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Abstract
Introduction Obstructive sleep apnoea (OSA) results in significant cardiovascular consequences. Level of hypoxia and degree of sympathetic activation are postulated to play a role.
The Apnoea Hypopnoea Index (AHI) is used as a tool to assess severity of OSA. However, it does not measure depth or duration of hypoxia and may underestimate the risk of complications. The aim of this study was to evaluate the relationship between AHI and the burden of hypoxia.
Method This was a retrospective study, using data from noxturnal sleep studies. Equal numbers of each OSA severity, defined by AHI, were selected consecutively from 122 adult patients who underwent sleep studies between Dec 2020 and May 2021. Demographic data, AHI and percentage time spent with oxygen saturations <90% (%T<90%) were recorded. Excel was used for analysis and Spearman’s rank used to calculate the correlation coefficient (rho, r).
Result AHI was compared to%T<90% (figure 1) showing a moderate positive correlation (r0.6). Subgroup analysis demonstrated a moderate correlation in the severe group (r0.67), whereas only a very weak correlation in the moderate and mild groups (r0.19 and 0.16 respectively). There was no significant difference in the%T<90% in the moderate group compared to those with an AHI 30-60 (mean(SD) 14.86(20.15) and 17.96(17.91) P=0.067) despite these patients having different categories of OSA severity.
Percentage time spent with O2 saturations <90% (%T<90) vs AHI
Conclusion This study suggests that AHI inadequately reflects degree of hypoxic burden, and therefore is an incomplete measure of OSA disease severity. The results demonstrate patients with moderate OSA have a burden of hypoxia similar to many of those with severe disease. In these patients, AHI may inadequately reflect the risk of future complications resulting from hypoxia. Further research is needed to develop an alternate measure of severity to accurately reflect this risk, a composite of AHI and hypoxic burden would be a first step.
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