Influence of sampling interval on the evaluation of nocturnal blood pressure in subjects with and without obstructive sleep apnoea

Blood pressure (BP) variability during sleep is high in obstructive sleep a pnoea syndrome (OSAS). How BP sampling interval affects the estimate of mea n nocturnal EP in OSAS and control subjects was investigated. Nine subjects with apnoea/hypopnoea index (AHI) <5 and 18 OSAS patients wit h AHI >30 underwent nocturnal polysomnography with beat-by-beat BP monitori ng. Mean nocturnal BP was evaluated averaging: a) all systolic (Ps) and dia stolic (Pd) BP values; b) Ps and Pd sampled every 5, 10, 15, 20, and 30 min , The sampling starting point was repeatedly shifted, and several mean BP e stimates for each sampling interval were obtained. Differences (Delta Ps an d Delta Pd) between means obtained by sampling BP and by averaging all BP v alues were calculated. In both groups Delta Ps and Delta Pd scatter increased as sampling interval increased; their variance was always higher in OSAS subjects (p<0.001). Ov er 95% of <Delta>Ps and Delta Pd were <5% of the beat-by-beat mean values a t all sampling intervals in controls, but this occurred only at sampling in tervals <less than or equal to>10 min in OSAS subjects. To conclude, for each blood pressure sampling time, a larger number of inac curate nocturnal mean blood pressure estimates are obtained in obstructive sleep apnoea syndrome than in control subjects. Obstructive sleep apnoea sy ndrome subjects require more frequent blood pressure measurements to obtain a similar accuracy in nocturnal blood pressure evaluation.