What is SleepImage™?
- What is CPC?
- How does it work?
- What is Frequency Analysis?
- What is the Image of Sleep?
- What are Advantages of SleepImage?
- What are the Limitations?
What is CPC?
What is "good" sleep? For the sleep specialist, good sleep might be defined as sleep with high efficiency, normal sleep architecture and the absence of sleep disruptions. For the patient, good sleep is waking up feeling refreshed. This vague description of sleep is directly associated with the subjective way in which sleep has been measured in the past. Unlike other aspects of our health, such as temperature, blood pressure, or weight, we could not objectively measure sleep - until now.
Now there is a simple and easy way to measure "sleep quality" at home that identifies how well we sleep. This new measure of sleep was developed by Dr. Robert Thomas and colleagues at the Beth Israel Deaconess Medical Center, an affiliate of the Harvard Medical School, is called cardio pulmonary coupling, and it is now available exclusively with the SleepImage system.
During normal, healthy, stable sleep the heart beat will slow down and speed up in tune with regular respiration. We call this cardiopulmonary coupling (CPC) - the "coupling" between the heart rate and the breathing rate.
However, during poor breathing the heart rate and the breathing rate become less "in tune" and the way the two signals are coupled together will change. This relationship, or synchrony, between the heart rate changes and sleep disruption is seen all the time in conventional sleep testing and is a well known pattern observed by sleep professionals as a person sleeps. However, repetitive sleep disruptions, caused most commonly by obstructive sleep apnea events, cause the heart rate to vary with the abnormal respiratory changes at a different frequency. Other sleep disrupting events also cause the heart rate and respiration to vary, such as pain, noise, medical conditions, leg jerks (periodic limb movements or PLMS) and other recurring arousals.
Dr. Thomas made use of these observations by recognizing that the normal heart variability coupled with respiration, occurred at a frequency range of 0.10 - 0.4 Hz. Within this range, called "high frequency coupling", stable Non-REM (Non-Rapid Eye Movement) sleep was observed. High Frequency Coupling reflects good or "stable sleep" and is associated with lower blood pressure and a strong, regular breathing rhythm.
Thomas also recognized that when there is poor or un-stable sleep, during disrupted breathing for example, the heart rate and breathing rate coupling occurs at lower frequencies levels in the 0.01 to 0.1Hz frequency range. He also observed that there is very little overlap between the high frequency "stable sleep" and Low Frequency Coupling, or "unstable sleep". This allows the two conditions, high frequency (stable sleep) and low frequency (unstable sleep) to be easily differentiated from one another.
The next step was to develop an analysis method that would take these changing frequencies and display them in a consistent way that could be easily seen and understood. For this Dr Thomas turned to Frequency Analysis.
How does SleepImage work?
In conjunction with the EKG data collected by the M1™ sleep recorder, the SleepImage system incorporates cardiopulmonary coupling (CPC) technology to measure patient sleep quality. The recorded data from the easy to apply M1 recorder is securely uploaded to the SleepImage.com website for review by a physician. The data can be uploaded by the Physician or the patient from any PC with a Windows based operating system. Information such as Body Position, Snore, Actigraphy, and an image of stable vs. unstable sleep, and indication of the presence of Obstructive, Complex, and Central sleep apnea is available immediately.
SleepImage is ideal for use in the sleep lab to screen patients for Sleep Apnea , but also as a tool to convince patients that they require a PSG sleep test. SleepImage is also a perfect screener for Anesthesia, and within the diabetic community as a wellness tool. To see examples on how SleepImage can make your business more profitable, please review the three case scenarios below.
What is frequency analysis?
The hard work is done by the software using variations of the Fast Fourier Transform algorithm (FFT). In other words, think of the FFT as a way of "curve fitting" data, that oscillates up and down, to a sine wave. The FFT finds the best fit sine wave to the varying heart rate data and respiratory data. The result is a mathematical representation of the actual wave form which can then be divided into its constituent parts. Sine waves are characterized by amplitude, frequency, and phase. By determining the sine wave characteristics of the heart rate data and the respiratory data, the analysis determines if they are indeed coupled, how strongly, and at what frequency. The result allows us to graphically distinguish between the "stable" (high frequency) and "unstable" (low frequency) sleep.
What is the Image of Sleep?
Signals that are in phase at the same frequency will add together to become high narrow peaks. Signals out of phase with mixed frequency will cancel out and show up as low and broad peaks. Displaying these over time results in the typical display of the CPC data called a spectrogram. Time is on the horizontal (or x-axis) and frequency is on the vertical (or y-axis).
Each black horizontal line represents a particular coupling frequency. If the line is horizontal there is no coupling, when the line jumps there is coupling, the higher the jump the more coupling is present. With all the high frequency coupling it is clear that this individual is sleeping well.
The image of High and Low Frequency coupling is referred to as a Spectrogram.
A Picture of Healthy, Quality Sleep
While this view looks at the data from the "front" we can also turn the graphic to look at it from the side to look at a more detailed view of the low frequency range. The graphic below shows the low frequency of the "front" view turned 90 degrees.
It can be seen that the coupling occurs over a range of different frequencies and is not very strong.
Looking at the whole night's spectrogram in this way gives an immediate visual picture of the sleep quality.
If we looked at a person with poor sleep the picture is very different.
A Picture of Unhealthy, Poor Sleep Quality
Virtually no presence of High Frequency coupling (stable sleep), with dominant presence of Low Frequency coupling (unstable sleep), indicating that this person had a very poor night's sleep. In addition if we look at the data from the side, or 90 degree angle, we see that the Low Frequency coupling has wide (broad) and varied frequencies that are typical of irregular sleep disruptions, called "Broad Band" coupling.
Another type of patient is displayed in the spectrogram below. The patient is clearly not sleeping well as a result of dominant Low Frequency coupling.
If we rotate the Low Frequency coupling data 90 degrees, we see a slightly different picture than the first example of poor sleep.
Instead of the widely distributed peaks in the previous example, this example shows that some of the peaks line up and form taller sharper peaks. This is because the coupling between the breathing and heart rate is much closer and more powerful. They are more in tune. This is caused by much more regular rhythms. These regular rhythms are typical of a breathing disorder known as Central Sleep Apnea (CSA) and a particular form of this called Cheyne Stokes. The breathing and heart rate are closely tied together and the result is taller sharper peaks that will line up over a relatively narrow frequency range and highlighted in this graphic.
In some patients with Obstructive Sleep Apnea (OSA), the CRA is unmasked by CPAP when the obstructive events are eliminated. The spectrogram shows a transition from the low frequency "broad band" coupling with low wide peaks, to the low frequency "Narrow band" coupling, tall narrow peaks (shown in red). This mixed picture has been called Complex Sleep Apnea Syndrome (CompSAS). This is a condition that is predominantly seen in patients that are being treated with PAP for obstructive sleep apnea and then start having these more dangerous CSA events. Up until the availability of cardio pulmonary coupling this was very difficult to differentiate without the therapy being in place.
As a result, in addition to identifying the quality of sleep, the spectrogram display of low frequency coupling may provide more specific information about the type of sleep disordered breathing.
Clearly, once you are oriented, this image of sleep will show that "a picture is worth a thousand waves."
Advantages of SleepImage
- Provides a very simple way to obtain information on sleep quality without requiring multiple leads and manual data analysis.
- Simple for the patient and physician with very little hookup required.
- Automated with analysis that occurs in less than 90 seconds
- Presents graphics and numerical displays of results
- Displays sleep quality and other sleep parameters over days, weeks, or months.
- Interpretation is simple and intuitive
Limitations of CPC
Although Sleepimage is effective on most people there are some limitations:
- The analysis cannot be used in patients with atrial fibrillation, ventricular bigeminy, demand ventricular or biventricular pacing or any condition that distorts the EKG signal.
- No specific respiratory event is identified, in other words it does not define specific events but rather takes a big picture of the sleep
- There is no differentiation between sleep disorders that disrupt sleep continuity.