Confused about how to understand the results of your sleep study?
A sleep study measures many different variables during sleep. What is focussed on depends on the main problem that is being investigated. If the problem is snoring or suspected sleep apnea, then measurements of breathing are the focus, with other measurements giving an idea of the impact of breathing problems on sleep and the cardiovascular system. However, for insomnia, the focus is on what the brain is doing during sleep, with the other measurements more about ruling out other problems.
Determining the severity and impact of sleep apnea
Whilst when looking at a sleep study as a diagnostic test for snoring or sleep apnoea the main variables that give an idea about severity are to do with breathing. They are:
- Airflow – nasal pressure is measured using small tubes in the nose
- Respiratory effort – bands around the chest and abdomen measure the effort used in breathing
- Oxygen levels – a probe on the finger measures oxygen levels in the blood
Whilst a sleep study measures the number of times the airway is partially or completely blocked and counts these and then reports them as the number of times per hour there are difficulties with breathing, the other important factors from a sleep apnoea are what this does to the cardiovascular system such as heart rate and oxygen levels and the impact this has on sleep itself and whether this causes sleep disturbance then tiredness throughout the day. The other variables measured include:
- EEG (electro-encephalography): brain waves are used to determine sleep stage and what different systems in the brain are doing throughout the night
- EMG (electro-myography): muscle tone in chin and jaw muscles helps determine sleep stage and look for things like clenching and grinding of jaw muscles
- EOG (electro-oculography): eye movements are used to help determine sleep stage
- ECG (electro-cardiography): heart rhythm and heart rate give an idea of what is happening in the cardiovascular system
- Leg movements – are measured electrically. There are a range of different movements that occur at different frequencies and measuring the frequency of leg movements helps to differentiate them
- Body position – breathing differs in different positions throughout the night. For some people, snoring or sleep apnea is worse when sleeping on their back
- Noise – snoring or sleep talking can be measured
- Video – allows movements, like leg movement or sleep talking / sleep walking to be looked at
Measurements of breathing
The way breathing is measured during a sleep study is most commonly now with small plastic tubes that sit inside the nose and measure nasal pressure. This shows periods where the airflow is either reduced or stops completely because of partial or complete blockage in the back of the airway. Each time this occurs for more than 10 seconds it is counted as an event, either an hypopnoea for a partial blockage or an apnoea for a complete blockage. The total number of apnoeas and hyponeas are then added up and divided by the total time of sleep to give an apnoea-hypopnoea index (AHI) or number of times per hour there are partial or complete blockages of the airway. This measurement gives an idea of the severity of sleep apnoea. In general, an apnoea-hypopnoea index of:
- <5 = normal
- 5-15 = mild obstructive sleep apnea
- 15-30 = moderate obstructive sleep apnea
- >30 = severe obstructive sleep apnea
The apnoea-hypopnoea index (AHI) should be clearly visible in your sleep study report (it’s sometimes called the respiratory disturbance index – RDI) and the overall number does give some indication of the severity of sleep apnoea. There are times, however, when this can be a little misleading such as if breathing problems are more so when sleeping on the back but there is only a short period of sleeping on the back seen during that particular night. Therefore, it is also important to look at the apnoea-hypopnoea index in different body positions and different sleep stages just to make sure that if someone were to spend the night with more time in a particular position or sleep stage the result would not be significantly different. These will usually be shown in a table in your sleep study report.
Impact of sleep apnea
Whilst the apnoea-hypopnoea index gives an idea of the severity of sleep apnoea other factors can given an idea of the impact that it may be having on other body systems.
Oxygen saturation: During a sleep study a small probe usually on the tip of the finger measures the amount of oxygen in the blood, the oxygen saturation. If this is low it indicates that there is not sufficient air getting to the lungs to be able to provide oxygen to the rest of the body. Levels above 91% are considered normal and if there are frequent drops to well below this it is of greater concern as lowered oxygen saturation is one of the factors that predicts cardiovascular risk from sleep apnoea. However, it is not the only factor as other experiments which have given people additional oxygen during sleep to normalise oxygen levels have not completely removed the cardiovascular risk associated with sleep apnoea.
Heart rate: There are often variations in heart rate during sleep as part of the normal sleeping process with increases and decreases in heart rate as we switch between different sleep stages. However, frequent rapid increases in heart rate such as those that occur when there is a partial or complete blockage of the upper airway can also put a strain on the cardiovascular system and lead to heart problems over time. Most sleep studies report the average heart rate and in a diagram of the sleep study (hypnogram) may show rapid increases in heart rate with episodes of airway narrowing, or other trends in heart rate across the night. If sleep is healthy and working well, heart rate should gradually reduce across the night.
Sleep architecture: Sleep usually follows a predictable pattern, initially with lighter non-REM sleep (N1) then transitioning into somewhat deeper sleep (N2) and into deeper sleep (N3). This process occurs over around one hour and then you transition back through N2 sleep to dreaming (REM) sleep. Each of these patterns makes up one sleep cycle and sleep cycles are usually of 90 to 120 minutes duration. Across one night there will be three to five sleep cycles giving a fairly predictable pattern to sleep. Looking at the pattern of sleep visually in the hypnogram can give an idea of whether the overall pattern of sleep is preserved or whether it is disrupted. Sleep patterns, commonly called sleep architecture, can be disrupted for a range of reasons including sleep apnoea but also other medical conditions and a variety of medications.
The hypnogram is a visual way of summarising the above measures in a sleep study and is usually included as part of the sleep study report. It is a visual representation of many of the measurements during sleep and then allows someone experienced in looking at sleep studies to quickly spot any patterns or abnormalities on a particular study. I find one of the helpful things about hypnograms is looking at how things evolve across the night and how things relate to each other in different sleep stages and body positions.
So don’t be afraid to look at your sleep study report if you get a copy of it. There are many variables in the report so do not get too bogged down in the detail but use the apnoea-hypopnoea index (AHI) to get an idea of the overall severity then look at oxygen saturation, changes in heart rate and overall sleep architecture which give an indication of the impact sleep apnoea or other sleep disturbing phenomena are having on sleep itself. You should feel free to discuss your sleep study results in this sort of detail with your health professional who can help explain any other aspects that may be on your study or that you would like to further discuss.
Related posts & links:
- What is sleep apnea?
- Why treat sleep apnea?
- What are the different sleep disorders?
- Understanding your sleep study – from the American Sleep Apnea Association
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