Nataša Honzíková

Time versus frequency domain techniques for assessing baroreflex sensitivity.

Background Newer techniques to evaluate baroreflex sensitivity (BRS) are based on the analysis of blood pressure (BP) and heart rate (HR) time series in the time or frequency domain. These novel approaches are steadily gaining popularity, since they do not require injection of vasoactive substances, nor do they rely on a complex experimental set-up. Aim This review outlines and compares some basic features of the latest methods to assess spontaneous baroreflex function. Results Modern techniques for the estimation of spontaneous BRS are based on a variety of signal processing schemes and derive information on the baroreflex function from different perspectives. Thus factors such as respiration and other non-stationary agents may have different influences on the estimates provided by each of these approaches. Notwithstanding such individual specificity, however, it has been observed that in several physiological and pathophysiological conditions these techniques often provide comparable information on BRS changes over time, particularly when the estimates are averaged over time windows of a few minutes. Conclusions Due to the general agreement in the pattern of BRS among most modern methods, it seems reasonable to employ the most validated of these techniques, for which data obtained in several studies are already available.

The importance of high-frequency paced breathing in spectral baroreflex sensitivity assessment

Objective Computation of the low-frequency (LF) blood pressure variability (BPV) to heart rate variability (HRV) transfer-index is a common method to assess baroreflex sensitivity (BRS), tacitly assuming that all LF-HRV is caused by baroreflex feedback of LF-BPV. However, respiration may also cause HRV by mechanisms not involving the baroreflex. Application of narrow-band (controlled) high-frequency breathing would keep such non-baroreflex-mediated HRV best out of the LF band. Spontaneous breathing, because of its broad-band character, might cause extra, non-baroreflex-mediated, HRV in the LF band, while paced LF breathing would even concentrate most non-baroreflex-mediated HRV in the LF band. Our study addresses the likely resulting BRS overestimation. Design We recorded HRV and BPV in 20 healthy young subjects in the sitting position. We varied the sympathovagal balance by gradual leg-lowering from horizontal till 60°. At each angle the subjects performed controlled 0.10 Hz, spontaneous, and controlled 0.25 Hz respiration. Results Resting BRS values were 15.5(7.2), 13.1(3.7), and 11.6(6.2) ms/mmHg, respectively. Both the 15/min and the free breathing values differed significantly, P < 0.01 and P = 0.04, from the 6/min breathing value. With lowered legs, the BRS values were 8.2(3.4), 8.3(2.9), and 8.3(3.4) ms/mmHg, respectively. Conclusion Controlled 6/min breathing caused significant BRS overestimation under resting conditions. For the group, spontaneous respiration yielded acceptable BRS values, but individual BRS values deviated sometimes considerably. Conversely, with gravitational load, the respiratory pattern had only minor impact on BRS. Our results demonstrate that the risk of an overestimated BRS value is realistic as long as respiration is not controlled and of high-frequency.

Baroreflex analysis in diabetes mellitus: linear and nonlinear approaches

The aim of our study was to employ novel nonlinear synchronization approaches as a tool to detect baroreflex impairment in young patients with subclinical autonomic dysfunction in Type 1 diabetes mellitus (DM) and compare them to standard linear baroreflex sensitivity (BRS) methods. We recorded beat-to-beat pulse interval (PI) and systolic blood pressure (SBP) in 14 DM patients and 14 matched healthy controls. We computed the information domain synchronization index (IDSI), cross-multiscale entropy, joint symbolic dynamics, information-based similarity index (IBSI) in addition to time domain and spectral measures of BRS. This multi parametric analysis showed that baroreflex gain is well-preserved, but the time delay within the baroreflex loop is significantly increased in patients with DM. Further, the level of similarity between blood pressure and heart rate fluctuations was significantly reduced in DM. In conclusion, baroreflex function in young DM patients is changed. The quantification of nonlinear similarity and baroreflex delay in addition to baroreflex gain may provide an improved diagnostic tool for detection of subclinical autonomic dysfunction in DM.

Vibration plethysmography: a method for studying the visco-elastic properties of finger arteries.

Vibration plethysmography records changes in vascular volume produced by fast vibrations of cuff pressure. From these, waveforms of dynamic vascular compliance (DVC) are obtained. A total of 46 recordings of DVC, photo-electric plethysmogram (PG), cuff pressure (CP), and indirect blood pressure (BP) are performed on two adjacent fingers (third and fourth) in 23 healthy subjects. The shape and polarity of the DVC waveform markedly depends upon CP or transmural pressure (TP) (TP=BP-CP). The correlation coefficient between DVC and PG waveforms is nearly −1 at negative mean TP, near zero at zero TP, and approaches +1 at positive TP. For CP moving between systolic and diastolic BP, the DVC waveform shows a diastolic peak, with its maximum close to the zero value of instantaneous TP. xy-diagrams of PG against TP and of DVC against TP plotted for the diastolic phase of single pulses reveal a close coincidence of the DVC peak with the maximum slope of the PG/TP curve. A similar relationship appears when slow changes in PG and the amplitude of PG pulse waves are plotted against mean TP.

Influence of age, body mass index, and blood pressure on the carotid intima-media thickness in normotensive and hypertensive patients

Abstract We investigated whether body mass index and blood pressure have an additive influence on the carotid intima-media thickness (IMT). In 27 patients treated for hypertension (47.2±8.7 years) and 23 normotensive subjects (44.1±8.1 years), 24-h recording of blood pressure was performed. The carotid IMT was determined by ultrasonography and baroreflex sensitivity by a spectral method from 5-min recordings of blood pressure. Significant differences between hypertensive and normotensive subjects were observed for carotid IMT (0.60±0.08 vs. 0.51±0.07 mm; p<0.001) and baroreflex sensitivity (3.5±1.8 vs. 5.6±2.1 ms/mm Hg; p<0.001). Hierarchical multiple regression analysis (p<0.01) showed that carotid IMT was positively correlated with age (p<0.001) and body mass index (p<0.05) in normotensive subjects. The increased carotid IMT in hypertensive patients was not additively influenced by either age or body mass index. Baroreflex sensitivity decreased with age (p<0.01) and with carotid IMT (p<0.05) in normotensive subjects only. Multiregression analysis showed that an additive influence of age and body mass index on the development of carotid IMT is essential only in normotensive subjects. In hypertensive subjects the influence of blood pressure predominates, as documented by a comparison of the carotid IMT between hypertensive and normotensive subjects.

Ageing and Cardiac Autonomic Status

Recent studies have shown the significance of autonomic changes for the diagnosis and prognosis of many cardiovascular diseases. A consensus exists that, in the setting of acute myocardial ischaemia, sympathetic hyperactivity facilitates the onset of malignant arrhythmias, whereas vagal activation can exert an antifibrillatory effect. Thus, evaluation of the activity of the autonomic nervous system can contribute to more refined post-myocardial infarction risk stratification. Decreased heart rate variability (HRV) which is a marker of lower parasympathetic and/or higher sympathetic activity and depressed baroreflex sensitivity (BRS) indicate an increased risk of sudden cardiac death (SCD) in these patients.1–3 BRS is also decreased in patients with essential hypertension and therapeutic normalisation of blood pressure is not accompanied by BRS increase.4,5 On the other hand, both BRS and HRV physiologically decline with age6 which complicates the evaluation of cardiac autonomic status in the elderly.

Individual features of circulatory power spectra in man.

SummaryRhythms of resting fluctuations of circulatory parameters in man reveal a considerable interindividual variability. We posed the question whether these rhythms are long-term individual characteristics. In nine healthy subjects aged 19–23 years the blood pressure and the finger blood flow were recorded by indirect continuous methods, together with cardiac intervals and respiratory movements. These recordings were repeated in each subject after 1 year. The power spectra of all the parameters recorded were calculated for 5-min periods. The shape of spectra and the division of power into four ranges of frequencies were compared to the spectra recorded after 1 year in each subject and the degree of similarity was evaluated by means of correlation analysis. The average measures of similarity (correlation coefficients) were high, cardiac intervals 0.527, systolic pressure 0.782, pulse pressure 0.755, diastolic pressure 0.709, mean blood pressure 0.673, blood flow 0.818 and respiration 0.627. All these values were higher than values obtained by comparison of spectra of two individuals chosen randomly. The differences were statistically significant for cardiac intervals (Wilcoxon test:P<0.05), pulse pressure (P<0.05) and respiration (P<0.01). These results have shown that interindividual variability of circulatory and respiratory spectra was greater than the intraindividual one. The resting circulatory rhythms are very stable individual features.

Non-baroreflex mediated heart rate variability causes overestimation of baroreflex sensitivity

Spectral assessment of baroreflex sensitivity (BRS) is hampered by non-baroreflex mediated heart rate variability (HRV), which adds to the baroreflex mediated HRV. Relating HRV (baroreflex output) to blood pressure variability (BPV, baroreflex input) may therefore yield BRS values that are too large. The authors propose to overcome this problem by computing BRS in the Mayer waves (0.05-0.15 Hz), while keeping the non-baroreflex mediated HRV in the high frequency band by 15/min (0.25 Hz) metronome breathing.

Power spectra of blood pressure and heart rate fluctuations during mental load

Abstract The aim of the present paper was a search of a respiration‐independent factor changing the power spectra of circulatory parameters during mental load. Blood pressure, heart rate and respiratory movements were recorded in 12 subjects (age 19–23) in 4 experimental periods, 5 minutes each: at rest ‐ period 1 ‐ and during a mental load (discrimination between 3 tones of different pitch) ‐ period 2. Then the record of respiration from periods 1 and 2 was replayed on the oscilloscopic screen and the subject had to reproduce his own previous respiratory movements ‐ period 3 and 4. The power spectra of all parameters recorded in periods 1 and 3, and 2 and 4 were compared; the reproduction of the respiration as a week mental load was taken into consideration. There is some tendency of mental load to damp the power spectral density of circulatory parameters in all frequency ranges (minutes waves, 10‐second rhythm and respiratory oscillations), but in some subjects expressive opposite changes occurred. We d...

Four signal processing techniques for continuous baroreflex determination

Heart rate baroreflex sensitivity (BRS) is given as a ratio of inter-beat intervals (IBI) and systolic blood pressure (SBP) variations. This paper compares the feasibility of four techniques for continuous BRS determination: sequence technique, modified complex demodulation and alpha index technique and LP model technique. Blood pressure of healthy subjects (age 21-22 years) was recorded at rest (3 min), during exercise (0.5 W/kg of body weight, 9 min) and at rest (6 min) after exercise. The controlled breathing (20 breaths per min) was used at rest. To test the methods thirty-eight records were analyzed. The sequence technique determines the BRS from spontaneous variations of IBI and SBP in time. The complex demodulation technique determines the BRS as a ratio of the IBI and SBP signal amplitudes filtered by a band-pass filter with a 0.1 Hz frequency center. The alpha techniques and the LP techniques are spectral techniques, In the last two techniques a window of a few tenths of seconds duration simultaneously slides from the beginning to the end of both IBI and SBP signals. For each window the spectra of both signals are computed. The standard Fourier transform is used to determine the spectra for the alpha index technique. The ratio of amplitudes of the dominant spectral components in IBI and SBP spectre having near frequencies gives the BRS at the central window time and the corresponding frequencies. The linear prediction (LP) technique determines the spectrum as the inverse of the whitening filter transfer function. The residue at poles of the IBI and SBP LP spectral functions are determined. The ratio of IBI and SBP transfer functions corresponding to residua of frequency near paired poles gives the BRS at these frequencies. Determined values are attached to the center time of the window. All used techniques of BRS determination yield similar results. Averaged curves of the BRS changes are identical with those of IBI, but they differ from those of SBP and diastolic blood pressure.