Polysomnography(PSG) has been the standard approach to diagnosing OSA. PSG consists of recording a number of physiologic parameters on the patient while they sleep. A typical set of parameters include:
However, this test is very expensive and is usually only available at major medical centres. Also, you have to sleep in the hospital and someone will be watching you as you sleep to record any unusual behaviour and to adjust the equipment when necessary. You may not have your usual night's sleep under these circumstances.
You should be aware that different labs may record different sets of parameters. Different labs may also use different criteria for determining when an abnormal event has occurred. Experts may also disagree on how to score the sleep stages.
Some labs have protocols where you are constrained to sleep in certain positions for a set amount of time. Finally, as a cost-cutting measure, you may be subjected to a so-called split-night study where your sleep is recorded for the first 4 hours. If this part of the recording shows clear evidence of OSA, you are started on a trial of CPAP on the same night. Now that dental appliances are available for the treatment of OSA, some labs may start to do split-night studies using these devices in place of CPAP.
In the US, a PSG costs you or your insurer ~ $2,000. The insurer will usually only pay for a single PSG for any policy holder. In Canada, the PSG is often covered by Medicare.
In an attempt to address the problems of cost and access, a number of home monitoring devices have been developed for use in diagnosing OSA. Although the medical profession has been slow to accept this technology, economics and the fact that the majority of patients with OSA remain undiagnosed and untreated are driving a shift towards greater use of this technology. In fact, Dr. J.R. Stradling, a leading OSA researcher in the U.K. now says that:
"Such simple sleep study techniques have always been compared with PSG. However, this is incorrect. Full PSG missed the partial forms of sleep apnea (snoring induced arousals for example) and is not a good gold standard. The correct way to evaluate sleep monitoring equipment is to assess its ability to recognize those patients with sleep related upper airways problems that will respond to definitive treatment. In future, response to treatment (such as nasal CPAP) is likely to be the gold standard against which the success of diagnostic tests is judged."
His opinions are contained in an article that is soon to be published in Sleep and are sure to generate controversy.
Home sleep monitoring can vary in complexity from oximetry alone to the recording of 6 or more parameters, including EEG, and EOG. Most systems do not include the EEG or EOG since sleep staging is not that important in diagnosing the typical patient with OSA. Treatment is based on clinical findings and the severity of the OSA as reflected by the AHI (Apnea/Hypopnea Index - see below). These patients have no difficulty falling asleep and fall back to sleep quite quickly even when waking up several times at night. Therefore, one can make the assumption that time in bed is nearly equal to time asleep when calculating the severity indices. Differentiation of REM from Non-REM sleep is important for the small number of patients who have so-called REM-Specific OSA. Some insurers will pay for home sleep monitoring, many do not. However, this situation will change dramatically in the near future with managed care. A large HMO in Puget Sound, Washington is already using home sleep monitoring as the principal method for diagnosing OSA.
|A Typical Home Monitoring Device for use in Patients with OSA||A Sample Recording Showing Severe Sleep Apnea|
At the Vancouver Sleep & Breathing Centre, we record the following parameters during home sleep monitoring (Channels are as shown in the sample recording above):
This allows us to confirm the diagnosis of OSA and grade the severity in terms of the Apnea/Hypopnea Index (AHI). The AHI is calculated by counting up the number of apneas and hypopneas that occur during the night and dividing by the time in bed. We make the assumption that time in bed is a good estimate of time asleep. This is a good assumption in patients with OSA who have no difficulty falling back to sleep even if they wake up several times during the night.
When the AHI is greater than 15, the patient is likely to complain of excessive daytime sleepiness.
When the AHI is greater than 30, the patient is at increased risk for cardiovascular complications. These patients are also likely to show rapid progression of their disease if left untreated.
The AHI must also be interpreted in the context of the individual patient. For example, a patient with heart disease who has an AHI of 20 needs closer follow-up than one who has no heart disease.
Sudden limb activity, oxygen desaturations, and sudden changes in heart rate when closely associated with apnea or hypopnea can provide useful measures for determining whether the patient is responding to treatment. The doctor can elect to monitor only the relevant parameters in the follow-up study.
Oximetry alone is often touted as a good way of screening patients for OSA. However, oximetry has the following limitations:
Not all oximeters are created equal. The oximeters which are commonly in use for screening for OSA use sampling intervals of 10 - 15 seconds. However, these long sampling intervals make the devices insensitive for the detection of brief apneas. The minimum acceptable sampling interval for oximeters used for this purpose should be 5 - 6 seconds. Most multiparameter home monitoring devices use oximeters with sampling intervals of 2 - 3 seconds.
It may be more difficult to determine whether changes in oxygen saturation are real or due to artifact when you are recording only the oxygen signal. Trying to make oximetry more sensitive by using a drop in oxygen saturation of 2 % as the criteria for detecting an abnormal event increases the chance of interference from movement artifacts.
At the Vancouver Sleep & Breathing Centre, we use oximetry to:
The last step is taken as an extra measure of safety for the patient who only snores. This is because no one can predict with absolute certainty who will have OSA. Those who show significant desaturations are recommended to have a more detailed sleep study.
Epworth Sleepiness Scale
Calculated Probability of Having OSA
Stanford Method for Predicting OSA
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