Clinical Science (1998) 94,245-253 (Printed in Great Britain)
245
Applicability of new techniques in the assessment of arterial
baroreflex sensitivity in the elderly: a comparison with
established pharmacological methods
Martin A. JAMES, Ronney B. PANERAP and John F. POTTER
University Departments of Medicine for the Elderly, Glenfield Hospital. Leicester LU 9QP. U.K., and
*Medical Physics, Leicester Royal Infirmary. Leicester E I 5 W , U.K.
(Received 10 March/30 October 1997; accepted 10 November 1997)
1. There has been considerable interest in techniques recently developed for the study of arterial
baroreceptor-cardiac reflex sensitivity based on
analysis of spontaneous baroreflex sequences and
on spectral analysis. This study examined how these
newer techniques agreed with the established
pharmacological methods in elderly subjects.
2. In 20 elderly subjects [lo hypertensive (clinic
blood pressure 180 4/88 2 mmHg) and 10 normotensive (clinic blood pressure 136 L 3/73 2
mmHg)], we assessed baroreflex sensitivity from
spontaneous sequences of increasing and decreasing
blood pressure and pulse interval and their mean,
and from spectral analysis to derive a, the index of
overall baroreflex gain. Pharmacological baroreflex
sensitivity was derived from the blood pressure and
pulse interval responses to depressor (sodium nitroprusside) and pressor (phenylephrine) stimuli, and
their mean.
3. Baroreflex sensitivity was significantly lower in
the hypertensive group by the pharmacological,
sequence and spectral methods (all P 0.05).
4. There was acceptable agreement between
pharmacological
baroreflex
sensitivity
and
sequences of the same direction, but with some systematic bias. There was also reasonable agreement
between pharmacological and spectral baroreflex
sensitivity and close agreement without bias
between sequence and spectral methods.
5. The newer and established techniques demonstrate acceptable agreement in the elderly, albeit
with some systematic bias. Pharmacological
methods have enjoyed historical precedence but
newer techniques give equivalent results, and are
preferable in some circumstances. The newer techniques may be more descriptive of the spontaneous
behaviour of the arterial baroreflex at rest rather
than under artificially stimulated conditions.
*
*
-=
Key words: aged, barwellex,hypertension, n
w
INTRODUCTION
The estimation of arterial baroreceptor-cardiac
reflex sensitivity (BRS) has for over 25 years formed
an important part of the study of cardiovascular
disease and particularly hypertension since the first
descriptions of a reproducible method by the Oxford
group [l, 21. The method assumes a static linear
relationship between stimulus (change in systolic
blood pressure) and response [change in pulse interval (PI)] and has changed little since its first description, although newer practical techniques such as
the non-invasive registration of beat-to-beat blood
pressure (BP) using the Finapres device have been
introduced [3]. However, more recently, two alternative analyses have been developed which obviate the
need for drug-induced BP disturbances in the
assessment of BRS. The first consists of the computerized scanning of long recordings of simultaneous BP and PI for sequences in which BP and PI
are either rising (+BP/+PI) or falling (-BP/-PI)
in parallel for at least three beats, representing the
spontaneous activity of the arterial baroreflex at rest
[4-61. The second method consists of the frequency
domain analysis of the variability in BP and PI by
power spectral analysis [7]. This enables the linkage
or cross-spectrum between the BP and PI signals to
be quantified in terms of amplitude or gain, phase
(the time shifts between the two signals) and coherence. Previous studies have suggested that in
humans coherence is acceptable in two frequency
regions usually referred to as low frequency (LF
0.05-0.15 Hz, sometimes called the mid-frequency)
and high frequency (HF 0.20-0.35 Hz) [&lo].
Recent work has summarized the interrelation of BP
and PI in the frequency domain in terms of the
indices aLF and aHF and their mean a, descriptive
of the overall activity of the baroreflex [9, 101.
Given our understanding of the arterial baroreflex
based on existing pharmacological methods, the
, phenylephrine, pressoreceptors,spectrum analys~~.
Abbmviations: BP, blood pressure; BRS. baroreceptor-cardiac reflex senritivity; CI, confidence interval; HF. hieh frequency, LF. low frequency; PI. pulse interval.
Comspomkmc: Dr Martin James, Department of Medicine for the Elderly, Royal Devm and Exeter Hospnal (Heavitree), Exeter EX1 2ED, U.K.
246
M. A. Jamese-t al.
question arises as to the comparability of results
obtained by the two newer methods. This issue has
been addressed at least in part in a number of
studies. A recent study in a small group of young
normotensive subjects [111 comparing sequence
analysis with pharmacological methods (with phenylephrine and sodium nitroprusside injection) found
considerable agreement between BRS assessed by
these two time domain techniques. Two other
studies [8, 91 have described a close relation
between BRS from spectral analysis and that
derived from the phenylephrine injection method.
Two recent studies [6, 121 compared BRS values
from sequence and spectral analysis and both found
significant correlations between sequence BRS and
aLF, UHF and the overall index a. In contrast,
another recent study [13] in a large group of young
normotensive and borderline hypertensive subjects
found substantial systematic bias between BRS
results obtained from the phenylephrine method
compared with sequence and spectral estimates,
with consequent poor agreement between the
respective methods. However, there is only one published study which has included a comparison of the
two newer methods in a small group of older subjects [14], which described the results as ‘superimposable’ without attempting to quantify the level
of agreement. Thus the purpose of the present study
was to examine the comparability of results obtained
by the traditional drug-based methods with those
from the newer sequence and spectral techniques in
a group of elderly subjects across a sufficiently wide
range of BPs to include both hypertensive and normotensive subjects, and the present study is the first
to simultaneously
examine pharmacological,
sequence and spectral methods in this age group.
METHODS
We studied 22 elderly (aged over 60years) subjects of age 6 8 k 1 years (meankSEM; range
60-75 years) who were non-smokers and free from
major disease. Hypertensive subjects were recruited
from among those referred to a large teaching
hospital for assessment and management of their
hypertension, and normotensive subjects from the
spouses and friends of the hypertensive subjects or
from the respondents to advertisements in the local
press. All subjects were active and ambulant and living independently in the community. None was taking any medication with known cardiovascular
effects, and none of the hypertensive subjects had
previously received any antihypertensive medication.
Subjects with a history or clinical evidence of other
cardiovascular disease including atrial fibrillation,
other disorders associated with autonomic dysfunction or other major illness were excluded. No subject in the normotensive group had any history of
hypertension or pre-eclampsia. All subjects gave
written informed consent to participate in the study,
which received local ethical committee approval and
was conducted in accordance with the Declaration
of Helsinki.
Sustained clinic BP was characterized from a total
of nine readings, averaged from three readings
taken on each of three visits over a 6 week run-in
period. BP was measured with the subject supine
after a minimum 5 min rest using a conventional
sphygmomanometer (diastolic Korotkoff phase V)
and a cuff of appropriate size. Subjects with an average sustained clinic BP of systolic BP 2 160 mmHg
and/or diastolic BP 2 9 0 mmHg were regarded as
hypertensive, and thus 11 subjects (8 female, 3 male)
were classified as hypertensive and 11 (4 female, 7
male) as normotensive.
Laboratory studies
All subjects attended the research laboratory for a
morning session in the post-absorptive state, having
refrained from alcohol and caffeine for at least 12 h.
Subjects wore light clothing and the laboratory temperature was thermostatically controlled to 20-22°C.
Subjects rested supine for a minimum of 30min
after the insertion of a cannula into a dorsal hand
vein, and were fitted with chest leads for recording
of the continuous surface ECG (model CR7;
Cardiac Recorders Ltd, London, U.K.). The appropriate-sized finger cuff of the Finapres 2300 noninvasive beat-to-beat BP recording device (Ohmeda
Monitoring Systems, Englewood, Colorado, U.S.A.)
was fitted to the middle finger of the non-dominant
hand, which rested throughout on an adjustable
support at the level of the heart.
After achievement of a satisfactory BP signal from
the Finapres and the stabilization of BP at the same
level for at least lOmin, subjects went through a
three-stage study protocol. During the first stage,
three periods of continuous BP and PI were
recorded, each of 5 min duration, for sequence and
spectral analysis, with the subject resting supine and
maintaining a respiratory rate above 12 breathdmin,
and without any external stimulation such as talking.
During each of these periods the servo self-adjust
mechanism of the Finapres was disabled, but the
device was recalibrated between each recording
period. If there was any substantial discrepancy
between BP at the end of any recording period and
the BP after recalibration, that record was discarded
and a repeat recording made after a further stable
period, although in practical terms this was only
required on a very small number of occasions [3,
151.
The two subsequent stages consisted of BRS testing by the established pharmacological methods:
1. The BP and PI response to phenylephrine injection [l]. An initial bolus dose of 50pg of phenylephrine was progressively increased in 50 pg steps as
necessary (up to a maximum of 200 pg) to achieve a
peak BP rise of 20-40 mmHg, and the effective dose
New baroreflex techniques in the elderly
was repeated to obtain a minimum of three adequate responses. Bolus injections of 0.9% saline
were interspersed at random between the drug
injections and the subject was blinded to the nature
of each injection.
2. The BP and PI response to a graded sodium
nitroprusside infusion [16-18]. The infusion was
commenced at 0.25 pg.min-' -kg-' and increased
(by 0.25 pg*min-'.kg-' each minute) until a fall in
BP of at least 20 mmHg was observed.
Data analysis
The analogue outputs from the Finapres and
ECG were routed to a dedicated personal computer
fitted with a 12-bit analogue-digital converter
sampling at 200 Hz per channel. Purpose-written
software permitted the recording, calibration and
editing of the digitized signal and the derivation for
later off-line analysis of beat-to-beat data for systolic, mean arterial and diastolic BP together with the
PI from both the Finapres and ECG signals. Temporal resolution was enhanced by a parabolic interpolation between sampling points on the digitized
series of both PI signals. PI values were derived primarily from the Finapres signal, with a visual correction facility to allow comparison with the ECG
signal. Baseline recordings were scanned by computer for spontaneous sequences of three or more
beats in which either BP and PI rose in parallel
(+BP/+PI or 'up' sequences) or fell in parallel
(-BP/-PI
or 'down' sequences) [4, 6, 191. We
selected sequences on the basis of a stimulus of a
minimum change in systolic BP of 0.5 mmHg per
beat, but made no similar stipulation regarding the
response in order not to bias the analysis towards
higher values of BRS. Systolic BP change was correlated with PI change for the immediately succeeding
beat (a 'lag' of 1) [20, 211 and only sequences with a
statistically significant correlation (P<0.05) were
included in the analysis. The automated analysis
derived separate values of BRS for +BP/+PI
sequences (up-BRS), - BP/- PI sequences (downBRS) and pooled results (sequence-BRS). In addition, data were obtained on the average length and
correlation coefficient of sequences, the percentage
of the entire recording represented by baroreflex
activity and the number of sequences of each type.
Power spectral analysis was performed on the
baseline recordings after low-pass filtering with an
eighth-order Butterworth digital filter with a cut-off
frequency of 20 Hz. Recordings with more than four
ectopic beats were rejected. The beat-to-beat
recordings of systolic BP and PI were interpolated
with a third order polynomial and then resampled at
a rate of 2 samples/s to render signals with a uniform time base. Linear trends were removed and a
20% tapering cosine window was applied at the
extremities of both signals. The signals were transformed to the frequency domain with a fast Fourier
147
transform algorithm using 512 samples. Separate
power spectra were computed for systolic BP and
PI, and the spectral BRS index a (spectral-BRS) was
computed from the mean of the square roots of the
ratios of the spectral powers of PI and systolic BP in
the LF (0.05-0.15 Hz) and HF (0.20-0.35 Hz) bandwidths [9, 101. The cross-spectrum was smoothed
with a 9-point triangular window yielding spectral
and coherence function estimates with 40 degrees of
freedom per subject. The mean coherence in the
cross-spectrum of PI and systolic BP was also
derived from the three data segments in each subject, with an a priori lower limit for the coherence
set at 0.40 [22].
The BRS was derived for the pharmacological
techniques from the linear regression of PI on systolic BP for the 'ramp' response using the original
method of Smyth et al. [l], but including both
inspiratory and expiratory beats [23]. Values were
obtained for each of the minimum three effective
phenylephrine doses and averaged to derive a final
value for phenylephrine-BRS for each individual.
Similar analysis of the 'ramp' portion of the sodium
nitroprusside response yielded a value for nitroprusside-BRS, and the mean of these two values (pharmBRS) represented the BRS estimated by drug-based
techniques in each study subject.
Statistical methods
Spectral analysis parameters are expressed both in
absolute terms (either mmHg2 or ms2) and after
removal of the very-low-frequency component
(<0.05 Hz) as normalized units (NU) [7]. All
numerical values are expressed either as meanfS.E.M., or as median (interquartile range) for
non-parametric data, after testing for normality
using the Shapiro-Wilk W' test. Between-group
comparisons were made using either Student's
unpaired two-tailed t-test or the Mann-Whitney
test, with a value for P<O.O5 regarded as statistically
significant. Agreement between methods is
expressed both in terms of linear regression analysis
with the 95% confidence interval (CI) for the slope
(fi) to test the hypothesis of numerical equivalence,
and with Bland-Altman plots of the difference
between two methods as a function of the mean of
the two methods [24].
RESULTS
Of the 22 subjects studied, results from two (one
hypertensive, one normotensive) were excluded
from analysis because of persistent ectopic activity
throughout the study protocol. Table 1 shows the
baseline characteristics and clinic BP for the hypertensive and normotensive groups. Clinic heart rate
was not significantly different between the two
groups.
M. A. Jameset al.
240
Tabk I. Baseline characteridcs, clinic BP and phvmacdoslol
*
BRS
for the llrpertmrivc and nonnotensive groups. *Different by study
design. For definitions of the different paameters of BRS see text. NS, not
signifwt.
Hypertensive
sub*
Nomtensive
subjects
between normotensive and hypertensive subjects,
but values for aHF and spectral-BRS were significantly higher in the normotensive group (Table 3).
Agreement between methods
P
n
10
10
Age 0
69f2
66+2
NS
Clinic
Systolic BP (mmHg)
Diastolic BP (mmHg)
Heart rate (beatslmin)
180+4
88*2
65+2
136k3
73+2
66+3
<o.o001*
<o.o001*
NS
Nitroprusside-BRS (msImmHg)
PhenykphrineBRS (mslmmHg)
Phann-BRS (mslmmHg)
3.7f0.7
3.9k0.5
3.8f 0.5
5.7+ 0.4
8.6 f I .4
7.2 f 0.9
0.03
0.009
0.004
Baroreflex sensitivity tests
Table 1 also shows the results from the pharmacological BRS estimation in the study subjects. There
was a statistically significant difference between the
normotensive and hypertensive groups in the drugbased parameters phenylephrine-BRS, nitroprusside-BRS and the overall pharm-BRS. Within each
group, however, there was no significant difference
between the BRS from a pressor or a depressor
stimulus.
Figure 1 serves to illustrate the sequence analysis
technique. Sequence analysis was performed on
records of mean total length 1010 & 29 beats, and
the results are given in Table 2. There was no significant difference between hypertensive and normotensive subjects in the number of baroreflex
sequences observed or in the percentage of the
entire recording represented by baroreflex slopes,
but in the normotensive subjects the down
sequences tended to be of slightly longer duration.
There were no differences between up-BRS and
down-BRS within either group, but all sequencederived BRS parameters were significantly different
between normotensive and hypertensive subjects.
Results of spectral analysis are given in Table 3
for the LF and H F bandwidths and the overall a
index spectral-BRS. There was no difference in
overall PI variability between the hypertensive and
normotensive groups [total PI power 782
(512-2159) ms2 and 660 (493-1089) ms2 respectively, P > 0.21, but there was a significant increase in
overall systolic BP variability in the hypertensive
group as shown by an increase in total systolic BP
power compared with the normotensive group [46.3
(34.6-57.4) mmHg2 versus 19.9 (15.8-29.1) mmHg2,
P = 0.0061. This increased variability was dispersed
among all frequency regions, as suggested by the
higher absolute but similar relative powers (as NU)
for systolic BP within either specified LF or H F frequency region. There was a tendency for values of
UHF to be higher than aLF by a mean
2.71 ms/mmHg (P = 0.07). aLF was not different
Agreement was tested between methods involving
a stimulus in the same direction where such a distinction could be made, i.e. with the sequence and
pharmacological BRS parameters. Thus nitroprusside-BRS was compared with down-BRS, and phenylephrine-BRS with up-BRS. Pooled results were
also compared (pharm-BRS with sequence-BRS).
Results of the comparison of pharmacological and
sequence methods are shown in Fig. 2. The upper
panels show linear regression, with the equivalent
Bland-Altman plots in the lower panels.
-
IJI140,
I
2
I
I
k
.:.:.
0
Bu) loo30
6
80
130
180
230
280
330
380
Beat number
----I
I
Beat number
1000
,:
mean slope
110
120
130
Systolic pressure ( mmHg !
10
Fig. 1. kat-to-but recordings of WO~K
blood prrrrure (top prml)
and puke interval (Pl; middle panel) during one 5min period.
Sequences detected (either tBPItPI or -BPI-PI) we shown as bdd lines
in h e upper panels and as data points with regression lines for indiiud
sequences in the lower panel. Also shown is the mean dope for all
sequences (sequence-66).
249
New baroreflex techniques in the elderly
Down-BRS showed significant systematic bias
compared with nitroprusside-BRS (mean 3.2 k0.7
ms/mmHg higher, P = 0.002) but the two values
were linearly related, and the slope was not significantly different from unity but with wide confidence
limits (regression equation: down-BRS = 2.19+ 1.22
nitroprusside-BRS, R2= 0.43, P = 0.002; 95% CI for
/3 0.52-1.92; Fig. 2, left panels). In contrast, up-BRS
showed less bias compared with phenylephrine-BRS
(mean difference 0.97 k0.54 ms/mmHg, P = 0.09)
but although the two measurements were linearly
related, the slope was significantly different from
unity, thus rejecting the hypothesis of numerical
Table 3. Results from power rpccerl ady& for thc h.ypactcnriVe
andnonnotanrm
' group Data are presMed as means+S.E.M. or
medians (interquartile t q e s ) . For definitions of the different p"meten of
BRS see text. NS, not rigniicant.
Table 2. Results hwn sequence analysis for the hyperleMive and
notmotenhe group. %BK percentage of the entire recording
represented by b m f l e x sequences. For definitions of the different
parametwsof BRS see text. NS, not significant.
Hvpertensive
subpar
P
PI power
Down (-BP/-PI) sequences
No. of sequences
length ( b . 4
Correlation coefficient
Down-BRS (ms/rnrnHg)
up (tBPItPI) seqwnces
No. of sequences
sequence lensth (bw
Correlation coefficient
UpBRS (mslrnrnHg)
NS
41 f6
5.0k0.2
0.95 f 0.01
5.7f0.7
39f6
6.4f0.4
0.92f0.01
10.1 f 1.4
53f6
5.4f0.2
5.0 f0.6
46f6
5.8f 0.3
0.93f0.0I
9.5f 1.2
0.005
24.4f2.7
5.3 f0.6
26.6 k4.0
9.8f I.2
NS
0.006
0.94 f 0.01
%BRS
sequence-BRS
l61
LF (0.05-0.I5 Hz)
Systdic BP power
Nwmotensiw
subpar
Coherence
aLF
0.04
NS
0.013
HF (0.20-0.35HI)
Systdic BP power
NS
NS
NS
PI power
Coherence
aHF
Speccal-BRS
Hypertensiw
subjects
Normotenrive
subpar
7.2(5.6-10.2)
29.8 (25.2-34.3)
I30 (95-366)
3 I .3(29.2-32.4)
0.52(0.45-0.65)
5.0 0.6
4.0 (2.7-5.5)
35.7 (24.6-45.0)
I55 (107-212)
33.7(22.0-40.3)
0.53 (0.48-0.54)
6.4f0.7
0.014
2.3 (I .6-3.5)
7.53 (6.58-9.70)
65 (37-173)
12.8(6.86-17.3)
0.47(0.43-0.64)
4.8f0.5
I.O (0.3-2.8)
4.72(1.72-10.7)
69(36-227)
14.4(7.37-31.9)
0.53 (0.47-0.56)
12.1 f2.0
NS
NS
NS
NS
NS
4.9f0.5
9.4f I.3
0.008
P
NS
NS
NS
NS
NS
0.006
l81
Q)
0
v)
L
m
0
9 1 2 1 6
6
3
3
0
BRS (nitroprusslde)
3
6
9
12 15 18
0
BRS (phenylephrine)
3
9 1 2 1 5
6
BRS (pharmacological)
v)
-5
c
-10
-10
0
4
8
42
46
Mean SNPldown seq
(wppndr) d B b n d - a
-10
0
4
8
12
Mean PElup req
16
0
4
8
12
16
Mean P h a d s e q
pkb (kmrppndr) forthc compukon d BRS hwn the phumpcd0eK.l d s e q q w n c c
n#thodr. Ldhon dots: d u r n nimwKurride ISNP) versu~'down'I-BP/-PI) sequences;middle dots: phenvkphrine (Powsus 'up' (tBP/tPI) sequencer; righanost
Fig. 2-.
pktt
M. A. James et d.
250
equivalence between the results from the two
methods ( p = 0.71, 95% CI 0.46-0.97; Fig. 2, middle
panels). Comparing the pooled sequence and
pharmacological results, there was a significant bias
in favour of higher values for sequence-BRS (mean
difference 2.1 f0.5 ms/mmHg, P = O.OOOl), but the
two variables were linearly related with a slope of
close to unity (B = 1.17, 95% CI 0.85-1.49; Fig. 2,
right panels). Within this there were still some
results with a large discrepancy; for example one
subject with pharm-BRS of 1.8 ms/mmHg and
sequence-BRS of 6.1 ms/mmHg. Nonetheless, as
shown in Table 2, sequence analysis was consistently
able to demonstrate a difference between the hypertensive and nonnotensive groups in the same
manner as the drug-based method.
In view of the overall estimate of BRS represented by the spectral-BRS and the inherent
inability of spectral methods to differentiate
between rising and falling baroreflex sequences,
comparisons were confined to those between spectral-BRS and the pooled results pharm-BRS and
sequence-BRS. These are shown in Fig. 3, again
depicting linear regression in the upper panels and
Bland-Altman plots in the lower. The level of
agreement between the different methods was
generally very high. Spectral-BRS did show significant positive bias compared with pharm-BRS (mean
difference 1.65 f 0.46 ms/mmHg, P = 0.002) and the
95% CI for the regression slope included unity
(/
=I
1.16, 95% CI 0.80-1.52). Again, there were a
few individuals in whom agreement was not close;
for example one subject with a pharm-BRS of
8.8 ms/mmHg and spectral-BRS of 17.0 ms/mmHg.
Comparison of sequence-BRS with spectral-BRS
showed an even closer linear relation between the
two parameters (fl=O.97, 95% CI 0.80-1.14), with
no systematic bias (mean difference = 0.42
0.28 ms/mmHg, P = 0.15), indicating a high level of
agreement between the sequence and spectral parameters and sustaining the hypothesis of numerical
equivalence between the methods.
DISCUSSION
This study in elderly subjects across a broad range
of clinic BPs suggests that results obtained from
computerized sequence and power spectral analyses
show an acceptable level of agreement with BRS
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3. Regrerriin pbk (upper paneb) and Bland-Altmul plots (lower p a d s ) for the comparison of BRS from power
spectd analysk and the phannrcdogiul and sequence methodr. Left plots: spectral analysis versus pharmacologd data; nght
plots: s p c t d analps versus seqwnce analps. Closed symbols, hypertensive subjects; open symbols, m t e n s i v e subjects. Upper
panels: dotted line, line of unity (perfect agreement); dashed line, rsgreSron line for the whde group. Lower panels: dotted line, mean
Fig.
difference; dashed lines,
2 S.D.s of the difference.
New baroreflex techniques in the elderly
obtained from the more traditional pharmacological
methods and analysis, with persisting systematic bias
in some cases. While the phenylephrine technique
was the first of the methods to be described [l], it
cannot be regarded as a true ‘reference’ value, and
so the issue of bias in one or other direction is confined to whether results obtained by different
methods can be regarded as numerically equivalent
or interchangeable [24].
The findings from the present study thus agree
substantially with those of other workers who have
similarly addressed the issue of comparability albeit
in groups of younger normotensive or mild hypertensive subjects [6, 8, 9, 11-13]. However, a further
examination of these studies requires us to first consider the methodology of comparisons between
methods [24]. This has largely been performed using
only standard correlation coefficients, a method that
solely tests for a linear relationship between two
variables (and compares the slope of the relation
with the null hypothesis of a slope of zero) and does
not assess agreement. We have therefore elected to
compare methods using two statistical tests, firstly by
comparison of linear regression with a model with a
slope of unity (to test for a constant linear relation
between methods, i.e. numerical equivalence or
complete ‘agreement’), and secondly by examining
Bland-Altman plots. The difference between this
approach and simple correlation is well illustrated if
one considers the available data in the literature. In
one recent paper [13] good agreement is claimed
between BRS as assessed by the two newer techniques and phenylephrine-BRS on the basis of a
statistically significant correlation. An examination
of the data, however, demonstrates that the agreement between the two methods is far from good, as
seen both from the substantial scatter when the individual data points are plotted with regression slopes
significantly different from 1, and from the substantial systematic bias when the methods are compared.
In the recent work of Parlow et al. [ll] it is possible
to assess agreement between the sequence approach
and pharmacological BRS measures, and this is
shown to be good in a small group of young normotensive subjects, some of whom had extremely high
BRS. However, in the one published study which
included a comparison of sequence and spectral
techniques (using ctLF) in a group of eight elderly,
mild hypertensive subjects [14] (who tend to show
much lower values for BRS), the results are
described only as ‘superimposable’ (indicating only
that the group means were similar) with no further
data given with which to assess agreement. Agreement between results obtained by different methods
may also be affected by the reproducibility of the
different techniques, a topic also not usually
addressed in such studies. In our laboratory the noninvasive methods have a between-visit reproducibility of approximately 17% (n = 56), (S. L. Dawson,
M. A. James, P. Weston, R. B. Panerai and J. F.
Potter, unpublished work), similar to that recently
251
reported in young normotensive subjects [25]. Despite these reservations regarding previous studies,
the data from the present study lead us to suggest
that the acceptable agreement between the traditional drug-based and newer methods should permit
their interchangeability in various clinical and
experimental situations, although it does not allow
the absolute BRS values obtained in studies using
the different techniques to be regarded as equivalent.
Nonetheless it remains that the traditional and
newer techniques have important methodological
and clinical differences, not the least of which being
that the sequence and spectral techniques involve
the automated computerized analysis of BP and PI
whereas there still remains some potential subjectivity when selecting the segments of interest in the
response to pharmacological stimuli despite the use
of standardized criteria [l, 231. In physiological
terms the pharmacological method involves the slow
onset of an artificial perturbation of the baroreflex
beyond the usual limits operating at rest, whereas
sequence analysis studies the smaller fluctuations
that occur in the subject at rest and free from
external stimulation. The majority of sequences seen
in the present study were of six or fewer beats,
whereas it is the express intent of drug-based
methods to provoke a sustained ‘ramp’ of perhaps
15 to 20 beats duration, raising the possibility that
the reflex response under these stimulated conditions is different from the much smaller baroreflexmediated adjustments that occur under basal
conditions. Indeed it is this important property of
the drug-based method which enables the behaviour
of the arterial baroreflex over the full range to be
studied, permitting sigmoidal fitting of the stimulusresponse relation and allowing parameters such as
threshold and saturation ranges to be described [ll,
26, 271. This is inherently not possible with analysis
of spontaneous sequences which operate within a
relatively narrow pressure range over the linear portion of the baroreflex curve. These points would
tend to suggest a potential dichotomy in the
approach to BRS estimation. The drug-based techniques, and particularly the phenylephrine injection
method, have come to be regarded as the ‘gold
standard’ in the assessment of arterial BRS, but this
is more likely to be due to their historical precedence than any view that they better describe the
spontaneous behaviour of baroreflex mechanisms in
the resting subject. Perhaps now it should be
accepted that the ‘old‘ and ‘new’ techniques have
different and complementary roles in extending our
understanding of cardiovascular neural control.
Some consideration is required at this point of the
different methodologies involved in computerized
sequence analysis [4, 6, 19, 211. Unlike other studies,
we have attempted to make the minimum number
of stipulations regarding what qualifies as a
‘sequence’, and in particular specified a sequence
only in terms of the stimulus (in our case
252
M. A James e-t al.
0.5 mmHg/beat sustained over a minimum of three
beats) and not also in terms of the response. To also
create minimum criteria for the response (usually a
change in PI of 4 ms [4, 191) would bias the analysis
towards higher BRS, particularly with the shorter
sequences. Furthermore, in the current study we
have stipulated only that the sequence satisfies the
criterion of linear dependence between stimulus and
response (i.e. a statistically significant regression)
and not specified a preset minimum coefficient of
determination. Again this protects against the exclusion of sequences of low BRS, a factor which is particularly pertinent in the study of elderly subjects
who demonstrate much lower BRS than younger
subjects.
The premise upon which spectral analysis of BRS
is based is different from that which underpins the
other techniques [9, 28-30]. Whereas the time
domain methods are based upon a simplified, openloop model of the transfer function between systolic
BP and PI which usually ignores other influences
such as respiration and rate of change of pressure,
spectral analysis examines the non-causal linkage
between the spectra of systolic BP and PI within an
integrated closed-loop model of the cardiovascular
system which accommodates other potential neural
and non-neural influences on both BP and PI [9, 28,
29, 311. Recent work from our group [32] studying
spectral analysis of a linearized model reconstructed
from the sequence analysis has shed light on some
common features that may account for the agreement between the two techniques, with the LF band
being associated with the longer sequences and the
H F band corresponding with the shorter sequences.
It would therefore seem appropriate when making
comparisons between sequence and spectral
methods to use the overall spectral index a [9, 10,
321. We have also recently observed in a group of
elderly subjects [33] that after phenylephrine injection, spectral analysis detected substantial increases
in power, phase and coherence in the very-low-frequency region corresponding with a slow (approximately 0.025 Hz) oscillation provoked by vasoactive
drug injection, and that as a result there was good
agreement between phenylephrine-BRS and the
spectral ratio a in the very-low-frequency region.
This suggests that phenylephrine injection induces
a baroreflex-mediated very-low-frequency link between systolic BP and PI that is not present at rest
and not described by sequence or spectral analysis
of resting recordings, thus accounting in part for the
discrepancies between results obtained by different
methods.
Study limitations
In our comparison of the different BRS techniques, the potential limitations of the study need to
be considered. We chose to make the pharmacological comparison using phenylephrine bolus injection
and sodium nitroprusside infusion given that these
were the most commonly used techniques on which
our understanding of BRS in the elderly was based
[16-18, 231, but this does not allow us to directly
compare the pharmacological BRS response to a rising and falling systolic BP. In this age group, technical difficulties can arise in data analysis because of
low values for BRS and a relatively lower signalto-noise ratio than in younger subjects, but this
study has demonstrated the feasibility as well as the
comparability of non-invasive baroreflex studies in
the elderly. The use of the Finapres method of BP
registration in the elderly has previously been found
to be acceptable in comparisons with intra-arterial
recordings [34, 351. Further mention is also required
here on the issue of coherence. Most investigators
have used a value of 0.50 as an arbitrary threshold
for assuming significant linear dependence in the
cross-spectrum between systolic BP and PI based on
earlier work [28, 361. This issue is often overlooked
in other studies using spectral techniques despite the
observation that the sample distribution of coherence is dependent on the number of degrees of freedom available for its estimation at each frequency
[37]. We have observed that, for the conditions of
our study, the lower 95% CI for coherence is
actually 0.40 [22]. However, under other circumstances with fewer degrees of freedom this value
may rise to 0.80, and in each case the confidence
limits for coherence should be computed and matched to the data analysis procedure.
In conclusion, our study comparing the newer and
longer-established techniques in a group of elderly
subjects across a broad range of clinic BPs indicates
that an acceptable measure of agreement exists
between the different methods for the assessment of
BRS. This should enable the application of spectral
and sequence analyses in clinical and experimental
situations where the use of drug injections to provoke substantial BP fluctuations would be inadvisable, for example, in subjects with very high BP, or in
conditions such as pre-eclampsia, acute stroke or
myocardial infarction. Indeed one could go further
and propose that the newer techniques could supersede drug-based methods for the description of the
spontaneous behaviour of baroreflex-heart rate
mechanisms under basal conditions. However, the
pharmacological methods will retain a role where a
fuller description of the entire baroreflex stimulusresponse relationship is required.
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