Effects Of Orthostatic Stress Research Articles

Orthostatic Stress Induced Changes in Heart Rate Variability, Pulse Transit Time and QRS Duration

Studies have shown the application of non-invasive cardiovascular parameters like heart rate variability (HRV), pulse transit time (PTT) and QRS duration. However no information is available about effects of orthostatic stress on these parameters. Our objective was to study the changes in HRV, PTT and QRS duration caused by orthostatic stress. Electrocardiogram (ECG) and photoplethysmogram (PPG) data was collected from 22 subjects in supine and upright positions. Comparison of parameters in supine and upright positions revealed that every parameter was effected by orthostatic stress. High frequency power of HRV showed the largest response (75%), while QRS duration had the smallest response of (7%). We found that gender played a significant role in PTT, R-R interval and QRS duration values, with male subjects showing higher values of all three parameters. Subject height also played a distinguishing role, with taller subjects showing higher values of R-R interval and PTT. The results allow us to better understand the interaction between various parameters and how they reflect the hemodynamic changes caused by orthostatic stress.

188: The effect of orthostatic stress on brain sparing phenomena in asymmetrical and symmetrical fetal growth restriction

188: The effect of orthostatic stress on brain sparing phenomena in asymmetrical and symmetrical fetal growth restriction

The effect of orthostatic stress on multiscale entropy of heart rate and blood pressure

Cardiovascular control acts over multiple time scales, which introduces a significant amount of complexity to heart rate and blood pressure time series. Multiscale entropy (MSE) analysis has been developed to quantify the complexity of a time series over multiple time scales. In previous studies, MSE analyses identified impaired cardiovascular control and increased cardiovascular risk in various pathological conditions. Despite the increasing acceptance of the MSE technique in clinical research, information underpinning the involvement of the autonomic nervous system in the MSE of heart rate and blood pressure is lacking. The objective of this study is to investigate the effect of orthostatic challenge on the MSE of heart rate and blood pressure variability (HRV, BPV) and the correlation between MSE (complexity measures) and traditional linear (time and frequency domain) measures. MSE analysis of HRV and BPV was performed in 28 healthy young subjects on 1000 consecutive heart beats in the supine and standing positions. Sample entropy values were assessed on scales of 1–10. We found that MSE of heart rate and blood pressure signals is sensitive to changes in autonomic balance caused by postural change from the supine to the standing position. The effect of orthostatic challenge on heart rate and blood pressure complexity depended on the time scale under investigation. Entropy values did not correlate with the mean values of heart rate and blood pressure and showed only weak correlations with linear HRV and BPV measures. In conclusion, the MSE analysis of heart rate and blood pressure provides a sensitive tool to detect changes in autonomic balance as induced by postural change.

Tilt-table testing: transient loss of consciousness discriminator or epiphenomenon?

Syncope accounts for 1–1.5% of emergency room visits and up to 6% of general hospital admissions.1–4 Patients with syncope present with a blackout,5 or transient loss of consciousness (T-LOC), and may present in family practice, cardiology, general medicine, accident and emergency, neurology, geriatric medicine, and psychiatry. Generally, a patient with T-LOC can only give limited details of the event, covering premonition and aftereffects. An eye-witness account is essential for filling in details during T-LOC. However, there are important causes of T-LOC other than syncope, e.g. generalized epilepsy and psychogenic blackouts. When patients present having had T-LOC, clinicians seek a test that will discriminate between these important causes. Is tilt-table testing such a test? Tilt testing is a simple, non-invasive test with a low risk of complications.6 The physiological and pathophysiological effects of orthostatic stress by tilt testing have been known for 60 years.7 In 1986, Kenny et al .8 observed a high rate of tilt-induced syncope, with hypotension and bradycardia, in patients with syncope of previously unknown origin when compared with controls. Subsequently, many reports profiled the use of the test, and guidelines and recommendations emerged.6 However, clinical practice featured a very variable yield from a wide range of protocols, varying in duration and angle of tilt, the use of drug provocation to increase yield, and most importantly, the type of patients studied. Such factors presumably underlie the variable yield of tilt-table testing, which is from 26 to 87%.9 The overall reproducibility of a negative response (85–94%)10–14 is higher than that of an initial positive response (31–92%). The impact of these variations in protocol and patient clinical types, and their effect on the interpretation of a test result for T-LOC discrimination, is considered below. Most studies using tilt …

Cerebral hemodynamics during orthostatic stress assessed by nonlinear modeling

The effects of orthostatic stress, induced by lower body negative pressure (LBNP), on cerebral hemodynamics were examined in a nonlinear context. Spontaneous fluctuations of beat-to-beat mean arterial blood pressure (MABP) in the finger, mean cerebral blood flow velocity (MCBFV) in the middle cerebral artery, as well as breath-by-breath end-tidal CO2 concentration (P(ET(CO2))) were measured continuously in 10 healthy subjects under resting conditions and during graded LBNP to presyncope. A two-input nonlinear Laguerre-Volterra network model was employed to study the dynamic effects of MABP and P(ET(CO2)) changes, as well as their nonlinear interactions, on MCBFV variations in the very low (VLF; below 0.04 Hz), low (LF; 0.04-0.15 Hz), and high frequency (HF; 0.15-0.30 Hz) ranges. Dynamic cerebral autoregulation was described by the model terms corresponding to MABP, whereas cerebral vasomotor reactivity was described by the model P(ET(CO2)) terms. The nonlinear model terms reduced the output prediction normalized mean square error substantially (by 15-20%) and had a prominent effect in the VLF range, both under resting conditions and during LBNP. Whereas MABP fluctuations dominated in the HF range and played a significant role in the VLF and LF ranges, changes in P(ET(CO2)) accounted for a considerable fraction of the VLF and LF MCBFV variations, especially at high LBNP levels. The magnitude of the linear and nonlinear MABP-MCBFV Volterra kernels increased substantially above -30 mmHg LBNP in the VLF range, implying impaired dynamic autoregulation. In contrast, the magnitude of the P(ET(CO2))-MCBFV kernels reduced during LBNP at all frequencies, suggesting attenuated cerebral vasomotor reactivity under dynamic conditions. We speculate that these changes may reflect a progressively reduced cerebrovascular reserve to compensate for the increasingly unstable systemic circulation during orthostatic stress that could ultimately lead to cerebral hypoperfusion and syncope.

Effects of Orthostatic Stress on Forearm Endothelial Function in Normal Subjects and in Patients With Hypertension, Diabetes, or Both Diseases

Sympathetically mediated vasoconstriction, to compensate for reduced venous return and cardiac output, characterizes the circulatory adaptation to head-up tilting (HUT). It has not been clarified whether this is coupled with a modulating endothelial vasorelaxation and whether diseases causing endothelial dysfunction, such as diabetes and hypertension, may impair this counterregulatory mechanism. In patients with hypertension (group 1), diabetes (group 2), or both diseases (group 3) and in healthy control subjects (12 subjects per group) we investigated the brachial artery vasodilating response to the release of distal circulatory arrest (DCA) while they were supine and during 60 degrees HUT. The supine increase in lumen was smaller (P<.01) in groups 1 (+4.5%+/-1.5%), 2 (+4.8%+/-1.4%), and 3 (+3.9%+/-1.3%) than in the control group (+8.6%+/-1.6%). Vasorelaxation by nitroglycerin was similar in each population. During HUT, the lumen response to DCA was enhanced (P<.01 v supine) in control subjects (+15.4%+/-2.5%) and group 1 (+10.0+/-2.4%) and was reduced (P<.01 v supine) in groups 2 (+2.9%+/-0.5%) and 3 (+2.1%+/-0.4%), even though the hyperemic reaction to DCA was similar. The ratio of lumen changes to changes in flow (mm/mL/min x 1000) during reactive hyperemia to DCA increased (P<.01) with HUT, compared with that in the supine position, in control subjects (1.75v1.19) and group 1 (1.61v0.95), and decreased (P<.01) in groups 2 (0.62v0.87) and 3 (0.48v0.77). The HUT posture is characterized by an increased endothelium-dependent, flow-mediated vasodilation as a possible modulator of the neural vasoconstriction. This effect is persistent but blunted in hypertension and is abolished in diabetes, either alone or in association with high BP. Thus, vasoconstrictor factors could remain unmodulated during an event such as orthostasis, making the risk posed by these disorders more critical.

Influence of posture on the Valsalva manoeuvre

The objective of the present study was to evaluate the influence of posture on the responses of blood pressure (BP) and heart rate (HR) to the Valsalva manoeuvre (VM). Neurohumoral activation, as well as changes in intravascular and intracardiac volumes and pressures, are well known effects of orthostatic stress. These changes are likely to have significant effects on cardiovascular reflexes, such as the response to the VM. However, the influence of posture on the VM has not been intensively evaluated, except for a few studies involving small sex- and age-selected case series. We therefore investigated the effects of posture on the VM in a larger non-selected group of healthy control subjects. In 19 healthy volunteers (ten female/nine male; age range 20-72 years, mean age 43 years), two reproducible VMs (40 mmHg; 15 s) were performed after 10 min of supine rest, 10 min of sitting and 10 min of standing. HR and BP were monitored continuously. End-diastolic volume, total peripheral resistance and cardiac output were calculated at baseline for each position. We found that assuming an upright position resulted in increases in total peripheral resistance and HR, accompanied by decreases in end-diastolic volume and cardiac output. The fall in BP during early phase II and the BP overshoot during phase IV were clearly more pronounced with increasing orthostatic stress, whereas the rise in BP during late phase II remained unchanged; pulse pressure was more compressed during phase II, but higher during phase IV. The Valsalva ratio was not significantly affected, but baroreflex gain (calculated from early phase II) was significantly decreased in the upright position. While a reduced late phase II was observed on one occasion in each of the lying and sitting positions, three abnormal responses were observed during standing. We conclude that posture has a significant influence on BP responses to the VM, probably resulting from changes in the intrathoracic blood volume. Standing results in a lower rate of 'flat-top' responses, but also seems to reduce the specificity of this test. Sympathetic activation in the upright position seems to blunt baroreflexes, leading to similar HR responses in spite of larger changes in BP.

Influence of posture on the Valsalva manoeuvre

The objective of the present study was to evaluate the influence of posture on the responses of blood pressure (BP) and heart rate (HR) to the Valsalva manoeuvre (VM). Neurohumoral activation, as well as changes in intravascular and intracardiac volumes and pressures, are well known effects of orthostatic stress. These changes are likely to have significant effects on cardiovascular reflexes, such as the response to the VM. However, the influence of posture on the VM has not been intensively evaluated, except for a few studies involving small sex- and age-selected case series. We therefore investigated the effects of posture on the VM in a larger non-selected group of healthy control subjects. In 19 healthy volunteers (ten female/nine male; age range 20–72 years, mean age 43 years), two reproducible VMs (40 mmHg; 15 s) were performed after 10 min of supine rest, 10 min of sitting and 10 min of standing. HR and BP were monitored continuously. End-diastolic volume, total peripheral resistance and cardiac output were calculated at baseline for each position. We found that assuming an upright position resulted in increases in total peripheral resistance and HR, accompanied by decreases in end-diastolic volume and cardiac output. The fall in BP during early phase II and the BP overshoot during phase IV were clearly more pronounced with increasing orthostatic stress, whereas the rise in BP during late phase II remained unchanged; pulse pressure was more compressed during phase II, but higher during phase IV. The Valsalva ratio was not significantly affected, but baroreflex gain (calculated from early phase II) was significantly decreased in the upright position. While a reduced late phase II was observed on one occasion in each of the lying and sitting positions, three abnormal responses were observed during standing. We conclude that posture has a significant influence on BP responses to the VM, probably resulting from changes in the intrathoracic blood volume. Standing results in a lower rate of ‘flat-top’ responses, but also seems to reduce the specificity of this test. Sympathetic activation in the upright position seems to blunt baroreflexes, leading to similar HR responses in spite of larger changes in BP.

Effect of perturbations and a meal on superior mesenteric artery flow in patients with orthostatic hypotension

Our aims were to evaluate to role of superior mesenteric blood flow in the pathophysiology of orthostatic hypotension in patients with generalized autonomic failure. Methods: Twelve patients with symptomatic neurogenic orthostatic hypotension and 12 healthy controls underwent superior mesenteric artery flow measurements using Doppler ultrasonography during head-up tilt and tilt plus meal ingestion. Autonomic failure was assessed using standard tests of the function of the sympathetic adrenergic, cardiovagal and postganglionic sympathetic sudomotor function. Results: Superior mesenteric flow volume and time-averaged velocity were similar in patients and controls at supine rest; however, responses to cold pressor test and upright tilt were attenuated ( p<0.05) in patients compared to controls. Head-up tilt after the meal evoked a profound fall of blood pressure and mesenteric blood flow in the patients; the reduction of mesenteric blood flow correlated ( r=0.89) with the fall of blood pressure in these patients, providing another manifestation of failed baroreflexes. We make the novel finding that the severity of postprandial orthostatic hypotension regressed negatively with the postprandial increase in mesenteric flow in patients with orthostatic hypotension. Conclusion: Mesenteric flow is under baroreflex control, which when defective, results in, or worsens orthostatic hypotension. Its large size and baroreflexivity renders it quantitatively important in the maintenance of postural normotension. The effects of orthostatic stress can be significantly attenuated by reducing the splanchnic–mesenteric volume increase in response to food. Evaluation of mesenteric flow in response to eating and head-up tilt provide important information on intra-abdominal sympathetic adrenergic function, and the ability of the patient to cope with orthostatic stress.

807-2 Effect of Age on Left Ventricular Diastolic Filling Patterns During Orthostatic Stress

Although numerous studies have demonstrated reduced early diastolic left ventricular (LV) peak filling velocity IE) and accentuated late filling velocity (A) with advancing age in the supine position, the effect of orthostatic stress on age-associated diastolic filling patterns is unknown. Accordingly, 30 healthy normotensive volunteers ages 19 to 90 years from the Baltimore Longitudinal Study of Aging underwent sequential Doppler echocardiography after 3 minutes each in the supine, seated, and standing positions. In the overall sample, standing was accompanied by an increase in heart rate (HR) of 8.8 ± 1.7 beats/min, and decreases in E(20.6 ± 2.7 cm/sl. A 150 ± 2.7 cm/s)and LV diastolic dimension (LVDD) (7.8 ± 0.8 mm), x ± SEM, each p &lt; 0.001 versus supine values. Systolic blood pressure (SBP) and atrial filling fraction (AFF) were unaffected by posture. Correlation coefficients versus age for relevant Doppler and hemodynamic variables are shown. E A E/A AFF HR SBP LVDD Supine -0.58 † 0.80 † -0.78 † 0.84 † 0.01 0.56 † -0.14 Sit -0.36 0.77 † -0.83 † 0.78 † -0.04 0.37 * -0.07 Stand -0.24 0.60 † -0.69 † 0.71 † -0.32 0.47 † 0.09 ΔSupine → Stand 0.53 † -0.46 † 0.56 † -0.17 -0.61 † 0.22 0.40 * * p &lt; 005 † p &lt; 001 Thus, orthostatic stress abolishes the reduction of E and attenuates the exaggerated A observed with advancing age in the supine position. probably because of the blunted standing-induced HR increase in older subjects. However, the supine age-associated increase in AFF is unaffected by orthostasis.

Effects of orthostatic stress on peripheral capillary filtration in mild congestive heart failure after healing of myocardial infarction

Patients with heart failure have impaired baroreflex control of the peripheral circulation with attenuated vasoconstrictor response during orthostatic stress. The aim of this study was to test if this impaired barorefiex control not only affects the arterial, but also the capillary bed. Blood flow and capillary filtration were measured in the forearm (plethysmography) in 7 normal subjects and 7 patients with mild congestive heart failure (New York Heart Association functional class II). Measurements were done with the subjects supine and during headkup tilt at 45°. While supine, forearm vascular resistance and capillary filtration coefficient did not differ significantly between the groups. In the control subjects, tilt decreased capillary filtration coefficient by 14 ± 3% (p <0.02), and Increased forearm vascular resistance by 88 ± 37% (p <0.02); in contrast, patients with heart failure had an increase in capillary filtration coefficient of 26 ± 5% (p <0.02) and only increased the forearm vascular resistance by 10 ± 1%, (p = NS, p = 0.26). Our data provide evidence that patients with mild heart failure, in contrast to normal subjects, Increase the peripheral capillary filtration during orthostatic stress. The data indicate that impaired barorefiex mechanisms might influence the capillary filtration and it is suggested that impaired barorefiex control of the peripheral circulation can contribute to formation of edema in patients with heart failure.