Resting heart rate predicts cardiac autonomic modulation during passive head-up tilt in subjects without cardiovascular diseases.

Background Resting heart rate (HR) and its variability (HRV) reflects the cardiac sympathovagal balance that is stimulated by head-up tilting. HRV is influenced by the level of HR, but how much HRV offers additional information about cardiac autonomic tone than HR alone remains unresolved. We examined the relation of resting HR with HRV during head-up tilt. Methods. Hemodynamics of 569 subjects without known cardiovascular diseases and medications with direct cardiovascular effects were recorded using whole-body impedance cardiography, radial pulse wave analysis, and electrocardiography-based HRV analysis during passive head-up tilt. Results. Higher low frequency to the high-frequency ratio (LF/HF) of HRV (reflecting sympathovagal balance) was associated with higher HR in supine (p < .05, both linear regression analysis and variance analysis comparing HR tertiles) and upright postures (p < .001, linear regression analysis). The association of HR with HRV during tilt-testing remained significant when the HR dependence of HRV was mathematically weakened by dividing the HRV power spectra with the fourth power of the average RR-interval. Conclusion. Higher resting HR is related to higher LF/HF both supine and upright, reflecting elevated sympathetic influence on cardiac autonomic modulation. Lower resting HR is associated with lower resting LF/HF, while the differences in LF/HF between the HR tertiles were minor during head-up tilt, suggesting a greater change in cardiac sympathovagal balance in response to upright posture in those with lowest resting HR. Altogether, resting HR well predicts HRV levels during head-up tilt.Trial registration: Clinicaltrialsregister.eu 2006-002065-39, first registered 5 May 2006. ClinicalTrials.gov NCT01742702, first registered 5 December 2012.

The type of the functional cardiovascular response to upright posture is associated with arterial stiffness: a cross-sectional study in 470 volunteers.

BACKGROUND: In a cross-sectional study we examined whether the haemodynamic response to upright posture could be divided into different functional phenotypes, and whether the observed phenotypes were associated with known determinants of cardiovascular risk. METHODS: Volunteers (n = 470) without medication with cardiovascular effects were examined using radial pulse wave analysis, whole-body impedance cardiography, and heart rate variability analysis. Based on the passive head-up tilt induced changes in systemic vascular resistance and cardiac output, the principal determinants of blood pressure, a cluster analysis was performed. RESULTS: The haemodynamic response could be clustered into 3 categories: upright increase in vascular resistance and decrease in cardiac output were greatest in the first (+45 % and -27 %, respectively), smallest in the second (+2 % and -2 %, respectively), and intermediate (+22 % and -13 %, respectively) in the third group. These groups were named as 'constrictor' (n = 109), 'sustainer' (n = 222), and 'intermediate' (n = 139) phenotypes, respectively. The sustainers were characterized by male predominance, higher body mass index, blood pressure, and also by higher pulse wave velocity, an index of large arterial stiffness, than the other groups (p < 0.01 for all). Heart rate variability analysis showed higher supine and upright low frequency/high frequency (LF/HF) ratio in the sustainers than constrictors, indicating increased sympathovagal balance. Upright LF/HF ratio was also higher in the sustainer than intermediate group. In multivariate analysis, independent explanatory factors for higher pulse wave velocity were the sustainer (p < 0.022) and intermediate phenotypes (p < 0.046), age (p < 0.001), body mass index (p < 0.001), and hypertension (p < 0.001). CONCLUSIONS: The response to upright posture could be clustered to 3 functional phenotypes. The sustainer phenotype, with smallest upright decrease in cardiac output and highest sympathovagal balance, was independently associated with increased large arterial stiffness. These results indicate an association of the functional haemodynamic phenotype with an acknowledged marker of cardiovascular risk. TRIAL REGISTRATION: ClinicalTrials.gov NCT01742702.

Metabolic syndrome is associated with decreased heart rate variability in a sex-dependent manner: a comparison between 252 men and 249 women.

Impaired heart rate variability (HRV) is associated with increased risk of cardiovascular disease, but evidence regarding alterations of HRV in metabolic syndrome (MetS) remains elusive. In order to examine HRV in MetS, we subjected 501 volunteers without atherosclerosis, diabetes or antihypertensive medication, mean age 48 years, to passive head-up tilt. The subjects were divided to control men (n = 131), men with MetS (n = 121), control women (n = 191) and women with MetS (n = 58) according to the criteria by Alberti et al. (Circulation, 2009, 120, 1640). In unadjusted analyses (i) men and women with MetS had lower total power and high-frequency (HF) power of HRV than controls whether supine or upright (P<0·05 for all). (ii) Supine low-frequency (LF) power of HRV was lower in men (P = 0·012) but not in women (P = 0·064) with MetS than in controls, while men and women with MetS had lower upright LF power of HRV than controls (P <0·01 for both). (iii) The LF:HF ratio did not differ between subjects with and without MetS. After adjustment for age, smoking habits, alcohol intake, height, heart rate and breathing frequency, only the differences in upright total power and HF power of HRV between women with MetS and control women remained significant (P<0·05). In conclusion, reduced total and HF power of HRV in the upright position may partially explain why the relative increase in cardiovascular risk associated with MetS is greater in women than in men. Additionally, the present results emphasize that the confounding factors must be carefully taken into consideration when evaluating HRV.

Voluntary liquorice ingestion increases blood pressure via increased volume load, elevated peripheral arterial resistance, and decreased aortic compliance.

We investigated the haemodynamic effects of two-week liquorice exposure (glycyrrhizin dose 290-370 mg/day) in 22 healthy volunteers during orthostatic challenge. Haemodynamics were recorded during passive 10-minute head-up tilt using radial pulse wave analysis, whole-body impedance cardiography, and spectral analysis of heart rate variability. Thirty age-matched healthy subjects served as controls. Liquorice ingestion elevated radial systolic (p < 0.001) and diastolic (p = 0.018) blood pressure and systemic vascular resistance (p = 0.037). During orthostatic challenge, heart rate increased less after the liquorice versus control diet (p = 0.003) and low frequency power of heart rate variability decreased within the liquorice group (p = 0.034). Liquorice intake increased central pulse pressure (p < 0.001) and augmentation index (p = 0.002) supine and upright, but in the upright position the elevation of augmentation index was accentuated (p = 0.007). Liquorice diet also increased extracellular fluid volume (p = 0.024) and aortic to popliteal pulse wave velocity (p = 0.027), and aortic characteristic impedance in the upright position (p = 0.002). To conclude, in addition to increased extracellular fluid volume and large arterial stiffness, two weeks of liquorice ingestion elevated systemic vascular resistance and augmentation index. Measurements performed at rest may underestimate the haemodynamic effects of liquorice ingestion, as enhanced central wave reflection and reduced chronotropic response were especially observed in the upright position.

Increased Cardiac Workload in the Upright Posture in Men: Noninvasive Hemodynamics in Men Versus Women.

BACKGROUND: Men and women differ in the risk of cardiovascular disease, but the underlying mechanisms are not completely understood. We examined possible sex-related differences in supine and upright cardiovascular regulation. METHODS AND RESULTS: Hemodynamics were recorded from 167 men and 167 women of matching age (≈45 years) and body mass index (≈26.5) during passive head-up tilt. None had diabetes mellitus or cardiovascular disease other than hypertension or used antihypertensive medication. Whole-body impedance cardiography, tonometric radial blood pressure, and heart rate variability were analyzed. Results were adjusted for height, smoking, alcohol intake, mean arterial pressure, plasma lipids, and glucose. Supine hemodynamic differences were minor: Men had lower heart rate (-4%) and higher stroke index (+7.5%) than women (P<0.05 for both). Upright systemic vascular resistance was lower (-10%), but stroke index (+15%), cardiac index (+16%), and left cardiac work were clearly higher (+20%) in men than in women (P<0.001 for all). Corresponding results were observed in a subgroup of men and postmenopausal women (n=76, aged >55 years). Heart rate variability analyses showed higher low:high frequency ratios in supine (P<0.001) and upright (P=0.003) positions in men. CONCLUSIONS: The foremost difference in cardiovascular regulation between sexes was higher upright hemodynamic workload for the heart in men, a finding not explained by known cardiovascular risk factors or hormonal differences before menopause. Heart rate variability analyses indicated higher sympathovagal balance in men regardless of body position. The deviations in upright hemodynamics could play a role in the differences in cardiovascular risk between men and women. CLINICAL TRIAL REGISTRATION: URL: http://www.clinicaltrials.gov. Unique identifier: NCT01742702.

Tidal breathing flow-volume curves with impedance pneumography during expiratory loading.

Diagnosis of asthma in the preschoold children is difficult due to lack of objective lung function tests suitable for this age group. Impedance pneumography (IP) is a mode of measurement that may potentially enable ambulatory 24h recording of tidal breathing indices and respiratory dynamics that are known to relate to small airway obstruction. The aim of this research was to induce changes in breathing control and mechanics and study the ability of IP to reproduce TBFVC and track its changes under potentially difficult conditions. This was achieved by a comparison of direct mouth pneumotachograph (PNT) and IP tidal breathing flow-volume curves (TBFVC) during free breathing and expiratory loading obtained from 17 young lung-healthy subjects. The expiratory loading produced strong and significant changes in the respiratory pattern and mouth pressure. The agreement of PNT and IP normalized TBFVCs was found excellent having the highest distance between the normalized TBFVCs of (mean ± SD) 7.4 % ± 3.6 % and 6.2 % ± 3.0 % during free and loaded breathing, respectively. The agreement was not affected by the presence of the expiratory load despite it poses multiple potential hazards for the IP measurements. We conclude that by using correct electrode placement and cardiac filtering, IP was able to accurately reproduce and track changes in normalized TBFVCs under normal and abnormal respiratory conditions in healthy adult subjects.

Novel electrode configuration for highly linear impedance pneumography.

Impedance pneumography (IP) is a non-invasive respiration measurement technique. Emerging applications of IP in respiratory medicine use the measured signal to monitor pulmonary flow and volume parameters related to airway obstruction during tidal breathing (TB). This requires a high impedance change (ΔZ)-to-lung volume change (ΔV) linearity. Four potential electrode configurations were tested on 10 healthy subjects. Only the novel configuration where the electrodes were placed in both the thorax and the arms yielded a highly linear ΔZ/ΔV in all subjects. The presented electrode configuration may expand the clinical use of IP from the conventional tidal volume estimation to flow measurement.