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Comparison of Auscultatory and Oscillometric Blood Pressures
Myung K. Park, MD;
Shirley W. Menard, RN, PhD;
Cheng Yuan, PhD
Arch Pediatr Adolesc Med. 2001;155:50-53.
ABSTRACT
Objective To study the differences in blood pressure readings between the auscultatory
and oscillometric (Dinamap model 8100; Critikon, Tampa, Fla) methods.
Design Survey of 2 blood pressure instruments.
Setting Public schools.
Participants Seven thousand two hundred eight schoolchildren aged 5 through 17 years.
Main Outcome Measure Blood pressure levels.
Results For all children combined, Dinamap systolic
pressure readings were 10 mm Hg higher (95% confidence interval, -4
to 24 mm Hg) than the auscultatory systolic pressure readings. Dinamap diastolic
pressure readings were 5 mm Hg higher (95% confidence interval, -14
to 23 mm Hg) than the auscultatory Korotkoff phase V diastolic pressure readings.
Conclusion These findings preclude the interchange of readings by the 2 methods.
Caution must be exercised in the diagnosis of hypertension when an automated
device is used.
INTRODUCTION
AN IMPORTANT element in pediatric practice is the routine measurement
of blood pressure (BP). Since the accuracy of an oscillometric device in reflecting
direct arterial pressure was reported in the neonate,1, 2
infant and child,1, 3, 4
and adult,5 the Dinamap monitor (Critikon,
Tampa, Fla) has become widely used in pediatric patient care. The comparison
of BP values obtained during office visits against normative values is used
as a screen for asymptomatic hypertension, assuming that those values are
comparable whether or not the same methods for measurement were used. In a
previous study, we reported that the systolic pressure levels obtained by
the Dinamap method (model 1846SX) were 6 mm Hg higher (P<.05) than those obtained by the auscultatory method in fifth-grade
children.6 It is not known if there is also
a clinically important magnitude of difference in BP levels measured by the
2 devices for other age groups or how the newer model (8100) of Dinamap device
compares with auscultatory BP measurement. This information is important in
view of the wide clinical use of the new model of the Dinamap monitor.
The San Antonio (Tex) Triethnic Children's Blood Pressure Study was
conducted to test for possible ethnic differences in BP levels among 3 major
ethnic groups of children. We used both the auscultatory method and Dinamap
model 8100 because the latter was devoid of observer-related variability in
BP readings. We found that there were significant differences between BP readings
obtained by the 2 methods. This report addresses the magnitude and clinical
implications of these differences.
METHODS
In the San Antonio Triethnic Children's Blood Pressure Study, both oscillometric
and auscultatory BP readings were obtained on 7208 schoolchildren aged 5 through
17 years. A letter of invitation was sent to the parents of each child enrolled
in kindergarten through 12th grade. The parents of the participating children
signed their consent and the children assented to participate in the study.
The University of Texas Health Science Center's institutional review board
approved the proposal. There were 3356 boys (46.6%) and 3852 girls (53.4%)
of whom 4215 were Mexican American (58.5%), 2040 were non-Hispanic white (28.3%),
and 953 were African American (13.2%).
The Dinamap model 8100 was used to measure BP, which was either preceded
or followed by auscultatory measurement of BP. The order of the measurement
was randomized so that about half of the children had their BP measured by
the auscultatory method first followed by the Dinamap method (A rotation),
and the remainder of the children had their BP measured first by the Dinamap
followed by the auscultatory methods (D rotation). Two Dinamap monitors were
used in this study. Research nurses who took oscillometric BP readings were
taught to use the Dinamap monitor through in-service training, familiarization
with the manual, and "hands-on" practice. The oscillometric device was calibrated
for accuracy by the bioengineering department of our institution once every
month or whenever a malfunction was suspected.
The auscultatory method of BP measurement used a Baumanometer mercury
gravity sphygmomanometer (WA Bauman Co, Copiague, NY). Because of well-known
observer-related variability,7 only those research
nurses who successfully completed the 6-hour Instructor's Course for Blood
Pressure Determination offered by the American Heart Association, Texas Affiliate,8 were selected to measure auscultatory BP. To ensure
acceptable consistency among these nurses, a small group of children had their
BP measured by several nurses. Two nurses, whose obtained readings were most
similar and not statistically different, were selected to do the BP measurements.
In the pilot study, the means of the systolic BP readings differed by 0.5
mm Hg (95% confidence interval [CI], –11.2 to 12.2) and the means (95%
CI) of the diastolic BP readings differed by 2.4 mm Hg (95% CI, -11.6
to 16.5 mm Hg).
Three BP readings were obtained by each method on the right arm, with
the patient sitting. The width of the BP cuff was selected to be 40% to 50%
of the circumference of the upper arm as recommended by the American Heart
Association9 and the Working Group of the National
High Blood Pressure Education Program (NHBPEP).10
This cuff selection method was shown to be appropriate for the oscillometric
BP measurement in children by a previous study.4
Blood pressure cuffs manufactured by the Critikon Company were easily adaptable
to the Baumanometer and were used for both the auscultatory and oscillometric
methods. Dinamap BP readings were unknown to the observers at the auscultatory
BP station.
The mean of the 3 BP readings was used for statistical analyses using
the SAS Software (SAS Institute Inc, Cary, NC). For the analysis of the difference
between BP readings by the auscultatory and the Dinamap methods, the mean
of the 3 BP readings by the auscultatory method was subtracted from the mean
of those by the Dinamap method for each subject, and the mean and 95% CI of
the difference were computed. For diastolic pressure, only the Korotkoff phase
V diastolic pressure (K5) was compared with the oscillometric BP readings,
as recommended by the Working Group of the NHBPEP.10
Because of an unexpectedly large difference observed in BP readings
between the Dinamap model 8100 and auscultatory methods, we compared measurements
by this model with those by the earlier model (1846SX) in 2 different ways.
First, we compared BP readings obtained by the auscultatory and Dinamap methods
in an earlier study6 with those from the current
study for fifth-grade children from the same school district. In that study,
BP readings obtained by the model 1846SX and the auscultatory method were
compared in the fifth-grade children.6 Second,
BP readings were obtained by both models of the Dinamap monitor in 48 17-year-old
students and the results were compared to test whether the 2 models gave significantly
different readings. Triplicate BP readings were obtained in both arms simultaneously
using model 1846SX on 1 arm and model 8100 on the other arm. The arm used
for each model was alternated from subject to subject.
The difference in BP readings between the 2 different devices was related
to the mean of those pressures suggested by Bland and Altman.11
The rating system for automated BP devices recommended by the British Hypertension
Society12 was used to evaluate the Dinamap
model 8100. This system grades the degree of agreement with auscultatory measurements
based on the percentage of comparisons that fall within particular ranges
of difference between the 2 measures. A grade of A indicates the differences
to be relatively small in most comparisons, whereas B and C grades indicate
a progressively lower degree of agreement.12
A grade of D indicates an even lower degree of agreement, (less than 45% of
the measurement difference is less than 5 mm Hg12).
RESULTS
Blood pressure readings obtained by the 2 Dinamap monitors (both model
8100) were similar. The mean (95% CI) of the systolic pressure by machine
A was 111.4 mm Hg (92.2-130.6 mm Hg) and that by machine B was 112.0 mm Hg
(92.8-131.2 mm Hg). Auscultatory BP readings obtained by the 2 observers were
also similar. The mean (95% CI) of the systolic pressure obtained by observer
1 was 100.2 mm Hg (80.8-119.6 mm Hg) and that obtained by observer 2 was 102.8
mm Hg (83.0-122.6 mm Hg). The diastolic K5 readings obtained by the 2 observers
were 56.2 mm Hg (37.2-75.2 mm Hg) and 56.3 mm Hg (40.6-72.0 mm Hg), respectively.
For a given method, BP readings were similar between children who had their
BP measured first by the auscultatory method followed by the Dinamap monitor
(A rotation) and those children who had their BP measured in the reverse order
(D rotation) (Table 1). Regardless
of the order of measurements, Dinamap systolic and diastolic pressures were
higher than auscultatory measurements for both the A and D rotation groups.
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Table 1. Auscultatory and Dinamap Blood Pressure Readings According
to the Order of Measurements*
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There were large and clinically important differences in BP readings
between the Dinamap and auscultatory methods throughout the ages studied (Table 2). The overall average systolic
pressure obtained by the Dinamap was consistently higher than that obtained
by the auscultatory method. The mean (95% CI) differences were 10.2 mm Hg
(–3.5 to 24.0 mm Hg) for systolic pressure and 4.7 mm Hg (–13.5
to 22.9 mm Hg) for diastolic pressure for all age groups combined (Table 2). The mean differences between
the 2 BP methods were also similar among the ethnic groups and between the
genders (Table 3).
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Table 2. Mean Difference Between Dinamap* and Auscultatory Blood Pressure
Readings According to Age Group
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Table 3. Mean Difference in Dinamap* and Auscultatory Blood Pressure
Readings by Ethnicity and Sex
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Figure 1 shows the difference
in systolic pressure between the 2 methods (Dinamap minus auscultatory) in
9- to 10-year-old boys plotted against the means of systolic pressures by
the 2 methods. The figure was a typical plot for boys and girls of all age
groups and is shown as a representative plot. The figure shows that the difference
is independent of the BP readings observed in the study population. The differences
in the diastolic pressure were also unrelated to the level of BP.
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Figure 1. Magnitude of difference in systolic
blood pressure readings by the Dinamap and auscultatory methods plotted against
the means of systolic blood pressure readings by the 2 methods for boys aged
9 to 10 years. Two broken horizontal lines represent the 95% confidence intervals
for the mean difference. The solid horizontal line represents the mean difference.
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When the results of the current study were compared with those of the
earlier study,6 the mean ages of the fifth
graders were comparable between the 2 studies (11.4 vs 11.6 years). The group
means of systolic and diastolic pressures obtained by the Dinamap model 8100
were 3.5 mm Hg higher and 6.8 mm Hg higher, respectively, than those obtained
by the model 1846SX in the 1990 study.6 Auscultatory
pressures did not differ significantly between the 2 cohorts. In the current
study, the mean auscultatory systolic pressure was 1.2 mm Hg lower and the
mean K5 pressure was 1.7 mm Hg lower than those found in the earlier study
(Figure 2). When readings obtained
by the 2 models of the Dinamap monitor were compared for the 17-year-old subjects,
the mean of the 3 BP readings by model 8100 was 1.8 mm Hg higher (95% CI,
–10.2 to 13.0) for systolic pressure and 1.3 mm Hg lower (95% CI, –11.4
to 8.8) for diastolic pressure than those obtained by model 1846SX.
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Figure 2. Comparison of blood pressure levels
of fifth-grade students from 2 studies. Dinamap model 1846SX was used in the
1990 study6 and Dinamap model 8100 (Critikon,
Tampa, Fla) in the current study. Although the populations were different,
they are both fifth-grade students from the same school district. BP indicates
blood pressure; DIN, Dinamap; AUS, auscultatory; and K5, Korotkoff phase V
diastolic pressure.
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According to the British Hypertension Society grading system,12 the systolic pressure readings of the Dinamap monitor
8100 achieved grade D. When compared with auscultatory K5 pressures, Dinamap
diastolic pressure also achieved grade D. The grade was D each time, whether
examined according to sex, ethnicity, or age.
COMMENT
In this study, we demonstrated that there are large, clinically important
differences between BP levels measured by the auscultatory method and those
measured by the Dinamap model 8100, a popular oscillometric device. Thus,
BP obtained by 1 method is not interchangeable with that obtained by the other.
Although a significant, but small, difference in BP readings between the 2
methods might have been expected based on our earlier study in fifth-grade
children,6 the large difference we observed
was unexpected. In the earlier study, using the Dinamap monitor model 1846SX,
the mean systolic pressure was 6.4 mm Hg higher than the auscultatory systolic
pressure and the mean diastolic pressure was 3.4 mm Hg lower than the auscultatory
K5 diastolic pressures.6 In the current study,
the magnitude of the difference in systolic pressure was 10 mm Hg, which was
higher than that reported in the earlier study,6
whereas the Dinamap diastolic pressure was 5 mm Hg higher than the auscultatory
K5 diastolic pressure. The reasons for the large difference in systolic BP
levels measured by the 2 methods are not clear. The difference may be due
either to higher BP readings by model 8100 than the earlier model 1846SX,
lower auscultatory systolic pressure readings by the current research team
than the previous team, or a combination of the 2. Different models use different
algorithms, measure different quantities,13, 14
and thus may result in different BP readings.
O'Brien et al15 compared the Dinamap
model 8100 with auscultatory BP readings using BP data from 86 subjects aged
15 to 80 years with a wide range of BP readings. According to the British
Hypertension Society evaluation system,12 the
Dinamap model 8100 achieved a grade B for systolic pressure and a grade D
for diastolic pressure.15 In our study, the
Dinamap model 8100 achieved a grade D for both systolic and diastolic pressures.
The reason(s) for the different ratings between the study of O'Brien et al
and this study are not clear. The different age groups with different BP ranges
and different auscultatory BP observers could have been contributing factors.
In both children and adults, however, BP readings by the auscultatory and
Dinamap methods were shown to be not interchangeable, at least for the model
8100. However, these data do not address whether the auscultatory or oscillometric
measurement of BP, or which model of Dinamap, is more accurate. The important
point, however, is that different methods can yield different values and that
abnormal BP readings must be interpreted with this inconsistency in mind.
In conclusion, systolic pressure levels measured by the Dinamap model
8100 are approximately 10 mm Hg higher than those assessed by the auscultatory
method in children aged 5 to 17 years. Dinamap diastolic pressures were on
the average 5 mm Hg higher than auscultatory K5 pressures. This finding precludes
the interchange of BP readings by the 2 methods and indicates that caution
must be exercised in the diagnosis of hypertension when this device is used.
Similarly, the BP readings obtained by other automated BP measuring devices
should not be considered interchangeable with auscultatory pressure readings.
AUTHOR INFORMATION
Accepted for publication August 29, 2000.
This study was supported by grant MCJ-480612-0 from the Maternal and
Child Health Program (Title V, Social Security Act), Health Resources and
Services Administration, US Department of Health and Human Services, Washington,
DC.
We thank John M. Johnson, PhD, for his critical review of the manuscript.
From the Department of Pediatrics, School of Medicine (Dr Park), School
of Nursing (Dr Menard), and the Department of Computing Resources (Dr Yuan),
University of Texas Health Science Center, San Antonio.
Corresponding author and reprints: Myung K. Park, MD, Department
of Pediatrics, MSC 7734, University of Texas Health Science Center, 7703 Floyd
Curl Dr, San Antonio, TX 78229-3900 (e-mail: parkm{at}uthscsa.edu).
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