The last option for patients in severe cardiogenic shock refractory to medical therapy or even mechanical resuscitation is venoarterial extracorporeal membrane oxygenation (va-ECMO). The concept of ECMO was introduced decades ago, but currently faces a renaissance with increasing demand and improved success rates according to the Extracorporeal Life Support Organization registry.1 Among the different possible configurations of ECMO implementation in emergency situations outside the operating room, femoral cannulation is the mostly used because of its simplicity and time-saving.2 However, concern exists whether retrograde aortic ECMO flow can increase cardiac afterload and thus leads to left ventricular distension and jeopardizes ventricular recovery (Figure 1). Left ventricular unloading on ECMO seems to be indicated under these circumstances, but the cohort of patients who benefit from left ventricular venting is unclear. There are reports describing a need in >60% of patients; nonetheless, these numbers derive from a pediatric population with congenital heart diseases, which cannot be extrapolated to an adult population with different diseases.3 By contrast, there is the own experience where a left atrial or ventricular vent is a true exception. Accordingly, the question arises whether to vent every patient treated with an ECMO for cardiogenic shock. There are many reasons why this is to be denied. First, ECMO should remain as simple as possible to minimize complications. Venting the left ventricle is invasive, can increase the likelihood of bleeding, and can potentially be harmful.
This review discusses which patients may benefit from left ventricular unloading and describes the different possibilities to vent the left ventricle on ECMO.
General Measures to Prevent Left Ventricular Distension on ECMO
There are several possibilities to decrease the likelihood of left ventricular distension on ECMO. Interestingly, there is hardly any literature available, and most of the recommendations are derived from acute heart failure guidelines like the one recently published by the European Society of Cardiology and on institutional experiences.4 The first step in establishing proper ECMO function is to eliminate the risk factors for left ventricular distension, including high afterload after inadequate vasopressor therapy, complete cardiac arrest with incomplete venous drainage, and aortic valve incompetence (Table 1).
As a high afterload can hinder left ventricular ejection and aortic valve opening, it is important to prevent high arterial pressures. The benefits of afterload reduction have been shown in heart failure patients, for example, during exercise.5 Decreasing afterload leads to a decrease in workload and O2 consumption. In case of an extremely poor left ventricular function, it is advisable to administer inotropes with a sufficient mean arterial pressure of 50–60 mm Hg. Inotropes such as epinephrine generally improve contractility and ventricular ejection by increasing intracellular calcium and muscular tension. The use of levosimendan, an intracellular calcium sensitizer, is also reasonable for patients on ECMO as has been demonstrated in patients experiencing acute ischemic heart failure.6 Similarly, some patients benefit from the use of arterial vasodilators such as sodium nitroprussid or a sympatholytic α1-receptor antagonist in a sound balance with inotropes. It remains a matter of discussion whether recovering patients with an improving left ventricular function on ECMO benefit from higher mean arterial pressures.
The pump flow is another important factor. Frequently, there is a tendency to keep patients on flow rates of ≥5 L/min in the belief that these high flow rates are beneficial. However, taking physiologic lactate levels, normal pH levels, and regular central venous saturations as a guide, flow rates of 2.5–4 L/min are probably sufficient in most cases. The lower pump flow rates also reduce the perfusion-related afterload. Intraaortic balloon pumping (IABP) concomitant to retrograde aortal perfusion is seen controversial as the inflated balloon in the descending aorta might hinder proper perfusion. Moreover, the recently published SHOCK II trial that randomized 600 patients in severe cardiogenic shock due to myocardial infarction for either conservative therapy or additional IABP could not demonstrate a survival benefit for the IABP application.7 The extrapolation of these findings to ECMO patients is difficult, but recent animal studies demonstrated that the IABP significantly reduced mean arterial pressure as well as oxygen saturation in the coronary sinus during simultaneous operation of retrograde ECMO flow and IABP.8 Similarly, our group does not favor the use of both support systems simultaneously.
Volume overload should be avoided, and hemofiltration should be considered liberally. Daily chest radiographs and echocardiographic controls are recommended to detect a fluid overload early. A Swan-Ganz catheter can be inserted to measure the pulmonary capillary wedge pressure to detect high left ventricular filling pressures as an indicator for left ventricular distension. No general recommendations applicable for all va-ECMO patients exist for ventilator settings. We suggest ventilation with low tidal volumes after the best positive end-expiratory pressure (PEEP) concept to keep the lung open. A higher PEEP is advisable in patients with a beginning pulmonary edema. An early extubation is aspired if applicable. The risk of infection, especially pulmonary infection, is encountered with antibiotics from the beginning of support.
Indication to Vent the Left Ventricle
The indication to vent the left ventricle is controversially discussed. In a pediatric population, the threshold for left ventricular venting is rather low as the infantile myocardium is extremely vulnerable to distension disordering the myocardial architecture and interstitial scaffolding. It is also reasonable to insert a vent prophylactically in pediatric ECMO patients.9
The situation in adults is different. Due to an incomplete unloading, some degree of left ventricular distention occurs in nearly every patient requiring extracorporeal life support, and this is well tolerated after the general measures previously mentioned. An indication for a left-sided vent is given in hearts without obvious ejection and a closed aortic valve, in which ventricular stasis is likely to develop and ventricular thrombus formation may take place. Therefore, repeated echocardiographic examinations are recommended specifically during the initial period. Another indication is the significant aortic valve regurgitation as retrograde aortic flow causes left ventricular distension. Furthermore, right ventricular function has an impact on left ventricular distension but to a lesser extent. Thus, an optimal venous drainage is important. The most pressing reason for vent placement is an uncontrolled pulmonary edema (“white lung”) after left ventricular overload, requiring a combination of ventricular unloading and optimized respiratory therapy. The subsequent paragraphs explain and discuss different approaches to vent the left ventricle (Table 2, Figure 2).
Open Chest Postcardiotomy Failure
Nowadays, only minorities of patients develop a postcardiotomy failure, which is still associated with a high mortality.10 However, an open chest situation offers different possibilities compared with closed chest extracorporeal support. Mechanical circulatory support can be applied in different ways, starting with intraaortal balloon pumping, and then going to temporary left ventricular assist devices (LVADs) such as the CentriMag System (Levitronix, Zurich, Switzerland) or any other type of centrifugal pump attached to the heart in LVAD configuration from the apex or the atrium of the left heart to the aorta.11 Intraaortic balloon pumps do not provide full circulatory support. Temporary LVADs provide adequate circulatory support and unload the left ventricle, but they enclose the disadvantage of being implanted in highly vulnerable hearts (e.g., after an acute myocardial infarction) with a considerable risk of bleeding in a time-consuming manner. Another important disadvantage of temporary LVADs is the univentricular support. In the case of right heart dysfunction, a biventricular support is necessary, either with two pump chambers or with ECMO. Extracorporeal membrane oxygenation is considered the preferred therapeutic modality for patients in postcardiotomy failure as it is much simpler and faster to install and also provides additional pulmonary support with benefits for critical patients.10 Extracorporeal membrane oxygenation therapy is also favorable from the economical standpoint.
There are two ways to connect an ECMO system, when weaning from cardiopulmonary bypass is not possible. If left ventricular distension is not primarily an issue, percutaneous cannulation (e.g., through the femoral vessels) is an elegant and simple technique. For the subclavian approach, a surgical dissection of the vessels is necessary. With both the methods, the chest can be closed, lowering the risk of mediastinal infection. When ventricular distension already presents before termination of cardiopulmonary bypass, left ventricular unloading is important and a central cannula placement suggested. Drainage of the inferior vena cava/right atrium can be achieved via direct cannulation or with a long percutaneous cannula from the groin, whereas arterial return should be directed into the ascending aorta or right subclavian artery.
Left ventricular unloading can be performed by introducing a 16 to 20 Fr flexible cannula through the right upper pulmonary vein using purse strings (Figure 3). The vent line is connected to the venous drainage limb with a Y-connector. The negative pressure generated by the pump facilitates drainage. In a pediatric patient population, the pulmonary artery might be more appropriate because the pulmonary veins are small and introducing a vent might cause problematic narrowing. As venting through the pulmonary artery is highly effective to unload the right and left ventricles, it is also advised in adults in case of severe biventricular failure.12 Overall, the indication to vent the left ventricle should be very liberal for patients with postcardiotomy failure in the OR with an open chest.
Surgical Minimal Invasive Left Heart Decompression on Peripheral ECMO
Left heart decompression on peripheral ECMO can be performed surgically using minimally invasive surgical techniques. However, these techniques are mostly rather sophisticated because the failing dilated heart is highly vulnerable for laceration leading to fatal rupture. An additional disadvantage is the risk to damage a coronary artery with consecutive myocardial ischemia. Basically, there are two surgical options: the subxiphoidal approach and the anterolateral approach. The subxiphoidal approach is advised in patients requiring ECMO support in the postoperative period after cardiac surgery because no additional incision needs to be done. The previous median incision can be reopened in its lower part and a retractor (e.g., an internal mammary retractor) is placed to elevate the left costal margin. After exposing the left apex pledgeted polypropylene sutures with a large needle are used to place purse-string sutures penetrating the entire myocardial wall. A 16 to 20 Fr cannula or sump suction is placed in the left ventricle and tunneled percutaneously or through the subxiphoidal incision to the extracorporeal side, as has been described by Guirgis et al.13
The anterolateral approach is favored in case of peripheral ECMO. The left heart apex can be located with fluoroscopy or echocardiography and exposed via an anterolateral thoracotomy through the fifth intercostal space. Similar to the subxiphoidal approach, a vent is placed in the left ventricle using purse-string sutures. The best region for cannulation is somewhat above the true apex, as the latter has a typically thin myocardial wall.14
Percutaneous Venting of the Left Ventricle Through the Pulmonary Artery and Retrograde Through the Aortic Valve
Avalli et al.15 described a smart and obviously quite effective technique to decompress the left and also the right heart by percutaneously introducing a venous cannula into the pulmonary artery. The authors describe a case of a middle-aged woman experiencing a profound myocardial depression after acute myocarditis. The patient was placed on ECMO using a femoral access. Transesophageal echocardiography demonstrated a dilated nonejecting left ventricle with an apical thrombus. To decompress the left heart, a 6 Fr angiographic catheter was introduced through the right jugular vein in Seldinger technique and advanced into the right pulmonary artery. With the help of a guide wire, a 15 Fr venous cannula (Medtronic Biomedicus 50 cm, Minneapolis, MN) was advanced and positioned in the common pulmonary artery and connected to the venous limb of the ECMO circuit. The authors do not disclose the achieved flow rates, but the total ECMO flow rates increased by 0.5 L/min. The cannula was pulled back to the superior vena cava and removed on day 9 of support. The patient was successfully weaned after 16 days with significantly improved left ventricular function. The possibility to vent the left ventricle through the pulmonary artery was already described in the 90s by Rossi et al.16 in a sheep model. In another bovine animal study, the pulmonary artery was vented through a long self-expanding Smart Cannula (Smartcanula LLC, Lausanne, Switzerland), which can be inserted through a small access and expands in situ up to 36 Fr.17 By placing a Smart Cannula in the pulmonary artery venting, the right and left ventricles are realized in addition to venous drainage. However, this was only shown in animals to date, and proper placement of the cannula might be problematic.
Another percutaneous technique is a transaortic technique described by Fumagalli et al.18 The authors punctured the left subclavian artery and inserted a J-tipped wire into the left ventricle because of a white lung and deteriorating arterial blood gases in a young women experiencing refractory cardiogenic shock. Subsequently, a 17 Fr pediatric cannula was inserted into the left ventricle under fluoroscopic guidance to vent the left ventricle. The authors monitored pulmonary wedge pressure that decreased after insertion of the transaortic vent. The patient was transferred on ECMO to a nearby transplant center where the patient was successfully transplanted after 7 days of support. However, the authors did not discuss the difficulty to pass a closed aortic valve with a wire without damaging it. In addition, the subclavian artery is anatomically behind the subclavian vein, and puncture of the vessel is sometimes associated with damage to the subclavian vein leading to a bleeding problem in an anticoagulated patient.
Percutaneous Transseptal Venting of the Left Heart
The largest published series of transeptal left atrial decompression was published from the Michigan group in 2006 by Aiyagari et al.19 This case series consists of seven patients, indicating that the body of literature on this topic is small. These seven patients were composed of 5 patients younger than 18 years, and the oldest patient had an age of 28 years (weight ranges 7.4–94 kg). The patient cohort had cardiac failure with elevated left heart pressures on ECMO, but the presence of pulmonary edema was not the primary indication for left ventricular unloading. The pediatric intensivists and cardiologists decided to decompress the left heart without specifying their indication. All procedures were accomplished in the catheterization laboratory under fluoroscopic guidance. The access was realized through the femoral vein. Initially an 8 to 10 Fr venous sheath was placed, and transseptal puncture was performed using the standard Brockenbrough transseptal needle and a 6 to 8 Fr transseptal sheath. The 8 Fr transseptal sheath was left in place in the youngest patient (<2.5 years), while in all other patients the transseptal sheath was exchanged for a larger cannula. The left atrial drain was connected to the venous limb of the ECMO circuit. Median procedural time to place the left atrial drain was 51 min (range 42–145 min), with no procedural complications. The left atrial drain flow ranged from 183 to 724 ml/min/m2. On average, left atrial cannula flow represented 5–32% of total ECMO flow. Three of the seven patients were discharged home. The authors state that the most significant finding from this series is the importance of an adequately sized left atrial drain for satisfactory decompression. The authors emphasized to place larger cannulas whenever possible. The fact that younger patients fared better might be due to the fact that the younger the higher the recoverability from cardiac and pulmonary injury after LA decompression.
Other possibilities to vent the left atrium interventionally are a blade balloon septostomy20 and atrial stenting.21 Stenting the left atrium seems to be advantageous over a simple blade balloon septostomy because a controlled atrial septal defect is due to a preferred size with a durable patency and unrestricted atrial communication for left heart decompression, whereas a simple balloon septostomy tends to close spontaneously sooner or later. Complications of atrial stenting include stent malpositioning due to a thin and unstable atrial septum as well as impingement of adjacent structures such as pulmonary venous return. In case of myocardial recovery, surgical correction is needed because no interventional stent closing device is available to date. Therefore, stenting of the atrial septum is mainly recommended for patients who will not recover (e.g., patients being bridged to transplantation or to a LVAD).
A smart and probably also effective method is the combined use of the TandemHeart (Cardiac Assist, Inc., Pittsburgh, PA) with an extracorporeal oxygenator integrated into the circuit resulting in an ECMO circuit.22 The TandemHeart drains blood from the left atrium through a 72 cm long cannula, placed transseptally via the femoral vein. An extracorporeal miniaturized centrifugal pump pumps blood retrograde into the aorta through the femoral artery. However, the impact of integrating an oxygenator into the circuit might hinder sufficient circulatory support because of a pressure drop across the oxygenator. Placement of the transseptal cannula may be difficult under emergency situations.
Impella LP for Left Ventricular Unloading on ECMO
Another possibility to unload the left ventricle is a synergistic combination of ECMO and a temporary and minimally invasive implanted extracorporeal LVAD. The device consists of a microaxial pump that is mounted on the tip of a catheter with a connected cable to a console. The concept was already explored in the 1990s with the Hemopump (Medtronic, Inc.) but did not improve the survival of patients with severe heart failure.23,24 After further technical refinements, the concept has been revived with the Impella system (Abiomed Inc., Danvers, MA) that is available in two sizes providing 2.5 and 5.0 L/min support.23 The actual pump consists of an inflow tip positioned into the left ventricle and an outflow into the aorta. The system is introduced via the femoral artery usually using a cut down ideally through a vascular graft. A direct comparison of outcome of patients in cardiogenic shock managed with either the Impella microaxial pump or the ECMO was published in 2011 by the group around Moss from Vancouver.25 Patients with biventricular failure or impaired oxygenation received predominantly ECMO. Weaning rate and 30 day mortality were slightly higher in the ECMO group (ECMO vs. Impella 47% vs. 41%; 44% vs. 38%). An experimental study by Kawashima et al.26 compared the degree of left ventricular unloading with an Impella pump and ECMO in a canine animal model by measuring pressure/volume relationships. The Impella clearly managed unloading of the left ventricle better than ECMO, which led to the clinical use of the concept.27 The authors described a case of a 70 year-old man in acute cardiogenic shock complicated by severe pulmonary edema, secondary to LV distension and severe mitral regurgitation who recovered with the use of ECMO followed by a percutaneous Impella placement. The patient was successfully weaned from both systems and survived. Another similar case report described severe complications.28 The authors had difficulties to pass the device through the closed aortic valve, which led to a kinking of the Impella purger and caused a fatal pump stop. Placement of the pump also caused a relevant aortic insufficiency. Further risks include limb ischemia and bleeding, and dislocation into the ascending aorta, for example, by turning the patient during nursing procedures.28
Patients in cardiogenic shock refractory to inotropes are increasingly stabilized by percutaneous placement of ECMO using the femoral vessels.29 According to the literature, 10–60% of patients develop severe left ventricular distension, which compromises ventricular recovery and leads to pulmonary edema. The highest incidences of left ventricular decompression are described in neonates and children. By contrast, our center with >500 adult va-ECMO runs applied left ventricular decompression in 1% of cases only. This may be explained by a strict prophylaxis of left heart distension, mainly afterload reduction.
It is difficult to predict the recovery potential of a patient on ECMO because there are only limited data available. However, according to our experience, there are some factors enabling judgment of the recovery potential. First, the underlying heart disease implies a certain degree of recovery potential. A patient experiencing an acute myocarditis is associated with a high recovery potential, whereas patients experiencing a dilative cardiomyopathy are not.30,31 In general, all acute but treatable diseases (e.g., myocarditis, recanalized acute myocardial infarction) have a higher recovery potential compared with end-stage diseases, such as chronic heart failure or uncorrected structural heart diseases. Second, the effectiveness of support and the end-organ response to the O2 delivery provide further hints. Patients with prolonged low venous saturations (e.g., <55%) with long-lasting elevated lactate levels and an acidosis have a low recovery potential, whereas patients with immediate normalization of arterial blood gases have a high recovery potential. Third, echocardiography stratifies the recovery potential. Patients with an improving ventricular function on ECMO have a much better recovery potential than patients with highly dilated ventricles and no ejection, closed aortic valve, and ventricular stasis.
Our institutional preference to unload the left ventricle on ECMO is centrally through a sternotomy by introducing a venting catheter through the right upper pulmonary vein because large cannulas can be introduced leading to an effective and secure unloading (Figure 3). Surgeons are used for this surgical approach; it is fast and compared with other methods easy to perform without the need for different specialities. This approach offers an additional advantage in case of prolonged need of unloading. Appropriate patients without obvious neurologic deficits experiencing severe ventricular distension and pulmonary edema are better treated with a ventricular assist device (VAD) type configuration of support. Ventricular assist device cannulas are implanted into the left apex and the ascending aorta that are attached to a temporary extracorporeal centrifugal pump (e.g., Rotaflow; Maquet, Hirrlingen, Germany) (Figure 4). Draining from the left ventricle leads to a perfect unloading. Also, by adding an oxygenator to the temporary circuit supports pulmonary function. This policy offers time to perform additional diagnostic tests to assess all other organ function, especially the neurologic function. After assessing a regular neurologic function, a paracorporeal assist device is attached to the cannulas and the patient is further bridged to transplant.
Left ventricular decompression is promising in patients with a high potential of recovery like in pediatric patients or myocarditis patients of any age. Different methods are applicable, each with advantages and disadvantages. In case of suspected slow recovery or no recovery with normal neurologic function, early VAD implantation is advisable.
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