit to minimize the embolic load delivered to the patient.
• Other components of CPB Machine:
  • "LV vent" is a cannula that suctions blood out of the left ventricle to prevent its dilatation (per LaPlace's law, Tension = {Pressure * Radius}/Wall thickness; increases in radius increase tension and myocardial oxygen consumption). The blood originates from Thebesian and bronchial veins that empty directly into the left ventricle.
  • "Cardiotomy suction" is a suction cannula that removes and salvages undiluted or "clean" blood from the open cardiac chambers or the surgical field.
  • "Cellsaver" system collects and washes red blood cells from diluted field blood and blood that has been exposed to potentially harmful elements (e.g., inflammatory cytokines).
  • "Cardioplegia pump" administers a cardioplegic solution.
• Monitoring and access:
  • Arterial line is placed prior to initiation of CPB: The flow on bypass is usually nonpulsatile and thus a noninvasive BP cuff will not work.
  • Large-bore intravenous access is also established in case of significant fluid shifts and/or blood loss prior to or after CPB.
  • Central venous access is routinely placed after induction; a pulmonary artery catheter (PAC) may be considered.
  • Transesophageal echocardiography probe is placed as indicated by the procedure in the absence of contraindications.
• Anesthetic management:
  • Balanced anesthesia with shorter-acting narcotics (fentanyl), short-acting intravenous agents, such as propofol, and modern volatile agents are utilized. Volatile agents appear to improve cardiac cell conditioning when administered prior to the onset of ischemia.
  • High-dose narcotic techniques are not commonly used today.
  • Supplemental techniques have increased in popularity (e.g., paraspinal blocks) to reduce the use of systemic agents, improve analgesic control, and improve postoperative pulmonary function.

• Systemic inflammatory response syndrome (SIRS): can result from cardiac surgery with CPB.
  • Possible causes include contact of the blood components with the artificial surface of the bypass circuit, ischemia-reperfusion injury, endotoxemia, and operative trauma.
  • Data indicate that a complex sequence of events leads to the activation of leukocytes and endothelial cells, which is responsible for cell dysfunction in different organs.
  • There may be some benefit of steroid use in patients undergoing CPB (see additional reading).
• Complications of CPB (3) [B]:
  • Capillary leak syndrome secondary to inflammatory response (see above)
  • Hemolysis
  • Clotting of blood can block the circuit (particularly the oxygenator) or send a clot into the patient.
  • Air embolism
  • Exsanguination (loss of blood perfusion of tissues) if a line becomes disconnected.
  • Type I CNS events in 3–6% of patients.
  • Long-term cognitive dysfunction in up to 15% of patients.
  • Acute respiratory distress syndrome in up to 1.5% of patients.
  • Renal dysfunction, with 1–2% of patients requiring hemodialysis.
• As a consequence, CPB is only used during the several hours a cardiac surgery may take. Most oxygenators come with a manufacturer's recommendation that they are only used for a maximum of 6 hours, although they are sometimes used for up to 10 hours. When longer periods of cardiopulmonary support are required, extra-corporeal membrane oxygenation (ECMO) or a ventricular assist device (VAD) should be utilized.

• Initiation of bypass is defined as the moment that the patient's venous blood is allowed to flow through the venous cannula, away from the heart into the bypass machine. A clamp is typically placed on the venous cannula while preparing for CPB; thus removal of the clamp usually marks initiation.
  • Use of CPB requires suppression of the clotting cascade with heparin because the components of the bypass pump and the surgical wound are powerful stimuli for thrombus formation.
  • Heparin is given intravenously before placement of the aortic cannula and the activated clotting time (ACT) is checked; the targeted ACT varies with the type of CPB used (heparin-coated versus nonheparin coated) and the type of surgical procedure.
  • After acceptable pump flows have been achieved and the mean BP is stabilized, the aorta is cross-clamped, cold cardioplegia (see below) is infused, and the heart is also topically cooled with a saline slush solution to stop it from beating.
• Cardioplegia is a solution that contains a high concentration of potassium and other electrolytes. It is designed to arrest and protect the heart muscle. The solution is typically administered every 20–30 minutes throughout the on-bypass time when there is no blood flow through the heart.
  • Anterograde cardioplegia is administered through the coronary artery os and the flow is anterograde.
  • Retrograde cardioplegia is administered through a catheter in the right atrium directly into the coronary sinus. The flow is retrograde through the coronary veins.
• Optimal conduct of CPB:
  • Currently, the management of patients during CPB varies substantially by institution and practitioner (surgeon, anesthesiologist, perfusionist involved).
  • Evidence-based guidelines for best perfusion practices have been developed (1)[A,B], (2) [A]. The guidelines describe physiologic parameters (mean arterial BP, pump flow rate, hematocrit, temperature) and technologies (heparinized versus nonheparinized circuits, arterial line filters, pulsatile versus nonpulsatlie pumps, centrifuge versus roller pumps) that may optimize perfusion.
• Alpha stat versus pH stat:
  • Alpha stat arterial blood gases are not temperature corrected. During hypothermia on bypass, an alkaline shift occurs as carbon dioxide leaves the gas form and dissolves in the blood (less kinetic energy to keep in gas form; resulting in decreased PaCO₂). This leads to a leftward shift of the oxyhemoglobin dissociation curve that remains uncorrected; carbon dioxide is not added to the system and total body carbon dioxide remains the same. Alpha stat is most commonly used in adults.
  • pH stat arterial blood gases are temperature corrected. Carbon dioxide is added to the oxygenator to correct for the alkaline drift at lower temperatures and increases the PaCO₂ to normal levels. This counteracts the leftward shift of the oxyhemoglobin dissociation curve. It is most commonly used in neonatal and infant cardiac surgery as it may offer neurologic protection for this patient population.
• Comparison of adult and pediatric CPB: There are significant differences in the conduct of CPB between adult and pediatric patients (see Table 1).

  Table 1. Comparison of adult versus pediatric cardiopulmonary bypass

  CPB Parameter	Adult	Pediatric
  Temperature	Rarely below 30°C	Commonly below 30°C
  Circulatory arrest	Rarely used	Commonly used
  Dilutional effect of pump prime	25–30%	150–300%
  Additives to pump prime	Blood only occasionally (e.g., low hematocrit)	Blood, albumin routinely
  Systemic perfusion pressure	50–80 mm Hg	20–50 mm Hg
  Influence of alpha stat versus pH stat	Minimal (at moderate hypothermia)	Marked (at deep hypothermia)
  Glucose regulation
  Hypoglycemia	Rare	Common
  Hyperglycemia	Frequent	Less common

• Deep hypothermic circulatory arrest (DHCA) describes maintaining the body without any perfusion (blood flow).
  • It is most frequently used in pediatric cardiac surgery and aortic arch aneurysm repairs in adults.
  • The patient is put on the CPB and cooled to induce total body hypothermia, which is the prerequisite for initiation of DHCA.
  • Additional measures are instituted in an attempt to further reduce the metabolic requirement of the brain and provide brain protection during DHCA: Ice packing of the head, administration of barbiturates and mannitol.
  • Finally, blood flow to all organs, including the brain, is stopped.
  • In adults, DHCA may offer brain protection for up to 30–45 minutes.
• Termination of CPB:
  • Weaning from CPB is only begun once the patient's core temperature is adequate (about 36.5°C) and ventilation is re-established.
  • The aortic cross-clamp is removed and the heart beat most often resumes spontaneously; temporary pacing wires can be used if heart rate is inadequate.
  • Weaning from CPB is usually done gradually: The pump flow rates are slowly reduced, while the amount of blood flow back into the heart is slowly increased at the same time ("partial bypass").
  • Once the patient is completely off CPB and vital signs are stable, venous and arterial cannulae are removed.
  • Protamine is then administered to reverse the heparin-induced anticoagulation. Always start with a test dose of protamine (typically 10 mg IV) before giving the full dose in case of an allergic reaction (severe pulmonary hypertension requiring reinstitution of the CPB).

• Coronary Artery Disease
• Coronary Artery Bypass Grafting
• Circulatory Arrest
• Cardioplegia
• Left Ventricular Assist Device
• Postoperative Cognitive Dysfunction
• pH Measurements

Sascha Beutler , MD, PhD

Daniel Castillo , MD