Spectral analysis of blood pressure variability in heart transplant patients

Identification of patients prone to hypotension during hemodialysis based on the analysis of cardiovascular signals

Intradialytic hypotension (IDH) is a major complication during hemodialysis treatment, and therefore it is highly desirable to identify, at an early stage during treatment, whether the patient is prone to IDH. Heart rate variability (HRV), blood pressure variability (BPV) and baroreflex sensitivity (BRS) were analyzed during the first 30 min of treatment to assess information on the autonomic nervous system. Using the sequential floating forward selection method and linear classification, the set of features with the best discriminative power was selected, resulting in an accuracy of 92.1%. Using a classifier based on the HRV features only, thereby avoiding that continuous blood pressure has to be recorded, accuracy decreased to 90.2%. The results suggest that an HRV-based classifier is useful for determining whether a patient is prone to IDH at the beginning of the treatment.

Relationship Between Cerebral Blood Flow and Blood Pressure in Long-Term Heart Transplant Recipients

Heart transplant recipients are at an increased risk for cerebral hemorrhage and ischemic stroke; yet, the exact mechanism for this derangement remains unclear. We hypothesized that alterations in cerebrovascular regulation is principally involved. To test this hypothesis, we studied cerebral pressure-flow dynamics in 8 clinically stable male heart transplant recipients (62±8 years of age and 9±7 years post transplant, mean±SD), 9 male age-matched controls (63±8 years), and 10 male donor controls (27±5 years). To increase blood pressure variability and improve assessment of the pressure-flow dynamics, subjects performed squat–stand maneuvers at 0.05 and 0.10 Hz. Beat-to-beat blood pressure, middle cerebral artery velocity, and end-tidal carbon dioxide were continuously measured during 5 minutes of seated rest and throughout the squat–stand maneuvers. Cardiac baroreceptor sensitivity gain and cerebral pressure-flow responses were assessed with linear transfer function analysis. Heart transplant recipients had reductions in R-R interval power and baroreceptor sensitivity low frequency gain ( P <0.01) compared with both control groups; however, these changes were unrelated to transfer function metrics. Thus, in contrast to our hypothesis, the increased risk of cerebrovascular complication after heart transplantation does not seem to be related to alterations in cerebral pressure-flow dynamics. Future research is, therefore, warranted.

Cardiovascular variability in Mexican patients with Parkinson's disease

Cardiovascular variability (CVV) has been evaluated in patients with Parkinson's disease (PD) in other countries and exhibit ethnic differences. Objective We investigated heart rate variability (HRV) and blood pressure variability (BPV) in Mexican patients with PD. Method We further compared HRV and BPV between this group and young healthy controls (YHC) in order to estimate, for the first time in our country, the magnitude of the difference. Twenty patients were examined. Time- and frequency-domain CVV parameters were studied during supine rest (SR), active standing (AS) and controlled breathing. These measurements were compared to those of 20 YHC. Results In the three conditions tested, our study showed a decrease in almost all HRV parameters in PD patients; on the contrary, decreased BPV parameters were found less frequently and only during SR and AS. Conclusion Our results indicate that HRV is impaired in PD. Some BPV parameters are also diminished.

Heart rate and blood pressure variability as markers of sensory blockade with labour epidural analgesia

PURPOSE

To evaluate the correlation between the progression of somatosensory blockade and changes in autonomic outflow following the onset of labour epidural analgesia.

METHODS

Twelve labouring parturients consented to participate in the study. Baseline electrocardiogram, blood pressure (BP) and respiratory rate were recorded for ten minutes. The epidural consisted of 0.125% bupivacaine with 50 microg of fentanyl (total volume 20 mL). Measurements were repeated for ten minutes after initiation of the block. The level of sensory block was measured bilaterally with loss of sensation to ice at two-minute intervals. Wavelet transform was used to obtain heart rate (HR) and BP variability every two minutes following the loading dose of epidural medication. High frequency power of HR variability was used to assess changes in parasympathetic activity. The total power of BP variability was used to assess changes in sympathetic activity. A nonparametric repeated measures ANOVA was used for the variability data, and a Spearman rank correlation test was used to evaluate the relationship between the sensory block and HR and BP variability.

RESULTS

The sensory block progressed to T9 at ten minutes post-epidural and was the mirror image of the decrease in total power of BP variability. High frequency power of HR variability increased to a plateau at six minutes post-epidural. A significant correlation was found between the increase in sensory block and the observed decrease in BP variability (r = -1.000, P = 0.0028).

CONCLUSION

In this study of labouring parturients, BP variability correlated with the progression of both sympathetic and somatosensory block following epidural anesthesia, while HR variability was shown to be a surrogate marker of increased parasympathetic activity.

Mechanisms of blood pressure and heart rate variability: an insight from low-level paraplegia

It is still unclear whether the low-frequency oscillation in heart rate is generated by an endogenous neural oscillator or by a baroreflex resonance. Our aim was to investigate this issue by analyzing blood pressure and heart rate variability and the baroreflex function in paraplegic subjects with spinal cord injury below the fourth thoracic vertebra. These subjects were selected because they represent a model of intact central neural drive to the heart, with a partially impaired autonomic control of the vessels. In our study, arterial blood pressure and ECG were recorded in 33 able-bodied controls and in 33 subjects with spinal cord lesions between the fifth thoracic and the fourth lumbar vertebra 1) during supine rest (lowest sympathetic activation), 2) sitting on a wheelchair (light sympathetic activation), and 3) during exercise (moderate sympathetic activation). Blood pressure and heart rate spectra, coherence, and baroreflex function (sequence technique) were estimated in each condition. Compared with controls, paraplegic subjects showed a reduction of the low-frequency power of blood pressure and heart rate, and, unlike controls, a 0.1-Hz peak did not appear in their spectra. Sympathetic activation increased the 0.1-Hz peak of blood pressure and heart rate and the coherence at 0.1 Hz in controls only. Paraplegic subjects also had significantly lower baroreflex effectiveness and greater blood pressure variability. In conclusion, the disappearance of the 10-s oscillation of heart rate and blood pressure in subjects with spinal cord lesion supports the hypothesis of the baroreflex nature of this phenomenon.

Automatic stride interval extraction from long, highly variable and noisy gait timing signals

This paper presents a probabilistic algorithm for automatically extracting the stride interval time series from long, highly variable and noisy two-state timing signals. Long interstride temporal records are of particular interest in nonlinear dynamical analysis of gait. The proposed method consists of probabilistic estimation and extraction followed by post-extraction filtering. With noisy timing signals from 10 children with Spastic Diplegia, no statistical differences in the numbers of extracted strides (p=0.94), the mean stride intervals (p=0.55) and the scaling exponents (p=0.94) (a measure of temporal heterogeneity) were found between series extracted by hand and by the probabilistic algorithm. The method is robust to noise and violations of normality. Results support the use of probabilistic extraction as an alternative to laborious manual extraction.

Functional restitution of cardiac control in heart transplant patients

Cardiovascular control is fundamentally altered after heart transplantation (HT) because of surgical denervation of the heart. The main goal of this work was the noninvasive characterization of cardiac rate control mechanisms after HT and the understanding of their nature. We obtained 25 recordings from 13 male HT patients [age = 28-68 yr, time after transplant (TAT) = 0.5-62.5 mo]. The control group included 14 healthy men (age = 28-59 yr). Electrocardiogram, continuous blood pressure (BP), and respiration were recorded for 45 min in the supine position and then during active change of posture (CP) to standing. The signals were analyzed in the time domain [mean and variance of heart rate (HR) and rise time of HR in response to CP] and the frequency domain [low and high frequency (LF and HF)]. Our principal finding was the consistent pattern of evolution of the HR response to standing: from no response, via a slow response (>40 s, TAT > 6 wk), to a fast increase (<20 s, TAT > 24 mo). HR response correlated with TAT (P < 0.001). LF correlated with HR response to CP (P < 0.0001); HF and HR did not. An important finding was the presence of very-high-frequency peaks in the power spectrum of HR and BP fluctuations. Extensive arrhythmias tended to appear at the TAT that corresponds to the transition from slow to fast HR response to CP. Our results indicate a biphasic evolution in cardiac control mechanisms from lack of control to a first-order control loop followed by partial sympathetic reinnervation and, finally, the direct effect of the old sinoatrial node on the pacemaker cell of the new sinoatrial node. There was no indication of vagal reinnervation.

Fractal characterization of complexity in temporal physiological signals

This review first gives an overview on the concept of fractal geometry with definitions and explanations of the most fundamental properties of fractal structures and processes like self-similarity, power law scaling relationship, scale invariance, scaling range and fractal dimensions. Having laid down the grounds of the basics in terminology and mathematical formalism, the authors systematically introduce the concept and methods of monofractal time series analysis. They argue that fractal time series analysis cannot be done in a conscious, reliable manner without having a model capable of capturing the essential features of physiological signals with regard to their fractal analysis. They advocate the use of a simple, yet adequate, dichotomous model of fractional Gaussian noise (fGn) and fractional Brownian motion (fBm). They demonstrate the importance of incorporating a step of signal classification according to the fGn/fBm model prior to fractal analysis by showing that missing out on signal class can result in completely meaningless fractal estimates. Limitation and precision of various fractal tools are thoroughly described and discussed using results of numerical experiments on ideal monofractal signals. Steps of a reliable fractal analysis are explained. Finally, the main applications of fractal time series analysis in biomedical research are reviewed and critically evaluated.

Decreased fractal component of human heart rate variability during non-REM sleep

The physiological significance of the fractal component of short-term, spontaneous heart rate variability (HRV) in humans remains unclear. The aim of the present study was to gain further information about the respective fractal components by extracting them from HRV, blood pressure variability (BPV), and instantaneous lung volume (ILV) time series via coarse graining spectral analysis in nine healthy subjects during waking and sleep states. The results show that the contribution made by the fractal component to the total variance in the beat-to-beat R-R interval declined significantly as the depth of non-rapid eye movement (non-REM) sleep increased, that the ILV time series was largely periodic (i.e., nonfractal), and that BPV was unaffected by sleep stage. Finally, the fractal component of HRV during REM sleep was found to be quite similar to that seen during waking. These results suggest that mechanisms involving electroencephalographic desynchronization and/or conscious states of the brain are reflected in the fractal component of HRV.

Sympathetic nervous system activity and cardiovascular homeostatis during head-up tilt in patients with spinal cord injuries

The relationship between sympathetic nervous system activity and cardiovascular responses to head-up tilt in patients with spinal cord injuries and in able-bodied subjects was studied. Twenty-seven adults, nine in each of the three groups (tetraplegia, paraplegia, and able-bodied subjects) were tilted 70 degrees, head up, for 12 minutes after 20 minutes supine rest. Differences between steady-state measurements of mean arterial pressure, stroke volume, and sympathetic nervous system activity were estimated in both positions. Sympathetic nervous system activity was reflected by the low-frequency peak of the blood pressure variability spectrum. From supine rest to head-up tilt, low-frequency power increased in able-bodied subjects (median, 0.42 mm Hg2, p = 0.003), which was different (p = 0.015) from patients with tetraplegia and paraplegia (-0.15 and -0.10 mm Hg2, respectively). Stroke volume and mean arterial pressure decreased in patients with tetraplegia (-40% and -9 mm Hg, respectively; p = 0.008, both variables) more than in able-bodied subjects (-33%, 11 mm Hg, respectively) or patients with paraplegia (-24%, 8 mm Hg, respectively). Results indicated increased sympathetic nervous system activity during head-up tilt in able-bodied subjects, but not in patients with paraplegia or tetraplegia, whereas patients with tetraplegia, but not paraplegia, showed poorer cardiovascular homeostasis than able-bodied subjects. This suggests that patients with paraplegia maintained cardiovascular homeostasis during head-up tilt without increased sympathetic nervous system activity.

Short-term variability of pulse pressure and systolic and diastolic time in heart transplant recipients

In heart transplant recipients (HTR), short-term systolic blood pressure variability is preserved, whereas heart rate variability is almost abolished. Heart period is the sum of left ventricular ejection time (LVET) and diastolic time (DT). In the present time-domain prospective study, we tested the hypothesis that short-term fluctuations in aortic pulse pressure (PP) in HTR were related to fluctuations in LVET. Seventeen male HTR (age 48 +/- 6 yr) were studied 16 +/- 11 mo after transplantation. Aortic root pressure was obtained over a 15-s period using a micromanometer both at rest (n = 17) and following the cold pressor test (CPT, n = 14). There was a strong positive linear relationship between beat-to-beat LVET and beat-to-beat PP in all patients at rest and in 13 of 14 patients following CPT (each P < 0.01). The slope of this relationship showed little scatter both at rest (0.34 +/- 0.07 mmHg/ms) and following CPT (0.35 +/- 0.09 mmHg/ms, P = not significant). Given the essentially fixed heart period, DT varied inversely with LVET. As a result, in 13 of 17 HTR at rest and in 12 of 14 HTR following CPT, there was a negative linear relationship between beat-to-beat PP and DT. In conclusion, our short-term time-domain study demonstrated a strong positive linear relationship between LVET and blood pressure variability in male HTR. We also identified a subgroup of HTR in whom there was a mismatch between PP and DT.

Assessment of the primary effect of aging on heart rate variability in humans

Beat-to-beat heart rate variability (HRV), reflecting cardiac autonomic control mechanisms, is known to change with age. However, the degree to which this change is mediated by aging per se or by physiologic changes characteristic of normative aging is still unclear. This study was designed to examine the association of aerobic fitness, body habitus or obesity, and blood pressure with age-related changes in HRV. Resting HRV data was recorded from 373 healthy subjects (124 men, 249 women; age range, 16-69 y) and analyzed by coarse-graining spectral analysis to decompose the total spectral power into its harmonic and fractal components. The low- and high-frequency (LF, 0.0-0.15 Hz; HF, >0.15 Hz) harmonic components were calculated from the former, whereas the latter was used to calculate the integrated power (FR) and the spectral exponent , beta, which were, in turn, used to evaluate the overall complexity of HRV. Factor analysis was performed to test whether potentially age-related changes in the components of HRV might be observed secondarily through other variables affecting HRV. Significant (p <0.05) age-related changes in the harmonic (HF and LF) and fractal (FR and beta) components of HRV were generally consistent with those described in the literature. In addition, factor analysis showed that there was a unique common factor that primarily explained correlations among age, HF, and beta (p <0.05) without the contributions from LF, FR, aerobic fitness, body habitus or obesity, and blood pressure. It was concluded that, in this population-based sample, age-related changes in HF and beta, both of which reflect vagal modulation of heart rate, were primarily mediated by aging per se and not by physiologic changes characteristic of normative aging.

Physiological mechanisms in regulation of blood pressure fast frequency variations

Spectral analysis of blood pressure fast oscillations (short-term variability), both in humans and animals, reveals three major frequential domains: the very low-, low- and the high-frequency domain. In this paper, experimental data providing evidence for physiological mechanisms involved in the regulation of blood pressure oscillations such as sympathetic nervous system, renin-angiotensin system, NO, respiration, heart function, and circulating blood volume are reviewed. In addition, novel evidence is provided by the author for vasopressin modulation of the low- and high-frequency blood pressure components. This review suggests that the multiplicity of factors involved in the genesis of the blood pressure spectral components imply utmost caution in interpreting spectral results.

Complexity of middle cerebral artery blood flow velocity: effects of tilt and autonomic failure

We examined spectral fractal characteristics of middle cerebral artery (MCA) mean blood flow velocity (MFV) and mean arterial blood pressure adjusted to the level of the brain (MAPbrain) during graded tilt (5 min supine, -10 degrees, 10 degrees, 30 degrees, 60 degrees, -10 degrees, supine) in eight autonomic failure patients and age- and sex-matched controls. From supine to 60 degrees, patients had a larger drop in MAPbrain (62 +/- 4.7 vs. 23 +/- 4.5 mmHg, P < 0.001; means +/- SE) and MFV (16.4 +/- 3.8 vs. 7.0 +/- 2.5 cm/s, P < 0.001) than in controls. From supine to 60 degrees, there was a trend toward a decrease in the slope of the fractal component (beta) of MFV (MFV-beta) in both the patients and the controls, but only the patients had a significant decrease in MFV-beta (supine: patient = 2.21 +/- 0.18, control = 1.99 +/- 0.60; 60 degrees: patient = 1.46 +/- 0.24, control = 1.62 +/- 0.19). The beta value of MAPbrain (MAPbrain-beta; 2.19 +/- 0.05) was not significantly different between patients and controls and did not change with tilt. High and low degrees of regulatory complexity are indicated by values of beta close to 1.0 and 2.0, respectively. The increase in fractal complexity of cerebral MFV in the patients with tilt suggests an increase in the degree of autoregulation in the patients. This may be related to the drop in MAPbrain. The different response of MFV-beta compared with that of MAPbrain-beta also indicates that MFV-beta is related to the regulation of cerebral vascular resistance and not systemic blood pressure.

Does low-frequency variability of heart period reflect a specific parasympathetic mechanism?

Low frequency (LF, approximately 0.1 Hz) spontaneous oscillations of heart period in humans have been attributed to and correlated with the sympathetic efferent control of the heart. However, this interpretation is controversial, because sympathetic blockade does not suppress these oscillations, while parasympathetic blockade strongly affects them. The sympathetic origin of LF of arterial pressure, on the contrary, has been convincingly demonstrated. Four 10 min cycle-by-cycle time series of R-R interval (RR), and systolic (SAP) and diastolic (DAP) arterial pressure were produced by automatic analysis of data obtained with non-invasive methods in 10 healthy humans during supine rest and while standing, both before and after beta 1-selective blockade (atenolol). Time series were analysed by autoregressive transfer function analysis. beta-blockade failed to induce systematic changes on the power of the LF peak of RR, in any condition. The coherence between RR and SAP in the same region remained high (0.77 +/- 0.03) and a constantly negative phase (approximately 50-60 degrees, corresponding to a delay of 1-2 heart beats of RR on SAP) was always seen. beta-blockade decreased the power of the LF peak of SAP, increased the transfer function gain between SAP and RR at LF, and the HF power of RR. We conclude that LF oscillations of RR are not directly generated by the sympathetic drive to the heart but reflect mainly the parasympathetic activity. The results suggest that the LF oscillations of the vagal outflow, and of RR, are generated by the baroreceptor reflex, driven by sympathetically-induced blood pressure LF waves.

Clinical relevance of heart rate variability

Heart rate variability (HRV) has recently become a popular noninvasive research tool in cardiology. Clinical assessment of HRV is frequently based on standard long-term ambulatory electrocardiograms, whereas physiologic studies employ spectral analysis of short-term recordings under controlled conditions. From a general point of view, HRV can be used in clinical practice to estimate (1) the integrity of cardiac autonomic innervation, (2) the physiologic status of cardiac autonomic activity, and (3) the vulnerability to various cardiac arrhythmias resulting from autonomic imbalance. Clinical relevance of HRV has been clearly demonstrated in only two clinical conditions: (1) impaired HRV can be used alone or in a combination with other factors to predict risk of arrhythmic events after acute myocardial infarction, and (2) decrease in HRV is a useful clinical marker for evolving diabetic neuropathy. Substantial advances of our knowledge are required to establish and promote clinical applications in other areas of clinical medicine. To accomplish this task, proper hypotheses should be studied and appropriate techniques selected.

Spectral pattern of neonatal cerebral blood flow velocity: comparison with spectra from blood pressure and heart rate

The cerebral blood flow velocity (CBFV) frequency spectra were studied in 106 premature and term newborns (gestational age range. 24-42 wk) and compared with the heart rate (HR) and mean arterial blood pressure (BP) spectra over the 0.005-0.5 Hz range. CBFV, BP, and HR were shown to have similar but not identical spectral patterns. Adjustment of a l/f model to these spectra produced highly significant fittings, but the residuals were not independent. This condition was met only by the CBFV and BP spectra over a limited frequency range (0.005-0.06 Hz). These results provide a characterization of the CBFV spectra for a much larger population of newborns than hitherto available, indicating that under certain conditions CBFV and BP might show the properties of chaotic systems. In infants without major complications, gestational age (GA) did not have a significant influence on the CBFV spectrum, whereas the spectral power to 0.5 Hz of both BP and HR was found to increase with GA. The spectral power increased over the first 24 h of postnatal life for all three variables: only CBFV showed a significant spectral change in the low frequency (LF, 0.02-0.08 Hz) range. A matched group comparison, adjusted for GA and postnatal age, indicated a reduction in CBFV LF power for term infants with birth asphyxia when compared with normal infants, which was not reproduced in the HR spectra.

Heart rate and blood pressure variability in cardiac diseases: pharmacological implications

Even at rest, blood pressure and heart fluctuate continuously around their mean values. Considerable interest has recently focused on the assessment of spontaneous in fluctuations in heart rate and blood pressure, i.e., heart rate and blood pressure variability, using time or frequency domain indexes. Heart rate variability has been extensively studied in cardiovascular disease and has emerged as a valuable parameter for detecting abnormalities in autonomic cardiovascular control, evaluating the prognosis and assessing the impact of drug therapy on the autonomic nervous system in patients with myocardial infarction, congestive heart failure or a heart transplant. In contrast, until the recent development of noninvasive methods for continuous blood pressure recording, blood pressure variability received little attention, and this parameter remains to be evaluated in cardiovascular disease.