Using information hidden in the beat-to-beat changes of the heart's electrical signals, researchers at Beth Israel Deaconess Medical Center (BIDMC) have developed an inexpensive method to assess the stability and quality of sleep, which could be used to help understand the mechanisms of sleep control and diagnose sleep disorders, as well as to test the efficacy of sleep aids and other medications.
Known as a "sleep spectrogram," the novel graph is based on data obtained solely from a simple electrocardiogram (ECG). The spectrogram is described in a study in the Sept. 1, 2005 issue of the medical journal Sleep, which currently appears on-line.
"This new ECG-based approach is important because it promises to provide an affordable and readily achievable way to monitor sleep stability in a wide range of conditions, including sleep apnea, depression, fibromyalgia, heart failure and stress," explains cardiologist Ary Goldberger, MD, Director of the Margret & H.A. Rey Institute for Nonlinear Dynamics in Medicine and the study's senior author.
The new study, led by sleep researcher Robert Thomas, MD, of BIDMC's Division of Pulmonary, Critical Care and Sleep Medicine, identified two distinct types of behavior exhibited throughout the course of a person's sleep, the first being stable and restful, the second being unstable and aroused. The results show that conventional approaches to categorize non-REM (non-rapid-eye-movement) sleep into grades of depth do not capture this potentially important dimension.
"Among healthy adults, physiological behaviors will show relatively abrupt shifts – a occurring over minutes – between both stable and unstable sleep, but the stable stage clearly dominates," explains Goldberger, who is also a Professor of Medicine at Harvard Medical School. "But," he adds, "in a variety of disease states, the spectrogram shows that an unstable sleep pattern is predominant, and among patients with severe cases of sleep apnea, virtually all of the patient's non-REM sleep is unstable."
The creation of the spectrogram could serve as an important complement to traditional sleep staging, which shows cycles of rapid eye movement (REM) and non-REM sleep and is obtained through polysomnography, a series of measurements that require the use an electroencephalogram (EEG) to record patients' brain waves.
"Spectrograms can uncover information that is not provided by traditional sleep scoring," notes Thomas. "Polysomnograms are both expensive and time-consuming. The spectrogram, therefore, could provide a new way of looking at sleep and offer doctors an alternative to the REM/non-REM sleep scoring system.
Furthermore, we have more recent data showing that the patterns in humans and rodents are remarkably similar, and suggesting that what we may be observing is a fundamental and conserved sleep mechanism."
In addition to Goldberger and Thomas, study coauthors include BIDMC engineer Joseph Mietus and C.K. Peng, PhD, of BIDMC's Cardiovascular Division.