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Centrifugal pumps also are used for ECMO support (Figure 53-5). Advanced technology in this area has resulted in many centers changing from roller-head to centrifugal pumps. Some centrifugal pump heads contain a spinning rotor that is controlled by magnets within and without the cone. Blood enters the cone at the apex and is propelled tangentially to the base of the pump head, where it is expelled. The longer blood sits in the centrifugal head and the faster the head spins, the more hemolysis will be created. It is also postulated that as blood clots develop in the membrane oxygenator over time and increase outflow resistance, this scenario may also increase hemolysis within the rotor head. Hemolysis has limited the use of centrifugal pumps. Newer pump designs have small heads that require little priming volume and technical advancements in generating the “spinning” motion, which may be associated with less hemolysis than in the past. Use of low-resistance hollow fiber oxygenators also may reduce hemolysis and make centrifugal pumps even more efficient.
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. Thus centrifugal pumps can be placed at any level relative to the patient, which makes transporting the patient who is being treated with ECMO easier than with roller-head devices. The active suction effect of centrifugal heads can create levels of negative pressures as high as −200 to −700 within the venous inlet tubing. High amounts of negative pressure generated by centrifugal pumps can result in hemolysis, endothelial damage of cannulated vessels, or cavitation of air. Monitoring of venous pressure with alarm limits to signal when excessive negative pressure is occurring is common in most centers. Monitoring venous return and understanding the function of centrifugal pumps are keys to safe and appropriate use of this form of support. Another advantage of centrifugal pumping devices is that air and debris are trapped within the vortex of the pump head and may be less likely to embolize to the patient than with roller-head circuits. This advantage is countered by some reports that centrifugal pumps generate more microbubbles than do roller-head devices. A recent publication examining causes of hemolysis noted that the direct interaction of blood with air and high negative pressure (such as occurs with cardiotomy suction during bypass surgery) resulted in the greatest amount of hemolysis. Centrifugal circuits contain a flow probe that displays how much blood is being returned to the patient. Servoregulation occurs by the same method as with roller pumps, in that venous and arterial pressure limits and goals are set and the pump (or the specialist) adjusts flow to maintain set goals. Another advantage of centrifugal pumps is that if there is occlusion of the circuit past the centrifugal head, no high back pressure is exerted to result in rupture of the circuit. Obstruction to forward flow, however, will result in increased hemolysis from the blood trapped in the centrifugal head. The components of ECMO circuits used with centrifugal pumps can be completely coated with heparin, which may limit bleeding or the need for exogenous heparin. Although centrifugal pumps are easy to set up and operate, lack of familiarity with them compared with roller-head pumps requires that close attention to patient physiology and pump interaction be maintained. The newer designs of centrifugal systems allow miniaturization such that transport systems can be more compact and much less cumbersome than in years past.
Advances in ACLS will likely be drawn from continuing work with artificial heart or ventricular support devices. One intriguing report from Japan noted successful use of a self-regulating ECMO device that contained two sac-type blood pumps that are placed in parallel and use compressed air to eject blood. This pump is completely self-regulated and provides pulsatile ECMO return. Three neonatal patients with respiratory failure were successfully supported in Japan with use of this device.
Whether by semiocclusive contact between pump roller heads and ECMO tubing or by advancement through a centrifugal head, blood moves past the pumping device to the membrane oxygenator, where gas exchange occurs. Oxygenated blood then flows through a heat exchanger, where it is rewarmed or cooled depending on the clinical situation, and then it returns to the patient.
Centrifugal pumps have almost completely substituted roller pumps for long-term applications. Blood enters the pump by the vortexing action of spinning impeller blades or rotating cones that are coupled magnetically with an electric motor and, when rapidly rotating, generate a pressure differential that causes the blood to move. Blood flow depends not only on the rotational rate of the pump but also on the preload (directly, a decrease in preload corresponds to a decrease in flow and vice versa) and afterload (inversely, a decrease in afterload corresponds to an increase in flow and vice versa).
The potential problems of a centrifugal pump head include stagnation and heating of blood in the pump head, leading to thrombus at low flows if the outlet line is occluded, or cavitation and hemolysis when the inlet line is occluded. New pump head designs have reduced the magnitude of many of these problems.
Conventional centrifugal pumps, such as the Biomedicus (Medtronic, Eden Prairie, ME), the Jostra Rotaflow (Maquet Cardiopulmonary, Rastatt, Germany), and the Levitronix (Levitronix, Waltham, MA), provide excellent short-term support via a CPB circuit extracorporeal membrane oxygenation (ECMO), or as a short-term VAD. They require direct cardiac cannulation and are therefore particularly with an extracorporeal membrane oxygenation circuit suited to support after cardiac surgery. In addition, a percutaneous centrifugal pump (Tandem Heart Percutaneous Transseptal Ventricular Assist (PTVA) System [Cardiac Assist, Pittsburgh, PA]) has been utilized. This uses groin cannulation: left atrial cannulation is made via the inferior vena cava and across the interatrial septum, and return is made via the femoral artery. These centrifugal pumps are prone to causing thrombus formation and have limited durability and significant heat generation by their bearings. This limits their use to around 10 to 14 days, and pump head changes may be required during this time.
A new generation of bearingless centrifugal pumps has been developed and is undergoing phase-two clinical trials. They all have magnetically or hydrodynamically suspended rotors or both. Like the Berlin Heart Incor, they have only one moving part, are frictionless (so parts do not wear out), and cause less heat generation and potentially less thrombus formation. They include the HeartMate III (Thoratec, Pleasanton, CA); the VentrAssist (Ventracor, Queensland, Australia [see Fig. 22-7]); the CorAide (Arrow International, Tucson, AZ); the HeartQuest (World Heart, Oakland, CA); and the Terumo (Terumo, Tokyo, Japan).
Currently utilized centrifugal pumps include the BioMedicus, CentriMag (Levitronix, Zurich, Switzerland), RotaFlow (Jostra, Hirrlingen, Germany), and the Capiox (Terumo, Ann Arbor, MI). The term _centrifugal pump_ is not always synonymous with VAD, because centrifugal pumps may also be used with an oxygenator to construct an ECMO circuit. Without an oxygenator, they may be used for right, left, or biventricular support (except in infants, when size limitations can preclude the presence of two pumps 37) and offer the advantage of excellent ventricular unloading and decreased wall stress, optimizing the chances of myocardial remodeling and recovery. Unloading the left ventricle can also decrease left ventricular cavity size and improve septal configuration, resulting in improved tricuspid valve function and right ventricular inflow. If used as a VAD without an oxygenator and heat exchanger, reduced systemic anticoagulation is required compared with ECMO usage.
Centrifugal pumps offer the advantage of decreased trauma to red blood cells and a less pronounced systemic inflammatory response compared with roller pumps. A centrifugal VAD spins, creating a vortex, with negative pressure at the inlet drawing blood into the cone and positive pressure at the outlet allowing ejection at the base (Fig. 19-2). Cardiac output from a centrifugal pump depends on preload, afterload, and the rotational speed of the pump. Because increases or decreases in preload and afterload can affect pump flow without changes in the rotational speed, a flow probe is necessary. Excessive negative inlet pressures (hypovolemia) must be avoided because air can be entrained into the circuit. The lowest pump speed possible to maintain desired flow is used to maximize pump efficiency and minimize hemolysis. Compared to ECMO, centrifugal pumps are less expensive and have faster setup times, reduced priming volumes, and less anticoagulation requirements. Disadvantages of centrifugal pumps include the potential for thrombus formation in the circuit, nonpulsatile flow, and limited duration of usage (usually <3 weeks). Additionally, children must remain sedated and immobilized while receiving centrifugal pump support.
In an attempt to simplify postoperative management and improve neurologic outcomes, Shen and colleagues have advocated the use of routine VAD support after stage I Norwood palliation without the use of an oxygenator, allowing the aortopulmonary shunt to act as the sole source of pulmonary blood flow and allowing maintenance of lower levels of anticoagulation. Twenty-three children received VAD support for an average of 3 days after initial palliative surgery, with no attempt made to balance systemic and pulmonary circulations. Lactate levels were used to guide VAD flows, with support weaned as lactate levels normalized. Eighty-seven percent of children survived to hospital discharge, and neurodevelopmental testing performed before a bidirectional Glenn procedure was normal in all survivors.40
Excellent outcomes with centrifugal VAD support have been reported in children who required postcardiotomy ventricular MCS of limited duration, such as those with anomalous origin of the left coronary artery from the pulmonary artery, and in those with ventricular failure due to cardiomyopathy. In addition, fewer postsupport neurologic complications were noted in children who received VAD support compared with those who received ECMO for similar indications.
The BioMedicus centrifugal pump (Medtronic BioMedicus, Eden Prairie, MN, USA) can be used for cardiopulmonary bypass, ECMO, or ventricular assistance; therefore this centrifugal pump should not be synonymous with VAD. This nonpulsatile VAD concept has been in existence since the late 1970s but only recently has been used more frequently in children. Although the terminology VAD is used, it also can be used to describe other types of devices that support the ventricle and thereby leads to some confusion. The VAD experience in the pediatric patient population in the United States is limited to the nonpulsatile centrifugal type device, described in the following paragraph, but includes other systems elsewhere in the world, particularly Europe.
This device evolved as the need for a mechanical support device superior to conventional ECMO became necessary, especially for patients with isolated ventricular failure. Common uses for VAD as ventricular support include myocarditis or dilated cardiomyopathy, acute rejection after transplantation, older infants after arterial switch operation, and children after surgery for correction of anomalous coronary artery from the pulmonary artery. Experience with single-ventricle patients is limited but has been positive.
With this VAD, an extracorporeal centrifugal pump with acrylic rotator cones produces a vortex continuous flow (Figure 25-2), which creates a negative pressure that enables blood to move. The pump is magnetically coupled to a driver that controls the RPM. The BP-80 and BP-50 models have volumes of 80 and 50 ml, respectively, with the former capable of maximal flow of 10 L/minute. Heparin-bonded tubing can be used to minimize the need for aggressive anticoagulation. Compared with roller pumps, there is less trauma to red blood cells and less pronounced inflammatory response. In addition, centrifugal pumps are advantageous in the event of air entrainment because the acrylic cones create an air trap (as bubbles accumulate in the middle of the vortex). Since the advent of the centrifugal pump device for pediatric patients, the evolution of the centrifugal pump has led to development of superior rapid deployment type devices. A flow probe is necessary with the centrifugal pump because the pump is afterload dependent, and RPM in and of itself may not correlate accurately with flow. Mean arterial pressure is maintained by varying intravascular volume and RPM on the VAD console. An oxygenator can be placed in cases of coexisting pulmonary failure. Cannulation is performed via the left atrium and aorta (left ventricular assist) or right atrium and pulmonary artery (right ventricular assist). Mechanical support can be performed with a minimally invasive technique that obviates the need for sternotomy or neck cannulation.
Advantages of the VAD include its relative ease of use compared with ECMO, ease of implantation, fast setup time, low priming volume, and low-level anticoagulation because it usually does not have an oxygenator or a heat exchanger. There is purported evidence that myocardial recovery may be superior to ECMO (although this is controversial if the left atrium is decompressed in ECMO support). Disadvantages include its shorter duration of usage, occasional thrombus formation in the circuit, and its nonpulsatile flow nature. In addition, the right ventricle must have adequate function because it supplies preload to the left ventricle supported by the VAD. There is a size limitation if biventricular support is necessary.
Coated circuits, closed systems and centrifugal pumps previously have been demonstrated individually to reduce inflammatory and hemostatic activation, although contradictory results exist. The salutary effect of the minimized circuit probably results from several factors. Reducing priming volume and minimizing contact of blood with polymers and air in a closed system is novel. Retrograde autologous priming is also an important factor in red blood cell conservation and minimization of hemodilution. Combined with the shorter tubing length, there is also a theoretical reduction in clotting factor consumption and complement activation triggering the systemic inflammatory response encountered after exposure to bypass. Centrifugal pumps reduce zones of stagnant blood flow and are believed to be less traumatic to erythrocytes than a roller pump, hence, lower hemolysis is expected. With the aim of reducing the inflammatory cascade, the first designs were of a closed system without a blood–air interface; therefore, they included no pump suction devices. Offpump suction was utilized almost exclusively, combined with cell-saver technology to provide autologous transfusion of blood that would previously have been returned via the bypass circuit. This posed the disadvantage of removing potentially large blood volumes from the combined patient-bypass circulation and leaving the surgeon without a true bloodless field. The development of a ‘semi-closed’ and hybrid or dual systems with integrated arterial filters followed.
In contrast to C-CPB, a venous reservoir is missing in most of these minimized closed circuits. Thus, venous air removal is limited and avoidance of bubble embolism is a major concern. Because of the importance of managing venous air and microemboli in the circuit, newer low-prime systems with active bubble removal capability have been developed and introduced into routine clinical use. These active bubble removal systems include microemboli-sensing systems that remove air from the venous line through the venous bubble trap automatically.
There are numerous sources of microemboli including the venous cannulation site, patient anomalies, administration of medication via the sample port into the venous line, blood sampling, active kinetic drainage using a centrifugal pump to drain the patient, and manipulation of the heart during surgery while on bypass. However, sensitive emboli detection technology did not show any difference in air-handling capacities in conventional or minimized circuits.
We also need to list limitations of mini-CPB. The closed system without reservoir needs special skills from the perfusionists, as the intravascular volume of the patient acts as the reservoir. Repositioning of the operating table is sometimes necessary to correct insufficient flow rates. This may interfere with the progress of the surgical work. Volume regulation can be difficult. A filled heart, together with backflow during suturing of a distal anastomosis, can be troublesome. The blood pressure has to be maintained during retrograde autologous priming and active communication when CPB is initiated is necessary to determine adequacy of perfusion because the circuit has been primed with the patient’s blood. The effect of the minimal circuit volume and retrograde autologous priming may be obviated if too much crystalloid volume infusion is administered before and during the case.
There are specific instances including the administration of cardioplegic solution, discontinuation of vent drainage, and importantly cardiac manipulation, particularly pulling the heart superiorly and to the right for access to the circumflex coronary artery system, that can impede venous drainage and lower perfusion flows. Drainage issues can also occur with vigorous traction on the left atrium during mitral valve surgery.
Setting up a minimized circuit program requires good planning, cross-discipline work and co-operation. Complete training and absolute co-operation between surgeon, anesthesiologist, nurse and perfusionists are warranted. These products are company-dependent. Mini-CPB systems can be used only on the heart–lung machines made by the same company.
In early attempts, routine, simple CABG cases should be chosen. Auxiliary packs may be opened as a safety feature before the operation. Nevertheless, it is difficult to correlate the benefits of a novel device demonstrated in laboratory studies with significant clinical benefits in patients.
In ECLS, roller pumps and centrifugal pumps are used. Both techniques have theoretical advantages in terms of complications and biocompatibility. Roller pumps may induce high arterial pressure levels, thereby leading to disruption of connections, as well as negative venous pressure levels, with the risk of endothelial damage in the cannulated veins. These potential complications emphasize the need for thorough pressure monitoring and regulation. Centrifugal pumps act through a spinning rotor to generate blood flow. This technique avoids high pressures in the case of distal circuit occlusion, but it may induce shear stress and turbulence to blood cells that may lead to hemolysis and thromboembolic complications. In fact, the roller pump was preferred in a 2002 survey among US neonatal ECMO centers; 95% of those surveyed claimed to use those devices. In centers specializing in ECMO treatment in adults, the use of centrifugal pumps seems to be standard practice.
Since the first application of extracorporeal circulation in 1954, efforts have been made to develop efficient oxygenators. Two different basic techniques have been used: bubble oxygenators, operating with direct contact between blood and gas; and membrane oxygenators. The application of bubble oxygenators is limited to a few hours because these devices damage blood cells. Bubble oxygenators were replaced by membrane oxygenators, which were significantly improved in oxygenation performance: up to more than 200 mL oxygen/minute/m 2. Today, heparin-coated silicone membrane oxygenators as well as hollow fiber oxygenators are in use for ECMO in patients with ARDS. Although many oxygenators are available from different manufacturers, the basic principle is similar: blood flows into a manifold region of the oxygenator, from which point it is distributed among the microporous fibers of the device. Oxygen is conducted in the opposite direction through the fibers, and oxygenation and CO 2 removal are provided by diffusion (Fig. 27.1). Advances in oxygenator development will aim to reduce shear stress on blood cells, blood side pressure, and priming volume while maximizing gas transfer. The common problem of plasma leakage depends on the type of the oxygenator and appears to be additionally influenced by the inflammatory status of the patient.
Most commonly used short-term devices are centrifugal pumps providing continuous flow. They include the Bio-Medicus Biopump (Medtronic), CentriMag and PediMag (Thoratec), RotaFlow (Maquet Cardiovascular, Wayne, NJ), and TandemHeart (CardiacAssist, Pittsburgh, PA). The Impella (Abiomed, Danvers, MA) is a continuous-flow axial device. The term _centrifugal pump_ is not always synonymous with VAD because a centrifugal pump may also be used with an oxygenator to construct an ECMO circuit. Centrifugal pumps offer the advantage of excellent ventricular unloading and decreased wall stress, optimizing the chances of myocardial remodeling and recovery. Unloading the left ventricle can also decrease LV size and improve septal configuration, resulting in improved tricuspid valve function and RV inflow. Decreased trauma to red blood cells and a less pronounced systemic inflammatory response are also observed compared with roller pumps. A centrifugal pump spins, creating a vortex, with negative pressure at the inlet drawing blood into the cone and positive pressure at the outlet allowing nonpulsatile ejection at the base. Cardiac output from a centrifugal pump depends on preload, afterload, and the rotational speed of the pump. A flow probe is necessary because increases or decreases in preload and afterload can affect pump flow without changes in rotational speed. Excessive negative inlet pressures (hypovolemia) must be avoided because air can be entrained into the circuit. The main limitation of centrifugal pumps is the inability to provide long-term support related to issues with thrombosis, bleeding, and infection
The RotaFlow pump (see Fig. 21.2A) is an extracorporeal, centrifugal, continuous-flow device that has a rotating mechanism levitated in three magnetic fields with one point bearing, allowing laminar flow and reducing mechanical friction, heat production, and clotting potential compared with the Bio-Medicus pump. It can be used in patients of all sizes, irrespective of body surface area (BSA), and can flow up to 10 L/minute. It has a small priming volume (32 mL), surface area, and passage time, minimizing hemodilution and blood trauma, and can be used along with a membrane oxygenator as an ECMO circuit. It is approved by the U.S. Food and Drug Administration (FDA) for up to 6 hours of use. However, one report described 2 months of support using the RotaFlow in an infant with a dilated cardiomyopathy.
The PediMag (see Fig. 21.2B) is the pediatric version of the CentriMag. It is an extracorporeal, centrifugal, continuous-flow device for children who weigh less than 20 kg. The device has a bearingless, magnetically levitated technology with no points of contact. It has a priming volume of only 14 mL and can provide up to 1.5 L/minute of flow. The PediMag is approved by FDA for up to 6 hours of support and is also commonly used as part of an ECMO circuit.
The TandemHeart is an extracorporeal, centrifugal, continuous-flow device with a priming volume of 10 mL and is capable of flows up to 5 L/minute, with a hydrodynamic fluid bearing supporting the spinning rotor (see Fig. 21.2C). Although size requirements (>40 kg) preclude its use in most children, it is advantageous because it can be placed percutaneously through the femoral vessels in either the OR or the cardiac catheterization laboratory, with a transseptal extended-flow cannula allowing entry from the femoral vein into the left atrium. The arterial cannula can be placed directly into the femoral artery in larger patients, and in patients who weigh less than 80 kg, a vascular graft to the femoral artery may be cannulated to avoid lower extremity vascular compromise. It is FDA approved for up to 6 hours of support.
The Impella is a microaxial continuous-flow device contained in a single-pigtail catheter with three pump sizes: 2.5 L/minute (Impella 2.5 via 12F), 3.3 L/minute (Impella CP via 14F), and 5 L/minute (Impella 5.0 via 21F), respectively (Fig. 21.3). The smaller pump is designed for use in adults requiring partial LV support during high-risk cardiac catheterizations and ablation procedures. It can also provide full LV support in pediatric patients. The Impella is inserted retrograde through a femoral artery; with the device inlet zone resting in the LV cavity where blood is collected and propelled into the aorta, the deployment is performed under direct vision by fluoroscopy and transesophageal echocardiography (TEE). It has been placed into the ascending aorta in smaller patients via a sternotomy. Pediatric experience is limited to small case series, and the smallest patient reported to be supported was 10 years old, weighing 21 kg with a BSA of 0.93 m 2.
The two pumps used most commonly for CPB are roller pumps and centrifugal pumps. Roller pumps have the advantages of simplicity, low cost, ease and reliability of flow calculation, and the ability to pump against increased resistance without reducing flow. Disadvantages include the need to assess occlusiveness, spallation or fragmentation of the inner tubing surface (potentially producing particulate arterial emboli), potential for pumping large volumes of air, and ability to create large positive and negative pressures. Compared with roller pumps, centrifugal pumps offer the advantages of less air pumping potential, less ability to create large positive and negative pressures, less blood trauma, and virtually no spallation. Disadvantages of centrifugal pumps include a greater cost, the lack of occlusiveness (creating the possibility of accidental patient exsanguination), and afterload-dependent flow that requires constant flow measurement. In the setting of short-term CPB for cardiac surgery, it remains uncertain whether the selection of a roller pump over a centrifugal pump, or of any specific centrifugal pump over another, has clinical importance. Pulsatile perfusion may prove to be beneficial in the future, but further outcome data and technical improvements are needed.
The TandemHeart has four components: a transseptal cannula, a centrifugal pump, a femoral arterial cannula, and a control console. A 21 Fr cannula is inserted into the right femoral vein, advanced to the right atrium, and finally into the left atrium via a transseptal puncture (Fig. 46.6A). The fenestrated cannula aspirates blood from the left atrium via a large end hole and 14 smaller side holes (Fig. 46.6B). Blood flows to a 15 to 19 Fr arterial perfusion cannula inserted into the common femoral artery. The flow of blood is propelled by an extracorporeal centrifugal pump containing a spinning impeller. The pump has both a motor chamber and a blood chamber that are separated by a polymer membrane. An electromagnetic motor rotates the impeller between 3000 and 7500 RPM. The size of the arterial cannula determines the maximum flow rate. The 15 Fr arterial cannula can support flow rate up to 3.5 L/min, whereas the 19 Fr arterial cannula can achieve flow up to 5 L/min. Heparinized saline flows continuously into the lower chamber of the pump, providing lubrication, cooling, and preventing thrombus formation. An external controller controls the pump and contains a 60-minute backup battery in case of power failure. An FDA-approved oxygenator can be added to the circuit to provide oxygenation in addition to circulatory support.
The hemodynamic effects of the TandemHeart are superior to the IABP (see Table 46.2). Similar to the Impella device and unlike the IABP, the TandemHeart does not require a trigger or timing based on the cardiac cycle. As the TandemHeart device works in parallel with the LV, any intrinsic CO from the LV is additive to the support of the device. By virtue of unloading the LV, the TandemHeart results in increased CO, increased mean atrial pressure, decreased PCWP, and decreased central venous pressure. Both the LV and RV have decreased filling pressures, resulting in reduced ventricular workload and oxygen demand, and an increase in cardiac power index. However, due to an increase in afterload and a decrease in preload, ventricular contraction may decrease. As a result, the LV often provides only a minimal contribution to CO, resulting in relatively nonpulsatile arterial pressure tracing.
The amount of cardiac support provided by TandemHeart can be increased or decreased by changing the RPM on the centrifugal pump. Although only FDA approved for 6 hours, in practice, devices are often used for a week or more. Weaning is facilitated by monitoring hemodynamic stability while slowly reducing the RPM on the centrifugal pump. If hemodynamic stability is confirmed, the pump can be turned off and removed.
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