Reproducibility of Measures of Visit-to-Visit Variability in Blood Pressure after Transient Ischaemic Attack or Minor StrokeHoward S.C. · Rothwell P.M.
Stroke Prevention Research Unit, Department of Clinical Neurology, University of Oxford, Oxford, UK
Background: In certain patients in routine practice, blood pressure (BP) measurements differ substantially from week to week or month to month. Although often assumed to be random, such variability could provide information on underlying pathology or prognosis. In order to be informative, however, visit-to-visit BP variability would have to be neither random (i.e. it should be reproducible over time within individuals) nor artefactual (i.e. it should not be an artefact of the method/timing of measurement, for example). Methods: We quantified visit-to-visit variability in BP and explored potential confounding factors by analysing repeat measurements obtained every few months during follow-up in two large trials in patients with a transient ischaemic attack (TIA) or minor ischaemic stroke: the UK-TIA Aspirin Trial (effect of aspirin, effect of season and day of the week of measurement) and the European Carotid Surgery Trial (ECST – effect of carotid endarterectomy). By comparing different periods of follow-up, we also determined the reproducibilities of mean and several different measures of variability for both systolic (SBP) and diastolic BP (DBP). Results: The mean absolute difference between adjacent SBP readings was 14.7 mm Hg in the UK-TIA Trial and 16.0 mm Hg in ECST. Visit-to-visit variability in both SBP and DBP were independent of the potentially confounding factors studied, but reproducibility of all the variability measures was statistically significantly greater than zero. Reproducibility (intraclass correlation) of standard deviation of SBP was 0.32 (p < 0.0001) in the UK-TIA Trial and 0.18 (p = 0.0007) in ECST. Consequently, classification of patients with high (top quintile) or low (bottom quintile) variability was consistent over time (observed/expected = 2.21, 95% confidence interval 1.71–2.85, p < 0.0001, and 1.65, 1.23–2.21, p = 0.0007, respectively). Reproducibility increased with the number of measurements used to calculate variability, and was independent of any correlation with mean BP. Conclusions: Visit-to-visit variability in BP in these populations was reproducible, independently of any correlation with mean BP, demonstrating that visit-to-visit intra-individual BP variability is not random.
© 2009 S. Karger AG, Basel
Prof. P.M. Rothwell
Stroke Prevention Research Unit, Department of Clinical Neurology
Level 6, West Wing, John Radcliffe Hospital
Headington, Oxford OX3 9DU (UK)
Tel. +44 1865 231 610, Fax +44 1865 234 639, E-Mail firstname.lastname@example.org
S.C.H. was funded by the UK Medical Research Council and the Oxford Partnership Comprehensive Biomedical Research Centre.
Received: February 2, 2009
Accepted: April 27, 2009
Published online: July 24, 2009
Number of Print Pages : 10
Number of Figures : 0, Number of Tables : 4, Number of References : 22
Vol. 28, No. 4, Year 2009 (Cover Date: September 2009)
Journal Editor: Hennerici M.G. (Mannheim)
ISSN: 1015-9770 (Print), eISSN: 1421-9786 (Online)
For additional information: http://www.karger.com/CED