Pediatr Crit Care Med. Author manuscript; available in PMC 2014 Jun 1.
Published in final edited form as:
doi: 10.1097/PCC.0b013e318292dd10
NIHMSID: NIHMS470863
The publisher's final edited version of this article is available at Pediatr Crit Care Med
See other articles in PMC that cite the published article.
Abstract
The extracorporeal membrane oxygenation (ECMO) circuit is made of a number of components that have been customized to provide adequate tissue oxygen delivery in patients with severe cardiac and/or respiratory failure for a prolonged period of time (days to weeks). A standard ECMO circuit consists of a mechanical blood pump, gas exchange device, and a heat exchanger all connected together with circuit tubing. ECMO circuits can vary from simple to complex and may include a variety of blood flow and pressure monitors, continuous oxyhemoglobin saturation monitors, circuit access sites and a bridge connecting the venous access and arterial infusion limbs of the circuit. Significant technical advancements have been made in the equipment available for short and long term ECMO applications. Contemporary ECMO circuits have greater biocompatibility and allow for more prolonged cardiopulmonary support time, while minimizing the procedure-related complications of bleeding, thrombosis and other physiologic derangements that were so common with the early application of ECMO. Modern era ECMO circuitry and components are simpler, safer, more compact and can be used across a wide variety of patient sizes from neonates to adults.
Keywords: extracorporeal life support, pump, oxygenator, cannula
Extracorporeal membrane oxygenation (ECMO) is a modified form of cardiopulmonary bypass used to provide adequate tissue oxygen delivery in patients with severe cardiac and/or respiratory failure. ECMO involves draining blood from the venous circulation, pumping it through an artificial lung where oxygen is added and carbon dioxide is removed, and then returning the warmed blood back to either the venous or arterial circulation. ECMO and the associated management protocols will mechanically support a patient and allow for optimization of all aspects of care for the period of time necessary for recovery of native cardiac and/or respiratory function. The ECMO circuit consists of a number of components that have been customized to allow patients to be supported for a prolonged period of time (days to weeks), as opposed to a number of hours during cardiopulmonary bypass. Significant technical advancements have been made in the equipment available for short and long term ECMO applications.
ECMO Circuit Design
A standard ECMO circuit consists of a mechanical blood pump, gas exchange device (membrane oxygenator), and a heat exchanger all connected together with circuit tubing between the venous access cannula and either the arterial (VA) or venous (VV) infusion cannula (Fig. 1) (1). Circuit tubing used is made from a polyvinylchloride (PVC) - based plastic compound. ECMO circuits can vary from simple to complex and may include a variety of blood flow and pressure monitors, continuous oxyhemoglobin saturation monitors, circuit access sites and a bridge connecting the venous access and arterial infusion limbs of the circuit. Blood is exposed to a large surface area as it moves through the ECMO circuit, causing significant heat loss, and children on ECMO require direct warming of the blood to maintain body temperature.
Standard ECMO circuit. Venous blood drains from the patient, passes through a venous saturation sensor and a bladder before being pumped to the oxygenator/heat exchanger device. The oxygenated, warmed blood passes the ECMO circuit bridge before infusing back into the patient into the arterial (VA) or venous (VV) system. There are multiple infusion and access ports as well as pressure and flow monitors along the way. Adapted from (1).
Individual centers customize their ECMO circuits to suit their patient population and program needs. In general, the less complicated the circuit, the easier it will be to manage and the fewer the connectors and stopcocks, the less potential sites of flow turbulence and blood stasis. ECMO circuits need to be functional and portable, yet ideally, one should attempt to limit the size of the foreign surface of the circuit. This is because when blood is exposed to the non-biologic surfaces of a circuit, a complex biologic response is initiated involving both the coagulation pathway and the inflammatory response pathway (). The tubing of the circuit can be coated with a biocompatible lining to reduce the systemic inflammatory response and risk of thrombosis and bleeding (), although no coating to date has been shown to eliminate this reaction completely. Shorter tubing may also optimize venous drainage, as there is less resistance to blood flow (). ECMO circuits usually have a variety of monitors including an integrated blood pump flow monitor which measures total circuit blood flow, as well as separate ultrasonic flow detectors that can be placed on venous drainage and arterial infusion limbs of the circuit. The best way to monitor ECMO circuits is without interrupting the blood path. This means that flow, saturations, pressures and blood chemistry should ideally be measured without the inclusions of cuvettes, connectors or added tubing into the blood flow path. Circuit pressures can be measured at three important locations within the ECMO circuit. Venous access pressures measured before a centrifugal blood pump (see below) help ensure excessive suction is avoided and help determine the adequacy of venous drainage and circuit volume. Circuit pressures are also commonly measured before and after the gas exchange device and this is also where most centers have circuit access sites. If both pressures rise, this indicates increased resistance to flow post-oxygenator and may be a result of obstruction to the inflow cannula. An increase in the difference between the pre- and post-oxygenator (transmembrane) pressures suggest an increase in the resistance inside the oxygenator. Oxyhemoglobin saturation of circuit blood is most useful on the venous access limb of the ECMO circuit, especially in VA support, to help assess the adequacy of oxygen delivery and support.
ECMO circuits can include a bridge of circuit tubing connecting the proximal venous access limb of the circuit to the proximal arterial infusion limb of the circuit. This bridge is used to recirculate blood through the circuit if the patient needs to be removed temporarily from support. It is also particularly useful in VA support during weaning of flow to low levels of support. The patient can be clamped off from the ECMO circuit to determine if adequate gas exchange and hemodynamics can be maintained while flow continues through the bridge. There are a number of modifications to the bridge that have been employed, including an open bridge (regulating flow with a thumb clamp), placing a clamp on the bridge to occlude flow totally, and a bridge that is closed with stopcocks which can be opened when needed (). All forms of clamping, reopening and circulating through the bridge may increase turbulence in the system and disturb cerebral blood flow, especially when the patient is cannulated via the cervical vessels. Complete removal of the bridge is now favored in some centers.
ECMO circuits can be crystalloid-primed for use relatively quickly, and stored safely for up to 30 days (). If time permits, most pediatric patients will receive a blood-primed ECMO circuit. The composition of the blood prime varies from center to center but usually contains a mixture of packed red blood cells, albumin, fresh frozen plasma, unfractionated heparin, and calcium to replace that bound by citrate in the banked blood.
Pumps
A pump is an essential component of the ECMO circuit. Semiocclusive roller pumps have been the standard for decades, but have mainly been replaced by novel centrifugal pumps. The ECMO pump must provide flow appropriate for the patient (typically 75 – 150 mL/kg/min for infants and children), within a safe range of pressures to avoid hemolysis. Outlet pressure is a function of the pump speed and the resistance in the tubing, cannula, and arterial pressure of the patient. The risk of high outlet pressure is rupture of the circuit. The circuit should withstand pressure of 600 mm Hg, but 300 mm Hg is considered the upper safe limit for most applications. The risk of low inlet pressure is hemolysis, which occurs when gas cavitates out of the blood (more than 600 mm Hg suction). In roller pumps the suction is limited to the siphon from the patient to the pump (typically 100–150 mm Hg). Centrifugal pumps can generate 600 mm Hg suction for brief periods whenever the drainage is occluded and RPMs are over 4000. Measuring inlet pressure closer to the pump gives a greater absolute pressure than by measuring at the cannula connection ().
Roller pumps could generate direct suction on the venous catheter. In practice, this problem is avoided by the inclusion of a small collapsible bladder positioned at the lowest point of the venous line. The bladder (or a transducer directly in the venous line) is attached to an electrical switch that slows or stops the roller pump when a threshold suction is reached, then restarts the pump instantly when the filling pressure exceeds the pump suction (i.e., the venous drainage flow exceeds the pump flow). The suction on the venous cannula is the siphon created by the distance from the patient to the floor (typically 100–150 cm H2O). Whenever the bladder collapses (or the transducer senses subatmospheric pressure) and the pump stops, the suction effect of the siphon between the patient and the level of the bladder stops, avoiding any continuing suction on the right atrium. Also, because the pump motor is slowed or turned off whenever the bladder is collapsed, the pump cannot generate excessive suction in the blood between the pump and the bladder (which would cause cavitation and hemolysis). Thus, this bladder and electrical switching mechanism provides servo-regulation and some measure of safety for prolonged perfusion with a roller pump. The pump is adjusted to provide flow for the desired level of gas exchange or cardiac support. As long as the venous drainage is adequate, suction force is acceptable and the desired flow is delivered. If venous drainage is impeded for any reason (eg. hypovolemia, pneumothorax, kinking of the venous catheter), the pump stops and an alarm sounds. Flow resumes as soon as venous drainage is reestablished. Early in the course of extracorporeal circulation, the operator increases the flow to the point at which the pump is stopped by servo-regulation, thus identifying the physical limitation of venous drainage for the system. This flow rate is usually considerably greater than the flow actually required for extracorporeal support. However, if maximal flow through the system is inadequate after improving volume status, the venous catheter must be assessed. This may result in attempts to manipulate catheter position, replace it with a different design or larger cannula, or add a second catheter to gain more flow. The problems with roller pumps are: a sizable heavy motor is required, the tubing can wear or rupture in the pump head, and there is no limit to infusion pressure so there is always a risk of blowout.
Centrifugal pumps, in which a spinning rotor generates flow and pressure, are replacing roller pumps in many centers. New models have long pump head durability and can be used for prolonged extracorporeal circulation. Unlike roller pumps, the motor can be light and small, the components do not wear out, and the perfusion pressure is limited by the revolutions per minute (RPM), so return line pressure is low and circuit rupture is extremely rare. In long term ECMO, or prolonged in-vivo experiments, a superiority in blood-handling characteristics has been identified with centrifugal pumps, as long as inlet pressures are monitored and do not become excessive (7). The potential problems of a centrifugal pump head include stagnation and heating in the pump head, leading to thrombus at low flows or if the outlet line is occluded, and cavitation and hemolysis when the inlet line is occluded. New pump head designs have reduced the magnitude of many of these problems. Mendler and colleagues initially introduced the Maquet Rotaflow pump (Maquet, Hirrlingen, Germany) as a new centrifugal pump in 1995. This pump was able to reduce heat generation of the bearing and seal, and improve hydraulic efficiency with less ensuing blood damage than other commercially available devices (). These and other novel centrifugal pumps for long term use have a hole in the center of the rotor which solves the stagnation, thrombosis, and heat problems with older centrifugal pumps. When centrifugal pumps are used for cardiac surgery, the pump attaches directly to the venous reservoir and servo-regulation is provided by level sensing or by the pump operator. In ECMO, the pump inlet line is attached directly to the venous cannula. Many times a day, the venous drainage may be transiently but completely occluded (e.g., during coughing, hypovolemia, or kinking, manifested as 'chattering' of the venous line). When the venous line is occluded, the rotor keeps spinning, evacuating blood from the pump head and creating a vacuum in the pump head which causes cavitation and hemolysis. This happens in a second (milliseconds at high RPM), so no servo-regulation system is fast enough to prevent cavitation. The problem can be minimized by incorporating a collapsible bladder, such as the Better Bladder (Circulatory Technology, Oyster Bay, NY), in the venous line to act as a mini reservoir by servo-regulating pump RPM based on inlet pressure sensing, which prevents continuing suction when the line is occluded for more than a few seconds (). However, this adds further complexity and prime volume to the circuit. In practice, many centers deal with the problem by limiting the amount of RPM and reducing the flow when the venous line is chattering. Novel centrifugal pumps such as Maquet Rotaflow (Maquet) and Cobe Revolution (Cobe Cardiovascular, Inc., Arvada, CO) have small priming volumes and minimize thrombus formation, heat generation and hemolysis associated with traditional centrifugal pumps (). Hemolysis may be caused by heat generation and thrombus formation in the pump head, stagnant or turbulent blood flow zones in the pump head, oxygenator or other places in the circuit, shear stress caused by high blood flow velocities, excessive suction, and circuit thrombosis. The Thoratec Centrimag pump (Thoratec, Pleasanton, CA) features bearing-less technology; the rotor is levitated into the housing by the magnetic force generated by the motor, hence minimizing friction and improving hemocompatibility. The risk of thrombus formation is reduced by uniform unidirectional flow and less stagnation, while reduced shearing stress attenuates hemolysis. A number of studies have shown a reduction in circuit-related complications and hemolysis when comparing the use of novel centrifugal pumps to either roller pumps or traditional centrifugal pumps (, ). Other reports have found increased hemolysis and renal failure with centrifugal systems, although these studies included older centrifugal systems and were not focused on newer devices ().
Complete ECMO pump systems are available from a number of commercial vendors including: Maquet (Hirrlingen, Germany), Medos (Stolberg, Germany), Sorin (Mirandola, Modena, Italy) and Thoratec (Pleasanton, CA). All use a modified centrifugal pump, are theoretically safer, more portable and some contain a complete ECMO system with all necessary components in an integrated, more compact product.
Oxygenators
ECMO circuits have a gas exchange device called an oxygenator, to add O2 and remove CO2 from blood. It may contain several different biomaterials including silicone rubber as in the classic membrane lungs used for years, polypropylene hollow fiber devices for short term use, newer compressed surface polymethylpentene (PMP), as well as polyvinylchloride, polyurethane and stainless steel. The surface area and blood path mixing determine the maximum oxygenation capacity of any gas exchange device. The rated flow of an oxygenator is defined by the amount of desaturated (75%) blood that can be nearly fully saturated (95%) per minute. The Kolobow silicone rubber membrane lung has been the standard oxygenator used for ECMO applications for almost 50 years (). It is constructed of a flat reinforced sheet of silicone rubber membrane envelope wrapped around a wire mesh in a spiral coil. Blood and gas flow in counter-current directions within the silicone lung and gas exchange occurs by diffusion across the membrane. This membrane oxygenator is very effective at exchanging O2 and CO2, but it is necessary to have a variety of sizes available to support different size patients. A separate heat exchanger is required for smaller silicone membrane lungs. The silicone lung has a high resistance to flow which limits the maximum blood flow that could be otherwise obtained through the device and makes them less suitable for use with centrifugal pumps. By nature of its design and high resistance a silicon oxygenator is much harder to de-air, so it takes longer to prime and is more difficult for transports. Despite these limitations, the Kolobow membrane lung and roller pump served the needs of the ECMO community for decades.
Hollow fiber PMP oxygenators are extremely efficient at gas exchange and demonstrate minimal plasma leakage; have relatively low resistance to blood flow, making them easy to prime; and are well suited for use with centrifugal blood pumps (Fig. 2) (). The design for bundling the hollow fibers with gas inside and blood outside the fibers within the oxygenator, the winding technique used, and the blood flow pattern through the device are all key factors in decreasing the priming volume. By making the oxygenators more compact and optimizing the blood flow path, it is possible to decrease the surface area of the membrane and heat exchanger, thus reducing its potential for thrombus formation and inflammatory activation. The low prime volume enables a center to utilize one device for all size of patients and these circuits can be left assembled and crystalloid-primed, with the benefit of support implementation within minutes. The early experience with the PMP devices established them to be robust and long-lasting, with limitation of the inflammatory response and decreased transfusion requirements, making them well suited for long-term use (). There are a number of PMP membrane oxygenators in commercial use including the Quadrox-iD (Maquet, Hirrlingen, Germany), Hilite LT (Medos, Stolberg, Germany), Lilliput 2 (Sorin, Mirandola Modena, Italy), and the Biocube (Nipro, Osaka, Japan). Many of these devices are marketed in both pediatric and adult sizes; however, many centers prefer to use one size of oxygenator for all patients. These new generation oxygenators also contain an integrated heat exchange device, making it possible to precisely control body temperature without the need for additional components.
Contemporary ECMO Circuit. This circuit has been improved over by the years and incorporates a radial blood pump, a PMP oxygenator and a simple minimal circuit. Adapted from ()
Vascular Cannulas
Like other components of the modern ECMO circuit, vascular cannulas have undergone significant design changes which have improved efficiency of blood flow and overall performance. Several types of cannulas are currently available in a variety of sizes, with distinct features that may be used to adopt a cannulation strategy that is customized to the unique requirements of individual patients. Single-lumen cannulasare used to provide venous and arterial access for patients receiving VA ECMO or multiple site venous access for patients receiving VV extracorporeal support. Most cannulas are manufactured from biocompatible polyurethane, which may be coated with heparin or non-heparin polymers that may reduce platelet activation and the inflammatory response at the blood-cannula interface (). Cannulas are available in sizes ranging from 6F (2 mm diameter) to 51F (17 mm diameter). Most cannulas are manufactured with wire-reinforced bodies that are designed to prevent luminal occlusion. In addition, some incorporate a malleable wire along the length of the cannula wall, which may be used to customize the angle of insertion and position within the heart when used during open-chest ECMO. Various cannula tip configurations are available. Right-angle metal tip cannulas, commonly used for venous drainage during cardiopulmonary bypass, provide excellent right atrial drainage during post-cardiotomy support. Cannulas that employ a long, multi-fenestrated flexible tip are useful for jugular and femoral venous drainage, whereas cannulas that contain a single end-hole or a short fenestrated tip may be used for arterial or venous vascular access. Many cannulas are designed for percutaneous vascular access using cannula-specific guide wire introducer sets. Overall cannula length and cross-sectional area, which impart an inherent resistance to blood flow, must be considered during cannula selection to achieve optimal venous drainage.
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Consequently, cannula selection should be based on the estimated level of support (flow rate) to be provided and the size of the vessels to be accessed.
Dual-lumen cannulas provide venovenous support via a single jugular venous access site. Blood is removed from the patient via one lumen and then returned to the patient via a smaller lumen. There are currently three commercially available dual-lumen ECMO cannulas. The OriGen (Austin, TX) dual-lumen cannula is available in 12F, 15F, and 18F sizes and capable of providing extracorporeal gas exchange for patients up to 12 kg. The OriGen cannula may be inserted percutaneously using a guide wire or directly into the internal jugular vein through an incision in the overlying soft tissues. Proper positioning of the tip of the cannula near the inferior cavoatrial junction is necessary for optimal cannula performance. Blood drainage and return ports are spatially separated to decrease recirculation of blood within the ECMO circuit, which reduces overall efficiency of gas exchange (). A significant limitation of the OriGen cannula is that it is manufactured from non-wire-reinforced polyurethane that is subject to structural deformation after insertion. Kinking and collapse of the cannula caused by excessive suction can result in catastrophic interruption of ECMO flow. Refinement of the catheter to alleviate this problem is underway. The Covidien ECMO (Mansfield, MA) cannula is also manufactured from non-wire-reinforced polyurethane. It is only available in 14F overall diameter, limiting its use to neonatal patients with respiratory failure. The Avalon (Rancho Dominguez, CA) Bi-Caval dual-lumen cannula is manufactured from a wire-reinforced silicone polyurethane polymer that resists structural deformation. It may be inserted through the internal jugular vein via open surgical or percutaneous techniques. A unique feature of the Avalon cannula is that, when properly inserted, the distal tip is placed in the inferior vena cava, which facilitates simultaneous removal of blood from the superior and inferior vena cavae and return of blood to the right atrium. Echocardiographic or fluoroscopic imaging is necessary to verify proper cannula placement. The Avalon cannula is available in sizes ranging from 13F to 31F, which enables it to be used to support neonates through to adults. However, some centers have reported delayed atrial perforation when using the 13F cannulas, which may limit use of the Avalon cannula to older infants, children, and adults.
Interhospital Transport Circuits
The growth in the field of ECMO transport is one that parallels that of other ECMO circuitry. Specific circuits containing smaller, more biocompatible components have expanded the possibilities of patient transport on ECMO. Circuit simplification utilizing PMP membrane oxygenators and centrifugal pumps has allowed safer movement of patients from facilities that do not provide ECMO to those equipped with the expertise to support patients for extended periods. The field of transportation continues to evolve with purpose-built transport systems such as the Cardiohelp (Maquet, Hirrlingen, Germany) (), further simplifying the setup and monitoring of patients moving around or in between hospitals.
Conclusion
Modern era ECMO circuitry and components are simpler, safer, more compact and can be used across a wide variety of patient sizes from neonates to adults. Today’s ECMO circuits have greater biocompatibility and allow for more prolonged cardiopulmonary support time while minimizing procedure-related complications of bleeding, thrombosis and other physiologic derangements that were so common with the early application of ECMO.
Footnotes
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dr. Bartlett has received financial support from the National Institutes of Health. The authors have not disclosed any potential conflicts of interest.
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Contributor Information
Laurance Lequier, Director, ECMO Program, Stollery Children’s Hospital, Associate Clinical Professor Pediatrics, University of Alberta, Edmonton, AB, Canada.
Stephen B. Horton, Associate Professor, Director of Perfusion, Faculty of Medicine, Department of Paediatrics, The University of Melbourne, Royal Children’s Hospital, Melbourne, Australia.
D. Michael McMullan, Associate Professor of Surgery, Director, Mechanical Cardiac Support and ECMO, Seattle Children’s Hospital, Seattle, WA.
Robert H Bartlett, Professor of Surgery, Emeritus, University of Michigan, Ann Arbor, MI.
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Ecmo Machine
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When you install a sprinkler system, you might use PVC or polyethylene piping to deliver water to the sprinkler heads. If you use PVC, the typical diameter of the pipes is between 1/2 inch and 2 inches. The right size pipe depends on the overall capacity of your system.
It's All About Flow Rate
To determine the right size pipe for your needs, you must calculate the flow rate of the receptacle from which the water will flow. The simplest way to do this is to get a one-gallon bucket and a stopwatch. Place the bucket under the faucet and turn it on full blast at the same time you turn on the stopwatch. When the bucket is filled, stop the stopwatch and note the time. Divide the number of gallons, in this case, 1, by the time it took to fill the bucket, and then multiply that number by 60 seconds. That number is flow rate in terms of gallons per minute, or GPM.
For schedule 40 PVC, use 1/2-inch pipe for a 4 GPM system, 3/4-inch pipe for 8 GPMs, 1-inch pipe for 13 GPMs, 1 1/4-inch pipe for 22 GPMs, 1 1/2-inch pipe for 30 GPMs, and 2-inch pipe for 50 GPMs. For schedule 80 PVC, use 1/2-inch pipe for 3 GPMs, 3/4-inch pipe for 6 GPMs, 1-inch pipe for 11 GPMs, 1 1/4- inch pipe for 20 GPMs, 1 1/2-inch pipe for 26 GPMs, and 2-inch pipe for 46 GPMs.
References (2)
About the Author
Nicole Vulcan has been a journalist since 1997, covering parenting and fitness for The Oregonian, careers for CareerAddict, and travel, gardening and fitness for Black Hills Woman and other publications. Vulcan holds a Bachelor of Arts in English and journalism from the University of Minnesota. She's also a lifelong athlete and is pursuing certification as a personal trainer.
Cite this Article Choose Citation Style
Vulcan, Nicole. 'What Is the Right Size PVC Pipe for a Sprinkler System?' Home Guides | SF Gate, http://homeguides.sfgate.com/right-size-pvc-pipe-sprinkler-system-101492.html. 29 December 2018.
Vulcan, Nicole. (2018, December 29). What Is the Right Size PVC Pipe for a Sprinkler System? Home Guides | SF Gate. Retrieved from http://homeguides.sfgate.com/right-size-pvc-pipe-sprinkler-system-101492.html
Vulcan, Nicole. 'What Is the Right Size PVC Pipe for a Sprinkler System?' last modified December 29, 2018. http://homeguides.sfgate.com/right-size-pvc-pipe-sprinkler-system-101492.html
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Published online 2014 Oct 9. doi: 10.1007/s13244-014-0357-x
PMID: 25296619
This article has been cited by other articles in PMC.
Abstract
Extracorporeal membrane oxygenation (ECMO) is increasingly being used in adults following failure to wean from cardiopulmonary bypass, after cardiac surgery or in cases of severe respiratory failure. Knowledge of the different types of ECMO circuits, expected locations of cannulas and imaging appearance of complications is essential for accurate imaging interpretation and diagnosis. Commonly encountered complications are malposition of cannulas, adjacent or distal haemorrhage, stroke, stasis thrombus in access vessels, and distal emboli. This article will describe the imaging appearance of different ECMO circuits in adults as well as commonly encountered complications. If a CT (computed tomography) angiogram is being performed on these patients to evaluate for pulmonary embolism, the scan may be suboptimal from siphoning off of the contrast by the ECMO. In such cases, an optimal image can be obtained by lowering the flow rate of the ECMO circuit or by disabling the circuit for the duration of image acquisition.
Key Points
• Femoroatrial VV ECMO: femoral vein drainage cannula and right atrial return cannula.
• Femorofemoral VV ECMO: return and drainage cannulas placed in femoral veins.
• Dual-lumen single cannula VV ECMO: via the right IJ/Femoral vein with the tip in the IVC/SVC.
• Peripheral VA ECMO: peripheral venous drainage cannula and peripheral arterial return cannula.
• Central VA ECMO: direct right atrial drainage cannula and aortic return cannula.
Keywords: Extracorporeal membrane oxygenation, Cannulas, Thorax, Radiography, CT, Echocardiography
Introduction
Extracorporeal membrane oxygenation (ECMO) refers to the life support system utilised in pulmonary or cardiopulmonary support for gas exchange []. The goal of the system is to oxygenate the patient’s blood while removing carbon dioxide. Also referred to as extracorporeal life support, ECMO is well established in neonatal respiratory failure. Use in adults has increased and ECMO is commonly used following failure to wean from cardiopulmonary bypass after cardiac surgery or in cases of severe respiratory failure [2].
Initial use of ECMO was largely in neonates given findings of increased survival in severely hypoxic infants but no significant benefit to severely ill adults [, ]. The results of these initial studies were due in large part to the reversibility of severe respiratory illness in neonates and non-reversible ventilator-associated lung injury present in adults []. Today, several inclusionary and exclusionary criteria help guide ECMO application in adults (Table 1).
Table 1
TYPE | Indications | Common complications |
---|---|---|
Central VA | After failure to wean from cardiopulmonary bypass After sternotomy | Mediastinal haemorrhage More invasive May predispose to aortic stasis thrombus |
Peripheral VA | When cardiac and pulmonary support is required | Large bore arterial cannulas may predispose to occlusion Malpositioned cannulas Carotid cannulation may contribute to stroke |
VV ECMO | Direct support of gas exchange-respiratory failure; ARDS | Malpositioned cannulas DVT in the cannulated limb |
Complete discussion of paediatric ECMO is beyond the scope of this article. Neonatal and paediatric ECMO encompasses different cannula sizes, configurations and complications. For example, in neonates arterial cannulation is limited to placement via sternotomy or via cut down technique for carotid cannulation. In such small patients, peripheral cannula placement is exceedingly difficult. Additionally, initiation of VA ECMO usually encompasses carotid or internal jugular vein ligation, a technique that is not well tolerated in adults given the risk of stroke []. Similarly, ECMO in the non-neonatal paediatric population may or may not require carotid or jugular ligation. Imaging considerations in paediatric ECMO have been previously discussed in the literature []. For the purposes of this discussion, only the imaging of adults on ECMO will be addressed.
In 2009, the CESAR study found improved outcomes in adults treated with ECMO versus conventional therapy []. This study randomised patients with severe but reversible respiratory failure to either conventional therapy with intermittent positive pressure ventilation or therapy with ECMO. With the primary end point being severe disability or death at 6 months, the CESAR study demonstrated improved survival and decreased morbidity in patients randomised to ECMO.
There are two types of ECMO: veno-arterial (VA) and veno-venous (VV) []. VA ECMO refers to siphoning of deoxygenated blood from a vein with return of oxygenated blood into an arterial vessel. VV ECMO refers to siphoning of deoxygenated blood from a venous vessel and return of oxygenated blood to a systemic venous vessel or the right atrium. The catheters used in ECMO for blood exchange are referred to as ‘cannulas’, a term used in this context to avoid confusion with other vascular catheters. ECMO cannulas may assume variable configurations on radiographs and CTs. Familiarity with differing positions and access sites is important when interpreting images. Additionally, a multitude of complications may arise intrinsic to either ECMO or cannula placement.
The ECMO circuit
The circuit consists of a pump, blender, oxygenator, control console, heater/cooler and two cannulas. The blender mixes oxygen with CO2. This mixture of gas usually consists of 95 % oxygen and 5 % CO2 and is referred to as the ‘sweep gas’. The pump is usually a centrifugal or roller pump and is used to transport blood throughout the circuit. The heater/cooler aids in temperature regulation of extracorporeal blood. The cannulas provide direct transport of oxygenated and deoxygenated blood to and from the patient.
The initial oxygenators consisted of venous blood pools that were oxygenated with oxygen bubbles. These first clinical attempts at extracorporeal life support began in the 1950s and were largely unsuccessful beyond several hours as direct exposure of blood to oxygen led to life-threatening coagulopathy, haemolysis, and multi-organ failure. In 1959, Burns developed the definitive solution involving a silicone membrane that separated blood from oxygen and allowed for indirect oxygenation of blood []. This was a major step in extracorporeal life support. In the years following (1972–1976) the first published reports of the successful use of extracorporeal life support emerged []. Modern membranes are largely made from polymethyl-pentene, which is lower in resistance and causes less consumption of blood products [].
Indications for initiation of adult ECMO
VA ECMO is used for both cardiac and pulmonary support such as acute cardiac failure or failure to wean from cardiopulmonary bypass after cardiac surgery []. VV ECMO is used for reversible respiratory failure with normal cardiac function. The most common use is in acute respiratory distress syndrome, which may be secondary to severe pneumonia or influenza []. More specifically, eligibility was defined in the CESAR trial by a Murray score >3, a pH of less than 7.20 and having a reversible disease process.
The Murray score consists of four factors including (1) the PaO2/FiO2 ratio, (2) PEEP (positive end expiratory pressure), (3) dynamic lung compliance in ml/cmH20 and (4) number of quadrants infiltrated on radiographs [] (Fig. 1b). Each element is graded on a scale of 0–4. The scores are then averaged and the mean score is used to grade the severity of respiratory disease; 0 implies no significant disease and 4 implies the most severe disease. Normal for the above elements are PaO2/FiO2 ≥ 40 kPa, PEEP ≤ 5, compliance ≥ 80 and 0 quadrants infiltrated on the radiograph [].
Illustration (a) and radiograph (b) of femoroatrial VV ECMO demonstrate a right femoral drainage cannula and right atrial return cannula. Frontal radiographs show the expected locations of the right atrial return cannula (b, arrow) and right femoral drainage cannula (c, arrow). While the Murray score depends on several factors, this patient would receive a 4 for the chest radiograph element as there is four-quadrant infiltration
Contraindications to ECMO
Relative contraindications include high pressure ventilation, high FiO2, limited vascular access and organ dysfunction that would lead to poor quality of life. In patients with pre-existing disease such as metastatic cancer or severe irreversible brain injury, ECMO may be contraindicated. Absolute contraindications include any conditions that preclude anticoagulation [].
Imaging modalities used in evaluation of ECMO
Transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE) play crucial roles during initial cannula placement. A full discussion on the role of echocardiography in ECMO is discussed by Platts et al. in “The Role of Echocardiography in the Management of Patients Supported by Extracorporeal Membrane Oxygenation” [].
Radiographs of the chest and abdomen are useful initial examinations for cannula position. They may reveal malposition or unintended migration. They often offer the first clue towards complications such as haemothorax, pneumothorax or mediastinal fluid collections.
Ultrasound is a portable, focussed and readily available modality that can be used to evaluate for insertion site haematomas, peri-cannula thrombus or deep venous thrombus. Spectral colour Doppler is a powerful tool to evaluate distal limb perfusion as the arterial return cannula can lead to obstruction/occlusion with decreased forward flow in the access artery. It can also be used for evaluating thrombus in the access vein. Interpretation of spectral Doppler waveforms with comparison of flow in the contralateral vessel can help in differentiating slow flow, which may be due to a systemic cause such as poor cardiac output, from slow flow in the access artery from either the access cannula itself or thrombus.
CT is reserved for evaluation of complications that cannot be fully evaluated by radiographs, ultrasound or Doppler imaging. While lacking the functional data of spectral Doppler, CT provides excellent anatomic detail. This may be used in evaluating cannula position or malposition, haematoma formation, haemothorax, stroke or arterial/venous thrombus in large vessels. A full summary of the different imaging modalities and their indications are provided in Table 2.
Table 2
Imaging modality | Uses | Key characteristic |
---|---|---|
Radiograph | Cannula positioning Pneumothorax and haemothorax Evaluation of quadrants for Murray scoring. | Initial study Widely available Comparison to priors for change |
Echocardiography | During initial placement of ECMO cannulas | Evaluate recovery of cardiac function |
Ultrasound ± Doppler | Evaluation of vascular patency Haemothorax Haematoma at cannulation site | Comparison of cannulated and non-cannulated vessels aids in evaluation of abnormal vascular waveforms |
NCCT | Malpositioned cannulas Haematoma Stroke Pulmonary/Abdominal infection Aortic stasis thrombus | Maintain resting respiratory rate to minimise motion artefact |
CECT | Pulmonary stasis thrombus Aortic stasis thrombus | Switch ECMO circuit to minimal flow status or stop the ECMO pump for the duration of the acquisition |
NCCT non-contrast computer-aided tomography, CECT contrast-enhanced computer aided tomography
VV ECMO
VV ECMO is usually placed percutaneously and peripherally. Several configurations are seen. In the ‘femoroatrial’ configuration the drainage cannula is introduced via the femoral vein and advanced to the level of the diaphragm, below the hepatic veins []. The return cannula is introduced via the internal jugular vein and advanced to the level of the superior vena cava (SVC)-atrial junction (Fig. 1). The tip is directed towards the tricuspid valve. This particular configuration is optimal to minimise recirculation [], a phenomenon in which a portion of the returning oxygenated blood is prematurely siphoned back to the ECMO circuit. Recirculation will result in less oxygenated blood returning to the pulmonary circulation and thus the systemic circulation.
A configuration in which the drainage cannula and return cannula are introduced via the femoral veins is termed 'femorofemoral ECMO'. In this configuration the drainage cannula may be placed on one side and the return cannula on the other. Alternatively, both cannulas may be introduced on the same side. In either case the drainage cannula is advanced to the distal inferior vena cava (IVC) and the return cannula is advanced to the right atrium [].
Another configuration of VV ECMO is the single cannula, dual lumen ECMO (Fig. 2a) []. This cannula is usually inserted via the right internal jugular vein with the tip advanced to the IVC. Alternatively this cannula may be inserted via the femoral vein with the tip advanced to the SVC. The atrial return port is identified on radiographs as a lucency in the cannula (Fig. 2b, c). Rarely, this dual lumen cannula may be inserted via the left internal jugular vein if the right is scarred or thrombosed. The distal tip of this cannula drains deoxygenated blood and a separate opening at the level of the right atrium allows for oxygenated blood return.
Illustration (a) and radiograph (b) of dual lumen, single cannula VV ECMO demonstrate the drainage cannula tip in the IVC and a sidehole in the right atrium, which is used for blood return. A magnified view of the right atrium and cannula (c) demonstrates the atrial opening identified by the slight discontinuity of the cannula (arrow)
VA ECMO
Compared with VV ECMO, the VA variation is associated with a higher incidence of bleeding, a greater need for transfusion, an increased need for reoperation and greater resource utilisation []. Despite this, VA ECMO is preferred when cardiac support is required in addition to pulmonary support and also after sternotomy. The two main types are central and peripheral. Central refers to cannula placement in the mediastinum and peripheral VA ECMO refers to placement outside of the mediastinum []. A summary comparison between VA and VV ECMO can be found in Table 3.
Table 3
Va Ecmo Physiology
Optimal cannula tip position | Drainage cannula | Return cannula | Key characteristics |
---|---|---|---|
VA ECMO | |||
Central | Right atrium | Aorta | Direct insertion into the mediastinal vessels |
Peripheral | Distal IVC or SVC, before the cavoatrial junction | Proximal femoral artery, axillary artery, subclavian artery | Insertion of cannulas in peripheral vessels |
VV ECMO | |||
Femorofemoral | Distal IVC, at the level of the diaphragm | Right atrium via the same or opposite iliofemoral vein | Less recirculation and improved flow |
Femoroatrial | Distal IVC, at the level of the diaphragm | Distal SVC/right atrium via the SVC | Optimal to minimise recirculation |
Dual lumen, single cannula | IVC, below the diaphragm | Right atrium via the SVC | For urgent pulmonary support |
IVC inferior vena cava, SVC superior vena cava, DVT deep vein thrombus
Also known as mediastinal VA ECMO, central VA ECMO involves direct placement of cannulas into the aorta (return cannula) and right atrium (drainage cannula) through an open sternum (Fig. 3). This is usually done when there is failure to wean from cardiopulmonary bypass in the operating room as direct access is readily available. If left ventricular (LV) function is poor, a drainage catheter may be placed in the LV (Fig. 4) to aid in decompression []. Left ventricular unloading is indicated when there is an extremely poor ejection fraction with a closed aortic valve []. This predisposes the patient to stasis of blood in the ventricle and subsequently thrombus formation. Patients with aortic regurgitation may also have a LV drain as retrograde flow of blood may compromise ventricular recovery after the initial insult []. Finally, patients with severe pulmonary oedema may also require an LV drain to help decrease the pulmonary afterload []. The drain can be introduced directly into the left ventricle after sternotomy. Alternatively, the LV drain can also be introduced via the femoral artery or right upper pulmonary vein. The LV drain is connected to the ECMO circuit along with the drainage cannula.
Illustration (a) and radiograph (b) of mediastinal (central) VA ECMO. Note the drainage cannula in the right atrium (thick arrow) and the return cannula in the ascending aorta (thin arrow)
Sagittal reformat from a CT demonstrating a left ventricular drain (arrow) terminating in the left ventricle. This drain aids in decompression when left ventricular function is poor
In peripheral VA ECMO, the arterial cannula may be seen projecting over the femoral or axillary arteries. At our institution, carotid artery cannulation is less commonly used in adults. The arterial cannulas are usually advanced to the level of the iliac arteries or abdominal aorta [], and usually multiple surgical clips are used for anchoring that are readily visualised on radiographs. The venous cannulas may be seen on the same side as the arterial cannula but are advanced closer to the right atrium, terminating in the SVC or IVC (Fig. 5). Often a distal perfusion catheter is placed in the access artery to prevent distal perfusion defects and limb ischaemia (Fig. 6).
Illustration (a) and radiograph (b) demonstrating peripheral VA ECMO. The drainage cannula is advanced in the distal IVC to the level of the diaphragm. The arterial return cannula is not advanced very far. Radiograph (b) demonstrating the arterial cannula (black arrow) in the proximal femoral artery with multiple surgical clips used for anchoring. The full extent of the venous drainage cannula (white arrow) can be better assessed with a chest radiograph (not shown)
Illustration (a) and radiograph (b) demonstrating a distal arterial perfusion cannula (thin arrow) in a patient on peripheral VA ECMO. The arterial return cannula (thick arrow) projects over the axillary artery and points proximally as compared to the perfusion cannula which points distally
Complications
Cannula malposition
Initial placement of ECMO cannulas is usually confirmed by echocardiography and the position reaffirmed by radiographs. The cannulas are anchored by sutures to prevent displacement or malposition. Any change in the position of the cannulas on follow-up radiographs should be noted. As noted above, for VV ECMO, venous drainage cannulas should be placed in the proximal SVC or IVC and return cannulas should be placed in the right atrium. Although discerning artery from vein may be difficult on radiographs, any change in cannula positioning with reference to adjacent bony landmarks should prompt the radiologist to enquire about ECMO malfunction. If suspected, CT and ultrasound are better suited to evaluate cannula dislodgement and/or displacement. A superior approach cannula placed too inferiorly may traverse the right atrium into the IVC and obstruct hepatic outflow (Fig. 7a). Alternatively, an inferior approach cannula placed too high may obstruct the SVC (Fig. 7b).
Malpositioned cannulas. Chest radiograph of a 54-year-old male status post MI on peripheral VA ECMO a demonstrates the tip of a superior approach venous ECMO cannula projecting too inferiorly over the IVC (thin arrow). A chest radiograph from a different patient b shows the tip of an inferior approach venous cannula projecting over the lower SVC, much higher than the expected location of the right atrium
Gas emboli
Gaseous microemboli can be introduced into the ECMO circuit during cannula placement, from inadequate priming of the filter system, from the perfusionist accessing the circuit (for blood draws or medication injection), from the pump or from turbulent flow within the tubing [, ]. Smart 2d cutting software crack. Only large air emboli may be seen on conventional imaging and only a few cases exist in the literature. Gaseous microemboli are not well seen on conventional imaging and in fact require specialised ultrasound machines for detection in the circuit [].
Access vessel obstruction/occlusion
Vascular complications are seen in 10–16.9 % of patients on ECMO []. Clinical presentation may include pallor, diminished or absent pulses, compartment syndrome or gangrene. In our experience, spectral Doppler ultrasound is the imaging modality of choice for evaluation of peripheral vessels in this setting.
The large size of arterial cannulas poses a significant risk of occlusion []. Initial examinations with spectral Doppler in the artery distal to the cannula may demonstrate an arterial waveform with a prolonged systolic peak and diminished amplitude (pulsus parvus et tardus) (Fig. 8a). However, it is normal for the peak and mean velocities to decrease by 30–50 % after ECMO placement []. Occlusion may also show lack of flow as well as absent waveforms (Fig. 8b). Given the risk of occlusion, some surgeons may choose to place arterial catheters in the distal extremity to augment downstream limb perfusion. Options include placement of an anterograde catheter in the common femoral artery/axillary artery or placement of a retrograde catheter in the tibial artery [, ]. It is important to note that vascular waveforms also depend on myocardial function, particularly left ventricular function. A poor ejection fraction may also produce dampened waveforms or accentuate partial occlusion by a cannula. This can be assessed by comparing waveforms in the contralateral limb.
Arterial vascular obstruction. Spectral Doppler image of the right axillary artery (a) in a 42-year-old female on VA ECMO demonstrates diminished amplitude and prolonged systolic peaks of the arterial waveforms, classic for pulsus parvus et tardus. In a different patient, B flow ultrasound (b) shows complete occlusion of the radial artery (arrow) just distal to the bifurcation
In venous obstruction, colour Doppler may show a lack of flow or absence of normal phasicity on spectral Doppler imaging. Although much consideration is given to arterial perfusion, venous drainage should be an equally important consideration. If the venous cannula is large enough to obstruct venous return, severe limb oedema and ischaemia result []. As such, one may see an additional catheter in the distal access vein that assists with venous drainage. Comparing flow in the non-cannulated extremity vessel will again help elucidate whether the suspected flow abnormality is due to poor cardiac function or true occlusion.
Venous and arterial thrombus
The incidence of deep venous, pulmonary and arterial thrombosis in patients on ECMO is not well documented in the literature. In fact experience with these entities is restricted mainly to case reports. In cases of acute arterial thrombus in the setting of peripheral VA ECMO, spectral Doppler imaging will show lack of flow, absent waveforms and an abrupt cutoff of distal vessels. In venous occlusion by thrombus, spectral Doppler imaging may show thrombus within or surrounding the cannula. There may also be enlargement of the affected vein and extension of the thrombus distally.
With poor left ventricular function in patients on VA ECMO, inadequate flow may lead to stasis thrombus forming in the left ventricle, left ventricular outflow tract and ascending aorta proximal to the return cannula. Non-contrast CT will demonstrate high-density thrombus in the ascending aorta, proximal to the insertion of the arterial cannula (Fig. 9). Additionally, similar cases of thrombus forming in the aortic root and descending aorta have been described in the literature but are rare []. Common clinical presentations are biventricular failure and failure to wean from ECMO. Not surprisingly, case reports also document significant mortality.
Arterial stasis thrombus. Non-contrast reconstructed CT image in a 67-year-old male on VA ECMO after a failed heart transplant demonstrates intraluminal high attenuation material in the proximal tubular ascending aorta compatible with thrombus
Cerebral ischaemia and stroke
Up to 50 % of patients on ECMO demonstrate severe neurological sequelae []. It is however unclear whether neurological damage is secondary to ECMO itself or secondary to the patients inciting reason for hospitalisation []. Autopsies performed on patients who did not survive ECMO demonstrated evidence of hypoxic-ischaemic injury or injury in a vascular distribution []. The clinical suspicion for stroke may be obscured in ECMO patients given the multitude of other systemic or metabolic derangements usually encountered in ICU patients. For new-onset neurological symptoms, CT may be used to identify cerebral infarcts (Fig. 10).
Stroke in a 48-year-old male on VA ECMO with mental status change. CT of the head demonstrates a large hypodensity in the distribution of the right MCA compatible with subacute infarct
Haemorrhage
Haemorrhage can occur at cannulation insertion sites as a direct result of puncture or distally due to coagulopathy. For central VA ECMO the earliest clue will be new widening of the cardiomediastinal silhouette on the chest radiograph. Haematoma is better appreciated on CT as a high-attenuation collection adjacent to the cannula (Fig. 11). For peripheral VA ECMO, ultrasound may show a heterogeneous collection surrounding an echogenic cannula
Haematoma. Axial CT in a 44-year-old male on mediastinal ECMO for heart failure after myocardial infarction shows mediastinal haematoma (thin arrow) tracking along the arterial cannula (thick arrow) up to the aortic arch
Distal haematoma may be seen as haemothorax or haemorrhagic ascites. It is usually multifactorial. Factors such as anticoagulation, consumption of complement and prothrombotic substances in addition to recent cannulation may cause subacute haemothorax to develop over time. Radiographs may demonstrate new effusion and/or displacement of mediastinal structures. Ultrasound will demonstrate echogenic fluid that can be confirmed by CT as high-attenuation fluid.
Optimizing CT angiogram for pulmonary emboli
Poor right ventricular function will predispose patients to slow flow within the pulmonary arterial system and thrombus formation. Evaluating these patients remains a challenge. As intra-venous contrast is injected, the venous cannula of the ECMO circuit siphons off contrast before it can opacify the pulmonary arteries []. Evaluation of pulmonary embolism using standard protocols may lead to suboptimal opacification of the main pulmonary artery and a non-diagnostic examination (Fig. 12a). If clinical suspicion for pulmonary embolism remains high and initial CT angiography with the ECMO circuit fully operational leads to a non-diagnostic examination, the solution may be obtained in two ways.
Pulmonary embolism. Axial CT image of a 44-year-old male on central VA ECMO a shows insufficient opacification of the pulmonary arteries using the standard protocol secondary to siphoning of IV contrast by ECMO. Diagnositc images are obtained in the same patient b after putting the circuit in a minimal flow state (500 ml/min) for 15 s with reinjection of IV contrast. Thrombus is seen in the main pulmonary artery (b, arrow)
The first is to reduce pump flow to the ECMO circuit typically to 500 cc/min for 15–25 s, inject IV (intravenous) contrast and place the region of interest locator at the main pulmonary artery. Minimal flow will preclude thrombus formation in the cannulas. With the ECMO circuit in low-flow status, IV contrast will opacify the pulmonary arteries []. As cardiac function may be poor in these patients, CT acquisition may be manually triggered when the region of interest demonstrates a peak or a plateau in the opacification of the main pulmonary artery. This plateau will vary according to right ventricle function and may be different for each patient. Triggering the scan will vary accordingly. Scan parameters are as follows: kVp and mAs are as determined from the initial scouts. IV contrast is injected at 4–5 cc/s (contrast volume = 4–5 cc × [delay time of the scanner from initiation of bolus trigger + scan duration]) [].
If pulmonary arterial opacification is suboptimal with the above technique, a repeat scan should be considered with the circuit completely disabled for the duration of the CT acquisition. This will typically be 10–15 s for a 64-slice MDCT. The pulmonary arteries will be opacified in a near anatomic manner depending on cardiac function. Bolus tracking as described above may be used.
Maquet Ecmo Circuit
Conclusion
ECMO cannula configurations are extremely variable. The radiologist should be familiar with the expected locations of cannulas in VV ECMO, peripheral VA ECMO and central VA ECMO. As such, the radiologist should be able to comment on malposition and prospectively search for complications such as vascular obstruction. Additionally, there should be a high index of suspicion for haemorrhage, venous thrombus, stasis thrombus and cerebral ischaemia. Familiarity with the appropriate imaging modalities and common complications will be useful for the interpreting radiologist.
Contributor Information
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