Impact of daily artificial gravity on autonomic cardiovascular control following 60-day head-down tilt bed rest

Impaired cardiovascular autonomic control following space flight or immobilization may limit the ability to cope with additional hemodynamic stimuli. Head-down tilt bedrest is an established terrestrial analog for space flight and offers the opportunity to test potential countermeasures for autonomic cardiovascular deconditioning. Previous studies revealed a possible benefit of daily artificial gravity on cardiovascular autonomic control following head-down tilt bedrest, but there is a need for efficiency in a long-term study before an artificial gravity facility would be brought to space. We hypothesized that artificial gravity through short-arm centrifugation attenuates functional adaptions of autonomic function during head-down tilt bed rest. 24 healthy persons (8 women, 33.4 ± 9.3 years, 24.3 ± 2.1 kg/m2) participated in the 60-day head-down tilt bed rest (AGBRESA) study. They were assigned to three groups, 30 min/day continuous, or 6(5 min intermittent short-arm centrifugation, or a control group. We assessed autonomic cardiovascular control in the supine position and in 5 minutes 80° head-up tilt position before and immediately after bed rest. We computed heart rate variability (HRV) in the time (rmssd) and frequency domain, blood pressure variability, and baroreflex sensitivity (BRS). RR interval corrected rmssd was reduced supine (p = 0.0358) and during HUT (p = 0.0161). Heart rate variability in the high-frequency band (hf-RRI; p = 0.0004) and BRS (p < 0.0001) decreased, whereas blood pressure variability in the low-frequency band (lf-SBP, p = 0.0008) increased following bedrest in all groups. We did not detect significant interactions between bedrest and interventions. We conclude that up to daily 30 min of artificial gravity on a short-arm centrifuge with 1Gz at the center of mass do not suffice to prevent changes in autonomic cardiovascular control following 60-day of 6° head-down tilt bed rest. Clinical Trial Registration: https://drks.de/search/en/trial/DRKS00015677, identifier, DRKS00015677.

Effects of daily artificial gravity training on orthostatic tolerance following 60-day strict head-down tilt bedrest

PURPOSE

Orthostatic intolerance commonly occurs following immobilization or space flight. We hypothesized that daily artificial gravity training through short-arm centrifugation could help to maintain orthostatic tolerance following head-down tilt bedrest, which is an established terrestrial model for weightlessness.

METHODS

We studied 24 healthy persons (eight women; age 33.3 ± 9.0 years; BMI 24.3 ± 2.1 kg/m2) who participated in the 60-days head-down tilt bedrest (AGBRESA) study. They were assigned to 30 min/day continuous or 6 × 5 min intermittent short-arm centrifugation with 1Gz at the center of mass or a control group. We performed head-up tilt testing with incremental lower-body negative pressure until presyncope before and after bedrest. We recorded an electrocardiogram, beat-to-beat finger blood pressure, and brachial blood pressure and obtained blood samples from an antecubital venous catheter. Orthostatic tolerance was defined as time to presyncope. We related changes in orthostatic tolerance to changes in plasma volume determined by carbon dioxide rebreathing.

RESULTS

Compared with baseline measurements, supine and upright heart rate increased in all three groups following head-down tilt bedrest. Compared with baseline measurements, time to presyncope decreased by 323 ± 235 s with continuous centrifugation, by 296 ± 508 s with intermittent centrifugation, and by 801 ± 354 s in the control group (p = 0.0249 between interventions). The change in orthostatic tolerance was not correlated with changes in plasma volume.

CONCLUSIONS

Daily artificial gravity training on a short-arm centrifuge attenuated the reduction in orthostatic tolerance after 60 days of head-down tilt bedrest.

Voices from Health Care Providers: Assessing the Impact of the Indian Assisted Reproductive Technology (Regulation) Act, 2021 on the Practice of IVF in India

The regulatory vacuum in the field of ART in India was filled when in December 2021, the Assisted Reproductive Technology (Regulation) Act, 2021 (ART Act) (https://egazette.nic.in/WriteReadData/2021/232025.pdf) and the Surrogacy (Regulation) Act, 2021 (SR Act) were passed. We surveyed medical professionals to understand their knowledge, attitude and perception towards the Acts and to offer an initial, snapshot assessment of their impact on the medical community. The government has already signalled its intent to implement the Acts and has published several notifications/gazettes to clarify and amend the issues surrounding the Acts (https://artsurrogacy.gov.in/NationalArtSurrogacy/faces/HomePage.xhtml#). We hope that these responses will help to voice the thoughts, concerns and suggestions from of ART service providers for ART to further clarify and rationalise the laws. Infertility is already a much stigmatised problem which deserves to be a higher public health priority. While the laws are a welcome step, changes in both laws is are the need of the hour to make ART more accessible, available and affordable to the millions of couples who need these services and for the health care providers who to be able to deliver them.

Effects of targeted therapy on blood gas analysis in pulmonary arterial hypertension – a retrospective analysis

Background

In pulmonary arterial hypertension (PAH), there is an excessive respiratory drive, which leads to inefficient ventilation with subsequent hypocapnia. Changes in blood gas analysis (BGA) may correlate with symptom severity and outcome. Data on impact of targeted PAH therapy on BGA are scarce and it remains unclear if these values can be used for risk stratification purposes.

Purpose

To assess capillary BGA in PAH at diagnosis and to evaluate the effect of targeted therapy on classical and calculated BGA parameters.

Methods

147 patients (62.3±17.1 years; female/male ratio: 66.7/33.3%) with newly diagnosed PAH were treated with targeted PAH therapy. 12-month follow-up included assessment of capillary blood gases and clinical variables. Patients underwent repeat RHC after 15.9±15.5 months.

Results

At diagnosis, both pcO2 (66.8±1.3 mmHg) and pcCO2 (32.7±0.5 mmHg) were slightly reduced. At 12-month follow-up, pcCO2 increased significantly to values within normal range (35.3±0.4 mmHg), while pcO2 showed no significant changes. After using the formula to calculate standard pcO2, baseline values were even lower, but there were significant improvements at 12-month follow-up, also reaching normal values. Changes in pcCO2 and standard pcO2 at diagnosis correlated with hemodynamics and survival at follow-up. Repeated RHC demonstrated significant reductions in mean PAP (48.9±1.2 to 39.9±1.0 mmHg; −18.4%), and PVR (11.3±0.7 to 6.2±0.3 WU; −45.1%), and an increase in cardiac index (2.1±0.04 to 2.6±0.1 ml/min/m2; +23.8%) (all p&lt;0.05). Hemodynamic improvements correlated with improved clinical parameters, including 6-minute walking distance (344±9 to 393±9 m), NTproBNP serum levels (2.163±219 to 772±84 ng/l, both p&lt;0.05) and WHO-FC at 12 months, resulting in improved risk status.

Conclusions

Targeted PAH therapy leads to significantly improved cardiopulmonary hemodynamics with subsequent increase in pcCO2, presumably due to less hyperventilation. Changes in pcCO2 and standard pcO2 (but not pcO2) correlate with hemodynamics and survival, potentially serving as non-invasive parameters for risk assessment and therapeutic response. The discrepancy between pcO2 and standard pcO2 at diagnosis suggests that pcO2 is upregulated by the hyperventilatory state. Standard pcO2 represents an easy-to-calculate parameter that can help more accurately identify PAH patients requiring O2 therapy in addition to targeted therapy. Further studies are needed in this context.

Funding Acknowledgement

Type of funding sources: None.

[Leiomyoma of the esophagus : A further indication for robotic surgery?]

BACKGROUND

Leiomyomas of the esophagus are rare tumors but the most common benign lesion of the esophagus originating from smooth muscle cells. The symptoms are mainly determined by the size of the tumor and are caused by dysphagia and/or retrosternal pain. The majority of patients are however asymptomatic. The diagnostics include esophagoscopy, endosonography and chest computed tomography. Surgery is considered the treatment of choice and ideally involves enucleation of the tumor but may lead to esophagectomy. In addition to the classical open procedures, minimally invasive procedures are also used. Regardless of the selected procedure, a lesion of the mucosa should be avoided.

OBJECTIVE

A review of the literature on thoracoscopic and robotic resections in the treatment of leiomyomas was carried out and an illustration of a clinical case is presented.

MATERIAL AND METHODS

A review of minimally invasive surgical treatment of esophageal leiomyomas is presented. The literature search was carried out in PubMed for publications of thoracoscopic and robotic-assisted thoracic enucleation of leiomyomas of the esophagus. In addition, the robotic-assisted thoracic enucleation of a horseshoe-shaped leiomyoma in the middle third of the esophagus is described.

RESULTS

The enucleation of the esophageal leiomyoma was carried out through a right-sided robotic-assisted operation with one lung ventilation. The surgery time was 143 min. There were no intraoperative or postoperative complications. On the 3rd postoperative day a light diet was started and the thorax drainage was removed. Histopathology confirmed a leiomyoma. The patient was discharged on the 5th postoperative day and free of complaints.

CONCLUSION

Robotic-assisted surgery for leiomyomas of the esophagus is a safe procedure. Taking the available data into account, robotic-assisted thoracic enucleation of leiomyomas was characterized by less mucosal lesions, general complications and a lower conversion rate as well as a shorter hospital stay compared to classical thoracoscopic enucleation. Thus, robotic-assisted surgery can be the method of choice for leiomyomas of the esophagus.

Correction to 'On the possibility (or lack thereof) of agreement between experiment and computation of flows over wings at moderate Reynolds number'

[This corrects the article DOI: 10.1098/rsfs.2016.0076.].

On the possibility (or lack thereof) of agreement between experiment and computation of flows over wings at moderate Reynolds number

The flight of many birds and bats, and their robotic counterparts, occurs over a range of chord-based Reynolds numbers from 1 × 10⁴ to 1.5 × 10⁵. It is precisely over this range where the aerodynamics of simple, rigid, fixed wings becomes extraordinarily sensitive to small changes in geometry and the environment, with two sets of consequences. The first is that practical lifting devices at this scale will likely not be simple, rigid, fixed wings. The second is that it becomes non-trivial to make baseline comparisons for experiment and computation, when either one can be wrong. Here we examine one ostensibly simple case of the NACA 0012 aerofoil and make careful comparison between the technical literature, and new experiments and computations. The agreement (or lack thereof) will establish one or more baseline results and some sensitivities around them. The idea is that the diagnostic procedures will help to guide comparisons and predictions in subsequent more complex cases.

Cardiovascular changes in parabolic flights assessed by ballistocardiography

This paper presents a comparison of the cardiovascular changes observed in microgravity as compared to ground based measurements. The ballistocardiogram (BCG), the electrocardiogram (ECG) and the transthoracic impedance cardiogram (ICG) were recorded on five healthy subjects during the 57th-European Space Agency (ESA) parabolic flight campaign. BCG is analyzed though its most characteristic wave, the IJ wave complex that can be identified along the longitudinal component of BCG and which has been demonstrated to be linked to cardiac ejection. The timings between the contraction of the heart and the ejection of blood in the aorta are analyzed via the time delay between the R-wave of the ECG and the I and J-waves of BCG (RI and RJ intervals respectively). Our results show that the IJ complex presents a larger amplitude in weightlessness and suggest that stroke volume (SV) increases in microgravity. We assume that ballistocardiography is an efficient method to assess the ventricular performance.

Three dimensional Ballistocardiogram and Seismocardiogram: what do they have in common?

3D-body accelerations, i.e. Ballistocardiograms (BCG) and Seismocardiograms (SCG), ECG and Impedance-cardiograms (ICG) were recorded on healthy volunteers participating to the European Space Agency (ESA) 59th parabolic flight campaign. In the present paper we document the similarities and differences that can be seen in the seismo- and ballisto-cardiogram signals in different positions (standing and supine) under normal gravity condition as well as during the weightlessness phases (0G) of a parabolic flight. Our results demonstrate that SCG and BCG both present a similar three dimensional (3D) nature, with components of the BCG having lower frequency content than the SCG. The recordings performed in the 0G environment are the one with the smoothest shape and largest maximum magnitude of the Force vector. The differences seen between SCG and BCG stress further the importance for the need of using different nomenclature for the identification of peaks in both signals.

Pacemaker current inhibition in experimental human cardiac sympathetic activation: a double-blind, randomized, crossover study

Hyperpolarization-activated, cyclic nucleotide-gated 4 (HCN4) channels comprise the final pathway for autonomic heart rate (HR) regulation. We hypothesized that HCN4 inhibition could reverse autonomic imbalance in a human model of cardiac sympathetic activation. Nineteen healthy men ingested oral metoprolol+reboxetine, ivabradine+reboxetine, or placebo+reboxetine in a double-blind, randomized, crossover fashion. We assessed HR, blood pressure (BP), stroke volume, and cardiac output during rest and profound orthostatic stress. HR variability, BP variability, and baroreflex sensitivity were analyzed. Metoprolol, but not ivabradine, decreased resting HR and BP. Ivabradine attenuated the HR increase to orthostatic stress, albeit to a lesser extent than metoprolol. Stroke volume and cardiac output at a given HR were significantly lower with metoprolol. Unlike metoprolol, ivabradine did not affect HR variability, BP variability, or baroreflex sensitivity. Ivabradine attenuates sympathetic influences on HR at the sinus node level, leaving myocardial sympathetic activation unopposed. Reversal of parasympathetic dysfunction by ivabradine appears limited.

Three dimensional ballisto- and seismo-cardiography: HIJ wave amplitudes are poorly correlated to maximal systolic force vector

Ballistocardiography was recorded in 3-D on a free floating astronaut in space as well as on healthy volunteers participating to the ESA 55(th) and DLR 19(th) parabolic flights campaigns. In this paper we demonstrate further the usefulness of recording and analyzing ballistocardiograms (BCG) in three dimensions. The spatial curves of the displacement, velocity and acceleration vectors are analyzed instead of their individual 2-D components. The maximum magnitude of the force vector is shown to be poorly correlated to the HI and IJ wave amplitude traditionally computed on the longitudinal (feet-to-head) component of acceleration (uni-dimensional BCG). We also suggest that kinetic energy and work are useful parameters to consider for a physiological interpretation of the 3D-BCG. The technique presented is invariant from the axis of representation and provides important novel physiological information. We stress further the need of 3D recordings and analysis techniques for Ballisto- and Seismo-cardiography.

Timing and source of the maximum of the transthoracic impedance cardiogram (dZ/dt) in relation to the H-I-J complex of the longitudinal ballistocardiogram under gravity and microgravity conditions

The transthoracic impedance cardiogram (ICG) and the acceleration ballistocardiogram (BCG) measured close to the center of mass of the human body are generated by changes of blood distribution. The transthoracic ICG is an integrated signal covering the whole thorax and spatial resolution is poor. Comparison between both signals can be used to elucidate the source of the ICG signal. We recorded the ECG, ICG, and BCG simultaneously in healthy subjects under resting and microgravity conditions during parabolic flights. The time interval between the R-peak of the ECG and the maximum of the ICG (R-dZ/dtmax) and the time interval between the R-peak of the ECG and the I-peak in the BCG (R-I) differed significantly (p<0.0001). The I-peak in the BCG always occurred earlier during systole than dZ/dtmax. The delay of dZ/dtmax ranged between 23 and 28 ms at rest and was lowest under microgravity conditions (12 ± 4 ms, p<0.02). Our results suggest that both signals have different sources. Combination of modern imaging techniques with classical non invasive approaches to detect changes of blood distribution may provide new insights into the complex interaction between blood flow and mechanocardiographic signals like the BCG.

3D-ballistocardiography in microgravity: comparison with ground based recordings

3D-Ballistocardiograms ECG and Impedancecardiograms (ICG) were recorded on 5 healthy volunteers participating to the European Space Agency (ESA) 57(th) parabolic flights campaigns. Comparisons are made between the baseline recordings performed on the ground and the recordings made during the microgravity phases of a parabolic flight. The spatial curves of the displacement, velocity and acceleration vectors, instead of their individual components are used to compute the magnitude of the force vector, kinetic energy and work during the cardiac cycle. Our hypothesis is that the 3D-BCG provides parameters correlated with the timings of ejection (PEP, LVET). Although our subject population is limited (N=5), this is the first study of BCG to be performed with N>1. Our results suggest that microgravity decrease the complexity of the 3D displacement curve and that peaks in curvature are consistently present in microgravity and on the ground. However they do not seem to be perfectly related to the classical cardiac ejection timings from ICG.

Three dimensional ballistocardiography: methodology and results from microgravity and dry immersion

Balistocardiography was recorded in 3-D on a free floating astronaut in space as well as on healthy volunteers participating to a dry immersion study in a terrestrial laboratory. We demonstrate a new technique suitable for the analysis of 3-D BCG. The spatial curve of the displacement vector is analyzed instead of the three components of acceleration. The technique presented is invariant from the axis of representation and provides important novel physiological information.

Osmosensitive mechanisms contribute to the water drinking-induced pressor response in humans

BACKGROUND

Water drinking elicits a sympathetically mediated pressor response in multiple-system atrophy patients through an unknown mechanism. We reasoned that gastrointestinal distention, hyposomotic stimulation, or both contribute to the water-induced pressor response.

METHODS

We compared the response to normal saline and water on blood pressure in 10 patients with probable multiple-system atrophy. Patients featured moderate to severe autonomic dysfunction. EKG and finger arterial blood pressure were recorded continuously, and 500 mL normal saline and distilled water were each given in a single-blinded fashion. Fluids were applied through a previously inserted nasogastric tube within a 5-minute period.

RESULTS

Blood pressure began to increase within 10 minutes after water administration and reached a maximum after 20 minutes. Blood pressure did not change after saline administration. The blood pressure change after 20 minutes was 8 +/- 9/2 +/- 5 mmHg with water and -1 +/- 11/-1 +/- 7 mmHg with normal saline administration (p = 0.02 between interventions). Heart rate did not change with either intervention.

CONCLUSION

Ingestion of water elicits a greater pressor response than the ingestion of normal saline. Thus, gastric distention is probably not the crucial mechanisms for the water-induced pressor response. Instead, the response may be mediated through osmosensitive afferent structures in the gastrointestinal tract, portal vein, and liver.

The autonomous regulation system functional reserves evaluation in 7-day head down bedrest

The heart rate variability estimation was used for autonomous regulation diagnostic in the 7-day head down bedrest experiment (HDBR). The new device "Pneumocard" and the fix respirations test were applied. The growth of sympathetic activity of autonomous nervous system and reduction of functional reserves of regulation mechanisms were observed in the microgravity modeling by HDBR. The analysis of the individual data had shown, that at two volunteers with most significant growth of SI after the experiment the orthostatic intolerance was observed. The data received in experiment confirm a hypothesis that growth of sympathetic activity in microgravity is accompanied by reduction of regulation mechanisms functional reserves is prognostic unfavorable.

[Hypotension--harmless or dangerous? The "orthostasis test" gives the answer]

Patients with non-specific symptoms that manifest when standing, are commonly seen in the doctor's office. The suspected diagnosis orthostatic dysregulation, can be established on the basis of a careful history, physical examination and a standing test. Frequent symptoms of orthostatic dysregulation are orthostatic hypotension, neurocardiogenic syncope and orthostatic intolerance. If a disorder of the autonomic nervous system is suspected, the need for further investigations is evaluated on an individual basis. The aim of such further diagnostic procedures is to differentiate between mild forms of autonomic disturbance that are unlikely to affect the patient's prognosis (mild orthostatic intolerance, neurocardiogenic syncope) and severe forms of autonomic failure, to identify the affected parts of the autonomic nervous system (partial or global autonomic failure) and, whenever necessary, to initiate appropriate treatment.

A cross-system comparison of bacterial and fungal biomass in detritus pools of headwater streams

The absolute amount of microbial biomass and relative contribution of fungi and bacteria are expected to vary among types of organic matter (OM) within a stream and will vary among streams because of differences in organic matter quality and quantity. Common types of benthic detritus [leaves, small wood, and fine benthic organic matter (FBOM)] were sampled in 9 small (1st-3rd order) streams selected to represent a range of important controlling factors such as surrounding vegetation, detritus standing stocks, and water chemistry. Direct counts of bacteria and measurements of ergosterol (a fungal sterol) were used to describe variation in bacterial and fungal biomass. There were significant differences in bacterial abundance among types of organic matter with higher densities per unit mass of organic matter on fine particles relative to either leaves or wood surfaces. In contrast, ergosterol concentrations were significantly greater on leaves and wood, confirming the predominance of fungal biomass in these larger size classes. In general, bacterial abundance per unit organic matter was less variable than fungal biomass, suggesting bacteria will be a more predictable component of stream microbial communities. For 7 of the 9 streams, the standing stock of fine benthic organic matter was large enough that habitat-weighted reach-scale bacterial biomass was equal to or greater than fungal biomass. The quantities of leaves and small wood varied among streams such that the relative contribution of reach-scale fungal biomass ranged from 10% to as much as 90% of microbial biomass. Ergosterol concentrations were positively associated with substrate C:N ratio while bacterial abundance was negatively correlated with C:N. Both these relationships are confounded by particle size, i.e., leaves and wood had higher C:N than fine benthic organic matter. There was a weak positive relationship between bacterial abundance and streamwater soluble reactive phosphorus concentration, but no apparent pattern between either bacteria or fungi and streamwater dissolved inorganic nitrogen. The variation in microbial biomass per unit organic matter and the relative abundance of different types of organic matter contributed equally to driving differences in total microbial biomass at the reach scale.

Limited effect of systemic beta-blockade on sympathetic outflow

Central beta-adrenoreceptors may augment sympathetic outflow. We tested the hypothesis that beta-blockade attenuates central sympathetic outflow by inhibiting central adrenoreceptors. We studied 18 healthy controls (4 female, 14 male; age, 26+/-6 years, body mass index, 23+/-3 kg/m(2)). ECG, brachial, and finger arterial blood pressure, muscle sympathetic nerve activity, and respiration were measured continuously before and during complete beta-blockade. Subjects received a total intravenous dose of 0.21 mg/kg of propranolol in 15 minutes. Spontaneous baroreflex slopes were calculated using the sequence technique (BRSup, BRSdown). The sympathetic baroreflex slope was determined at baseline using phenylephrine and sodium nitroprusside infusions. The subjects underwent cold pressor testing before and during beta-blockade. The R-R interval increased from 861+/-119 ms at baseline to 952+/-141 ms during beta-blockade (P<0.01). Blood pressure was 117+/-9/65+/-8 mm Hg at baseline and 117+/-10/67+/-8 mm Hg during beta-Blockade (P=NS). beta-Blockade did not affect baroreflex sensitivity (BRSup: 21+/-10 versus 28+/-11 ms/mmHg, P<0.1; BRSdown: 17+/-8 versus 20+/-8 ms/mmHg, P=NS). Muscle sympathetic nerve activity increased significantly during beta-blockade (number of bursts/100 beats: 32+/-9 versus 40+/-14, P<0.05), compared with baseline. However, the operating points of the parasympathetic and sympathetic baroreflex during beta-blockade were on the baroreflex curves obtained at baseline. beta-Blockade blunted the heart rate response to cold pressor testing; blood pressure and muscle sympathetic nerve activity responses were similar. Our study demonstrates that propranolol does not cause an acute decrease in sympathetic activity in normotensive young subjects. This, observation is not consistent with an important tonic stimulatory effect of beta-adrenoreceptors in the brain.

Autonomic nervous system function in patients with monogenic hypertension and brachydactyly: a field study in north-eastern Turkey

Laboratory studies in patients with autosomal-dominant hypertension and brachydactyly showed increased sensitivity to sympathetic stimuli and severe abnormalities in baroreflex buffering. To further elucidate the mechanisms by which impaired baroreflex sensitivity could influence blood pressure (BP), we conducted autonomic testing under field conditions. We studied 17 hypertensive affected (13 to 48 years, BMI 22.7 +/- 6.5 kg/m(2), 160 +/- 23/98 +/- 15 mm Hg) and 12 normotensive non-affected (9 to 60 years, BMI 24.0 +/- 4.7 kg/m(2), 120 +/- 16/70 +/- 10 mm Hg) family members. Pulse intervals and finger BP were measured using the Portapres device. Valsalva ratio, the blood pressure overshoot during phase IV of the Valsalva manoeuver, the Ewing coefficient (RR30/15 ratio), and heart rate and BP variability were similar in affected and non-affected family members. Overall, baroreflex sensitivity calculated using the cross-spectral (BRSLF, BRSHF) and sequence techniques (BRS+, BRS-) was not different between the groups. However, in younger family members, BRS+ was 12 +/- 3.7 and 22 +/- 13 msec/mm Hg in affected and in non-affected family members, respectively. The decline in BRS with age and with increasing blood pressure was absent in affected family members. We conclude that autonomic reflex testing conducted under field conditions is not impaired in patients with monogenic hypertension and brachydactyly. However, noninvasive testing showed impaired baroreflex control of heart rate at a young age. The reduced BRS in young family members with moderate arterial hypertension may suggest that the impaired baroreflex function is not secondary to the hypertension but rather a primary abnormality, which aggravates the progression of hypertension.

Spontaneous baroreflex sensitivity and heart rate variability are not superior to classic autonomic testing in older patients with type 2 diabetes

BACKGROUND

Early detection of cardiac autonomic neuropathy (CAN) permits individual risk stratification. Spontaneous heart rate variability (HRV) and baroreflex sensitivity (BRS) are suggested to be superior to classic autonomic testing in that they detect CAN earlier, with greater reliability, and do not require the patient's undue attention.

METHODS

To test that hypothesis, we studied 53 diabetic patients (mean age, 55 years) and 38 age-matched healthy control subjects (HC). Subjects underwent deep breathing, Valsalva maneuver, and orthostatic testing. Each abnormal test was counted as 1 point. A change in systolic blood pressure during standing of more than 10 mm Hg was graded with a single point; a decrease of more than 20 mm Hg received 2 points. A total score of zero was regarded as no CAN (noCAN), a score > or =4 as severe CAN (sCAN), and scores of 1 to 3 as mild CAN (mCAN). Spontaneous BRS was determined using the sequence technique. HRV was calculated as coefficient of variation (CV), high frequency power (HF) and low frequency power (LF).

RESULTS

Mean group values for HRV and BRS were: CV = 3.9+/-1.3; 4.0+/-1.3; 2.4+/-1.1; and 1.2+/-0.4; BRS = 8+/-3; 8+/-5; 5+/-2; and 2+/-2 msec/mm Hg for HC n = 38, noCAN n = 15, mCAN n = 26, and sCAN n = 12, respectively. BRS was similar in HC and patients with noCAN. In sCAN, BRS detected only 10 of 12 patients. HRV and BRS did not improve reclassification based on discriminant analysis.

CONCLUSION

BRS and HRV did not detect CAN in older diabetic patients better than classic autonomic testing.

Interaction between beta-adrenergic receptor stimulation and nitric oxide release on tissue perfusion and metabolism

Nitric oxide (NO) may be an important modulator of sympathetic tone. We used im and sc microdialysis in humans to characterize the interaction of NO synthase inhibition and adrenoreceptor stimulation on tissue perfusion, metabolism, and norepinephrine release. Microdialysis probes were perfused with L- or D-nitro-L-arginine-methyl-ester (100 micromol/L) followed by incremental doses of isoproterenol, epinephrine, or nitroprusside. Blood flow was estimated based on the ethanol dilution technique. In muscle, the increase in blood flow with isoproterenol was abolished by L-NAME. The ethanol ratio was 0.03 +/- 0.011 with D-NAME and 0.075 +/- 0.014 with L-NAME during isoproterenol treatment (1 micromol/L). The effect was less pronounced in adipose tissue. The vasodilatory effect of nitroprusside was similar with D- and L-NAME. L-NAME augmented isoproterenol- and epinephrine-induced glycerol release. Dialysate glycerol during 1 micromol/L isoproterenol was 47 +/- 6.7 micromol/L with D-NAME and 72 +/- 15 micromol/L with L-NAME. In skeletal muscle, dialysate norepinephrine during 1 micromol/L isoproterenol treatment was 0.73 +/- 0.17 and 1.3 +/- 0.15 nmol/L with D- and L-NAME, respectively. We conclude that NO synthase inhibition attenuates beta(2)-adrenoreceptor-mediated vasodilation and enhances beta-adrenoreceptor-mediated lipolysis. These effects are in part mediated through an increase in interstitial norepinephrine concentrations. The data are consistent with the idea that in humans, NO is important in modulating and ameliorating sympathetic effects in peripheral tissues.

Genetic influences on baroreflex function in normal twins

Blood pressure and heart rate are strongly influenced by genetic factors; however, despite the pivotal role of genetics in short-term cardiovascular regulation, little is known about the genetic contribution to baroreflex function. We assessed genetic influence on baroreflex sensitivity (BRS) in 149 twin pairs (88 monozygotic of age 33+/-13 years and BMI 23+/-4 kg/m(2) and 61 dizygotic of age 33+/-11 years and BMI 24+/-4 kg/m(2)). ECG and finger arterial blood pressures were measured continuously under resting conditions. BRS values were calculated by use of cross-spectral analysis (baroreflex slope calculated as mean value of transfer function between systolic blood pressure and the R-R interval in the low-frequency band [BRSLF] and baroreflex slope calculated as the mean value of transfer function between systolic blood pressure and R-R interval in the respiratory frequency band [BRSHF]) and the sequence technique (BRS+, BRS-). Heritability (h(2)) was estimated with a path-modeling approach. BRS values did not differ significantly between groups (monozygotic, BRSLF, 17+/-13; BRSHF, 21+/-18; BRS+, 19+/-16; and BRS-, 21+/-15, and dizygotic, BRSLF, 16+/-9; BRSHF, 20+/-14; BRS+, 18+/-10; and BRS-, 20+/-11 ms/mm Hg), and were significantly correlated (P:<0.001). When variances and covariances for monozygotic and dizygotic twins were compared, significant correlations were found for BRS in monozygotic (range, r=0.38 to 0.48) but not in dizygotic twin pairs (r=-0.03 to 0.09). Thus, BRS is heritable; the variability can be explained by genetic influences (P:<0.01; h(2) range, 0.36 to 0.44). The genetic influence on BRS remained strong after correction for BMI and blood pressure. Therefore, BRS is strongly genetically determined, probably by different genes than are resting blood pressure and BMI.

Severely impaired baroreflex-buffering in patients with monogenic hypertension and neurovascular contact

BACKGROUND

We identified a family with a monogenic syndrome of hypertension, brachydactyly, and neurovascular contact of the brain stem. Neurovascular contact of the ventrolateral medulla may lead to arterial hypertension by interfering with baroreflex function.

METHODS AND RESULTS

In 5 patients with monogenic hypertension (18 to 34 years old), we conducted detailed autonomic function tests. Blood pressure during complete ganglionic blockade was 134+/-4.9/82+/-4.1 mm Hg and 90+/-6/49+/-2.4 mm Hg in patients and in control subjects, respectively. During ganglionic blockade, plasma vasopressin concentration increased 24-fold in control subjects and <2-fold in patients. In patients, cold pressor testing, hand-grip testing, and upright posture all increased blood pressure excessively. In contrast, muscle sympathetic nerve activity was not increased at rest or during cold pressor testing. The phenylephrine dose that increased systolic blood pressure 12.5 mm Hg was 8.0+/-2.0 microg in patients and 135+/-35 microg in control subjects before ganglionic blockade and 5.4+/-0.4 microg in patients and 13+/-4.8 microg in control subjects during ganglionic blockade.

CONCLUSIONS

In patients with monogenic hypertension and neurovascular contact, basal blood pressure was increased even during sympathetic and parasympathetic nerve traffic interruption. However, sympathetic stimuli caused an excessive increase in blood pressure. This excessive response cannot be explained by increased sympathetic nerve traffic or increased vascular sensitivity. Instead, we suggest that baroreflex buffering and baroreflex-mediated vasopressin release are severely impaired.

Reference values of indices of spontaneous baroreceptor reflex sensitivity

Spontaneous baroreceptor reflex sensitivity (BRS) is a well established method for determining baroreflex function, which can be used to assess the potential impact on survival after myocardial infarction, to detect autonomic dysfunction in diabetic patients and in human essential hypertension. The assessment of impaired spontaneous baroreflex function in individual patients contains important clinical information, but age-dependent reference values are still lacking. In the present study we evaluated spontaneous BRS in healthy human controls to determine reference values as a function of age. Two hundred and sixty-two healthy volunteers divided into six age groups (I: <20 years, f = 11, m = 9, II: 20-29 years, f = 42, m = 37, III: 30-39 years, f = 23, m = 37, IV: 40-49 years, f = 27, m = 22, V: 50-59 years, f = 19, m = 17, VI: 60-69 years, f = 5, m = 13). Electrocardiograms (ECG) and finger arterial BP were measured with each subject in the supine position (sup, 7 min) and during deep breathing (dB, 6/min, 15 cycles). BRS was assessed using the sequence technique and the alpha coefficients as obtained from a power spectrum density estimate. Due to the normal logarithmic distribution of the BRS, the limits for impaired baroreflex function at rest were defined from logarithmic data. The limits for the BRS at rest (P = .025) were calculated as (-0.0283 x age) + 2.5198 for the sequence technique. We did not find significant differences in BRS among the female and male healthy volunteers. Our analysis of the six age groups showed the expected significant decrease in BRS, which was most prominent at the transition from group III (<40 years) to group IV (<50 years). BRS at rest and during deep breathing as well as sequential and spectral BRS indices did differ significantly. The results underline the necessity of reference values to evaluate impaired baroreflex function in individual patients.

Oscillatory patterns in sympathetic neural discharge and cardiovascular variables during orthostatic stimulus

BACKGROUND

We tested the hypothesis that a common oscillatory pattern might characterize the rhythmic discharge of muscle sympathetic nerve activity (MSNA) and the spontaneous variability of heart rate and systolic arterial pressure (SAP) during a physiological increase of sympathetic activity induced by the head-up tilt maneuver.

METHODS AND RESULTS

Ten healthy subjects underwent continuous recordings of ECG, intra-arterial pressure, respiratory activity, central venous pressure, and MSNA, both in the recumbent position and during 75 degrees head-up tilt. Venous samplings for catecholamine assessment were obtained at rest and during the fifth minute of tilt. Spectrum and cross-spectrum analyses of R-R interval, SAP, and MSNA variabilities and of respiratory activity provided the low (LF, 0.1 Hz) and high frequency (HF, 0.27 Hz) rhythmic components of each signal and assessed their linear relationships. Compared with the recumbent position, tilt reduced central venous pressure, but blood pressure was unchanged. Heart rate, MSNA, and plasma epinephrine and norepinephrine levels increased, suggesting a marked enhancement of overall sympathetic activity. During tilt, LF(MSNA) increased compared with the level in the supine position; this mirrored similar changes observed in the LF components of R-R interval and SAP variabilities. The increase of LF(MSNA) was proportional to the amount of the sympathetic discharge. The coupling between LF components of MSNA and R-R interval and SAP variabilities was enhanced during tilt compared with rest.

CONCLUSIONS

During the sympathetic activation induced by tilt, a similar oscillatory pattern based on an increased LF rhythmicity characterized the spontaneous variability of neural sympathetic discharge, R-R interval, and arterial pressure.

Yohimbine in neurally mediated syncope. Pathophysiological implications

In this study, we evaluated if increased sympathetic stimulation is an essential requirement for the development of neurally mediated syncope (NMS) by manipulating overall sympathetic outflow in subjects susceptible to tilt-induced syncope. Eight previously characterized patients with recurrent NMS (five females and three males; 34+/-2 yr) were recruited from the Vanderbilt Syncope Unit and eight age-matched controls underwent initial administration of clonidine (CLO) or yohimbine (YHO). This was done, prospectively, to determine doses of these agents that would increase or decrease plasma norepinephrine levels by >/= 30%. On a different day, in all subjects we determined intraarterial blood pressure, EKG and muscle sympathetic nerve activity (MSNA) both supine and during upright tilt. After this, subjects randomly received either CLO or YHO, and 3 h later another tilt was performed. After 1 wk, a similar procedure with the other drug was performed. During the two basal tilts, all the control subjects completed the study, whereas all the NMS patients developed syncope. Reduction in sympathetic tone by CLO resulted in a decreased tolerance to tilt in three out of eight controls and in all the NMS patients. In contrast, YHO not only increased basal plasma NorEpi levels and MSNA, but also prevented syncope in seven out of eight patients. In a selected population of patients, increased sympathetic activity is not a prerequisite for the development of syncope. Yohimbine-induced enhancement of sympathetic tone in patients with NMS improves orthostatic tolerance and raises the possibility that this drug may be a useful agent in the treatment of NMS.

Impaired activation of the baroreflex loop as early sign of sympathetic damage in diabetics with normal heart rate variability at rest

The objective of the study was to define the impairment of sympathovagal balance in patients with diabetes mellitus (DM) and coronary heart disease (CHD) compared to healthy controls (HC) showing similar heart rate variability (HRV) at supine rest. 88 DM (41 m, 47 f; age 62 +/- 1 years; BMI 27.1 +/- 1.5 kg/m2; HbA1c 7.9 +/- 0.4%), 49 CHD (27 m, 22 f; age 62 +/- 1 years; BMI 27.1 +/- 1.6 kg/m2; HbA1c 5.2 +/- 0.1%) and 16 HC (8 m, 8 f; age 59 +/- 1 years; BMI 26.4 +/- 0.5 kg/m2; HbA1c 5.0 +/- 0.1%) were investigated. Time series of heart period duration (HPD) were obtained during 2 min deep breathing (6/min), 5 min of supine rest and for 5 min at upright position using a RR memory device (BHL 6000, Baumann-Haldi Switzerland, modified ECG lead, 1 kHz). Mean HPD, coefficient of variation (CV), total power (TP) and integral power in the HF (0.15 to 0.5 Hz), MF (0.05 to 0.15 Hz) and LF (0.015 to 0.05 Hz) frequency bands as well as (MF-HF)/(MF + HF) as spectral index were calculated. As to be expected we found significantly lower values of CV, TP and HPD in DM compared to HC. The CV of HRV did not differ significantly between DM and CHD but TP and HPD of CHD patients were significantly higher in comparison to DM. Therefore, the deterioration of HRV was most pronounced in the DM group. For further analysis we calculated data of subjects with CV's in the upper quartile (> or = 3.52) of the CV at supine rest. The aim of this procedure was to compare subjects with similar high HRV at supine rest. With this method we obtained from all subjects 12 HC, 11 DM and 12 CHD. These DM had a significant decrease of CV, TP and the integral power at the HF frequency band during active orthostasis compared to HC and CHD. The spectral index increased significantly during standing in HC and CHD but was unchanged in DM. These changes were accompanied by a nearly similar increase of HRV during deep breathing. In conclusion, DM with normal reaction to deep breathing did not activate the sympathetic baroreflex loop during active orthostasis. This could be an early sign of sympathetic dysfunction in DM with normal HRV at supine rest.