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Cardiovascular disease (CVD) is increased in moderate to severe sleep-related breathing disorder (SRBD). Interleukin 6 (IL6) promotes atherogenesis and elevated levels occur in SRBD. Soluble IL6 receptor (sIL6R) exerts widespread effects compared with IL6 alone. The authors examined the relationship between SRBD severity, IL6 and sIL6R levels, and morning and evening variability for each cytokine.
Subjects (n = 385) underwent overnight polysomnography and morning and evening IL6 and sIL6R measurements. The primary exposure was moderate to severe SRBD, defined as a respiratory disturbance index (RDI) ⩾30, compared with four other categories with less severe SRBD. The primary analyses were morning IL6 and sIL6R levels. Statistical models adjusted the outcomes for confounding subject characteristics such as waist circumference and comorbidities. The authors postulated that levels increase with overnight SRBD stresses and therefore adjusted for evening levels.
Adjusted morning and evening IL6 levels were not significantly higher in moderate to severe SRBD. Adjusted morning (p = 0.001) and evening (p = 0.01) sIL6R values were significantly higher in moderate to severe SRBD. This relationship was stronger for morning levels. Morning levels remained significantly higher even when evening levels were adjusted for (p = 0.02).
Limitations of the study are the potential for analytic variability of measurements and possible unmeasured confounders. The authors suggest that sIL6R may be a useful biomarker for overnight SRBD stresses and identifying patients with a higher CVD risk. A lack of specificity of an inflammatory molecule for clinical screening is a concern. Studies to assess sensitivity, positive predictive value and the effect of treating SRBD on reducing sIL6R levels and subsequent CVD risk would be of interest.