Sympathetic hyperactivity mediates the association between obstructive sleep apnea and hypertension: age effect.

BACKGROUND

Sympathetic hyperactivation is one of underlying mechanisms of obstructive sleep apnea (OSA) associated hypertension. The association between OSA and hypertension is stronger in young and middle-aged than older adults. We hypothesized that sympathetic hyperactivity mediates the association between OSA and hypertension and this effect is moderated by age.

METHODS

OSA was defined as an apnea-hypopnea index (AHI) ≥5 events/hour based on an overnight polysomnography. Time under 90% oxygen (T90) was used to assess hypoxic severity. Twenty-four-hour urinary norepinephrine was used to assess overall sympathetic activity. Blood pressure was averaged from morning and evening measurements. Age cut off to define older adult was >50 years (the 75th percentile of the overall subjects).

RESULTS

Among the 166 OSA patients, each 10-event/hour increase in AHI and 1-h increase in T90 were associated with 1.63-μg (p = 0.087) and 4.76-μg (p < 0.001) increase in 24-h urinary norepinephrine with adjustments. Furthermore, each 10-event/hour increase in AHI and 1-h increase in T90 were associated with 2.59-μg (β = 0.22, p = 0.019, interaction-p = 0.075) and 6.36-μg (β = 0.37, p < 0.001, interaction-p = 0.017) increase in 24-h urinary norepinephrine respectively in young and middle-aged OSA patients, but not in older patients. Similarly, either AHI or T90 was associated with higher blood pressure among young and middle-aged (all p < 0.001), but not in older patients. These associations of OSA and hypertension in the young and middle-aged patients were mediated by 24-h urinary norepinephrine concentrations with mediation effect ranging 7.67∼13.41%.

CONCLUSIONS

Sympathetic hyperactivity mediates the association between OSA and hypertension in young and middle-aged, but not in older adults.