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Ann Thorac Surg 1996;61:448-451
© 1996 The Society of Thoracic Surgeons
McGowan Center, Artificial Heart and Lung Program, Department of Surgery, Pittsburgh, Pennsylvania, and Nimbus, Inc, Rancho Cordova, California
Abstract
Background. We are developing a miniaturized centrifugal blood pump for use as a temporary cardiac assist device in neonatal and pediatric sized patients. This pump has a very low priming volume of 13 mL. A small motor stator has also been designed, which resulted in a device that can be placed very close to the patient, thereby minimizing overall circuit volume.
Methods. Testing to date has included in vitro hemodynamic performance, in vitro hemolysis generation, and in vivo evaluation in 5 lambs weighing 5.5 to 21 kg. Two lambs underwent peripheral cannulation from external jugular vein to carotid artery, whereas 3 others were cannulated from left atrium to carotid artery.
Results. In vitro data demonstrated pump capacity spanning 0.3 to 3.0 L/min and very low hemolysis generation at these conditions. In vivo, the pump functioned satisfactorily for periods up to 148 hours, and the bypass appeared to be well tolerated by the animals. Plasma free hemoglobin levels remained less than 25 mg/dL during all animal experiments. All devices were thrombus-free at explantation.
Conclusions. We conclude that this device has merit as an alternative to current oversized systems used for neonatal and pediatric cardiac assistance. In addition, a chronic neonatal lamb model in which to evaluate pediatric circulatory assist devices has been developed successfully.
Use of a mechanical circulatory support system in neonates and small children is often associated with operations performed to correct congenital heart defects. In the United States, approximately 2,500 neonates annually with congenital heart defects have highly complex malformations requiring high-risk operations [1]. In approximately 2% to 2.5% of these, postcardiotomy mechanical circulatory support is warranted [2]. A common opinion regarding devices currently approved for this class of patients is that they are disproportionately large and, in general, lack the technical innovation associated with mechanical circulatory support systems used in adult care [1–3]. The specific aims of our program are directed at applying new and innovative blood pump technology to create a clinical product that meets an existing, important need in pediatric cardiac care.
Material and Methods
System Hardware
The basis for this new technology is Nimbus' experience with miniature centrifugal blood pumps, and breakthroughs the company recently has made in applying these to create a safe, effective, and low-cost clinical product for adult cardiopulmonary bypass. The pediatric pump concept is based on using a disposable, miniature extracorporeal blood pump connected into the circulation via inflow and outflow cannulas. This pump mates to a relatively small power module placed in close proximity to the patient. A remote console powers and controls the blood pump.
CENTRIFUGAL BLOOD PUMP.
Figure 1is a photograph of an actual pump head. This unit is a one-time-use pump, which mates to a power module as shown in Figure 2
. With the pump placed in the module as shown, the motor's rotor, which is incorporated into the pump bearing housing, is centered within the bore of the motor coil set. Use of a smaller motor stator allows the pump system to be placed immediately adjacent to a patient, thereby further reducing the priming volume for cardiac assistance. This rotor, a two-pole permanent magnet, is fixed to the pump drive shaft, which in turn is mounted to an impeller comprising three open, swept-back blades. Two ball bearings support the drive shaft loads radially and axially. A lip-type seal ensures that blood products do not enter the bearing housing.
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The range of flow specified for the pump implies that it should be suitable for patients ranging in size from a few kilograms to approximately 15 kg.
BLOOD PUMP CONTROLLER.
The controller is the source of the three-phase excitation current delivered to the pump motor, with speed being set manually on the front panel. A digital display on the panel indicates motor speed and the average level of electrical current being drawn by the motor. The controller contains a built-in battery capable of powering the system for approximately 60 minutes when fully charged.
The motor is a three-phase, two-pole configuration operated basically as an alternating current synchronous machine. Thus motor speed, which is determined by the frequency of the three-phase current applied to the coils, is unaffected by torque delivered to the pump drive shaft. Motor commutation is achieved using back-electromotive-force sensing. As seen in Figure 2
, the motor coil set is contained in a finned housing to enhance heat removal. The housing is formed from standard extruded aluminum tubing.
In Vitro Testing
HYDRODYNAMIC PERFORMANCE.
Flow characteristics of the pediatric pump were mapped using a recirculating loop flowing fresh bovine blood. For a specified motor (pump) speed, loop flow resistance was set with a clamp, and delivered pump flow and corresponding pressure differential were measured for various clamp settings, up to full shut-off. This process was repeated over a range of pump speeds.
HEMOLYSIS GENERATION.
Hemolysis potential of the pediatric pump was investigated using fresh bovine blood that was heparinized, filtered, and adjusted to a hematocrit of 35%. Each hemolysis run was 1 hour in duration. A test involved operating a blood pump at a specified flow rate and after-load pressure in a loop. Periodic samples were collected and analyzed for plasma free hemoglobin level using a cyanmethemoglobin reagent. Results were determined as the average rate of liberation of hemoglobin in grams per day (eg, the index of hemolysis).
In Vivo Testing
ANIMAL MODEL.
We use sheep as the species in which to test pediatric blood pumps. The use of sheep also permits us to compare this pump with other devices evaluated in the same species. All animals used in these experiments received humane care in compliance with the ``Guide for the Care and Use of Laboratory Animals'' published by the National Institutes of Health (NIH publication 85-23, revised 1985).
Surgical implantation has been straightforward. Lambs are anesthetized with an ultrashort thiobarbiturate (Brevital Na; Eli Lilly Co, 10 mg/kg), intubated (5F to 6F diameter cuffed tube), and maintained in a surgical plane of anesthesia with isoflurane (0.5% to 2.0%) in oxygen and air (1:1), delivered under positive-pressure ventilation (25 breaths/min; tidal volume, 10 to 15 mL/kg; mean inspiratory pressure, 20 cm H2O; positive end-expiratory pressure, 3 cm H2O). The pump is interfaced to the animal via a left fifth interspace thoracotomy, with pump inflow coming from a cannula in the left atrium or left ventricle and outflow to the left carotid artery. This has allowed us to capture all, or nearly all, of the cardiac output at the anticipated pump speed.
The pump is positioned very close to the animal to minimize overall priming volume and surface area of the extracorporeal circuit. Heparin is administered continuously to keep activated clotting times at 1.5 to 2.0 times normal. A sling method of restraint was fashioned so that the animals did not need to be anesthetized and ventilated during the observation period. This has resulted in a satisfactory animal model and a protected pump circuit. As illustrated in Figure 3, animals have adapted well to the sling. Feeding has not been a problem as these animals take readily to a nursing bottle, and the older ones will eat hay as well.
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In Vitro
HYDRODYNAMIC PERFORMANCE.
Figure 4 shows a plot of the measured differential pressure-flow rate characteristics, covering a speed range of 2,500 to 6,000 rpm. Particular conditions the pump would likely encounter when used for pediatric cardiac support are illustrated by these data. As seen, the pump can be used to provide left ventricular assistance for a neonate (0.30 L/min), for an infant (1.5 L/min), and for a larger child (3.0 L/min).
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contains a summary of the in vitro hemolysis tests. These data indicate that this pump is atraumatic to red blood cells over the range of hemodynamics anticipated for this patient population.
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Pump function has been good, and the system has operated without interruption throughout each animal trial. In the present series of animals flows ranged from as low as about 300 mL/min to 2.0 L/min. Figure 5 presents daily snapshots of pump performance during one experiment that continued for 148 hours. These data are typical of the pump hemodynamics achieved in the present in vivo series.
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. On average, free plasma hemoglobin level remained less than 20 mg/dL.
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There were only minor, transient perioperative alterations in kidney and liver biochemistries in the 5 animals. No manifestation of renal or hepatic dysfunction was observed in the postoperative periods. The animals ate and drank (nursed) as would nonoperated similarly aged lambs. Urine output was unremarkable for this size animal and species.
At autopsy, there was no evidence of widespread thromboembolic complications in any lamb. The kidneys of the lamb perfused for 148 hours contained evidence of small infarcts, but this was not found in the other organs examined. In all 5 animals, the liver and brain were normal, as were the other abdominal organs.
All pump circuits were carefully disconnected from each animal at the time of autopsy. Buffered saline solution was perfused through the circuit until all visible traces of blood were absent. Figure 7 is an explant photograph of the pump removed after 148 hours of operation. One notes the absence of any thrombus on the vanes or along the blood flow path within the pump. This was a consistent finding in these experiments.
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). Comment
Mechanical circulatory support systems are becoming a more commonly used method of treatment for adult-sized patients requiring temporary cardiac support. This covers applications ranging from short-term postoperative support to bridge to cardiac transplantation cases that extend out for many months in duration. On the other hand, cardiac support device technology is comparatively lacking for neonatal and pediatric applications. As noted by Pennington in his recent articles [2, 3], none of the ventricular assist devices that have been used successfully in adults are available in sizes appropriate for infants and children.
We have now initiated a program to develop a pediatric blood pump that would provide temporary support (3 to 7 days) of the failed circulation of neonates and pediatric patients. An important virtue of our design is the very small size and priming volume (13 mL) of the disposable pump head. Also, by its design, the pump can be placed adjacent to the patient, further reducing extracorporeal blood volumes and surface areas in contact with blood. In the in vitro and in vivo studies conducted to date, we were able to document satisfactory and reliable pump hemodynamic performance over a wide range of hemodynamic situations that span the intended clinical use of the pump. Hemolysis generation was quite low in vitro and in vivo. In the 5 animal studies, hematologic indices were essentially unchanged and satisfactory organ function was preserved. Finally, we have documented satisfactory seal performance in our longest two experiments of 76 hours and 148 hours.
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This work was supported in part by a phase I SBIR award from the National Heart, Lung and Blood Institute, National Institutes of Health (HL 51667).
Footnotes
Presented at The Third International Conference on Circulatory Support Devices for Severe Cardiac Failure, Pittsburgh, PA, Oct 28-30, 1994.
Address reprint requests to Dr Litwak, Department of Surgery, University of Pittsburgh, 300 Technology Dr, Pittsburgh, PA 15219.
References
This article has been cited by other articles:
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  A K Mahmood, J M Courtney, S Westaby, M Akdis, and H Reul Critical review of current left ventricular assist devices Perfusion, September 1, 2000; 15(5): 399 - 420. [PDF]
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= rpm [/1,000]; 
= current [A]; 
=flow [L/min]; x = differential pressure [/100 mm Hg].)
