A NEW DEVICE TO REPAIR THE ‘HEART STRINGS’
The mitral valve lies between the left ventricle and left atrium of the heart, and functions to prevent blood flowing back the way it came. But around 10% of people aged 75 or more suffer from ‘mitral regurgitation’1, where the valve fails to close properly and lets some of the blood leak back into the left atrium. This reduces the efficiency of the heart, which means it has to work harder – and the increased stress can eventually lead to heart failure.
Stretched heart strings
The mitral valve is connected to muscles in the wall of the heart by the ‘chordae tendinae’: tendon-like chords known commonly as the ‘heart strings’. Mitral regurgitation is often caused by the chords becoming stretched out, like a “swimming suit when you use it too much”2, says Costanza Culmone, a PhD researcher at Bio-Inspired Technology Group (BITE) at Delft University of Technology in the Netherlands.
Culmone is working on a device that could replace these chords – potentially without the need for open-heart surgery, which is a risky proposition in older patients. Such surgery typically involves stopping the heart while a machine takes on the job of pumping blood around the patient’s body – a procedure deemed too dangerous to be carried out on around 50% of people with mitral regurgitation3.
Several devices exist to replace the heart strings without the need for open-heart surgery4, but nobody has yet mastered the holy grail of minimally invasive ‘transcatheter’ repair, where the repair device is threaded along a blood vessel to the heart. By incorporating design constraints adapted to transcatheter delivery, Culmone’s eventual aim is that her device – known as ChoRe – can be made small and flexible enough to be delivered via a catheter. “It’s quite difficult,” says Culmone, “But it’s possible.”
The ChoRe device
The ChoRe prototype that Culmone developed consists of the delivery device and the implant itself – a replacement chord made of polytetrafluoroethylene (ePTFE) and a pre-tied knot made from polyester thread. Each faulty chord is replaced in a multi-step procedure. First, the ChoRe device fires the replacement chord from the inside of the heart through the ventricle wall. On the far side, an accordion-like device called a pledglet collapses to firmly anchor the new chord.
Then, the device draws the thread up and clamps it onto the leaflet of the mitral valve itself via the pre constructed knot. The ChoRe device then allows the length of the replacement chord to be adjusted before the knot is drawn closed.
This pre-tied knot that is used to fix the artificial cord to the valve is a key part of Culmone’s device, and the part that was hardest to get right. “The main issue for this procedure, in general, is the fixation on the [valve] leaflets,” says Culmone. “Because they are really thin, and they're really damaged. It's quite difficult.” The pre-constructed knot that Culmone used was inspired by a suture-closure device she had read about in the literature5, and consists of multiple loops that can be tightened to ensure the cord is the correct length.
Ex vivo testing
Culmone and her colleagues created a larger than normal prototype ChoRe device using 3D printing. “For this project, making the instrument using a conventional manufacturing process would have cost a lot of time – it can take years. With 3D printing, if it doesn't work, you can just modify your design and then make a new one.” That said, Culmone recognises that 3D printing has its limitations, particularly with creating very small and detailed designs. She made her prototype twice the normal size to compensate for this.
The team then tested the 3D-printed prototype in ex vivo bovine hearts6. They successfully attached 10 replacement chords using the device, with around 3 minutes needed to attach each chord. The next steps will be to improve the device design, especially how the replacement chord is fixed to the valve after adjustment of the chord length.
Then the team plan to produce a “real life-sized” prototype and perform further animal tests. When it comes to shrinking the device to a size that could be used for human trials, Culmone says that she will have to rely on conventional manufacturing techniques. Nevertheless, she is excited by how quickly the 3D printing field is developing. “It will get better and better over the years. They're also trying to produce biocompatible materials for 3D printing, which I think is key. The potential is so high.”
Culmone’s device was partly inspired by conversations with her house mates. “I was living with some other PhDs from other departments, even faculties,” she says, including an aerospace engineer and an electrical engineer, as well as an architect. “We were speaking about my project, and I really found inspiration from them. It seems weird, but when you brainstorm, and you have people that don't really know anything about what you're doing, they have a wider view of solutions.” She recommends that other would-be inventors search outside their field for inspiration. “Have a look around – sometimes it’s really inspiring. Speaking with other people is important.”
WHEN YOU BRAINSTORM, AND YOU HAVE PEOPLE THAT DON'T REALLY KNOW ANYTHING ABOUT WHAT YOU'RE DOING, THEY HAVE A WIDER VIEW OF SOLUTIONS.
Nkomo, V. T. et al. Burden of valvular heart diseases: a population-based study. Lancet. 2006; 368: 1005–1011.
Interview with Costanza Culmone, September 2019.
Mirabel, M. et al. What are the characteristics of patients with severe, symptomatic, mitral regurgitation who are denied surgery? Eur Heart J. 2007; 28: 1358–1365.
Seeburger, J. et al. Off-pump transapical implantation of artificial neo-chordae to correct mitral regurgitation: the TACT Trial (Transapical Artificial Chordae Tendinae) proof of concept. J Am Coll Cardiol. 2014; 63: 914–919.
Ramponi, F. et al. Total percutaneous cardiopulmonary bypass with Perclose ProGlide. Interact Cardiovasc Thorac Surg. 2011; 13: 86–88.
Culmone, C. et al. ChoRe: A device for trans-catheter chordae tendineae repair. Proc Inst Mech Eng H. 2019; 233: 712–722.