Download Continuous vs Intermittent Blood Pressure

Continuous vs Intermittent Blood Pressure Measurements
in Estimating 24-Hour Average Blood Pressure
MARCO DI RIENZO, P H . D . , GUIDO GRASSI, M.D.,
ANTONIO PEDOTTI, P H . D . , AND GIUSEPPE MANCIA,
M.D.
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SUMMARY In the past few years noninvasive automatic blood pressure (BP) recorders have been
increasingly used to estimate patients' 24-hour BP more accurately than by one or few isolated
measurements. However, these recorders only allow BP to be intermittently measured at intervals
between 5 to 30 minutes, which means that the number of values collected over 24 hours (10 to 100)
remains a tiny fraction of the thousands of values that occur during the same period. To determine
whether this represents a limitation to this approach, BP was recorded intraarterially for 24 hours
(Oxford method) in 20 ambulant hypertensive patients. A beat-to-beat analysis of the BP recording
was provided by a computer, and the average 24-hour systolic, diastolic, and mean BP values were
compared with those obtained by analyzing single BP waves of the same recording at intervals of 5,10,
15, 30, and 60 minutes. In each subject the average 24-hour BP values obtained by the beat-to-beat
analysis closely corresponded to those obtained by the analysis performed at 5-, 10-, 15-, or 30-minute
intervals. In most subjects, this was the case also when the analysis was performed at 60-minute
intervals. Thesefindingsdemonstrate that intermittency of measurements does not limit the accurate
assessment of true average BP. Indeed, accurate assessment can be achieved at intervals as much as 30
or 60 minutes apart. (Hypertension 5: 264-269, 1983)
KEY WORDS • blood pressure variability
intraarterial blood pressure recording
automatic blood pressure recording
I
TABLE 1. Age, Sex, 24-Hour Mean Arterial Pressure
and Standard Deviation for MAP (SD) in 20 Subjects
N recent years noninvasive automatic blood pressure recorders have been developed in an attempt
to record blood pressure more accurately than has
been possible with instruments affording isolated measurements1"5 often collected under psychologically unfavorable conditions. 4 ' 6 ^ Most of these new recorders
still present limitations with regard to accuracy.9"13
Also, there are problems with the intermittency of the
readings, usually at intervals of 5 to 30 minutes, which
means a maximum of a few hundred values in 24
hours. This number is a minute fraction of the many
thousand values that make up a 24-hour blood pressure
profile. Our study investigates whether this limitation
affects the accuracy of the results.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Methods
Our study was performed on 20 hospitalized subjects all with uncomplicated untreated essential hypertension of mild or moderate degree. In each subject
arterial blood pressure was invasively recorded for 24
hours with the Oxford technique,14-15 the arterial cath-
Istituto di Clinica Medica IV, Universita di Milano, Centre di
Rsiologia Clinica e Ipertensione, Ospedale Maggiore, CNR, and
Centre di Bioingegneria, Fondazione Don Gnocchi, Milano, Italy.
Address for reprints: Professor Giuseppe Mancia, Clinica Medica IV, Policlinico, Via F. Sforza 35, 20122 Milano, Italy.
Received April 5, 1982; revision accepted October 7, 1982.
Age
(yrs)
45
52
41
29
51
36
Sex
M
M
M
F
M
F
24
M
48
43
63
50
33
43
42
38
47
54
38
45
68
F
F
M
M
M
F
M
F
M
M
F
M
F
MAP
(mm Hg)
110.7
144.5
105.2
108.3
100.5
103.9
100.3
113.9
119.3
97.5
130.4
92.0
117.5
188.5
105.6
130.5
100.3
76.7
83.9
140.5
(MAP),
SD
(mm Hg)
9.9
17.2
12.8
19.4
15.4
14.5
11.5
18.5
18.3
12.9
18.4
16.2
15.7
15.3
13.9
17.1
10.8
12.3
12.4
10.7
MAP refers to the average 24-hour values obtained in each subject by continuous analysis of the direct blood pressure recording.
SD also refers to data obtained by continuous analysis.
264
INTERMITTENT VS CONTINUOUS BLOOD PRESSURE ASSESSMENTS/ Rienzo et al.
eter being implanted percutaneously in a radial artery.
During the recording the subjects were free to move
within the hospital area and to engage in recreational
and social activities as inpatients not confined to bed.
Each recording was analyzed by a computer (PDP
11/34) that sampled the blood pressure signal every 60
msec and calculated for each pressure wave the systolic, diastolic, and mean arterial pressures. These values were stored on a magnetic disk to be later analyzed
by two different methods. First, the average systolic,
diastolic, and mean arterial pressures were calculated
for the whole 24-hour recording period by making use
265
of all available values, i.e., the values of all pressure
waves. Second, the average systolic, diastolic, and
mean arterial pressures over 24 hours were also calculated by making use of the data of single pressure
waves regularly taken at intervals of 5, 10, 15, 30, or
60 minutes. The results obtained by this latter method,
which simulated intermittent blood pressure measurements, were compared with those obtained by the former method in which measurements were virtually
continuous.
In each subject, calculation of average 24-hour systolic, diastolic, and mean arterial pressures was ac-
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A SBP
mmHg
0
i-^-^V-
-
•--
- 15
+ 10
*"
m vr
-10
+ 10
0--»=- =j,
- 10
+ 1O
]0
0 -10-
- 10J
5
10
15
30
60
5
10
15
30
60
periodical
0
— 24 h SBP by continuous
5
10
15
30
60
10
15
30
60mm
measurements
recording
FIGURE I. Evaluation of the average 24-hour systolic blood pressure (SBP) value obtained by taking into account all SBP values in
the 24 hours and by considering SBP values at intervals of 5, 10, 15, 30, and 60 minutes throughout the same period of time. The
results obtained via the second method of analysis (indicated by the points) are shown as differences from the result obtained via the
former method of analysis (indicated by the dashed horizontal line intersecting the 0 point). Data from the 20 subjects are separately
shown.
266
HYPERTENSION
companied by calculation of their respective 24-hour
standard deviations. Also, in this instance a comparison was made of the results obtained through continuous and variably intermittent analyses of the data. In
this way it was possible to assess whether intermittent
blood pressure measurements could evaluate blood
pressure variability precisely, a phenomenon that is
important for understanding cardiovascular regulation16"18 and which may also represent an independent
risk factor in hypertension.
The results were separately analyzed for each patient and also calculated as means ( ± SE) for the whole
group of patients. Before performing the 24-hour
blood pressure recording, all patients gave free and
informed consent to the procedure.
VOL 5, No
2,
MARCH-APRIL
1983
Results
The average 24-hour mean arterial pressure obtained in each subject by continuous analysis of the 24hour recording is shown in table 1. Comparison of the
data obtained by the continuous and intermittent measurements of the blood pressure tracing are shown in
figures 1,2, and 3. In each patient the intermittent
measurements provided average values for systolic,
diastolic, and mean arterial pressure that were close
(and often identical) to those obtained by the continuous measurements. This occurred when the intervals
between the intermittent measurements were set at 5,
10, or 15 minutes, and also (with a slightly greater
approximation) when the intervals were set at 30 minutes. Even prolongation of the intervals to 60 minutes
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A DBP
mm Hg
10
lO
r
Or
-
«
-10-
0-10+ O-i
10-
1O
1©
- 1OJ
0 -10-
5
0
10
15
30 60
5
10 15 30 60
5 10 15 30
p e r i o d i c a l measurements
— 24 h DBP by continuous
60
10
15
30
60min
recording
FIGURE 2. Evaluation of the average 24-hour diastolic blood pressure (DBP) value obtained by taking into account all DBP values
in the 24 hours and by considering DBP values at intervals of 5, 10, 15, 30, and 60 minutes throughout the same period of time.
Explanations as for figure I. Data from the same subjects as in figure 1.
INTERMITTENT VS CONTINUOUS BLOOD PRESSURE ASSESSMENTS; Rienzo el al.
caused only small differences to appear in only few
subjects. When expressed as average data for all patients (fig. 4), the 24-hour readings provided by the
intermittent measurements at 60-minute intervals differed from those obtained through continuous measurements by no more than 2-3 mm Hg. For the intermittent measurements at shorter intervals, these
differences were 1 mm Hg or less.
The results obtained for mean arterial pressure variability are shown in table 1 and figure 5, which compare the data obtained by continuous and intermittent
measurements. The 24-hour standard deviations of
mean arterial pressure that were obtained by intermittent and continuous measurements differed much more
than the average 24-hour mean arterial pressure values. When the intervals between the intermittent meas-
267
urements were 5, 10, or 15 minutes, the differences
with the data obtained by continuous measurement
were in the range of ± 10%. When the intervals were
of 30 or 60 minutes, the differences remained within a
similar range in a number of subjects, but when the
intervals were 24 hours, the differences increased in a
number of patients, to produce erratic results. Similar
results were obtained for standard deviations of systolic and diastolic blood pressure.
Discussion
Our observations demonstrate that over a 24-hour
period the average blood pressure values from measurements at intervals from 5 to 30 or 60 minutes do not
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A MAP
mmHg
10
lO
0 -10
+ 10-1
(
0 -10
+ 10
0 - 1OJ
+1O
1©
- 1OJ
+ 10-1
- 10 J
5
0
10
15
30
60
5
10
15 30 60
5 10 15 30 6 0
periodical measurements
24 h MAP by continuous
10
15 30
60min
recording
FIGURE 3. Evaluation of average 24-hour mean arterial pressure (MAP) values obtained by taking into account all pressure values
in the 24 hours, and by considering MAP values of the pressure waves occurring at intervals of 5, 10, 15, 30, and 60 minutes
throughout the same period of time. Explanations as for figure I. Data from the same subjects as in figure I.
268
HYPERTENSION
DBP
SBP
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0
15
30
No
2, MARCH-APRIL
MAP
5
10
15
30
60
periodical measurements
— 24 h by continuous recording
10
VOL 5,
60
10
15
30
FIGURE 4. Mean(± SEM) data for SBP, DBP, and MAP obtained from the individual data shown in figures 1, 2,
and 3. Explanations as in the preceding figures; 24-hour average (± SE) values for systolic, diastolic, and mean
arterial pressure obtained by analysis of all values were, respectively, 174.5 ± 6.7, 101.2 ± 3.4 and 125.4
± 4.3 mm Hg.
+ 24
+ 200
20
180
16
z
^ 160
E
E
c
o
- 140
c
o
•
.? 6
a
•o
c
60
•o
•o
J 40
•o
c
"
<
2
0 -
20
0 20
-
- 40
4
10
0
15
30
60
periodical
5
10
measurements
15
30
60min
24 h SD by continuous recording
FIGURE 5. Evaluation of 24-hour standard deviation for MAP obtained by taking into account all pressure
values in the 24 hours, and by considering only the pressure waves occurring at intervals of 5, 10, 15, 30, and 60
minutes. The results obtained via the second method of analysis are shown for each subject as absolute (left panel)
and percent (right panel) differences from those obtained via the former method of analysis (for each subject
represented by the dashed horizontal line intersecting the 0 point). The 24-hour average (± SE) standard deviation
for MAP was 15.1 ± 1.1 mm Hg.
1983
INTERMITTENT VS CONTINUOUS BLOOD PRESSURE ASSESSMENT/^ Rienzo et al.
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differ substantially from mean values obtained by continuous measurement. This finding implies that a large
number of blood pressure values are not necessary to
achieve accurate mean values, a factor that may considerably reduce the complexity and cost of computer
programs and facilities. Also, and more important,
these results reinforce the use of noninvasive techniques that enable blood pressure levels to be automatically assessed at selected intervals. The accuracy of the
single blood pressure readings provided by these devices is still under scrutiny. However, it is important
that intermittency of blood pressure assessment does
not itself represent a limitation in estimating the true
average blood pressure of patients. It is also important
that such an assessment can be obtained even by increasing the interval of the sampling up to 30 or 60
minutes. This may justify future adoption of automatic
blood pressure measurements more widely spaced,
with less disturbance to the patient. It may also simplify the construction of the recorders and avoid some
technical difficulties.
With regard to the second issue of our study, the
standard deviations of the 24-hour blood pressure values were also not too different when derived by intermittent blood pressure measurements or by continuous
blood pressure analysis. However, for intervals of
measurements of 5 to 15 minutes, these differences
encompassed a range of ± 10%, and this was much
more than the ± 1 % to 2% difference found for the
mean blood pressure values. Furthermore, in a number
of subjects (about 30%) the differences in the standard
deviation for blood pressure became larger, and often
much so, for intermittencies of 30 or 60 minutes. This
may be explained simply in statistical terms: according
to the inference formulas, when the sampling size is
reduced the potential error in estimating the true variance of the population under study progressively increases, and the rate of this increase largely exceeds
that which characterizes the error in estimating the
mean of the same population. At any rate, it is clear
that intermittent blood pressure measurements may assess blood pressure variability less precisely than is the
case with mean blood pressure values. In particular
serious errors may occur if blood pressure assessments
are too far apart from each other.
269
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Continuous vs intermittent blood pressure measurements in estimating 24-hour average
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M di Rienzo, G Grassi, A Pedotti and G Mancia
Hypertension. 1983;5:264-269
doi: 10.1161/01.HYP.5.2.264
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