(Circulation. 1999;100:1071-1076.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
Prediction of Coronary and Cerebrovascular Morbidity and Mortality by Direct Continuous Ambulatory Blood Pressure Monitoring in Essential Hypertension
From the Department of Cardiovascular Medicine, Northwick Park and St Mark's Hospital NHS Trust and Institute for Medical Research, Harrow, Middlesex (R.S.K., A.B., R.S., A.L.); Department of Health, Whitehall, London (J.D.S.); and Department of Medical Statistics and Evaluation, Imperial College School of Medicine, Hammersmith Hospital, London (C.D.), UK.
Correspondence to Dr A. Lahiri, MB, BS, MSc, MRCP, FACC, FESC, Cardiovascular Research Institute, University of Leicester, Leicester Royal Infirmary, Leicester LE 2 TLX, UK.
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Abstract |
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Background—The goal of this study was to assess the prognostic value of ambulatory versus clinic blood pressure measurement and to relate cardiovascular risk to ambulatory systolic and diastolic blood pressure levels.
Methods and Results—The study population consisted of 688 patients 51±11 years of age who had undergone pretreatment 24-hour intra-arterial ambulatory blood pressure monitoring on the basis of elevated clinic blood pressure. A total of 157 first events were recorded during a 9.2±4.1-year follow-up period. The predictive value of a regression model containing age, sex, race, body mass index, smoking, diabetes mellitus, fasting cholesterol level, and previous history of cardiovascular disease was significantly improved by the addition of any ambulatory systolic or diastolic blood pressure parameter (whether 24-hour, daytime, or nighttime mean) or pulse pressure, whereas inclusion of baseline clinic blood pressure variables did not enhance the prediction of events. The most predictive models contained the ambulatory systolic blood pressure parameters. In the model containing 24-hour mean ambulatory systolic blood pressure (P=0.001), age (P<0.001), male sex (P<0.001), South Asian origin (P=0.008), diabetes mellitus (P=0.05), and previous cardiovascular disease (P<0.001) were additional independent predictors of events. Whereas 24-hour ambulatory systolic blood pressure was linearly related to the incidence of both coronary and cerebrovascular events, 24-hour ambulatory diastolic blood pressure exhibited a positive linear relationship with cerebrovascular events but a curvilinear relationship with coronary events.
Conclusions—Ambulatory blood pressure is superior to clinic measurement for the assessment of cardiovascular risk; there is no reduction in coronary risk at lower levels of ambulatory diastolic blood pressure.
Key Words: blood pressure • prognosis • cardiovascular diseases
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Introduction |
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Over the last 50 years, it has become increasingly recognized that isolated clinic blood pressure measurements may not be adequately representative of the daily blood pressure load away from the medical environment.1 2 With frequent readings taken during routine daily activities, 24-hour ambulatory blood pressure monitoring offers a more valid assessment of an individual's true blood pressure level.3 Consequently, cross-sectional studies have generally shown ambulatory blood pressure to be more closely related to target organ damage than clinic-based measurements,4 but longitudinal data are limited. The intra-arterial method of ambulatory blood pressure monitoring is a powerful research tool and remains the reference standard by which noninvasive monitors are validated.5 6 It provides beat-to-beat measurements of blood pressure over a 24-hour period in truly ambulant patients and therefore represents the most accurate measure of the daily hemodynamic burden imposed by blood pressure. In contrast, noninvasive auscultatory and oscillometric monitors rely on intermittent recordings at predetermined time intervals, and these may not be accurate during unrestricted physical activity. The aims of this study were to assess the prognostic value of ambulatory versus clinic blood pressure measurement for the prediction of subsequent morbid events in patients with essential hypertension and to relate the risk of all-cause, coronary, and cerebrovascular events to the level of ambulatory systolic and diastolic blood pressures.
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Methods |
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From January 1, 1979, to January 1, 1993, 723 patients were subjected to 24-hour intra-arterial ambulatory blood pressure monitoring at our institution; their demographic profile has recently been published.7 The racial composition of the patient population studied reflects the local population of the London boroughs of Harrow, Brent, and Ealing, which includes mainly white subjects but also South Asian (Indian subcontinent) and African-Caribbean subjects. All patients were originally referred for the management of hypertension because of persistently elevated clinic blood pressure taken over a period of weeks to months in a primary care setting. At each hospital visit, a single blood pressure measurement was taken by a nurse or technician using the conventional auscultatory technique after 5 to 10 minutes of semisupine rest in a warm environment. The point of disappearance of auscultatory sounds was taken as the diastolic blood pressure. This was followed by a full medical history and physical examination by a physician. Secondary causes of hypertension were excluded, as far as possible, in all patients by measurement of serum urea, creatinine, and electrolytes and of urinary catecholamines; chest x-ray; and more recently, intravenous renal digital subtraction angiography. Baseline clinic blood pressure was taken as the mean of
2 untreated readings at separate clinic visits in the 4 weeks before
or after the intra-arterial study. Those in whom clinic
systolic blood pressure was 140 mm Hg or
diastolic blood pressure was 90 mm Hg were
requested to undergo 24-hour intra-arterial ambulatory
blood pressure monitoring within 2 months. Antihypertensive medication
either had not been started or had been withdrawn in the 8 weeks
preceding intra-arterial blood pressure monitoring. The
method of 24-hour intra-arterial ambulatory blood pressure
monitoring was approved by the Harrow Health Authority Ethical
Committee, and patients were required to give written informed consent
before the procedure. General practitioners were informed
of test results, and antihypertensive therapy was generally recommended
if 24-hour ambulatory systolic blood pressure was 140
mm Hg or diastolic blood pressure was 90 mm Hg.
Treatment of lower ambulatory blood pressure readings was more
conservative and discretionary. Subsequent assessment of blood pressure
control and treatment was largely left to the individual general
practitioners or hospital physicians and was based on
clinic blood pressure measurements, in keeping with standard
clinical practice.
Intra-Arterial Blood Pressure Monitoring
The technique of intra-arterial blood pressure
recording used in this laboratory has been well
documented,8 as has the method of
analysis.9 Blood pressure was recorded from a
fine brachial artery cannula with a specially designed
transducer/perfusion unit and an Oxford Medilog Mark I tape
recorder. The equipment was designed so that patients were fully
ambulant and able to carry out their normal daily activities away from
the hospital environment. The 24-hour tape recordings were
analyzed on a custom-built hybrid computer by use of a program
that calculated mean hourly blood pressure and heart rate. Mean
(24-hour) systolic and diastolic
intra-arterial blood pressures were calculated by averaging
the 24 hourly systolic and diastolic readings.
Blood pressure and heart rate variabilities were expressed as the SD of
mean hourly systolic and diastolic blood pressures
and heart rate, respectively. Daytime mean systolic and
diastolic blood pressures were defined as the average of
the hourly blood pressure readings from 6 AM to 10
PM; nighttime mean blood pressures, between 10
PM and 6 AM.10 The nocturnal
decreases in systolic and diastolic blood pressures
were calculated by subtracting respective nighttime mean from daytime
mean blood pressure readings. Nondippers were defined as those who did
not exhibit a reduction in mean systolic and
diastolic blood pressures by 10% from day to night; the
remaining subjects were classified as dippers.10 The
white-coat effects on systolic and diastolic blood
pressures were defined as initial clinic blood pressure measurements
minus respective daytime mean ambulatory readings.
Follow-Up Evaluation
The study patients have been intermittently reviewed over the
years to record clinic blood pressure, drug therapy, and the
occurrence of interim cardiovascular events. Ethical
approval for the most recent follow-up, performed during an 18-month
period from 1994 to 1996, was gained from the hospital ethics committee
before patients or their family practitioners were
contacted. To obtain complete mortality data, the dates and certified
causes of interim deaths were obtained from the National Health Service
Central Register, Southport, UK. Hospital records of all patients
were also scrutinized. Survivors were invited to attend a follow-up
evaluation for documentation of events, clinic blood pressure
measurement on current treatment, serum creatinine
estimation, and fasting cholesterol level determination.
General practitioners of the nonattenders were sent a
questionnaire for details about these patients.
Documented events consisted of noncardiovascular death, coronary death (myocardial infarction or ischemia, ventricular fibrillation, or cardiac failure), cerebrovascular death, peripheral vascular death, nonfatal myocardial infarction, nonfatal stroke, and coronary revascularization.
Statistical Analysis
The clinical variables analyzed included age, sex,
race, body mass index, diabetes mellitus, smoking, fasting
cholesterol level, and previous
cardiovascular disease. The follow-up period was
defined as the time interval between 24-hour intra-arterial
ambulatory blood pressure monitoring and last follow-up in
uncomplicated patients or the development of a first morbid event.
Continuous variables were expressed as mean±SD, and categorical
variables were given as the percentage of patients so affected. The
study population was dichotomized into those with and without events.
Comparison of clinical variables and blood pressure data between
these 2 groups was made by univariate Cox
proportional-hazards analysis. In multivariate
analysis, a baseline Cox regression model containing age, sex
(men versus women), race (South Asians versus whites and
African-Caribbeans versus whites), smoking (smokers versus nonsmokers),
diabetes mellitus, previous cardiovascular disease, and
fasting cholesterol level was devised for predicting the
time to experiencing a first event. A series of regression models was
then created, individually adding each blood pressure
parameter to the clinical variables to assess whether
any of these parameters could enhance the predictive value
of the model with the use of the likelihood ratio test. The
2 value obtained from this test
represented the difference in the log likelihood between
the baseline model and the model containing the blood pressure
parameter; the higher the 2 value,
the better the fit of the model for prediction of events. Quadratic
functions of 24-hour mean ambulatory systolic and
diastolic blood pressures and clinic systolic and
diastolic blood pressures were also incorporated into the
regression models to assess for curvilinearity between these
variables and subsequent events. Hazard ratios with 95% CIs were
derived for each variable, and a value of P<0.05 was
considered significant.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The study population consisted of 688 patients (440 men, 248 women) with follow-up data, of whom 528 were white, 106 were South Asian, and 54 were African-Caribbean. A total of 157 first events were recorded during a mean follow-up period of 9.2±4.1 years, including 32 noncardiovascular deaths, 27 coronary deaths, 10 cerebrovascular deaths, 4 peripheral vascular deaths, 46 nonfatal myocardial infarctions, 20 nonfatal strokes, and 18 coronary revascularization procedures.
Comparison of Demographic Data
Table 1
gives a
comparison of the baseline demographic data in those with and without
events. The group with events was significantly older and had a higher
fasting cholesterol level than the group without events. A
significantly greater proportion of men than women, smokers than
nonsmokers, diabetics than nondiabetics, and those with than those
without a previous history of cardiovascular disease
experienced a subsequent event. In addition, proportionately fewer
African-Caribbean subjects (9%) developed an end point compared with
whites (23%; P<0.001) and South Asians (28%;
P<0.001).
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Comparison of Hemodynamic Data
Table 2 gives a comparison of the
baseline blood pressure data in those with and without events. All
components of systolic blood pressure and pulse pressure
(initial clinic, 24-hour mean, daytime mean, and nighttime mean) were
significantly greater in those who experienced a morbid event. The most
discriminatory diastolic blood pressure
parameter between the 2 groups was nighttime mean
diastolic blood pressure. The group of patients with events
had a significantly greater proportion of nondippers and smaller
nocturnal decreases in both systolic and diastolic
blood pressures compared with those without events. Heart rate
variability was also significantly lower in those who developed a
morbid end point.
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Cardiovascular Risk According to Level of
Ambulatory Systolic and Diastolic Blood
Pressures
Table 3 and Figures 1 and 2
summarize the incidence of all-cause, coronary, and
cerebrovascular events in the 688 patients according to strata of
24-hour mean ambulatory systolic and diastolic
blood pressures, respectively. These indicate that the relationships
between 24-hour mean ambulatory systolic blood pressure and
each category of event were approximately linear. In contrast, the
pattern of risk for 24-hour mean ambulatory diastolic blood
pressure was dependent on the type of event. For all-cause and
coronary events, a plateau effect was observed in the event
rates at lower levels of ambulatory diastolic blood
pressure, indicating no risk reduction below a certain level of
diastolic blood pressure. However, the relationship between
ambulatory diastolic blood pressure and cerebrovascular
events was essentially linear, with proportionate reductions in the
incidence of fatal and nonfatal stroke with a lower the level of
diastolic blood pressure.
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Multivariate Analysis
Table 4 lists the blood pressure
parameters that significantly improved the prediction of
events when added individually to a baseline regression model
containing clinical variables, with respective
2 and probability values. Whereas ambulatory
systolic and diastolic blood pressure
parameters (whether 24-hour, daytime, or nighttime mean)
and ambulatory pulse pressure provided independent information and
significantly improved the fit of the model for the prediction of
events, the addition of clinic blood pressure measurements to the
baseline model did not enhance the prediction of events. The regression
models with the best predictive value, as reflected by the
2 values, contained the ambulatory
systolic blood pressure parameters. Addition of the
quadratic function of 24-hour ambulatory diastolic blood
pressure [24-hour (diastolic blood
pressure)2] to the baseline model also
significantly enhanced the prediction of subsequent events, indicating
a curvilinear relationship with subsequent
cardiovascular risk. The equivalent function for
systolic blood pressure did not exhibit any independent
prognostic value. Neither the measures of blood pressure and heart rate
variability (including hourly SDs of blood pressure and heart rate,
degree of nocturnal decrease in blood pressure, and nondipper status)
nor the white-coat effect of systolic and diastolic
blood pressures was able to provide any incremental prognostic
information for the prediction of future events.
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In the regression model containing 24-hour ambulatory
systolic blood pressure, age (P<0.001), sex
(P<0.001), race (South Asians versus whites,
P=0.008), diabetes (P=0.05), previous history of
cardiovascular disease (P<0.001), and
24-hour ambulatory systolic blood pressure (P=0.001)
were independent predictors of time to a first morbid event (Table 5).
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Discussion |
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Since the introduction of ambulatory blood pressure monitoring >30 years ago, only 3 longitudinal studies have assessed the prognostic value of this test. Perloff et al11 measured daytime ambulatory blood pressure using a semiautomatic device in 1076 hypertensive subjects and showed that ambulatory blood pressures that were higher than predicted from a regression line between ambulatory blood pressure and office blood pressure were associated with an adverse outcome. Importantly, however, use of a patient-activated, semiautomatic, indirect blood pressure recorder did not allow measurement of nocturnal blood pressure. Verdecchia et al12 remedied this limitation by performing 24-hour, intermittent, noninvasive ambulatory blood pressure monitoring in 1187 subjects with essential hypertension and 205 normotensive control subjects. They showed that the risk conferred by white-coat hypertension was similar to normotension, but in those with ambulatory hypertension, particularly in female nondippers, the event rate was significantly higher. Finally, Ohkubo et al13 showed ambulatory blood pressure monitoring to be superior to clinic measurement for the prediction of all-cause mortality in a general Japanese population. However, these findings are not directly applicable to Western populations given that only 13% of deaths were attributable to heart disease. Moreover, clinic blood pressure was taken at only 1 sitting by an automated device.
The present study provides the longest follow-up data evaluating the prognostic value of ambulatory blood pressure monitoring and differs from previous reports in providing the only substantial data obtained with the intra-arterial technique. In our study, ambulatory blood pressure was evaluated as a continuous variable, thereby allowing assessment of the relationships between stratified levels of ambulatory blood pressure and subsequent cardiovascular risk. Linear and nonlinear relationships between ambulatory blood pressure and cardiovascular morbidity and mortality have not previously been evaluated in a hypertensive population but have important treatment implications. Our findings indicate that ambulatory blood pressure monitoring yields greater prognostic information than clinic blood pressure measurements. The addition of any 1 of the 24-hour, daytime, or nighttime mean ambulatory systolic or diastolic blood pressure parameters or ambulatory pulse pressure provided independent information for the prediction of all-cause events. This partly reflects the high degree of colinearity between systolic and diastolic blood pressures, making it difficult to determine the relative contribution of each of these components of blood pressure with disease risk. However, accumulating epidemiological data suggest that systolic blood pressure may confer greater prognostic information than diastolic blood pressure, particularly in the elderly. Our results lean toward this view, with the most predictive regression models containing the ambulatory systolic blood pressure parameters. These findings reaffirm the importance of systolic blood pressure in the pathogenesis of cardiovascular complications and counter the traditionally held assumption that diastolic blood pressure is the main determinant of cardiovascular disease. Previous studies have shown that a blunted decrease in nocturnal blood pressure is associated with left ventricular hypertrophy,10 14 coronary heart disease, and cerebrovascular manifestations.12 15 In the present study, although the proportion of nondippers was greater and the levels of nocturnal decrease in systolic and diastolic blood pressures were lower in the group of patients with compared with those without events, these parameters failed to provide independent prognostic information after adjustment for clinical variables.
Analysis of the relationships between 24-hour ambulatory systolic blood pressure and all-cause, coronary, and cerebrovascular events revealed a positive linear relationship with each category of event. Previous studies have also reported a positive linear relationship between systolic pressure and the incidence of stroke16 17 and found the lowest risk of a coronary event to be associated with the lowest systolic blood pressure.18 19 In contrast, 24-hour ambulatory diastolic blood pressure demonstrated a curvilinear relationship with all-cause and coronary events, indicating no reduction in risk below a threshold level of diastolic blood pressure. Much controversy surrounds the notion that lowering diastolic blood pressure too far in certain patients with hypertension may provoke a coronary event. Proponents of this J-curve phenomenon have emphasized the consistency of this finding across a variety of populations with hypertension20 21 and formed the hypothesis that an inappropriately low, treated diastolic blood pressure may lead to myocardial ischemia in predisposed subjects because of a failure of autoregulation in the coronary circulation.20 Others argue that the effect occurs as a result of an irregularity in small sets of data compounded by biased methods of analysis22 and suggest that the association between low blood pressure and mortality merely reflects a deterioration in general health rather than a treatment-induced causal relationship.23 Our finding of a nonlinear relationship between baseline pretreatment 24-hour ambulatory intra-arterial diastolic blood pressure and subsequent coronary events adds to the body of evidence in favor of a curved relationship, which has been observed in untreated populations.23 For cerebrovascular events, most studies have found the lowest diastolic pressures to be associated with the lowest event rates.16 19 24 25 The positive linear relationship between 24-hour ambulatory diastolic blood pressure and cerebrovascular events in the present study concurs with these findings.
In addition to the ambulatory blood pressure parameters, age, male sex, South Asian origin, diabetes mellitus, and previous history of cardiovascular disease were independent predictors of events. The increased risk of subsequent morbidity and mortality in South Asians compared with their white counterparts is consistent with epidemiological data from the United Kingdom and elsewhere, showing that South Asians are particularly prone to coronary heart disease and have a higher mortality from cerebrovascular disease than white subjects.26 27
Most of the prognostically important baseline demographic parameters were assessed in this study, but family history of cardiovascular disease was inadequately documented and hence not taken into account. Although the much-quoted New Zealand chart for assessment of cardiovascular risk28 does not include an evaluation of family history, data from the Framingham study suggest that this variable is an independent risk factor for coronary heart disease.29 It is therefore possible that an evaluation of family history of cardiovascular disease in our study may have altered the findings of multivariate analysis, but the inclusion of noncardiovascular death as an end point may have dampened its effects. A second limitation of this study was the lack of a formalized protocol for the assessment of blood pressure control and administration of antihypertensive drug therapy. Both of these aspects of hypertension management were left entirely to the discretion of the attending physician of the hypertension clinic or to the family practitioner. Blood pressure control was based on clinic blood pressure measurements as part of standard clinical practice; it was not considered justifiable on ethical grounds to repeat intra-arterial blood pressure monitoring, particularly in the face of developing noninvasive technologies for ambulatory blood pressure measurement in subsequent years. The vast majority of patients in this study were subjected to many switches in drug therapy, having been treated with a variety of regimens over the years, as is inevitably the case in large, longitudinal, cohort studies of this nature. Consequently, it was not possible to reliably evaluate the effect of specific classes of antihypertensive drug therapy on prognosis. Despite the importance of such treatment on events, highly significant relationships between baseline ambulatory blood pressure and subsequent risk remained, indicating only partial modification of risk by treatment in these patients. Similar independent relationships were not observed with clinic blood pressure. This study therefore shows that assessment of the 24-hour blood pressure load, particularly systolic blood pressure, provides a superior method of identifying high-risk patients.
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Acknowledgments |
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This study was supported by an educational grant from the Northwick Park Hospital Cardiac Research Fund. This study was performed under the invaluable guidance and support of the late Dr E.B. Raftery. We would like to thank Christopher Kinsey and the staff at the NHS Central Register, Southport, UK, for their assistance with data collection.
Received December 22, 1998; revision received June 7, 1999; accepted June 17, 1999.
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 R. Klein, B. E. K. Klein, S. E. Moss, and S. M. Meuer Retinal Emboli and Cardiovascular Disease: The Beaver Dam Eye Study Arch Ophthalmol, October 1, 2003; 121(10): 1446 - 1451. [Abstract] [Full Text] [PDF]
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 D. L. Clement, M. L. De Buyzere, D. A. De Bacquer, P. W. de Leeuw, D. A. Duprez, R. H. Fagard, P. J. Gheeraert, L. H. Missault, J. J. Braun, R. O. Six, et al. Prognostic Value of Ambulatory Blood-Pressure Recordings in Patients with Treated Hypertension N. Engl. J. Med., June 12, 2003; 348(24): 2407 - 2415. [Abstract] [Full Text] [PDF]
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 J. H. Young, M. J. Klag, P. Muntner, J. L. Whyte, M. Pahor, and J. Coresh Blood Pressure and Decline in Kidney Function: Findings from the Systolic Hypertension in the Elderly Program (SHEP) J. Am. Soc. Nephrol., November 1, 2002; 13(11): 2776 - 2782. [Abstract] [Full Text] [PDF]
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 A. Covic and D. Goldsmith Ambulatory blood pressure monitoring: an essential tool for blood pressure assessment in uraemic patients Nephrol. Dial. Transplant., October 1, 2002; 17(10): 1737 - 1741. [Full Text] [PDF]
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Y. Yamamoto, I. Akiguchi, K. Oiwa, M. Hayashi, T. Kasai, and K. Ozasa Twenty-four-Hour Blood Pressure and MRI as Predictive Factors for Different Outcomes in Patients With Lacunar Infarct Stroke, January 1, 2002; 33(1): 297 - 305. [Abstract] [Full Text] [PDF]
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R. S. Khattar, J. D. Swales, C. Dore, R. Senior, and A. Lahiri Effect of Aging on the Prognostic Significance of Ambulatory Systolic, Diastolic, and Pulse Pressure in Essential Hypertension Circulation, August 14, 2001; 104(7): 783 - 789. [Abstract] [Full Text] [PDF]
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E. Ritz, V. Schwenger, M. Zeier, and I. Rychlik Ambulatory blood pressure monitoring: fancy gadgetry or clinically useful exercise? Nephrol. Dial. Transplant., August 1, 2001; 16(8): 1550 - 1554. [Full Text] [PDF]
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P. Verdecchia, G. Schillaci, G. Reboldi, S. S. Franklin, and C. Porcellati Different Prognostic Impact of 24-Hour Mean Blood Pressure and Pulse Pressure on Stroke and Coronary Artery Disease in Essential Hypertension Circulation, May 29, 2001; 103(21): 2579 - 2584. [Abstract] [Full Text] [PDF]
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 R. S KHATTAR and J. D SWALES Pulse pressure and prognosis Heart, May 1, 2001; 85(5): 484 - 486. [Full Text]
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 F. A McAlister and S. E Straus Evidence based treatment of hypertension: Measurement of blood presssure: an evidence based review BMJ, April 14, 2001; 322(7291): 908 - 911. [Full Text]
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