(Hypertension. 2000;35:887.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
From the Département de Néphrologie et dHypertension Artérielle (J.P.F., M.L., M.D.), EA 645 Université C. Bernard, Hôpital E. Herriot, Lyon, France; CNRS ESA 814 (C.C., M.P.G., C.Z.P.), Faculté de Pharmacie, Lyon, France; and Médecine du Travail (P.Q.), Rhodia, Saint-Fons, France.
Correspondence to Jean Pierre Fauvel, Département de Néphrologie et dHypertension artérielle, Hôpital E. Herriot, 69437 Lyon, France. E-mail jean-pierre.fauvel{at}chu-lyon.fr
| Abstract |
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Key Words: baroreflex blood pressure men spectral analysis stress, mental
| Introduction |
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| Methods |
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Mental Stress
Mental stress was induced by a computerized version of the
CWT.9 Successive series of 4 color words written in
incongruent colors appeared in a random order on the screen. The
subjects had to type the color of the word on selected keys from the
keyboard. An audio signal was provoked if a wrong response was made.
The examiner encouraged subjects to perform the stress test at their
maximum speeds but kept an emotionally neutral attitude throughout the
test.
BP Measurement
The experiment was conducted in quiet room with a controlled
temperature at 20°C. BP was recorded using a Finapres device. The
cuff was wrapped around the forefinger of the nondominant arm, which
was resting on a table, the level of which was adjusted to obtain
<5 mm Hg difference from the previously determined BP (by
mercury sphygmomanometer). The equipped arm of the subject, sitting in
front of the screen computer, was held in the same position throughout
the procedure. After a 2-minute period for familiarization, the
automatic calibration was switched off, and BP was recorded for 15
minutes. Each patient was recorded during 10 minutes at rest and 5
minutes during the mental stress.
Signal Acquisition and Data Processing
Signal acquisition and data processing were previously
described.14 In brief, the analog output from the Finapres
was connected to the analog-to-digital converter (ATMIO 16H, National
Instrument) to perform data acquisition, storage, and analysis.
Finapres signals were sampled at a rate of 100 Hz with 8 precision
bits. Our own algorithm to detect SBP is accurate enough to compute HR,
so ECG signal recording was not necessary. Data processing was
carried out on a 4-minute recording both at rest and during
stress after a delay of 1 minute and 30 seconds, respectively (Figure 1). Rest and stress data were obtained
during these two 4-minute periods. These delays were chosen to obtain
stationary conditions during rest and stress (Figure 1) as
necessary to perform a fast Fourier transformation (FFT). Frequency
domain analyses of HR and SBP oscillations were
performed by spectral analysis using the FFT algorithm. The FFT
was applied on 342 points from a 4-minute recording resampled
at 1.4286 Hz (0.7 seconds) and completed to 512 points by the
zero-padding technique to enhance the spectral resolution (1.7
MHz).15 BP variability was expressed as standard deviation
and as power spectra in the mid-frequency (MF) band (0.07 to 0.14 Hz)
and in the high-frequency (HF) band (0.14 to 0.40 Hz). As previously
described, the BRS was estimated by the computation of the average
modulus of the transfer function between SBP and HR spectra in the 2
frequency bands.14
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SBP and HR time series were first low-passfiltered at 0.5 Hz and then resampled at 2 Hz. Slopes of the linear regression between opposite changes in SBP and 1 beatdelayed HR changes during at least 3 consecutive beats were selected for P<0.05. The mean slope of the selected sequences was reported to provide a reliable index of the BRS.16
Statistical Analysis
Data are expressed as mean±SEM in text, tables, and figures.
Normal distribution of each parameter was tested by use of
the Kolmogorov-Smirnov test. For the data for which the distribution
was normal (SBP, HR, and their standard deviations; stress-induced
increase in SBP and HR; and spectral BRS and sequence BRS at rest and
stress), a Student t test and a Pearson correlation were
used. If the distribution was not normal (spectral power in the MF and
HF bands), a nonparametric rank test for paired data
(Wilcoxon) or a Spearman rank correlation test was applied.
| Results |
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Stress-Induced Variations in SBP and HR Levels, Variabilities, and
BRS at Rest
Stress induced a significant increase in SBP (22.4±0.1 mm Hg,
P<0.001) and HR (7.6±0.04 bpm, P<0.001).
Stress-induced increases in SBP and HR were related neither with age
nor with their basal values. Although SBP and HR standard deviations
were not altered by stress, spectral indices were significantly
modified (Figure 2). Stress-induced
increase in SBP was significantly correlated with its resting SD and
power spectrum in the MF band (Table 2). Spectral BRS and sequence BRS were
highly significantly correlated (r=0.66,
P<0.001). Stress-induced increase in SBP was related
neither with the spectral BRS nor with the sequence BRS. Stress-induced
increase in HR was correlated with spectral BRS but not with the
sequence BRS at rest (Table 2).
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| Discussion |
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The accuracy of BP Finapres recordings has been assessed and compared with intra-arterial BP recordings18 at rest and during sympathetic stimulation.11 The accuracy of Finapres BP determination implies a simultaneously determined BP with a validated technique. In the present study, Finapres resting BP was adjusted with a sphygmomanometer. However, because we focused on BP variability, absolute BP levels should not alter our results. The sympathetic stimulation was elicited by use of a computerized version of the CWT that involved sensory rejection and that has been used as a model of defense reaction in humans.19 The CWT provoked a steady and reproducible increase in HR and BP associated with an increase in plasma catecholamines10 and in muscle sympathetic nerve activity.20 However, because BP increase was not related to catecholamine release,10 these indices were not determined in the present study. The magnitude of the stress-induced increase in SBP (22.4 mm Hg) confirmed that our version of the CWT provided a major sympathetic stimulation. The main function of the CWT, instead of mental arithmetic,5 17 was to elicit a sustained increase in BP (Figure 1) that could allow spectral analysis of BP and HR signals. Thus, in our experimental conditions, our methods were valid in the determination of cardiac BRS and indices of BP and HR variabilities. BRS assessed the efficiency of the cardiac sympathetic and parasympathetic pathways to buffer BP variations via HR. Evidence suggests that BRS should limit BP reactivity to profound challenge.21 22 Thus, BP would be expected to rise more in subjects with lower BRS. Because cardiac BRS was not related to the BP stress reactivity, our results do not support this hypothesis. Adjustment for age and basal SBP did not alter our results.
In response to a peripheral vasoconstriction, the cardiac baroreflex decreases HR through parasympathetic stimulation. Opposite hemodynamic cardiac patterns were observed in response to emotional stress that provoked a vasoconstriction associated with an increase in HR. For a few beats after the onset of the stressing stimulation, the cardiac baroreflex appeared to be overcome, because HR and BP increased simultaneously. Interestingly, Zhang et al23 reported a delay in BP increase in intact rats compared with sinoaortic-denervated rats that involved the role of the cardiac BRS during the critical period after the onset of the sympathetic stimulation. The BRS could be involved in the kinetics of the pressor response. To verify this hypothesis, it would be of interest to study the relation between BRS and the delay to obtain the plateau of stress BP. Unfortunately, in humans, because many individual parameters may interfere (eg, alert reaction to the entry of the physician, comprehension of the test, and sagacity), the slope of the BP increase cannot be precisely measured during a psychological stress test. Stress induced a centrally mediated stimulation on vasomotor centers and induced a parasympathetic withdrawal, as elegantly shown by Zhang et al.23 These authors showed, with the use of sinoaortic-denervated rats, that intact baroceptor afferents limited the centrally mediated sympathetic BP response to stress but did not influence the parasympathetic withdrawal. Our results confirmed, in humans, these findings. The magnitude of the stress-induced increase in BP, which was not influenced by the cardiac baroreflex, could be modulated by the arterial baroreflex. Consistently, resting BP variability (standard deviation and spectral power in the MF band), which is influenced not only by the cardiac baroreflex but also by the arterial sympathetic tone, was slightly but significantly related to the stress-induced BP increase in accordance with the reports of Parati et al5 and Mounier-Vehier et al.24
The prolonged sympathetic stimulation led to a BP and HR plateau.
Because spectral and sequence methods could determine BRS during
stress, it has been suggested that BP is regulated by inverse
variations of HR during the plateau. Furthermore, the calculated BRS
during stress was lower than during resting conditions, as reported by
Tulen et al.25 Although not recorded, the increase in
respiratory frequency that has been reported to be
2 breaths per
minute (see Reference 2626 ) could not explain the stress-induced
decreases in BRS and in HF bands observed only with greater increases
in breathing frequency.27 BP regulation during stress
could be compared with what is observed in established hypertension,
eg, a resetting of the BRS that becomes able to buffer BP variations
but at a higher level. In humans, the stress-induced sustained BP
increase was preserved even during a sustained stimulation of
arterial baroreceptors produced by infusing
phenylephrine,28 and BRS during stress was
reported to be impaired.29 In light of this result, the
stress-induced reduction in BRS should originate from a central
inhibition of the cardiac BRS8 rather than from a
diminished sensitivity of the arterial baroreceptors.
In conclusion, resting BP variability was slightly but significantly related to stress-induced BP reactivity. Resting cardiac BRS that limits BP variability at rest did not blunt BP increase produced by a mental challenge. Our results suggest that a centrally mediated sympathetic stimulation overcame cardiac autonomic regulation at the onset of the stressing stimulation and emphasized the role of the sympathetic vasoconstriction in the pressor response. During the sustained sympathoexcitatory stimulus, a cardiac baroreflex controls BP variations but at a lower sensitivity. Because BP reactivity to challenge was reported to be associated with an unfavorable cardiovascular profile, our results are of major clinical interest.
| Acknowledgments |
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Received October 25, 1999; first decision November 18, 1999; accepted November 29, 1999.
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