Time course analysis of baroreflex sensitivity during postural stress

Determination of baroreflex sensitivity during the modified Oxford maneuver by trigonometric regressive spectral analysis

BACKGROUND

Differences in spontaneous and drug-induced baroreflex sensitivity (BRS) have been attributed to its different operating ranges. The current study attempted to compare BRS estimates during cardiovascular steady-state and pharmacologically stimulation using an innovative algorithm for dynamic determination of baroreflex gain.

METHODOLOGY/PRINCIPAL FINDINGS

Forty-five volunteers underwent the modified Oxford maneuver in supine and 60° tilted position with blood pressure and heart rate being continuously recorded. Drug-induced BRS-estimates were calculated from data obtained by bolus injections of nitroprusside and phenylephrine. Spontaneous indices were derived from data obtained during rest (stationary) and under pharmacological stimulation (non-stationary) using the algorithm of trigonometric regressive spectral analysis (TRS). Spontaneous and drug-induced BRS values were significantly correlated and display directionally similar changes under different situations. Using the Bland-Altman method, systematic differences between spontaneous and drug-induced estimates were found and revealed that the discrepancy can be as large as the gain itself. Fixed bias was not evident with ordinary least products regression. The correlation and agreement between the estimates increased significantly when BRS was calculated by TRS in non-stationary mode during the drug injection period. TRS-BRS significantly increased during phenylephrine and decreased under nitroprusside.

CONCLUSIONS/SIGNIFICANCE

The TRS analysis provides a reliable, non-invasive assessment of human BRS not only under static steady state conditions, but also during pharmacological perturbation of the cardiovascular system.

Baroreflex sensitivity is higher during acute psychological stress in healthy subjects under β-adrenergic blockade

Acute psychological stress challenges the cardiovascular system with an increase in BP (blood pressure), HR (heart rate) and reduced BRS (baroreflex sensitivity). β-adrenergic blockade enhances BRS during rest, but its effect on BRS during acute psychological stress is unknown. This study tested the hypothesis that BRS is higher during acute psychological stress in healthy subjects under β-adrenergic blockade. Twenty healthy novice male bungee jumpers were randomized and studied with (PROP, n=10) or without (CTRL, n=10) propranolol. BP and HR responses and BRS [cross-correlation time-domain (BRSTD) and cross-spectral frequency-domain (BRSFD) analysis] were evaluated from 30 min prior up to 2 h after the jump. HR, cardiac output and pulse pressure were lower in the PROP group throughout the study. Prior to the bungee jump, BRS was higher in the PROP group compared with the CTRL group [BRSTD: 28 (24-42) compared with 17 (16-28) ms·mmHg-1, P<0.05; BRSFD: 27 (20-34) compared with 14 (9-19) ms·mmHg-1, P<0.05; values are medians (interquartile range)]. BP declined after the jump in both groups, and post-jump BRS did not differ between the groups. In conclusion, during acute psychological stress, BRS is higher in healthy subjects treated with non-selective β-adrenergic blockade with significantly lower HR but comparable BP.

Causal relationships between heart period and systolic arterial pressure during graded head-up tilt

In physiological conditions, heart period (HP) affects systolic arterial pressure (SAP) through diastolic runoff and Starling's law, but, the reverse relation also holds as a result of the continuous action of baroreflex control. The prevailing mechanism sets the dominant temporal direction in the HP-SAP interactions (i.e., causality). We exploited cross-conditional entropy to assess HP-SAP causality. A traditional approach based on phases was applied for comparison. The ability of the approach to detect the lack of causal link from SAP to HP was assessed on 8 short-term (STHT) and 11 long-term heart transplant (LTHT) recipients (i.e., less than and more than 2 yr after transplantation, respectively). In addition, spontaneous HP and SAP variabilities were extracted from 17 healthy humans (ages 21-36 yr, median age 29 yr; 9 females) at rest and during graded head-up tilt. The tilt table inclinations ranged from 15 to 75° and were changed in steps of 15°. All subjects underwent recordings at every step in random order. The approach detected the lack of causal relation from SAP to HP in STHT recipients and the gradual restoration of the causal link from SAP to HP with time after transplantation in the LTHT recipients. The head-up tilt protocol induced the progressive shift from the prevalent causal direction from HP to SAP to the reverse causality (i.e., from SAP to HP) with tilt table inclination in healthy subjects. Transformation of phases into time shifts and comparison with baroreflex latency supported this conclusion. The proposed approach is highly efficient because it does not require the knowledge of baroreflex latency. The dependence of causality on tilt table inclination suggests that "spontaneous" baroreflex sensitivity estimated using noncausal methods (e.g., spectral and cross-spectral approaches) is more reliable at the highest tilt table inclinations.

Gravity, the hydrostatic indifference concept and the cardiovascular system

Gravity, like any acceleration, causes a hydrostatic pressure gradient in fluid-filled bodily compartments. At a force of 1G, this pressure gradient amounts to 10 kPa/m. Postural changes alter the distribution of hydrostatic pressure patterns according to the body's alignment to the acceleration field. At a certain location--referred to as hydrostatically indifferent--within any given fluid compartment, pressure remains constant during a given change of position relative to the acceleration force acting upon the body. At this specific location, there is probably little change in vessel volume, wall tension, and the balance of Starling forces after a positional manoeuvre. In terms of cardiac function, this is important because arterial and venous hydrostatic indifference locations determine postural cardiac preload and afterload changes. Baroreceptors pick up pressure signals that depend on their respective distance to hydrostatic indifference locations with any change of body position. Vascular shape, filling volume, and compliance, as well as temperature, nervous and endocrine factors, drugs, and time all influence hydrostatic indifference locations. This paper reviews the physiology of pressure gradients in the cardiovascular system that are operational in a gravitational/acceleration field, offers a broadened hydrostatic indifference concept, and discusses implications that are relevant in physiological and clinical terms.

Trigonometric regressive spectral analysis reliably maps dynamic changes in baroreflex sensitivity and autonomic tone: the effect of gender and age

Background

The assessment of baroreflex sensitivity (BRS) has emerged as prognostic tool in cardiology. Although available computer-assisted methods, measuring spontaneous fluctuations of heart rate and blood pressure in the time and frequency domain are easily applicable, they do not allow for quantification of BRS during cardiovascular adaption processes. This, however, seems an essential criterion for clinical application. We evaluated a novel algorithm based on trigonometric regression regarding its ability to map dynamic changes in BRS and autonomic tone during cardiovascular provocation in relation to gender and age.

Methodology/Principal Findings

We continuously recorded systemic arterial pressure, electrocardiogram and respiration in 23 young subjects (25±2 years) and 22 middle-aged subjects (56±4 years) during cardiovascular autonomic testing (metronomic breathing, Valsalva manoeuvre, head-up tilt). Baroreflex- and spectral analysis was performed using the algorithm of trigonometric regressive spectral analysis. There was an age-related decline in spontaneous BRS and high frequency oscillations of RR intervals. Changes in autonomic tone evoked by cardiovascular provocation were observed as shifts in the ratio of low to high frequency oscillations of RR intervals and blood pressure. Respiration at 0.1 Hz elicited an increase in BRS while head-up tilt and Valsalva manoeuvre resulted in a downregulation of BRS. The extent of autonomic adaption was in general more pronounced in young individuals and declined stronger with age in women than in men.

Conclusions/Significance

The trigonometric regressive spectral analysis reliably maps age- and gender-related differences in baroreflex- and autonomic function and is able to describe adaption processes of baroreceptor circuit during cardiovascular stimulation. Hence, this novel algorithm may be a useful screening tool to detect abnormalities in cardiovascular adaption processes even when resting values appear to be normal.

Operational point of neural cardiovascular regulation in humans up to 6 months in space

Entering weightlessness affects central circulation in humans by enhancing venous return and cardiac output. We tested whether the operational point of neural cardiovascular regulation in space sets accordingly to adopt a level close to that found in the ground-based horizontal position. Heart rate (HR), finger blood and brachial blood pressure (BP), and respiratory frequency were collected in 11 astronauts from nine space missions. Recordings were made in supine and standing positions at least 10 days before launch and during spaceflight ( days 5– 19, 45– 67, 77– 116, 146– 180). Cross-correlation analyses of HR and systolic BP were used to measure three complementary aspects of cardiac baroreflex modulation: 1) baroreflex sensitivity, 2) number of effective baroreflex estimates, and 3) baroreflex time delay. A fixed breathing protocol was performed to measure respiratory sinus arrhythmia and low-frequency power of systolic BP variability. We found that HR and mean arterial pressure did not differ from preflight supine values for up to 6 mo in space. Respiration frequency tended to decrease during prolonged spaceflight. Concerning neural markers of cardiovascular regulation, we observed in-flight adaptations toward homeostatic conditions similar to those found in the ground-based supine position. Surprisingly, this was not the case for baroreflex time delay distribution, which had somewhat longer latencies in space. Except for this finding, our results confirm that the operational point of neural cardiovascular regulation in space sets to a level close to that of an Earth-based supine position. This adaptation level suggests that circulation is chronically relaxed for at least 6 mo in space.

Human vagal baroreflex mechanisms in space

Although astronauts' cardiovascular function is normal while they are in space, many have altered haemodynamic responses to standing after they return to Earth, including inordinate tachycardia, orthostatic hypotension, and uncommonly, syncope. Simulated microgravity impairs vagal baroreceptor-cardiac reflex function and causes orthostatic hypotension. Actual microgravity, however, has been shown to either increase, or not change vagal baroreflex gain. In this study, we tested the null hypothesis that spaceflight does not impair human baroreflex mechanisms. We studied 11 American and two German astronauts before, during (flight days 2-8), and after two, 9- and 10-day space shuttle missions, with graded neck pressure and suction, to elicit sigmoid, vagally mediated carotid baroreflex R-R interval responses. Baseline systolic pressures tended to be higher in space than on Earth (P = 0.015, repeated measures analysis of variance), and baseline R-R intervals tended to be lower (P = 0.049). Baroreceptor-cardiac reflex relations were displaced downward on the R-R interval axis in space. The average range of R-R interval responses to neck pressure changes declined from preflight levels by 37% on flight day 8 (P = 0.051), maximum R-R intervals declined by 14% (P = 0.003), and vagal baroreflex gain by 9% (P = 0.009). These measures returned to preflight levels by 7-10 days after astronauts returned to Earth. This study documents significant increases of arterial pressure and impairment of vagal baroreflex function in space. These results and results published earlier indicate that microgravity exposure augments sympathetic, and diminishes vagal cardiovascular influences.

Variability in cardiovascular control: the baroreflex reconsidered

Although blood pressure control is often viewed as a paradigmatic example of a "homeostatic" biological control system, blood pressure levels can fluctuate considerably over shorter and longer time scales. In modern signal analysis, coherence between heart rate and blood pressure variability is used to estimate baroreflex gain. However, the shorter the measurement period, the more variability this gain factor reveals. We review evidence that this variability is not due to the technique used for the estimation, but may be an intrinsic property of the circulatory control mechanisms. The baroreflex is reviewed from its evolutionary origin, starting in fishes as a reflex mechanism to protect the gills from excessively high pressures by slowing the heart via the (parasympathetic) vagus nerve. Baroreflex inhibition of cardiovascular sympathetic nervous outflow is a later development; the maximally possible extent of sympathetic activity probably being set in the central nervous system by mechanisms other than blood pressure per se. In the sympathetic outflow tract not only baroreflex inhibition but also as yet unidentified, stochastic mechanisms decide to pass or not pass on the sympathetic activity to the periphery. In this short essay, the "noisiness" of the baroreflex as nervous control system is stressed. This property is observed in all elements of the reflex, even at the--supposedly--most basic relation between afferent receptor nerve input and efferent--vagus--nerve output signal.

Vestibular control of arterial blood pressure during head-down postural change in anesthetized rabbits

This study was undertaken to elucidate neural control of the arterial blood pressure (ABP) in head-down postural change which causes both stimulation to the vestibular system and head-ward fluid shift. Experiments were carried out with urethane-anesthetized rabbits. The animal was mounted on a tilting table, tilted to 45 degrees head-down in 5 s, and kept at the position for 5 min. The head-down rotation (HDR) induced a transient decrease in ABP (10 +/- 3 mmHg; mean +/- SE), and then the pressure gradually recovered toward the pre-HDR level during the 5 min at the head-down position. Pretreatment with hexamethonium bromide, a ganglionic transmission blocker, suppressed the HDR-induced drop of ABP, suggesting that the ABP drop was induced by an inhibition of autonomic neural outflows. Renal sympathetic nerve activity (RSNA) decreased considerably after 1.6 +/- 0.2 s from the onset of HDR, which was followed by the ABP drop. Aortic depressor nerve activity (ADNA), an afferent for baroreceptor reflex, increased significantly during the rotation, but the peak of ADNA increase was 3.2 +/- 0.5 s after the initiation of the HDR. Therefore, the suppression of RSNA seems to be induced mainly by a quicker mechanism than baroreceptor reflex. In order to test the possibility, we examined changes in ABP and RSNA during HDR using vestibular-lesioned rabbits. In these rabbits, RSNA and ABP did not change significantly during HDR. These results suggest that vestibular organs play a role in the transient drop in ABP induced by HDR through the suppression of sympathetic nerve outflows.

Dynamic cerebral autoregulation in homozygous Sickle cell disease

BACKGROUND AND PURPOSE

Sickle cell disease (SCD) is associated with cerebral hyperperfusion and an increased risk of stroke. Also, both recurrent microvascular obstruction and chronic hemolysis affect endothelial function, potentially interfering with systemic and cerebral blood flow control. We addressed the question whether cerebrovascular control in patients with SCD is affected and related to hemolysis.

METHODS

Systemic and cerebrovascular control were studied in 18 patients with SCD and 10 healthy subjects. Dynamic cerebral autoregulation was evaluated by transfer function analysis assessing the relationship between mean cerebral blood flow velocity and mean arterial pressure.

RESULTS

Normal baroreflex sensitivity and postural cardiovascular reflex responses indicated integrity of systemic cardiovascular control. In the low- (0.07 to 0.15 Hz) frequency region, mean arterial pressure variability was comparable for both groups, but a larger mean cerebral blood flow velocity variability in SCD (6.1 [4.6 to 7.0] versus 4.2 [2.6 to 5.2] [cm x s(-1)](2) x Hz(-1); P<0.05) indicated a reduced capacity to buffer the transfer of blood pressure surges to the cerebral tissue. Impairment of dynamic cerebrovascular control was confirmed by a reduced mean arterial pressure-to-mean cerebral blood flow velocity transfer function phase lead in SCD versus healthy subjects (32+/-17 degrees versus 50+/-19 degrees , P<0.05) that was unrelated to the severity of hemolysis.

CONCLUSIONS

In patients with SCD, dynamic cerebral autoregulation is impaired but appears unrelated to hemolysis.

Contrasting effects of isocapnic and hypocapnic hyperventilation on orthostatic circulatory control

The effects of hyperventilation (HV) on mean arterial pressure (MAP) are variable. To identify factors affecting the MAP response to HV, we dissected the effects of hypocapnic HV (HHV) and isocapnic HV (IHV) and evaluated the effects of acute vs. prolonged HHV. In 11 healthy subjects the cardio- and cerebrovascular effects of HHV and IHV vs. normal ventilation were examined for 15 min in the supine position and also for 15 min during 60° head-up tilt. The end-tidal CO2 of the HHV condition was set at 15–20 mmHg. With HHV in the supine position, mean cerebral blood flow velocity (mCBFV) declined [95% confidence interval (CI) −43 to −34%], heart rate (HR) increased (95% CI 7 to 16 beats/min), but MAP did not change (95% CI −1 to 6 mmHg). However, an augmentation of the supine MAP was observed in the last 10 min of HHV compared with the first 5 min of HHV (95% CI 2 to 12 mmHg). During HHV in the tilted position mCBFV declined (95% CI −28 to −12%) and MAP increased (95% CI 3 to 11 mmHg) without changes in HR. With supine IHV, mCBFV decreased (95% CI −14 to −4%) and MAP increased (95% CI 1 to 13 mmHg) without changes in HR. During IHV in the tilted position MAP was further augmented (95% CI 11 to 20 mmHg) without changes in CBFV or HR. Preventing hypocapnia during HV resulted in a higher MAP, suggesting two contrasting effects of HV on MAP: hypocapnia causing vasodepression and hyperpnea without hypocapnia acting as a vasopressor.

Mechanisms of orthostatic intolerance following very prolonged exercise

Nine men completed a 24-h exercise trial, with physiological testing sessions before (T1, ∼0630), during (T2, ∼1640; T3, ∼0045; T4, ∼0630), and 48-h afterwards (T5, ∼0650). Participants cycled and ran/trekked continuously between test sessions. A 24-h sedentary control trial was undertaken in crossover order. Within testing sessions, participants lay supine and then stood for 6 min, while heart rate variability (spectral analysis of ECG), middle cerebral artery perfusion velocity (MCAv), mean arterial pressure (MAP; Finometer), and end-tidal Pco2 (PetCO2) were measured, and venous blood was sampled for cardiac troponin I. During the exercise trial: 1) two, six, and four participants were orthostatically intolerant at T2, T3, and T4, respectively; 2) changes in heart rate variability were only observed at T2; 3) supine MAP (baseline = 81 ± 6 mmHg) was lower ( P < 0.05) by 14% at T3 and 8% at T4, whereas standing MAP (75 ± 7 mmHg) was lower by 16% at T2, 37% at T3, and 15% at T4; 4) PetCO2 was reduced ( P < 0.05) at all times while supine (−3–4 Torr) and standing (−4–5 Torr) during exercise trial; 5) standing MCAv was reduced ( P < 0.05) by 23% at T3 and 30% at T4 during the exercise trial; 6) changes in MCAv with standing always correlated ( P < 0.01) with changes in PetCO2 ( r = 0.78–0.93), but only with changes in MAP at T1, T2, and T3 ( P < 0.05; r = 0.62–0.84); and 7) only two individuals showed minor elevations in cardiac troponin I. Recovery was complete within 48 h. During prolonged exercise, postural-induced hypotension and hypocapnia exacerbate cerebral hypoperfusion and facilitate syncope.

Alterations in autonomic function and cerebral hemodynamics to orthostatic challenge following a mountain marathon

We examined potential mechanisms (autonomic function, hypotension, and cerebral hypoperfusion) responsible for orthostatic intolerance following prolonged exercise. Autonomic function and cerebral hemodynamics were monitored in seven athletes pre-, post- (<4 h), and 48 h following a mountain marathon [42.2 km; cumulative gain ∼1,000 m; ∼15°C; completion time, 261 ± 27 (SD) min]. In each condition, middle cerebral artery blood velocity (MCAv), blood pressure (BP), heart rate (HR), and cardiac output (Modelflow) were measured continuously before and during a 6-min stand. Measurements of HR and BP variability and time-domain analysis were used as an index of sympathovagal balance and baroreflex sensitivity (BRS). Cerebral autoregulation was assessed using transfer-function gain and phase shift in BP and MCAv. Hypotension was evident following the marathon during supine rest and on standing despite increased sympathetic and reduced parasympathetic control, and elevations in HR and cardiac output. On standing, following the marathon, there was less elevation in normalized low-frequency HR variability ( P < 0.05), indicating attenuated sympathetic activation. MCAv was maintained while supine but reduced during orthostasis postmarathon [−10.4 ± 9.8% pre- vs. −15.4 ± 9.9% postmarathon (%change from supine); P < 0.05]; such reductions were related to an attenuation in BRS ( r = 0.81; P < 0.05). Cerebral autoregulation was unchanged following the marathon. These findings indicate that following prolonged exercise, hypotension and postural reductions in autonomic function or baroreflex control, or both, rather than a compromise in cerebral autoregulation, may place the brain at risk of hypoperfusion. Such changes may be critical factors in collapse following prolonged exercise.

Effects of hyperglycemia on the cerebrovascular response to rhythmic handgrip exercise

Dynamic cerebral autoregulation (CA) is challenged by exercise and may become less effective when exercise is exhaustive. Exercise may increase arterial glucose concentration, and we evaluated whether the cerebrovascular response to exercise is affected by hyperglycemia. The effects of a hyperinsulinemic euglycemic clamp (EU) and hyperglycemic clamp (HY) on the cerebrovascular (CVRI) and systemic vascular resistance index (SVRI) responses were evaluated in seven healthy subjects at rest and during rhythmic handgrip exercise. Transfer function analysis of the dynamic relationship between beat-to-beat changes in mean arterial pressure and middle cerebral artery (MCA) mean blood flow velocity ( Vmean) was used to assess dynamic CA. At rest, SVRI decreased with HY and EU ( P < 0.01). CVRI was maintained with EU but became reduced with HY [11% (SD 3); P < 0.01], and MCA Vmean increased ( P < 0.05), whereas brain catecholamine uptake and arterial Pco2 did not change significantly. HY did not affect the normalized low-frequency gain between mean arterial pressure and MCA Vmean or the phase shift, indicating maintained dynamic CA. With HY, the increase in CVRI associated with exercise was enhanced (19 ± 7% vs. 9 ± 7%; P < 0.05), concomitant with a larger increase in heart rate and cardiac output and a larger reduction in SVRI (22 ± 4% vs. 14 ± 2%; P < 0.05). Thus hyperglycemia lowered cerebral vascular tone independently of CA capacity at rest, whereas dynamic CA remained able to modulate cerebral blood flow around the exercise-induced increase in MCA Vmean. These findings suggest that elevated blood glucose does not explain that dynamic CA is affected during intense exercise.