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Ann Thorac Surg 2001;71:S171-S175
© 2001 The Society of Thoracic Surgeons
a Thoratec Laboratories Corporation, Pleasanton, California, USA
b Department of Cardiac Surgery, California Pacific Medical Center, San Francisco, California, USA
Address reprint requests to Dr Hill, Department of Cardiac Surgery, California Pacific Medical Center, 2100 Webster, Suite 512, San Francisco, CA 94115
e-mail: jdhill{at}brown.cpmc.org
Presented at the Fifth International Conference on Circulatory Support Devices for Severe Cardiac Failure, New York, NY, Sept 15–17, 2000.
Abstract
Background. As patients are supported for longer durations with paracorporeal Thoratec left ventricular and biventricular assist devices (longest durations: 515 and 457 days, respectively), there is a need for implantable options.
Methods. We are developing a small, simple, and versatile intracorporeal ventricular assist device (IVAD) for left, right, or biventricular support as an alternative to the large, implantable, pulsatile left ventricular assist device (LVAD) systems available today. The new device is based on the Thoratec paracorporeal VAD that has been used in more than 1,400 patients weighing from 17 to 144 kg and for durations exceeding 1 year including patient discharge (using the portable driver).
Results. The IVAD has the same blood flow path and Thoralon polyurethane blood pumping sac as the paracorporeal VAD, but the housing is a smooth contoured, polished titanium alloy. The IVAD has a new sensor to detect when the pump is full and empty, and is controlled with the Thoratec TLC-II portable VAD driver, which is a small, briefcase-sized, battery-powered, pneumatic control unit. A small flexible (9 mm OD) percutaneous pneumatic driveline for each VAD is tunneled out of the body from the LVAD or right VAD in a pre- or intraperitoneal position. Small size and simplicity are the major advantages of the new device. The IVAD weight (339 g) and implanted volume (252 mL) are approximately one-half that of the current implantable pulsatile electromechanical LVAD systems.
Conclusions. The small size of the IVAD should not only allow support of a large range of patient sizes and body habitus, but also provide options for implantable left, right, or biventricular support. By implanting only the mechanically simple blood pump, the more complex control unit is external, where it can be serviced and replaced without surgery. The IVAD with the portable driver will be a viable alternative to large implanted electromechanical systems and should address a larger segment of the physically diverse patient population.
There is a need for both paracorporeal and intracorporeal ventricular assist devices (VADs). Paracorporeal devices may be preferable for short- to intermediate-term support, ie, for up to several months, while waiting for cardiac recovery or transplantation. They are also preferable for small body sizes, in which there are anatomic fit problems with large implantable devices. On the other hand, for long-term support and for use of VADs outside the hospital, implantable devices are preferable. Because the current paracorporeal Thoratec VAD system (Thoratec Laboratories Corporation, Pleasanton, CA) is being used for longer durations, and the new portable driver enables patients to be discharged home on VAD support, there is a need for implantable options of this technology. In addition, there are no currently available implantable biventricular assist systems, and this system could easily be adapted for these applications.
The paracorporeal Thoratec VAD system has been used in more than 1,400 patients for left, right, or biventricular VAD (LVAD, RVAD, or BVAD) support. Patients have been supported for more than a year on the system, including several patients discharged from the hospital. This system has regulatory approval for bridge-to-cardiac transplantation and postcardiotomy recovery of the natural heart [1–3]. It also has been used for bridge-to-recovery for myocarditis and cardiomyopathies [4]. This VAD blood pump is placed in a paracorporeal position on the patient’s anterior abdominal wall, with cannulas crossing the chest wall to connect to the heart and great vessels. The paracorporeal position is especially versatile and has been used for patients with a wide range of body sizes, the smallest being a 7-year old boy weighing 17 kg with a body surface area of 0.7 mm2, and the largest being a patient weighing 144 kg with a body surface area of 2.75 m2. In contrast to external bedside-based blood pumps, patients with paracorporeal devices can ambulate and move freely, especially with the portable drive unit [5–7]. The longest duration of a patient on a Thoratec paracorporeal VAD (PVAD) is 515 days, in apatient who was discharged from the hospital using the portable driver.
To fulfill the need for a versatile intracorporeal VAD that can benefit a diverse patient population, an implantable version of the Thoratec VAD is being developed. This new device maintains the internal flow geometry, blood pumping sac, valves, and cannula construction of the paracorporeal version, and will provide a small, lightweight intracorporeal pulsatile VAD applicable to LVAD or BVAD support in a wide range of patient sizes.
Design philosophy and features
The design approach concentrated on reducing device size, simplifying the implanted components, and using the clinically proved technology of the Thoratec PVAD system. The small size of the IVAD was to allow LVAD, RVAD, or BVAD support of small or large patients. The implanted components were simplified by keeping the complicated electromechanical components external to the body where they could be serviced or replaced. Basing the design on the clinically proved Thoratec PVAD accelerated the development and took advantage of the successful clinical history of the PVAD.
The device resulting from the design philosophy described above is shown in Figure 1. The overall size and shape of the IVAD facilitates implantation in a diverse patient population. The specific design features of the IVAD are illustrated in Figure 2. The low profile housing is fabricated from a titanium alloy known to have good tissue biocompatibility properties. The smooth external contours are designed to minimize dead space in the pre-peritoneal pocket and reduce the risk of infection. In addition, the surface of the device is polished to take advantage of potential bacteria colonization resistance associated with smooth metallic surfaces [8, 9].
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A small flexible percutaneous pneumatic line delivers and vents air that activates the IVAD. The 9 mm-diameter velour-covered line also carries an electrical cable for the sensor. At the distal end of the line, a special small-diameter fitting combines electrical and pneumatic connections. Extensive design efforts were made to achieve an 11-mm diameter for the fitting that is tunneled through the skin. The pneumatic line tubing is constructed of Thoralon, Thoratec’s proprietary polyurethane multipolymer, with wire reinforcement and polyester velour covering, the same materials that are used in the PVAD percutaneous cannulas. The polyester velour covering promotes tissue ingrowth, providing an effective barrier to infection and anchoring the VAD within the body.
The geometry of the blood path is identical between the clinically proven PVAD and the IVAD. The IVAD uses the same blood sac, actuation diaphragm, and unidirectional blood valves as the PVAD without modifications. Both the PVAD and IVAD have a smooth, seamless, flexible pumping chamber (blood sac) made of Thoralon. For both VADs, two monostrut valves with Delrin (DuPont, Wilmington, DE) occluder disks maintain unidirectional flow through the blood pump. The pumping chamber is separated from the air chamber by a polyurethane actuation diaphragm. The diaphragm serves as both a volume limiter and safety chamber. To prevent abrasion, silicone oil lubricates the surfaces where the diaphragm and VAD case contact the blood sac. The blood-contacting geometry of the valve housings, the components that connect the cannula to the VAD, are identical for the two pumps. The only difference is the PVAD valve housings are constructed of stainless steel, whereas the IVAD valve housings are constructed of a more damage-resistant titanium alloy. The effective stroke volume is the same for both pumps, 65 mL. A comparison of the physical dimensions for the Thoratec IVAD and PVAD is given in Table 1.
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The IVAD is designed for implantation in a pre-peritoneal or intraperitoneal position using common surgical techniques described in the literature [11, 12]. Curved 12.7-mm-diameter tunnelers have been designed to create a tunnel and allow passage of the pneumatic lines from the pocket to the exit site. Figure 3 demonstrates the proposed tunnel routes for biventricular implantation using right atrial or ventricular cannulation.
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The anatomic positioning of the IVAD has been evaluated using computer-simulated virtual fit trials and trials in human cadavers. The virtual fit trials used an anatomic model based on the National Library of Medicine’s Visible Human project (Visible Productions, Fort Collins, CO). Figure 4 shows computer models of the IVAD and cannulas positioned for biventricular support.
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Animal trials of the PVAD implanted in subcutaneous pockets and confirmatory implants of the IVAD have been carried out. The original PVAD trials were carried out with the paracorporeal pump implanted in the bodies of calves [13]. The PVAD was implanted in ten calves, all animals were sacrificed at 30 days and the average pump flow was 5.6 ± 0.5 L/min. The Thoralon blood sacs were free from thrombus on the blood contact surfaces. A study demonstrating biventricular assist with the PVAD has also been reported in the literature [6]. Although the blood flow paths are the same and the IVAD housing materials and design is much more suited to implantation, two confirmatory tests were performed with the IVAD. Two calves were implanted with the IVAD and sacrificed at 30 and 45 days. As expected, the blood sacs of both animals were free from thrombus and there were no pump failures during the implants. The optical sensor also performed as expected throughout the trials and the drivers were maintained in auto mode during the experiments.
In vitro testing of the IVAD has also been carried out and indicates the IVAD function is equivalent to the PVAD. With the IVAD connected to a mock circulatory loop and driven by a TLC-II in the fill-to-empty mode, the device output was measured with a flow probe on the arterial cannula. Figure 5 shows the measured IVAD output compared with the calculated flow. When the IVAD is completely emptying, as indicated by external driver interface signal, there is good correlation between the calculated value and the device (Fig 5). As expected, when the IVAD is not completely emptying, the measured flow begins to deviate from the calculated value that assumes complete full-to-empty operation.
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Comment
The versatility and simplicity of the intracorporeal VAD are advantageous. The IVAD weight and volume is approximately half of that of existing implantable pulsatile electromechanical left ventricular assist systems [14]. The IVAD blood pump is small enough to be used for left or right heart support, even in relatively small patients (Fig 3). Although it is smaller in size than other implantable VAD systems, it will also effectively support large patients, as has been shown with the paracorporeal VAD with a maximum output of 7.2 L/min. Only the small blood pump is implanted, leaving the more complex control units external where they can be serviced and replaced.
The smooth contours, polished titanium housing, and small, flexible velour-covered pneumatic drive line that accommodates long tunnel lengths should help reduce the risk of infection. The small profile and smooth contours of the IVAD should also reduce the risk of organ compression and pocket erosion. Substantiating these potential benefits, however, will require clinical trials.
Intracorporeal VAD systems are being used to discharge patents from the hospital while on VAD support with an acceptable quality of life [15–17]. Nevertheless, there is also a need for paracorporeal VADs, and the selection of a system should be dependent on the patient and indications for support. The Thoratec paracorporeal VAD has been used successfully in patients with body surface areas as small as 0.7 m2. In addition, as paracorporeal devices do not require abdominal surgical procedures, placement is simplified and removal is technically easier. Therefore, paracorporeal devices may be preferable for short- to intermediate-term applications, supporting patients for several months while awaiting myocardial recovery, for example. Although each paracorporeal VAD has two percutaneous lines (cannulas), serious ascending infections are rare with Thoratec cannulas, and there is no pump pocket that can become infected. Paracorporeal placement has disadvantages including risk of mechanical damage to the device, aesthetic considerations, and possibly a reduced patient quality of life. Therefore, having an intracorporeal version of the Thoratec VAD would complement the existing system.
The patient population that could benefit from mechanical ventricular assistance is diverse. Not only is there large variation in patient size and body habitus, but the support required also differs in degree and duration. The versatile IVAD system should not only support a large range of patient sizes and body habitus, but also provide an option for implantable biventricular support. The use of the TLC-II portable driver is already improving mobility of patients with paracorporeal VADs [7], including some patients discharged from the hospital. The IVAD will allow even greater mobility and will facilitate patient discharge for more patients. The IVAD with the portable driver will be a viable alternative to large implanted electromechanical systems and may help address a larger segment of the physically diverse patient population.
Footnotes
Dr Hill is a stockholder of Thoratec Laboratories. Drs Reichenbach and Farrar are salaried employees of Thoratec Laboratories.
References
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= measurements without complete empty indication.) (Courtesy of Thoratec Laboratories.)