Cannulation for ECLS
see also Extracorporeal Life Support
What are the preferred cannulation sites?
The main limitation on flow through the extracorporeal life support (ECLS) circuit is the blood drainage via the venous cannula. Therefore, cannulation sites should be chosen in order to accommodate the largest diameter venous cannula possible. Ultrasound allows assessment of vein size when considering whether a desired cannula may safely be placed.
Single site dual lumen cannulasare used in infants, children, and adults. This approach incorporates both drainage and reinfusion lumens into one unit inserted through the right internal jugular vein (RIJV).
Perhaps the greatest change in ECLS technology is the introduction of bicaval dual lumen cannulas which are now the most common mode of ECLS. The Avalon® cannula is wire reinforced and intended for the tip to sit in the inferior vena cava (IVC) with drainage holes in the superior and inferior vena cava and a reperfusion side hole in the right atrium. It is termed a bicaval cannula due to the presence of drainage holes in both cavae. A recent review by Zamora et al demonstrated an increase in utilization of single dual lumen cannulation compared to multiple site cannulation - especially in smaller and younger patients. Overall, survival was comparable at 64.4% versus 68.6% for single dual lumen and multisite cannulation, respectively . The single unit dual lumen catheters have demonstrated improved weight-adjusted flow performance when compared to traditional two site cannulation. However, in newborns and young children many centers have found that the Avalon cannula has been associated with multiple complications during cannulation including vascular or atrial perforation . As a result, we now perform all percutaneous cannulation procedures with fluoroscopic guidance, preferably in the operating room. In addition, since the reinfusion port is only a short distance from the tip, there is only a small segment of the cannula which is in the IVC. There have been a number of cases in young children where the tip of the cannula migrated into the hepatic veins, the right atrium or the right ventricle with potential loss of flow or perforation. As a result, some centers do not use the Avalon cannula in patients less than two years of age.
Prior to the advent of the Avalon cannula, multisite VV ECLS was preferred in adults and older children. The jugulo-femoral VV or the femoro-jugular VV approach were used such that either the femoral or jugular cannula served for drainage or reinfusion of blood . However, studies have determined that draining blood from the IVC via the femoral vein cannula and reinfusing into the right atrium via the internal jugular vein cannula is the most efficient in terms of reduced recirculation and enhanced gas exchange .
A less commonly used option is the femoro-femoral VV approach where a short cannula is placed via the femoral vein into the distal IVC for blood drainage and a long cannula is placed into the right atrium via the contralateral femoral vein for reinfusion. This configuration is more complex and is not commonly used .
This mode of support is used when cardiac support is required or in neonates and others where small vein size or other factors limit the placement of an adequate VV cannula. VA bypass removes blood from the systemic venous circulation and returns the blood to the systemic arterial circulation.
Femoral artery cannulation also potentially results in differential perfusion of the upper and lower body (i.e. North-South syndrome). With a femoral artery reinfusion configuration, the highly oxygenated blood delivered into the femoral artery cannula by the circuit perfuses the lower portion of the body while the relatively hypoxic blood pumped from the native heart perfuses the upper body, including the coronary and cerebral vessels. Placement of an additional RIJV cannula may allow delivery of oxygenated blood to the upper body. In this case, blood is drained from the contralateral femoral vein, and returned via the femoral artery for blood pressure support and the RIJV for gas exchange. Known as veno-arteriovenous (V-AV) ECLS, reinfusion can be adjusted to provide more flow via the arterial system for cardiovascular support or jugular vein for better gas exchange .
Stove pipe end-to-side graft axillary artery access has been used in adults and avoids issues with the North-South syndrome, but bleeding and hyperperfusion resulting in swelling and compartment syndrome of the ipsilateral upper extremity may result.
An alternative to peripheral vessel access for ECLS is transthoracic central cannulation. This technique allows for the placement of larger cannulae directly into the right atrium and aorta. This can be used for patients unable to come off cardiopulmonary bypass and cases where the peripheral vessels are of inadequate size. Some studies have suggested that central cannulation may be of benefit in patients with septic shock who require increased flow and hemodynamic support. Survival rates of 73% can be achieved without severe disability on long term follow up . Central cannulation is typically performed by pediatric cardiac surgeons.
With venoarterial ECLS, the left atrium and/or ventricle may become over distended due to the inability to eject against an increased afterload, resulting in pulmonary edema. In this instance, it is necessary to decompress the left atrium/ventricle either with balloon atrial septostomy or cannulation of the left ventricular apex.
Surgical Decision Making
How is the proper cannula size determined?
The success of ECLS is greatly dependent on adequate blood flow through the extracorporeal circuit. The cannula provides the interface between the circuit and the patient . The optimal cannula will maximize flow (80 to 120 mL/kg/min) and minimize damage to blood. Flow through the cannula depends on viscosity and the radius and length of the cannula, as well as the pressure drop across the cannula. The Poiseuille equation states the following:
In this equation, viscosity is the thickness or internal friction generated by a fluid, i.e., blood in the case of ECLS. Thus, all things being kept equal, the radius to the fourth power as well as the length of the cannula are the determining factors for blood flow.
The Reynolds number is a quantitative assessment of whether flow will be laminar or turbulent. Laminar flow occurs when a fluid flows in parallel. At high fluid velocities, some of the flow becomes turbulent which is characterized by rotational and chaotic flow. Cannulae have complex shapes: changes in diameter, step offs to connection tubing and drainage through side holes can all induce turbulent flow. There is increased internal friction to turbulent when compared to laminar flow which affects the effective flow in a cannula.
Montoya et al were the first to estimate the pressure-flow relationship for a cannula and to describe the M number where a higher M number has lower flow for a given pressure drop across the cannula . Sinard et al derived the M numbers for extracorporeal arterial and venous cannulae experimentally. In addition to the radius, length, and flow characteristics, the shape of the cannula is an important determinant of the M number. There are single or double lumen cannulae and nonwire or wire reinforced cannulae. Cannulae vary by brand, length, and the location of the drainage and reinfusion portals: addition of side holes will decrease the M number . Thus, each cannula has a distinct M number. Kinks in the cannula will increase the M number and, therefore, decrease flow. Importantly, wire reinforcement prevents kinking . The M numbers for a variety of individual cannulas, as well as the nomogram which allows one to identify expected flow for a variety of pressure drop and M number scenarios, is demonstrated.
It is important to use a venous cannula with the largest diameter and the shortest length possible. If intravascular volume and cannula positioning are adequate, the limiting factor in determining maximum ECLS flow is cannula resistance. Cannula size is reported according to its outer diameter. Identically sized cannulas may vary in their inner diameter because of differences in wall thickness. Venous cannulas typically have both end and side holes to optimize drainage and reduce the M number. Wire wound cannulas, such as the BioMedicus® (Medtronic, Minneapolis MN), are resistant to kinking . The typical pressure drop across the venous cannula is 100 cm H2O which relates to the siphon pressure generated by the height (approximately 100 cm) of the bed and patient above the bladder. If a bladder is not used, the pressure drop is related to that generated by the centrifugal pump which is typically maintained less than -100 mm Hg (-136 cm H2O).
Dual lumen veno-venous cannulas
Veno-venous (VV) ECLS is used in patients with adequate cardiac function. Deoxygenated blood is drained from one lumen of the venovenous cannula and is reinfused into the other lumen after passing through the circuit. Recirculation, where a portion of the oxygenated blood from the reinfusion cannula is immediately returned into the drainage cannula, is a concern during VV ECLS. The smallest double lumen cannula is 12 Fr. The most commonly used cannula of this type and size is made by OriGen®. The cannula works well, but it can bend and kink which affects flow. New, wire reinforced cannulas were developed by the same company to combat this problem, but currently the smallest is 13 Fr.
Arterial cannulas tend to have a smaller diameter due to smaller vessel size. For example, in full term neonates the desired arterial cannula size (10 Fr) is usually smaller than the venous cannula size (12 to 14 Fr).This translates into a higher pressure drop across the cannula . A high reinfusion cannula M number, however, does not limit circuit flow when compared to that of a drainage cannula because desired flows generally can be achieved via the pump, albeit at higher circuit pressures. Interestingly, with VA ECLS the pressure drop across the arterial cannula and the patient’s mean blood pressure are major determinants of circuit pressure. Arterial cannulas generally have only a single end hole. The cannula must be thick enough to resist kinking while remaining thin-walled to offer the least resistance possible .
What are the methods of vascular access?
The common methods of vascular access include open, percutaneous and semi-percutaneous approaches . With the open method, the vessels are dissected and proximal and distal control obtained. The distal vessels are ligated and the cannulae are inserted into the proximal vessels and secured. A Seldinger approach is used for the percutaneous method. With this technique the vessels are more likely to remain patent . Finally, the semi-percutaneous method provides cutdown visualization of the vessels combined with percutaneous access.
A PTFE graft can be used in a "stovepipe" end-to-side anastomosis to arterial vessels, with cannulation of the end of the graft, which allows continued arterial blood flow. This prevents the distal ischemia that can be seen with direct arterial cannulation.
Neuromuscular blockade is particularly important during preoperative preparation to prevent an air embolus during introduction of the cannulas. Sedation should also be administered if tolerated by the blood pressure. Additionally, heparin sodium (100 units/kg) is drawn up for subsequent administration. If there is a concern for anticoagulation, such as a low grade intraventricular hemorrhage, then a half dose may be planned. Local anesthesia (1% lidocaine or equivalent) is made available for incisional infiltration.
The patient is positioned with the head turned to the left side. A rolled towel or gel roll is placed under the shoulders and a radiograph plate is placed under the patient. The chest, neck and right side of the face are aseptically prepared and draped .
While more expeditious, the percutaneous or semi-percutaneous technique does carry a higher risk of complications, including serious events such as caval and/or atrial perforation. As a result, routine use of fluoroscopy and/or echocardiographic guidance, preferably in the operating room, is strongly recommended when percutaneous or semi-percutaneous approaches are used .
Steps of the Procedure
How is an open neck cannulation performed?
1% lidocaine is infiltrated in the area of the planned incision. A transverse cervical incision approximately one to two cm in length is made one finger’s breadth above the clavicle and over the lower aspect of the right sternocleidomastoid muscle. The platysma muscle and subcutaneous tissues are divided with electrocautery: cautery should be used generously during the procedure in order to avoid subsequent cannula site bleeding. The sternocleidomastoid muscle is exposed. A hemostat is used to bluntly divide the fibers of the sternocleidomastoid muscle (SCM) preferably in the area between the sternal and clavicular heads of the muscle. The internal jugular vein is observed in the base of the incision. Dissection and cautery are used to open the SCM along the fibers in a superior and inferior direction as the jugular vein is exposed. The omohyoid muscle may be seen superiorly and may be divided without consequence. Self retaining retractors can be used for retraction; two self-retaining retractors may be placed 90 degrees to each other so that the field is broadly exposed.
Dissection of the right internal jugular vein (RIJV)is typically performed first, taking care to avoid vessel spasm which can make introduction of a large venous cannula difficult, if not impossible. The investing tissue over the RIJV is divided with cautery in a superior and inferior direction. There is often a thyroid branch on the medial aspect of the RIJV which may be ligated to allow subsequent full dissection of the carotid artery. 2-0 silk sutures are carefully placed proximally and distally around the RIJV with care to avoid "sawing" the vein as the sutures are brought around. Dissection is performed delicately with the utmost care to minimize touching or irritating the vein. For example, the ends of the sutures are left lying free: hemostats are not placed on the sutures so that tension on the vein is avoided. The goal is to maximize the size of the vein following a gentle dissection.
The common carotid artery (CA) lies medial and mostly posterior to the RIJV when the head is turned to the left. The CA has no branches and can be dissected without concern for branch vessel injury or spasm. 2-0 silk ligatures are placed proximally and distally around the carotid artery.
Once vessel dissection is completed, heparin (100 units/kg) is administered intravenously and three minutes allowed for circulation. This is expected to achieve an activated clotting time (ACT) greater than 200 seconds although the ACT is not routinely monitored at this point in the procedure.
Prior to making an incision in the vessels, the appropriate cannulas are choosen. In neonatal venoarterial (VA) bypass the typical size of the arterial cannula is 10 French and venous cannula is 12 to 14 French. While the heparin is circulating, the arterial cannula is marked at a point such that the tip will lie at the ostium of the brachiocephalic artery. The venous cannula is similarly marked at a point where the tip will lie in the middle of the right atrium. Therefore, in a full term newborn a 2-0 silk suture is placed on the arterial cannula 2.5 cm from the tip and on the venous cannula 6 cm from the tip. An obturator is placed into the venous cannula and a tubing clamp occludes the circuit end of the arterial cannula.
The distal (cranial) RIJV is ligated and the proximal (caudad) vein is occluded by gently pulling up on the ends of the proximal suture. Specifically, no clamp is placed on the proximal vein. The assistant controls the distal suture which retracts the vein cranially and the proximal suture which gently occludes the lumen of the vein. The operator performs a venotomy close to the distal, ligating suture and the opening and proximal vein are gently dilated with the hemostat. The venous cannula is then introduced with the bevel down, occasionally twisting the cannula if side holes catch on the edge of the vein. A thin coat of water soluble lubricant may be applied to the surface of the venous cannula to aid with insertion. No undue force is used to prevent tearing the vein. Once the cannula is passed to the level of the marking suture, the cannula is secured using two circumferential 2-0 silk ligatures tied around the vein and cannula and over a small piece of Silastic® vessel loop. The vessel loop allows for the sutures to be sharply divided during decannulation without injury to the vessel. One end of one of the circumferential sutures is secondarily tied to one end of the marking suture on the cannula. The venous cannula obturator is removed and the operator’s thumb placed over the end of the cannula to control back bleeding. If the central venous pressure is reasonable, the cannula will easily fill to a level within the cannula when it is held above the patient. If it does not, one may gently compress the liver through the drapes which will usually enhance cannula filling. The cannula is allowed to back bleed and occluded with a tubing clamp. The venous and arterial connector tubing is obtained from the ECLS specialist. The venous connector is often marked with blue tape and has a uer lock side adapter which can be used to remove air bubbles from the venous aspect of the circuit. The arterial connector tubing is marked with red tape and has no Luer lock side adapter; because of the risk of air embolus, access is not allowed to the arterial side of the circuit in proximity to the patient. The venous connector tubing is carefully attached to the end of the cannula with the assistant firmly holding the tubing clamp on the cannula to prevent decannulation should sudden movements occur. The connector is filled with heparinized saline 1 unit/mL and debubbled with care to avoid introduction of air. Some of the heparinized saline is allowed to drain into the patient through the cannula in order to prevent cannula thrombosis in the interval before initiation of ECLS. A tubing clamp is placed on the circuit end of the connector tubing and the tubing clamp in turn attached to the drapes with a non-penetrating towel clamp, taking any tension off of the venous cannula at the cannulation site.
In a similar fashion the artery is ligated distally (cranially). Proximal control is obtained with an angled ductus clamp. A transverse incision is made distally toward the ligating suture and a hemostat used to gently dilate the proximal artery. 6-0 polypropylene sutures are placed full thickness on the proximal edge of the artery at approximately the 10 and 2 o’clock positions to prevent subintimal dissection and to hold the artery open during cannula insertion. The arterial cannula is inserted to the marking suture and secured in a similar fashion to the RIJV cannula. The operator places the thumb over the end of the cannula and the tubing clamp is released as the operator controls back bleeding. Once the cannula is mostly filled, the clamp is replaced. The arterial connector tubing is attached to the cannula and debubbled with heparinized saline. The clamp is then placed on the circuit end of the arterial connector tubing.
The ECLS specialist and surgeon then discuss which end, arterial or venous, is going to be connected first. The operator then will contaminate one hand as that portion of the circuit is grasped and connected to the respective connector tubing. The assistant debubbles the ends of the tubing with heparinized saline while they are being connected. Once both cannulas are connected, a time out is performed to ensure that the arterial cannula is attached to the arterial circulation, the venous cannula is attached to the venous side of the circuit and that there are no air bubbles in the tubing or cannula prior to going on bypass. ECLS is then initiated and the surgeon ensures that blood is infusing into the arterial cannula. If the patient’s status allows flow is slowly increased to allow slow mixing of circuit prime and patient blood. Blood pressure is monitored closely and hypotension managed with volume.
The wound is irrigated and closed with a continuous or interrupted monofilament suture. The cannulas are sutured individually to the skin on the neck and posterior to the ear with several 2-0 silk sutures. We have found that placing Duoderm® under the cannula, especially by the ear, prevents skin breakdown. Vigorous attention to pressure points is important to prevent decubitus formation. Special attention should be paid to affixing the cannulas securely to the bed in order to take tension off of the cannulation site.
How is a bicaval dual lumen cannula placed?
It is recommended that the bicaval dual lumen cannula only be placed under fluoroscopic guidance, preferably in the operating room, because of the risk of complications such as caval and/or atrial/ventricular perforation.
Ultrasound is used to access the RIJV with a 21 gauge needle followed by introduction of a 0.015" wire. A wire changer then allows the wire to be upsized to a 0.035" Amplatz stiff wire. An incision is made at the puncture site appropriate for the size of the cannula. The wire is advanced under fluoroscopic guidance into the inferior vena cava (IVC). A Kumpe catheter, which has an angled tip, may be of value in directing the wire into the IVC. Heparin 100 units/kg is administered with a half dose given if there is concern for bleeding. The dilators followed by the Avalon Elite®bicaval dual lumen catheter is advanced into the IVC below the hepatic veins under fluoroscopic guidance. Avalon cannula video link The reinfusion port is placed as low as possible in the right atrium in order to maximally engage the distal cannula within the IVC while still directing the reinfused blood toward the tricuspid valve. ECLS is initiated and echocardiography is used to document that the reinfusion is indeed directed toward the tricuspid valve. The cannula is anchored at the neck and posterior to the ear with a series of 2-0 silk sutures. It is critical that the reinfusion port is oriented in an anterior direction.
Routine echocardiographic surveillance during and after placement is strongly recommended to rule out malposition and/or hemopericardium with Avalon Elite®bicaval dual lumen catheter placement in children .
How is a non-bicaval dual lumen cannula placed by either an open or percutaneous approach?
For open, single cannula venovenous (VV) ECLS, the catheter is placed through the venotomy via cutdown and advanced to the mid right atrium as described for the venous cannula placement. While securing the catheter, it is crucial to maintain the reinfusion (red) port anteriorly to minimize the recirculation of reinfused oxygenated blood. Percutaneous placement is similar to that of the bicaval dual lumen cannula, but without requirement for advancement of the wire or cannula into the IVC. It is recommended that percutaneous placement be performed under fluoroscopic guidance, preferably in the operating room.
How is the semi-percutaneous technique for venovenous ECLS performed?
A transverse cervical incision is made 1.5 to 2 cm in length and 1 to 2 cm above the right clavicle between the heads of the SCM. The platysma muscle is divided and the anterior surface of the internal jugular vein is exposed with minimal dissection through the SCM. The cannula skin exit position is selected so that the cannula will lie behind the right ear when the head is turned to the midline. The needle is placed through the skin superior to the incision and into the internal jugular vein. A 0.035-inch diameter guidewire is advanced through the needle and the needle withdrawn. The skin exit site is then enlarged to the diameter of the cannula with a scalpel. A Teflon guiding obturator is placed over the guidewire into the vessel and right atrium. Heparin is administered and three minutes allowed for circulation. The cannula is advanced over the Teflon obturator into the vein under direct vision to confirm entrance into the vein. To minimize recirculation, the reinfusion (red) port of the cannula must be directed anteriorly so that reinfused blood flows toward the tricuspid valve. A series of skin sutures affix the catheter to the skin and the incision is closed with monofilament suture.
The semi-percutaneous technique does carry a higher risk of complications. As a result, routine use of fluoroscopy and/or echocardiographic guidance is strongly recommended .
How is a reperfusion cannula placed?
In older children and adults the femoral artery may be used for cannulation instead of the common carotid artery. With this approach ipsilateral limb ischemia may occur in as many as 52% of patients . In most patients, no specific criteria exist to predict those at risk for ischemia. Early consideration of a reperfusion cannula may decrease neurovascular complications. In children, the superficial femoral artery may be used for a reperfusion cannula which is then connected to the arterial side of the circuit . Alternatively, a posterior tibial artery reperfusion cannula may be placed .
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