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 indefinitely.
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.