Mechanical circulatory support has been successfully applied to patients with low cardiac output refractory to pharmacologic treatment for more than three decades. The aim is either to wean the patients from circulatory support after myocardial recovery or to bridge the patients to transplantation. With the first successful implantation of the MEDOS-System in a pediatric patient in 1995, a new device for ventricular assistance in pediatric and adult patients was introduced.1,2 Design and pump characteristics have been previously described.2–4 The different sizes of the pump chambers (9/10 cc, 22.5/25 cc, 54/60 cc and 72/80 cc) enable the use of the MEDOS ventricular assist device (VAD) without limitations regarding patients' age and body surface area.5 The MEDOS VAD can be used for biventricular assistance or as a right or left VAD alone. The aim is either to bridge to transplantation or to myocardial recovery.2 The MEDOS VAD has proven its efficacy for intermediate term assistance in pediatric patients.2
We report our experience with seven consecutive pediatric patients.
Materials and Methods
From January 2000 until January 2005, a total of seven MEDOS-Systems (Medos Medizintechnik GmbH, Stolberg, Germany) were implanted in pediatric patients at our center. Mean patient age was 7.3 ± 6.5 years (range 0.75–16.9 years). Five patients were male (71.4%); mean weight was 26.3 ± 21.7 kg (range 5.9–60 kg), and mean length was 113 ± 46 cm (range 63–182 cm), respectively. Indications for mechanical circulatory support were low cardiac output syndrome with beginning multiorgan failure in all cases; resuscitation before implantation was necessary in two cases. All patients were in functional NYHA status IV. The underlying primary disease was dilated cardiomyopathy in all patients, with two patients revealing a history of myocarditis. One patient was switched to the MEDOS VAD from extracorporeal membrane oxygenation to install intermediate-term assistance as bridge to transplantation. The primary destination was bridge to transplantation in all patients. The choice of the VAD size depended on the patients' body surface area. Chamber sizes were 9/10 cc in one patient, 22.5/25 cc in four patients, 54/60 cc in one patient, and 72/80 cc in one patient. The MEDOS implantation was always performed with cardiopulmonary bypass. The cannulation technique was right atrium to pulmonary artery for right VADs and left ventricular apex (n = 6) or left atrium (n = 1) to aorta. A left VAD was implanted in three patients, and three patients required biventricular support. One patient first received a left VAD but received extracorporeal membrane oxygenation for sudden right ventricular failure and was secondarily switched to biventricular assistance. Postoperative anticoagulation was performed with continuous intravenous heparin controlled by activated clotting time (target activated clotting time, 180 seconds) and was switched to Coumadin in selected patients.
For computer-assisted statistical analysis, SPSS 11.0 for Windows (SPSS Inc., Chicago, IL) was used to process the data. Values of continuous variables were expressed as means and standard deviations. Kaplan Meier analysis was used to illustrate actuarial survival data.
Perioperative survival was 100% (seven of seven patients). Mean duration of support was 20.4 ± 10.8 days, and ranged from 6 to 38 days. Postoperative complications occurred in six of seven patients (86%; two cerebral embolism/bleeding, two rethoracotomy, two exchange of pump chambers due to thrombus formation after 4 and 9 days). Two of seven patients were extubated under circulatory support (Figure 1). No technical defects of either the pump chambers or the driving unit were observed. Increased bilirubin levels decreased under ventricular assistance from 3.5 ± 2.6 mg/d before implantation to 2.1 ± 1.2 mg/d after 7 days. Creatinine levels were mildly elevated and showed a small but steady decline from 1.4 ± 0.8 mg/dl to 1.2 ± 0.6 mg/dl after 3 days of MEDOS support. Preoperative lactate concentrations were highly elevated in our patient collective particularly in patients who underwent MEDOS implantation after resuscitation. Lactate values fell from 6.8 ± 4.5 mg/dl at baseline to 1.4 ± 1.2 mg/dl after 3 days (Figure 2).
Table 1 demonstrates the postoperative course of the seven patients. Three patients (43%) died while waiting for a donor organ. Although they were eligible for heart transplantation on the German high urgency list, no organ offer was received. Causes of death were cerebral hemorrhage due to an intracerebral aspergilloma, cerebral infarction, and septic multiorgan failure. All patients who underwent subsequent heart transplantation (four of seven patients; 57.1%) were discharged from the hospital and are currently in good physical condition (NYHA I or II). Cumulative survival is displayed in Figure 3.
Left VAD therapy has demonstrated significantly improved 1-year-survival and quality of life compared with optimal medical management in adult patients with end-stage heart failure (Randomized Evaluation of Mechanical Assistance in Treatment of Chronic Heart Failure).6
Clinical experience with circulatory support in pediatric patients is limited, particularly regarding pulsatile systems.7
Our results demonstrate that mechanical circulatory support with the MEDOS-System can be successfully performed in pediatric patients for bridging to heart transplantation. The MEDOS is a pulsatile VAD for systemic and/or pulmonary circulatory support available in different sizes. Therefore, it can be used in patients from newborn to adult age. It was first successfully used in a pediatric patient in 1995 for myocardial recovery,2 and has since been used in a number of adult and pediatric patients either for weaning or for bridging to transplantation. The operation mode allows heart rates from 40 beats per minute upwards. The system should run full to empty to prevent blood stasis, turbulent flow and thus thromboembolic complications. Although the system is not heparin coated, the seamless chamber design and optimized valve design provide laminar flow dynamics with low thrombogenicity.
Under the currently available pumps, the MEDOS is a cost-effective uni- and biventricular support system for intermediate-term assistance (e.g., as bridge to transplantation). Secondary organ functions can be improved by mechanical circulatory assistance. As in all systems, hemorrhage and thromboembolic events are the most frequent complications. However none of our patients had to undergo repeated rethoracotomy. Thromboembolic complications correlate with an increased duration of support, so that donor organ shortage results in increased complication rates and mortality. This was also the case in our series of MEDOS VAD patients. Thromboembolic events might be reduced by the use of acetylic acid or glycoprotein IIb/IIIa receptor inhibitors and with monitoring of the individual thrombocyte function in the near future,8–10 as are used in adult patients with VADs but not yet validated for pediatric patients. Furthermore, heparin coating of the inner pump chamber surface might lower thromboembolic complication rates and decrease the frequency of pump chamber exchanges. The most important design improvement after animal experiments was a new trileaflet polyurethane valve, which was designed as a blood-pump valve with an emphasis on opening behavior and flow resistance. Hydrodynamic performance and sufficient durability of this new valve were confirmed by various in vitro tests.3 However, the valve sinuses were the regions in which thrombus formation was found in most patients, and further improvement should focus on this area and on inadequate flow patterns within the device.
To achieve favorable results, the implantation should be performed under cardiopulmonary bypass conditions because manipulations at the atrial cannula can result in air embolism when the MEDOS VAD is operating. The use of a left ventricular apex cannula seems to be advantageous for drainage of the left side. This technique also provides easier access, no risk of accidental perforation of the atrial septum, and a decreased risk of neurologic complications compared with atrial cannulation.11,12 Furthermore, negative pressure can be applied without the danger of occlusion of the cannula by the atrial wall. If weaning is the primary goal, potential myocardial damage caused by the apical approach has to be considered.
Patients who underwent MEDOS VAD implantation after resuscitation more often suffered from complications, underlining the importance of an adequate timing for mechanical circulatory support and its impact on survival.
In summary, the MEDOS VAD is an effective device for circulatory support in pediatric patients for bridging to transplantation. To further improve results of circulatory support in pediatric patients, guidelines should be defined based on multicenter experience collected in a registry.
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