Pediatric patients diagnosed as overweight and obese have an elevated risk of dyspnea.

We investigated whether pediatric patients with overweight and obesity are more likely to have dyspnea compared with those who are non-overweight. We collected de-identified data from TriNetX, a global federated multicenter research database, using both the UT Southwestern Medical Center and multinational Research Networks. Our analysis focused on patients aged 8-12 years. We identified overweight and obesity using ICD-10-CM codes E66 and dyspnea using code R06.0. Patients with overweight and obesity had a significantly higher risk of dyspnea compared with those who were non-overweight. This association was observed in both the UT Southwestern Network (risk ratio: 1.81, p < 0.001) and the Research Network (risk ratio: 2.70, p < 0.001). Furthermore, within the UT Southwestern Network, the risk was found to be higher in females compared with males (risk ratio: 2.17 vs. 1.67). These results have significant clinical implications, suggesting that clinicians should consider overweight and obesity as independent risk factors for dyspnea in pediatric patients after excluding other possible contributing factors.

Elevated risk of dyspnea in adults with obesity.

We investigated whether older adults (OA) with obesity are more likely to have dyspnea compared with OA without obesity, and whether OA with obesity are at a greater risk of having dyspnea compared with middle-aged (MA) and younger adults (YA) with obesity. We obtained de-identified data from the TriNetX UT Southwestern Medical Center database. We identified obesity and dyspnea using ICD-10-CM codes E66 and R06.0, respectively. Patients were separated into three age groups: OA, (65-75 y.o.), MA (45-55 y.o.), and YA (25-35 y.o). Within these groups, those with and without obesity or dyspnea were identified for analysis. The risk of dyspnea was greater in OA (risk ratio: 3.64), MA (risk ratio: 3.52), and YA (risk ratio: 2.76) with obesity compared with age-matched patients without obesity (all p < 0.01). The risk of dyspnea was greater in OA and MA with obesity compared with YA with obesity (both p < 0.001 vs. YA). These findings suggest that clinicians should consider obesity as an independent risk factor for dyspnea.

Noninvasive Assessment of Cardiac Output: Accuracy and Precision of the Closed-Circuit Acetylene Rebreathing Technique for Cardiac Output Measurement.

Background Accurate assessment of cardiac output is critical to the diagnosis and management of various cardiac disease states; however, clinical standards of direct Fick and thermodilution are invasive. Noninvasive alternatives, such as closed-circuit acetylene (C2H2) rebreathing, warrant validation. Methods and Results We analyzed 10 clinical studies and all available cardiopulmonary stress tests performed in our laboratory that included a rebreathing method and direct Fick or thermodilution. Studies included healthy individuals and patients with clinical disease. Simultaneous cardiac output measurements were obtained under normovolemic, hypovolemic, and hypervolemic conditions, along with submaximal and maximal exercise. A total of 3198 measurements in 519 patients were analyzed (mean age, 59 years; 48% women). The C2H2 method was more precise than thermodilution in healthy individuals with half the typical error (TE; 0.34 L/min [r=0.92] and coefficient of variation, 7.2%) versus thermodilution (TE=0.67 [r=0.70] and coefficient of variation, 13.2%). In healthy individuals during supine rest and upright exercise, C2H2 correlated well with thermodilution (supine: r=0.84, TE=1.02; exercise: r=0.82, TE=2.36). In patients with clinical disease during supine rest, C2H2 correlated with thermodilution (r=0.85, TE=1.43). C2H2 was similar to thermodilution and nitrous oxide (N2O) rebreathing technique compared with Fick in healthy adults (C2H2 rest: r=0.85, TE=0.84; C2H2 exercise: r=0.87, TE=2.39; thermodilution rest: r=0.72, TE=1.11; thermodilution exercise: r=0.73, TE=2.87; N2O rest: r=0.82, TE=0.94; N2O exercise: r=0.84, TE=2.18). The accuracy of the C2H2 and N2O methods was excellent (r=0.99, TE=0.58). Conclusions The C2H2 rebreathing method is more precise than, and as accurate as, the thermodilution method in a variety of patients, with accuracy similar to an N2O rebreathing method approved by the US Food and Drug Administration.

Aging women and their endothelium: probing the relative role of estrogen on vasodilator function.

Despite significant decreases in cardiovascular disease (CVD) mortality in the past three decades, it still remains the leading cause of death in women. Following menopause and the accompanying loss of estrogen, women experience a unique, accelerated rise in CVD risk factors. Dysfunction of the endothelium represents an important antecedent to CVD development, with rapid declines in endothelial vasodilator function reportedly taking place across the menopause transition. Importantly, the decline in endothelial function is independent of chronological age and is associated with estrogen deficiency. Estrogen-mediated effects, including increasing nitric oxide bioavailability and attenuating oxidative stress and inflammation, contribute to preserving endothelial health. This review will discuss studies that have probed the role of estrogen on endothelial vasodilator function in women at discrete stages of the menopause transition and the effects of estradiol supplementation in postmenopausal women. Estrogen receptor signaling is also an important aspect of endothelial function in women, and studies suggest that expression is reduced with both acute and prolonged estrogen deficiency. Changes in regulatory mechanisms of estrogen receptor-α expression as well as sensitivity to estrogen may underlie the differential effects of estrogen therapy in early (≤5 yr past final menstrual period) and late postmenopausal women (>5 yr past final menstrual period). Lastly, this review presents potential therapeutic targets that include increasing l-arginine bioavailability and estrogen receptor activation to prevent endothelial dysfunction in postmenopausal women as a strategy for decreasing CVD mortality in this high-risk population.

The Relationship Between Iron Deficiency Anemia and Sensorineural Hearing Loss in the Pediatric and Adolescent Population.

PURPOSE: A correlation between iron deficiency anemia (IDA) and sudden sensorineural hearing loss (SNHL) was described in adults. In this study, we examined if there is a relationship between IDA and hearing loss in the pediatric population. METHOD: This was a retrospective cohort study of data collected from the Informatics for Integrating Biology and the Bedside database from 2011 to 2016. Children and adolescents 4-21 years old seen at Penn State Milton S. Hershey Medical Center, Hershey, PA, were examined for hearing loss and IDA status. Hearing loss was determined by International Classification of Disease-9 and -10 codes, and IDA was determined by both low hemoglobin and serum ferritin levels for age and sex. RESULTS: We identified 20,113 patients. Prevalence of hearing loss and IDA was 1.7% and 2.3%, respectively. The prevalence of all hearing loss was 3.0% in the IDA cohort and 1.7% in those without IDA. Children and adolescents with IDA are at increased odds of developing SNHL (adjusted odds ratio: 3.67, 95% CI [1.60-7.30]). CONCLUSIONS: Children with IDA demonstrate increased likelihood of SNHL. Although correction of IDA in those with hearing loss has yet to be linked to improvements in hearing outcomes, screening for and correcting IDA among pediatric patients will positively affect overall health status.Supplemental Material: https://doi.org/10.23641/asha.5087071.

Association of Iron Deficiency Anemia With Hearing Loss in US Adults.

Importance: Hearing loss in the US adult population is linked to hospitalization, poorer self-reported health, hypertension, diabetes, and tobacco use. Because iron deficiency anemia (IDA) is a common and easily correctable condition, further understanding of the association between IDA and all types of hearing loss in a population of US adults may help to open new possibilities for early identification and appropriate treatment. Objective: To evaluate the association between sensorineural hearing loss (SNHL) and conductive hearing loss and IDA in adults aged 21 to 90 years in the United States. Design, Setting, and Participants: The prevalence of IDA and hearing loss (International Classification of Diseases, Ninth Revision codes 389.1 [SNHL], 389.0 [conductive hearing loss], and 389 [combined hearing loss]) was identified in this retrospective cohort study at the Penn State Milton S. Hershey Medical Center, Hershey, Pennsylvania. Iron deficiency anemia was determined by low hemoglobin and ferritin levels for age and sex in 305 339 adults aged 21 to 90 years. Associations between hearing loss and IDA were evaluated using χ2 testing, and logistic regression was used to model the risk of hearing loss among those with IDA. The study was conducted from January 1, 2011, to October 1, 2015. Main Outcomes and Measures: Hearing loss. Results: Of 305 339 patients in the study population, 132 551 were men (43.4%); mean (SD) age was 50.1 (18.5) years. There was a 1.6% (n = 4807) prevalence of combined hearing loss and 0.7% (n = 2274) prevalence of IDA. Both SNHL (present in 26 of 2274 individuals [1.1%] with IDA; P = .005) and combined hearing loss (present in 77 [3.4%]; P < .001) were significantly associated with IDA. Logistic regression analysis confirmed increased odds of SNHL (adjusted odds ratio [OR], 1.82; 95% CI, 1.18-2.66) and combined hearing loss (adjusted OR, 2.41; 95% CI, 1.90-3.01) among adults with IDA, after adjusting for sex. Conclusions and Relevance: Iron deficiency anemia was associated with SNHL and combined hearing loss in a population of adult patients. Further research is needed to better understand the potential links between IDA and hearing loss and whether screening and treatment of IDA in adults could have clinical implications in patients with hearing loss.

Respiratory modulation of human autonomic function on Earth.

KEY POINTS: We studied healthy supine astronauts on Earth with electrocardiogram, non-invasive arterial pressure, respiratory carbon dioxide concentrations, breathing depth and sympathetic nerve recordings. The null hypotheses were that heart beat interval fluctuations at usual breathing frequencies are baroreflex mediated, that they persist during apnoea, and that autonomic responses to apnoea result from changes of chemoreceptor, baroreceptor or lung stretch receptor inputs. R-R interval fluctuations at usual breathing frequencies are unlikely to be baroreflex mediated, and disappear during apnoea. The subjects' responses to apnoea could not be attributed to changes of central chemoreceptor activity (hypocapnia prevailed); altered arterial baroreceptor input (vagal baroreflex gain declined and muscle sympathetic nerve burst areas, frequencies and probabilities increased, even as arterial pressure climbed to new levels); or altered pulmonary stretch receptor activity (major breathing frequency and tidal volume changes did not alter vagal tone or sympathetic activity). Apnoea responses of healthy subjects may result from changes of central respiratory motoneurone activity. ABSTRACT: We studied eight healthy, supine astronauts on Earth, who followed a simple protocol: they breathed at fixed or random frequencies, hyperventilated and then stopped breathing, as a means to modulate and expose to view important, but obscure central neurophysiological mechanisms. Our recordings included the electrocardiogram, finger photoplethysmographic arterial pressure, tidal volume, respiratory carbon dioxide concentrations and peroneal nerve muscle sympathetic activity. Arterial pressure, vagal tone and muscle sympathetic outflow were comparable during spontaneous and controlled-frequency breathing. Compared with spontaneous, 0.1 and 0.05 Hz breathing, however, breathing at usual frequencies (∼0.25 Hz) lowered arterial baroreflex gain, and provoked smaller arterial pressure and R-R interval fluctuations, which were separated by intervals that were likely to be too short and variable to be attributed to baroreflex physiology. R-R interval fluctuations at usual breathing frequencies disappear during apnoea, and thus cannot provide evidence for the existence of a central respiratory oscillation. Apnoea sets in motion a continuous and ever changing reorganization of the relations among stimulatory and inhibitory inputs and autonomic outputs, which, in our study, could not be attributed to altered chemoreceptor, baroreceptor, or pulmonary stretch receptor activity. We suggest that responses of healthy subjects to apnoea are driven importantly, and possibly prepotently, by changes of central respiratory motoneurone activity. The companion article extends these observations and asks the question, Might terrestrial responses to our 20 min breathing protocol find expression as long-term neuroplasticity in serial measurements made over 20 days during and following space travel?

Respiratory modulation of human autonomic function: long-term neuroplasticity in space.

KEY POINTS: We studied healthy astronauts before, during and after the Neurolab Space Shuttle mission with controlled breathing and apnoea, to identify autonomic changes that might contribute to postflight orthostatic intolerance. Measurements included the electrocardiogram, finger photoplethysmographic arterial pressure, respiratory carbon dioxide levels, tidal volume and peroneal nerve muscle sympathetic activity. Arterial pressure fell and then rose in space, and drifted back to preflight levels after return to Earth. Vagal metrics changed in opposite directions: vagal baroreflex gain and two indices of vagal fluctuations rose and then fell in space, and descended to preflight levels upon return to Earth. Sympathetic burst frequencies (but not areas) were greater than preflight in space and on landing day, and astronauts' abilities to modulate both burst areas and frequencies during apnoea were sharply diminished. Spaceflight triggers long-term neuroplastic changes reflected by reciptocal sympathetic and vagal motoneurone responsiveness to breathing changes. ABSTRACT: We studied six healthy astronauts five times, on Earth, in space on the first and 12th or 13th day of the 16 day Neurolab Space Shuttle mission, on landing day, and 5-6 days later. Astronauts followed a fixed protocol comprising controlled and random frequency breathing and apnoea, conceived to perturb their autonomic function and identify changes, if any, provoked by microgravity exposure. We recorded the electrocardiogram, finger photoplethysmographic arterial pressure, tidal carbon dioxide concentrations and volumes, and peroneal nerve muscle sympathetic activity on Earth (in the supine position) and in space. (Sympathetic nerve recordings were made during three sessions: preflight, late mission and landing day.) Arterial pressure changed systematically from preflight levels: pressure fell during early microgravity exposure, rose as microgravity exposure continued, and drifted back to preflight levels after return to Earth. Vagal metrics changed in opposite directions: vagal baroreflex gain and two indices of vagal fluctuations (root mean square of successive normal R-R intervals; and proportion of successive normal R-R intervals greater than 50 ms, divided by the total number of normal R-R intervals) rose significantly during early microgravity exposure, fell as microgravity exposure continued, and descended to preflight levels upon return to Earth. Sympathetic mechanisms also changed. Burst frequencies (but not areas) during fixed frequency breathing were greater than preflight in space and on landing day, but their control during apnoea was sharply altered: astronauts increased their burst frequencies from already high levels, but they could not modulate either burst areas or frequencies appropriately. Space travel provokes long-lasting sympathetic and vagal neuroplastic changes in healthy humans.

Caloric restriction diminishes the pressor response to static exercise.

BACKGROUND: Astronauts in space consume fewer calories and return to earth predisposed to orthostatic intolerance. The role that caloric deficit plays in the modulation of autonomic control of the cardiovascular system is unknown. Therefore, the purpose of this study was to determine the effects of 6° head-down bedrest (an analog of spaceflight) with a hypocaloric diet (25 % caloric restriction) (CR) on autonomic neural control during static handgrip (HG) and cold pressor (CP) tests. Nine healthy young men participated in a randomized crossover bedrest (BR) study, consisting of four, two-week interventions (hypocaloric ambulatory, hypocaloric bedrest, normocaloric ambulatory, and normocaloric bedrest), each separated by 5 months. Heart rate (HR), arterial pressure, and muscle sympathetic nerve activity (MSNA) were recorded before, during, and after HG (40 % of maximum voluntary contraction to fatigue), post-exercise muscle ischemia (forearm occlusion), and CP. Bedrest and nutritional combinations were compared using two-way ANOVA with repeated measures. RESULTS: HR, MSNA, and the change in systolic blood pressure during HG were attenuated with caloric restriction, but post-intervention responses for all groups were similar during post-exercise muscle ischemia. CR was associated with a higher diastolic blood pressure during CP; however, HR was directionally opposite (i.e., increase with BR, decrease with CR). CONCLUSIONS: In summary 14-day caloric/fat restriction attenuated MSNA and pressor responses during isometric exercise to fatigue but not to post-exercise muscle ischemia. This indicates that the integrity of the metaboreflex is maintained whereas the influence of the mechanoreflex and/or central command may be reduced.

Incorporating Informatics for Integrating Biology and the Bedside (i2b2) into Predoctoral Trainee Curriculum to Evaluate Student-Generated Hypotheses.

As part of the Clinical and Translational Science Institute predoctoral TL1 training program at the Pennsylvania State University, a multidisciplinary team of predoctoral trainees representing the Chemistry, Neurosurgery, Nutritional Sciences, and Public Health Sciences departments were introduced to the NIH-sponsored Informatics for Integrating Biology and the Bedside (i2b2) database to test the following student-generated hypothesis: children with iron deficiency anemia (IDA) are at increased risk of attention deficit-hyperactivity disorder (ADHD). Children aged 4-12 and 4-17 years were categorized into IDA and control groups. De-identified medical records from the Penn State Milton S. Hershey Medical Center (HMC) and the Virginia Commonwealth University Medical Center (VCUMC) were used for the analysis. Overall, ADHD prevalence at each institution was lower than 2011 state estimates. There was a significant association between IDA and ADHD in the 4-17-year-old age group for all children (OR: 1.902 [95% CI: 1.363-2.656]), Caucasian children (OR: 1.802 [95% CI: 1.133-2.864]), and African American children (OR: 1.865 [95% CI: 1.152-3.021]). Clinical and Translational Science Award (CTSA) infrastructure is particularly useful for trainees to answer de novo scientific questions with minimal additional training and technical expertise. Moreover, projects can be expanded by collaborating within the CTSA network.

Caloric restriction decreases orthostatic tolerance independently from 6° head-down bedrest.

Astronauts consume fewer calories during spaceflight and return to earth with an increased risk of orthostatic intolerance. Whether a caloric deficiency modifies orthostatic responses is not understood. Thus, we determined the effects of a hypocaloric diet (25% caloric restriction) during 6° head down bedrest (an analog of spaceflight) on autonomic neural control during lower body negative pressure (LBNP). Nine healthy young men completed a randomized crossover bedrest study, consisting of four (2 weeks each) interventions (normocaloric bedrest, normocaloric ambulatory, hypocaloric bedrest, hypocaloric ambulatory), each separated by 5 months. Muscle sympathetic nerve activity (MSNA) was recorded at baseline following normocaloric and hypocaloric interventions. Heart rate (HR) and arterial pressure were recorded before, during, and after 3 consecutive stages (7 min each) of LBNP (-15, -30, -45 mmHg). Caloric and posture effects during LBNP were compared using two-way ANOVA with repeated measures. There was a strong trend toward reduced basal MSNA following caloric restriction alone (normcaloric vs. hypocaloric: 22±3 vs. 14±4 burst/min, p = 0.06). Compared to the normocaloric ambulatory, both bedrest and caloric restriction were associated with lower systolic blood pressure during LBNP (p<0.01); however, HR responses were directionally opposite (i.e., increase with bedrest, decrease with caloric restriction). Survival analysis revealed a significant reduction in orthostatic tolerance following caloric restriction (normocaloric finishers: 12/16; hypocaloric finishers: 6/16; χ2, p = 0.03). Caloric restriction modifies autonomic responses to LBNP, which may decrease orthostatic tolerance after spaceflight.

Evidence for the emergence of leg sympathetic vasoconstrictor tone with age in healthy women.

While muscle sympathetic nerve activity (MSNA) is elevated with advancing age, correlational evidence suggests that, in contrast to men, basal MSNA is not related to resting lower limb hemodynamics in women. However, limited data exists in women that have attempted to directly assess the degree of limb sympathetic vasoconstrictor tone, and whether it is altered with age. To address this issue, we measured changes in femoral artery vascular conductance (FVC) during an acute sympatho-inhibitory stimulus (-60 mm Hg neck suction, NS) in groups of healthy younger (n = 8, 23 ± 1 years) and older (n = 7, 66 ± 1 years) women. The percent change in FVC in response to NS was significantly augmented in the older (P = 0.006 vs. young) women. Although NS caused no significant change (3 ± 3%, P = 0.33) in FVC in the young women, there was a robust increase in FVC (21 ± 5%, P = 0.003) in the old women. Collectively, these findings provide evidence that in women, leg sympathetic vasoconstrictor tone emerges with age.

Pneumatic antishock garment inflation activates the human sympathetic nervous system by abdominal compression.

Pneumatic antishock garments (PASG) have been proposed to exert their blood pressure-raising effect mechanically, i.e. by increasing venous return and vascular resistance of the lower body. We tested whether, alternatively, PASG inflation activates the sympathetic nervous system. Five men and four women wore PASG while mean arterial pressure (MAP), muscle sympathetic nerve activity (MSNA), heart rate and stroke volume were measured. One leg bladder (LEG) and the abdominal bladder (ABD) of the trousers were inflated individually and in combination (ABD+LEG), at 60 or 90 mmHg for 3 min. By the end of 3 min of inflation, conditions that included the ABD region caused significant increases in MAP in a dose-dependent fashion (7 ± 2, 8 ± 3, 14 ± 4 and 13 ± 5 mmHg for ABD60, ABD+LEG60, ABD90 and ABD+LEG90, respectively, P < 0.05). Likewise, inflation that included ABD caused significant increases in total MSNA compared with control values [306 ± 70, 426 ± 98 and 247 ± 79 units for ABD60, ABD90 and ABD+LEG90, respectively, P < 0.05 (units = burst frequency × burst amplitude]. There were no changes in MAP or MSNA in the LEG-alone conditions. The ABD inflation also caused a significant decrease in stroke volume (-11 ± 3 and -10 ± 3 ml per beat in ABD90 and ABD+LEG90, respectively, P < 0.05) with no change in cardiac output. Neither cardiopulmonary receptor deactivation nor mechanical effects can account for a slowly developing rise in both sympathetic activity and blood pressure during ABD inflation. Rather, these data provide direct evidence that PASG inflation activates the sympathetic nervous system secondarily to abdominal, but not leg, compression.

The location of the human volume indifferent point predicts orthostatic tolerance.

The volume indifferent point (VIP) is the point within the circulation where blood volume does not change with changes in posture. Because both volume and pressure are unaffected by posture at this point, its location should dictate the filling gradient to the heart. Previously we identified a contribution of the splanchnic circulation to its location. We experimentally manipulated blood volume in the splanchnic region to quantify changes in the VIP. Furthermore, we determined the relationship between the VIP and an individual's tolerance to an orthostatic stress. In Protocol 1, we found that administration of the somatostatin analog octreotide acetate, which elicits relatively selective splanchnic vasoconstriction, induced a superior shift in the VIP (+1.9 +/- 3.3 cm, P = 0.03). This finding corroborates previous reports of improvements in tilt tolerance after octreotide and suggests it might be related to relocation of the VIP. In Protocol 2, application of -20 mmHg lower body negative pressure (LBNP) induced splanchnic pooling and moved the VIP inferiorly (-6.0 +/- 7.2 cm, P < 0.01). LBNP combined with head-up tilt significantly decreased tilt tolerance (median tilt time: 28.0 vs. 4.2 min; Chi (2) = 14.29, P < 0.01); the change in the VIP predicted the reduction in tilt time (Deltatilt time = 3.05 + 0.12 DeltaVIP, P = 0.03). Thus, individuals with the largest inferior shift in the VIP also demonstrated the largest decrease in tilt table tolerance. We conclude that the splanchnic circulation plays an important role in determining the location of the VIP and the location of the VIP is a determinant of tolerance to orthostatic stress.

Sympathetic and haemodynamic responses to lipids in healthy human ageing.

Plasma non-esterified fatty acids (NEFAs) activate the sympathetic nervous system and increase vascular resistance and blood pressure (BP); however, the response with ageing is not known. The objectives of this study were to characterize the cardiovascular, neural and endocrine responses to acute elevation of NEFA concentration. Seventeen healthy older volunteers (7 male and 10 female; age, 69 +/- 1 years; body mass index, 24 +/- 0 kg m(2); values are means +/- s.e.m.) received a 4 h intravenous infusion of the lipid emulsion Intralipid 20% or placebo (single-blind, randomized, balanced order) on two different days separated by at least 2 weeks. Muscle sympathetic nerve activity (MSNA), heart rate (HR), BP, cardiac output, leptin, insulin, aldosterone, angiotensin II and F(2)-isoprostanes were measured. The change in HR (+8.8 +/- 0.9 versus +3.0 +/- 0.9 beats min(1)), systolic BP (+13.9 +/- 2.2 versus +6.6 +/- 2.4 mmHg) and diastolic BP (+7.4 +/- 1.5 versus +1.3 +/- 0.8 mmHg) was significantly greater after Intralipid versus placebo infusions (P < 0.001). Lipid infusion increased MSNA burst frequency (+6.7 +/- 1.6 bursts min(1)), total MSNA (+45%; P < 0.001) and concentrations of insulin (+40%), aldosterone (+50%) and F(2)-isoprostanes (+80%), but not leptin. Hyperlipidaemia caused directionally opposite responses for insulin (increased) and calf vascular resistance (decreased) in men, whereas insulin and calf vascular resistance responses were severely blunted and non-existent, respectively, in women. We conclude that direct vascular mechanisms and central sympathetic activation contribute to the NEFA pressor response; though absolute values are higher, the change is not different compared with previous studies in a younger population.

Identification of the human electrical impedance indifferent point: a surrogate for the volume indifferent point?

Head-up tilt (HUT) redistributes approximately 700 mL of blood to the dependent regions. In a gravitational field, hydrostatic pressure is balanced against vascular compliance, resulting in a hydrostatic indifferent point (HIP) whose location should be coincident with an indifferent point for volume (VIP). Cardiac filling is determined by the hydrostatic gradient between the HIP/VIP and right atrium. We employed segmental impedance to examine blood volume redistribution during HUT and estimate the location of the VIP. During HUT, impedance increased in the chest and decreased in the lower abdomen, presumably due to blood volume shifts. Using a non-linear model to relate blood volume shifts to the hydrostatic gradient, we estimated the location of the electrical impedance indifferent point between the xyphoid process and iliac crest, at 64.5 +/- 2.6% of an individual's height. This method may provide a quantitative framework to assess the effects of blood volume distribution on tilt tolerance.

Non-esterified fatty acids increase arterial pressure via central sympathetic activation in humans.

Previous studies have shown that acute increases in plasma NEFAs (non-esterified fatty acids) raise SVR (systemic vascular resistance) and BP (blood pressure). However, these studies have failed to distinguish between CNS (central nervous system) mechanisms that raise sympathetic activity and paracrine mechanisms that increase SVR directly, independent of CNS involvement. The aim of the present study was to directly determine whether the sympathetic nervous system contributes to the pressor response to NEFAs. On 2 days separated by at least 2 weeks, 17 lean healthy volunteers (ten male/seven female; age, 22+/-1 years; body mass index, 23+/-1 kg/m2; values are means+/-S.E.M.) received a 4-h intravenous infusion of 20% Intralipid or placebo (in a single-blind randomized balanced order). MSNA (muscle sympathetic nerve activity), HR (heart rate), BP (oscillometric brachial measurement) and Q (cardiac output; acetylene rebreathing) were measured before and throughout infusion. The change in HR (+8.2+/-1.0 and +2.4+/-1.2 beats/min), systolic BP (+14.0+/-1.6 and +3.2+/-2.5 mmHg) and diastolic BP (+8.2+/-1.0 and -0.1+/-1.7 mmHg) were significantly greater after the 4-h infusion of Intralipid compared with placebo (P<0.001). The change in BP with Intralipid resulted from an increase in SVR (Q/mean arterial pressure; P<0.001) compared with baseline, without a change in Q. MSNA burst frequency increased during Intralipid infusion compared with baseline (+4.9+/-1.3 bursts/min; P<0.05), and total MSNA (frequencyxamplitude) was augmented 65% (P<0.001), with no change during placebo infusion. Lipid infusion increased insulin, aldosterone and F2-isoprostane, but not leptin, concentrations. On the basis of the concomitant increase in BP, MSNA and SVR, we conclude that central sympathetic activation contributes to the pressor response to NEFAs.

Evidence for reduced sympatholysis in leg resistance vasculature of healthy older women.

Inhibition of a sympathetic stimulus (i.e., sympatholysis) during forearm exercise is reduced with age in women. This age-related alteration has not been characterized in the lower extremity vasculature of women, and the potential for blunting of the conduit artery dilatory response to a sudden increase in shear stress [flow-mediated dilation (FMD)] has not been examined in older adults of either sex. In the present study, we assessed popliteal artery diameter and velocity (Doppler ultrasound) in 16 young (23 +/- 1 yr) and 14 older (69 +/- 1 yr) women after 5 min of distal calf occlusion (FMD), 3 min of hand immersion in ice water [cold pressor test (CPT)], and 5 min of distal calf occlusion combined with hand immersion in ice water (FMD+CPT). Peak popliteal conductance after 5-min ischemia was not significantly different in young vs. older women. During the combined stimulus (FMD+CPT), the magnitude of vasoconstriction in the calf (reduction in peak popliteal artery conductance) was similar (5-8%), despite reduced resting adrenergic sensitivity to CPT [young (Y): -27.3 +/- 3.8%; older (O): -15.8 +/- 2.2%; P < 0.05] and blunted muscle sympathetic nerve activity responses to CPT (Y: 12.7 +/- 3.6 bursts/min; O: 7.8 +/- 2.5 bursts/min; P < 0.05) in older women. Popliteal FMD, normalized to the shear stimulus, was attenuated by 60-70% in older women. Peak popliteal diameter, measured during the combined stimulus (FMD+CPT), was blunted in young but not in older women (Y FMD: 5.5 +/- 0.1 mm; Y FMD+CPT: 5.4 +/- 0.1 mm; P = 0.03; O FMD: 5.8 +/- 0.2 mm; O FMD+CPT: 5.8 +/- 0.2 mm). These results confirm previous findings of diminished reactivity in the conduit arteries of older humans and provide the first evidence of reduced sympatholysis in the leg resistance vasculature of older women.

Human cerebral autoregulation before, during and after spaceflight.

Exposure to microgravity alters the distribution of body fluids and the degree of distension of cranial blood vessels, and these changes in turn may provoke structural remodelling and altered cerebral autoregulation. Impaired cerebral autoregulation has been documented following weightlessness simulated by head-down bed rest in humans, and is proposed as a mechanism responsible for postspaceflight orthostatic intolerance. In this study, we tested the hypothesis that spaceflight impairs cerebral autoregulation. We studied six astronauts approximately 72 and 23 days before, after 1 and 2 weeks in space (n = 4), on landing day, and 1 day after the 16 day Neurolab space shuttle mission. Beat-by-beat changes of photoplethysmographic mean arterial pressure and transcranial Doppler middle cerebral artery blood flow velocity were measured during 5 min of spontaneous breathing, 30 mmHg lower body suction to simulate standing in space, and 10 min of 60 deg passive upright tilt on Earth. Dynamic cerebral autoregulation was quantified by analysis of the transfer function between spontaneous changes of mean arterial pressure and cerebral artery blood flow velocity, in the very low- (0.02-0.07 Hz), low- (0.07-0.20 Hz) and high-frequency (0.20-0.35 Hz) ranges. Resting middle cerebral artery blood flow velocity did not change significantly from preflight values during or after spaceflight. Reductions of cerebral blood flow velocity during lower body suction were significant before spaceflight (P < 0.05, repeated measures ANOVA), but not during or after spaceflight. Absolute and percentage reductions of mean (+/- s.e.m.) cerebral blood flow velocity after 10 min upright tilt were smaller after than before spaceflight (absolute, -4 +/- 3 cm s(-1) after versus -14 +/- 3 cm s(-1) before, P = 0.001; and percentage, -8.0 +/- 4.8% after versus -24.8 +/- 4.4% before, P < 0.05), consistent with improved rather than impaired cerebral blood flow regulation. Low-frequency gain decreased significantly (P < 0.05) by 26, 23 and 27% after 1 and 2 weeks in space and on landing day, respectively, compared with preflight values, which is also consistent with improved autoregulation. We conclude that human cerebral autoregulation is preserved, and possibly even improved, by short-duration spaceflight.