Prediction of the actual awake and asleep blood pressures by various methods of 24 h pressure analysis.

OBJECTIVE: To assess the relationships of daytime and night-time blood pressures and the day-night pressure differences, obtained by various analytical methods, with the actual awake and asleep pressures and the awake-asleep pressure difference. METHODS: Ambulatory blood pressure was successfully monitored by use of the SpaceLabs 90202 device in 91 healthy young men during a weekend, when they went to bed and awoke at variable and often unusual times. The actual 'awake' and 'asleep' blood pressures were calculated on the basis of these times, noted by the subjects. The 24 h recordings were further analysed by use of two clock time-independent methods (square-wave fitting; cumulative sum analysis) and by one 'wide' (A) and one 'narrow' (B) fixed-time method (daytime: 0700-2200 h or 1000-2000 h; night-time: 2200-0700 h or 0000-0600 h, for methods A and B, respectively). RESULTS: In the total study population, square-wave fitting (+1.1 +/- 1.5 mmHg, mean +/- SD) and cumulative sum analysis (2.4 +/- 1.9 mmHg) overestimated the actual awake systolic blood pressure and underestimated the asleep pressure (-0.7 +/- 1.7 and -1.8 +/- 1.8 mmHg, respectively). The fixed-time techniques underestimated the awake pressure (-2.6 +/- 3.1 and -0.2 +/- 3.2 mmHg for methods A and B, respectively) and overestimated the asleep pressure by +4.9 +/- 5.5 and +2.7 +/- 6.1 mmHg, respectively. The actual awake-asleep pressure difference was overestimated by square-wave fitting (+1.8 +/- 1.9 mmHg) and more so by cumulative sum analysis (+4.2 +/- 2.5 mmHg); the underestimation by the fixed time approach averaged -3.0 +/- 8.4 mmHg with method B and amounted to -7.5 +/- 8.3 mmHg with method A. Overall, the SD of the various differences between the estimated and the actual awake and asleep pressures, and consequently the limits of agreement, were larger for the fixed-time methods than for the clock time-independent techniques. In the 47 subjects who went to bed before 2400 h and awoke between 0600 and 1000 h, the results of the clock time-independent methods were similar to the results in the total study population, whereas the deviations of the fixed time pressures from the actual awake and asleep blood pressures were considerably reduced. The results were similar for diastolic blood pressure. CONCLUSIONS: Clock time-independent methods, particularly square-wave fitting, can predict the actual awake and asleep blood pressures and the awake-asleep pressure differences with reasonable accuracy and the results are independent of the awake-asleep pattern of the subjects. However, fixed-time methods are only reliable when the subjects go to bed and arise within well-defined periods, and yield more accurate results when the morning and evening phases are excluded from the daytime and night-time periods.

Heritability of conventional and ambulatory blood pressures. A study in twins.

Conventional and 24-hour ambulatory blood pressures were measured in 26 pairs of monozygotic twins and 27 pairs of dizygotic twins, all male, ages 18 to 38 years, to determine the heritability of blood pressure measured under various conditions. Conventional pressure was the average of three well-standardized measurements in the supine position, and ambulatory pressure was recorded during the subjects' normal activities by use of the SpaceLabs 90202 device. Heritability was assessed by classic methods and by model fitting and path analysis. In the latter approach, the percent genetic variance was 70% for mean 24-hour systolic pressure and 73% for diastolic pressure, which was similar to the results for the conventional pressures (64% and 73%, respectively). During the night, these estimates were 72% and 51% for systolic and diastolic pressures, respectively, and also the average pressures of the total awake daytime period were under partial genetic control (63% and 55%, respectively). The remaining variances could be attributed primarily to unique environmental influences. However, shared and nonshared environmental factors were predominant for the pressures during a fixed 6-hour afternoon period. We conclude that the heritability of blood pressure is relatively high in young adult healthy men, for standardized conventional pressure and the average 24-hour pressure. Genetic variance is somewhat higher for the asleep pressure than for the awake systolic pressure.