Beta-adrenergic control and inter-relationships between heart rate and blood pressure in neonatal lambs

Our aim was to develop a signal analysis method for revealing interrelationships between heart rate and blood pressure and for displaying the influence of autonomic nervous control on these signals in a chronic lamb model. A chronically instrumented neonatal lamb model was made to record ECG and direct arterial blood pressure (N=15). Continuous two-minute recordings of blood pressure (BP) and ECG were digitised. The instantaneous heart rate signal (IHR) was derived from the ECG. The IHR and BP signals were bandpass filtered. Autospectra, cross-spectra, coherence spectra and phase spectra for the signals were computed to study the relative magnitudes and inter-relationships of the cardiovascular signals under normal conditions and during beta-adrenergic blockade. It was noted that both in the BP and IHR there were oscillations at the frequency of <0·1 Hz and also at the respiratory rate around 0·6 Hz. Beta-blockade reduced the oscillations of the IHR in <30-day-old lambs. It did not affect the coherence spectra or the phase lag between the signals. During quiet sleep the variability of blood pressure was decreased. In over-30-day-old-lambs the beta-blockade did not affect the variabilities of the cardiovascular parameters. These findings indicate that in neonatal lambs the sympathetic control system is a major regulator of cardiovascular interactions.     

Effect of baroreceptor denervation on the autonomic control of arterial pressure in conscious mice

This study evaluated the role of arterial baroreceptors in arterial pressure (AP) and pulse interval (PI) regulation in conscious C57BL mice. Male animals, implanted with catheters in a femoral artery and a jugular vein, were submitted to sino-aortic (SAD), aortic (Ao-X) or carotid sinus denervation (Ca-X), 5 days prior to the experiments. After basal recording of AP, the lack of reflex bradycardia elicited by administration of phenylephrine was used to confirm the efficacy of SAD, and cardiac autonomic blockade with methylatropine and propranolol was performed. The AP and PI variability were calculated in the time and frequency domains (spectral analysis/fast Fourier transform) with the spectra quantified in low- (LF; 0.25-1 Hz) and high-frequency bands (HF; 1-5 Hz). Basal AP and AP variability were higher after SAD, Ao-X or Ca-X than in intact mice. Pulse interval was similar among the groups, whereas PI variability was lower after SAD. Atropine elicited a slight tachycardia in control mice but did not change PI after total or partial denervation. The bradycardia caused by propranolol was higher after SAD, Ao-X or Ca-X compared with intact mice. The increase in the variability of AP was accompanied by a marked increase in the LF and HF power of the AP spectra after baroreceptor denervation. The LF and HF power of the PI were reduced by SAD and by Ao-X or Ca-X. Therefore, both sino-aortic and partial baroreceptor denervation in mice elicits hypertension and a remarkable increase in AP variability and cardiac sympathetic tonus. Spectral analysis showed an important contribution of the baroreflex in the power of LF oscillations of the PI spectra. Both sets of baroreceptors seem to be equally important in the autonomic regulation of the cardiovascular system in mice.     

Rhythmic oscillations of blood pressure and R-R interval in patients with chronic renal failure

Rhythmic blood pressure (BP) and R-R interval (R-R) oscillations at low-mid and high-frequency bands (LF: 0·02–0·06 Hz; MF; 0·07–0·14 Hz; HF; 0·15–0·40 Hz) were compared between uraemic patients maintained on haemodialysis and control subjects. The LF and MF powere spectra of BP were attenuated more in patients than in controls. With subjects standing, the MF power spectrum of BP increased significantly in both groups. With subjects supine, the plasma norepinephrine concentration was higher, and its increment upon standing was greater in patients than in controls. Each R-R frequency power spectrum decreased more in patients than in control subjects. The HF power spectrum of R-R, i.e. a vagal tone index, systematically decreased upon standing in the control subjects but not in the patients. The linear coupling between BP and R-R oscillations was strongest in the HF band, decreasing in the MF and LF bands. Transfer function analysis indicated that, in uraemic patients, linear BP/R-R relationships were altered in the HF band but remained normal in the LF and MF bands. The present results suggest that, first, the decreased amplitude of Mayer waves, i.e. the MF power spectrum of BP, observed in uraemic patients can be attributed to low sensitivity of the vasculature to sympathetic stimuli, and, secondly, autonomic modulation of linear BP/R-R relationships is frequency-dependent.     

Wide-band spectral analysis of blood pressure and RR interval variability in borderline and mild hypertension.

The aim of this study was firstly to investigate whether indices of wide-band spectral analysis in borderline hypertensive (BHT) or mildly hypertensive (HT) subjects differ from those in normotensive (NT) subjects, and secondly to assess the predictive value of these indices for future hypertension. Electrocardiogram and intra-arterial 24 h ambulatory blood pressure (BP) were recorded in 32 NT, 29 BHT and 30 HT middle-aged men. From the recordings, a 16 h period was extracted for wide-band spectral analysis. A single spectrum of BP and RR interval (RRI) variability was computed for each period by the fast Fourier transform method. The slopes of the spectra were assessed on a log-log scale by linear fitting of the spectral values. Power spectral densities were calculated over regions of 0-0.003, 0.003-0.04, 0.04-0.15, 0.15-0.40 and 0-0.4 Hz. No between-group differences were found in the slopes of BP and RRI spectra. The between-group differences in spectral powers for BP variability were almost invariably significant. The spectral powers for RRI variability did not show between-group differences. Five years later, 22 NT, 22 BHT and 18 HT subjects were re-assessed using casual BP measurements. In a logistic regression model for the combined group of NT and BHT subjects who became HT (22 of 44) during the five-year period, none of the parameters of wide-band spectrum predicted the development of hypertension. In conclusion, parameters of wide-band spectral analysis may not be useful in predicting future hypertension in NT and BHT subjects. Because the BP level is a major factor influencing BP variability, the between-group differences in wide-band spectral powers in BP may be due to differences in BP level rather than differences in cardiovascular regulatory mechanisms.     

Spectral indices of human cerebral blood flow control: responses to augmented blood pressure oscillations

We set out to fully examine the frequency domain relationship between arterial pressure and cerebral blood flow. Oscillatory lower body negative pressure (OLBNP) was used to create consistent blood pressure oscillations of varying frequency and amplitude to rigorously test for a frequency- and/or amplitude-dependent relationship between arterial pressure and cerebral flow. We also examined the predictions from OLBNP data for the cerebral flow response to the stepwise drop in pressure subsequent to deflation of ischaemic thigh cuffs. We measured spectral powers, cross-spectral coherence, and transfer function gains and phases in arterial pressure and cerebral flow during three amplitudes (0, 20, and 40 mmHg) and three frequencies (0.10, 0.05, and 0.03 Hz) of OLBNP in nine healthy young volunteers. Pressure fluctuations were directly related to OLBNP amplitude and inversely to OLBNP frequency. Although cerebral flow oscillations were increased, they did not demonstrate the same frequency dependence seen in pressure oscillations. The overall pattern of the pressure–flow relation was of decreasing coherence and gain and increasing phase with decreasing frequency, characteristic of a high-pass filter. Coherence between pressure and flow was increased at all frequencies by OLBNP, but was still significantly lower at frequencies below 0.07 Hz despite the augmented pressure input. In addition, predictions of thigh cuff data from spectral estimates were extremely inconsistent and highly variable, suggesting that cerebral autoregulation is a frequency-dependent mechanism that may not be fully characterized by linear methods.     

Acoustic transmission of the aortic component of the second heart sound within the ascending aorta of dogs

The spectral characteristics of the acoustic transmission of the aortic component of the second heart sound within the ascending aorta was studied using a Millar dual-micromanometer catheter. The tip micromanometer was located close to the aortic valve leaflets while the second micromanometer was located 3 cm above the aortic valve. The frequency response of the transmission properties (amplitude and phase) of the blood and the aortic wall was modelled by an equivalent acoustic transmission system. The signal recorded by the tip micromanometer located near the aortic valve was considered to be the input signal of the equivalent system and the signal recorded by the second micromanometer was used as the output signal. Results of the spectral analysis of the input and output signals show that the acoustic transmissibility of blood in the ascending aorta is high at 20 Hz (the attenuation is negligible). Between 20 and 60Hz, the transmissibility decreases at a rate of −3dB per octave while between 60 and 120 Hz it decreases at a rate of −14dB per octave. Above 120Hz the transmissibility is low and the resulting attenuation is greater than 20 dB. The phase of the transfer function is shifted by −60°g at 20Hz and decreases at a mean rate of −2·0°Hz⁻¹ between 20 and 100Hz and −0·75°Hz⁻¹ up to 400Hz. The phase velocity of the sound transmission is relatively constant (5·5ms⁻¹) between 40 and 100Hz and increases up to 9ms⁻¹ at 300Hz.     

Stress cardiovascular/autonomic interactions in mice

Studies evaluated the role of the autonomic nervous system in the cardiovascular response to stress using radiotelemetric blood pressure (BP) recording coupled with autoregressive spectral analysis. Conscious male C57/BL6 mice with carotid arterial telemetric catheters were exposed to acute episodes of shaker stress before and after administration of cholinergic, beta1-adrenergic and alpha1-adrenergic receptor antagonists. Pulse interval (PI) and systolic arterial pressure (SAP) were analyzed for variance and the low frequency (LF: 0.1-1.0 Hz) and high frequency (HF: 1-5 Hz) spectral components. Stress (5 min) increased BP and heart rate (HR) as well as PI and SAP variability. PI variance increased from 41+/-6 to 75+/-14 ms2 while SAP variance increased from 25+/-5 to 55+/-9 mm Hg2. Autonomic blockade had specific effects on stress-induced changes in PI and SAP and their respective variability. Atropine reduced the tachycardia and abolished the increase in PI variance and its LF component. Data documents that in mice the cholinergic system is fundamental for the maintenance of HR variability. Atropine had no effects on the BP responses, either the increase in SAP or the variance associated with stress. Atenolol blocked the increase in PI and SAP variability induced by stress. Prazosin reduced the tachycardia produced by stress and blocked the increase in PI (only LF) and SAP variability. Using quantitative spectral analysis of telemetrically collected BP data in mice along with pharmacological antagonism, we were able to accurately determine the role of autonomic input in the mediation of the stress response. Data verify the role of sympathetic/parasympathetic balance in stress-induced changes in HR, BP and indices of variance.     

Acute and chronic stress influence blood pressure variability in mice

There is evidence that alterations in heart rate and blood pressure variability (BPV) are associated with cardiovascular disease. We used a mice model to investigate the effects of acute and chronic stress on blood pressure variability (BPV) and heat rate variability (HRV). Shaker stress was given acutely (5 min, 150 cycles/min) and chronically (3 days, 2 min stress, 150 cycles/min, 45 sessions/day) in male C57BLJ mice. Systolic arterial pressure (SAP) and pulse interval (PI) time series were submitted to autoregressive spectral analysis with variability measured in the low-frequency (LF, 0.1-1.0 Hz) and high-frequency (HF, 1-5 Hz) ranges. In the acute experiment, MAP was increased significantly in the first 10 min poststress period (99+/-2 vs. 113+/-2 mm Hg) and returned to control levels 30 min poststress. HR was significantly higher in the initial poststress period (537+/-12 vs. 615+/-20 bpm). These alterations were associated with a marked increase in BPV (21+/-4 vs. 55+/-11 mm Hg2) and in power of LF oscillations (18+/-3 vs. 42+/-7 mm Hg2). On the other hand, chronic stress exposure produced a reduction in BPV (16+/-4 vs. 6+/-1 mm Hg2) and LF oscillations (11+/-3 vs. 3+/-1 mm Hg2). HRV was not altered after either acute or chronic stress. Spontaneous baroreflex sensitivity (SBS), determined by cross-spectral analysis between PI and BP, was reduced significantly in acute stress (-50%), but unchanged in chronic stress. Our results show that acute stress produced changes in BPV that may be associated with increased sympathetic activity and a reduction in blood pressure buffering. Under chronic conditions, there is no alteration in baroreflex sensitivity while BPV is reduced. This is likely related to the combination of sympathetic activation in the face of vasculature alterations.     

Seasonal training and heart rate and blood pressure variabilities in young swimmers

To evaluate if changes in athletes’ physical fitness due to seasonal training are associated with changes in cardiovascular autonomic control, nine swimmers (three males and six females; aged 14–18 years) were evaluated before and after 5 months of training and competitions. Maximal oxygen consumption and ventilatory threshold were determined during a maximal test; heart rate (HR) and blood pressure (BP) variabilities’ power spectra were calculated at rest (supine and sitting positions) and in the recovery of two exercises at 25 and 80% pre-training At the end of the season: (a) and ventilatory threshold increased respectively by 12 and 34% (P<0.05); (b) at rest, HR decreased by 9 b min⁻¹ in both body positions, whereas BP decreased in supine position only by 17%. No change in low frequency (LF, 0.04–0.15 Hz) and high frequency (HF, 0.15–1.5 Hz) normalized powers and in LF/HF ratio of HR variability and in LF power of systolic BP variability was observed. In contrast, a significant increase in HF α-index (about 12 ms mmHg⁻¹) was found; (c) during recovery no change in any parameter was observed. Seasonal training improved exercise capacity and decreased resting cardiovascular parameters, but did not modify vagal and sympathetic spectral markers. The increase in α-index observed at rest after the season and expression of augmented baroreflex sensibility indicated however that HR vagal control could have been enhanced by seasonal training. These findings suggested that autonomic system might have played a role in short-term adaptation to training.     

Association of breathing movements to the variability of heart rate and blood pressure in foetal lambs

Heart rate (HR) variability and arterial blood pressure (BP) variability were analysed as functions of foetal breathing movements (FBMs) by means of power spectral analysis in seven foetal lambs during the third trimester of gestation. No evidence of FBM-related changes, either in mean HR, mean systolic or diastolic arterial pressures, were found. Mean arterial pulse pressure, HR variability, and BP variability increased during FBMs. The increase in BP variability occurred at frequencies higher than 0.35 Hz, i.e. those of FBMs. The increase in HR variability occurred at 0.07-1.0 Hz, i.e. at every frequency band except the lowest one. Thus, the increase in HR variability was not frequency-specifically related to FBMs. During FBMs the periodic variability of HR at frequencies > 0.35 Hz was only 10% of total HR variability. We suggest that the FBM-related changes of BP variability may be mediated by direct peripheral, hydraulic mechanisms. HR changes involve autonomic control systems: the vagal component of baroreflex seems to be relatively insensitive, whereas the very slow vasomotor component of HR variability is dominant.     

Human vagal baroreflex sensitivity fluctuates widely and rhythmically at very low frequencies

Arterial pressure fluctuates rhythmically in healthy supine resting humans, who, from all outward appearances, are in a 'steady-state'. Others have asked, If baroreflex mechanisms are functioning normally, how can arterial pressure be so variable? We reanalysed data from nine healthy young adult men and women and tested the hypotheses that during brief periods of observation, human baroreflex sensitivity fluctuates widely and rhythmically. We estimated vagal baroreflex sensitivity with systolic pressure and R–R interval cross-spectra measured over 15 s segments, moved by 2 s steps through 20-min periods of frequency- and tidal volume-controlled breathing. We studied each subject at the same time on three separate days, with fixed protocols that included two physiological states, supine and passive 40 deg upright tilt, before and after β-adrenergic, cholinergic, and angiotensin converting enzyme blockade. Minimum, mean and maximum (± s.d. ) supine control baroreflex sensitivities averaged 5 ± 3, 18 ± 6, and 55 ± 22 ms mmHg⁻¹. In most subjects, moderate ongoing fluctuations of baroreflex sensitivity were punctuated by brief major peaks, yielding frequency distributions that were skewed positively. Fast Fourier transforms indicated that baroreflex sensitivity fluctuations (expressed as percentages of total power) concentrated more in very low, 0.003–0.04 Hz, than ultra low, 0.0–0.003 Hz, frequencies (77 ± 7 versus 11 ± 8%, P ≤ 0.001, rank sum test). Autoregressive centre frequencies averaged 0.012 ± 0.003 Hz. The periodicity of very low frequency baroreflex sensitivity fluctuations was not influenced significantly by upright tilt, or by variations of autonomic drive or angiotensin activity. Our analysis indicates that during ostensibly 'steady-state' conditions, human vagal baroreflex sensitivity fluctuates in a major way, at very low frequencies.     

Frequency-domain analysis of cerebral autoregulation from spontaneous fluctuations in arterial blood pressure

The dynamic relationship between spontaneous fluctuations of arterial blood pressure (ABP) and corresponding changes in crebral blood flow velocity (CBFV) is studied in a population of 83 neonates. Static and dynamic methods are used to identify two subgroups showing either normal (group A, n=23) or impaired (group B, n=21) cerebral autoregulation. An FFT algorithm is used to estimate the coherence and transfer function between CBFV and ABP. The significance of the linear dependence between these two variables in demonstrated by mean values of squared coherence >0.50 for both groups in the frequency range 0.02–0.50 Hz. However, group A has significanlty smaller coherences than group B in the frequency ranges 0.02–0.10 Hz and 0.33–0.49 Hz. The phase response of group A is also significantly more positive than that of group B, with slopes of 9.3±1.05 and 1.80±1.2 rad Hz⁻¹, respectively. The amplitude frequency response is also significantly smaller for group A in relation to group B for the frequency range 0.25–0.43 Hz. These results suggest that transfer function analysis may be able to identify different components of cerebral autoregulation and also provide a deeper understanding of recent findings by other investigators.     

Beat-to-beat measurement and analysis of the R-T interval in 24 h ECG Holter recordings

This study assesses the feasibility of beat-to-beat measurement of the R-T interval in Holter ECG recordings. The low sampling rate of the Holter system was increased by a specific interpolating filter, and the precision and accuracy of two T-wave fiducial point (T-wave maximum: T_m, T-wave end: T_e) detection algorithms were compared. The results of the validation tests show better performance of the T_m measurement procedure in the presence of high noise levels. The overall process for the beat-to-beat R-T interval measurement was then tested on ECG Holter recordings collected during free and controlled respiration. Finally, the R-T_m and the corresponding R-R intervals were measured on 24h ECG recordings of healthy subjects and the spectral analysis was applied to the constructed series. Both R-R and R-T_m spectra show two main frequency components (low-frequency ∼0·1 Hz, high-frequency ∼0·25 Hz) changing in their power ratios continuously throughout the 24h period. The method described seems to provide a dynamic index of the sympatho-vagal balance at the ventricle that can be useful for a deeper understanding of ventricular repolarisation duration variability.     

Autonomic modulation of arterial pressure and heart rate variability in hypertensive diabetic rats

OBJECTIVE: The aim of the present study was to evaluate the autonomic modulation of the cardiovascular system in streptozotocin (STZ)-induced diabetic spontaneously hypertensive rats (SHR), evaluating baroreflex sensitivity and arterial pressure and heart rate variability. METHODS: Male SHR were divided in control (SHR) and diabetic (SHR+DM, 5 days after STZ) groups. Arterial pressure (AP) and baroreflex sensitivity (evaluated by tachycardic and bradycardic responses to changes in AP) were monitored. Autoregressive spectral estimation was performed for systolic AP (SAP) and pulse interval (PI) with oscillatory components quantified as low (LF:0.2-0.6Hz) and high (HF:0.6-3.0Hz) frequency ranges. RESULTS: Mean AP and heart rate in SHR+DM (131+/-3 mmHg and 276+/-6 bpm) were lower than in SHR (160+/-7 mmHg and 330+/-8 bpm). Baroreflex bradycardia was lower in SHR+DM as compared to SHR (0.55+/-0.1 vs. 0.97+/-0.1 bpm/mmHg). Overall SAP variability in the time domain (standard deviation of beat-by-beat time series of SAP) was lower in SHR+DM (3.1+/-0.2 mmHg) than in SHR (5.7+/-0.6 mmHg). The standard deviation of the PI was similar between groups. Diabetes reduced the LF of SAP (3.3+/-0.8 vs. 28.7+/-7.6 mmHg2 in SHR), while HF of SAP were unchanged. The power of oscillatory components of PI did not differ between groups. CONCLUSIONS: These results show that the association of hypertension and diabetes causes an impairment of the peripheral cardiovascular sympathetic modulation that could be, at least in part, responsible for the reduction in AP levels. Moreover, this study demonstrates that diabetes might actually impair the reduced buffer function of the baroreceptors while reducing blood pressure.     

Effect of ambient respiratory noise on the measurement of lung sounds

The effect of ambient sounds, generated during breathing, that may reach a sensor at the chest surface by transmission from mouth and nose through air in the room, rather than through the airways, lungs and chest wall, is studied. Five healthy male non-smokers, aged from 11 to 51 years, are seated in a sound-proof acoustic chamber. Ambient respiratory noise levels are modified by directing expiratory flow outside the recording chamber. Low-density · gas (HeO₂=80% helium, 20% oxygen) is used to modify airway resonances. Spectral analysis is applied to ambient noise and to respiratory sounds measured on the chest and neck. Flow-gated average sound spectra are compared statistically. A prominent spectral peak around 960 Hz appears in ambient noise and over the chest and neck during expiration in all subjects. Ambient noise reduction decreases the amplitude of this peak by 20±4 dB in the room and by 6±3.6 dB over the chest. Another prominent spectral peak, around 700 Hz in adults and 880 Hz in children, shows insignificant change, i.e. a maximum reduction of 3 dB, during modifications of ambient respiratory noise. HeO₂ causes an upward shift in tracheal resonances that is also seen in the anterior chest recordings. Ambient respiratory noise explains some, but not all, peaks in the spectra of expiratory lung sounds. Resonance peaks in the spectra of expiratory tracheal sounds are also apparent in the spectra of expiratory lung sounds at the anterior chest.     

Relationships between short-term blood-pressure fluctuations and heart-rate variability in resting subjects I: a spectral analysis approach

A method to attribute the short-term variability of blood pressure and heart rate of resting subjects to their various causes, using spectral techniques, is presented. Power spectra and cross-spectra are calculated for beat-to-beat values of R-R interval and blood pressure from subjects who were seated in a comfortable chair. Interval values as well as systolic, mean and pulse pressures show variations linked to respiration and to the so-called 10 s rhythm. The diastolic pressure values are scarcely influenced by respiration in the normal respiratory range (0·20–0·35 Hz), but do show 10 s variability. Relationships between pressure and interval variability which indicate that the 10 s variability in systolic pressure leads the interval variation by two to three beats become manifest in cross-spectra; however, no such lag is found between the respiration-linked variations in systolic pressure and intervals. It is argued that the technique presented provides a critical test for models of the fast regulation of the cardiovascular system.     

Computer analysis of heart rate variation and breathing movements in fetal lambs

A quantitative method for studying the frequency-specific relationships between heart rate (HR) and fetal breathing movements (FBM) was developed. The reactivity of periodic HR variation in relation to FBM was investigated by means of power spectral analysis. Seven fetal lambs were studied during the third trimester of gestation using a chronic animal model. HR variability increased at the rate of FBM, as shown by an increase of spectral density at >0·35 Hz in the HR autospectrum and in the cross-spectrum of HR and respirogram, as well as by an increase in the shorterm variability index CVS. FBM were associated with the increased HR variation in all but the lowest frequency bands (0·07-1·0 Hz). Although respiratory sinus arrhythmia was found, only 10 per cent of the total HR variability and 25 per cent of the joint-density of HR and respirogram appeared at <0·35 Hz during FBM. The greatest variation in both the HR and respirogram spectra appeared at <0·07 Hz. Although the low-frequency variability of HR and respirogram was simultaneous, it was on the whole not synchronised. The existence of multiple control systems that simultaneosly link the cardiac and respiratory control mechanisms to each other in the fetal lamb is postulated.     

Adaptive spectral analysis of cutaneous electrogastric signals using autoregressive moving average modelling

The recording of the human, gastric myoelectrical activity, by means of cutaneous electrodes is called electrogastrography (EGG). It provides a noninvasive method of studying electrogastric behaviour. The normal frequency of the gastric signal is about 0·05 Hz. However, sudden changes of its frequency have been observed and are generally considered to be related to gastric motility disorders. Thus, spectral analysis, especially online spectral analysis, can serve as a valuable tool for practical purposes. The paper presents a new method of the adaptive spectral analysis of cutaneous electrogastric signals using autoregressive moving average (ARMA) modelling. It is based on an adaptive ARMA filter and provides both time and frequency information of the signal. Its performance is investigated in comparison with the conventional FFT-based periodogram method. Its properties in tracking time-varying instantaneous frequencies are shown. Its applications to the running spectral analysis of cutaneous electrogastric signals are presented. The proposed adaptive ARMA spectral analysis method is easy to implement and is efficient in computations. The results presented in the paper show that this new method provides a better performance and is very useful for the online monitoring of cutaneous electrogastric signals.     

Optimal time-window duration for computing time/frequency representations of normal phonocardiograms in dogs

The optimal duration of the time-window used to compute the time-frequency representation (spectrogram) of the phonocardiogram was studied in four dogs by using intracardiac and thoracic measurements of the PCG. The power and cross-spectrograms of the intracardiac and thoracic PCGs were computed using a fast Fourier transform algorithm and a sine-cosine window with 10 per cent decaying functions. A coherence spectrogram was also computed for each dog to study the linear relationship between the two signals and determine the optimal time-window duration. Results show that the optimal range of the time-window duration is between 16 and 32 ms. A time-window shorter than 16ms spreads out low-frequency components into the higher frequencies and generates a spectrographic representation with poor frequency resolution (≥62·5 Hz). A window larger than 32 ms increases the frequency resolution but smears the spectrographic representation of the signal in the time domain and thus cannot correctly reflect the time-varying properties of the signal. In both cases, the amplitude of the coherence function computed between the left ventricular and the thoracic phonocardiograms is overestimated.     

Spectra of data sampled at frequency-modulated rates in application to cardiovascular signals: Part 1 analytical derivation of the spectra

Beat-to-beat cardiovascular signals, e.g. a series of systolic pressure values, can be considered as time series which are pulse amplitude modulated (PAM) and pulse frequency modulated (PFM). The latter process, due to variations in heart rate, causes the series to become non-uniformly spaced in time. If PAM is to be quantified by spectral analysis, the influence of PFM must be known. An analytical expression is therefore derived for the spectrum of sinusoids which are sampled according to the output event series of a linear integral pulse frequency modulator (IPFM). We conclude that two spectral components arise at the difference and sum of the PFM and PAM frequencies, f_p±f_x, with amplitudes proportional to the PFM modulation depth. These components appear as a DC component and as a first harmonic if both modulating frequencies are equal. In addition, a cluster of spectral components appears around the mean pulse frequency f_o (i.e. mean heart rate), at frequencies f_o-nf_p±f_x, which may leak into the signal band. From these theoretical considerations, we conclude that the amplitude spectrum of a sinusoidally varying systolic blood pressure series can contain up to 20–30% spurious components, owing to the heart rate modulation process.