variation in a subject with mean pressures 148/102 mmHg. As can be seen in the
figure, as the average pressures increase, the range of values also increases, so that
in the top panel where the pressures are lowest, systolic pressure ranges from 82 to
111 (29 mmHg spread) while diastolic ranges from 39 to 75 (36 mmHg spread). In
the middle panel, systolic pressures range from 107 to 147 (40 mmHg spread)
while diastolic pressures range from 59 to 101 (42 mmHg spread). Finally, in the
lower panel, where pressures are highest, systolic pressures range from 120 to 173
(53 mmHg spread), while diastolic pressures range from 74 to 127 (53 mmHg
spread). It is not uncommon to see a range of up to 100 mmHg over the course of
one waking–sleep cycle, even in subjects who would be termed “normal”
(Pickering et al. 1986 ). The extent of adaptable blood pressure variation can be
quite substantial. Thisfigure, however, also illustrates the heteroscedasticity in
blood pressure across people as previously mentioned, where there is a direct
relationship between the 24-h mean blood pressure and its variance (Pickering
1991 ). The heteroscedasticity in blood pressure is probably related to underlying
arterial structural differences between people, such as differences in arterial stiffness
(Pickering 1991 ) and/or differences in vasoactive hormone receptor density or
sensitivity (see van Berge-Landry et al. 2008 ). Thus, from the perspective of the
question, how much does blood pressure change for an adaptive purpose, the
answer is really: it depends.
The next questions after asking how much does blood pressure change are
perhaps: what external environmental and cognitive factors require blood pressure
adjustments, or more precisely a hidden adaptive response, and what is the average
size of that adaptive response? The following is a brief summary of the blood
pressure responses to some reported diary factors.
Situation of Measurement
The seminal studies of ambulatory blood pressure variation that employed auto-
matic monitors (e.g., Harshfield et al. 1982 ; Pickering et al. 1982 ) found that the
situation or location of measurement had a substantial influence on blood pressure.
Numerous subsequent studies conducted as either an evaluation of ecological
momentary factors or in a“natural experiment”framework consistently show that
the place, location, or situation of measurement is among the most important of the
factors requiring a hidden blood pressure adaptation. Those studies that have
evaluated people employed outside their home have found that blood pressures
measured at the place of employment tend to be the highest during the day, while
those measured during sleep are the lowest (see James2007afor review). The
elevation in pressure needed to meet the predictable demands of work is also
independent of the time of day, as several researchers have found that the average
ambulatory work pressure is highest when compared to other daily situations in
shift workers, who either exclusively worked a night shift or who worked a night
154 G.D. James