Does caffeine confound relationships among adrenergic tone, blood pressureand sleep apnoea?

The purpose of this study was to determine whether caffeine consumption con founds the relationship among adrenergic tone, as measured by urinary norep inephrine (NE), blood pressure (BP) and obstructive sleep apnoea (OSA). Dat a were analysed using correlation and regression analysis, analysis of cova riance and t-tests. Subjects included normotensives and hypertensives with and without OSA: 38 men, 23 women, aged 30-60 y; 100-150% of ideal body wei ght; without other major illness. Patients were studied using polysomnograp hy, caffeine consumption was assessed, 24-h urinary NE levels were examined and ambulatory BP was recorded. Patients with OSA (N=27) reported signific antly greater caffeine consumption than those without OSA (N=34) (295 vs. 1 03 mg, P=0.010), but caffeine was not significantly correlated with their a mbulatory BP. In contrast, NE excretion correlated with caffeine consumptio n (r=0.24, P=0.041), apnoea severity (r=0.65, P < 0.001) and BP (r=0.34, P < 0.005). Significant OSA-NE and BP-NE relationships remained even after co ntrolling for caffeine consumption. Patients with OSA consumed nearly three times the amount of caffeine as patients without OSA. While caffeine parti ally explains the increased adrenergic tone in patients with OSA and the re lationship between BP and NE, it does not appear to contribute significantl y to the relationship between OSA and elevated BP.