With wider adoption of mechanical circulatory support and subsequent longer duration of expected support, it is imperative to maximize the durability of not only the device, but also its peripheral components. In addition, because of the inevitability of device and component failures, it is critical to understand the incidence of these events, spectrum of interventions that are available, and the outcomes after these interventions. Clinical trial outcomes of the HeartMate II demonstrate no observed bearing or pump failure and a low need for pump replacement (6 events per 100 patient-years of support).1–4 Damage to the percutaneous driveline accounts for almost half of all HeartMate II pump replacements, which has been observed to occur in 3% of all HeartMate II implants.4 In the postclinical trial era, the incidence of percutaneous driveline has been reported to occur in 9.2% of HeartMate II implants with percutaneous driveline failure of the externalized portion of the cable observed in 87% of cases.5 Percutaneous driveline fracture is often preceded by trauma, overuse/bending of the driveline, aggressive tug, or excessive postimplant weight gain placing strain on wire integrity. Although this incidence is decreasing and the majority of lead failures are amenable to simple repair, understanding the durability and outcomes of patients after these interventions are important to optimize long-term outcomes.
Because driveline repair durability and impact on outcomes have not been specifically analyzed, the aim of this study was to 1) identify patients requiring driveline repair for fracture after HeartMate II implantation and 2) analyze outcomes, including intervention required, need for reintervention, and impact on patient survival.
The data collection process and analysis was performed after informed patient consent and approved by the University of Michigan and Mayo Foundation Institutional Review Board. From May 2004 to August 2014, 734 patients underwent continuous flow left ventricular assist device (LVAD) implantation at the centers that comprise the Mechanical Circulatory Support Research Network (University of Michigan Health System, Mayo Clinic College of Medicine, and Vanderbilt Heart). Of these, 560 patients (465 men) underwent primary VAD implantation with a HeartMate II. Median age at operation was 59 years (range 18–82 years).
Preoperative clinical characteristics are presented in Table 1. Early death occurred in 41/560 patients (7.4%). Follow-up was available in all 519 early survivors for a median of 13 months (maximum, 10.4 years). Patients requiring driveline repair to the external portion of the percutaneous driveline were identified and outcomes were analyzed according to the type of driveline dysfunction, intervention required, durability of repair, and impact on survival. At the time of implant, median age was 52 years (range 22–76 years), 11/17 patients were male and 8 were implanted as destination therapy. A history of a precedent traumatic event was individually sought in the medical record, once patients were identified. Adjudication of lead dysfunction and clinical outcomes were performed by the Principal Investigators at each institution, which collectively comprise the Mechanical Circulatory Support Research Network (FDP, JMS, SM). This study spans an era during which two design modifications were made to the percutaneous driveline. In June 2007, the external connector bend relief was made at its connection to the controller and in December 2010, the internal pump-end bend relief was reinforced.5
Approaches to driveline fracture depended on patient presentation and location of fracture; these included driveline repair (clamshell, silicone tape reinforcement), device exchange, and connection to ungrounded cables.
Follow-up information was obtained from subsequent clinic visits, and written correspondence from local physicians. Data were expressed either as mean ± standard error of the mean for normally distributed data or median with range for nonnormally distributed data. Proportions were made for categorical data. Inclusion was defined as any patient requiring intervention for percutaneous driveline fracture. Actuarial survival was determined using Kaplan-Meier analysis. Early operative mortality was defined as death occurring within 30 days of operation or at any time during the index hospitalization.
All patients underwent HeartMate II implantation through a midline sternotomy; operative data are presented in Table 1. Follow-up was available in all early survivors (n = 519) for a total of 940 patient-years of support (median 1.1 years, max. 10.4 years). Overall survival in this cohort was 83% at 1 year, 61% at 3 years, and 48% at 5 years and is presented in Figure 1. Percutaneous driveline fracture was identified in 17 patients (3.3%), eight of whom were implanted as destination therapy; an antecedent history of driveline trauma was elicited in all patients. The percutaneous driveline fractures were not associated with either the external connector bend relief at the controller or the internal pump-end bend relief. Furthermore, events did not correlate to earlier era before structural design with only two driveline fractures occurring before the second change in lead design in 2010; 5/239 (2.1%) occurred before 2010 and 12/321 (3.7%) occurred after 2010. All events included fractures of the silicone-coated portion of the driveline itself. Median time from LVAD implantation to driveline fracture was 1.3 years (range, from 4 months to 3.8 years). Two of these patients underwent device exchange as primary treatment, while 15/17 (88%) underwent driveline repair. Three of these 15 patients (20%) required a driveline reintervention, including device exchange (n = 1), rerepair (n = 1), and ungrounded cables (n = 1). Flow of treatment approaches employed in these patients is presented in Figure 2. Median time of support after driveline repair was 10 months (range, from 3 months to 5.4 years). There were no late deaths after driveline repair during the follow-up period with 14/15 patients (93%) active on support and 1 having undergone transplant.
Total events included 17 first-time events and 3 second-time events for a total number of 20 events; this resulted in an overall raw incidence of 20/519 (3.9%) for a total of 0.02 events per patient-year of support.
This study demonstrates a very low incidence of percutaneous driveline fracture after HeartMate II implantation (3.9%) and very good durability and survival after driveline repair. We did not observe involvement of either component of the percutaneous driveline, which was the subject of structural changes (external connector bend relief at the controller in June 2007 and internal pump-end bend relief in December 2010). Furthermore, we did not observe any difference in incidence of driveline fractures correlating to the differences in eras associated with these design changes with the majority of events occurring after 2010. This is in contradistinction to reports documenting a decrease in incidence after design changes mentioned before.5 However, similar to prior reports,5 the majority of lead fractures in our cohort were minor and amenable to simple repair with a low need for reintervention. Although in the minority, some patients may present in a fashion that necessitates urgent device exchange6 or emergency reconnection of driveline.7 Both percutaneous driveline fractures and successful driveline repair have been reported in other mechanical circulatory support devices, but these are limited to case reports.8–10
The HeartMate II driveline consists of six total wires paired in groups of two: yellow-green, brown-black, and red-orange (Figure 3). The impeller movement is driven by three magnets associated with the wire pairs. If any one wire in the group of two wires breaks or has a shielding defect, the pump can still function. If two in a pair break or have shielding defects, the pump will stop. Fractured drivelines or shielding defects may be preceded by trauma to the driveline (may see breaks in the silicone sheath covering the driveline or prior rescue tape application), overuse or excessive acute angle bending of driveline, aggressive tugging of driveline, or excessive weight gain straining wire integrity.
The original controllers (EPC) powering the HeartMate II did not have a built in mechanism within the controller to assess for driveline integrity. New controllers (PC) have an added function that sends a signal from the controller to the pump every five seconds to assess driveline integrity. If the controller senses a current change in the driveline wire that is sustained beyond an accepted tolerance level, the controller will alarm and display a driveline fault alarm. Because of PC sensitivity, faulty driveline alarms have been reported. If a patient has not experienced a faulty driveline alarm within 30 days of implant, it is less likely that they will experience a faulty driveline alarm.
Patient presentation can vary when a fractured percutaneous driveline is sustained. EPC patients with a single wire defect will not notice anything unusual when they are connected to battery power (DC). When they connect to wall power via their power module (AC), they will experience low flow alarms, speed drops below their low speed setting, which is often set to 600–1000 RPM below their fixed speed, and may feel syncopal during periods of wire separation or shielding defects. PC patients will also experience these symptoms but will additionally have a driveline fault alarm displayed on the controller screen regardless of whether they are on AC or DC power. If the wire defect is positional in nature and the patient is able to change position or move the driveline in such a way as to alleviate the shielding defect or cause the wire to reconnect, normal VAD function may temporarily be restored. If both wires in a pair are fractured, the pump will stop unless the patient is able to change positions or move the driveline in such a way that the wires touch enough to maintain a connection.
Intervention for percutaneous driveline fracture may also vary depending on the finding and patient presentation. EPC and PC patients may feel syncopal if they are not disconnected from wall power and transitioned to batteries. If patients have a single wire defect in any pair and are transitioned to batteries (DC), there should be resolution to speed drops because it is the grounding with AC power that causes pumps stoppage. Patient safety must be considered when deciding whether to download log files as this can only be done on power module. EPC patients will likely not experience alarms once transitioned to batteries if only one wire in a pair is involved. PC patients will likely not experience low flow alarms or speed drops but will still have a driveline fault alarm displayed on batteries. Because PC patients may experience driveline fault alarms that are just a reflection of an overly sensitive controller, it is helpful to assess patient stability. If the patient is not symptomatic and near a VAD center, it may be helpful for the patient to report to the nearest VAD center to have the log files downloaded and sent to the device company for assessment. The Heartline can be called to determine whether the log files are reflective of a genuine driveline fault requiring intervention. If there is nothing to suggest disrupted integrity of the driveline, the driveline fault alarm can be cleared or silenced. This does not guarantee that a future faulty driveline alarm may occur. Alternatively, the controller can be exchanged for the patient’s backup controller. One important factor to consider before exchanging controllers is awareness of whether the patient has a patched aortic valve or very little cardiac reserve. Should driveline fault alarms not recur on the new controller, it can be presumed that the driveline fault was an error of the initial controller. If the alarm persists, it is more likely that the driveline fault alarm is not an over sensing controller and may be genuinely reflective of driveline discontinuity.
The VAD center may consider application of splint taped over the driveline at area of concern if a visible anomaly is noted and should advise the patient to limit mobility if positional changes prompt alarms. It is important to contact the VAD device company representative to have a technician potentially flown to the VAD site to assess the driveline for repair. The technician will be able to isolate, which wire or wires are fractured if readily identifiable. This may be more challenging if the alarms were intermittent and it is unknown which positions prompt alarms.
If a fracture exists in the external part of the driveline, the driveline will be transferred beyond the area of discontinuity one wire at a time to a new driveline. If an internal fracture exists, the patient may require pump exchange or an ungrounded cable, which mimics battery support but allows patients to use wall power. It is important to remember that an ungrounded cable only “band-aids” the issue and does not correct the shielding defect or fracture. Rather, it eliminates the grounding effect of AC power. Our center has a few patients that have safely been maintained on ungrounded cables for years. It is important that the patient brings the ungrounded cable with them for any clinic visits or for rehospitalization as they cannot safely use a grounded cable that other VAD patients use.
This study has several important limitations that need to be acknowledged. Although the data were prospectively collected, it was retrospectively analyzed and, as such, possesses the inherent bias that is associated with this study design. This is basically a descriptive study with small number of events and firm conclusions may not be able to be drawn from this dataset. Offsetting these limitations is the fact that this is a large, multiinstitutional experience.
In summary, we demonstrate a low incidence of percutaneous lead fracture after HeartMate II implantation and excellent durability after driveline repair with uniform patient survival. The majority of lead fractures are amenable repair with a minority requiring device exchange or cardiac transplantation.
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Keywords:Copyright © 2017 by the American Society for Artificial Internal Organs
Driveline; driveline fracture; mechanical circulatory support