The methodology of blood pressure recording.

Blood pressure is a continuous physiological variable. The heart beats approximately 110,000 times in 24 h and each beat generates a systolic and a diastolic pressure. While directional changes over each 24 h period can be defined (Millar Craig, Bishop and Raftery, 1978), no two beats generate exactly the same pressures; beat-to-beat pressure variation appears to be a random process influenced by an almost infinite number of variables (Goldberg, 1977). Minute to minute variation also appears to be a random process, with changes in physical activity exerting a strong directional influence. Direct comparability is not seen in pressure samples of less than 3 h and even then underlying circadian rhythms have a pronounced influence. It is plainly inconceivable that any random measurement of blood pressure should be representative of any patient’s ‘blood pressure’; it cannot represent anything more than the blood pressure generated by the heart of that individual under the prevailing conditions (Figure 1).