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 apnoea syndrome (OSAS). How BP sampling interval affects the estimate of mean nocturnal BP in OSAS and control subjects was investigated. Nine subjects with apnoea/hypopnoea index (AHI) <5 and 18 OSAS patients with AHI >30 underwent nocturnal polysomnography with beat-by-beat BP monitoring. Mean nocturnal BP was evaluated averaging: a) all systolic (Ps) and diastolic (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 estimates for each sampling interval were obtained. Differences (deltaPs and deltaPd) between means obtained by sampling BP and by averaging all BP values were calculated. In both groups deltaPs and deltaPd scatter increased as sampling interval increased; their variance was always higher in OSAS subjects (p<0.001). Over 95% of deltaPs and deltaPd were <5% of the beat-by-beat mean values at all sampling intervals in controls, but this occurred only at sampling intervals < or =10 min in OSAS subjects. To conclude, for each blood pressure sampling time, a larger number of inaccurate nocturnal mean blood pressure estimates are obtained in obstructive sleep apnoea syndrome than in control subjects. Obstructive sleep apnoea syndrome subjects require more frequent blood pressure measurements to obtain a similar accuracy in nocturnal blood pressure evaluation.