Scoring of sleep respiratory events
There has been little discussion on scoring of apnoea events. It is recommended that they be scored with a thermistor (airflow sensor based on temperature changes), not with a nasal pressure transducer, which will overestimate the number of apnoea events (10).
There has been more discussion and change associated with scoring of hypopnoea, partly based on professional viewpoints, partly in connection with reimbursement schemes in the USA (11). The preliminary consequence of this is that the AASM still has two definitions of hypopnoea – one «recommended», in which hypopnoeas can now be scored if a 30 % amplitude reduction measured with a nasal pressure transducer is associated with desaturation of 3 % or EEG arousal, and one «acceptable», which does not include arousal, and the desaturation requirement is 4 %. In the earlier «Chicago criteria» of 1999 (Table 1) hypopnoeas could also be scored without a desaturation requirement (12).
The requirement of associated desaturations for scoring hypopnoeas has a strong effect on the apnoea-hypopnoea index. However, different oxymeters have different averaging times, which can vary from two to 21 seconds. A long averaging time results in fewer desaturation events, and a short averaging time results in more, but less reliable desaturations (13).
Experimental studies have proved the AASM’s arousal definition to be valid and reliable, but arousals are demanding to score. Reliability varies with scoring experience, and is probably somewhat poorer between different groups of raters (14). Arousal is also a dynamic variable, with a triggering threshold that is influenced to a large degree by preceding sleep shortage/sleep disruption (15).
Upper airway resistance syndrome, like obstructive sleep apnoea, is characterised by daytime sleepiness, which is assumed to be due to disrupted sleep at night. However, nasal flow measurement has proved to be more sensitive than oesophageal pressure manometry (16), and when coupled with liberal hypopnoea criteria, events that were previously perceived as respiration-related arousals could be scored as hypopnoeas with ordinary polysomnography.
In a study in which 423 patients were referred to sleep centres owing to clinical suspicion of obstructive sleep apnoea, all tests were scored according to the Chicago criteria of 1999. The readings were then rescored according to the two hypopnoea criteria of the AASM of 2007. Median values for the apnoea-hypopnoea index then varied from 8.3 to 25.1, and the proportion of hyponoea events from 25 % to 60 % (10, 12, 17).
In another survey, 37 relatively young patients with little tendency to desaturations received the diagnosis obstructive sleep apnoea on the basis of symptoms and sleep records interpreted according to the Chicago criteria (18). After therapy, the patients reported subjective improvement and a clear fall in the apnoea-hypopnoea index. After rescoring using the two hypopnoea definitions of the AASM of 2007, the improvement tendency was the same.
As a result of the requirement of 4 % desaturation in order to score a hypopnoea, 14 of the 37 received an initial apnoea-hypopnoea index of less than 5. These 14 would not have received the diagnosis obstructive sleep apnoea or therapy if the 4 % criterion had been used. The article concluded that the «4 % criterion» should not be used in studies of young people with healthy lungs and good baseline O₂ saturation. It also gives pause for thought that in a population-based study where there was a requirement of associated 4 % desaturation (conservative), it was found that in the group with an apnoea-hypopnoea index of between 0.1 and 4.9 (i.e. some are in the «normal» range), there was a slightly larger proportion with high blood pressure than in the supernormal group (with apnoea-hypopnoea index = 0) four years later (19).