Arterial baroreflex function and cardiovascular variability: interactions and implications

Relaxant effect of microtitan via regulation of autonomic nerve activity in mice

AIMS

It has been shown that microtitan may possibly affect the nervous system. In the present study, we examined the effect of microtitan on spontaneous activity during the sleeping period and on autonomic nervous activity in mice.

MAIN METHODS

Institute of Cancer Research (ICR) mice were divided into placebo and microtitan groups that were housed in chambers with rubber sheets impregnated with microtitan or placebo sheets. In both groups, spontaneous active movement, metabolic parameters, and heart rate variability (HRV) were measured.

KEY FINDINGS

Spontaneous activity during the light period was decreased for mice housed with microtitan sheets compared with placebo sheets. The urinary noradrenalin level was also reduced by microtitan. Heart rate variability was assessed by using a telemetry system and autonomic nervous activity was estimated. Power spectral analysis of R-R interval data revealed that the high frequency band, which shows parasympathetic activity, was significantly increased by microtitan, while the low frequency to high frequency power spectral ratio was decreased in the mice housed with microtitan sheets compared to the mice housed with placebo sheets.

SIGNIFICANCE

Microtitan promoted rest during the sleeping period by regulating autonomic nervous activity, which indicates that microtitan has a relaxant effect.

Voxel-based morphometry reveals an association between aerobic capacity and grey matter density in the right anterior insula

This study investigated the effects of aerobic capacity on brain structure and memory performance. A sample of 33 healthy young subjects completed (i) assessment of aerobic capacity based on blood-lactate concentration, (ii) structural magnetic resonance imaging (MRI) scanning and analysis of grey matter density using voxel-based morphometry (VBM) and (iii) a range of memory tests. Memory performance was not significantly associated with aerobic capacity. After adjusting for effects of age, gender and total intracranial volume, cortical grey matter density in the right anterior insula was strongly correlated with aerobic capacity. These findings are in line with studies implicating the insula in the cortical control of cardiovascular processes during both exercise and autonomic arousal. Interindividual differences in aerobic capacity are thus reflected in structural differences in brain regions involved in cardiovascular control, resembling structural changes associated with certain cognitive or motor skills.

BP variability and cardiovascular autonomic function in relation to forced expiratory volume: a population-based study

BACKGROUND

Cardiovascular autonomic dysfunction is associated with increased incidence of cardiovascular diseases. This population-based study explored whether low FEV(1) or low vital capacity (VC) is associated with autonomic dysfunction, as measured by spontaneous heart rate variability (HRV) and systolic BP variability (SBPV).

METHODS

SBPV and HRV were recorded during 5 min of controlled breathing in men and women who were 70 years of age. FEV(1) and VC were recorded in 901 subjects. Of them, information on HRV and SBPV was available in 820 and 736 subjects, respectively. Measures of autonomic function, that is, SBPV in the low-frequency (LF) and high-frequency (HF) domains, HRV, and baroreceptor sensitivity (BRS), were studied in sex-specific quartiles of FEV(1) and VC.

RESULTS

Low FEV(1) was associated with high SBPV in the HF domain. The mean SBPV-HFs were 5.2, 4.5, 4.1, and 3.8 mm Hg, respectively, in subjects with FEV(1) in the first (low), second, third, and fourth quartile (p < 0.001 [for trend]). This relationship persisted after adjustments for potential confounding factors. Low VC was significantly associated with high SBPV-HF in the crude analysis but not after adjustment for confounding factors. Neither FEV(1) nor VC showed any significant relationship with BRS, HRV, or SBPV in the LF domain.

CONCLUSION

In this population-based study, low FEV(1) was associated with high SBPV in the HF domain. It is suggested that high beat-to-beat variability in BP could contribute to the increased cardiovascular risk in subjects with moderately reduced FEV(1).

Acute application of bilevel positive airway pressure influences the cardiac autonomic nervous system

INTRODUCTION

Noninvasive positive pressure has been used to treat several diseases. However, the physiological response of the cardiac autonomic system during bilevel positive airway pressure (Bilevel) remains unclear.

OBJECTIVE

The aim of this study was to evaluate the heart rate variability (HRV) during Bilevel in young healthy subjects.

METHODS

Twenty men underwent 10-minute R-R interval recordings during sham ventilation (SV), Bilevel of 8-15 cmH(2)O and Bilevel of 13-20 cmH(2)O. The HRV was analyzed by means of the parallel R-R interval (mean R-Ri), the standard deviation of all R-Ri (SDNN), the root mean square of the squares of the differences between successive R-Ri (rMSSD), the number of successive R-Ri pairs that differ by more than 50 milliseconds (NN50), the percentage of successive R-Ri that differ by more than 50 milliseconds (pNN50), the low frequency (LF), the high frequency (HF) and SD1 and SD2. Additionally, physiological variables, including blood pressure, breathing frequency and end tidal CO(2), were collected. Repeated-measures ANOVA and Pearson correlation were used to assess the differences between the three studied conditions and the relationships between the delta of Bilevel at 13-20 cmH(2)O and sham ventilation of the HRV indexes and the physiological variables, respectively.

RESULTS

The R-Ri mean, rMSSD, NN50, pNN50 and SD1 were reduced during Bilevel of 13-20 cmH(2)O as compared to SV. An R-Ri mean reduction was also observed in Bilevel of 13-20 cmH(2)O compared to 8-15 cmH(2)O. Both the R-Ri mean and HF were reduced during Bilevel of 8-15 cmH(2)O as compared to SV, while the LF increased during application of Bilevel of 8-15 cmH(2)O as compared to SV. The delta (between Bilevel at 13-20 cmH(2)O and sham ventilation) of ETCO(2) correlated positively with LF, HF, the LF/HF ratio, SDNN, rMSSD and SD1. Acute application of Bilevel was able to alter the cardiac autonomic nervous system, resulting in a reduction in parasympathetic activity and an increase in sympathetic activity and higher level of positive pressure can cause a greater influence on the cardiovascular and respiratory system.

Evidence for functional alterations in the skeletal muscle mechanoreflex and metaboreflex in hypertensive rats

Exercise in hypertensive individuals elicits exaggerated increases in mean arterial pressure (MAP) and heart rate (HR) that potentially enhance the risk for adverse cardiac events or stroke. Evidence suggests that exercise pressor reflex function (EPR; a reflex originating in skeletal muscle) is exaggerated in this disease and contributes significantly to the potentiated cardiovascular responsiveness. However, the mechanism of EPR overactivity in hypertension remains unclear. EPR function is mediated by the muscle mechanoreflex (activated by stimulation of mechanically sensitive afferent fibers) and metaboreflex (activated by stimulation of chemically sensitive afferent fibers). Therefore, we hypothesized the enhanced cardiovascular response mediated by the EPR in hypertension is due to functional alterations in the muscle mechanoreflex and metaboreflex. To test this hypothesis, mechanically and chemically sensitive afferent fibers were selectively activated in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) decerebrate rats. Activation of mechanically sensitive fibers by passively stretching hindlimb muscle induced significantly greater increases in MAP and HR in SHR than WKY over a wide range of stimulus intensities. Activation of chemically sensitive fibers by administering capsaicin (0.01-1.00 microg/100 microl) into the hindlimb arterial supply induced increases in MAP that were significantly greater in SHR compared with WKY. However, HR responses to capsaicin were not different between the two groups at any dose. This data is consistent with the concept that the abnormal EPR control of MAP described previously in hypertension is mediated by both mechanoreflex and metaboreflex overactivity. In contrast, the previously reported alterations in the EPR control of HR in hypertension may be principally due to overactivity of the mechanically sensitive component of the reflex.

Are baroreflex events detected by invasive and non-invasive techniques coincident?

Cardiac baroreceptor sensitivity, a prognostic indicator for a range of diseases, such as myocardial infarction and stroke, may be estimated from spontaneous fluctuations of arterial blood pressure (BP) and heart rate using sequence analysis. We tested the hypothesis that BP values recorded with the non-invasive Finapres device do not always produce sequences coincident with sequences detected from central BP measurements. Finapres recordings of resting BP in the finger, ascending aorta (Millar catheter-tip transducer) and ECG were obtained from 34 patients undergoing coronary angioplasty, including 24 patients treated with betablockers. Coincidence of baroreflex sensitivity (BRS) sequences was expressed by the sensitivity of the Finapres to detect a simultaneously occurring sequence in aortic pressure. The influence of different criteria to detect and accept sequences from beat-to-beat values of systolic BP (SBP) and cardiac interval (RRi) on the Finapres sensitivity was also assessed. The Finapres was able to detect 70.7% of all three beat intra-arterial sequences when the selection criteria was based on the correlation coefficient between SBP and RRi (>0.85), but decreased to 27.5% when the P-value of the linear regression was limited to 0.05. Changing the thresholds for minimum changes in SBP and RRi also had significant effects on sensitivity, as well as in the corresponding values of BRS. Significant differences in BRS were obtained between invasive and non-invasive estimates, but there was no difference between non-invasive estimates calculated from coincident and non-coincident sequences. Non-invasive, compared with intra-arterial estimates of BRS by sequence analysis are not influenced by coincidence of sequences if acceptance of sequences is based on the correlation coefficient criteria (>0.85).

Verapamil buffering effect on the abrupt elevation in blood pressure, linkage with microcirculatory blood flow

1 We studied the effects of verapamil on sudden elevation in blood pressure, microcirculation and arterial baroreflex sensitivity (BRS). 2 Thirty experiments (10 controls and 20 with verapamil) were performed in rabbits sedated using pentobarbital infusion (5 mg kg(-1) h(-1)). 3 BRS, mean femoral artery blood pressure (MAP), heart rate (HR) and ear lobe skin microcirculatory blood flow, estimated using microphotoelectric plethysmography (MPPG), were simultaneously measured during 30 min of verapamil infusion (20 mug kg(-1) min(-1)). BRS was assessed from HR and MAP responses to intravenous phenylephrine (Ph) and by power spectral analysis using transfer function (TF) from MAP to the HR (BRS(Ph,TF)). 4 Verapamil significantly increased microcirculatory blood flow, and decreased BRS(Ph,TF) and phenylephrine-induced abrupt elevation in MAP (MAP(AE)). 5 A significant inverse correlation was found between verapamil-induced changes in MAP(AE), BRS and in microcirculatory blood flow, measured before phenylephrine blood pressure ramps (DeltaMAP(AE) with DeltaBRS(TF), r = -0.47, P < 0.036; DeltaMAP(AE) with DeltaMPPG, r = -0.49, P < 0.025). 6 These results suggest involvement of the arterial baroreflex and vascular blood pressure-buffering mechanisms, their enhancement by verapamil, and thus a potential benefit of verapamil in cardiovascular conditions where patients present with abrupt high elevations in blood pressure.

Oral glucose tolerance test reduces arterial baroreflex sensitivity in older adults

Although postprandial decreases in blood pressure are a common cause of syncope in the older adult population, the postprandial effects of the oral glucose tolerance test on blood pressure and the arterial baroreflex remain poorly characterized in older adults. Therefore, arterial blood pressure and the arterial baroreflex were studied in 19 healthy older adults (mean age 71.7 ± 1.1 years) who were given a standardized oral glucose load (75 g) or an isovolumetric sham drink during 2 separate sessions. All measures were taken for 120 min after treatment. Baroreflex function was assessed by using the spontaneous baroreflex method (baroreflex sensitivity, BRS). Subjects demonstrated a decrease in BRS after oral glucose that was not seen in the placebo session (two-way analysis of variance, p = 0.04). There was no significant change in systolic, mean, or diastolic blood pressure; together with a drop in BRS, this resulted in a significant tachycardia post glucose (two-way analysis of variance, p < 0.001). We conclude that healthy older adults can successfully maintain blood pressure during an oral glucose tolerance test despite a decrease in arterial BRS. Decreased BRS resulted in a tachycardic response to glucose.

Using 100% Oxygen does not Alter the Cardiovascular Autonomic Regulation during Non-invasively Simulated Haemorrhage in Healthy Volunteers

We tested the effect of 100% oxygen on heart rate (HR), arterial blood pressure (ABP), cardiac output (CO), stroke volume (SV), total peripheral resistance (TPR), HR variability (HRV), systolic blood pressure variability (SBPV) and baroreflex sensitivity (BRS) in 20 healthy volunteers during simulated haemorrhage induced by −40 mmHg lower body negative pressure (LBNP). HRV in the high frequency region (HRVHF), BRS, ABP and TPR were significantly increased, SBPV in the low frequency region (SBPVLF), CO and SV were unchanged, and HR was significantly decreased by 100% oxygen administration during normovolaemia. HRVHF, BRS, CO and SV were significantly decreased, SBPVLF and ABP were unchanged, and HR and TPR were significantly increased by LBNP during 21% or 100% oxygen administration. There were no significant differences in cardiovascular autonomic and haemodynamic responses to LBNP during 21% or 100% oxygen administration, suggesting that 100% oxygen does not alter normal cardiovascular autonomic responses during simulated haemorrhage.

Personal Interaction in the Vegetative State

Background and purpose: Brain processing at varying levels of functional complexity and emotional reactions to relatives are anecdotally reported by the caregivers of patients in a vegetative state. In this study, computer-assisted machine-learning procedures were applied to identify heart rate variability changes or galvanic skin responses to a relative’s presence. Methods: The skin conductance (galvanic skin response) and heart beats were continuously recorded in 12 patients in a vegetative state, at rest (baseline) and while approached by a relative (usually the mother; test condition) or by a nonfamiliar person (control condition). The cardiotachogram (the series of consecutive intervals between heart beats) was analyzed in the time and frequency domains by computing the parametric and nonparametric frequency spectra. A machine-learning algorithm was applied to sort out the significant spectral parameter(s). For all patients, each condition (baseline, test, control) was characterized by the values of its spectral parameters, and the association between spectral parameters values and experimental condition was tested (WEKA machine-learning software). Results and comments: A galvanic skin response was obtained in two patients. The machine-learning procedure independently selected the nu_LF spectral parameter and attributed each nu_LF measure to any of the three experimental conditions. 69.4% of attributions were correct (baseline: 58%; test condition: 75%; control. 75%). In seven patients, attribution changed when the subject was approached by the test person; specifically, sequential shifts from baseline to test condition (“the Mom effect”) to control condition were identified in four patients (30.0%); the change from test to control was attributed correctly in seven patients (58%). The observation of heart rate changes tentatively attributable to emotional reaction in a vegetative state suggest residual rudimentary personal interaction, consistent with functioning limbic and paralimbic systems after massive brain damage. Machine-learning proved applicable to sort significant measure(s) out of large samples and to control for statistical alpha inflation.

Heart Rate Response to Music

Background and rationale: Investigation of the brain’s emotional response to music is limited by methodological problems mainly related to the characterization of the emotions and concomitant brain conditions. In this study, artificial intelligence procedures were applied to identify significant music-induced changes in heart rate variability and to classify autonomic reactions to stimuli requiring complex brain operations. Both healthy subjects and traumatic brain-injury (TBI) patients were studied in order to test the method’s validity. Methods: 16 TBI patents and 26 healthy subjects were requested to listen to selected music samples while the heart beat was continuously recorded. The parametric and nonparametric frequency spectra were computed on the heart rate and the spectra descriptors were entered into a 1-R rules (very simple classification rules) data-mining procedure. Data-mining procedures independently classified the heart-rate spectral patterns and the emotions reported by subjects as positive, indifferent, or negative. Results and conclusions: The data-mining procedures sorted the nu_LF descriptor as the spectral parameter that allowed clustering the emotions reported by the subjects as positive and negative. Classification by nu_LF was comparable to that by self-reported emotions, with an overall correct classification by author in 76.0% of controls and 70% of patients. The identification of negative and positive emotions was correct in 81.3% and 68.9% of controls and in 65% and 74% of TBI patients, without significant differences between healthy subjects and TBI patients. This observation suggests that autonomic concomitants of emotions are detectable in response to complex emotional stimuli.

The effect of gender on autonomic and respiratory responses during sleep among both young and middle-aged subjects

BACKGROUND AND OBJECTIVES

Sleep affects the control of circulation and respiratory function. Gender and age are also known to have a profound impact on the neural control of circulation. We investigated whether gender affects sleep-related cardiovascular and respiratory responses and whether these vary according to healthy subjects being young or middle-aged.

METHODS

We studied 32 subjects: 8 women and 8 men aged 20-30 years (young), and 8 women and 8 men aged 50-60 years (middle-aged). Young women were under oral contraceptive therapy and middle-aged women were postmenopausal and not receiving hormonal replacement therapy. One-night polysomnography was used to assess RR variability during non-rapid eye movement (NREM) (stage 2) and rapid eye movement (REM) sleep. Low-frequency (LF) and high-frequency (HF) components, in normalized units (LFnu and HFnu) and LF/HF ratio were calculated on five-minute segments selected across the night and averaged for each sleep stage. The respiration frequency in NREM and REM sleep was also measured. Interaction between gender, age and sleep on autonomic and respiration variables was assessed by 2 x 2 x 2 analysis of variance (ANOVA).

RESULTS

Compared to men, women had a greater NREM-to-REM increment in LFnu (gender-by-state interaction, p<0.01), a greater decrement in HFnu (interaction, p<0.01) and a greater increment in LF/HF (interaction, p<0.05). Women also showed a more pronounced increase in respiratory frequency during REM sleep compared to men in both groups of age (gender-by-state interaction, F=7.1, p<0.05). No gender-by-age-by-state interaction was observed to affect autonomic and respiration variables.

CONCLUSION

NREM-to-REM excitatory cardiac and respiratory responses are more marked among women compared to men, regardless of their hormonal status and whether they are young or middle-aged.

Quantification of perfusion and permeability in breast tumors with a deconvolution-based analysis of second-bolus T1-DCE data

PURPOSE

To test the feasibility of using a second-bolus injection, added to a routine breast MRI examination, to measure regional perfusion and permeability in human breast tumors.

MATERIALS AND METHODS

In 30 patients with breast tumors, first a routine whole-breast T1-DCE sequence was applied, and the slice where the lesion enhanced maximally was located. At that slice position, T1-weighted MR images were acquired at 0.3-second resolution using a second-bolus dynamic inversion recovery (IR)-prepared turbo field echo (TFE) sequence. A pixel-by-pixel model-independent deconvolution of the relative signal enhancement was performed to estimate the tumor blood flow (TBF), tumor volume of distribution (TVD), mean transit time (MTT), extraction flow product (EF), and extraction fraction (E). In addition to this pilot study, a first appraisal of its sensitivity to tissue type was made on the basis of a small patient cohort.

RESULTS

In the malignant tumors, the parametric maps clearly delineated tumors from the breast tissue and enabled visualization of the heterogeneity. The deconvolution analysis provided objective parametric maps of tumor perfusion with a mean TBF (84 +/- 48 mL/100 mL/minute) in malignant tumors in the high range of literature values.

CONCLUSION

In terms of these perfusion values, our method appears promising to quantitatively characterize tumor pathophysiology. However, the number of patients was limited, and the separation between malignant and benign groups was not clear-cut. Additional parameters generated through compartment modeling may improve the tumor differentiation.

Baroreflex sensitivity in obesity: relationship with cardiac autonomic nervous system activity

OBJECTIVE

The aim of this study was to test the hypothesis that baroreflex sensitivity (BRS), assessed by indirect measurement of aortic pressure, is blunted in obesity. Additionally, the potential effect of cardiac autonomic nervous system (ANS) activity, aortic compliance, and metabolic parameters on BRS of obese subjects was investigated.

RESEARCH METHODS AND PROCEDURES

A group of 30 women with BMI>30 kg/m2 and a group of 30 controls with BMI<25 kg/m2 were examined. BRS was estimated by the sequence technique, cardiac ANS activity by short-term spectral analysis of heart rate variability (HRV), and aortic compliance by the method of applanation tonometry.

RESULTS

BRS was lower in obese women (9.18+/-3.77 vs. 19.63+/-9.16 ms/mm Hg, p<0.001). The median values (interquartile range) of the power of both the high-frequency and low-frequency components of the HRV were higher in the lean than in the obese participants [1079.2 (202.7 to 1716.9) vs. 224.1 (72.7 to 539.6) msec2, p=0.001 and 411.8 (199.3 to 798.0) vs. 235.8 (99.4 to 424.5) msec2, p=0.01 respectively]. Low-to-high-frequency ratio values were higher in the obese subjects [0.82 (0.47 to 2.1) vs. 0.57 (0.28 to 0.89), p=0.02]. Aortic augmentation values were not significantly different between lean and obese subjects. Multivariate analysis demonstrated a significant and independent association between BRS and age (p=0.003), BMI (p<0.001), and high-frequency power of HRV (p<0.001). These variables explained 72% of the variation of BRS values.

DISCUSSION

BRS is severely reduced in obese subjects. BMI, age, and the parasympathetic nervous system activity are the main determinants of BRS. Baroreflex behavior is of clinical relevance because an attenuated BRS represents a negative prognostic factor in cardiovascular diseases, which are common in obesity.

Mechanisms of intrinsic beating variability in cardiac cell cultures and model pacemaker networks

Heart rate variability (HRV) exhibits fluctuations characterized by a power law behavior of its power spectrum. The interpretation of this nonlinear HRV behavior, resulting from interactions between extracardiac regulatory mechanisms, could be clinically useful. However, the involvement of intrinsic variations of pacemaker rate in HRV has scarcely been investigated. We examined beating variability in spontaneously active incubating cultures of neonatal rat ventricular myocytes using microelectrode arrays. In networks of mathematical model pacemaker cells, we evaluated the variability induced by the stochastic gating of transmembrane currents and of calcium release channels and by the dynamic turnover of ion channels. In the cultures, spontaneous activity originated from a mobile focus. Both the beat-to-beat movement of the focus and beat rate variability exhibited a power law behavior. In the model networks, stochastic fluctuations in transmembrane currents and stochastic gating of calcium release channels did not reproduce the spatiotemporal patterns observed in vitro. In contrast, long-term correlations produced by the turnover of ion channels induced variability patterns with a power law behavior similar to those observed experimentally. Therefore, phenomena leading to long-term correlated variations in pacemaker cellular function may, in conjunction with extracardiac regulatory mechanisms, contribute to the nonlinear characteristics of HRV.

Effects of birth weight on spontaneous baroreflex sensitivity in adult life

BACKGROUND AND AIM

Several epidemiological studies have suggested a link between low birth weight and coronary heart disease; this may be partly due to the association between low birth weight and conventional risk factors. Among the factors involved in the regulation of cardiovascular homeostasis, baroreflexes play a crucial role. The objective of the present study was to investigate if baroreflex sensitivity (BRS) in adulthood is associated with birth weight.

METHODS AND RESULTS

Two hundred and eleven adults from Turin, Italy, aged 22-24 years, were examined in a cross sectional survey. Birth weight, blood pressure, pulse rate, family history of hypertension, anthropometric and environmental parameters and spontaneous baroreflex sensitivity were evaluated. In this study we observed a significant increase in baroreflex sensitivity across the tertiles of birth weight, even after correction for gender, blood pressure and heart rate; in a regression model, birth weight was positively and independently associated with BRS; moreover, BRS showed a significant negative correlation with adult pulse rate.

CONCLUSION

This finding may be helpful in understanding the association between low birth weight and cardiovascular disease outcome in later life, since baroreflex failure is associated with an increased cardiovascular morbidity and mortality.

Predictors of Decreased Spontaneous Baroreflex Sensitivity in Obstructive Sleep Apnea Syndrome

BACKGROUND

The impact of obstructive sleep apnea syndrome (OSAS) on the arterial baroreflex, and its significance, is still under debate. We investigated the baroreflex sensitivity (BRS) during sleep in well-selected OSAS patient and control subject cohorts

METHODS

We performed a prospective study of 10 non-OSAS subjects, 14 subjects with mild-to-moderate OSAS, and 14 male subjects with severe OSAS subjects. Groups were matched for age, body mass index, and other relevant variables. Subjects had no other disease and were not receiving regular medication. BP was monitored beat-by-beat (Portapres; Finapres Medical Systems; Amsterdam, the Netherlands) at night during polysomnography. Spontaneous BRS was assessed by the sequence technique. Heart-rate correction was also applied to calculate BRS at a heart rate (HR) of 60 beats/min (BRS-60) to account for intersubject variability in baseline HR. Eight suitable patients were treated with continuous positive airway pressure (CPAP), and BRS measurements were repeated 6 weeks later.

RESULTS

BRS and BRS-60 were significantly lower in patients with severe OSAS than in patients with mild-to-moderate OSAS and in non-OSAS subjects, and a separate sleep-stage analysis revealed this difference to be evident in stage 2 non-rapid eye movement sleep and during nocturnal wakefulness. There was no difference in BRS and BRS-60 between non-OSAS subjects and patients with mild-to-moderate OSAS. In multivariate analysis, the desaturation index was the only independent predictor of depressed BRS. CPAP therapy significantly improved the BRS measures.

CONCLUSION

Patients with severe OSAS demonstrate depressed BRS during sleep, which may contribute to the cardiovascular pathophysiology in OSAS patients.

Closed-loop minimal model analysis of the cardiovascular response to transient arousal from sleep in healthy humans

In a previous work we reported discrepancies in the cardiovascular response to arousal from NREM sleep between OSAS patients and healthy controls. The long lasting cardiac sympathetic increase observed in normals was not present in the OSAS group, whereas the peripheral vasculature reaction was similar between the two groups. Analysis of REM arousal revealed that there was a similar temporary cardiac sympathetic impairment in the control group. In this work we have implemented a model-based time domain system identification method to assess the mechanisms involved in this reaction to arousal from both NREM and REM sleep in a group of healthy subjects. The use of time-varying techniques has enabled us to characterize the arousal reaction by analyzing the change in shape of the impulse responses of the system. The mechanisms regulating respiration and vascular effects on heart rate (respiratory sinus arrhythmia or RSA and arterial baroreflex or ABR, respectively) were the most affected by NREM arousal, likely as a result of the return of the wakefulness stimulus. The effect observed on the cardiac influence on the vasculature (circulatory dynamics, CID) was attributed to a change in the dominant mechanism prevailing in its dynamics.

Leg crossing with muscle tensing, a physical counter-manoeuvre to prevent syncope, enhances leg blood flow

In patients with orthostatic intolerance, the mechanisms to maintain BP (blood pressure) fail. A physical counter-manoeuvre to postpone or even prevent orthostatic intolerance in these patients is leg crossing combined with muscle tensing. Although the central haemodynamic effects of physical counter-manoeuvres are well documented, not much is known about the peripheral haemodynamic events. Therefore the purpose of the present study was to examine the peripheral haemodynamic effects of leg crossing combined with muscle tensing during 70° head-up tilt. Healthy subjects (n=13) were monitored for 10 min in the supine position followed by 10 min in 70° head-up tilt and, finally, for 2 min of leg crossing with muscle tensing in 70° head-up tilt. MAP (mean arterial BP), heart rate, stroke volume, cardiac output and total peripheral resistance were measured continuously by Portapres. Leg blood flow was measured using Doppler ultrasound. Leg vascular conductance was calculated as leg blood flow/MAP. A significant increase in MAP (13 mmHg), stroke volume (27%) and cardiac output (18%), a significant decrease in heart rate (−5 beats/min) and no change in total peripheral resistance during the physical counter-manoeuvre were observed when compared with baseline 70° head-up tilt. A significant increase in leg blood flow (325 ml/min) and leg vascular conductance (2.9 arbitrary units) were seen during the physical counter-manoeuvre when compared with baseline 70° head-up tilt. In conclusion, the present study indicates that the physical counter-manoeuvre of leg crossing combined with muscle tensing clearly enhances leg blood flow and, at the same time, elevates MAP.

Blood pressure regulation in neurally intact human vs. acutely injured paraplegic and tetraplegic patients during passive tilt

We investigated autonomic control of cardiovascular function in able-bodied (AB), paraplegic (PARA), and tetraplegic (TETRA) subjects in response to head-up tilt following spinal cord injury. We evaluated spectral power of blood pressure (BP), baroreflex sensitivity (BRS), baroreflex effectiveness index (BEI), occurrence of systolic blood pressure (SBP) ramps, baroreflex sequences, and cross-correlation of SBP with heart rate (HR) in low (0.04-0.15 Hz)- and high (0.15-0.4 Hz)-frequency regions. During tilt, AB and PARA effectively regulated BP and HR, but TETRA did not. The numbers of SBP ramps and percentages of heartbeats involved in SBP ramps and baroreflex sequences increased in AB, were unchanged in PARA, and declined in TETRA. BRS was lowest in PARA and declined with tilt in all groups. BEI was greatest in AB and declined with tilt in all groups. Low-frequency power of BP and the peak of the SBP/HR cross-correlation magnitude were greatest in AB, increased during tilt in AB, remained unchanged in PARA, and declined in TETRA. The peak cross-correlation magnitude in HF decreased with tilt in all groups. Our data indicate that spinal cord injury results in decreased stimulation of arterial baroreceptors and less engagement of feedback control as demonstrated by lower 1) spectral power of BP, 2) number (and percentages) of SBP ramps and barosequences, 3) cross-correlation magnitude of SBP/HR, 4) BEI, and 5) changes in delay between SBP/HR. Diminished vasomotion and impaired baroreflex regulation may be major contributors to decreased orthostatic tolerance following injury.

Impaired baroreflex function in temporal lobe epilepsy

Changes of cardiovascular function are frequent in temporal lobe epilepsy (TLE). The baroreflex - the most important reflex for cardiovascular stability - has not been studied systematically in TLE. We evaluated cardiovascular variability and baroreflex function in TLE. In 22 TLE patients and 20 controls, we continuously monitored heart rate (HR) and blood pressure (BP). Time-domain parameters were derived from recordings at rest and from standard cardiovascular reflex tests. Spectral analysis determined sympathetic and parasympathetic modulation of HR and BP in the low (LF-power) and high frequency range (HF-power). We calculated the relative LF- and HF-powers of HR in relation to the sum of LF- and HF-powers. LF/HF-ratio of HR was assessed as a parameter of sympatheticovagal balance. LF-transfer function gain between BP and HR determined baroreflex function.Time-domain parameters did not differ between TLE patients and controls. Spectral analysis showed decreased absolute LF- and HF-powers but increased relative LF-power and LF/HF-ratio of HR in TLE. LF-transfer function gain between BP and HR was reduced in TLE (p<0.05). The reduction of absolute LF- and HF-powers indicates decreased total autonomic variability in TLE. However, increased relative LF-power and LF/HF-ratio of HR in TLE show a relative increase of sympathetic tone. Most importantly, we demonstrate an impaired baroreflex function in TLE. These cardiovascular autonomic abnormalities may contribute to cardiac arrhythmia in TLE.

Exercise pressor reflex function is altered in spontaneously hypertensive rats

In hypertension, exercise elicits excessive elevations in mean arterial pressure (MAP) and heart rate (HR) increasing the risk for adverse cardiac events and stroke during physical activity. The exercise pressor reflex (a neural drive originating in skeletal muscle), central command (a neural drive originating in cortical brain centres) and the tonically active arterial baroreflex contribute importantly to cardiovascular control during exercise. Each of these inputs potentially mediates the heightened cardiovascular response to physical activity in hypertension. However, given that exercise pressor reflex overactivity is known to elicit enhanced circulatory responses to exercise in disease states closely related to hypertension (e.g. heart failure), we tested the hypothesis that the exaggerated cardiovascular response to exercise in hypertension is mediated by an overactive exercise pressor reflex. To test this hypothesis, we used a rat model of exercise recently developed in our laboratory that selectively stimulates the exercise pressor reflex independent of central command and/or the arterial baroreflex. Activation of the exercise pressor reflex during electrically induced static muscle contraction in the absence of input from central command resulted in significantly larger increases in MAP and HR in male spontaneously hypertensive rats as compared to normotensive Wistar-Kyoto rats over a wide range of exercise intensities. Similar findings were obtained in animals in which input from both central command and the arterial baroreflex were eliminated. These findings suggest that the enhanced cardiovascular response to exercise in hypertension is mediated by an overactive exercise pressor reflex. Potentially, effective treatment of exercise pressor reflex dysfunction may reduce the cardiovascular risks associated with exercise in hypertension.

The link between cardiac autonomic activity and sleep delta power is altered in men with sleep apnea-hypopnea syndrome

We hypothesize that sleep apnea-hypopnea alters interaction between cardiac vagal modulation and sleep delta EEG. Sleep apnea-hypopnea syndrome (SAHS) is related to cardiovascular complications in men. SAHS patients show higher sympathetic activity than normal subjects. In healthy men, non-rapid eye movement (NREM) sleep is associated with cardiac vagal influence, whereas rapid eye movement (REM) sleep is linked to cardiac sympathetic activity. Interaction between cardiac autonomic modulation and delta sleep EEG is not altered across a life span nor is the delay between appearances of modifications in both signals. Healthy controls, moderate SAHS, and severe SAHS patients were compared across the first three NREM-REM cycles. Spectral analysis was applied to ECG and EEG signals. High frequency (HF) and low frequency (LF) of heart rate variability (HRV), ratio of LF/HF, and normalized (nu) delta power were obtained. A coherency analysis between HFnu and delta was performed, as well as a correlation analysis between obstructive apnea index (AI) or hypopnea index (HI) and gain, coherence, or phase shift. HRV components were similar between groups. In each group, HFnu was larger during NREM, while LFnu predominated across REM and wake stages. Coherence and gain between HFnu and delta decreased from controls to severe SAHS patients. In SAHS patients, the delay between modifications in HFnu and delta did not differ from zero. AI and HI correlated negatively with coherence, while HI correlated negatively with gain only. Apneas-hypopneas affect the link between cardiac sympathetic and vagal modulation and delta EEG demonstrated by the loss of cardiac autonomic activity fluctuations across shifts in sleep stages. Obstructive apneas and hypopneas alter the interaction between both signals differently.

Altered Autonomic Cardiovascular Regulation After Combined Deep and Superficial Cervical Plexus Blockade for Carotid Endarterectomy

Compromised cardiac autonomic modulation can produce cardiovascular disturbances. We investigated whether combined deep and superficial cervical plexus (CP) blockade for carotid endarterectomy (CEA) produces changes in autonomic cardiovascular regulation. To estimate alterations in cardiovascular autonomic control before and after combined CP blockade in 22 patients undergoing CEA, the heart rate (HR) variability, systolic blood pressure (SBP) variability, and baroreflex sensitivity were analyzed. We found that SBP (157 +/- 28 mm Hg versus 191 +/- 38 mm Hg before and after combined CP blockade, respectively) and HR (68 +/- 10 bpm versus 84 +/- 9 bpm) increased after combined CP blockade. The high frequency power of HR variability (3.7 +/- 0.9 versus 2.2 +/- 1.2 ln/ms2) decreased (decrease in parasympathetic drive), whereas the low frequency power of SBP variability (5.5 +/- 4.7 versus 8.6 +/- 9.4 mm Hg2) increased (increase in vascular sympathetic outflow). Baroreflex sensitivity decreased, and this decrease was negatively correlated with a SBP increase (r = -0.455). The present results suggest that combined CP blockade impairs autonomic cardiovascular homeostasis and suggests an association between combined CP blockade and intraoperative or postoperative adverse cardiovascular events in high-risk cardiac patients undergoing CEA that merits further studies.

Long-term alterations of heart rate dynamics after coronary artery bypass graft surgery

We tested the hypothesis that there may be long-term alterations in overall heart rate (HR) variability and in fractal HR behavior after coronary artery bypass graft (CABG) surgery. Reduced HR variability predicts morbidity in various patient populations. Continuous 24-h electrocardiograph recordings were performed in 25 elective CABG surgery patients 1 wk before the operation and 6 wk and 6 mo after. Seventeen of the patients also had recordings 12 mo after CABG. Time and frequency domain measures of HR variability were assessed, along with measurement of short-term fractal scaling exponent (alpha1), approximate entropy, and power-law relationship of relative risk interval variability (beta-slope). The high, low, very low, and ultra low frequency powers decreased significantly after the operation and remained at a significantly decreased level 6 wk and 6 and 12 mo after the operation than before (P = 0.01, P < 0.001, P < 0.001, and P < 0.001 for overall difference between the time points, respectively). The fractal scaling exponent alpha1 was at significantly more decreased 6 wk after (P < 0.05) CABG than before surgery but recovered to the preoperative level 6 mo after the operation. Long-term fractal organization (beta-slope) remained stable, but the overall complexity (approximate entropy) decreased toward more predictable HR dynamics during the study period (P < 0.01 after 1 yr). The predictive value of temporary and persistent long-term changes of the HR dynamics after CABG surgery for long-term outcome is not clear.

Spontaneous baroreflex cardiac sensitivity in end-stage liver disease: effect of liver transplantation

BACKGROUND AND OBJECTIVE

End-stage liver disease is associated with an imbalance in the autonomic nervous system. The purpose of this study was to estimate the effect of liver transplantation on this imbalance.

METHOD

The study involved 10 patients undergoing liver transplantation and 9 patients without liver impairment undergoing liver surgery. The spontaneous baroreflex sensitivity was measured before and 1 month after surgery for the liver surgery group; before and 1, 3, 6, 12 and 18 months after orthotopic liver transplantation.

RESULTS

The spontaneous baroreflex slope of patients with end-stage liver disease was decreased before liver transplantation compared to the liver surgery group (3.9 +/- 2.5 ms mmHg(-1) vs. 9.9 +/- 5.0 ms mmHg(-1), P = 0.002). The mean slope was significantly increased at 12 and 18 months compared to the pre-transplantation value (3.9 +/- 2.5 ms mmHg(-1) vs. 8.1 +/- 6.6 ms mmHg(-1) and 7.4 +/- 4.8 ms mmHg(-1), respectively; P = 0.042). Nevertheless, further analysis of individual data showed that only four patients exhibited a marked increase in their baroreflex slope 12 months after the liver transplantation whereas it remained decreased in the six others.

CONCLUSIONS

These results confirm that the baroreflex sensitivity is depressed in end-stage liver disease in line with an autonomic nervous system imbalance. The liver transplantation reverses this disturbance only in some patients.

Autonomic cardiovascular regulation in subjects with acute mountain sickness

The aims of this study were 1) to evaluate whether subjects suffering from acute mountain sickness (AMS) during exposure to high altitude have signs of autonomic dysfunction and 2) to verify whether autonomic variables at low altitude may identify subjects who are prone to develop AMS. Forty-one mountaineers were studied at 4,559-m altitude. AMS was diagnosed using the Lake Louise score, and autonomic cardiovascular function was explored using spectral analysis of R-R interval and blood pressure (BP) variability on 10-min resting recordings. Seventeen subjects (41%) had AMS. Subjects with AMS were older than those without AMS ( P < 0.01). At high altitude, the low-frequency (LF) component of systolic BP variability (LFSBP) was higher ( P = 0.02) and the LF component of R-R variability in normalized units (LFRRNU) was lower ( P = 0.001) in subjects with AMS. After 3 mo, 21 subjects (43% with AMS) repeated the evaluation at low altitude at rest and in response to a hypoxic gas mixture. LFRRNU was similar in the two groups at baseline and during hypoxia at low altitude but increased only in subjects without AMS at high altitude ( P < 0.001) and did not change between low and high altitude in subjects with AMS. Conversely, LFSBP increased significantly during short-term hypoxia only in subjects with AMS, who also had higher resting BP ( P < 0.05) than those without AMS. Autonomic cardiovascular dysfunction accompanies AMS. Marked LFSBP response to short-term hypoxia identifies AMS-prone subjects, supporting the potential role of an exaggerated individual chemoreflex vasoconstrictive response to hypoxia in the genesis of AMS.

Heart rate variability and spontaneous baroreflex sequences in supine healthy volunteers subjected to nasal positive airway pressure

To determine the dynamic effects of short-term nasal positive airway pressure (nPAP) on cardiovascular autonomic control, continuous recordings of noninvasively obtained hemodynamic measurements and heart rate variability (HRV) were obtained in 10 healthy subjects during frequency-controlled breathing (between 0.20 and 0.24 Hz) in supine posture under different pressures of nPAP ranging from 3 to 20 cmH(2)O. HRV was assessed using spectral analysis of the R-R interval. The slope of the regression line between spontaneous systolic blood pressure and pulse interval changes was taken as an index of the sensitivity of arterial baroreflex modulation of heart rate (sequence method). Application of nPAP resulted in a pressure-dependent decrease of cardiac output and stroke volume (P < 0.05, ANOVA) and in an increase in total peripheral resistance (P < 0.03, ANOVA). Hemodynamic changes under increasing nPAP were accompanied by a decrease in total power of HRV despite mean R-R interval remaining unchanged. The overall decrease in HRV was accompanied by a reduction across all frequency bands when absolute units were used (P < 0.01). When the power of low frequency and high frequency was calculated in normalized units, a diminished high frequency and an increased low-to-high frequency ratio were observed (P < 0.05). Compared with low levels of nPAP, pressure levels of >10 cmH(2)O were associated with a significant decline in the mean slope of spontaneous baroreceptor sequences (P < 0.04). These findings indicate that short-term administration of nPAP in normal subjects exerts significant alterations in R-R interval variability and spontaneous baroreflex modulation of heart rate.

Short- and Long-Term Joint Symbolic Dynamics of Heart Rate and Blood Pressure in Dilated Cardiomyopathy

Autonomic cardiovascular control involves complex interactions of heart rate and blood pressure. In patients with dilated cardiomyopathy (DCM), this control is impaired and parameters for its quantification might be of prognostic importance. In this paper, we introduce methods based on joint symbolic dynamics (JSD) for the enhanced analysis of heart rate and blood pressure interactions. To assess the coarse-grained dynamics beat-to-beat changes of heart rate and blood pressure are encoded in symbol strings. Subsequently, the distribution properties of short symbol sequences (words) as well as the scaling properties of the whole symbol string are assessed. The comparison of joint symbolic heart rate and blood pressure dynamics in DCM (n = 75) with those in healthy controls (n = 75) showed significant changes. Both, the distribution of words and the scaling properties indicate a loss in heart rate dynamics associated with blood pressure regulation in DCM. In conclusion, the analyses of short- and long-term JSDs provide insights into complex physiological heart rate and blood pressure interactions and furthermore reveal patho-physiological cardiovascular control in DCM.

Glucocorticoids act in the dorsal hindbrain to modulate baroreflex control of heart rate

Systemic corticosterone (Cort) modulates arterial baroreflex control of both heart rate and renal sympathetic nerve activity. Because baroreceptor afferents terminate in the dorsal hindbrain (DHB), an area with dense corticosteroid receptor expression, we tested the hypothesis that prolonged activation of DHB Cort receptors increases the midpoint and reduces the gain of arterial baroreflex control of heart rate in conscious rats. Small (3-4 mg) pellets of Cort (DHB Cort) or Silastic (DHB Sham) were placed on the surface of the DHB, or Cort was administered systemically by placing a Cort pellet on the surface of the dura (Dura Cort). Baroreflex control of heart rate was determined in conscious male Sprague Dawley rats on each of 4 days after initiation of treatment. Plots of arterial pressure vs. heart rate were analyzed using a four-parameter logistic function. After 3 days of treatment, the arterial pressure midpoint for baroreflex control of heart rate was increased in DHB Cort rats (123 +/- 2 mmHg) relative to both DHB Sham (108 +/- 3 mmHg) and Dura Cort rats (109 +/- 2 mmHg, P < 0.05). On day 4, baseline arterial pressure was greater in DHB Cort (112 +/- 2 mmHg) compared with DHB Sham (105 +/- 2 mmHg) and Dura Cort animals (106 +/- 2 mmHg, P < 0.05), and the arterial pressure midpoint was significantly greater than mean arterial pressure in the DHB Cort group only. Also on day 4, maximum baroreflex gain was reduced in DHB Cort (2.72 +/- 0.12 beats x min(-1) x mmHg(-1)) relative to DHB Sham and Dura Cort rats (3.51 +/- 0.28 and 3.37 +/- 0.27 beats x min(-1) x mmHg(-1), P < 0.05). We conclude that Cort acts in the DHB to increase the midpoint and reduce the gain of the heart rate baroreflex function.

Continuous Heart Rate Variability From an Implanted Device: A Practical Guide for Clinical Use

Continuous measurement of spontaneous heart rate variability is now possible from permanently implanted devices and may contain information useful in the chronic management of patients with congestive heart failure. Since heart rate variability semi-quantifies autonomic control of the heart, changes noted from continuous measurement provide insight into cardiac control system status, which may reflect clinical status of the patient. This review outlines clinical evidence supporting the use of heart rate variability derived from implanted electronic devices and suggests a practical application in an every day clinical setting for its use. Changes in continuously measured heart rate variability coupled with the absolute value of the standard deviation of the atrial-to-atrial activation period predict subsequent risk for decompensation and hospitalization. Prospective clinical trials will evaluate this approach and the use of such markers on health care utilization in populations of patients with long-term chronic heart failure.

Postnatal intermittent hypoxia alters baroreflex function in adult rats

Chronic perinatal intermittent hypoxia (IH) could have long-term cardiovascular effects by altering baroreflex function. To examine this hypothesis, we exposed rats (n = 6/group) for postnatal days 1-30 or prenatal embryonic days 5-21 to IH (8% ambient O2 for 90 s after 90 s of 21% of O2, 12 h/day) or to normoxia (control). Baroreflex sensitivity (BRS) and cardiac chronotropic responses were examined in anesthetized animals 3.5-5 mo later by infusing phenylephrine or sodium nitroprusside (6-12 microg/min iv, 1-2 min) during normoxia and after 18 min of acute IH (IHA). In controls after IHA, baroreflex gain was 42% (P < 0.05) less than during normoxia. BRS in the postnatal IH group during normoxia was approximately 50% less than in control rats and similar to controls after IHA. The heart rate response to phenylephrine in the IH group was also less than in controls (P < 0.05) and was not changed by IHA. BRS and heart rate responses in the prenatal IH group were similar to the normoxic control group. Vagal efferent projections to atrial ganglia neurons in rats after postnatal IH (n = 4) were examined by injecting tracer into the left nucleus ambiguous. After 35 days of postnatal IH, basket ending density was reduced by 17% (P < 0.001) and vagal axon varicose contacts by 56% (P < 0.001) compared with controls. We conclude that reduction of vagal efferent projections in cardiac ganglia could be a cause of long-term modifications in baroreflex function.

Pulse transit time measured from the ECG: an unreliable marker of beat-to-beat blood pressure

The arterial pulse-wave transit time can be measured between the ECG R-wave and the finger pulse (rPTT), and has been shown previously to have a linear correlation with blood pressure (BP). We hypothesized that the relationship between rPTT, preejection period (PEP; the R-wave/mechanical cardiac delay), and BP would vary with different vasoactive drugs. Twelve healthy men (mean age 22 yr) were studied. Beat-to-beat measurements were made of rPTT (using ECG and photoplethysmograph finger probe), intra-arterial radial pressure, PEP (using cardiac bioimpedance), and transit time minus PEP (pPTT). Four drugs (glyceryl trinitrate, angiotensin II, norepinephrine, salbutamol) were administered intravenously over 15 min, with stepped dosage increase every 5 min and a 25-min saline washout between agents. All subjects in all conditions had a negative linear correlation (R2 = 0.39) between rPTT and systolic BP (SBP), generally constant between different drugs, apart from four subjects who had a positive rPTT/SBP correlation with salbutamol. The 95% limits of agreement between measured and rPTT-predicted SBP were +/-17.0 mmHg. Beat-to-beat variability of rPTT showed better coherence with SBP variability than it did with heart rate variability (P < 0.001). PEP accounted for a substantial and variable proportion of rPTT (12-35%). Diastolic (DBP) and mean arterial BP (MAP) correlated poorly with rPTT (R2 = 0.02 and 0.08, respectively) but better with pPTT (rPTT corrected for PEP, R2 = 0.41 and 0.45, respectively). The 95% limits of agreement between measured and pPTT-predicted DBP were +/- 17.3 mmHg. In conclusion, the negative correlation between rPTT and SBP is generally constant, even with marked hemodynamic perturbations. However, the relationship is not reliable enough for rPTT to be used as a surrogate marker of SBP, although it may be useful in assessing BP variability. DBP and MAP cannot be predicted from rPTT without correction for PEP. The significant contribution of PEP to rPTT means that rPTT should not be used as a marker of purely vascular function.

Phase dynamics in cerebral autoregulation

Complex continuous wavelet transforms are used to study the dynamics of instantaneous phase difference delta phi between the fluctuations of arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) in a middle cerebral artery. For healthy individuals, this phase difference changes slowly over time and has an almost uniform distribution for the very low-frequency (0.02-0.07 Hz) part of the spectrum. We quantify phase dynamics with the help of the synchronization index gamma = (sin delta phi)2 + (cos delta phi)2 that may vary between 0 (uniform distribution of phase differences, so the time series are statistically independent of one another) and 1 (phase locking of ABP and CBFV, so the former drives the latter). For healthy individuals, the group-averaged index gamma has two distinct peaks, one at 0.11 Hz [gamma = 0.59 +/- 0.09] and another at 0.33 Hz (gamma = 0.55 +/- 0.17). In the very low-frequency range (0.02-0.07 Hz), phase difference variability is an inherent property of an intact autoregulation system. Consequently, the average value of the synchronization parameter in this part of the spectrum is equal to 0.13 +/- 0.03. The phase difference variability sheds new light on the nature of cerebral hemodynamics, which so far has been predominantly characterized with the help of the high-pass filter model. In this intrinsically stationary approach, based on the transfer function formalism, the efficient autoregulation is associated with the positive phase shift between oscillations of CBFV and ABP. However, the method is applicable only in the part of the spectrum (0.1-0.3 Hz) where the coherence of these signals is high. We point out that synchrony analysis through the use of wavelet transforms is more general and allows us to study nonstationary aspects of cerebral hemodynamics in the very low-frequency range where the physiological significance of autoregulation is most strongly pronounced.

Autonomic nervous system and blood pressure regulation in RGS2-deficient mice

Regulator of G protein signaling (RGS2) deletion in mice prolongs signaling by G protein-coupled vasoconstrictor receptors and increases blood pressure. However, the exact mechanism of the increase in blood pressure is unknown. To address this question we tested autonomic nervous system function and blood pressure regulation in RGS2-deficient mice (RGS2 −/−). We measured arterial blood pressure and heart rate (HR) with telemetry, computed time and frequency-domain measures for blood pressure and HR variability (HRV) as well as baroreflex sensitivity [BRS-low frequency (LF)], and assessed environmental stress sensitivity. Mean arterial blood pressure (MAP) was ∼10 mmHg higher in RGS2 −/− compared with RGS2 +/+ mice, while HR was not different between the groups, indicating a resetting of the baroreceptor reflex. Atropine increased MAP more in RGS2 −/− than in RGS2 +/+ mice while HR responses were not different. Urinary norepinephrine excretion was higher in RGS2 −/− than in RGS2 +/+ mice. The blood pressure decrease following prazosin was more pronounced in RGS2 −/− mice than in RGS2 +/+ mice. The LF and high-frequency (HF) power of HRV were reduced in RGS2 −/− compared with controls while BRS-LF and SBP-LF were not different. Atropine and atropine + metoprolol markedly reduced the HRV parameters in the time (RMSSD) and frequency domain (LF, HF, LF/HF) in both strains. Environmental stress sensitivity was increased in RGS2 −/− mice compared with controls. We conclude that the increase in blood pressure in RGS2 −/− mice is not solely explained by peripheral vascular mechanisms. A central nervous system mechanism might be implicated by an increased sympathetic tone. This state of affairs could lead to a baroreceptor-HR reflex resetting, while BRS remains unimpaired.

Increased sympathetic and decreased parasympathetic cardiovascular modulation in normal humans with acute sleep deprivation

Cardiovascular autonomic modulation during 36 h of total sleep deprivation (SD) was assessed in 18 normal subjects (16 men, 2 women, 26.0 +/- 4.6 yr old). ECG and continuous blood pressure (BP) from radial artery tonometry were obtained at 2100 on the first study night (baseline) and every subsequent 12 h of SD. Each measurement period included resting supine, seated, and seated performing computerized tasks and measured vigilance and executive function. Subjects were not supine in the periods between measurements. Spectral analysis of heart rate variability (HRV) and BP variability (BPV) was computed for cardiac parasympathetic modulation [high-frequency power (HF)], sympathetic modulation [low-frequency power (LF)], sympathovagal balance (LF/HF power of R-R variability), and BPV sympathetic modulation (at LF). All spectral data were expressed in normalized units [(total power of the components/total power-very LF) x 100]. Spontaneous baroreflex sensitivity (BRS), based on systolic BP and pulse interval powers, was also measured. Supine and sitting, BPV LF was significantly increased from baseline at 12, 24, and 36 h of SD. Sitting, HRV LF was increased at 12 and 24 h of SD, HRV HF was decreased at 12 h SD, and HRV LF/HF power of R-R variability was increased at 12 h of SD. BRS was decreased at 24 h of SD supine and seated. During the simple reaction time task (vigilance testing), the significantly increased sympathetic and decreased parasympathetic cardiac modulation and BRS extended through 36 h of SD. In summary, acute SD was associated with increased sympathetic and decreased parasympathetic cardiovascular modulation and decreased BRS, most consistently in the seated position and during simple reaction-time testing.

System identification: a multi-signal approach for probing neural cardiovascular regulation

Short-term, beat-to-beat cardiovascular variability reflects the dynamic interplay between ongoing perturbations to the circulation and the compensatory response of neurally mediated regulatory mechanisms. This physiologic information may be deciphered from the subtle, beat-to-beat variations by using digital signal processing techniques. While single signal analysis techniques (e.g., power spectral analysis) may be employed to quantify the variability itself, the multi-signal approach of system identification permits the dynamic characterization of the neural regulatory mechanisms responsible for coupling the variability between signals. In this review, we provide an overview of applications of system identification to beat-to-beat variability for the quantitative characterization of cardiovascular regulatory mechanisms. After briefly summarizing the history of the field and basic principles, we take a didactic approach to describe the practice of system identification in the context of probing neural cardiovascular regulation. We then review studies in the literature over the past two decades that have applied system identification for characterizing the dynamical properties of the sinoatrial node, respiratory sinus arrhythmia, and the baroreflex control of sympathetic nerve activity, heart rate and total peripheral resistance. Based on this literature review, we conclude by advocating specific methods of practice and that future research should focus on nonlinear and time-varying behaviors, validation of identification methods, and less understood neural regulatory mechanisms. Ultimately, we hope that this review stimulates such future investigations by both new and experienced system identification researchers.

Integration of autonomic and local mechanisms in regulating cardiovascular responses to heating and cooling in a reptile (Crocodylus porosus)

Reptiles change heart rate and blood flow patterns in response to heating and cooling, thereby decreasing the behavioural cost of thermoregulation. We tested the hypothesis that locally produced vasoactive substances, nitric oxide and prostaglandins, mediate the cardiovascular response of reptiles to heat. Heart rate and blood pressure were measured in eight crocodiles (Crocodylus porosus) during heating and cooling and while sequentially inhibiting nitric-oxide synthase and cyclooxygenase enzymes. Heart rate and blood pressure were significantly higher during heating than during cooling in all treatments. Power spectral density of heart rate and blood pressure increased significantly during heating and cooling compared to the preceding period of thermal equilibrium. Spectral density of heart rate in the high frequency band (0.19-0.70 Hz) was significantly greater during cooling in the saline treatment compared to when nitric-oxide synthase and cyclooxygenase enzymes were inhibited. Cross spectral analysis showed that changes in blood pressure preceded heart rate changes at low frequencies (< 0.1 Hz) only. We conclude that the autonomic nervous system controls heart rate independently from blood pressure at higher frequencies while blood pressure changes determine heart rate at lower frequencies. Nitric oxide and prostaglandins do not control the characteristic heart rate hysteresis response to heat in C. porosus, although nitric oxide was important in buffering blood pressure against changes in heart rate during cooling, and inhibition caused a compensatory decrease in parasympathetic stimulation of the heart.

Cardiac neuronal hierarchy in health and disease

The cardiac neuronal hierarchy can be represented as a redundant control system made up of spatially distributed cell stations comprising afferent, efferent, and interconnecting neurons. Its peripheral and central neurons are in constant communication with one another such that, for the most part, it behaves as a stochastic control system. Neurons distributed throughout this hierarchy interconnect via specific linkages such that each neuronal cell station is involved in temporally dependent cardio-cardiac reflexes that control overlapping, spatially organized cardiac regions. Its function depends primarily, but not exclusively, on inputs arising from afferent neurons transducing the cardiovascular milieu to directly or indirectly (via interconnecting neurons) modify cardiac motor neurons coordinating regional cardiac behavior. As the function of the whole is greater than that of its individual parts, stable cardiac control occurs most of the time in the absence of direct cause and effect. During altered cardiac status, its redundancy normally represents a stabilizing feature. However, in the presence of regional myocardial ischemia, components within the intrinsic cardiac nervous system undergo pathological change. That, along with any consequent remodeling of the cardiac neuronal hierarchy, alters its spatially and temporally organized reflexes such that populations of neurons, acting in isolation, may destabilize efferent neuronal control of regional cardiac electrical and/or mechanical events.

Relationship between renal sympathetic nerve activity and renal blood flow during natural behavior in rats

The purpose of the present study was to determine the relationship between renal sympathetic nerve activity (RSNA) and renal blood flow (RBF) during normal daily activity in conscious, chronically instrumented Wistar rats ( n = 8). The animal's behavior was classified as rapid eye movement (REM) sleep, non-REM (NREM) sleep, quiet awake, moving, and grooming states. On average RSNA was lowest during REM sleep, which was decreased by 39.0 ± 3.2% ( P < 0.05) relative to NREM sleep, and rose linearly with an increase in activity level in the order of quiet awake (by 10.9 ± 1.8%, P < 0.05), moving (by 29.4 ± 2.9%, P < 0.05), and grooming (by 65.3 ± 3.9%, P < 0.05) relative to NREM sleep. By contrast, RBF was highest during REM sleep, which was increased by 4.8 ± 0.7% ( P < 0.05) relative to NREM sleep and decreased significantly ( P < 0.05) by 5.5 ± 0.6 and 6.6 ± 0.5% during moving and grooming states, respectively, relative to NREM sleep. There was a significant ( P < 0.05) inverse linear relationship between the percent changes in RSNA and RBF and between those in RSNA and renal vascular conductance. Furthermore, renal denervation ( n = 8) abolished the changes in RBF induced by different natural behavioral activities. These results suggest that the changes in RSNA induced by natural behavioral activities had a significant influence on RBF.

A method for heart rate-corrected estimation of baroreflex sensitivity

OBJECTIVE

The relationship between the prevailing heart rate (HR) and the baroreflex sensitivity (BRS) is described in the present study together with a method for individual HR-corrected estimations of BRS.

DESIGN

HR and BRS, determined with the sequence method, were measured in ten young healthy subjects during rest, stress, standing and bicycle exercise, i.e. at a wide range of HRs.

RESULTS

BRS decreased exponentially with increasing HR. The relationship between the natural logarithm of BRS and HR was linear in each individual and could be described by the equation of a straight line. The equation describing the individual BRS-HR relationship could be derived either from BRS and HR measured during steady-state conditions or from the slope and average HR of the individual sequences occurring throughout the experimental protocol. The latter method was preferable since it did not require recordings during steady-state conditions. In order to eliminate the influence of differences in HR on BRS when comparing BRS between subjects, the equation describing the individual BRS-HR relationship was used to calculate BRS at a HR of 60 bpm, BRS(60), which ranged from 9.5 to 30.1 ms/mmHg for the 10 subjects.

CONCLUSIONS

Considering the dramatic effect of a small difference in HR on BRS, especially at lower HRs, BRS should be estimated at a wide range of HRs in order to determine the HR-corrected BRS from the individual HR-BRS relationship. Otherwise, comparisons of BRS between different individuals, study groups or following drug treatment or other interventions would be highly dependent on differences in HR and thereby easily misinterpreted.