New York, NY - Decreases in intrathoracic impedance related to fluid build-up in the chest, monitored by technology included in an implantable electronic device, may provide an early warning of impending decompensation in patients with heart failure, suggests a report in the August 9, 2005 issue of Circulation [1].

The study "is the first to confirm the feasibility and clinical usefulness of monitoring intrathoracic impedance in ambulatory heart-failure patients," write Dr Cheuk-Man Yu (Chinese University of Hong Kong, China) and associates. In their series of 33 patients with HF of NYHA class 3 or 4 who were implanted with such a monitor, they observed a continuous pattern of decreasing impedance consistently associated with later hospitalization for acute decompensation. In the hospital, impedance changes were closely tied to pulmonary capillary wedge pressure as well as the clearing of pulmonary volume overload or edema after diuretic therapy, according to the authors.

Implantable devices can collect a lot of data that clinicians don't really know how to use to the patient's advantage, observed HF-specialist Dr Michael Felker (Duke University, Durham, NC). But of the various kinds under study, intrathoracic impedance may be closest to potential use, he told heartwire. If it could be shown that impedance decreases were truly predictive of accumulating intrathoracic fluid and impending decompensation, something could be done about it ahead of time.

It's conceivable that there is something about the biology of heart failure that, once the process starts, it can't be headed off.

"That makes perfect sense, but it needs to be demonstrated. It's conceivable that there is something about the biology of heart failure that, once the process starts, can't be headed off," Felker said. On the other hand, the study from Yu and associates appears to be "proof of concept that it might work in a reasonably sized population of real-world heart-failure patients." The technique's sensitivity for predicting hospitalization for fluid overload in the study was about 77%. "That's reasonable but it's not great. It still misses a potentially significant number of exacerbations. So I think it's promising, but there's still a lot to be worked out before it can be widely adopted."

Once the device was implanted in the study's patients, all of whom had a history of significant HF-related events and were on optimal drug therapy, it measured intrathoracic impedance every six hours during chronic management and every 30 minutes during any hospitalization. Patients and their physicians were always blinded to the impedance data.

After a one-month interval for determination of impedance reference values, 10 patients were hospitalized for HF a total of 24 times over a mean follow-up of 20 months. The first exacerbation struck a mean of three days before hospitalization, whereas intrathoracic impedance started to decline an average of 15.3 days before exacerbations (p<0.001).

Diuretic therapy during all 17 CCU admissions was associated with a 17.1% rise in impedance and 45.1% decrease in pulmonary artery wedge pressure: p<0.001 for both improvements and for the interaction between diuresis-associated impedance and both wedge-pressure and net fluid loss.

The next step, said Felker, should be a randomized study comparing patients managed with or without the monitor or whose physicians do or don't have access to the impedance data. "That would be a way to demonstrate that it doesn't just provide information, but that it provides actionable information that makes a clinical difference."

The Medtronic impedance-monitoring technology, called OptiVol, is available in the company's InSync Sentry cardiac resynchronization therapy-defibrillator (CRT-D) device, which became commercially available last year. Although the Sentry's volume-status monitoring function is being studied in several ongoing clinical trials, patients in the trial of Yu et al were implanted with conventional pacemakers upgraded with the OptiVol feature.