What if a person who needs a pacemaker no longer had to undergo surgery to have one implanted?

Imagine having no wires threaded into the heart, no pulse generator implanted under the skin and, potentially, no repeat procedures to deal with implanted hardware?

Researchers have developed an experimental wearable pacemaker that uses focused ultrasound to stimulate the heart without placing a device or electrical leads inside the body. The system, developed by scientists from the University of Southern California, Massachusetts Institute of Technology and collaborating institutions, has been tested in engineered human heart cells and animals.

They report that they were able to control cardiac activity and restore normal heart rhythm in rats with abnormal heartbeats. They also carried out experiments using pig tissue to assess whether the technology could work at a scale closer to the human heart.

The findings, published in Nature Biomedical Engineering, could open a new direction in the treatment of cardiac rhythm disorders. But there is an important caveat: this is still experimental technology, not a replacement for the pacemakers used in hospitals today.

The device is described as a non-invasive ultrasound pacemaker, or NUP. It is designed to adhere to the chest and use focused ultrasound to reach the heart. This is a significant departure from conventional cardiac pacing.

Traditional pacemakers work by delivering electrical impulses to the heart, usually through leads positioned in or around the heart and connected to a pulse generator implanted under the skin. The new system uses sound.

More specifically, it relies on a technique called sonogenetics, in which specially engineered cells are made responsive to ultrasound. The researchers modified heart muscle cells, known as cardiomyocytes, so they could express a mechanosensitive ion channel called MscL-G22S.

When focused ultrasound reaches the modified cells, the channels respond to the mechanical force by opening. Calcium ions then enter the cells, calcium is crucial to the way heart muscle contracts.

What the researchers have done is to create a system in which sound waves can activate specially modified heart cells and trigger contraction.The result is something that looks remarkably different from the familiar image of a pacemaker.

No electrical wire has to reach the heart. Instead, ultrasound travels through the chest to stimulate the target cells. But getting sound into the heart isn’t enough. It has to get to the right place. The heart is constantly moving. It also sits beneath skin, fat, muscle and other tissues. Any system designed to stimulate it from outside the body therefore has to be capable of accurately locating and targeting cardiac tissue.

The researchers addressed this by combining ultrasound stimulation with imaging. Their wearable system can identify the relevant region of the heart and direct focused ultrasound towards it. According to the study, the system achieved spatial targeting of less than 1 millimetre and was capable of controlling stimulation frequencies of up to 9 Hz.

In rats with abnormal heart rhythms, the researchers were able to use ultrasound stimulation to restore sinus rhythm. That level of control is important because cardiac pacing isn’t simply about making the heart beat.

A pacemaker has to deliver stimulation in a controlled and reliable way. An inaccurate or poorly timed stimulus could be ineffective or potentially dangerous. The experimental system is therefore designed not merely to stimulate the heart, but to do so with considerable precision.

However, there is no ultrasound pacemaker available for patients today. The work remains at the experimental stage. Researchers reported safety testing in rats during daily activities over an eight-month period and carried out additional experiments using ex vivo pig models to examine the technology at a larger scale.

But success in laboratory models and animals does not establish that the technology is safe or effective in humans. There is also a fundamental requirement that makes this approach very different from simply sticking an ultrasound patch onto an ordinary heart. The target heart cells have to be made responsive to ultrasound. That involves genetic modification so the cells express the ultrasound-sensitive ion channel.

The researchers investigated the genetic safety of their approach, but moving from experimental studies to human treatment would require extensive additional research, clinical trials and regulatory approval.
Implanted cardiac devices have transformed the treatment of people with certain rhythm disorders. But implantation is still a medical procedure, and implanted hardware can bring complications, including problems involving leads and the need for procedures associated with device management.

A genuinely non-invasive pacing system could change that equation. Instead of putting hardware inside the body, doctors could eventually have the option of delivering cardiac stimulation from outside it.

The researchers’ system combines stimulation with ultrasound imaging. That creates the possibility of a future wearable device capable of doing more than simply delivering a stimulus. It could potentially monitor cardiac activity, identify an abnormal rhythm and respond automatically.

One wearable device could eventually combine monitoring, diagnosis and treatment. That would represent a major change in the way some cardiac rhythm disorders are managed.

For now, however, patients should not expect their cardiologist to prescribe an ultrasound patch in place of a conventional pacemaker. The technology has not yet made that leap. What the researchers have demonstrated is an early proof of concept that raises a bigger question about the future of cardiac pacing.

The future, however, may sound very different. It may not involve a wire inside the heart. It may involve sound travelling through the body.