Monitoring Sleep one Z at a Time Monitoring Sleep one Z at a Time
National Institute of Biomedical Imagining and Bioengineering

A few good zzz’s make you feel better during the day, help your brain nimbly accomplish mental tasks, and hone the immune system to fight disease. But for the nearly 70 million Americans who suffer from sleep disorders, their sleep offers little chance to dream. In the U.S., sleep deprivation and disorders carry a price tag of over $100 billion annually in lost productivity, medical expenses, sick leave, and property damage. The National Highway Traffic Safety Administration estimates that 71,000 injuries each year are related to drowsy drivers.

Poor-quality sleep often results from sleep apnea – a condition in which individuals stop breathing for short periods of time throughout the night. Sleep researchers are now giving considerable attention to how and why this condition occurs. According to the American Sleep Apnea Association, sleep apnea is as common as adult diabetes, with as many as 12 million diagnosed cases in the U.S. Many more go undiagnosed and untreated.

This sleep spectrogram was produced using novel sleep quality analysis software. The upper part of the figure shows two views of central sleep apnea during a single night. The heart rate changes and breathing line up in single file, like a one-note marching band. This pattern, called "narrowband" coupling, predicts that continuous positive airway pressure treatment is at high risk of failing. The lower part of the figure shows two views of relatively pure obstructive apnea during a single night. The heart/lung couplings have a more "broadband" or variable pattern. Continuous positive airway pressure will likely work well.

Assessing individuals who suffer from poor sleep may get easier in the coming year as a new device based on technology developed by researchers at the Beth Israel Deaconess Medical Center in Boston finds its way to sleep labs and doctors’ offices. The new device, designed by Embla Systems, a Denver-based sleep diagnostic equipment firm, may also help cardiologists more precisely track heart failure and track therapies that reduce its impact since heart failure is often associated with poor-quality sleep. Because of the device’s small size and portability, it may also permit at-home monitoring of sleep quality.

The Heart, Lung, and Sleep Connection

When patients have trouble sleeping, often the first question asked by clinicians is “How do you feel?” This is not necessarily the most precise approach since many patients who experience poor sleep may get used to feeling fatigued. The next step may be to initiate a sleep study, which requires that patients go to a special sleep lab where they will stay overnight. Connecting a patient to the 16 to 20 electrodes takes about half an hour. Sensors are placed on the head, face, chin, torso, and legs. Belts wrap around the patient’s chest and abdomen with tubes positioned at the patient’s nostrils, and a probe attaches to the finger. Additional sensors may be placed in the nose, upper lip, and neck.

“Sleep studies are intrusive and not carried out in a patient’s natural environment,” says Ary Goldberger, a cardiologist and co-developer of the new technology. “The cost of the studies is also a problem.” An initial study carries a price tag of up to $2,500. To reach a larger population, Goldberger’s team – including Joseph Mietus, a bioengineer, Chung-Kang Peng, a statistical physicist, and Robert Thomas, a sleep researcher – wanted to develop a point-of-care device that could easily travel from a doctor’s office to a patient’s home.

Working with Embla Systems, the team reduced the number of sensors that attach to a patient’s chest to just the three needed to obtain a single-lead, continuous electrocardiogram that measures the heart’s electrical activity. An algorithm (a set of mathematical equations) developed by Goldberger’s team analyzes the information transmitted from the body and provides a three-dimensional graph, called a sleep spectrogram, showing heart fluctuations and breathing rates and how the heart and lungs talk to each other during sleep. “When analyzed in this new way we can uncover the physiology of sleep,” says Goldberger.

When we sleep, we alternate between periods of stable and unstable sleep. These states correspond to changes in breathing patterns and heart rate. By capturing the dynamic relationship between breathing and heart rate, called cardiopulmonary coupling (CPC), the researchers create a map of sleep throughout the night rather than simply counting how many times a patient stops breathing. “The sleep spectrogram shows unstable or poor-quality sleep and stable or high-quality sleep, and allows us to dissect different causes of apneas [which occur during unstable sleep],” explains Goldberger. Stable or good sleep is associated with high-frequency coupling between the heart and lungs where heart beats are in synch with each breath. Unstable or poor-quality sleep is associated with low-frequency coupling in which breathing and heart rate coupling occurs over periods of 20-50 seconds.

The automated analysis provides “ruthless neutrality” when it comes to assessing sleep, the researchers say. Traditional sleep studies keep track of such sleep parameters as when the patient fell asleep, the number of times breathing was interrupted, how often the patient was roused, and when the patient moved into a deep sleep. The study data are then rated by a sleep technologist. “If you had seven different technologists, you would get seven different analyses,” says David Baker, President of Embla Systems. Goldberger, Thomas, and Baker point out that the new device is not meant to replace conventional sleep studies, but to complement them.

Monitoring Heart Failure

There are three kinds of sleep apnea: obstructive – caused by a blockage (usually soft tissue in the throat that collapses); central – occurs when the brain fails to signal the muscles to breathe (associated with conditions such as heart failure); and mixed or complex – a combination of the two. With the ability to detect all three, Goldberger and Thomas have extended their studies to examine the impact of drug and alternative therapies on heart failure, a progressive condition in which the heart’s ability to pump blood throughout the body decreases.

No method currently exists to monitor the sleep of heart failure patients, but Goldberger thinks that sleep is a golden opportunity to track whether a drug is helping or hindering a patient’s condition. “The amount of good-quality sleep for anyone with substantial heart failure is severely reduced,” he says. “This approach would allow you to treat a patient [or change drug therapies] before they ended up in the emergency room or intensive care unit,” explains Goldberger.

A recent study completed by Gloria Yeh, Goldberger, Thomas, and colleagues assessed the effects of 12 weeks of a Tai Chi exercise program on the sleep quality of heart failure patients. Using sleep spectrograms, the researchers found that the exercise program improved sleep stability and that more stable sleep may have a beneficial impact on blood pressure and other heart-related functions. “This technology gives us a way to quantify what’s happening physiologically between the heart and lungs through the window of sleep,” says Thomas.

Beyond Clinical Use

The new device, which debuts at the Sleep 2008 meeting in Baltimore, will be delivered to sleep labs, sleep physicians, and primary care physicians in the U.S. by the end of this year. The device, currently about the size of a cell phone, would include data storage so that information taken through the night could be transmitted to a doctor’s office. This would be of particular use for the 13 million U.S. patients who must wear masks attached to continuous positive airway pressure machines that blow air down the patient’s airway. Weight gain or loss over time can alter pressure needs. The new device would allow clinicians to remotely monitor the sleep quality of those patients and change their pressures as needed rather than requiring patients to return to a sleep lab for assessment. Baker anticipates medical insurance reimbursement for the device for in-home use. He also sees an opportunity for the device in sports. Teams could track how well the team or key players sleep, and then manage their schedules for optimal performance. “We’re creating a paradigm shift in how sleep management is done,” says Baker. “Everybody sleeps. Wouldn’t you want to know the quality of that part of your life?”

This work is supported in part by the National Institute of Biomedical Imaging and Bioengineering.