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Sleep StudyThese tests are performed if there is a good history of sleep apnoea - stopping breathing at night for more than a few seconds, waking at night in a panic or short of breath, or if there is a raised Epworth sleepiness score. The patient needs to come into hospital overnight so that the test can be carried out [simply a matter of attaching probes]. At home screening tests are also available - see later.In children all we can test is simple blood oxygen levels via a probe on the finger, which are helpful in making a decision regarding apnoea and are usually sufficient to broadly establish a diagnosis, which is useful in planning surgery [usually adeno- tonsillectomy]. Adults can be more extensively tested with blood oxygen levels and lung air flow analysis to correlate stopping breathing with a drop in oxygen levels, and a rise in pulse rate. Further additions to this polysomnography include strain guages for chest movement, eye movement analysis, EEG for sleep levels and a microphone for snoring. The findings of sleep studies are relatively easy to interpret. Essentially we look for a significant drop in blood oxygen levels [below 90%] which occurs at the same time as significant flow reduction through the nose and mouth. This flow reduction can be classified as hypopnoea [20% reduction in flow] or apnoea [50% reduction in flow]. If flow changes occur without significant desaturation [drop in oxygen levels] these are not counted, since they may be positional, or not long enough to cause problems. The number of true hypopnoeas and apnoeas are added together and divided by the number of hours sleep. This figure is called the respiratory disturbance index [RDI]. A figure of 5 and below is insignificant , 6-40 is mild to moderate apnoea, greater than 40 is severe apnoea. Sometimes screening tests are useful before sleep studies to save time and reduce cost. The screening test we use can be taken home by the patient and sent to us in the post. |
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