Chapter 38: Extracorporeal Life Support

The concept of extracorporeal life support (ECLS, commonly called extracorporeal membrane oxygenation [ECMO]) has been available since the introduction of cardiac bypass during cardiac surgery. The first use of ECMO outside of the operating room occurred in the late 1960s. These first applications were to patients with severe refractory hypoxemia and the acute respiratory distress system (ARDS). Based on the initial successful application of ECMO to patients with ARDS in the 1970s, a randomized controlled trial comparing the use of ECMO to standard conventional management in patients with ARDS was conducted. This study failed to show benefit for the use of ECMO, with mortality in each group of about 90%. As a result, the use of ECMO in adult ARDS was not considered an option by most centers. In the 1980s a number of groups began using neonatal ECMO in an attempt to reduce the mortality in near-term infants from meconium aspiration, diaphragmatic hernia, sepsis, pneumonia, and other causes of severe respiratory failure. By the 1990s, based on the success of ECMO in neonates, more centers began using ECMO in pediatric and adult patients with severe respiratory failure. By 2000, the use of ECMO expanded to cardiac patients in all age groups. The increased use of ECMO in ARDS has been supported in recent years by the successful use of ECMO in the H1N1 epidemic of 2009. Today, ECMO is also used as a bridge to lung and heart transplantation and as a means of ensuring lung protective ventilation in patients in whom CO₂ elimination is markedly compromised.

There are two approaches to ECLS: VV and VA. With VV ECMO, blood is removed from a major vein, passed through a pump and oxygenator, and back to the patient via a major vein. In VA ECMO, blood is removed from a major vein but, after passing through a pump and oxygenator, is returned via a major artery.

Venovenous ECMO

This approach to ECMO is primarily designed to support the respiratory system. Blood is removed from a large vein, usually a femoral vein with the catheter frequently extending into the inferior vena cava, and returned to another large vein, the contralateral femoral or the superior vena cava via the jugular vein (Figure 38-1). Since the arterial circulation is not affected, normal pulsatile blood flow is maintained. This approach requires a normal functioning heart. Thus, blood flows through the pulmonary circulation. Gas exchange is a combination of the effect of gas exchange via the ECMO system and the respiratory system. The amount of gas exchange occurring in each area is dependent on the amount of blood flow diverted through the ECMO circuit. The greater the ECMO blood flow, the less contribution to gas exchange by the respiratory system.

VV ECMO has the advantage of not invading the arterial circulation, so the risk of air embolism is minimized. Pulmonary perfusion is also maintained, and pulsatile blood flow is maintained to the kidneys. The major disadvantages to this approach are that cardiac function must be relatively normal. Patients with severely compromised cardiac function are not candidates for VV ECMO. Catheter position is critical. Recirculation can occur, nullifying the effects of ECMO. Careful monitoring of the oxygenation and ventilation is important. The ability of VV ECMO to oxygenate the patient is less than that of VA ECMO for the same blood volume diverted to the ECMO system. The patient must be anticoagulated, and bleeding is a major potential complication.

Venoarterial (VA) ECMO

With venoarterial (VA) ECMO, blood is removed from a large vein (femoral or jugular) similar to VV ECMO, but blood is returned to circulation via a large artery, frequently the carotid artery in neonates and children or the femoral artery in adults (Figure 38-2). VA ECMO is designed to support the failing heart. With this approach, essentially 100% of the cardiac output can be diverted through the ECMO system. As a result, pulsatile blood flow is lost. All gas exchange occurs via the ECMO system.

The advantages of this approach are independence from cardiac function, ability to divert the entire blood volume, maximized oxygenation capability, and a marked decrease in the need for ventilatory support. The major disadvantages are the need to invade the arterial circulation and the risk of air embolism. Pulsatile blood flow is lost, and thus, pulse oximeters do not work. Normal pulmonary and renal pulsatile flow is lost. The patient must be anticoagulated, and thus, bleeding is a major potential complication.

There are three parts of an ECMO system regardless of the approach used to provide ECMO: the pump, the oxygenator, and the cannulas used to establish vascular access. Original ECMO systems used roller pumps to move blood through the circuit. These pumps are still in use for neonates. However, most adult and pediatric ECMO systems use a centrifugal pump. The major problem with roller pumps is that blood is moved by compressing the circuit between the roller and its casing as the pump rotates. This places stress on the circuit and can potentially cause circuit leaks. In addition, the compression destroys red blood cells and platelets, requiring transfusions. Neither of these concerns exists with centrifugal pumps.

Original oxygenators were very large—some a meter in diameter. Newer models are much smaller, approximately 25 cm long and 5 in in diameter. All oxygenators operate by the countercurrent principle; that is, blood flows in one direction on one side of a semipermeable membrane and gas moves in the opposite direction on the other side of the membrane. Oxygen and carbon dioxide move across the membrane in opposite directions. Oxygen diffuses from the gas to the blood, and carbon dioxide diffuses from the blood to the gas. The gas flowing through the oxygenator is referred to as the sweep flow. Dependent on the specific oxygenator, sweep flows of up to 15 L/min can be set. Because of the physiology of oxygen transport by the blood, compared to carbon dioxide, oxygenators are more efficient in removing carbon dioxide than adding oxygen. In some cases with VV ECMO in which the oxygenation needs are great, carbon dioxide is added to the sweep flow to avoid severe hypercapnia and alkalosis. Efficient carbon dioxide removal is a reason that VV ECMO has been recently promoted for the management of patients with chronic obstructive pulmonary disease (COPD) in severe hypercapnic respiratory failure.

A number of monitors can be added to the ECMO system to ensure safety. Pre- and post-membrane pressure and blood gases can be measured continuously, and a blood warmer is added to the system to maintain the returning blood at body temperature. The ECMO system can be used to rapidly adjust a patient's body temperature. With VA ECMO diverting nearly 100% of the blood volume, body temperature can be changed in a short time.

A number of different types of cannulas have been designed for ECMO. Most are single-lumen cannulas. Recently a double-lumen bicaval cannula has become popular for VV ECMO where the goal is to mobilize the patient after stabilization of ECMO. This cannula is placed into the external jugular vein and passed into the vena cava. It drains blood from both the superior and inferior vena cava, and directs the return of blood directly across the tricuspid valve (Figure 38-3). The major concern with this type of cannula is recirculation of blood if the catheter is not correctly placed. The advantage to this type of cannula is mobility. Patients can be mobilized with less concern regarding altered blood flow resulting from kinking of the catheter, as would occur with a femoral catheter.

VV or VA ECMO is very effective in removing CO₂. The CO₂ level in blood returning from an ECMO system can be decreased to almost zero. As a result, it does not require a large diversion of the cardiac output by a VV ECMO system to markedly reduce the Paco₂. The movement of 1 to 2 L/min of cardiac output through a VV ECMO system can decrease the Paco₂ by 20 mm Hg or more. Experimental pump-less VV ECMO systems might be used to manage the hypercarbia common in patients in an exacerbation of COPD, severe acute asthma, severe ARDS where lung protective ventilation is at its limit, and in patients awaiting lung transplantation who require rehabilitation. The systems that are being developed require adequate cardiovascular reserves and have very low resistance to blood flow (≤ 15-mm Hg pressure drop across the oxygenator at 3 L/min flow).

ECMO is used with neonates, children, and adults. ECMO was originally designed for management of severe respiratory failure. However, there is an increasing trend for the use of ECMO in patients with cardiac failure and chronic respiratory failure (Table 38-1).

Neonates

ECMO was first used primarily in neonates. The overall number of neonates having received ECMO is much larger than that of pediatric and adult patients combined, but the use of ECMO in neonates is decreasing. This is attributed to better overall management of neonates with cardiopulmonary failure. The introduction of surfactant and lung protective ventilatory strategies, as well as improvement in the medical management of neonates, has reduced the yearly number of patients requiring ECMO since its peak in the mid 1990s. The primary indications for ECMO in neonates are congenital diaphragmatic hernia, meconium aspiration, persistent pulmonary hypertension of the newborn, respiratory distress syndrome, sepsis, and severe cardiac anomalies/cardiac surgery.

Pediatrics

The number of pediatric patients treated with ECMO has increased each year since the mid-1980s. Early use of ECMO in pediatric patients has been primarily for severe respiratory failure whether it was from ARDS, pneumonia, or other causes. However, today the primary reason that pediatric patients are placed on ECMO is postcardiac surgery, in most cases for the correction of severe cardiac anomalies.

Adults

Few ECMO cases in adults were reported before 1990. However, by the mid 1990s, the number of adult ECMO cases started to rise and there has been major growth in the number of adult ECMO cases since 2009. This is due to the positive results reported by many centers on the use of ECMO for the management of severely ill patients as a result of the H1N1 epidemic of 2009, and the increase in the use of ECMO to manage patients with heart failure. In addition, centers are using ECMO as a bridge to transplantation, and for the management of exacerbations of COPD and asthma.

The number of pediatric patients, and adults, managed with ECMO is expected to continue to rise as the number of centers offering ECMO increases. This increase in the use of ECMO will be in three specific patient populations. The first and largest increase will be in patients with heart failure. Many centers with a large heart failure/cardiac surgical program will likely develop ECMO programs. The second area of increase will be in patients with acute hypercapnia who cannot be adequately managed with lung protective ventilation, or in patients with chronic lung disease failing conventional management or requiring rehabilitation as transplant candidates. The third area will be the management of severe refractory hypoxemia of any cause. Clinical challenges such as the H1N1 epidemic will continue to promote the use of ECMO in this group of patients. The expectation is that the number of centers offering ECMO will continue to increase.