Ann Thorac Surg 2003;76:2109-2111
© 2003 The Society of Thoracic Surgeons
How to do it
Special considerations on the implantation technique for the MicroMed-DeBakey ventricular assist device axial pump
Georg M. Wieselthaler, MDa*,
Heinrich Schima, PhDa,
Ernst Wolner, MDa
a Department of Cardiothoracic Surgery and Ludwig Boltzmann Institute for Cardiosurgical Research, University of Vienna, Vienna, Austria
Accepted for publication March 17, 2003.
* Address reprint requests to Dr Wieselthaler, Department of Cardiothoracic Surgery, University of Vienna, Waehringer Guertel 18-20, A-1090, Vienna, Austria
e-mail: georg.wieselthaler{at}akh-wien.ac.at
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Abstract
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Implantable continuous flow axial pumps were introduced to clinical settings in November 1998 with the MicroMed-DeBakey ventricular assist device. Axial pumps continuously unload the failing left ventricle and accurate positioning of the inflow cannula prevents substantial reductions in pump flow. Considerations on the implantation technique are made to minimize surgical trauma, to prevent ventricular collapse, to optimize inflow conditions (flow increased from 4.3 ± 0.6 to 6.7 ± 0.3 L/min), and to facilitate the subsequent transplantation.
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Introduction
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Recently, a new generation of mechanical blood pumps became available for clinical application [1]. Implantable axial flow impeller pumps such as the MicroMed-DeBakey ventricular assist device (VAD) hold the potential for small size, low noise, and absence of a compliance chamber. Although these pumps produce continuous flows patients with implanted axial flow pumps show flow patterns with low pulsatility due to the remaining function of the natural left heart. In contrast to pulsatile blood pumps continuous unloading of the left ventricle with axial flow pumps can easily cause obstruction and suction. That may result in a dramatically reduced pump output and elevated hemolysis. As previously reported, intermittent obstruction of the inflow was observed in our first 2 patients causing substantial flow restrictions resulting in low mixed venous oxygen saturation and limited exercise capacity. Analysis of pump flow, echocardiography, and extensive examination of the excised hearts after successful transplantation revealed an improper positioning of the inflow cannula. Taking into account the geometry of a dilated heart we modified the implantation technique and that resulted in a significant improvement of pump flow in the patients described here.
Since November 1998 21 male and 4 female patients (mean age, 50 ± 13 years; range, 14 to 65) in end-stage heart failure received a MicroMed-DeBakey axial pump as a bridge to transplantation. Thirteen patients were successfully transplanted after a mean pump duration of 140 ± 54 days (range, 67 to 226), 9 patients died after a mean pumping period of 93 ± 76 days owing to patient-related causes, and 3 patients are still on pump support after 110 ± 87 days.
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Technique
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Similar to other implantable left ventricular assist devices (LVADs) [2–4] the implantation is performed through a median sternotomy during extracorporeal circulation with the beating heart. Owing to the small size of the pump (especially after the recently performed reduction of inflow cannula length) the skin incision does not exceed the length of the sternum. As an advantage over the bulkier pulsatile devices the rectus fascia is incised longitudinally in the linea alba over a distance less than 2 cm. No preparation of a distinct pump pocket is required. The right visceral pleura should stay intact and is therefore dissected precisely from the pericardium to place the outflow graft later on between the two layers. That should guarantee good tissue discrimination properties for the later transplantation and to prevent adhesions of the lung to the posterior sternum surface. After longitudinally opening the pericardium and extending the incision in a T-shaped manner along the diaphragm, cannulation for extracorporeal circulation is performed.
The heart’s apex is exposed with the patient in the Trendelenburg position. The insertion site for the inflow cannula has to be chosen extremely carefully near the tip of the apex, close laterally to the left anterior descending artery. This position should avoid later obstruction of the inflow because the cannula is placed close (and parallel) to the septum and at a safe distance from the free lateral wall and papillary muscles. A Dacron (C. R. Bard, Haverhill, PA) sewing ring with circumferencially placed buttressed sutures is fixed around the insertion site (Fig 1).
After excising the apical core within the ring great care must be taken to excise additional trabeculations inside the ventricle that could potentially obstruct parts of the inflow. The inflow cannula is inserted into the ventricle and placed in a strict parallel position to the septum with sufficient distance of the inflow orifice to the lateral wall and the papillary muscles (Fig 2).
This setting should remain when the exposure of the heart is released and the pump is placed in the space previously prepared caudally to the diaphragm. The power cable is tunneled subcutaneously across the abdomen and exits the skin cranially to the right iliac crest. Sharp kinking must be avoided to prevent damage to the thin and flexible cable.
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Fig 1. Exact positioning of the inflow cannula insertion site in relation to the left anterior descending coronary artery (arrows).
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Fig 2. The inflow cannula is placed in a position strictly parallel to the septum with sufficient distance of the inflow cannula orifice from the lateral wall and the papillary muscles.
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The outflow graft is placed extrapericardially and anastomosed end to side to the ascending aorta. The system is easily de aired through the clamped outflow graft and started. Then extracorporeal circulation is gradually discontinued. In order to avoid strong adhesions of the Dacron ring on the apex the inflow cannula insertion site is wrapped with a 2-mm Gore-Tex (W. L. Gore & Associates, Flagstaff, AZ) patch. An additional 20-mm Gore-Tex graft is placed around the reinforcement of the outflow graft. This guarantees easy access for the upcoming transplantation procedure. Reconstruction and closure of the pericardium finalizes the implantation procedure.
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Results
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In our first 2 patients, who were operated on before the operation technique was modified, substantial restrictions of inflow with consecutive low pump outputs and limited exercise capacity were observed. Echocardiography and examination during explants showed evidence of improper position of the inflow cannula toward the septum. During continuous unloading of the massive dilated left ventricles, the rather thin free lateral walls and parts of the anterior papillary muscle were flexing toward the septum and easily obstructed the inflow cannula (Fig 3).
After these first two implants the new technique was established. In the next patients the achieved mean flow increased from 4.3 ± 0.6 to 6.7 ± 0.3 L/min. In contrast to the first 2 patients no suction episodes were observed. At the time of transplantation easy excess to intrathoracic cannulation for extracorporeal circulation and accelerated heart explantation was possible.
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Fig 3. Improper positioning of the inflow cannula toward the septum with partial obstruction of the inflow orifice can cause substantial flow restrictions.
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Comment
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The MicroMed DeBakey VAD was the first implantable axial flow pump available for clinical application in November 1998. With adequate cannulation this VAD system can completely replace left ventricular function and provide good patient recovery and complete restitution of end-organ function particularly if high pump flows are achieved [5].
Compared with pulsatile blood pumps these pumps are far smaller, and owing to the continuous flow characteristics the inflow and outflow cannulas require a smaller diameter. However owing to this smaller diameter of inflow cannula and inflow orifice, occlusion by surrounding endocardium and trabeculae may occur more easily and cause substantial reduction of pump flow. Furthermore owing to the continuous pumping characteristics and the high achievable vacuum an eventual suction status could persist even for longer periods. Therefore any potential obstruction of the inflow cannula has to be avoided. Exact positioning of the inflow cannula close to and strictly parallel to the septum avoids obstruction even with full unloading conditions (Fig 4),
which could occur by changing filling conditions and contraction characteristics during the circardian cycle, rest, and exercise, and by de-remodeling of the enlarged ventricle during long-term support. With the described modified technique we could achieve in all patients sufficient high pump flows for daily life and exercise.
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Fig 4. Exact positioning of the inflow cannula close to and strictly parallel to the septum avoids obstruction even during full, continuous unloading conditions.
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References
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- Wieselthaler G.M., Schima H., Hiesmayr M., et al. First clinical experience with the DeBakey VAD continuous-axial flow pump for long-term bridge to transplantation. Circulation 2000;101:356-359.[Abstract/Free Full Text]
- Loisance D., Cooper G.J., Deleuze P.H., et al. Bridge to transplantation with the wearable Novacor left ventricular assist system: operative technique. Eur J Cardiothorac Surg 1995;9:95-98.[Abstract]
- Vigano M., Martinelli L., Minzioni G., Rinaldi M. Pagani F. Modified method for Novacor left ventricular assist device implantation. Ann Thorac Surg 1996;61:247-249.
- McCarthy P.M., Smedira N.O., Vargo R.L., et al. One hundred patients with the HeartMate left ventricular assist device: evolving concepts and technology. J Thorac Cardiovasc Surg 1998;115:904-912.[Abstract/Free Full Text]
- Wieselthaler G.M., Schima H., Lassnigg A., et al. Lessons learned from the first clinical implants of the DeBakey VAD axial pump: a single center report. Ann Thorac Surg 2001;71(Suppl):S139-143.[Abstract/Free Full Text]
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Abstract
Introduction
