Lower Body Positive Pressure Research Articles

Effects of Lower Body Positive Pressure on Blood Pressure and Heart Rate in College Age Students

Elucidating the Effects of 40 Minutes of Lower Body Positive Pressure on Blood Pressure and Heart Rate in College Age Students Lower Body Positive Pressure (LBPP) has become a popular exercise modality treatment in the physical therapy rehabilitation setting for lower extremities in post-injury/operative patients. PURPOSE: The purpose of this study was to investigate the effects of 40 minutes LBPP at a percent unloading of 75%, 50%, and 25% body weight on resting SBP/DBP and HR in a standing position in the Alter-G. METHODS: Subjects included 10 college-age students (3 males and 7 females) who were screened for this investigation and signed an approved Informed Consent Form through the University Institutional Review Board. All subjects were asked to sit for 5 minutes prior to testing. Any subject with a resting SBP/DBP greater than 139 mmHg/88 mmHg was excluded from participation in the study. Resting HR, SBP and DBP measurements were recorded every ten minutes at each percent body weight by using an electronic Omron Blood Pressure Monitor (Module HEM-790IT). Barometric pressure inside the Alter-G cockpit was recorded by a Thomas Scientific Hygrometer Thermometer Barometer DP. RESULTS: A repeated measures multivariate analysis of variance (MANOVA) was performed across conditions to analyze data at the p<.05 significance level. Mean barometric pressure within the Alter-G increased significantly from 755.85 mmHg at 100% body weight (BW) to 766.8 mmHg, 778.88 mmHg, and 790.92 mmHg at 75% BW, 50% BW, and 25% BW, respectively, (p<.001). Mean systolic blood pressure varied from 112.8 mmHg at 100% BW to 111.6 mmHg, 114.7 mmHg, and 114.3 mmHg at 75% BW, 50% BW, and 25% BW, respectively. Mean diastolic blood pressure varied from 76.3 mmHg at 100% BW to 74.8 mmHg, 75.8 mmHg, 75.7 mmHg at 75% BW, 50% BW, and 25% BW, respectively. Systolic and diastolic blood pressures were found not to be significant across conditions (p=.619), (p=.642). Mean heart rate varied from 81.6 bpm at 100% BW to 76.3 bpm, 74.1 bpm and 72.3 bpm at 75% BW, 50% BW, and 25% BW, respectively. Heart rate was found not to be significant through MANOVA analyses, (p<.161). CONCLUSION: The clinical relevance of such findings indicates that the LBPP Treadmills will not adversely affect systolic and diastolic parameters in a college-age population with 40 minutes of LBPP. Future studies should now include an older population with similar time exposure to LBPP to see if similar findings are found.

Influence Of Unweighting On Muscle Oxygenation And Vo2 During Running On A Lower-body Positive-pressure Treadmill

The influence of unweighting using a lower body positive pressure treadmill (PPT) on oxygen consumption (VO2) has been previously explored. However, it is unknown how different body weight settings (BWSet) in a PPT influence muscle oxygen saturation. PURPOSE:To describe the relationship between tissue oxygenation and VO2 while running across multiple BWSet on a PPT. METHODS: Six experienced runners participated in this study. Tissue saturation index (TSI), oxygenated hemoglobin (O2Hb), and deoxygenated hemoglobin (HHb) levels were obtained using a near infrared spectroscopy (NIRS) unit secured on the right vastus lateralis. VO2 data were collected using a metabolic cart. Subjects ran at a self-reported “easy” training pace for 4 minutes at 40, 60, 80, and 100% BWSet (randomized order), followed by 4 minutes of rest. Preceding each stage, subjects walked at 3 mph for 1 minute at 100% BWSet. The NIRS data from the last 30 seconds of each running stage were averaged and data from the last 30 seconds of each walking stage were subtracted to serve as a reference measurement. Linear mixed effects models were used to determine whether BWSet influenced VO2 and NIRS parameters. For the first model, BWSet was analyzed as a categorical variable and pairwise comparisons were then used to determine differences between each BWSet. For the second model, all data were normalized to 100% BWSet and analyzed as a continuous variable to allow development of regression models for each parameter. Slopes of the regression equations were then compared using Z-score estimates. RESULTS: When the data were modeled using BWSet categorically, BWSet significantly influenced VO2, TSI, O2Hb, and HHb (p<0.005 for each), and pairwise comparisons revealed significant (p<0.05) differences between each BWSet. When normalized data were modeled using BWSet as a continuous variable, there was a significant linear relationship for each parameter (p<0.005), but the slope for all NIRS parameters (β range: 0.010-0.12) were significantly greater than that for VO2 (β = 0.006). CONCLUSIONS: The results show that VO2 and tissue oxygenation change at different rates for varying BWSet on a PPT. Further research is necessary to determine if altered tissue oxygenation achieved on PPT influences training response.

Influence of Body Weight Support and Direction of Locomotion on Preferred Gait

Previously, no research has investigated differences in preferred gait characteristics during forwards and backwards running at varying body weight (BW) support. PURPOSE: To understand how BW support and direction of locomotion influence descriptors of preferred gait. METHODS: Subjects (n=8; 27.0±11.7 years, 173.8±3.5 cm, 74.0±8.7 kg) performed forward and backward running on a lower body positive pressure treadmill at different levels of BW support (i.e., 0%-80% body weight support). Subjects preferred running speed (PS), preferred stride frequency (PSF), and rate of perceived exertion (RPE) were measured. PS, PSF, and RPE were analyzed using a 2(exercise mode: forward and backward locomotion) x 5(BW support: 0%, 20%, 40%, 60%, and 80%) repeated measures analysis of variance (α= 0.05). RESULTS: PS, PSF, and RPE were not influenced by the interaction of exercise mode and BW support (p > 0.05). PS was different between modes (p < 0.001) and between BW support conditions (p < 0.01). For example, PS increased approximately 3.8%-8.5% and 3.0%-11.9% with decreasing BW support for forward and backward running, respectively. Additionally, PS during forward running was higher than that of backward running at reduced BW support (e.g., 3.4±.9 m/s and 2.5±.5 m/s for forward and backward running at 80% BW support, respectively). Furthermore, PSF (p< 0.001) and RPE (p< 0.01) were different between BW conditions but not different between direction of locomotion (p>0.05). Specifically, PSF (e.g., a decrease of 13.2% during forward running at 0% BW support vs. 80% BW support) and RPE (e.g., a decrease of 19.9% during forward running at 0% BW support vs. 80% BW support) during forward and backward running decreased with decreasing BW support. CONCLUSION: It seems that BW support may have more of an influence on preferred gait characteristics than direction of locomotion since PSF and RPE were not different between directions of locomotion.

Effects of unweighting and speed on in-shoe regional loading during running on a lower body positive pressure treadmill

The purpose of this study was to determine how unweighted running on a lower body positive pressure treadmill (LBPPT) modifies in-shoe regional loading. Ten experienced runners were fit with pressure distribution measurement insoles and ran at 100%, 120%, and 140% of self-reported easy training pace on a LBPPT at 20%, 40%, 60%, 80%, and 100% body weight percentage settings (BWSet). Speeds and BWSet were in random order. A linear mixed effect model (p<0.05 significance level) was used to compare differences in whole foot and regional maximum in-shoe plantar force (FMAX), impulse, and relative load distribution across speeds and BWSet. There were significant main effects (p<0.001) for running speed and BWSet for whole foot Fmax and impulse. The model revealed 1.4% and 0.24% increases in whole foot FMAX (times body weight) and impulse, respectively, for every unit increase in body weight percentage. There was a significant main effect for BWSet on Fmax and relative load (p<0.05) for each of the nine foot regions examined, though four regions were not different between 80% and 100% BWSet. There was a significant (p<0.001) main effect for BWSet on forefoot to rear foot relative load. Linear relationships were found between increases in BWSet and increases in-shoe Fmax and impulse, resulting from regional changes in foot pressure which represent a shift towards forefoot loading, most evident <80% BWSet. Estimating in-shoe regional loading parameters may be useful during rehabilitation and training to appropriately prescribe specific speed and body weight levels, without exceeding certain critical peak force levels while running.

The influence of Metabo‐ and Baroreceptors on Postexercise Heat Loss

We studied the relative and separate contribution of metabo‐ and baroreflex activity to heat loss during postexercise recovery. Seven males (21 ± 2 years) performed 15 min of running (90% heart rate max) in the heat (35°C). During the 60 min postexercise period, four 1‐min bouts of isometric handgrip exercise (IHG) were followed by 2 min of forearm ischemia (toactivate metaboreceptors, OCC), each separated by 15 min. This was repeated on separate days during a normal seated recovery (CON) and during the application of lower body positive (LBPP, +40 mmHg) or negative (LBNP, ‐20 mmHg) pressure starting at 13 min of recovery. Forearm sweat rate and cutaneous vascular conductance (CVC) were measured continuously. For all conditions CVC returned to pre‐dynamic exercise resting levels at 45 min of recovery (P &gt; 0.05) whereas sweating remained elevated for the duration of recovery (P &lt; 0.05). Relative to pre‐IHG resting CVC was reduced during IHG at 15 min of recovery and subsequently returned to pre‐IHG levels during OCC in CON only. No effect of IHG on CVC was observed for LBPP or LBNP. However LBNP caused an OCC‐induced decrease in CVC from pre‐IHG levels at 15 min of recovery only (P &lt; 0.05). Sweating increased during IHG and remained elevated at similar levels during OCC for all conditions in the final 30 min of recovery (P &lt; 0.05). We show that metaboreceptors influence CVC during the early stages of recovery which is modulated to a certain extent by baroreceptor activity. In contrast, sweating is influenced by metaboreceptor activation in the mid‐to‐late stages of recovery independent of changes in baroreceptor activity. Support: Natural Sciences and Engineering Research of Canada.

Conversion table for running on lower body positive pressure treadmills.

Lower body positive pressure (LBPP) or antigravity treadmills are becoming increasingly popular in sports and rehabilitation settings. Running at a decreased body weight (BW) reduces metabolic cost, which can be offset by running at faster speeds. To date, however, little is known about how much faster someone must run to offset the reduced metabolic cost. This study aimed to develop a user-friendly conversion table showing the speeds required on an LBPP treadmill to match the equivalent metabolic output on a regular, non-LBPP, treadmill across a range of body weight supports. A total of 20 recreational runners (11 males, 9 females) ran multiple 3-minute intervals on a regular treadmill and then on an LBPP treadmill at 6 different BWs (50-100%, 10% increments). Metabolic outputs were recorded and matched between the regular and LBPP treadmill sessions. Using regression analyses, a conversion table was successfully created for the speeds from 6.4 to 16.1 km·h (4 to 10 mph) in 0.8 km·h (0.5 mph) increments on the regular treadmill and BW proportions of 50, 60, 70, 80, 90, and 100% on an LBPP treadmill. The table showed that a greater increase in speed on the LBPP treadmill was needed with more support (p < 0.001) but that the proportion increase was smaller at higher speeds (p < 0.001). This research has implications for coaches or practitioners using or prescribing training on an LBPP treadmill.

Accuracy of unloading with the anti-gravity treadmill.

Body weight (BW)-supported treadmill training has become increasingly popular in professional sports and rehabilitation. To date, little is known about the accuracy of the lower-body positive pressure treadmill. This study evaluated the accuracy of the BW support reported on the AlterG "Anti-Gravity" Treadmill across the spectrum of unloading, from full BW (100%) to 20% BW. Thirty-one adults (15 men and 16 women) with a mean age of 29.3 years (SD = 10.9), and a mean weight of 66.55 kg (SD = 12.68) were recruited. Participants were weighed outside the machine and then inside at 100-20% BW in 10% increments. Predicted BW, as presented by the AlterG equipment, was compared with measured BW. Significant differences between predicted and measured BW were found at all but 90% through 70% of BW. Differences were small (<5%), except at the extreme ends of the unloading spectrum. At 100% BW, the measured weight was lower than predicted (mean = 93.15%, SD = 1.21, p < 0.001 vs. predicted). At 30 and 20% BW, the measured weight was higher than predicted at 35.75% (SD = 2.89, p < 0.001), and 27.67% (SD = 3.76, p < 0.001), respectively. These findings suggest that there are significant differences between reported and measured BW support on the AlterG Anti-Gravity Treadmill®, with the largest differences (>5%) found at 100% BW and the greatest BW support (30 and 20% BW). These differences may be associated with changes in metabolic demand and maximum speed during walking or running and should be taken into consideration when using these devices for training and research purposes.

Influence of short-term unweighing and reloading on running kinetics and muscle activity.

In running, body weight reduction is reported to result in decreased lower limb muscle activity with no change in the global activation pattern (Liebenberg et al. in J Sports Sci 29:207-214). Our study examined the acute effects on running mechanics and lower limb muscle activity of short-term unweighing and reloading conditions while running on a treadmill with a lower body positive pressure (LBPP) device. Eleven healthy males performed two randomized running series of 9 min at preferred speed. Each series included three successive running conditions of 3 min [at 100 % body weight (BW), 60 or 80 % BW, and 100 % BW]. Vertical ground reaction force and center of mass accelerations were analyzed together with surface EMG activity recorded from six major muscles of the left lower limb for the first and last 30 s of each running condition. Effort sensation and mean heart rate were also recorded. In both running series, the unloaded running pattern was characterized by a lower step frequency (due to increased flight time with no change in contact time), lower impact and active force peaks, and also by reduced loading rate and push-off impulse. Amplitude of muscle activity overall decreased, but pre-contact and braking phase extensor muscle activity did not change, whereas it was reduced during the subsequent push-off phase. The combined neuro-mechanical changes suggest that LBPP technology provides runners with an efficient support during the stride. The after-effects recorded after reloading highlight the fact that 3 min of unweighing may be sufficient for updating the running pattern.

A case with below knee amputation due to diabetic foot gangrene using a lower body positive pressure treadmill for prosthetic rehabilitation

A case with below knee amputation due to diabetic foot gangrene using a lower body positive pressure treadmill for prosthetic rehabilitation

Metabolic accommodation to running on a body weight-supported treadmill.

Body weight-supported treadmill training using positive air pressure has become increasingly popular, but little is known about the metabolic adaptations to these treadmills. This study aimed to evaluate the existence and length of a metabolic accommodation period to running on a lower body positive pressure (LBPP) treadmill. A total of eight recreational runners (5 males and 3 females) ran 15 min trials (5 min at 50, 70, and 90 % body weight) on the AlterG Anti-gravity(®) P200 treadmill. No verbal instruction was given on how to run on the device. Their trial pace corresponded to 70-80 % of their velocity measured at [Formula: see text]O2max on a standard treadmill. Trials were continued until no significant metabolic change was observed. Two-way repeated measures analysis of variance was used to analyze changes in [Formula: see text]O2 across trials and levels of unloading. Participants completed 7 trials. Comparing trial 1 to the average of trials 5, 6, and 7, [Formula: see text]O2 decreased from 29.6 ± 3.8 to 23.6 ± 4.4 ml/kg/min at 50 % body weight (~20 % reduction), from 33.7 ± 4.5 to 29.2 ± 5.1 ml/kg/min at 70 % body weight (~13 % reduction), and from 41.0 ± 7.7 to 36.6 ± 5.6 ml/kg/min at 90 % body weight (~11 % reduction). No significant reduction occurred after trial 4 at any level of support. An accommodation effect of running on a treadmill with LBPP was observed and reached after 60 min of running (4 trials of 15 min). The accommodation effect was the largest at the greatest level of body weight support. These data suggest the importance of an accommodation period for reliable measures of metabolic cost to be made.

Muscle Activity During Running With Different Body-Weight-Support Mechanisms: Aquatic Environment Versus Body-Weight-Support Treadmill

Body-weight (BW) support during running can be accomplished using deep-water running (DWR; 100% BW support) and a lower-body positive-pressure (LBPP) treadmill. To compare lower-extremity muscle activity during DWR and running on an LBPP treadmill at matched stride frequency. Eight subjects (40 ± 6.5 y, 173 ± 7.2 cm, 66.9 ± 11.7 kg) completed 4 running conditions all at a preferred stride frequency that was determined while running with no support. Two conditions were running on the LBPP treadmill at 60% and 80% of BW, and the other 2 conditions were different DWR styles: high knee (DWR-HK) and cross-country (DWR-CC). Average (AVG) and root-mean-square (RMS) electromyography (rectus femoris, biceps femoris, gastrocnemius, and tibialis anterior) were each compared among conditions (repeated-measures analysis of variance). Results for AVG and RMS variables were identical for statistical tests for each muscle. Rectus femoris electromyography during DWR-HK was lower than that of DWR-CC (P < .05) but not different than either 60% BW or 80% BW (P > .05). Biceps femoris electromyography was less during DWR-HK than DWR-CC (P < .05) but greater during DWR-HK than either BW 60% or BW 80% (P < .05). Neither gastrocnemius nor tibialis anterior electromyography differed between conditions (P > .05). Neither the mechanism of BW support nor style of DWR influenced gastrocnemius or tibialis anterior muscle activity during running at the same stride frequency. However, rectus femoris and biceps femoris muscle activity were influenced by not only the mechanism of BW support but also the style of DWR.

Pathogenesis of obstructive sleep apnoea in hypertensive patients: role of fluid retention and nocturnal rostral fluid shift.

Obstructive sleep apnoea (OSA) is highly prevalent in hypertensive patients, particularly those with drug resistance. Evidence from animal experiments, epidemiologic studies and clinical trials strongly suggest a causal link. Mechanistic studies argue for increased sympathetic neural activity and endothelial dysfunction. However, disturbances in fluid volume regulation and distribution may also be involved in the pathogenesis of these two conditions. Several studies have shown a high prevalence of OSA in fluid-retaining states including hypertension, a direct relationship between the severity of OSA and the volume of fluid displaced from the legs to the neck during sleep, and a decrease in upper airway cross-sectional area in response to graded lower body positive pressure. Treatments targeting fluid retention and redistribution, including diuretics, mineralocorticoid antagonists, exercise, and possibly renal denervation lower blood pressure and reduce the apnoea-hypopnoea index, a measure of OSA severity. From these observations, it has been postulated that during the daytime, excess fluid collects in the lower extremities due to gravity, and on lying down overnight is redistributed rostrally to the neck, where it may narrow the upper airway and increase its collapsibility, predisposing to OSA when pharyngeal dilator muscle activity decreases during sleep. This article discusses the associations between OSA and hypertension and reviews the evidence for fluid accumulation and its nocturnal rostral redistribution in the pathogenesis of OSA in hypertensive patients.

Influence of rostral fluid shift on upper airway size and mucosal water content.

Fluid displacement from the legs during recumbency while in bed might narrow the upper airway (UA) in association with nuchal fluid accumulation that may contribute to the pathogenesis of obstructive sleep apnea (OSA). The aim of this study was to test the hypothesis that rostral fluid displacement from the legs causes a greater decrease in UA cross-sectional area (UA-XSA) and a greater increase in UA mucosal water content (UA-MWC) and internal jugular venous volume (IJVVol) in subjects with OSA than in those without OSA. Subjects underwent baseline assessment of leg fluid volume (LFV) measured by bio-electrical impedance, as well as UA-XSA and UA-MWC by magnetic resonance imaging. They were then randomly assigned to a 20-min period either with or without application of lower body positive pressure (LBPP) of 40 mm Hg, followed by a 15-min washout period, after which they crossed over to the other arm of the study. Measurements of LFV, UA-MWC, and UA-XSA were repeated after each arm of the study. In 12 subjects without sleep apnea, UA-XSA increased and UA-MWC decreased significantly, whereas in 12 subjects with OSA, UA-XSA decreased and UA-MWC increased significantly in response to LBPP. The changes in UA-XSA and UA-MWC in response to LBPP differed significantly between the 2 groups (p = 0.006 and p < 0.001, respectively), despite similar changes in LFV and IJVVol. Our results suggest that rostral fluid shift may contribute to the pathogenesis of OSA at least partly through narrowing of the UA due to transudation of fluid into the UA mucosa.

The Role of Autonomic Function on Sport Performance in Athletes With Spinal Cord Injury

Devastating paralysis, autonomic dysfunction, and abnormal cardiovascular control present significant hemodynamic challenges to individuals with spinal cord injury (SCI), especially during exercise. In general, resting arterial pressure after SCI is lower than with able-bodied individuals and is commonly associated with persistent orthostatic intolerance along with transient episodes of life-threatening hypertension, known as "autonomic dysreflexia." During exercise, the loss of central and reflexive cardiovascular control attenuates maximal heart rate and impairs blood pressure regulation and blood redistribution, which ultimately reduces venous return, stroke volume, and cardiac output. Thermoregulation also is severely compromised in high-lesion SCI, a problem that is compounded when competing in hot and humid conditions. There is some evidence that enhancing venous return via lower body positive pressure or abdominal binding improves exercise performance, as do cooling strategies. Athletes with SCI also have been documented to self-induce autonomic dysreflexia before competition with a view of increasing blood pressure and improving their performance, a technique known as "boosting." For health safety reasons, boosting is officially banned by the International Paralympics Committee. This article addresses the complex issue of how the autonomic nervous system affects sports performance in athletes with SCI, with a specific focus on the potential debilitating effects of deranged cardiovascular control.

The effect of hypercapnia on static cerebral autoregulation.

Hypercapnia impairs cerebrovascular control during rapid changes in blood pressure (BP); however, data concerning the effect of hypercapnia on steady state, nonpharmacological increases in BP is scarce. We recruited fifteen healthy volunteers (mean ± SD: age, 28 ± 6 years; body mass, 77 ± 12 kg) to assess the effect of hypercapnia on cerebrovascular control during steady‐state elevations in mean arterial BP (MAP), induced via lower body positive pressure (LBPP). Following 20 min of supine rest, participants completed 5 min of eucapnic 20 and 40 mm Hg LBPP (order randomized) followed by 5 min of hypercapnia (5% CO2 in air) with and without LBPP (order randomized), and each stage was separated by ≥5 min to allow for recovery. Middle cerebral artery blood velocity (MCAv), BP, partial pressure of end‐tidal carbon dioxide (PETCO2) and heart rate were recorded and presented as the change from the preceding baseline. No difference in MCAv was apparent between eupcapnic baseline and LBPPs (grouped mean 65 ± 11 cm·s−1, all P >0.05), despite the increased MAP with LBPP (Δ6 ± 5 and Δ8 ± 3 mm Hg for 20 and 40 mm Hg, respectively, both P <0.001 vs. baseline). Conversely, MCAv during the hypercapnic +40 mm Hg stage (Δ31 ± 13 cm·s−1) was greater than hypercapnia alone (Δ25 ± 11 cm·s−1, P =0.026), due to an increased MAP (Δ14 ± 7 mm Hg, P <0.001 vs. hypercapnia alone and P =0.026 vs. hypercapnia +20 mm Hg). As cardiac output and PETCO2 were similar across all hypercapnic stages (all P >0.05), our findings indicate that hypercapnia impairs static autoregulation, such that higher blood pressures are translated into the cerebral circulation.

A Conversion Table For Running On Lower Body Positive Pressure (LBPP) Treadmills

A Conversion Table For Running On Lower Body Positive Pressure (LBPP) Treadmills

Effect of Weight Support on Tibial Accelerations During a Lower-Body Positive Pressure Treadmill Cadence-Controlled Run

Effect of Weight Support on Tibial Accelerations During a Lower-Body Positive Pressure Treadmill Cadence-Controlled Run

Cardiovascular models of simulated moon and mars gravities: head-up tilt vs. lower body unweighting.

In this study we compare two models [head-up tilt (HUT) vs. body unweighting using lower body positive pressure (LBPP)] to simulate Moon, Mars, and Earth gravities. A literature search did not reveal any comparisons of this type performed previously. We hypothesized that segmental fluid volume shifts (thorax, abdomen, upper and lower leg), cardiac output, and blood pressure (BP), heart rate (HR), and total peripheral resistance to standing would be similar in the LBPP and HUT models. There were 21 subjects who were studied while supine (simulation of spaceflight) and standing at 100% (Earth), 40% (Mars), and 20% (Moon) bodyweight produced by LBPP in Alter-G and while supine and tilted at 80 degrees, 20 degrees, and 10 degrees HUT (analogues of Earth, Mars, and Moon gravities, respectively). Compared to supine, fluid shifts from the chest to the abdomen, increases in HR, and decreases in stroke volume were greater at 100% bodyweight than at reduced weights in response to both LBPP and HUT. Differences between the two models were found for systolic BP, diastolic BP, mean arterial BP, stroke volume, total peripheral resistance, and thorax and abdomen impedances, while HR, cardiac output, and upper and lower leg impedances were similar. Bodyweight unloading via both LBPP and HUT resulted in cardiovascular changes similar to those anticipated in actual reduced gravity environments. The LBPP model/Alter-G has the advantage of providing an environment that allows dynamic activity at reduced bodyweight; however, the significant increase in blood pressures in the Alter-GC may favor the HUT model.

Simulations of gravitational stress on normovolemic and hypovolemic men and women.

Earth-based simulations of physiologic responses to space mission activities are needed to develop prospective countermeasures. To determine whether upright lower body positive pressure (LBPP) provides a suitable space mission simulation, we investigated the cardiovascular responses of normovolemic and hypovolemic men and women to supine and orthostatic stress induced by head-up tilt (HUT) and upright LBPP, representing standing in lunar, Martian, and Earth gravities. Six men and six women were tested in normovolemic and hypovolemic (furosemide, intravenous, 0.5 mg x kg(-1)) conditions. Continuous electrocardiogram, blood pressure, segmental bioimpedance, and stroke volume (echocardiography) were recorded supine and at lunar, Martian, and Earth gravities (10 degrees, 20 degrees, and 80 degrees HUT vs. 20%, 40%, and 100% bodyweight upright LBPP), respectively. Cardiovascular responses were assessed from mean values, spectral powers, and spontaneous baroreflex parameters. Hypovolemia reduced plasma volume by approximately 10% and stroke volume by approximately 25% at supine, and increasing orthostatic stress resulted in further reductions. Upright LBPP induced more plasma volume losses at simulated lunar and Martian gravities compared with HUT, while both techniques induced comparable central hypovolemia at each stress. Cardiovascular responses to orthostatic stress were comparable between HUT and upright LBPP in both normovolemic and hypovolemic conditions; however, hypovolemic blood pressure was greater during standing at 100% bodyweight compared to 80 degree HUT due to a greater increase of total peripheral resistance. The comparable cardiovascular response to HUT and upright LBPP support the use of upright LBPP as a potential model to simulate activity in lunar and Martian gravities.

Single‐unit muscle sympathetic recordings identify in human heart failure unique fibers discharging in response to both unloading and loading of cardiopulmonary receptors (LB735)

In healthy men, multi‐unit muscle sympathetic nerve activity (MSNA) has been shown to integrate two populations of single‐units exhibiting directionally opposite discharge to acute selective perturbations in atrial pressure. The behaviour of these single‐units was consistent during application of lower body negative and positive pressure (LBNP and LBPP). The aim of the present study was to compare single‐unit discharge characteristics of patients with and without heart failure (HF). We measured central venous pressure and single‐unit MSNA in 6 HF patients (LVEF: 24±6%; 54±13 years [mean±SD]) and 9 healthy controls (56±8 years) before and during 7 minutes of LBNP and LBPP (±10mmHg). These interventions had no effect on stroke volume, heart rate or systemic blood pressure (All P&gt;0.05). Twelve HF and 22 control single‐units were identified. In controls, single‐unit responses to LBNP and LBPP exhibited either ‘anticipated’ (increased with LBNP and decreased with LBPP) or ‘paradoxical’ (decreased with LBNP and increased with LBPP) firing behavior. In HF, the majority (8/12; 67%) of single‐units exhibited a unique U‐shaped discharge pattern, with ‘anticipated’ responses to LBNP but ‘paradoxical’ responses to LBPP. This novel observation provides evidence for the emergence in HF of a population of cardiopulmonary mechanoreceptors exerting consistently sympathoexcitatory efferent firing characteristics.Grant Funding Source: Supported by the Canadian Institutes of Health Research (PJM) and Canadian Research Chairs program (JSF)