Predictors of overnight Blood Pressure (BP) changes in normal subjects
Recent evidence has suggested that obstructive sleep apnoea (OSA) may affect the normal overnight fall in blood pressure more so than the average diurnal figure. The aspect of OSA responsible for higher BP is far from clear, possibilities being hypoxia, sleep disturbance or increased inspiratory effort. This abstract reports an interim analysis from a community survey of 326 randomly selected men and women aged 35-65 addressing this issue. Following random selection from a GP practice's patient records, subjects were visited at home by a specially trained nurse. Husbands and wives were usually studied on the same occasion. Questionnaires were administered and anthropometric measurements made. BPs were measured with an automatic device three times by the nurse, three times by the subject themselves (after instruction), and again three times the following morning by the subject immediately on arousal. Overnight recordings were made using the RM50 recorder (SaO2, snoring (throat microphone), posture, heart rate, thoracic movement, and indirect beat to beat blood pressure (pulse transit time)). These data were processed to give >4%SaO2dip rate, snores/hr, mean inspiratory effort across the night (pulsus paradoxus), and autonomic (blood pressure) 'arousals'/hr. The difference between the morning and evening BP measured by the subjects was calculated. The relationship between morning to evening BP change and potential predictors was explored using linear modelling (SAS), any non-normally distributed data was logged first. Evening to morning systolic BP change averaged -9.5mmHg (95th centiles, - 32 to +11). Independent predictors of this evening to morning change in SBP were average overnight inspiratory effort (P<0.0001) and >4%SaO2 dip rate (P=0.03); autonomic arousals, snoring, mean blood pressure and anthropometric variables were not independent correlates. Inspiratory effort, arousals and snoring were inter-correlated, as would be predicted. This suggests that inspiratory effort may be one of the characteristics of OSA and its variants that influence BP beyond the sleeping period.