Removing the Tether: Improving the Patient’s Quality of Life with a VAD
June 1, 2015 - Lori Lucke, Systems Engineering Fellow; and Vlad Bluvshtein, Senior Electrical Engineer
The growing patient population with advanced heart failure – and the limited number of donor organs – has stimulated the use of ventricular assist devices (VADs). However, postoperative management of VADs and their support systems can be challenging and costly.
Patients with severe heart disease may be implanted with a VAD, a pump that is placed inside the chest to help the heart circulate blood to the body. The current design tethers the patient to an external system controller which connects via a percutaneous cable from the outside of the body through the skin to the implanted pump as shown in Figure 1. The percutaneous cable is a leading cause of infection for VAD patients.
Since 1996, Minnetronix has been working to improve the quality of life for VAD patients through next-generation VAD development. Our efforts have focused on:
- Eliminating the percutaneous driveline and its related complications
- Minimizing the inconvenience of cables
- Improving usability and patient comfort
The Minnetronix transcutaneous energy transfer system (TETS) is designed to replace the percutaneous cable to reduce the infection risk as shown in Figure 2a. Using a set of power transfer coils, one external, and the other implanted, our TET system eliminates the need for the percutaneous cable. The external coil magnetically couples through the skin to the implanted coil to transfer power. To support the coils, the external system controller is split into two controllers, an implanted VAD controller and an external power controller. The patient wears the external power controller that is attached via a cable to the external coil. The implanted VAD controller connects to the implanted coil. The small, flexible, cool coils support continuous power transfer to the VAD with no tissue damage. The patient coil set implements the inductors in an “air” gap power transformer where the “air” gap is replaced by the skin barrier as shown by the schematic representation of the system in Figure 2b.
Improving patient comfort has been a driving force behind Minnetronix’ enhancements to the TET system. A significant inconvenience with the current TET configuration is the external power controller. In addition to adding extra bulk and weight, the controller makes sleeping especially difficult. To minimize discomfort and sleep disruption, Minnetronix has developed an additional TET configuration to solve this problem utilizing a distributed transformer (patent pending).
By implementing a distributed transformer, shown in Figure 3a, it is possible to extend the power transfer to other parts of the body and adapt the coils’ shape and size. In this way, the external controller can be moved away from the patient. The power transfer occurs in two locations with the first occurring between the patient coil set. The second occurs using another configurable coil set. The distributed transformer consists of the two coil sets connected by a capacitor network. The advantage with this system is that the second set of coils no longer has a fixed physical size. They can be designed in multiple configurations to allow the patient freedom from the direct connection to the power controller. By arranging the location of the components within the distributed transformer, the patient only needs to wear a single cable and coils as shown in Figure 3b.
One example of the distributed transformer is shown in Figure 4a. With this approach it is possible to design a system that provides freedom from a direct cable connection to an external controller. A patient could easily roll over while sleeping without losing power to the VAD. For this to work properly, the configurable coil set is designed as follows. Coil B is shaped as a large coil that can wrap around the patient’s waist to form a belt. Coil A becomes a set of coils that are placed in an array on top of the patient’s bed. The external power controller is connected to coil array A in the patient’s bed. The power transfer occurs first between a coil within coil array A to coil B located around the patient’s waist. From there, the power is transferred through the patient’s skin via the patient coil set. An example of this configuration is shown in Figure 4b. Since coil B is wrapped around the patient, the patient can roll around the bed and always maintain close proximity to a coil within the array of coils in coil set A. The belt gives the patient rotational freedom when sleeping on a mattress. The power controller is tethered to the array in the mattress rather than to the patient. Only a single coil in the coil array needs to be active to transfer power to the patient. An additional advantage of this method is that the radiation of power occurs only at the coil pairs, rather than across large distances, thus reducing exposure to large magnetic fields.
The distributed transformer approach provides many meaningful patient benefits, improving ease-of-use and quality of life compared to current VAD systems. The approach is not limited to just a sleeping application but can also be designed for similar use in a chair. More comfortable methods for storing and carrying the external controller can be developed, including a backpack or purse. The distributed transformer system remains compatible with the single coil pair system. The patient can remove the intermediate coils and return to the configuration with only a single external coil.
Since our early work with the LionHeart VAD system and the Penn State Total Artificial Heart, Minnetronix continues to raise the bar for power and control of mechanical circulatory support systems. In addition to our proprietary TET system that enables a full-implantable VAD system, we developed the distributed transformer approach. Minnetronix remains committed to improving the quality of life for VAD patients by providing innovative, patient-centric, next-generation VAD development.