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Ann Thorac Surg 1998;65:470-473
© 1998 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

LVAD Power Delivery: A Percutaneous Approach to Avoid Infection

Oxford Heart Centre, John Radcliffe Hospital, Oxford, United Kingdom

Accepted for publication August 18, 1997.

Mr Westaby, Oxford Heart Centre, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, United Kingdom.

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Background. Driveline infection limits the event-free survival of patients with a left ventricular assist device. With the evolving prospect of improved left ventricular assist devices in the bridge-to-transplantation or recovery setting, we sought to reduce the risk of driveline complications.

Methods. As part of the Oxford Jarvik 2000 research program, we developed a carbon and then titanium pedestal to transmit the electric wires through the skin. In a sheep model, the pedestal was brought out through the skin of the shoulder (n = 10) or the scalp (n = 9) with underlying fixation to the skull. Exit wounds were carefully inspected for healing and infection. Power cable durability tests were performed in 6 additional animals without an implanted pump.

Results. The cumulative observation period was 1,491 days (mean time, 78 days; range, 14 days to 198 days). There was no difference in observation period between the two groups. Infection (n = 2) and impaired healing (n = 5) occurred in the mobile tissues at the shoulder. Skull-mounted pedestals were free from infection or healing problems. The electric cables were not interrupted by repeated neck flexion (cumulative observation period, 588 days). The carbon pedestal was replaced by a titanium pedestal when the head butting of the sheep fractured the carbon.

Conclusions. The combination of rigid fixation and highly vascular scalp skin reduces the risk of percutaneous driveline infection and may solve an important outstanding problem in use of left ventricular assist devices.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Left ventricular assist devices (LVADs) are now used for permanent circulatory support on an outpatient basis as well as for bridging to transplantation or myocardial recovery [1] [2]. Our early experience with the Thermo Cardiosystems Inc LVAD in patients not eligible for transplantation was complicated by driveline infection despite initial healing. This problem detracts from quality of life and may eventually lead to death from pump infection or septicemia. Although transcutaneous power delivery through induction coils is an option for the future [3] [4], the efficiency of energy transmission must improve before clinical application. We therefore sought to increase the reliability of percutaneous power delivery in the Jarvik 2000 Heart by using a system modified from artificial-hearing technology. In cochlear implants with a percutaneous carbon button screwed to the skull, there is a documented implant success (with minimal superficial infection) of greater than 90% at 10 years with some patients having had an implant for longer than 20 years [5].

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
The Jarvik 2000 Heart is a compact axial-flow impeller pump with an outflow Dacron graft for anastomosis to the descending thoracic aorta (Fig 1). In the Oxford system, percutaneous power is delivered from external batteries through a controller unit. Internal electric wires are brought through the pleural cavity to the apex of the chest and then through the neck to the base of the skull, where a percutaneous titanium pedestal transmits fine electric wires through the skin of the scalp (Fig 2). The pump structure and the surgical technique for implantation have been described previously [6] [7]. In parallel with the in vivo assessment of pump function, we have developed the electric system with studies to determine the optimal site for percutaneous transmission and the durability of the subcutaneous power cables.

View larger version (110K): [in this window] [in a new window]   The Jarvik 2000 axial-flow impeller pump with vascular graft, electric cable, and skull-mounted carbon pedestal.

View larger version (104K): [in this window] [in a new window]   Detail of the carbon pedestal with bone screws and external electric connection, which can be replaced at skin level in the event of disruption.

The sheep were anesthetized with fentanyl, intubated, and ventilated with an inspired oxygen fraction of 100%. A venous catheter and an arterial catheter were introduced into the left internal jugular vein and the left common carotid artery, respectively. Lidocaine hydrochloride infusion and boluses of bretylium tosylate were used to suppress left ventricular irritability during cardiac manipulation.

The animals were positioned for a left thoracotomy. The fleece was removed from the scalp to the left flank, and the skin was prepared thoroughly with povidone-iodine solution. A left thoracotomy was undertaken to gain access to the descending thoracic aorta and the apex of the heart. Before the left pleural cavity was entered, the power cable with carbon pedestal was tunneled under the scapula toward the midline. In the first 10 animals, the carbon pedestal was used to transmit the electrical wires through the skin of the middle of the shoulder. This site was chosen to prevent the animals from chewing the cable.

In the second group of 9 animals, the power cable with a carbon (n = 3) or titanium pedestal (n = 6) was tunneled in a zigzag fashion to the occipital region where a semicircular skin flap was fashioned over the base of the skull. The periosteum of the skull was elevated, and the external table of the occiput was excavated to recess the power cable beneath the percutaneous pedestal. The pedestal itself was then screwed to the outer table of the skull, providing complete immobility in contrast to the shoulder pedestals situated in mobile subcutaneous tissue.

With the electrical system in place, the Jarvik 2000 pump was implanted by anastomosing the Dacron graft to the descending thoracic aorta and inserting the pump through a cuff into the apex of the left ventricle. This was achieved by excising apical left ventricular muscle with a coring knife while the heart supported the circulation. Cardiopulmonary bypass was not required.

Once the pump had been inserted into the apex of the left ventricle, the percutaneous power cable was connected to the external controller and battery power supply. After the sheep recovered consciousness, intermittent positive-pressure ventilation was discontinued, and the animals were moved to a stall where mains power replaced the batteries. Mains power was transmitted through an armored cable from a swivel in the ceiling to allow free mobility about the stall. The armored cable inserted directly into a vest made to carry the portable controller (Fig 3). Six additional animals (used for the cable durability testing) had a loop of cable only (no pump) and were not fitted with a vest or mains power cable.

View larger version (144K): [in this window] [in a new window]   External power system to the sheep. The power cable is connected to a controller mounted on the vest. Power is brought to the controller through an armored swivel cable suspended above the pen.

View larger version (14K): [in this window] [in a new window]   Distribution of the implanted cables (A) with a working pump and (B) to test only the integrity of the subcutaneous cable.

Analysis of Infection
The driveline exit sites were inspected carefully each day. Cultures of any serous or purulent discharge from exit sites were grown. Uncomplicated wound healing was defined as complete healing around the percutaneous pedestal with complete absence of exudate. If a driveline complication was identified, the postoperative day was recorded, and the actuarial incidence and the linearized rate of complications were calculated. Time-related incidence of driveline infection was analyzed according to the Kaplan-Meier method. Data between groups were compared using either unpaired t test or Fisher’s exact test where appropriate. A p value of less than 0.05 was considered significant.

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
The cumulative observation period was 1,491 days (mean time, 78 days; range, 14 days to 198 days). There was no significant difference in mean observation period between the two groups (88 days for skull pedestals versus 70 days for shoulder pedestals; p = 0.46).

All 9 animals with skull-mounted pedestals had rapid healing at the percutaneous exit sites despite the more complicated and prolonged surgical procedure. Of the first 3 sheep with skull-mounted pedestals, 2 fractured the carbon by butting their heads against the wall of the stall. Subsequently the material of the pedestal was changed to titanium and the site of implantation moved further dorsally on the occiput. In contrast, 5 of the 10 sheep with a mobile subcutaneously based shoulder pedestal had persistent nonhealing of the exit site, but cultures and histopathologic studies provided no evidence of infection (50%; p = 0.03 by Fisher’s exact test). Four of the 5 animals showed erythema around the driveline and had a clear exudate. The other had development of poor tissue ingrowth around the pedestal, which, although not infected, required surgical revision.

All the animals with skull pedestals were completely free from driveline infection after a total observation period of 796 days (Fig 5). The shoulder position was associated with reduced infection-free survival (100% for skull pedestals versus 80% for shoulder pedestals) at 164 days after implantation. Two sheep with percutaneous exit sites in the shoulder position had development of a driveline infection (20%; p = 0.47 by Fisher’s exact test). One animal contracted local staphylococcal infection at the site of the percutaneous carbon pedestal 7 days after pump implantation. After a 2-week course of antibiotic treatment, the exudate became culture negative, and the pedestal was encapsulated with granulation tissue. At 48 days, there was an external disruption of power delivery, and the pump stopped. The animal was sacrificed, and postmortem examination showed no sign of deep infection. The second animal had a purulent discharge from the exit site at the shoulder 10 days postoperatively. Several courses of antibiotics failed to clear the infection. Subsequently, a persistent high-grade fever and malaise developed, and the sheep was sacrificed on the 34th postoperative day. Postmortem examination showed a deep infection around the driveline pocket, which communicated with the left pleural cavity along the cable. All antemortem and postmortem cultures grew Staphylococcus aureus.

View larger version (147K): [in this window] [in a new window]   Well-healed site of skull-mounted carbon pedestal 150 days after implantation.

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
In the current Food and Drug Administration–approved LVADs (Thermo Cardiosystem Inc, Woburn, MA, and Novacor, Oakland, CA), external electric power requires percutaneous line delivery, and driveline infection is common in patients supported for long periods. Previous studies [8] [9] have documented positive driveline cultures in 50% of patients successfully bridged to transplantation and obvious clinical infection in 30%. The risk of driveline infection, which may occasionally preclude transplantation, must be weighed against the risk of death caused by progressive heart failure. Currently about 30% of patients being bridged to transplantation are rejected for cardiac transplantation because of infection problems, and 29% of those with major driveline problems die while waiting for a donor organ [9] [10].

Although driveline infection may be related to the patient’s overall clinical condition, the thick, stiff structure penetrating mobile skin and subcutaneous tissue contributes to the risk. Both localized and ascending driveline infections are usually preceded by nonhealing of the exit site when the rigid cable moves with the patient’s activity. The cases of 2 of our patients with a Thermo Cardiosystems Inc LVAD illustrate this. In 1 of them, initial complete skin healing broke down after the patient coughed during a viral infection or increased activity at home. In the other, a diabetic, an ascending Candida infection with fungemia developed. Given these difficulties with existing devices where an electric cable is combined with an air vent, the small-diameter power cable for the Jarvik 2000 Heart is a major improvement. Nevertheless, all percutaneous devices such as venous cannulas that pass through mobile skin, subcutaneous tissue, and fat can cause skin breakdown and infection because of movement of the device.

In our efforts to develop a user-friendly LVAD for permanent outpatient left ventricular support or bridge-to-myocardial recovery, we adopted the principles used successfully by Parkin [5] for artificial-hearing technology. Parkin’s cochlear implant operations achieved reproducible long-term freedom from infection by combining an exit site through highly vascular scalp skin with immobility by fixing a biocompatible pyrolitic carbon pedestal to the underlying skull. Rigid fixation in relation to the overlying skin allows prompt and secure healing. We initially employed a pyrolitic carbon pedestal but changed to titanium after the pedestal shattered on impact. Although this is unlikely to occur in patients, the strength of titanium provides additional insurance against this problem.

In our experiments, all animals with a skull-fixed pedestal were completely free from driveline-related events. On the other hand, nonhealing and infection problems were encountered with the mobile shoulder exit site. Our results suggest that the infection-resistant conditions in artificial-ear technology can be successfully translated to LVAD electric drivelines, although clearly not with combined air vents.

For long-term patient use, we intend to screw the percutaneous pedestal to the outer table of the temporal bone in the same position as for the cochlear implants. A discrete and detachable electric wire will pass behind the ear and below the shirt to a battery and controller system in a vest. This should be only a little more obtrusive than an old-fashioned hearing aid. For the bridge-to-transplant setting, an alternative position may be the sternum or iliac crest, although these sites have a deeper covering of fat and subcutaneous tissue. A larger pedestal on the iliac crest may cause discomfort during some activities.

To date, our experience with the Jarvik 2000 axial-flow impeller pump suggests long-term mechanical reliability with minimal hemolysis or clotting at impeller speeds between 8,000 and 12,000 rpm [6]. This small, silent axial-flow impeller pump, which fits within the apex of the failing left ventricle, greatly increases the scope of mechanical circulatory support for both adults and children. In addition to bridging to transplantation, the pump can be used before end-stage disease to promote myocardial recovery in certain conditions where the potential for this exists [1] [2]. If driveline complications can be avoided, there is the possibility of permanent mechanical support as an alternative to cardiac transplantation.

	Acknowledgments

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
We thank Miss Katherine L. Ely for editorial assistance.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Doctor Jarvik is the owner of Jarvik Heart Inc.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 

1. Westaby S, Jin XY, Katsumata T, Taggart D, Coats AJC, Frazier OH Mechanical support in dilated cardiomyopathy: signs of early left ventricular recovery. Ann Thorac Surg 1997;64:1303-1308.[Abstract/Free Full Text]
2. Frazier OH, Benedict CR, Radovancevic B, et al. Improved left ventricular function after chronic left ventricular unloading. Ann Thorac Surg 1996;62:675-682.[Abstract/Free Full Text]
3. Macris MP, Parnis SM, Frazier OH, Fuqua JM, Jr, Jarvik RK Development of an implantable ventricular assist system. Ann Thorac Surg 1997;63:367-370.[Abstract/Free Full Text]
4. McCarthy PM, Portner PM, Tobler HG, Starnes VA, Ramasamy N, Oyer PE Clinical experience with the Novacor ventricular assist system. J Thorac Cardiovasc Surg 1991;102:578-587.[Abstract]
5. Parkin JL Percutaneous pedestal in cochlear implantation. Ann Otol Rhinol Laryngol 1990;99:796-800.[Medline]
6. Westaby S, Katsumata T, Evans R, Pigott D, Taggart D, Jarvik RK The Jarvik 2000 Oxford System. Increasing the scope of mechanical circulatory support. J Thorac Cardiovasc Surg 1997;114:467-474.[Abstract/Free Full Text]
7. Kaplon RJ, Oz MC, Kwiatkowski PA, et al. Miniature axial flow pump for ventricular assistance in children and small adults. J Thorac Cardiovasc Surg 1996;111:13-18.[Abstract/Free Full Text]
8. McCarthy PM, Schmitt SK, Vargo RL, Gordon S, Keys TF, Hobbs RE Implantable LVAD infections: implications for permanent use of the device. Ann Thorac Surg 1996;61:359-365.[Abstract/Free Full Text]
9. Griffith BP, Kormos RL, Nastala CJ, Winowich S, Pristas JM Results of extended bridge to transplantation: window into the future of permanent ventricular assist devices. Ann Thorac Surg 1996;61:396-398.[Abstract/Free Full Text]
10. Oaks TE, Pae WE, Miller CA, Pierce WS Combined Registry for the Clinical Uses of Mechanical Ventricular Assist Pumps and the Total Artificial Heart in Conjunction With Heart Transplantation: fifth official report—1990. J Heart Lung Transplant 1991;10:621-625.[Medline]

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