(Hypertension. 1998;32:430-436.)
© 1998 American Heart Association, Inc.
Statistical Base Value of 24-Hour Blood Pressure Distribution in Patients With Essential Hypertension
Osamu Tochikubo;
Satoshi Hishiki;
Eiji Miyajima;
; Masao Ishii
From the Second Department of Internal Medicine, Urafune Hospital of
Yokohama City University (O.T., S.H., E.M.), and the Second Department of
Internal Medicine, School of Medicine, Yokohama City University (M.I.),
Yokohama, Japan.
Correspondence to Osamu Tochikubo, MD, Second Department of Internal Medicine, Urafune Hospital of Yokohama City University, 3-46 Urafune-cho, Minami-ku, Yokohama 232, Japan.
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Abstract
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Abstract—The purpose of this study was to calculate
statistically the minimum (base) blood pressure (BP) of nighttime
(sleep-time) BP values obtained by ambulatory BP monitoring (ABPM) and
to investigate its clinical significance. Twenty-four-hour
recording of ECG with ABPM was performed directly (n=89) or
indirectly (n=117) in 206 patients with essential hypertension. A
telemeter was used for the direct method and a multi-biomedical
recorder (TM2425) was used for indirect measurement. First, minimum
heart rate (HR0=60/RR0) was determined from
sleep-time ECG. The mean product of sleep-time
diastolic BP (DBP) and pulse interval (RR) was divided by
RR0 to obtain DBP0
[DBP0=(DBPxRR)s/RR0]. The correlation
between systolic BP (SBP) and DBP was used to determine
SBP0 corresponding to DBP0. Statistical base
mean BP (MBP0) was calculated from these values, and its
reproducibility and relation to hypertension severity were
investigated. MBP0 values were similar to true
base values of sleep-time MBP obtained by the direct method (mean±SD
difference, 2.0±4.2 mm Hg). Direct MBP0 criteria
predicted hypertension severity (mild, moderate, or severe target organ
damage) more accurately (predictive accuracy, 89%) than daytime MBP
criteria (53%, P<0.01). Almost the same results were
obtained using indirect MBP0 criteria. Day-to-day indirect
MBP0 variation (mean absolute difference) was smaller
(2.4±1.8 mm Hg) than day-to-day daytime and nighttime MBP
variation (6.3±5.3 and 5.4±3.4 mm Hg, respectively; n=61,
P<0.01), and the correlation coefficient between day-to-day
variations of daytime MBP and physical activity (measured by an
acceleration sensor) was 0.38 (P<0.05). In
conclusion, statistical base BP was almost equal to true base (minimum)
BP of sleep-time BP distribution. It was closely related to the
severity of hypertensive organ damage, was highly reproducible, and is
considered likely to serve stochastically and
physiologically as a
representative BP value in an individual subject.
Key Words: blood pressure monitoring, ambulatory • sleep • hypertension, essential
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Introduction
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The enormous fluctuations occurring in blood pressure
(BP) over a 24-hour period can involve as many as 100 000 directly
measured intra-arterial BP values and 
100 measurements
made indirectly by ambulatory BP monitoring (ABPM). A rational
statistical way of treating these large numbers of values is
fundamental to BP evaluation. Stochastically, each office BP or
indirect ABPM value is only a sample of the 24-hour direct BP values
occurring in an individual subject. Consequently, determining which of
the measurement values to adopt as a representative BP
value for an individual subject requires examination of the extent to
which each value fulfills the following essential conditions.
First, the value must be logically sound, stochastically as well
as physiologically. This means it must be a
statistical parameter with a
physiological basis. Second, to be clinically
useful and applicable, it must be closely related to hypertension
severity (organ damage). Third, it must be able to be measured
noninvasively in clinical practice. Fourth, its measurement values must
be highly reproducible and must demonstrate only slight day-to-day
variation. Fifth, ideally, it should evaluate the prognosis and risk of
hypertensive vascular complications.
Sir F. Horace Smirk1 reported that basal BP is more
intimately related to hypertension severity than casual BP. Applying
the same line of thought to 24-hour intra-arterial BP
values, we reported base BP (that is, minimum BP occurring during
nighttime sleep) to be a physiologically and
stochastically important BP value.2 3 4 5 6 7 However, because
base BP can be measured only with a large number of samples by the
direct method, we found it necessary to devise a method of calculating
statistical base BP with a small number of samples obtained indirectly
by means of ABPM, and we have investigated the extent to which values
obtained in this way satisfy the first 4 of the conditions set forth
above.
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Methods
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Patient Groups
This investigation was carried out in 206 patients (107 men and
99 women between 30 and 74 years of age) with hypertension (office
systolic BP [SBP] >140 mm Hg and diastolic
BP [DBP] >90 mm Hg on at least 3 visits) who were taking no
medication or for whom medication had been withdrawn at least 2 weeks
previously. These patients satisfied the following inclusion criteria:
no valvular defect, myocardial infarction, or secondary cause
of hypertension. They were subdivided into 3 groups (Table 1
). Group 1 (mild group with World Health
Organization [WHO] stage I) consisted of subjects with no objective
signs of hypertensive organic change.8 Their chest x-rays
showed a cardiothoracic ratio (CTR) of <50%. The sum of
SV1 and RV5 on their ECG was <4.0 mV, the
echocardiographic left ventricular mass
(LVM) index was <134 g/m2 for male subjects or 110
g/m2 for female subjects,9 and their
Keith-Wagner-Barker (KW) ophthalmic category was lower than grade 1.
They demonstrated no cerebral or renal hypertensive complications
(plasma creatinine level was <91.5 µmol/L). Group 3
(severe group) consisted of patients with accelerated hypertension
whose KW ophthalmic category was grade 3 or whose plasma
creatinine level was >152 µmol/L. Group 2 (WHO
stage II) was intermediate between these 2 (moderate group with LV
hypertrophy and without retinopathy, plasma
creatinine <152 µmol/L). Table 1 shows numbers of
subjects and gender, age, mean office BP, and main clinical
characteristics for each group.
Methods consisted of direct and indirect ABPM measurements. Direct
measurements were made in 89 patients (47 men and 42 women; 37 from
group 1, 38 from group 2, and 14 from group 3) after hospitalization.
Indirect BP measurements were made in 117 patients (60 men and 57
women; 52 from group 1, 52 from group 2, and 13 from group 3) at the
outpatient clinic. All subjects gave informed consent to participation
in the study, and the study protocol was approved by the ethics
committee of the Yokohama City University Department of Internal
Medicine.
Direct BP Measurement
Although it has been reported previously,6 10 we
here briefly outline the procedure whereby direct BP measurements were
made. Using a system developed in our laboratory, we used the
telemetric method to perform continuous 24-hour monitoring of
intra-arterial pressure and ECG under almost unrestricted
conditions. With patients under local anesthesia, a
stiff-walled Teflon catheter was inserted into the left brachial artery
and connected to a strain-gauge transducer (Statham P50, Gould Statham
Instruments Inc) attached to the chest wall at the level of the heart.
BP calibrations were performed with a sphygmomanometer at 0, 100, and
200 mm Hg both before and after measurement. To prevent
coagulation, a portable microinfusion pump (NEC Sanei Instruments Ltd)
continuously flushed the catheter with heparinized saline solution. A
V5 lead was used to record ECG waves, which, together
with BP waveform, were telemetrically transmitted by means of a
portable transmitter (model 1429, NEC Sanei Instruments Ltd). Both
paper and tape recordings were collected (CR-31 portable tape
recorder, TEAC Co). An analog-to-digital converter was used to
input BP and ECG waves into a computer at a sampling rate of 1 kHz for
subsequent computer processing to determine beat-to-beat heart rate
(HR), SBP, and DBP.
In accordance with the hospital schedule, sleep-time (nighttime) was
defined as the hours between 9 PM and 6 AM. The
remainder of the 24-hour period was regarded as waking time (daytime).
The 24-hour trendgrams and sleep-time frequency histograms of BP and HR
were made by computer (Figure 1A and 1B).
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Figure 1. Representative tracings produced by
computer show 24-hour intra-arterial SBP and DBP trendgrams
(A). Upper tracings are BP trendgrams of mean±SD plotted every 3
minutes. Middle and lower graphs are trendgrams of maximum and minimum
values (0.5% extremity of frequency histograms) during 1 hour.
Histograms show SBP and DBP frequency during sleep-time (B). The points
of the 0.5% lower probability integral were taken as true base
(minimum) SBP and DBP.
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Measurement of True Base BP Values From Direct Recordings
The minimum values (values corresponding to the 0.5%
lower probability integral during 1 hour) of intra-arterial
SBP and DBP were almost the same as baseline values during
sleep-time2 (Figure 1A
). Sleep-time frequency
distributions for intra-arterial SBP, DBP, and HR were
compiled; values corresponding to the 0.5% lower probability integral
were taken as true base SBP and DBP2 3 (Figure 1B).
Measurement of Estimated Base BP Values From Direct
Recordings
Because true base BP values can be calculated only on the basis
of large numbers of directly obtained samples (about 30 000 samples),
we devised a new method for determining statistical (estimated) base BP
values from a small number of either directly or indirectly obtained
samples. HR can be measured noninvasively by ECG, which makes it
possible to derive the minimum value (lowest 1% value;
HR0) from the HR frequency histogram obtained during
sleep-time. This determines the corresponding pulse interval (RR) or
RR0 (RR0=60/HR0 is the lowest 1%
value of HR histogram during sleep-time) (Figure 2). Base value DBPxRR (product of
DBP and RR) was almost equal to mean sleep-time DBPxRR and mean
waking-time DBPxRR.4 Using this value, we calculated the
mean of DBPxRR during sleep-time: (DBPxRR)s. Then, dividing this by
RR0, we obtained the statistical base DBP
(DBP0):
 | (1) |
Because of the high positive coefficient of correlation
(r) between direct SBP and DBP (r=0.70 to
0.98),11 we used the linear-regression formula
SBP=axDBP+b (where a is the regression coefficient and b is constant
value) with sleep-time values to obtain the statistical base SBP
(SBP0) corresponding to DBP0:
 | (2) |
Next, statistical base mean BP (MBP0) was calculated
from DBP0 and SBP0
[MBP0=(SBP0-DBP0)/3+DBP0].
This method for obtaining statistical base BP values is applicable to
BP values obtained both directly and indirectly (Figure 2A and 2B). In
this study, its accuracy was evaluated first using directly obtained BP
values and then with indirect BP measurements.
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Figure 2. Graphs show the methods of calculating direct (A)
and indirect (B) statistical base SBP and DBP by computer.
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Indirect BP Measurement
The portable multi-biomedical recorder (TM2425, A&D Co Ltd)
can simultaneously record 24-hour indirect BP and ECG
and measure body position, motion (acceleration), and temperature. The
BP accuracy of TM2425 was validated according to and satisfied the
criteria of the Association for the Advancement of Medical
Instrumentation.12 13 Body motion (activity) was measured
by an acceleration pickup sensor, and cumulative values for 1 minute
were recorded at 18-millisecond intervals at frequencies ranging
from 1 to 10 Hz in the vertical direction, with a sensitivity of
4.1x10-3 m/s2.12 SBP and DBP
were recorded at 30-minute intervals, and HRs were recorded
beat-to-beat on ECG by TM2425. The process of obtaining indirect
statistical base BP by computer is as follows (Figure 2B). First, a
sleep-time HR distribution was compiled from the ECG obtained with
TM2425. Arrhythmic beats were automatically excluded.12 HR
frequency distribution, mean, SD, skewness (sk), and minimum (lowest
1%) value were calculated by computer, and HR0 and
RR0 (60/HR0) were derived. After indirect
DBP0 was obtained by means of Equation 1, Equation 2 was
used to arrive at indirect SBP0 (Figure 2B). The
coefficient of correlation (r) between sleep-time SBP and
DBP was often smaller than the corresponding value obtained with the
direct method, but Equation 2 was applied only when r
0.6
(n=82). In instances in which r<0.6 (n=35), the following
equations (arrived at stochastically from least-squares linear
regression analysis performed on 82 patients in whom
correlation between SBP and DBP was r>0.6 were used for
estimating SBP0):
SBP0=DBP0x[(SBP/DBP)s+0.05] and
(SBP/DBP)s=mean SBP/DBP ratio during sleep-time. The difference between
SBP0 derived from these equations and SBP0
derived from Equation 2 was 1.5±4.9 mm Hg in the 82 patients.
The indirect measurements were performed during the subjects' daily
routines, and sleep-times were determined from their diaries.
With the aim of investigating the reproducibility of statistical base
BP, 24-hour measurements were performed twice (Figure 3) using TM2425 for 61 patients (31 men
and 30 women). The second measurement was made more than 1 week after
the first (median interval of 3 weeks; range, 1 week to 2 months).
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Figure 3. 24-Hour trendgrams show body position, SBP, DBP,
pulse rate (PR), temperature (TEMP), and physical activity (ACT)
monitored by TM2425 in a patient. A, first day; B, second day. ACT
indicates ACT measured by a ceramic acceleration pick up sensor;
ACTW, mean ACT per minute during daytime; G, acceleration
of gravity (m/s2); and  ,
MBPW/MBP0.
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Echocardiographic Measurements
Because LV hypertrophy is related to hypertension
severity, echocardiography was used to examine LVM.
All subjects underwent standard M-mode and 2-dimensional
echocardiography performed by a cardiologist using
an echocardiograph equipped with a 2.5-MHz and a 3.6-MHz
imaging transducer (SONOS 2500 ultrasound system, Hewlett-Packard Co).
As recommended by the American Society of
Echocardiography,14 LV dimensions were
derived from 2-dimensionally guided M-mode tracings. LVM was calculated
using the Penn convention15 and adjusted for body surface
area (LVM index). Echocardiograms were read in a blinded fashion
without knowledge of other findings.
Statistical Analysis
Standard statistical methods, including paired 2-sample
t test, nonpaired t test, F test,
2 test, sensitivity test, specificity
test,16 and ANOVA were used. Least-squares linear
regression analysis and Bland-Altman plotting method were
performed for 2 variables. MBP criteria (cutoff point) were
determined by discriminant analysis17 to
discriminate well between the groups [discriminant functions:
Z12=a12MBP+b12,
Z23=a23MBP+b23; Z12
(a12 and b12) and Z23
(a23 and b23) were determined to allocate a
patient to group 2 if Z12 >0 and to group 3 if
Z23 >0]. Predictive accuracy was calculated as follows:
(n1+n2+n3)/total number;
n1, n2, and n3 were numbers of
patients classified accurately into each group (1, 2, and 3) by MBP
criteria. Values were expressed as mean±SD, and values of
P<0.05 were considered significant. The Multiple
Statistical Analysis Program (Social Survey Research
Information Co, Ltd) was used for calculations.
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Results
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Comparison Between True Base BP and Statistical Base BP From
Direct Recordings
The correlation between the true base DBP and statistical base DBP
(direct DBP0) by the direct method was high
(r=0.93). The mean difference between them was 2.6±4.8
mm Hg. A high coefficient of correlation (r=0.93) was also
found between the true base SBP and statistical base SBP (direct
SBP0). The difference between them was 1.0±6.0
mm Hg. MBP calculated from direct DBP0 and
SBP0 was taken as direct MBP0. The difference
between true base MBP and direct MBP0 assessed by paired
t test and Bland-Altman plotting was not significant,
although MBP0 tended to be higher than true base MBP (mean
difference, 2.0±4.2 mm Hg) (Figure 4).
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Figure 4. Scatterplot shows Bland-Altman plotting between
true base MBP and statistical base MBP (direct MBP0). The
mean difference between them is 2.0±4.2 mm Hg.
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Statistical Base BP and Hypertensive Target Organ Damage
Waking-time mean MBP was obtained and designated MBPW.
Investigation was then performed to determine whether statistical base
MBP or MBPW is more closely related to hypertension (organ
damage) severity (Figure 5, Table 2). MBP criteria (MBPW=110 to
129 mm Hg; MBP0=89 to 109 mm Hg) were
established to discriminate well between the groups by discriminant
analysis (Table 2).
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Table 2. Relationship Between MBP Criteria and Classified
Groups According to Degree of Hypertensive Organ Damage
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Sensitivity and specificity for direct MBP0 criteria were
almost equal to or higher than those for direct MBPW
criteria (Figure 5, Table 2). The predictive accuracy (sum of
accurately classified number/total numberx100) of direct
MBP0 criteria (79/89x100=89%) was significantly higher
than that of direct MBPW criteria (47/89x100=53%). These
results suggest that direct MBP0 criteria predict
hypertension (target organ) severity more precisely than direct
MBPW criteria.
In this case, MBPW is expressed as a function of
xMBP0
(=MBPW/MBP0),5 and it becomes
essential to know whether is independently related to hypertension
severity. The absence of a significant difference in values among
the 3 groups (Figure 5) indicates that contains no information
related to hypertension severity. A negative coefficient of correlation
(r=-0.43, P<0.01) was observed between direct
MBP0 and (Figure 5, bottom).
Almost the same results were obtained from indirect measurements
(Figure 6, Table 2). There was a high
correlation between indirect MBP0 and
echocardiographic LVM index (r=0.75,
P<0.01; Figure 6). The correlation coefficient
(r) between indirect MBP0 and indirect
MBPW was 0.72, and r between indirect
MBPW and LVM index was 0.61.
Mean physical activity (acceleration) during waking hours
(ACTw) in group 3 patients was less than in groups 2 and
group 1 (8.4±2.4: 14.2±6.8: 18.7±6.4x10-3
m/s2 per minute; P<0.05).
Day-to-Day Variation of Statistical Base BP
To investigate the reproducibility, we used a multi-biomedical
recorder (TM2425) to observe daily (day-to-day) variation (mean
absolute difference between values obtained in 2 measurements) in
indirect MBP0 and MBPW (Figure 7). Day-to-day indirect MBP0
variation was significantly less (2.4±1.8 mm Hg) than daily
indirect MBPW variation (6.3±5.3 mm Hg,
P<0.001). Furthermore, day-to-day indirect MBP0
variation was significantly less than daily nighttime indirect MBP
variation (5.4±3.4 mm Hg, P<0.01).
There was a significant correlation (r=0.38,
P<0.05) between day-to-day MBPW variation
(first-day MBPW- second-day MBPW) and physical
activity variation (first-day ACTw-second-day
ACTw, where ACTw is mean acceleration during
waking hours).
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Discussion
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Because of the great influence of daily activity and duration of
sleep, the mean of nighttime BP values and the day-night differences
demonstrate low reproducibility.18 19 20 To compensate for
this fault, we propose statistical base BP as a new index
representative of nocturnal sleep-time BP. This study
investigates the extent to which this new index serves as an individual
representative BP value satisfying the conditions set
forth in the introduction.
Sir F.H. Smirk first stated that for purposes such as measuring basal
metabolism, basal BP was more closely related to
hypertensive severity and mortality than casual BP, but supplemental BP
(casual BP minus basal BP) was not related to them.1
Applying this concept to ABPM measured values, we have specified
minimum sleep-time BP (base BP) and have repeatedly studied its
significance.2 3 4 5 6 7 Instead of manifesting a gaussian
distribution, intra-arterial BP and HR values obtained
during nighttime and daytime are distributed asymmetrically with a
levopositional mode (mode<mean) in a manner approximating a gamma
distribution that has a minimum value (location
parameter).8 9 We found that base BP was
almost the same as the location (L) parameter of the BP
frequency distribution, which approximated a gamma
distribution.2 3 The L parameters took
approximately the same values for both nocturnal sleep-time and
waking-time BP distributions and seemed to be the starting point for
their gamma distributions.3 True base BP corresponds to an
actual measured value of the L parameter,2
which is an important parameter of these BP
distributions.
BP reaches a minimum value during the electroencephalographic slow-wave
phase.3 6 21 The base BP waveform manifesting itself at
this time takes the most efficient pulse interval (RR) for producing a
maximum per-minute waveform area, which indicates a maximum per-minute
blood flow in relation to BP value.5 Moreover, at this
time, base BP demonstrates a higher correlation with total
peripheral vascular resistance and arterial
elastic modulus than MBPW.5 DBP0
and SBP0, which are statistical base BP values, are the
estimated values for the true base BP values of each of their frequency
distributions and are thought to satisfy the stochastical and logical
conditions set forth in the introduction.
Every BPi (i=1 ... n) during daytime and nighttime may be
expressed by the formula BPi=ixbase BP
(i=incre-mental ratio). The i value,
which can be analyzed by means of the formula
i
FI+BI (FI indicates cardiovascular
function index; BI, baroreflex function index),5 7 may
vary because of the changes of cardiovascular and
baroreflex responses to physical and mental activity. Because
(MBPW/MBP0) is unrelated to hypertension
(target organ) severity and correlates with physical activity
(group 3 patients had lower and lower physical activity), using
MBP0 is considered more logical than using MBPW
(xMBP0) for evaluation of hypertension severity. Figure 3 shows typical variations of value and indirect MBPW
caused by changes in physical activity (acceleration) monitored in 1
patient with TM2425. Although the means of waking-time BP and nighttime
BP are also statistical parameters, because they are
greatly influenced by daily activities, their values may be changed
by alterations in such factors as external temperature, insufficient
sleep, duration of sleep, physical activity (acceleration), emotional
stress, and autonomic nervous activity.12 18 19
Indirect MBP0 values demonstrated less day-to-day variation
than the indirect MBPW (xMBP0) and
nighttime mean MBP values (Figure 7).
This means that daily activities and sleep duration may exert no
influence on statistical base BP. It is therefore more logical to use
it instead of mean nighttime BP as a value
representative of sleep-time BP.
The circadian change in BP in human subjects is well recognized, with
BP levels being lower at night, and this observation has led to the
perception of hypertensive subjects as dippers (with a nocturnal BP
fall or dip) and nondippers.22 23 There are many reports
that nighttime BP is closely related to end-organ damage, which is more
severe in nondippers than dippers.24 25 However, the
reproducibility of BP dipping (nighttime BP/daytime BP) is
poor.19 We suggest that the statistical base BP, instead
of mean nighttime BP, may be useful for evaluating the extent of the
dipping phenomenon (dipping corresponds to value).
It is true that we should compare indirectly measured statistical base
BP with true base BP obtained by means of the direct method, but such
an investigation would be very difficult because of the great burden
imposed on subjects by the need for simultaneous 24-hour
measurement of both direct and indirect BP. In addition, there are
ethical reasons for rejecting such a practice. Our inability to perform
this kind of study prevents our being able to conclude yet whether
statistical indirectly measured base BP is consistent with true
base BP value obtained with the direct method.
Nonetheless, statistical base BP is considered significant as a BP
value that can serve as a normalized BP value free of the effects of
environmental conditions. It reflects true basal BP, and because it
approximately fulfills the first 4 of the 5 conditions set forth above
(logical, clinical, noninvasive, and highly reproducible conditions),
statistical base BP is believed to be suitable as an individual
representative BP value. Determination of its relation
to the second and fifth conditions (indicating severity and prognosis
of hypertension) requires further study involving noninvasive BP
measurement in a much larger number of subjects. The present study
is only the first step toward achieving this ultimate goal.
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Acknowledgments
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The authors wish to thank Shigehiro Ishizuka and Kenichirou
Yasaka, Research and Development Section, A&D Co, Ltd, Tokyo, Japan,
for providing the computer program used for statistical base BP
calculations of TM2425 recordings. This program is commercially
available.
Received March 5, 1998;
first decision March 19, 1998;
accepted April 13, 1998.
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Abstract
Introduction
, group 1; 
, group 2; 
, group 3. Mean 
y2-y1
