Comparison of agreement between different measures of blood pressure in primary care and daytime ambulatory blood pressure

Editor - The white-coat phenomenon among treated and untreated
hypertensive patients is difficult to define in everyday clinical practice
and therefore the article highlighting the difference between blood
pressure measured by general practitioners and values recorded in the home
environment is welcomed [1]. Importantly, this could also apply to
hospital physicians. However, we must caution against the dogmatic
conclusion that ‘It is time to stop using high blood pressure readings
documented by general practitioners to make treatment decisions’ that
resulted in The Times to report “GPs ‘cause’ high blood pressure” [2].

This white-coat effect is more common in the elderly hypertensive
subjects [3]. Although the authors report an average difference of
18.9/11.4mmHg between clinic and mean daytime ambulatory recordings, they
did not specify whether there is an age-related factor. We, therefore,
question whether the conclusion can be applied equally to young and
elderly hypertensive patients?

Furthermore, current advice regarding the treatment targets and
thresholds of high blood pressure, published by the British Hypertension
Society [4], are based on clinical studies using clinic readings.
Crucially, it has never been established how home readings relate to these
treatment targets.

A consensus has suggested that the upper limit of normal for mean
daytime ambulatory blood pressure and home monitoring is 135/85mmHg and
therefore equivalent to a clinic blood pressure of 140/90mmHg [5].
However, equivalent values have not been determined for treatment targets
in hypertensive subjects with diabetes or chronic renal failure.
Significantly, morbidity or mortality outcome data to support ambulatory
or home readings is lacking. An ambulatory blood pressure sub-study of the
Syst-Eur trial presented by Staessen at the American Society of
Hypertension Scientific Meeting in May 2002 demonstrated that the best
predicator of cardiovascular outcome is mean nighttime ambulatory blood
pressure. However, this has not been translated into guidelines and the
study population was restricted to subjects with systolic hypertension
aged over 65 years.

Therefore, while acknowledging the superiority of blood pressure
reading in a home environment, we would caution against using these
readings at face value to determine treatment for hypertension across all
age-groups. For example, reducing blood pressure control by at least 5/5
mmHg in patients at high cardiovascular risk would be inappropriate.

Essentially, clinic blood pressure readings can be unreliable; but we
await newer guidelines incorporating ambulatory and home blood pressure
readings based on clinical outcome studies before relieving doctors of
their duty to measure blood pressure.’

References:

1. Little P, Barnett J, Barnsley L, Marjoram J, Fitzgerald-Barron A, Mant
D. Comparison of agreement between different measures of blood pressure in
primary care and daytime ambulatory blood pressure, BMJ 2002;325:254-7

2. Hawkes N. GPs ‘cause’ high blood pressure. The Times 2002 Aug 2

3. Mansoor GA, McCabe EJ, White WB. Determinants of the white-coat effect
in hypertensive subjects. Journal of Human Hypertension 1996:10(2):87-9.

4. Ramsay L, Williams B, Johnston G, MacGregor G, Poston L, Potter J et
al. Guidelines for management of hypertension: report of the third working
party of the British Hypertension Society. J of Human Hyper 1999; 13:569-
592.

5. Staessen JA, Thijs L. Development of diagnostic thresholds for
automated self-measurement of blood pressure in adults. First
International Consensus Conference on Blood pressure Self-Measurement.
Blood Press Monit 2000; 5(2): 101-9.

Adrian G. Stanley,

Clinical Lecturer in Medicine

Bryan Williams,

Professor of Medicine and Director of Cardiovascular Research Institute

There are no competing interests to declare.