The variability of arterial pressure.

Abstract Direct arterial pressure has been recorded continuously on magnetic tape in totally unrestricted patients going about their normal routine outside hospital for periods of 24 hours. On replaying these tapes data are derived in digital form of systolic, diastolic, and mean pressure, plus pulse interval, all on a beat-by-beat basis; a computer program then performs averaging and statistical analysis of these data. The frequency distribution of blood pressure over a 24-hour period was plotted and showed a bi-modal curve, the lesser mode being due predominantly to sleep. This bimodality was unaffected by the average 24-hour pressure, but was modified by the length of sleep. The variability of arterial pressure was determined from the standard deviation by averaging pressure over each hour in 10 patients (five men, five women). They were selected to give a range of different levels of arterial pressure and had never received drugs. Throughout the 24 hours the variability of systolic pressure was significantly greater than that in diastolic pressure. This variability was most acutely affected by physical exercise. During sleep both systolic and diastolic pressure fell by an average of 20 per cent of the waking pressure and both were less variable than during waking. Spontaneous rhythmic changes of pressure occurring approximately every 90 to 120 minutes accounted for the greatest variability during sleep and were probably due to REM sleep.