| Summary: | <h4>Introduction</h4> <p>When obstructive sleep apnoea (OSA) does not occur throughout sleep, there must be factors influencing its presence or absence. These are most likely to be sleep stage, posture, and pre-sleep alcohol, amongst others. We hypothesised that as OSA severity increases, the likelihood of positional OSA (POSA) would also decrease.</p> <h4>Methods</h4> <p>Laboratory sleep studies of 39 patients with OSA were manually reviewed to calculate supine and non-supine oxygen desaturation indices (ODI). The usual definition for POSA was used, a ratio of supine to non-supine ODI of 2</p> <h4>Results</h4> <p>The mean age was 53.2 (SD 12.4) years, the BMI was 35.0 (SD 8.9) kg/m2, and 74% were male. The median supine ODI was 54.3 (IQR 25.7/73.5) and non-supine ODI 18.7 (IQR 8.6/38.4). The overall prevalence of POSA was 56%. The prevalence of POSA for ODIs of <40 was 68%, and 35% if ≥40.</p> <h4>Conclusions</h4> <p>An ODI ≥40, compared to <40, halved the likelihood of POSA from 68% to 35%. Although there is clearly a relationship between overall ODI and POSA, it is not strong enough to diagnose an individual’s POSA. However the relationship provides a useful way to pre-screen trial subjects to enrich for POSA.</p>
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