Study Using Novel Index Links OSA Severity to Higher Cardiovascular Disease Risk

Patients in upper quartiles of the sleep breathing impairment index, a novel gauge of obstructive sleep apnea (OSA) severity, had higher 10-year Framingham cardiovascular risk than patients whose symptoms were classified as less severe.

Obstructive sleep apnea (OSA) has been linked to a variety of complications, including increased risk of cardiovascular disease (CVD) based on the apnea-hypopnea index (AHI) to gauge OSA severity, but past research has led to conflicting data, and AHI itself has been scrutinized as a gauge of OSA severity.

A recent study found that a potentially more appropriate novel metric, the sleep breathing impairment index (SBII), was associated with higher 10-year CVD risk after adjusting for multiple potential confounding factors.

The AHI is based on the number of apnea and hypopnea events per hour of sleep and is used extensively to measure OSA severity and predict clinical outcomes. But authors of the study, published in the journal Nature and Science of Sleep, note that recent research supports the notion that AHI only provides limited information, “ignoring the pathophysiologic elements contained in the event-associated desaturation.” Therefore, they opted to utilize SBII to identify correlations between OSA severity and CVD risk.

The SBII used polysomnographic data containing detailed information, including the frequency, duration, and degree of analyzed obstructive apnea and hypopnea as well as associated desaturation events.The metric was calculated using the sum of the duration and associated desaturation of each event, which was then divided by total sleep time.

A total of 140 male participants with a median age of 40 were enrolled in the study. All had no pre-existing CVDs at baseline and were not using lipid-lowering medication or insulin. Exclusion criteria also included OSA treatment before study enrollment and chronic obstructive pulmonary disease, restless legs syndrome, or narcolepsy. The main outcome of interest was any association between SBII and moderate-to-high Framingham 10-year CVD risk.

Each patient underwent overnight polysomnography from 11 p.m. to 6 a.m., which was recorded and analyzed by a sleep laboratory technician to identify apnea and hypopnea events. Apnea was defined as cessation of airflow for 10 or more seconds, and hypopnea was defined as a 30% or greater decrease in airflow lasting for at least 10 seconds and accompanied by an associated 3% or greater oxygen desaturation. An AHI of less than 5 was categorized as non-OSA; 5-15 was considered mild OSA; 15-30 was considered moderate; 30-60 was severe; and more than 60 was classified as very severe.

Overall, 80 study participants had a moderate-to-high Framingham CVD risk, and participants with SBII in the third and fourth quartile were more likely to have a moderate-to-high CVD risk. After adjusting for potential confounding factors—including BMI, excessive daytime sleepiness, AHI, lowest SpO2, percent of time spent with SpO2 at less then 90%, endothelial dysfunction, and insulin resistance—the association between moderate-to-high CVD risk and SBII in 2 highest quartiles was still significant.

Unadjusted odds ratios were 3.24 in the third quartile (95% CI, 1.22-8.63) and 5.54 in the fourth (95% CI, 1.98-15.52). Adjusted odds ratios were 6.28 (95% CI, 1.10-36.04) and 11.78 (95% CI, 1.25-111.38), respectively. AHI measures were not significantly correlated with CVD risk in this study.

Study limitations included the small sample size from a single center and the fact that all participants were male. The predictive nature of the Framingham CVD risk algorithm is also a limitation, and it is unknown if long-term follow-up will show the same association. And although the SBII is more detailed than the AHI, it does not incorporate some known OSA elements and does not reflect the differences between the biological effects of apneas and hypopneas, the study authors note.

Despite the limitations, they conclude that the novel SBII index is associated with 10-year CVD risk and may be a more appropriate measure than the AHI to characterize OSA severity in clinical practice and research.

“The SBII, as a novel index to measure OSA severity, was associated with an increased 10-year CVD risk after adjusting for multiple potential confounding factors,” the authors wrote. “The SBII is believed to be a more comprehensive measure to better capture the pathophysiologic OSA diversity when evaluating OSA severity and the association with clinical sequelae. Thus, future studies are warranted to test the effect of CPAP on OSA-related outcomes based on this novel metric.”


Cao W, Luo J, Huang R, Xiao Y. The association between sleep breathing impairment index and cardiovascular risk in male patients with obstructive sleep apnea. Nat Sci Sleep. Published online January 11, 2022. doi:10.2147/NSS.S343661