Effect of head-down tilt on cardiovascular control in healthy subjects: a spectral analytic approach

On the construct validity of interoceptive accuracy based on heartbeat counting: Cardiovascular determinants of absolute and tilt-induced change scores

Interoceptive accuracy (IAcc) as assessed with the heartbeat counting task (IAcc_HBCT) may be affected by a range of factors including (1.) the ability to adequately detect cardiac signals, indicated by IAcc in a heartbeat discrimination task (IAcc_HBDT), (2.) cardiac signal properties, affected by sympathetic and parasympathetic tone, and (3.) non-interoceptive processes, including time estimation accuracy (TEAcc). In the current study we investigated the contribution of these factors to absolute and Δ IAcc_HBCT scores, induced by passive head-up and head-down tilt in 49 healthy individuals. A set of hierarchical regression models showed IAcc_HBDT scores as the strongest and, across different orthostatic (tilt) conditions, most stable (positive) predictor of absolute and Δ IAcc_HBCT scores. Neither indicators of cardiac signal properties (except for HR in head-down-tilt), nor TEAcc predicted absolute or Δ IAcc_HBCT scores. These findings support the convergent and discriminant validity of absolute and Δ IAcc_HBCT scores.

The Effect of Body Position on Measures of Arterial Stiffness in Humans

BACKGROUND

The dynamics ofpulsatile waveforms travelling the central aorta are governed by pressures and arterial compliance. Arterial stiffness, the inverse of compliance, is an independent risk factor for cardiovascular disease and has been suggested as a superior risk index compared to brachial blood pressure (BP). Arterial stiffness is typically measured via carotid-femoral pulse wave velocity (cfPWV) in the supine position; however, different body positions alter orthostatic column height, impacting heart rate and BP. The purpose of this investigation was to examine different body positions and associated measures of cfPWV.

METHODS

Measures of resting cfPWV were acquired simultaneously with BP during supine, head-up tilt (HUT), head-down tilt (HDT), and Fowler's position, all at 45 degrees from vertical.

RESULTS

Relative to supine, cfPWV was increased 1.1 ± 1.0 and 1.5 ± 1.1 m/s (both p ≤ 0.001) in HUT and Fowler's positions, respectively. Supine to HDT cfPWV was unaltered (p = 0.1), despite an increase in mean arterial pressure (MAP) (10 ± 9 mm Hg). When cfPWV was normalized to MAP, the same effects persisted (p ≤ 0.001).

CONCLUSION

Increasing orthostatic column height by changing posture independently increases resting cfPWV, concurrent with increases in BP. This data demonstrates the impact of body position on measures of central artery stiffness, which may have clinical relevance.

Disentangling cardiovascular control mechanisms during head-down tilt via joint transfer entropy and self-entropy decompositions

A full decomposition of the predictive entropy (PE) of the spontaneous variations of the heart period (HP) given systolic arterial pressure (SAP) and respiration (R) is proposed. The PE of HP is decomposed into the joint transfer entropy (JTE) from SAP and R to HP and self-entropy (SE) of HP. The SE is the sum of three terms quantifying the synergistic/redundant contributions of HP and SAP, when taken individually and jointly, to SE and one term conditioned on HP and SAP denoted as the conditional SE (CSE) of HP given SAP and R. The JTE from SAP and R to HP is the sum of two terms attributable to SAP or R plus an extra term describing the redundant/synergistic contribution to the JTE. All quantities were computed during cardiopulmonary loading induced by −25° head-down tilt (HDT) via a multivariate linear regression approach. We found that: (i) the PE of HP decreases during HDT; (ii) the decrease of PE is attributable to a lessening of SE of HP, while the JTE from SAP and R to HP remains constant; (iii) the SE of HP is dominant over the JTE from SAP and R to HP and the CSE of HP given SAP and R is prevailing over the SE of HP due to SAP and R both in supine position and during HDT; (iv) all terms of the decompositions of JTE from SAP and R to HP and SE of HP due to SAP and R were not affected by HDT; (v) the decrease of the SE of HP during HDT was attributed to the reduction of the CSE of HP given SAP and R; (vi) redundancy of SAP and R is prevailing over synergy in the information transferred into HP both in supine position and during HDT, while in the HP information storage synergy and redundancy are more balanced. The approach suggests that the larger complexity of the cardiac control during HDT is unrelated to the baroreflex control and cardiopulmonary reflexes and may be related to central commands and/or modifications of the dynamical properties of the sinus node.

Effect of respiration on Korotkoff sounds and oscillometric cuff pressure pulses during blood pressure measurement

Blood pressure (BP) measurement accuracy depends on consistent changes in Korotkoff sounds (KorS) for manual measurement and oscillometric pulses for automated measurement, yet little is known about the direct effect of respiration on these physiological signals. The aim of this research was to quantitatively assess the modulation effect of respiration on Korotkoff sounds and oscillometric pulses. Systolic and diastolic blood pressures were measured manually from 30 healthy subjects (age 41 ± 12 years). Three static cuff pressure conditions were studied for two respiratory rates. Cuff pressure [with oscillometric pulses (OscP)], ECG, chest motion respiration [respiration signal (Resp), from magnetometer] and Korotkoff sounds (KorS, from digital stethoscope) were recorded twice for 20 s. The physiological data were evenly resampled. Respiratory frequency was calculated from Resp (f_R), OscP (f_O) and KorS (f_K) from peak spectral frequency. There was no statistically significant difference between f_R and f_O or f_K. Respiratory modulation was observed in all subjects. OscP amplitude modulation changed significantly between the two respiratory rates (p < 0.05) and between the three cuff pressures (p < 0.0001), and decreased significantly with decreasing cuff pressure (p < 0.05). The phase shift between Resp and modulation of OscP was statistically significant with respiratory rates (p < 0.05), but not with cuff pressures. It is accepted that BP in individuals is variable and that this relates to respiration; we now show that this respiration modulates oscillometric pulse and Korotkoff sound amplitudes from which BP is measured.

Autonomic mediation of the interdependence between variability signals of heart rate and blood pressure in the lizard Gallotia galloti

Autonomic nervous system (ANS) involvement in the mediation of the synchronization between beat-to-beat RR interval variability (RRIV) and systolic blood pressure variability (SBPV) signals of the lizard Gallotia galloti (Oudart, 1839) was investigated through linear and nonlinear time series analysis methods in a pharmacological blockade context. The ANS blockers used were atropine, prazosin, and propranolol. The interdependence between the signals was quantified by means of the magnitude-squared coherence (MSC), which measures amplitude and phase linear synchronization; the phase lag index (PLI), which evaluates the phase synchronization; and the index L, which quantifies the generalized linear and nonlinear synchronization. Atropine decreased the PLI in the low-frequency (LF: 0.01–0.05 Hz) range; prazosin decreased the MSC in the medium-frequency (MF: 0.06–0.15 Hz) range; and propranolol did not alter any of the interdependence measures. It is suggested that (i) the cholinoceptor activity mediates the phase cardiovascular synchronization in the LF range; (ii) the α1-adrenoceptor activity mediates the amplitude and phase linear cardiovascular synchronization in the MF range; and (iii) the β-adrenoceptor activity plays no role in mediating any dynamics of cardiovascular synchronization in the studied frequency range. When comparing these results with those in mammals, a lesser overall autonomic involvement in the mediation of the studied cardiovascular interdependences is seen in reptiles.

Autonomic changes in preoperative uncomplicated diabetic patients with postural changes

Diabetic autonomic neuropathy is a critical complication frequently encountered in anaesthetic and surgical practice. Power spectral analysis is a noninvasive tool for monitoring frequency analysis of heart rate variability (HRV) and autonomic control of the heart. This study examined HRV changes in preoperative diabetic patients without overt signs of autonomic dysfunction and in matched controls (n=18 per group). HRV values at -15 degrees, 0 degrees, 15 degrees, 45 degrees and sitting positions were compared between groups and for each position. HRV in diabetic patients was lower than in controls at all positions (absolute units). Low-frequency power (normalized units) and the low-frequency/high-frequency ratio increased significantly at 45 degrees and in sitting positions in controls but not in diabetic patients. Pre-existing autonomic derangements in diabetic patients without overt clinical symptoms can be aggravated by high-degree tilting or sitting positions. Consequently, great care should be taken during the intra- and perioperative management of these patients.

Association between cardiovascular autonomic neuropathy and left ventricular hypertrophy in diabetic haemodialysis patients

BACKGROUND

Patients with diabetic nephropathy are likely to have neurological complications including cardiovascular autonomic dysfunction, which is related to increased risk of mortality. We investigated whether cardiovascular autonomic neuropathy is associated with left ventricular hypertrophy (LVH) in diabetic haemodialysis patients.

METHODS

Holter electrocardiography was carried out for 24 h with time and frequency domain analyses of heart rate variability in 154 diabetic (age 62+/-11 years) and 63 non-diabetic haemodialysis patients (62+/-10 years). The left ventricular mass index (LVMI) was determined by echocardiography. We used the percentage of differences exceeding 50 ms between adjacent normal RR intervals (pNN50) in time domain analysis and the power in the high-frequency range (HF: 0.15-0.40 Hz) in frequency domain analysis as indicators of parasympathetic activity.

RESULTS

The mean LVMI was greater in diabetic than in non-diabetic patients (168+/-63 vs 144+/-54 g/m(2), P<0.01). LVMI inversely correlated with pNN50 (r = -0.270, P = 0.0007, n = 154) and HF (r = -0.277, P = 0.0005, n = 154) in diabetic patients, but not in non-diabetic patients. By multiple logistic analysis, LVH was strongly associated with pNN50 (odds ratio 0.088; 0, <2%; 1, >/=2%) and HF (odds ratio 0.058; 0, <500 ms(2); 1, >/=500 ms(2)) in diabetic patients.

CONCLUSIONS

Impaired parasympathetic activity, which indicates cardiovascular autonomic neuropathy, was associated with the presence of LVH in diabetic haemodialysis patients. The co-existence of cardiovascular autonomic neuropathy and LVH may be one of the key factors for the high incidence of cardiovascular events in diabetic haemodialysis patients.

Differential analysis in the time domain of the baroreceptor cardiac reflex sensitivity as a function of sequence length

Analysis of baroreceptor sensitivity (BRS) in the time domain through the spontaneous sequence method used a measure of BRS based on the average of all sequences detected, without making any distinction in cardiac cycle length. In this article, we study differentially the functioning of the baroreflex as a function of the length of the cardiac sequences (3, 4, 5, or 6 cardiac cycles). One hundred and four students performed three mental stress tasks: mental arithmetic, memory, and visual attention. The results show that (1) as sequence length decreases, the relationships between BRS and indexes of vagal cardiac control increase, (2) the BRS associated with the more short sequences (3 and 4 beats) is the most vulnerable to mental stress, particularly the mental arithmetic task, and (3) BRS increases progressively as sequence length decreases. These results suggest that the nature and functioning of the baroreflex differ as a function of the length of the cardiac sequences.

A common origin of the very low frequency heart rate and blood pressure variability--a new insight into an old debate

The purpose of this study was to assess the exact temporal and amplitude relationship between very low frequency heart rate variability waves and very low frequency blood pressure variability waves. We developed a computerized system based on a modified proportional-integral controller for the controlled increase of heart rate by isoproterenol. Heart rate and blood pressure were measured continuously in conscious tethered rats. Using time domain methods, we found that the very low frequency heart rate variability waves and the very low frequency blood pressure variability waves are irregular, while at the same time strikingly 1:1 synchronized with each other. In 78% of the cases, the phase between the peaks of the very low frequency heart rate variability waves and very low frequency blood pressure variability waves was negative (blood pressure leads). Their amplitudes were linearly related with a degree of hysteresis. As blood pressure went up, heart rate went down. Our results suggest with a high degree of probability that the very low frequency heart rate variability waves do not cause very low frequency blood pressure variability waves, and that these two signals are probably driven by the same autonomic nervous system controller/oscillator.

Antihypertensive monotherapy and cardiovascular responses to an acoustic startle stimulus

To determine contribution of the autonomic nervous system to cardiovascular reactivity to noise, acoustic startle stimulus (110 dB, 1-20 kHz, 0.150 s) was administered to 35 subjects (19 women, 16 men) with mild essential hypertension. Among these patients, 10 were unmedicated and 25 were receiving long-term monotherapy (10 were taking 100 mg atenolol, 5 were taking 10 mg prazosin, and 10 were taking 50 mg losartan daily). Polygraphic recordings were obtained in supine position. Blood pressure (BP) and heart rate (HR) levels were stable until the noise was administered. In the unmedicated group BP and HR were elevated during the first 10 s. BP returned to resting levels after this period. The calculated hemodynamic indexes showed a biphasic change in total peripheral resistance (TPR), with an overall vasoconstriction associated with the BP rise phase, preceding a delayed vasodilation. The lowest HR changes were observed in the beta-blocker group with increases of 6 beats/min and 3 beats/min after the first and second noise stimulations, compared with 10 beats/min and 5 beats/min in the unmedicated group. Prazosin significantly reduced the BP rises to 7 mm Hg and 6 mm Hg for systolic BP and diastolic BP after the first stimulation compared with 22 mm Hg and 17 mm Hg in the untreated group (p < 0.01). The second stimulation after prazosin determined -5 mm Hg and 1 mm Hg changes for systolic BP and diastolic BP respectively, compared to rises of 13 mmHg for systolic BP and 10 mmHg for diastolic BP in the untreated group (p < 0.01). The hemodynamic percentage changes resulting from the first stimulation indicated prazosin markedly reduced the noise-induced rise in TPR (p < 0.05). No effect of beta-blocker was detectable using percentage changes. The rises in BP were amplified in the losartan-treated subjects compared with the other groups. Because of a low resting TPR in this group, the percentage changes in TPR resulting from noise were amplified in the subjects treated with the AT1 receptor antagonist. In conclusion the acoustic startle stimulus appeared as a simple and reliable procedure for inducing transient increases due to a rise in TPR. Cardiovascular responses differed according to the antihypertensive monotherapy, with a limited effect of noise in the prazosin-treated group.

Cross-spectral analysis of heart rate and blood pressure modulations

The cross-spectral analysis of heart rate (HR) and blood pressure (BP) variabilities provides "amplitude" and "phase" related measures. Compared to the amplitude measure, that is the baroreflex gain, the phase related measure characterizing the time lag between HR and BP oscillations has been studied to a much lesser extent. A population of 103 patients (73 men, 30 women, aged 53 +/- 12, range 20-82 years) referred for the management of coronary artery disease and/or hypertension were studied. In each subject, electrocardiogram and BP recordings were obtained in the supine and sitting positions of 5 minutes of rest (spontaneous respiration), 3 minutes of controlled respiration at 0.1 Hz (slow-controlled respiration), and 3 minutes of controlled respiration at 0.33 Hz (fast-controlled respiration). The frequency of maximum coherence (above the arbitrary threshold of 0.5) of BP and RR interval variabilities was searched between 0.033-0.133 Hz and 0.200-0.400 Hz to obtain baroreflex gain and phase shift in low and high frequency bands, respectively. Mean phase shifts of -79.1 and -67.0 degrees (-2.4 and -2.1 s) were found during slow-controlled respiration in the supine and sitting body positions, respectively. The mean phase shift between systolic BP and RR interval in the low frequency band was found between 83 and -109 degrees for body positions and respiration regimes. The actual baroreflex related time lag between systolic BP and RR variations was found between 3.5 and 5.1 seconds. The study concludes that the appropriate, and not always easy, selection of the frequency of maximum coherence between BP and HR oscillation is crucial for an accurate cross-spectral assessment of baroreflex sensitivity.

Arterial baroreflex influence on heart rate variability: a mathematical model-based analysis

The influence of the arterial baroreflex on the heart rate variability is analysed by using a mathematical model of heart rate baroreceptor control. The basic mechanisms of the model, sufficient to elicit heart rate variability include: systemic circulation, a non-pulsatile cardiac pump and nonlinear negative feedback simulating arterial baroreflex closed-loop control of the heart rate (-3 bpm/mmHg as maximum reflex sensitivity). The latter reproduces, through two distinct delayed branches (0.8 and 2.8 s), the short-term autonomic control effected respectively by sympathetic and parasympathetic divisions on the sinus node. By means of this model, two distinct self-sustained oscillatory components with incommensurate frequencies (0.1 and 0.26 Hz) are reproduced. Frequencies of these two oscillatory components closely agree with the main heart rate rhythms in humans (0.09 +/- 0.01 Hz and 0.26 +/- 0.01 Hz). When sympathetic-mediated regulation prevails over parasympathetic activity, simulated heart rate oscillation is characterised by a low frequency (approximately 0.1 Hz). On the other hand, a high-frequency oscillatory component (approximately 0.26 Hz) appears when enhanced vagal activation or partial inhibition of the sympathetic control is simulated. When both autonomic divisions are operative, both low- and high-frequency components are present and the heart rate oscillates quasi-periodically. This variability in heart rate at different frequencies is reproduced without including outside perturbations and is due to the nonlinear delayed structure of the closed-loop control. Bifurcation theory of nonlinear system is used to explain the high sensitivity of the heart rate oscillatory pattern to model parameter changes.

Autonomic control of cardiovascular dynamics during weightlessness

Measuring cardiovascular dynamics is a new method of assessing the autonomic regulation of the cardiovascular system, it provides an easily-implemented non-invasive way of monitoring the effects of weightlessness on this regulatory function. The major findings of studies on cardiovascular dynamics during actual or simulated weightlessness are presented, taking into account the recent consensus on this approach. Future improvements of these studies are discussed.

Contribution of renal nerves to renal blood flow variability during hemorrhage

We have examined the role of the renal sympathetic nerves in the renal blood flow (RBF) response to hemorrhage in seven conscious rabbits. Hemorrhage was produced by blood withdrawal at 1.35 ml.min(-1).kg-1 for 20 min while RBF and renal sympathetic nerve activity (RSNA) were simultaneously measured. Hemorrhage was associated with a gradual increase in RSNA and decrease in RBF from the 4th min. In seven denervated animals, the resting RBF before hemorrhage was significantly greater (48 +/- 1 vs. 31 +/- 1 ml/min intact), and the decrease in RBF did not occur until arterial pressure also began to fall (8th min); however, the overall percentage change in RBF by 20 min of blood withdrawal was similar. Spectral analysis was used to identify the nature of oscillations in each variable. Before hemorrhage, a rhythm at approximately 0.3 Hz was observed in RSNA, although not in RBF, whose spectrogram was composed mostly of lower-frequency (< 0.25 Hz) components. The denervated group of rabbits had similar frequency spectrums for RBF before hemorrhage. RSNA played a role in dampening the effect of oscillations in arterial pressure on RBF as the transfer gain between mean arterial pressure (MAP) and RBF for frequencies > 0.25 Hz was significantly less in intact than denervated rabbits (0.83 +/- 0.12 vs. 1.19 +/- 0.10 ml.min(-1).mmHg-1). Furthermore, the coherence between MAP and RBF was also significantly higher in denervated rabbits, suggesting tighter coupling between the two variables in the absence of RSNA. Before the onset of significant decreases in arterial pressure (up to 10 min), there was an increase in the strength of oscillations centered around 0.3 Hz in RSNA. These wer accompanied by increases in the spectral power of RBF at the same frequency. Arterial pressure fell in both groups of animals, the dominant rhythm to emerge in RBF was centered between 0.15 and 0.20 Hz and was present in intact and denervated rabbits. It is speculated that this myogenic in origin. We conclude that RSNA can induce oscillations in RBF at 0.3 Hz, plays a significant role in altering the effect of oscillations in arterial pressure on RBF, and mediates a proportion of renal vasoconstriction during hemorrhage in conscious rabbits.

Analysis of oscillatory components of short-term heart rate variability in hemodynamically stable and unstable patients during hemodialysis

Short period oscillatory components embedded in heart rate variability (HRV) were studied during hemodialysis induced hypovolemia in 15 hypotension-resistant (stable) and 15 hypotension-prone (unstable) patients. Hemodialysis was undertaken so that a similar blood volume reduction was induced in all patients (p > 0.05) without causing acute hypotension events. Autoregressive HRV power spectrums were calculated using an eigenanalysis-based approach. The frequencies of the main HRV rhythmic components were estimated through the Pisarenko harmonic decomposition. Percent changes during the hemodialysis in both heart rate and arterial pressure were similar in the stable and unstable groups (p > 0.05). The HRV spectral density showed markedly different power distributions. In the stable patients, power was mainly in the low frequency band (74+/-7 nU in the low frequency [LF] band vs. 21+/-6 nU in the high frequency [HF] band) whereas in stable patients, it was mainly in the high frequency band (39+/-10 nU in the LF band vs. 47+/-7 nU in the HF band). The frequency of the main oscillation was 0.1+/-0.02 Hz in stable patients and 0.18+/-0.04 Hz in unstable ones (p < 0.01). These HRV spectral parameters have a clear diagnostic value in discriminating between stable and unstable patients when their hemodynamic behaviors are similar.

Pathophysiologic basis for vasodepressor syncope

The current knowledge regarding the pathophysiologic basis of the vasodepressor response was reviewed. The balance of evidence indicates that the mechanoreceptor hypothesis seems unlikely to be the sole afferent alteration that leads to the vasodepressor response. Alternative afferent mechanisms should include neurohumoral mediated sympathoinhibition triggered by opioid mechanisms as well as impaired endothelial and NO responses to orthostatic stress in susceptible individuals. It is possible that impaired cardiovagal and sympathetic outflow control of arterial baroreceptors is enhanced by the aforementioned mechanisms. The role of central sympathoinhibition and vagal excitation triggered directly from pathways within the temporal lobe or triggered by alterations in regional cerebral blood flow should be considered as potential alternative mechanisms. Efferent autonomic outflow during vasodepressor syncope include sympathetic neural outflow withdrawal in addition to activation of parasympathetic outflow to the heart and abdominal viscera. Further human research is needed to understand the underlying mechanisms that result in the described neural and vascular responses.

The effects of midazolam and ephedrine on post-exercise autonomic chronotropic control of the heart in normal subjects

Benzodiazepines may induce hypotension by inhibiting the pressor response. Ephedrine has adrenergic effects on the circulation. After exercise, changes in cardiovascular control impair orthostatic tolerance. The impaired pressure response can be compensated for by chronotropic control of the heart. We studied the effect of midazolam and ephedrine on post-exercise cardiac autonomic chronotropic control in six 21-year-old female volunteers, who received single doses of 15 mg midazolam, 50 mg ephedrine, or placebo orally according to a placebo-controlled, double-blind, crossover design. After exercise, the subjects assumed the supine position for rest, then a -10 degrees head-down position followed by a 70 degrees head-up position. Power spectral analysis of heart rate variability for 7 min and steady-state brachial arterial blood pressure were measured in each position. After administration of midazolam, three subjects had an abnormal fall in their arterial blood pressure (with one presyncope) as a response to head-up tilt. Changes in heart rate variability exceeded those seen during placebo treatment (p < 0.01) and involved oscillations, suggesting activation of both sympathetic and parasympathetic dynamics. After ephedrine administration, arterial blood pressure increased during head-down tilt, but parasympathetic dynamics to the heart were dampened. Head-up tilt induced increased sympathetic stimulation of the heart and a sympathicotonic cardiovascular response (p < 0.01). In conclusion, midazolam induced unexpectedly great changes in dynamic cardiac control during cardiovascular stimulation. Ephedrine increased tonic sympathetic activity and stabilized the neural circulatory control of the heart by immobilizing dynamic parasympathetic activation.

Effect of hemodialysis on cardiovascular rhythms in end-stage renal failure

The purpose of this investigation was to determine non-invasively the alteration in autonomic cardiovascular control observed in end-stage renal disease (ESRD) patients submitted to a hemodialysis (HD) treatment. The effect of HD on finger blood pressure (BP) and heart rate (HR) variability was studied by means of spectral analysis in 12 ESRD patients at supine and during 45 degrees head-up tilt. Amplitude spectra of BP and HR rhythmicity were estimated and integrated amplitudes of the low (60 to 140 mHz, Mayer waves) and high frequency (area under the curve at mean respiration rate +/- 50 mHz) components were computed. The overall variability of a signal was given by the total area under the curve of the spectrum between 20 and 500 mHz. The transfer function was calculated between systolic BP and HR fluctuations using cross-spectral analysis. The principle findings are as follows: (1) Before HD, overall variability in systolic and diastolic BP as well as in the low frequency component in BP spectra were markedly reduced in ESRD patients compared to control subjects. Dialysis produced an immediate improvement in overall BP variability and the LF BP rhythmicity. (2) Before HD, there was an alteration of the HR spectral profile in uremic patients. (3) Chronic uremia and HD induced no changes in the transfer gain characteristic of modulation of HR by systolic BP. Taken together, these findings suggest that volume depletion in ESRD patients during a dialysis treatment is responsible for the improvement of overall BP variability and its spectral components.