Wireless Heart Monitor Fine-Tunes Cardiac-Failure Treatment
A new implanted electromechanical pressure sensor reduced the number of hospital readmissions for chronic heart-failure patients by 30 percent
By Katherine Harmon | February 10, 2011
A new pressure-sensing technology is helping doctors to read some of the
innermost secrets of the heart. This cardiac monitor uses electromechanical
dynamics to detect crucial pressure levels in a patient's pulmonary artery and
relay those readings to physicians wirelessly, obviating the need for clunky
moving parts or
external power sources.
For the some 5.8 million people in the U.S. faced with chronic heart failure, trips back to the hospital can be frequent, costly and dangerous. Medications and dosages are usually prescribed based on most recent clinical checkups, and health is monitored more or less via patient symptoms and weight fluctuations, which can indicate rapid fluid buildup in the heart—a condition that often precipitates heart failure.
But the new implantable wireless device might provide better early warning, research suggests. The information helps physicians track the patients' heart health and adjust medication accordingly, reducing the number of heart-related hospital visits. Results of the recent clinical trial, published February 10 in The Lancet, show that chronic heart-failure patients who were having their ticker tracked daily had 30 percent fewer hospital readmissions over six months (39 percent over the full 15-months of the study). The research was backed by CardioMEMS, the company that makes the device.
"This study represents a major advance in the management of patients with heart failure," says Gregg Fonarow, who is associate chief of the cardiology division of the University of California, Los Angeles, and was not involved in the research. "This trial opens the door to a new era where remote monitoring of hemodynamics can be employed to enhance outcomes of patients with heart failure."
Weak hearts, weaker measures
As more people survive heart attacks, the proportion of the population living with weakened hearts has grown over the past few decades, making chronic heart failure an increasingly common condition.
Patients with heart failure often wind up in the hospital when they start to feel short of breath, which occurs as excess fluids put pressure on the arteries and lungs. At that point, assessment and treatment is often expensive and invasive—for instance, pressure readings are taken with an inserted catheter.
"We're always fixing stuff that's already broken," says Jay Yadav, a cardiologist and CEO of CardioMEMS. Treating heart failure could be cheaper and easier if a doctor could detect a patient's deterioration before serious symptoms set in, he notes.
"Traditional methods of monitoring the symptoms and signs of heart failure are not very sensitive," Fonarow says. These measures, such as daily weight fluctuation or shortness of breath, are often noted too late to avoid a hospital stay or an invasive procedure. A 2010 telemonitoring study of more than 1,600 patients with heart failure who reported weight changes to their doctors found that this reading did not cut down on the number of deaths.
Instead the new monitors, Fonarow notes, "allows for proactive management to address fluid accumulation early and to potentially prevent hospitalization for worsening heart failure."
Its daily reports promise a much more specific and exacting opportunity to practice personalized medicine, tailoring treatment to individuals as their condition changes, notes William Abraham, a cardiologist at The Ohio State University Medical Center who is a consultant for CardioMEMS and co-author of the new study.
Sensing the pressure
Older implantable cardiac devices have a solid track record of keeping heart patients alive longer. But these devices need power sources, such as internal batteries (which require changing) or electrical leads (which can break).
The new device has no internal power source and no major moving parts so, as Abraham points out, "there's not much to wear out." It uses microelectromechanical systems (MEMS) technology to measure subtle changes in the surrounding artery's pressure.
The developers based the device on technology created to sense the pressure
changes in jet engines,
research that was funded by the Defense Advanced Research Projects
Administration (DARPA). They married that with advances out of the
Massachusetts Institute of Technology in passive wireless sensing, creating a
chiplike circuit that could be manufactured on silicon wafers. The result is a
small sealed sensor that is expected to stay functional after implantation
The device, which is 15 millimeters long and 3.5 millimeters wide, is implanted like a stent, entering through a leg vein. It is then dropped into the pulmonary artery, and blood pressure helps nestle it into place. The nitinol (nonmagnetic nickel–titanium alloy) anchoring loops hold it against the artery wall, where it does not seem to hamper blood flow.
Inside the sensor a glass membrane shifts ever so slightly (about a nanometer) as pressure outside the device varies. This atomic-scale movement changes the voltage across the central capacitor, which alters the amount of energy being transferred to an internal coil, thereby forming a resonating circuit.
The resonance is the key to obtaining the internal reading. Patients with the implanted device lay over a receiver console encased in a pillow. The base station uses what Yadav describes as "a ping-and-listen approach," sending out blips of radio-frequency energy 100,000 times a second. The sensor takes in this energy and resonates some of it back, conveying any changes in pressure with a shift in frequency over previous readings. The process takes about 18 seconds, ensuring that data are gathered across at least a few breath cycles, which can impact pressure readings.
The information is then sent via modem to a secure centralized database, and doctors can access their patients' information via a Web site. If a patient's artery pressure is detected to be rising beyond an individualized threshold, the system can send alerts out to the relevant doctor who can then decide whether to adjust medication levels or take another course of action.
Putting internal medical devices to the test can be tricky, especially when they require surgery. To avoid having to conduct sham surgeries and to keep the study blinded, the researchers implanted the sensor in all of the 550 trial subjects, but for the 280 control group individuals data were not relayed to their doctors. All of the participants would sync up daily and then receive feedback from their treating physicians, who had scripts to follow so they did not reveal whether or not they were actively receiving data. All patients were also monitored and treated based on standard care practices. Doctors used hormonal, diuretic and vasodilator drugs to help get high arterial pressure under control.
Patients who were being actively monitored during the trial had a greater reduction in pulmonary artery pressure, fewer hospital readmissions, more days outside of the hospital and a reported higher quality of life. If they were readmitted for heart issues, the monitored group's average stay was shorter (about 2.2 days) than the unmonitored group (3.8 days).
The new device, which Abraham estimates costs about $15,000, including implantation, could save money in the long run if it is approved by the U.S. Food and Drug Administration, which is currently reviewing the application. The in-hospital cost of treating heart failure in the U.S. tops $19.5 billion each year, and patients who die from heart failure spend more than $117,000 on average in hospital costs in the last six months of life alone.
The most recent study, in a companion essay published in the same issue of The Lancet, Henry Krum, director of Monash University's Center of Cardiovascular Research and Education in Therapeutics, noted that the study examined only a subset of heart-failure patients. And as Fonarow points out, "further studies will be needed to see if this wireless monitoring would also be beneficial in patients with milder heart failure."
The study also only used hospital admission as a measuring stick for the device's effectiveness. Researchers will have to examine more data to determine just what aspects of the monitored group's intervention helped prevent additional admissions—and whether the wireless monitor was effective in reducing the likelihood of dying from chronic heart failure.
But many in the cardiology field seem hopeful that similar technology will
help improve individualized medical care in the near future. As Krum concluded
in his essay: "We are only at the beginning of
this revolution in patient monitoring."
The sensor technology also has the potential to improve medical treatment for other common conditions. Similar MEMS sensors are already keeping tabs on stents implanted to treat aortic aneurysms. The sensors can also track healing in bone fractures and swelling in the brain after an accident or illness, Yadav notes, and could be useful in other conditions, such as liver disease or urological troubles.
Abraham envisions a future where implanted sensors could communicate with other internal devices, such as pacemakers, permitting fine-tuned calibration in real-time as a person's condition changes day to day, hour to hour, beat to beat.