The first such device reported in Circulation, manufactured by St. Jude Medical, measures the left atrial pressure directly through a transseptal transducer implanted from the leg area. This small pressure sensor is attached to the intraatrial septum (the wall between the two upper chambers of the heart) and is connected to a pacemaker-like device implanted under the skin of the lower abdomen. This device then radios its signal to a hand-held device (that looks remarkably like an old Palm Pilot organizer) that corrects for changes in barometric pressure and displays the pressure recordings. It’s measurements correlated well with Swan Ganz catheter pressure recordings of estimated left atrial pressure.
The second device reported in the Journal of the American College of Cardiology, uses a novel catheter-delivered pressure sensor that is chronically implanted in the pulmonary artery (the same site that harbors the Swan Ganz catheter when it is installed) called the CardioMEMS Heart Failure Sensor (CardioMEMS Inc., Atlanta, Georgia). This device consists of a 3-dimensional coil and a pressure-sensitive capacitor encased within a hermetically sealed, fused silica capsule completely covered in medical-grade silicone. Two wired nitinol (nickle-titanium alloy) loops prevent sensor distal migration in to the pulmonary artery branches. Alterations in the pressure inside of the artery alters the baseline resonant frequency emitted by the device and then, using an external antenna that powers the internal device using electromagnetic coupling non-invasively (i.e., it has no power requirements), the signal is transmitted outside of the body and transduced into a waveform. The external device similarly calibrates for external barometric pressure changes. This device, too, had excellent correlation to Swan Ganz pressure recordings.
Each are fascinating devices, especially for a biomedical engineering geek like myself.
But the most important question to be asked, especially as these devices break through the development hurdles, is will they improve the care we provide patients?
Recent experience with a similar device, the Chronicle implantable hemodynamic monitor (Medtronic, Inc.), failed to gain support from the FDA because the device did not demonstrate an improvement in patient outcomes when their heart failure management was guided by the device’s pressure recordings. It was difficult for physicians to determine from continuous recordings when the threshold for changing therapy was required. Will these newer devices reach a similar fate? Time will tell.
But one thing is certain: if these devices do not affect heart failure management outcomes, there will not be a market for them. This is not to say that our care of heart failure patients might not be enriched in a subgroup of difficult-to-manage patients using these devices: say, those with severe renal insufficiency. But the stakes are high for these companies to determine which patients’ lives might be benefited from their use and to design an appropriate prospective, randomized clinical study to prove the point. Perhaps Dr. Silver said it best in his editorial in JACC:
"...there really are 2 additional "overriding" questions from which we should not be dissuaded. We are always excited to welcome new technology to our diagnostic and potentially therapeutic armamentarium; we need, however, to be aware that such devices herald in a new era where serial recording of the patient in a 'compensated,' 'decompensated,' and every state in between may now be available. We need to sort out how and when to retrieve these signals, how to incorporate them into daily clinical routines and disease management strategies, and how to use information technology to help us decide what signals are the actionable early warning signs of a patient with a forthcoming clinical event. The devices without a suitable infrastructure will overwhelm the doctors and nurses monitoring these devices, and any potential benefit will be obscured by the burden of the data load."-Wes
Ritzema J, et al. "Direct Left Atrial Pressure Monitoring in Ambulatory Heart Failure Patients Initial Experience With a New Permanent Implantable Device." Circulation 2007;116:2952-2959.
Verdejo HE, et al. "Comparison of a Radiofrequency-Based Wireless Pressure Sensor to Swan-Ganz Catheter and Echocardiography for Ambulatory Assessment of Pulmonary Artery Pressure in Heart Failure." J Am Coll Cardiol, 2007; 50:2375-2382.
Silver MA. "New Approaches to Hemodynamic Measurement: Cool Devices But a Shaky Infrastructure" J Am Coll Cardiol, 2007; 50:2383-2384. (Subscription required)