AOPVPA RV RA LV LA Hemofilter Pump Lung Heparin O2 in CO2 out A Ventilator FiO2 PPlat/PEEP Monitor P V VO2, VCO2˙ ˙ ulateCalc DO2 liance,Comp PVRSVR, ˙ ˙ nitorMo P, CO,BP, PA SvO2 Sa, O2 globinHemo it[.]
Calculate ˙ O2 D ˙ O2, V˙ CO2 V Ventilator FiO2 PPlat/PEEP Monitor Flow P SAT ACT Monitor P V ˙ O2, V ˙ CO2 V Calculate ˙ O2 D ˙ Compliance, SVR, PVR CO2 out PA AO PV Pump LA RA Lung RV Monitor BP, PAP, CO, SvO2, SaO2 Hemoglobin LV O2 in Hemofilter Heparin A VV access: mixing ECMO flow and native venous flow in the right atrium ECMO flow + Native venous flow = Cardiac output No lung function Outlet Content 14 Sat 100 PO2 500 Systemic Content 12.3 Sat 90 PO2 50 CO2 out PA Pump RA AO PV LA Lung RV LV O2 in Inlet ECMO flow B Content Sat 70 PO2 40 Native venous flow Total venous return (CO) • Fig 56.1 (A) Cervical venoarterial (VA) extracorporeal membrane oxygenation (ECMO) cannulation (B) Venovenous (VV) ECMO cannulation. Calculate ˙ O2 D ˙ O2, V˙ CO2 V Ventilator FiO2 PPlat/PEEP Monitor Flow P SAT ACT Monitor P V ˙ O2 , V ˙ CO2 V Calculate ˙ O2 D ˙ Compliance, SVR, PVR CO2 out PA AO PV Pump RA LA Monitor BP, PAP, CO, SvO2, SaO2 Hemoglobin Lung LV RV O2 in Hemofilter Heparin C Calculate ˙ O2 D ˙ O2, V˙ CO2 V Ventilator FiO2 PPlat/PEEP Monitor Flow P SAT ACT Monitor P V ˙ O2, V ˙ CO2 V CO2 out PA RA Lung LA Pump RV O2 in AO PV LV Calculate ˙ O2 D ˙ Compliance, SVR, PVR Monitor BP, PAP, CO, SvO2, SaO2 Hemoglobin Heparin D • Fig 56.1, cont’d (C) Double-lumen VV support (Avalon or Crescent) (D) Femoral venoarterial ECMO ACT, Activated clotting time; AO, aorta; BP, blood pressure; CO, cardiac output; CO2, carbon dioxide; Do2, oxygen delivery; Fio2, fraction of inspired oxygen; LA, left atrium; LV, left ventricle; O2, oxygen; P, pressure; PA, pulmonary artery; PAP, pulmonary artery pressure; PEEP, positive end-expiratory pressure; PO2, partial pressure of oxygen; PPlat, plateau pressure; PV, pulmonary vein; PVR, pulmonary vascular resistance; RA, right atrium; RV, right ventricle; Sao2, arterial oxygen saturation; SAT, oxygen saturation; Sat, oxygen saturation; Svo2, mixed venous oxygen saturation; SVR, systemic vascular resistance; V, volume; Vo2, maximal oxygen uptake; VCo2, volume of exhaled carbon dioxide 660 S E C T I O N V Pediatric Critical Care: Pulmonary heart This catheter can be connected by a Y adapter into the venous drainage of the extracorporeal life support (ECLS) circuit to provide adequate left heart decompression.31 Patients with intact sternums who require left atrial decompression often are taken to the cardiac catheterization suite for a blade atrial septostomy, which allows the left heart to decompress into the right atrium and the blood to be drained into the venous ECMO cannula.31 Experience with use of an Impella device to provide left heart drainage or use of an intraaortic balloon pump (in larger patients) has also been successful Venovenous Extracorporeal Membrane Oxygenation VV ECMO differs from VA ECMO in that blood is both withdrawn and reinfused into the patient’s venous circulation (see Fig 56.1B); thus, adequate cardiac function must exist Cannulation can be introduced via either the cervical or femoral vessels, although new cannulas with improved flow dynamics allow for the use of other vessels as well.33 Currently, several types of multiplelumen single cannulas exist One such cannula, manufactured by Origen, is potentially available in sizes from 13 to 32 Fr and has been used often in infants This cannula is placed into the right IJ vein and requires only one surgical site The drainage and infusion lumens in this cannula are separated by a distance of a few centimeters Careful placement and orientation of the cannula can reduce recirculation of reinfused blood from the ECMO circuit, although some amount of recirculation (which will be discussed later in further detail) always occurs Other cannulas, especially popular for use with adults and larger children, are manufactured by Maquet (Avalon) and Medtronic (Crescent), and are up to 32 Fr These cannulas have two drainage lumens, one of which is positioned in the IVC and one in the SVC A reinfusion port that sits between the two lumens is directed at the tricuspid valve when placed properly, thus limiting recirculation of oxygenated blood Placement of this cannula requires meticulous assessment with echocardiography or fluoroscopy for safe and optimal performance Although experience with the Avalon cannula has been good, especially among adolescents and adults, late cardiac perforation or difficulty in maintaining proper placement has been reported in patients receiving the 13 Fr cannula, which has been reconfigured Effectiveness is not yet well documented Limited availability of small cannulas for VV ECMO in neonates and infants is still resulting in many patients receiving VA support The Crescent (24–32 Fr) has limited patient experience at this writing, but initial results are favorable The Crescent has a slightly larger return orifice, reinforced suture collars, and radiopaque markers to identify SVC, IVC, and return port sites Time will tell whether the advantages of these cannulas outweigh their increased cost Obstruction to venous drainage and deep vein thrombosis in the upper extremities are complications that may occur VV ECMO also can be provided via two (or more) separate access sites; this is the most popular mode in many centers Patients with VV cannulation may have venous blood drained from the right atrium via the IJ vein or from the IVC via the femoral vein Although more venous drainage usually can be obtained from a cervical cannula (which is usually shorter and larger than a cannula that can be placed into the femoral vein), the femoral site often may prove adequate, and draining from the femoral vessel and reinfusing into the IJ results in less recirculation Older children and adults also may undergo bilateral femoral cannulations, with one cannula placed into the high IVC and the other ending in the low IVC or iliac vessels Venoarteriovenous Extracorporeal Membrane Oxygenation The venoarteriovenous (VAV, sometimes also referred to as venovenoarterial) mode is a hybrid mode that combines VV support for pulmonary failure and partial VA support, with the advantage of percutaneous cannulation The typical circuit consists of a drainage cannula in the right femoral vein and two return cannulas connected via a Y connection, one in the right IJ vein and the second in a femoral artery Alternatively, a dual-lumen IJ venous cannula and carotid or subclavian arterial cannula can be used, which simplifies access Blood flow between the two return limbs is controlled to achieve approximately 30% flow into the arterial system by restricting the flow to the venous cannula (usually with a simple screw clamp) VAV support is ideally suited for larger pediatric patients who require partial cardiac support in addition to pulmonary support.34 Arteriovenous The concept of pumpless AV support of gas exchange was described over 40 years ago but was not feasible until the development of low-resistance, high-efficiency membrane lungs Usually placed in the femoral artery and vein, the patient’s systemic AV pressure gradient drives flow through a membrane lung without the need for a pump Because arterial blood is usually well oxygenated, oxygen transfer is limited, but carbon dioxide (CO2) is readily removed The applications of AV CO2 removal (AVCO2R) are identical to those of extracorporeal carbon dioxide removal (ECCO2R) without the need for an extracorporeal pump, that is, reduction in mechanical ventilatory support and control of hypercapnia A blood flow of 15% to 20% of cardiac output is required, and, in adults, a cardiac index of 3L/min per m2 and mean arterial blood pressure greater than 70 mm Hg are considered necessary to avoid the need for a pump in this method of support Another form of pumpless ECMO support is used in patients with severe pulmonary hypertension This form of paracorporeal lung assist uses the right ventricle to push blood flow via a cannula placed in the pulmonary artery through the low-resistance membrane lung where CO2 is removed and oxygen added The oxygenated return flows via a cannula placed in the left atrium and native heart ejection provides systemic blood flow Patients can be supported until their acute pulmonary hypertensive crisis is over or even to the point of heart/lung transplantation.33 Percutaneous Cannulation Although the historical approach to vessel access has been via an open procedure with placement of cannulas under direct vision, kits for percutaneous placement now exist in many sizes Percutaneous cannulation avoids the need for an open surgical site, thus decreasing surgical-site bleeding and risk of infection Ultrasonography is also helpful in guiding needle entry during vessel puncture, enhancing the chance of successful cannulation Use of fluoroscopy during percutaneous catheter insertion can help to identify aberrant guidewire placement and reduce the chance of vascular injury Percutaneous kits use a modified Seldinger technique with obturators increasing in size to dilate the vessel Once the appropriate size is reached, the cannula is passed into the vessel over the largest obturator Percutaneous cannulation carries with it the inherent risk of potentially tearing a large vessel during cannulation Although CHAPTER 56 Extracorporeal Life Support percutaneous cannulation in some centers is performed by nonsurgical personnel, surgical backup to perform an immediate cutdown for control of bleeding from a disrupted vessel may be needed Despite the obvious fear of vessel disruption, this complication occurs infrequently In one series of 100 patients who were cannulated by nonsurgical personnel, only two vascular complications occurred Both were associated with mortality.35 A more recent experience with 32 ECMO runs in neonates and children (3.0–17 kg, 13–27 Fr double-lumen cannulas) with percutaneous cannulation by intensivists also noted successful placement with no major adverse events.36 Extracorporeal Membrane Oxygenation Circuit The majority of ECMO circuits are incorporated with the ECMO pump and membrane lung, which makes setup and priming easier, although some require assembly Circuit configuration can be tailored for centers specifically as well Most manufacturers provide coating of their circuits to help prevent platelet activation and aggregation, although this has not totally prevented use of anticoagulation and the success of the surface coating is not well proven Over time, the complexity of circuits and access ports has been reduced to limit accidental exposure to air embolus from open stopcocks, especially as centrifugal systems have come into practice The number of connectors within the circuit has also been linked to fibrin deposition and hemolysis and may increase the risk of infection Venous Reservoir and Venous Saturation Monitor In cardiopulmonary bypass, use of an open venous reservoir allows venous filling pressures to be controlled and prevents excessive negative pressure in the bypass circuit to develop as blood is withdrawn from the heart ECMO operates in a closed system; thus, generation of high negative pressure as blood is withdrawn can be transmitted to the venous cannulation vessel This can induce endothelial damage, destruction of red blood cells with resultant hemolysis, or cause a decrease or cessation in venous return to the ECMO circuit To prevent these events, most centers use either a servoregulated pressure monitoring system that decreases or stops pump flow if excessive negative pressure is measured or employ a smaller version of a venous reservoir commonly referred to as a bladder In roller head devices, the bladder sits at the lowest point of the ECMO circuit and allows blood to collect and be drawn from it to the pump head It also acts as an air trap and normally has access ports to allow aspiration of any air collected in the bladder chamber The bladder can be used as a servoregulating device that helps to match forward flow to venous return Servoregulation by monitoring of direct negative and positive pressure within the ECMO circuit has eliminated the use of the bladder in many centers, although those that use roller pump systems most often still incorporate some sort of bladder within the circuit.37–39 Another important feature of the ECMO circuit is the venous saturation monitor, which is placed along the venous drainage line Most new pump systems incorporate this into their monitoring apparatus Monitoring venous saturation over time gives information regarding the balance of Do2 and extraction Use caution when interpreting venous saturation in the presence of left atrial drains or left-to-right intracardiac shunts in patients with adequate pulmonary function and gas exchange, as oxygenated blood returning to the left atrium will be directed into the venous drainage line and elevate observed venous saturation To obtain a 661 more accurate assessment of the patient’s venous saturation, monitoring should occur at a site proximal to entry of oxygenated blood into the circuit.39–41 Priming Although ECMO can be initiated with a crystalloid prime in infants or in circumstances in which ECMO is not emergent, blood priming may be optimal Because blood has been citrated and stored, it may be acidotic, depleted of calcium, and have a high potassium level Calcium (usually as calcium chloride), bicarbonate or tromethamine, and heparin are added during the blood priming procedure Electrolytes should be measured in the priming blood before bypass is initiated because disturbances of cardiac rhythm, or frank cardiac arrest can occur upon initiation of ECMO Hyperkalemia exists almost universally in the bloodprimed circuit despite buffering by calcium and bicarbonate The potassium level rarely causes systemic effects once the ECMO prime is diluted with the patient’s intrinsic blood volume Use of the freshest blood available also may lessen the degree of hyperkalemia in the primed circuit Rarely, hyperkalemia may be of such concern that blood must be washed prior to ECMO use or filtered via a hemofiltration device in the ECMO circuit to reduce the potassium level Newer centrifugal/hollow fiber systems have a priming volume much lower (,400 mL) than with older roller pump/silicone lung configurations and may also decrease the risk of hyperkalemia It should be noted, however, that sudden changes in cardiac rhythm or the “myocardial stun” phenomenon whereby the heart seems to stop contracting on initiation of ECMO are often related to hypocalcemia (or hyperkalemia) Medical management of these conditions must be implemented immediately Types of Pumps and Oxygenators Roller Head Pumps The majority of ECMO prior to 2012 was performed using a roller head semiocclusive pump.38–41 Venous blood is siphoned via gravity to the roller heads, which are enclosed in a box (the pump housing) Compression of the tubing inside the pump housing (called the raceway) by the rollers advances blood forward at high pressure to the membrane lung from which it is returned to the patient The gravity siphon, the need to periodically reposition tubing within the raceway to avoid rupture from excessive spallation, and the need for pressure monitoring with multiple access points within the circuit combine to require a large priming volume This increases the exposure to blood products and the associated inflammatory response from contact with the artificial surface of the circuit Hemolysis from red cell destruction within the raceway or other areas of turbulent flow in the circuit may also occur Roller pumps generate high pressure in the circuit distal to the raceway/roller heads Thus, acute interruption to forward flow as may occur with kinking of the arterial cannula or elevated resistance to blood flow on the high-pressure side of the circuit can result in immediate and potentially lethal circuit rupture Monitoring of the high-pressure side of the ECMO circuit is universal, with critical high limits for arterial line pressure determined based on tubing size and pump flow Pressures below 300 to 350 mm Hg are desired; safety mechanisms will stop the ECMO pump if line pressure limits are exceeded As air embolus into the patient can be lethal (especially with VA support), servoregulation to decrease or stop the pump if air bubbles are detected are often incorporated into the circuit Newer pumps may provide pulsatile flow The nonpulsatile nature of VA ECMO flow has been implicated 662 S E C T I O N V Pediatric Critical Care: Pulmonary in the renal dysfunction that is sometimes noted in patients treated with this modality The pulsatile flow of the newer pumps has yet to be effectively linked to native heart ejection, although this remains a goal for the future.42,43 While roller pump systems were the primary reported method of ECMO support until 2012, advances in centrifugal technology have resulted in many centers, especially outside the United States, eliminating roller pump ECMO At this writing, 600 to 700 patients still receive roller pump ECMO yearly, the majority being neonates.51 Centrifugal Pumps Centrifugal devices contain a magnetically controlled spinning head Blood enters via the ECMO circuit at the inner apex of the pump head and is propelled tangentially to the outer wall where it is ejected into the circuit and advances to the membrane lung and back to the body The longer blood sits in the centrifugal head, and the faster the head spins, the more red blood cells are exposed to shear stress, and hemolysis may be created In the past, centrifugal heads were large and heat was generated by the rotating head seated on a bearing Because blood remained in the head longer at the low flow rates that infants require, hemolysis was severe Although newer models have small heads with small priming volumes (e.g., 13 mL) and levitation may create less heat, some data have shown that hemolysis is still high at infant flow rates while other research has found patient size and flow rates not to be important factors It is also postulated that, as clots develop in the membrane lung over time and increase outflow resistance, hemolysis worsens in the rotor head Centrifugal pumps may also create microemboli that can reach the patient if they are not trapped in the membrane lung.44–47 Air bubble detectors are a component of most centrifugal systems Centrifugal pumps have been linked to renal failure and other adverse events in infants in some recent studies, although concerns that the learning curve to adjust to centrifugal support from roller head systems may be related to observed outcomes and not necessarily the pump itself Centrifugal pumps are very sensitive to preload and afterload; understanding the physiology of their operation is mandatory to providing optimal ECMO support.49,50,52 Despite some concerns, centrifugal pumps have many features that make them potentially safer and easier to operate than roller pump systems They not require gravity drainage; thus, they can be placed at any level as long as it is lower than the patient’s heart, and therefore circuit lengths can be shorter As centrifugal pumps advance blood only if the downstream pressure is less than that generated in the circuit, the absence of risk of tubing rupture if postpump sites are occluded is another advantage They are easier to transport, have a smaller footprint, and many have built-in pressure and flow monitoring systems that can help servoregulate flow These factors make centrifugal technology theoretically safer than roller devices and have fostered transition to a “single caregiver” model in some centers, which reduces or eliminates a specific ECMO specialist to monitor the system on a 24/7 bedside basis and has the bedside nurse provide both patient care and ECMO circuit oversight ECMO experts are available for troubleshooting or intermittent rounding purposes Since 2012, the use of centrifugal pumps has exceeded that of roller systems, and they have replaced roller systems in most countries outside the United States.55 Negative Inlet Pressure Both roller and centrifugal pumps can create high negative pressure on the venous inflow side (also referred to as the inlet) Loss of venous return with continuous rotation of either roller head or centrifugal pumps results in generation of high negative pressure in the ECMO tubing that can lead to hemolysis and cavitation as air is drawn out of solution As mentioned, blood flow to the centrifugal head is augmented by the “suction” effect of the spinning pump head and thus is not as dependent on gravity drainage as are roller head pumps The active suction effect of centrifugal heads can create levels of negative pressure as high as 2200 to 2700 mm Hg within the venous inlet tubing Monitoring of venous pressure with alarm limits to signal when excessive negative pressure is occurring is common in most centers Setting an appropriate venous line or inlet pressure is an important part of circuit management but seems frequently misunderstood Optimal servoregulation alarms or interventions require several factors that should be taken into consideration The most important fact is that the total resistance on the venous side of the circuit is the sum of all resistances encountered The major contributor to this negative pressure is the pressure drop across the venous cannula Most manufacturers provide pressure drop curves for each of their cannulas across a range of flow rates While these curves are created using water instead of blood—and thus not represent the effect of viscosity of blood—they provide a suitable surrogate for the resistance induced by the cannula Selecting a cannula that will provide the expected blood flow at a pressure drop of less than 100 mm Hg is recommended.48 Pressure drops greater than 100 mm Hg will result in an increased risk of hemolysis Another factor to consider is that animal research has shown that exposure of the right atrium to pressures more negative than 20 mm Hg can result in damage to the intima Taking these two points into consideration, if the venous cannula chosen to provide the expected blood flow rate has a pressure drop of 40 mm Hg from inlet to outlet, the venous servo limit should be set at 260 mm Hg to protect the right atrium from damage and provide adequate blood flow Thus, the venous pressure limit to be set is not a constant number but will vary from patient to patient based on cannula size and desired flow rates Clinicians also vary as to where best the venous pressure should be measured—at the junction of the catheter to the circuit tubing, which will reflect negative pressure closest to the heart, while others use the prepump-head site, which reflects the most negative pressure within the total circuit.45 Servoregulation of both inlet and outlet pressures are set, and the pump (or specialist) adjusts the revolutions per minute of the pump head or the desired flow to maintain set goals Membrane Lung Until recently, the predominant membrane lung was a silicone membrane envelope (with a plastic spacer screen inside) wound in a spiral fashion around a polycarbonate spool.5,46 Gas flows within the interior of the envelope, and blood flows between the turns in the membrane envelope Gas exchange occurs due to partial pressure differences in the gas and blood While this device is commonly referred to as the oxygenator, CO2 removal is also provided—thus, the correct term should be membrane lung Blood flow to the membrane lung is controlled by the pump setting While the silicone lung was extremely efficient in gas exchange, it had high resistance to blood flow; newer versions of the membrane lung have replaced it in ECMO support Hollow-Fiber Membrane Lung Almost overnight, new hollow-fiber membranes replaced the silicone lung during ECMO use.53,54 Early versions of these oxygenators were plagued by difficulties with plasma leakage that ... within the raceway to avoid rupture from excessive spallation, and the need for pressure monitoring with multiple access points within the circuit combine to require a large priming volume This... necessary to avoid the need for a pump in this method of support Another form of pumpless ECMO support is used in patients with severe pulmonary hypertension This form of paracorporeal lung assist... pressure within the ECMO circuit has eliminated the use of the bladder in many centers, although those that use roller pump systems most often still incorporate some sort of bladder within the