A primer for students regarding advanced topics in cardiothoracic surgery, part 1: Primer 6 of 7.

Central MessageCardiopulmonary bypass and extracorporeal membrane oxygenation are important modalities to support a patient's heart and/or lung function during cardiac surgery or cardiopulmonary failure. Cardiopulmonary bypass and extracorporeal membrane oxygenation are important modalities to support a patient's heart and/or lung function during cardiac surgery or cardiopulmonary failure. Cardiopulmonary bypass (CPB) was first successfully used by Dr John Gibbon Jr, in 1953 at the Jefferson Hospital in Philadelphia to repair an atrial septal defect.1Kurusz M. May 6, 1953: the untold story.ASAIO J. 2012; 58: 2-5https://doi.org/10.1097/MAT.0b013e31823ccfe7Crossref PubMed Scopus (10) Google Scholar As the name suggests, CPB was designed to substitute for the function of the heart and the lungs, thereby allowing a surgeon to operate on these structures in a relatively bloodless field. Specifically, CPB has 5 objectives:•To oxygenate and remove carbon dioxide from the blood (replace pulmonary function);•To provide sufficient systemic circulation (replace cardiac function);•To provide a bloodless surgical field by draining the cardiopulmonary system;•To thermoregulate the patient; and•To protect the heart during the operation with thermoregulation and cardioplegia delivery. A simplified schematic diagram of a CPB circuit is depicted in Figure 1. It is essential to understand the layout and the management of CPB to participate in the operative care of cardiac surgery patients. The CPB circuit typically provides a constant flow of oxygenated blood. To do so, the venous side of the circulation needs to be drained from the patient using drainage (or venous) cannula(s), placed via the superior and inferior vena cava or their confluence at the right atrium. Once the cannulas are inserted, venous blood drains passively into the reservoir of the system located lower than the patient. In cases where passive drainage is insufficient, active suction can be used to increase venous drainage. The blood is then oxygenated and ventilated with a membrane oxygenator, with carbon dioxide levels controlled using sweep flow (ventilating gas flow rate) before it is returned to the patient via an arterial cannula that can be positioned in the ascending aorta, axillary artery, femoral artery, or other arterial positions. Flow can be accomplished via a centrifugal pump or roller pump. Figure 2 depicts the relationship between afterload and flow rate for both pump types. Centrifugal pump is preload-dependent and afterload sensitive, whereas roller pump (further discussed in the Bloodless Field section) provides a relatively fixed flow rate as it works by positive fluid displacement. Among the feared complications in providing systemic perfusion is entraining air within the circuit and causing embolic complications such as stroke. All circuits have fail-safe devices so flow is immediately stopped when air is detected in the system. De-airing the circuit is beyond the scope of this section. The patient must be sufficiently anticoagulated to be placed on CPB (further discussed in the Consequences of Cardiopulmonary Bypass section). During CPB, venting or the removal of excess blood is important to prevent myocardial or vascular injury from the high pressures sometimes encountered during CPB. Multiple vents have been employed, but 2 are most common: left ventricular vent and aortic root vent (often termed root vent). The left ventricular vent is typically placed via the right superior pulmonary vein and passed through mitral valve into the left ventricular cavity. This vent drains the blood accumulated within left side of the heart from the bronchial and Thebesian veins. The aortic root vent allows suction to be applied to the aortic root, thereby indirectly emptying the left ventricle. Both vents have the dual role of evacuating air that has become trapped during surgery, thereby decreasing the risk of air embolism. Among the benefits of CPB is that blood in the operative field (along with the blood in the patient's circulatory system) can be returned to the CPB circuit and then back into the systemic circulation, thereby minimizing blood loss. By employing cardiotomy suckers, 1 of the roller pumps in the CPB circuit provides suction for the field. The sucker tips may be configured to be left in the dependent region in the operative field or be hand-held to enable directed aspiration of blood. Additionally, blood can be salvaged using a cell-saver device that filters and processes the blood and packages it in blood bags that can be intravenously administered by the anesthesia team intraoperatively or postoperatively in an intensive care unit. This approach further reduces any net blood loss during the procedure by allowing blood loss at all stages to be returned to the patient. The CPB circuit can heat and cool the oxygenated blood returning to the patient. Whereas the cardioplegia solution for myocardial protection is routinely cooled to 4 °C, the CPB circuit can cool and subsequently warm the patient in a systemic fashion. Systemic cooling can be of substantial benefit when performing surgery that is not safe metabolically at physiologic temperatures (discussed below). However, thermoregulation is associated with potential complications of which the surgeon and the perfusionist must be aware. Protection of the heart during surgery is vital. The CPB machine is able to cool and provide cardioplegia to the heart. Cardioplegia is designed to silence the electrical activity of the heart and reduce its metabolic rate. This process is accomplished through a cold (4 °C) solution with high potassium levels that reduces the electrochemical gradient leading to diastolic relaxation of the myocardium. Cardioplegia requires regular redosing, the timing of which depends on the type of cardioplegia being administered. The surgeon and perfusionist are in constant communication during the surgery regarding the timing, frequency, and volume of this redosing. Avoiding ischemic insult to the myocardium during cardiac surgery is a primary goal of techniques of myocardial protection. However, myocardial injury may occur. The right ventricle tends to be susceptible to ischemic injury primarily for 2 reasons. First, depending on the procedure, cardioplegia may be administered and perfuse the myocardium in a retrograde fashion through the coronary sinus. Compared with the left ventricle, the right ventricle has more direct venous return via the Thebesian veins that drain into the cardiac chamber and not into the coronary sinus. Thus, retrograde cardioplegia will often favor left heart protection compared with that of the right heart. Second, the right ventricle has an anterior position, leading to greater exposure to room air and operating room lights, influencing optimal myocardial temperature and increasing risk of ischemic insult. Thus, topical hypothermia using iced saline solution/slush may be placed over the heart to enhance myocardial cooling. An important approach to myocardial protection is infusion of antegrade cardioplegia directly into the coronary arteries. Antegrade cardioplegia is often delivered into the aortic root via 1 limb of the cardioplegia/root vent cannula (with the root vent clamped). This cannula is placed in the ascending aorta between the aortic crossclamp and a competent aortic valve. Delivery of the cardioplegia solution thus perfuses the myocardium via the coronary artery in an antegrade fashion. Alternatively, in patients undergoing aortic valve surgery or other surgery requiring an aortotomy, antegrade cardioplegia may be directly infused into the coronary ostia using handheld cannulas. Although beyond the scope of this primer, the atherosclerosis of the right and left coronary arteries can interfere with optimal antegrade cardioplegia administration. A key feature of the CPB circuit and one that distinguishes CPB from extracorporeal membrane oxygenation (ECMO) is that with the former the reservoir is open to the air. This system thus allows for cardiotomy suckers to be attached to the circuit, venous system to be passively drained, and potential for large volume infusions. A reservoir permits the surgeon and the perfusionist to regulate the amount of circulating volume in the patient. At the time of separating from CPB, the majority of the blood volume is generally returned to the patient to sustain hemodynamic stability. The CPB circuit is typically primed with crystalloid solution to ensure there is no air in the system prior to connecting to the patient. Upon initiating CPB, crystalloid will enter the patient’s blood volume leading to hemodilution and potential ischemic complications. One method proposed to minimize hemodilution is retrograde autologous priming performed after the patient is attached to the CPB circuit but before initiation of bypass. In this fashion, a portion of the patient’s blood is drawn into the circuit, displacing the crystalloid prime and reducing the hemodilution. As noted, the CPB system includes a reservoir that is open to the air. This air-fluid interface, as well as the tubing for the circuit, results in a procoagulant and inflammatory response. To mitigate the risk of thrombus formation within the system, a patient is systemically anticoagulated with heparin before cannulation and initiating CPB. The degree of anticoagulation is monitored by the activated clotting time (ACT). As a general rule (recognizing variability among surgeons), the target ACT should be >400 seconds. Of note, entrance of nonheparinized blood into the CPB circuit as well as other factors can influence the ACT; therefore, ACT is regularly monitored throughout the procedure. Because of the dynamics of coagulation and anticoagulation, as well as inherent risks of CPB such as platelet dysfunction and effects of hypothermia, underlying coagulopathies or consumption of clotting factors can contribute to bleeding in the postoperative setting. Many have proposed the use of thromboelastogram, which aids in identifying the coagulation issues and the blood component to administer (eg, platelets vs fibrinogen vs clotting factors). An example of a thromboelastogram and its diagnostic utility is shown in Figure 3. When weaning or separating from CPB, reversal of heparin is required with protamine. During infusion of protamine, one main concern is the potential for protamine reactions, of which there are 3 types.•Mild: A transient hypotension with administration•Moderate: An anaphylactic-like response with bronchial constriction•Severe: Profound pulmonary hypertension with systemic hypotension Strategies to avoid adverse reactions include an initial intravenous testing dose by anesthesia and a slow infusion of protamine. Significant hemodynamic instability and hypotension requires vasopressor support and consideration of reheparinization and resuming CPB. The air-fluid interface and tubing contact promotes an inflammatory environment. Additive effects include the mechanical shearing forces on the red blood cells and platelets from the CPB system and the use of cardiotomy suckers. These perturbations are more pronounced with roller pumps compared with centrifugal pumps, recognizing the tradeoffs of using the latter. The net result of CPB is reduced platelet function, decreased hematocrit level, and increased levels of free hemoglobin in the blood. One proposed mechanism is that nitric oxide reacts with free hemoglobin, reducing the levels of available nitric oxide and resulting in impaired vasodilation.2Satoh T. Xu Q. Wang L. Gladwin M.T. Hemolysis-mediated toxicity during cardiopulmonary bypass ameliorated by inhaled nitric oxide gas.Am J Respir Crit Care Med. 2018; 198: 1244-1246https://doi.org/10.1164/rccm.201806-1165EDCrossref PubMed Scopus (3) Google Scholar These events may lead to end-organ damage, in particular acute kidney injury.3Lei C. Berra L. Rezoagli E. Yu B. Dong H. Yu S. et al.Nitric oxide decreases acute kidney injury and stage 3 chronic kidney disease after cardiac surgery.Am J Respir Crit Care Med. 2018; 198: 1279-1287https://doi.org/10.1164/rccm.201710-2150OCCrossref PubMed Scopus (69) Google Scholar Although the surgery itself may only take a matter of hours, CPB and the stress of surgery activate cellular cascades, which may be prolonged and require intensive postoperative management. Successful separation from CPB involves the transition from pump-assisted circulation to spontaneous cardiopulmonary activity with adequate blood flow and tissue perfusion. This complex process requires the vigilance of the surgeon, anesthesiologist, and perfusionist. Fundamentally, it requires a stable cardiac rhythm, adequate hemostasis, and sufficient cardiac and respiratory function. Thus, the anesthesiologist utilizes transesophageal echocardiography to monitor for changes in contractility, air emboli, filling and/or distension of the cardiac chambers, and more. Meanwhile, the surgeon is visually inspecting the heart for contractility, stable rhythm, and hemostasis in the surgical field. In general, the venous drainage is gradually decreased (increasing the patient’s blood volume) while reducing the arterial flow rate of the pump. This process allows the patient’s own heart to take over the circulation. However, if the above-mentioned requirements are not met, the patient may not be able to be weaned from CPB. Older patients and those with preoperative left ventricular dysfunction, mitral regurgitation, coagulopathy, or longer ischemic time or duration of CPB are at increased risk of failure to wean.4Denault A.Y. Tardif J.C. Mazer C.D. Lambert J. BART InvestigatorsDifficult and complex separation from cardiopulmonary bypass in high-risk cardiac surgical patients: a multicenter study.J Cardiothorac Vasc Anesth. 2012; 26: 608-616https://doi.org/10.1053/j.jvca.2012.03.031Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar The issue(s) precluding separation from bypass should be identified and addressed, if possible. If the patient remains dependent on CPB, temporary support options can be employed, including intra-aortic balloon pump, ECMO, or ventricular assist devices. Operations where the patient is cooled significantly (20 °C or below) are termed deep hypothermic circulatory arrest (DHCA). The goal of DHCA is to decrease the metabolism of the entire body, most importantly the brain, to allow for periods of no systemic flow. This technique enables short durations of completely bloodless field, where conventional CPB is not sufficient. The main indications for DHCA are surgeries involving the aortic arch, such as ascending aortic dissection with an open distal anastomosis or arch aneurysm repair, and pulmonary thromboendarterectomy. A relationship between systemic temperature, estimated safe arrest time, and brain metabolism is depicted in Figure 4. One issue with DHCA is knowing when the patient is sufficiently cooled based on their metabolic activity. The use of neuromonitoring (see below) helps to direct DHCA cooling. Monitoring brain activity allows the surgeon to know when the patient is sufficiently hypothermic (ie, electrocerebral inactivity). Electrocerebral inactivity indicates that the metabolic activity of the brain is low enough that it is safe to arrest or cease circulatory support. Typically, the duration of DHCA employed is <30 or 40 minutes, with an increased rate of deficits associated with longer periods. The use of DHCA also adds a substantial amount of time to the procedure because of the time required for cooling and rewarming. The rewarming gradient (how cold the patient is compared to the temperature of the blood from the CPB circuit) is held around 10 °C or less and is adjusted so that warming is less than 0.5 °C per minute. Rapid rewarming can lead to damaging the brain and vasculature and protein denaturation. One strategy to limit cerebral ischemia is to selectively perfuse the brain during hypothermic circulatory arrest using antegrade cerebral perfusion (ACP) or retrograde cerebral perfusion. These techniques, where ACP may be employed with moderate hypothermic circulatory arrest, continue to be evaluated and often are dependent of the extent of surgery (ie, anticipated circulatory arrest time) and surgeon preference. In recent meta-analyses, the postoperative incidence of stroke, early mortality, and permanent neurological dysfunction was similar between ACP and retrograde cerebral perfusion.5Takagi H. Mitta S. Ando T. A contemporary meta-analysis of antegrade versus retrograde cerebral perfusion for thoracic aortic surgery.Thorac Cardiovasc Surg. 2019; 67: 351-362https://doi.org/10.1055/s-0038-1632389Crossref PubMed Scopus (7) Google Scholar,6Guo S. Sun Y. Ji B. Liu J. Wang G. Zheng Z. Similar cerebral protective effectiveness of antegrade and retrograde cerebral perfusion during deep hypothermic circulatory arrest in aortic surgery: a meta-analysis of 7023 patients.Artif Organs. 2015; 39: 300-308https://doi.org/10.1111/aor.12376Crossref PubMed Scopus (27) Google Scholar However, a trend toward decreased temporary neurological dysfunction was found in ACP.5Takagi H. Mitta S. Ando T. A contemporary meta-analysis of antegrade versus retrograde cerebral perfusion for thoracic aortic surgery.Thorac Cardiovasc Surg. 2019; 67: 351-362https://doi.org/10.1055/s-0038-1632389Crossref PubMed Scopus (7) Google Scholar,6Guo S. Sun Y. Ji B. Liu J. Wang G. Zheng Z. Similar cerebral protective effectiveness of antegrade and retrograde cerebral perfusion during deep hypothermic circulatory arrest in aortic surgery: a meta-analysis of 7023 patients.Artif Organs. 2015; 39: 300-308https://doi.org/10.1111/aor.12376Crossref PubMed Scopus (27) Google Scholar Given these findings, the ideal cerebral protection strategy should be individualized based on patient characteristics, surgeon preference, and hospital resources.7Qu J.Z. Kao L.W. Smith J.E. Kuo A. Xeu A. Iyer M.H. et al.Brain protection in aortic arch surgery: an evolving field.J Cardiothorac Vasc Anesth. 2021; 35: 1176-1188https://doi.org/10.1053/j.jvca.2020.11.035Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar The goal of neuromonitoring intraoperatively is to detect signs of central nervous system ischemia. The neuromonitoring techniques facilitate operative decision making to reduce neurological deficits after cardiac surgery. Neuromonitoring is employed in cases DHCA and aortic surgery (thoracic or thoracoabdominal). There are 3 complementary approaches of monitoring: electroencephalogram (EEG), somatosensory evoked potential (SSEP), and motor evoked potential (MEP). EEG assesses electrical silence during DHCA and potential ischemia during the operation. A key abnormal finding is asymmetrical changes in the EEG waveforms. SSEPs are used to determine spinal cord ischemia, particularly involving the posterior spinal arteries and dorsal column. SSEPs are assessed by stimulating sensory nerves at the extremities and seeing the propagation of that signal back to the brain. Finally, MEPs stimulate the motor cortex and observe the propagation through the descending motor tracts to the extremities. MEPs are sensitive to ischemia involving the anterior spinal arteries and, by extension, the radicular arteries of the aorta. These measurements help determine whether there is a risk of spinal cord ischemia and subsequent deficit; the findings allow the surgeon to intervene. For example, SSEPs are typically normal if perfusion is 20 mL/100 g/hour. Spinal cord damage occurs at rates of 10 mL/100 g/hour or lower. In between these 2 values, the SSEPs will be abnormal, allowing for interventions such as permissive hypertension. For MEPs, ischemia of the radicular arteries, and thus the anterior spinal artery, triggers abnormal signals. This modality is useful for procedures such as thoracic endovascular aortic repair for descending thoracic aortic pathology or thoracoabdominal aneurysm repair, during which flow to the radicular arteries may be compromised. If the MEPs are abnormal, a surgeon may decide to reimplant the intercostal arteries (for open surgical procedures), increase the blood pressure, or decrease the intracranial/spinal cord pressure. Another type of CPB is left heart bypass. As implied, this technique bypasses only the left side of the heart, maintaining normal flow through the right side and the native pulmonary circulation. Indications for this approach are open surgical procedures involving the descending thoracic or thoracoabdominal aorta. Left heart bypass allows for perfusion of the branches of the aorta that are not directly excluded during the surgery (ie, by the crossclamp) and requires close monitoring. Depending on the cannulation setup, the bypass circuit will not drain all the flow into the left atrium. Instead, there is a balance of how much blood to allowed into the left ventricle, and thus antegrade aortic flow, and how much blood will be diverted to the rest of the systemic circulation via distal aortic or arterial cannulation. Of note, in extensive descending and thoracoabdominal aortic surgery, left heart bypass may not be feasible, and conventional CPB and DHCA may be necessary. ECMO is a form of temporary circulatory support that has been used over the last decade following the H1N1 influenza pandemic. Since its clinical success in 1970,8Baffes T.G. Fridman J.L. Bicoff J.P. Whitehill J.L. Extracorporeal circulation for support of palliative cardiac surgery in infants.Ann Thorac Surg. 1970; 10: 354-363https://doi.org/10.1016/s0003-4975(10)65613-5Abstract Full Text PDF PubMed Google Scholar ECMO has become an accepted support modality for patients with respiratory and/or cardiac failure refractory to all other therapies. Although conceptually similar to CPB, modifications to the ECMO circuit allow for a longer duration of cardiopulmonary support than afforded by traditional CPB. ECMO is a closed circuit with no open reservoir where blood contacts air and does not require the same degree of anticoagulation as CPB. ECMO drains venous blood from the patient, exchanges gases, then returns the blood back to the patient. The main components of the ECMO circuit are the inflow and outflow cannulas (defined in relation to the pump, not the patient's body) for draining and returning blood, respectively, a pump, and an oxygenator. Other components include pressure and flow sensors, a heat exchanger for heating or cooling blood, and arterial or venous ports for drawing blood. Important ECMO concepts in a clinical setting include flow rate, speed, fraction of inspired oxygen, and sweep. Flow rate describes the volume of blood per minute being delivered. Depending on the patient's native cardiac output and the setting of the ECMO circuit, ECMO flow may account for a variable percentage of the patient’s total cardiac output. Flow is modulated using the pump speed (in rotations/minute). Sweep gas is how carbon dioxide is removed from the patient's blood and is accomplished by flowing a gas (typically 100% oxygen) through the oxygenator. Sweep gas flows through the oxygenator, which is a microporous tube, while the patient's blood flows around it. This interface between the sweep gas and the blood is where gas exchange occurs. The pressure gradient between the patient's blood flow and the sweep gas flow determines the level of oxygenation and ventilation. Increased sweep gas flow (measured in liters per minute) leads to increased decarboxylation of the blood. Thus, increasing the sweep can take over pulmonary function when a patient is very ill, and titrating the sweep down can be done to assess a patient’s ability to ventilate before decannulation. Finally, fraction of inspired oxygen, as in ventilators, is the percentage of oxygen in the gas being flowed through the oxygenator. This determines the partial pressure gradient between the patient’s blood gas and is thus important for both oxygenation and ventilation. There are numerous factors one must consider before initiating ECMO for a patient. These include the severity of the cardiac and pulmonary failure, whether or not it is responsive to more conservative treatment modalities, the overall prognosis of the patient, and the current resource utilization of the hospital system. The only absolute contraindication to ECMO is a preexisting condition that is incompatible with recovery, such as advanced malignancy or severe brain injury. Relative contraindications include very poor prognosis, advanced age, and severe coagulopathy. ECMO configurations can be generally classified into 2 different types (Figure 5).•Veno-venous (VV) ECMO: In VV ECMO, the circuit both drains the blood from and returns the blood to the patient's venous system. Because the blood is returned to the venous system prior to entering the pulmonary circulation, VV ECMO relies on the patient's native cardiac output to circulate the newly oxygenated blood throughout the body. The most common configuration of VV ECMO consists of the inflow cannula placed in a femoral vein with drainage holes extending all the way up the inferior vena cava, with an outflow cannula placed in the right internal jugular vein. However, other options exist, such as femoral-femoral cannulation or a double lumen catheter inserted into the right internal jugular vein. Notably, the ECMO circuit is established in series with the patient’s existing cardiopulmonary system and therefore does not augment cardiac function. VV ECMO is primarily indicated for patients with isolated respiratory failure. According to the Extracorporeal Life Support Organization, VV ECMO is indicated for patients in: hypoxemic respiratory failure (Pao2/fraction of inspired oxygen ratio <80 mm Hg) after optimal medical management or hypercapnic respiratory failure (pH <7.25), despite optimal conventional mechanical ventilation.9Tonna J.E. Abrams D. Brodie D. Greenwood J.C. Rubio Mateo-Sidron J.A. Usman A. et al.Management of adult patients supported with venovenous extracorporeal membrane oxygenation (VV ECMO): guideline from the Extracorporeal Life Support Organization (ELSO).ASAIO J. 2021; 67: 601-610https://doi.org/10.1097/MAT.0000000000001432Crossref PubMed Scopus (142) Google Scholar Specifically, bacterial pneumonia, viral pneumonia, and trauma-related acute respiratory distress syndrome are the most frequent indications.•Veno-arterial (VA) ECMO: In VA ECMO, the inflow cannula still drains blood from the venous system, but the outflow cannula is placed into the patient's arterial system, thus creating a parallel circuit that can effectively augment the patient's cardiac output (partial bypass support). VA ECMO can be placed peripherally, with the inflow cannula placed in a femoral or internal jugular vein and an outflow cannula placed in the femoral artery. This peripheral configuration results in retrograde flow in the aorta, which mixes with the native cardiac output, and has important implications for potential left ventricular strain, root stasis, pulmonary edema, and north-south syndrome. Other peripheral cannulation sites include the axillary or subclavian arteries. Central cannulation, on the other hand, involves drainage from the right atrium and outflow into the aorta, which provides antegrade flow in conjunction with the native cardiac output. This method may facilitate the transition from CPB to ECMO if the patient fails to wean from CPB intraoperatively. VA ECMO is indicated for patients suffering from cardiac failure with or without respiratory failure. It is most commonly used to support patients in cardiogenic shock (systolic blood pressure <90 mm Hg), including acute coronary syndrome, myocarditis, pulmonary embolism, or failure to wean from CPB. Depending on the clinical context, VA ECMO can be utilized as a bridge to recovery or as a bridge to definitive therapies, such as ventricular assist device implantation or cardiac transplantation.•Although VV and VA ECMO comprise the large majority of ECMO circuits used, there other circuits that utilize hybrid or parallel configurations that may be seen. These strategies may be useful if support on standard circuits is insufficient.10Shah A. Dave S. Goerlich C.E. Kaczorowski D.J. Hybrid and parallel extracorporeal membrane oxygenation circuits.J Thorac Cardiovasc Surg Tech. 2021; 8: 77-85https://doi.org/10.1016/j.xjtc.2021.02.024Abstract Full Text Full Text PDF Scopus (15) Google Scholar Veno-arteriovenous ECMO may be used if a patient supported by VV ECMO develops worsening cardiac function, necessitating placement of an arterial inflow cannula. Moreover, a patient supported by VA ECMO may lack adequate venous return, requiring placement of a second venous outflow cannula. Several factors contribute to ECMO-related complications, including the physiologic condition of the patient, the nature of the materials used in the circuit, the amount of anticoagulation required to prevent thrombosis within the system, and the challenges associated with managing the circuit over time. To simplify, the following are technical and nontechnical complications. Technical complications include vascular injury during cannulation, cannula malposition, and mechanical failure of the circuit due to air emboli or thrombus formation. Although individual events are relatively rare, when combined, nearly one-third of ECMO runs require replacement of the system due to technical issues. Common nontechnical complications include hemorrhage, neurologic injury, infection/sepsis, and thromboembolism. In general, VA ECMO is associated with higher rates of complications compared to VV ECMO, partially due to the cannulation of the arterial system. In particular, lower extremity ischemia can occur secondary to the return cannula placed in the femoral artery, which can be mitigated with a distal perfusion catheter. A summary of common complications as