mmHg Lower Body Negative Pressure Research Articles

Effect of Lower Body Negative Pressure (LBNP) on Blood Pressure in Awake Rats and Effects of Anesthesia

Lower body negative pressure (LBNP) has been used for decades in a variety of settings and species to model orthostatic challenge (OC) and baroreceptor unloading. However, due to physical constraints, measurements of blood pressure (BP) during LBNP have always been made in anesthetized animals, where cardiovascular reflexes are blunted or even blocked. In terminal procedures, for example, urethane is often used in an attempt to preserve some measure of autonomic reflex function during OC. More recently, investigators have used isoflurane in animals fitted with BP telemeters in order to perform longitudinal studies, but the extent to which various cardiovascular reflexes may be preserved is unknown. Here we have developed a novel method to apply lower body negative pressure in awake rats instrumented for BP measurement. Rats were placed into a tubular restraining device (5 cm diameter X 21 cm length), that is divided into an upper and lower chamber by two adjacent latex septa with small holes (~ 2 cm). Rats were positioned so that the septa form a seal at the lower margin of the thorax, thereby creating independent upper and lower chambers. Negative pressure was then applied to the lower chamber using a high-volume vacuum and the pressure controlled using an adjustable damper. BP measurements in response to progressive levels of LBNP (-3 to -15 mmHg) were first made in awake rats, followed by induction of anesthesia and repetition of the LBNP protocol. The effects of pentobarbital (50mg/kg), ketamine/xylazine (70/6 mg/kg) and isoflurane (3%) were determined on separate days. BP and heart rate (HR) responses are shown in the Table. We found that BP was remarkably preserved in awake rats up to at least -9 mmHg LBNP, and thereafter (at -12 and -15 mmHg) decreased modestly. There was a robust baroreceptor-mediated increase in HR beginning at -6 mmHg. By contrast, under each anesthetic, blood pressure decreased dramatically and progressively with increasing LBNP, and the reflex increases in HR were entirely blocked (and actually inverted). These data indicate that the awake rat has robust mechanisms to preserve cardiovascular function during OC, and that these mechanisms are severely compromised during anesthesia. University of Cincinnati College of Medicine Innovation Seed Grant. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Breathing 5% carbon dioxide for 1 minute increases cerebral blood flow during lower body negative pressure

Objective: Lower body negative pressure (LBNP) is an established surrogate of hypergravity to investigate responses to central hypovolemia, including the reduction in cerebral blood flow. Increased inspired carbon dioxide (CO2) causes cerebrovascular dilation and increases cerebral blood flow. We aimed to determine whether breathing 5% CO2 for 1 minute increased cerebral blood flow during LBNP. We hypothesized 5% inspired CO2 would increase cerebral blood flow during LBNP. Methods: We collected data from 15 (9 M/6 F, 31±8 yr, 25.6±3.2 kg/m2) healthy individuals. Subjects breathed room air or a mix of 5% CO2, 21% oxygen, balance nitrogen gas for 1 minute prior to initiation of 2 minutes of -60 mmHg LBNP and throughout the duration of LBNP. We allowed a 2-minute wash-out between trials. We measured middle cerebral artery velocity (MCAv) with transcranial Doppler ultrasound and continuous, non-invasive blood pressure (MAP) with a finger plethysmograph. We corrected MCAv for MAP and present it as cerebrovascular conductance index (CVCi). We also measure ventilation and gas exchange with a pneumotachometer and gas analyzer. Results: All measured variables were similar prior to the initiation of room air and 5% CO2 prebreathe ( p > 0.05). Inspired (1±1 vs. 36±1 mmHg) and end-tidal (38±4 vs. 47±3 mmHg) CO2 were greater during the 5% inspired CO2 trial ( p < 0.05, main effect of gas). LBNP caused an increase in heart rate, decrease in stroke volume, decrease in cardiac output, and an increase in total peripheral resistance ( p < 0.05, main effect of LBNP). These variables were similar regardless of inspiratory gas. MCAv (40±16 vs. 53±19 cm/s), MAP (94±10 vs. 99±11 mmHg), and CVCi (0.46±0.22 vs 0.56±0.24 cm/s/mmHg) were all greater during the 5% inspired CO2 trials ( p < 0.05, main effect of gas). Discussion: Breathing a mixture of 5% CO2 for 1 minute increased cerebral blood flow and blood pressure during a mimic of hypergravity stress. Despite technical challenges of integration into high-performance aircraft, increasing inspired CO2 could contribute to better tolerance to hypergravity stress. Funding was provided by the Air Force Offce of Scientific Research and King’s College London. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Effect of lower body negative pressure on cardiac and cerebral function in postural orthostatic tachycardia syndrome: A pilot MRI assessment.

Postural orthostatic tachycardia syndrome (POTS) is characterized by an excessive heart rate (HR) response upon standing and symptoms indicative of inadequate cerebral perfusion. We tested the hypothesis that during lower body negative pressure (LBNP), individuals with POTS would have larger decreases in cardiac and cerebrovascular function measured using magnetic resonance (MR) imaging. Eleven patients with POTS and 10 healthy controls were studied at rest and during 20 min of -25 mmHg LBNP. Biventricular volumes, stroke volume (SV), cardiac output (Qc), and HR were determined by cardiac MR. Cerebral oxygen uptake (VO2 ) in the superior sagittal sinus was calculated from cerebral blood flow (CBF; MR phase contrast), venous O2 saturation (SvO2 ; susceptometry-based oximetry), and arterial O2 saturation (pulse oximeter). Regional cerebral perfusion was determined using arterial spin labelling. HR increased in response to LBNP (p < 0.001) with no group differences (HC: +9 ± 8 bpm; POTS: +13 ± 11 bpm; p = 0.35). Biventricular volumes, SV, and Qc decreased during LBNP (p < 0.001). CBF and SvO2 decreased with LBNP (p = 0.01 and 0.03, respectively) but not cerebral VO2 (effect of LBNP: p = 0.28; HC: -0.2 ± 3.7 mL/min; POTS: +1.1 ± 2.0 mL/min; p = 0.33 between groups). Regional cerebral perfusion decreased during LBNP (p < 0.001) but was not different between groups. These data suggest patients with POTS have preserved cardiac and cerebrovascular function.

Sex Differences in Sympathetic Responses to Lower-Body Negative Pressure.

Trauma-induced hemorrhage is a leading cause of death in prehospital settings. Experimental data demonstrate that females have a lower tolerance to simulated hemorrhage (i.e., central hypovolemia). However, the mechanism(s) underpinning these responses are unknown. Therefore, this study aimed to compare autonomic cardiovascular responses during central hypovolemia between the sexes. We hypothesized that females would have a lower tolerance and smaller increase in muscle sympathetic nerve activity (MSNA) to simulated hemorrhage. Data from 17 females and 19 males, aged 19-45 yr, were retrospectively analyzed. Participants completed a progressive lower-body negative pressure (LBNP) protocol to presyncope to simulate hemorrhagic tolerance with continuous measures of MSNA and beat-to-beat hemodynamic variables. We compared responses at baseline, at two LBNP stages (-40 and -50 mmHg), and at immediately before presyncope. In addition, we compared responses at relative percentages (33%, 66%, and 100%) of hemorrhagic tolerance, calculated via the cumulative stress index (i.e., the sum of the product of time and pressure at each LBNP stage). Females had lower tolerance to central hypovolemia (female: 561 ± 309 vs male: 894 ± 304 min·mmHg [time·LBNP]; P = 0.003). At LBNP -40 and -50 mmHg, females had lower diastolic blood pressures (main effect of sex: P = 0.010). For the relative LBNP analysis, females exhibited lower MSNA burst frequency (main effect of sex: P = 0.016) accompanied by a lower total vascular conductance (sex: P = 0.028; main effect of sex). Females have a lower tolerance to central hypovolemia, which was accompanied by lower diastolic blood pressure at -40 and -50 mmHg LBNP. Notably, females had attenuated MSNA responses when assessed as relative LBNP tolerance time.

Dynamic changes in renal sodium handling during sympathetic stimulation in healthy human males

The temporal response of changes in renal sodium reabsorption during increased renal sympathetic nerve activity has not been investigated. Central hypovolemia by application of lower-body negative-pressure (LBNP) elicits baroreceptor mediated sympathetic reflexes to maintain arterial blood pressure. We hypothesized, that during 90 min LBNP, the renal sodium retention would increase rapidly, remain increased during intervention, and return to baseline immediately after end of intervention. Methods30 young, healthy, sodium loaded, non-obese males were exposed to −15 mmHg LBNP, −30 mmHg LBNP, −15 mmHg LBNP + renin blockade or time-control (0 mmHg LBNP) for 90 min. Urine was collected every 15 min during 90 min of intervention and 60 min of recovery to identify a possible relation between time of intervention and renal response. ResultsAll intervention groups exhibited a comparable reduction in distal sodium excretion at the end of the intervention (P = 0.46 between groups; −15 mmHg: −3.1 ± 0.9 %, −30 mmHg: −2.9 ± 0.6 %, −15 mmHg + aslikiren: −1.8 ± 0.6 %). −15 mmHg+Aliskiren resulted in a slower onset, but all groups exhibited a continued reduction in sodium excretion after 1 h of recovery despite return to baseline of renin, aldosterone, diuresis and cardiovascular parameters. ConclusionSympathetic stimulation for 90 min via LBNP at −30 mmHg LBNP compared to −15 mmHg did not result in a greater response in fractional Na+ excretion, suggesting that additional baroreceptor unloading did not cause further increases in renal sodium reabsorption. Changes in distal Na+ excretion were linear with respect to time (dose) of intervention, but seem to exhibit a saturation-like effect at a level around 4 %. The lack of recovery after 1 h is also a new finding that warrants further investigation.

A single high-fat meal impairs functional sympatholysis in young healthy adults

Introduction: A high-fat diet increases the risk of cardiovascular disease and mortality, but the underlying mechanisms remain incompletely understood. During exercise, sympathetically mediated vasoconstriction is dampened in active skeletal muscle due to the counteracting vasodilatory effects of metabolic by-products from muscle contraction. This phenomenon, termed functional sympatholysis, plays a critical role in the redistribution of cardiac output to contracting skeletal muscles to meet the metabolic demands of exercise. Whether a high-fat meal modulates the magnitude of sympatholysis is currently unknown. Herein, we tested the hypothesis that a high-fat meal would acutely impair functional sympatholysis in young healthy adults. Methods: In a randomized, controlled, and cross-over design, 14 healthy adults (8 women, 26±4 years), consumed either a high- (1030kcals, 91g fat) or a low-fat (1006kcals, 1g fat) meal on two separate days. Forearm blood flow (FBF; Doppler ultrasound) and beat-to-beat mean arterial pressure (MAP; finger photoplethysmography) were measured during sympathetic activation induced by -20 mmHg lower-body negative pressure (LBNP) applied at rest and simultaneously during rhythmic handgrip exercise at 15% and 30% maximum voluntary contraction (MVC). Measurements were taken pre-meal and at 1-, 2-, and 3-hours post-prandial. Forearm vascular conductance (FVC) was calculated as FBF/MAP and the magnitude of sympatholysis as the difference of LBNP-induced changes in FVC between handgrip and rest. Results: Prior to the high-fat meal, LBNP decreased resting FVC (Δ-57±10%), and this response was attenuated during exercise at 15% (Δ-18±8%) and 30% MVC (Δ-9±7%). After the high-fat meal, LBNP induced smaller decreases in resting FVC (1h: Δ-43±12%, 2h: Δ-45±12%, 3h: Δ-49±8%, all P≤0.01 vs. pre), but during exercise greater decreases in FVC were observed at both 15% (1h: Δ-22±10%, 2h: Δ-23±8%, 3h: Δ-25±8%, all P≤0.04 vs. pre) and 30% MVC (1h: Δ-15±8%, 2h: Δ-15±6%, 3h: Δ-16±6%, all P≤0.04 vs. pre). As a result, the high-fat meal decreased the magnitude of sympatholysis during rhythmic handgrip exercise at 15% (pre-prandial: 39±8%, 1h: 20±10%, 2h: 21±8%, 3h: 24±7%, all P≤0.001) and 30% MVC (pre-prandial: 48±11%, 1h: 28±10%, 2h: 30±11%, 3h: 33±8%, all P≤0.001). The low-fat meal did not change the magnitude of sympatholysis at either 15% (pre-prandial: 37±6%, 1h: 39±6%, 2h: 38±7%, 3h: 37±8%, all P≥0.07) or 30% MVC (pre-prandial: 45±10%, 1h: 47±8%, 2h: 47±9%, 3h: 46±7%, all P≥0.39). Conclusion: These findings demonstrate the capacity of a high-fat meal to acutely impair the ability of contracting skeletal muscle to offset the vascular responsiveness to α1-adrenoreceptor activation during low- and moderate-intensity rhythmic handgrip exercise. Natural Science and Engineering Research Council (NSERC) of Canada Discovery Grant (P.J.M) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Habitual physical activity improves vagal cardiac modulation and carotid baroreflex function in elderly women.

The impact of habitual physical activity on vagal-cardiac function and baroreflex sensitivity in elderly women is poorly characterized. This study compared vagal-cardiac modulation and carotid baroreflex (CBR) function in eight physically active (67.6 ± 1.9 years; peak O2 uptake 29.1 ± 2.5 mL/min/kg) versus eight sedentary (67.3 ± 1.8 years; peak O2 uptake 18.6 ± 0.9 mL/min/kg) elderly women. Heart rate (HR) variabilities and maximal changes of HR and mean arterial pressure (MAP) elicited by 5-s pressure pulses between +40 and -80 mmHg applied to the carotid sinus were measured at rest and during carotid baroreceptor unloading effected by -15 mmHg lower-body negative pressure (LBNP). HR variability was greater in active than sedentary women in both low (0.998 ± 0.286 versus 0.255 ± 0.063 bpm2; P = 0.036) and high (0.895 ± 0.301 versus 0.156 ± 0.045 bpm2; P = 0.044) frequency domains. CBR-HR gains (bpm/mmHg) were greater (fitness factor P < 0.001) in active versus sedentary women at rest (-0.146 ± 0.014 versus -0.088 ± 0.011) and during LBNP (-0.105 ± 0.014 versus -0.065 ± 0.008). CBR-MAP gains (mmHg/mmHg) tended to be greater (fitness factor P = 0.077) in active versus sedentary women at rest (-0.132 ± 0.013 versus -0.110 ± 0.011) and during LBNP (-0.129 ± 0.015 versus -0.113 ± 0.013). However, LBNP did not potentiate CBR-MAP gains in either sedentary or active women (LBNP factor P = 0.94), and it depressed CBR-HR gains in both groups (LBNP factor P = 0.003). CBR-HR gains in the sedentary women did not differ (sex factor P = 0.65) from gains reported in age-matched sedentary men, although CBR-MAP gains tended to be greater (sex factor P = 0.109) in the men. Thus, tonic vagal modulation indicated by HR variability and dynamic vagal responses assessed by CBR-HR gain were augmented in physically active women. Enhanced vagal-cardiac function may protect against senescence-associated cardiac electrical and hemodynamic instability in elderly women.

Cardio-respiratory interactions in response to lower-body negative pressure

The relationship between heart rate and blood pressure, as well as cardiorespiratory coupling, play a critical role in maintaining blood pressure and organ perfusion during conditions of blood loss. Traditional vital signs such as blood pressure, breathing rate, and oxygen saturation are poor markers of blood loss, making it difficult for medics to assess the severity of central hypovolemia. Monitoring hemorrhage is further complicated by the fact that some patients have a low tolerance to hemorrhage and would reach the point of cardiovascular collapse in less time than high tolerant individuals. Therefore, this study aimed to investigate the potential of the physiological interaction between heart rate and blood pressure, and cardiorespiratory coupling to track the progression of simulated hemorrhage, as well as distinguish individuals with low tolerance (LT) from the ones with high tolerance (HT) to hypovolemia. Nineteen subjects (age: 28 ± 6 years; height: 170 ± 7 cm; weight: 68 ± 10 kg) underwent a progressive lower body negative pressure (LBNP) protocol in which the participant was supine inside the chamber for 12 min (baseline) before 12 min of chamber decompression at –20, –30, –40, –50 and –60 mmHg followed by a 12 min recovery period. Twelve subjects reached presyncope before or during –60 mmHg LBNP stage and were considered low tolerant (LT, 12 participants), while the ones who completed –60 mmHg were considered high tolerant (HT, 7 participants). Continuous blood pressure (BP), respiration (RSP), and electrocardiogram (ECG) signals were acquired simultaneously during baseline and each LBNP stage. RR interval was calculated using ECG, while systolic blood pressure (SBP), and pulse pressure were derived from BP waveform. Wavelet transform coherence and convergent cross-mapping techniques were employed to study the physiological interdependence and the causal relationship between heart rate, blood pressure, and respiration. The interaction between blood pressure and heart rate in terms of gain, active gain, and fraction time active to maintain homeostasis was higher in the LT group during baseline, and LBNP simulated mild, moderate, and severe hemorrhage. The significant time of interaction between SBP and RSP, and the causal effect of blood pressure on respiration were higher in the HT group during baseline compared to the LT group. HT participants also had a higher causal effect of respiration on blood pressure during –30 and –40 mmHg compared to LT. Moreover, the HT group displayed a higher causal drive of respiratory-related changes in heart rate and heart rate mediated changes in respiration during severe simulated hemorrhage (–40 mmHg) compared to the LT group. The calculated metrics to distinguish between individual LT from HT subjects achieved a sensitivity of 58%–83%, an accuracy of 63%–84%, and an area under the ROC curve of 74%–86%, while the overlap of LT individual responses with HT was 0%–33%. These results indicate the potential of cardiorespiratory coupling, and heart rate and blood pressure interaction toward tracking the progression of hemorrhage and distinguishing individuals with low tolerance to hypovolemia from those with high tolerance. Measurements of such interactions could improve clinical outcomes for patients with low tolerance to hypovolemia and therefore reduce morbidity and mortality through early implementation of life-saving interventions.

The reciprocal relationship between cardiac baroreceptor sensitivity and cerebral autoregulation during simulated hemorrhage in humans.

A reciprocal relationship between the baroreflex and cerebral autoregulation (CA) has been demonstrated at rest and in response to acute hypotension. We hypothesized that the reciprocal relationship between cardiac baroreflex sensitivity (BRS) and CA would be maintained during sustained central hypovolemia induced by lower body negative pressure (LBNP), and that the strength of this relationship would be greater in subjects with higher tolerance to this stress. Healthy young adults (n=51; 23F/28M) completed a LBNP protocol to presyncope. Subjects were classified as high tolerant (HT; completion of -60mmHg LBNP stage, ≥20-min) or low tolerant (LT; did not complete -60mmHg LBNP stage, <20-min). R-R intervals (RRI), systolic arterial pressure (SAP), mean arterial pressure (MAP), and middle cerebral artery velocity (MCAv) were measured continuously. Cardiac BRS was calculated in the time domain (ΔHR/ΔSAP) and frequency domain (RRI-SAP low frequency (LF) transfer function gain), and CA was calculated in the time domain (ΔMCAv/ΔMAP) and frequency domain (MAP-mean MCAv LF transfer function gain). There was a moderate relationship between cardiac BRS and CA for the group of 51 subjects in both the time (R=-0.54, P<0.0001) and frequency (R=0.61, P<0.001) domains; there was a stronger relationship in the HT group (R=0.73) compared to the LT group (R=0.31) in the frequency domain (P=0.08), but no difference between groups in the time domain (HT: R=-0.73 vs. LT: R=-0.63; P=0.27). These findings suggest that an interaction between BRS and CA may be an important compensatory mechanism that contributes to tolerance to simulated hemorrhage in young healthy adults.

Noninvasive indicators of intracranial pressure before, during, and after long-duration spaceflight.

Weightlessness induces a cephalad shift of blood and cerebrospinal fluid that may increase intracranial pressure (ICP) during spaceflight, whereas lower body negative pressure (LBNP) may provide an opportunity to caudally redistribute fluids and lower ICP. To investigate the effects of spaceflight and LBNP on noninvasive indicators of ICP (nICP), we studied 13 crewmembers before and after spaceflight in seated, supine, and 15° head-down tilt postures, and at ∼45 and ∼150 days of spaceflight with and without 25 mmHg LBNP. We used four techniques to quantify nICP: cerebral and cochlear fluid pressure (CCFP), otoacoustic emissions (OAE), ultrasound measures of optic nerve sheath diameter (ONSD), and ultrasound-based internal jugular vein pressure (IJVp). On flight day 45, two nICP measures were lower than preflight supine posture [CCFP: mean difference -98.5 -nL (CI: -190.8 to -6.1 -nL), P = 0.037]; [OAE: -19.7° (CI: -10.4° to -29.1°), P < 0.001], but not significantly different from preflight seated measures. Conversely, ONSD was not different than any preflight posture, whereas IJVp was significantly greater than preflight seated measures [14.3 mmHg (CI: 10.1 to 18.5 mmHg), P < 0.001], but not significantly different than preflight supine measures. During spaceflight, acute LBNP application did not cause a significant change in nICP indicators. These data suggest that during spaceflight, nICP is not elevated above values observed in the seated posture on Earth. Invasive measures would be needed to provide absolute ICP values and more precise indications of ICP change during various phases of spaceflight.NEW & NOTEWORTHY The current study provides new evidence that intracranial pressure (ICP), as assessed with noninvasive measures, may not be elevated during long-duration spaceflight. In addition, the acute use of lower body negative pressure did not significantly reduce indicators of ICP during weightlessness.

Cardiovascular Responses to Mild Lower Body Negative Pressure during Spaceflight

IntroductionWeightlessness during spaceflight causes a chronic cephalad fluid shift that has been hypothesized to underlie numerous risks associated with spaceflight, including the development of ocular structural and functional changes and in rare cases venous thrombosis. Acute use of 25 mmHg lower body negative pressure (LBNP) during spaceflight can partially reverse this headward fluid shift and thus, may represent a promising countermeasure. However, there are limited data on how the cardiovascular system responds to sustained use of mild LBNP during spaceflight when cardiovascular deconditioning and adaptation may cause crewmembers to be more susceptible to orthostatic stressors than they would have been on Earth.PurposeThe purpose of this study was to quantify the heart rate (HR) and blood pressure response throughout ~60 minutes of exposure to 25 mmHg LBNP during spaceflight and determine if this response was augmented at later time points during 6‐month spaceflight missions.HypothesisWe hypothesized that crewmembers would experience an increase in HR during LBNP that would be sufficient to maintain mean arterial pressure (MAP) and this would not be augmented later in spaceflight.MethodsBrachial arterial pressure and HR were measured before and throughout ~60 min of 25 mmHg LBNP after ~45 days (FD45) and ~150 days (FD150) of spaceflight in 12 crewmembers. Total LBNP session times varied slightly due to logistical constraints. MAP and HR changes from baseline with LBNP exposure were analyzed with splines models. Marginal means were used for estimation and statistical comparisons across time and between flight days.ResultsBefore LBNP use, HR was 59 beats per min (BPM 95% CI: 56 – 62) and MAP was 93 mmHg (95% CI: 90 – 96). During LBNP exposure on FD45, HR increased by 9 BPM (95% CI: +7 – +10) at 15 min of exposure, and increased further to +10 BPM (95% CI: +8 – +12) at 30 min, and +11 BPM (95% CI: +9 – +14) at 45 min. On FD150, HR increased by 8 BPM (95% CI: +5 – +11) at 15 min of exposure but plateaued thereafter. MAP decreased by 5 mmHg (95% CI: ‐8 – ‐2) at 15 min and remained lower thereafter. MAP responses during ~60 min of LBNP exposure were not statistically different between FD45 and FD150. A total of 46 sessions of LBNP were successfully completed, and only a single case of mild hypotensive symptoms was reported, and this session was completed.ConclusionThese data suggest that the cardiovascular system can accommodate up to 60 min of sustained use of mild levels of LBNP during long‐duration spaceflight. There was a single report of mild hypotensive symptoms during use of LBNP by an individual who had low blood pressure prior to the start of the LBNP session. Thus, the absolute arterial blood pressure should be taken into consideration when determining if an LBNP session will be a potential challenge to the cardiovascular system during spaceflight.

Influence of head-up tile and lower body negative pressure on the internal jugular vein.

Head‐up tilt (HUT)‐induced gravitational stress causes collapse of the internal jugular vein (IJV) by decreasing central blood volume and through mass‐effect from the surrounding tissues. Besides HUT, lower body negative pressure (LBNP) is used to stimulate orthostatic stress as an experimental model. Compared to HUT, LBNP has less of a gravitational effect because of the supine position; therefore, we hypothesized that LBNP causes less of a decrease in the cross‐sectional area of the IJV compared to HUT. We tested the hypothesis by measuring the cross‐sectional area of the IJV using B‐mode ultrasonography while inducing orthostatic stress at levels of −40 mmHg LBNP and 60° HUT. The cross‐sectional area of IJV decreased from the resting baseline during both LBNP and HUT trials, but the LBNP‐induced decrease in the cross‐sectional area of IJV was smaller than that of HUT (right, −45% ± 49% vs. −78% ± 27%, p = 0.008; left, −49% ± 27% vs. −78% ± 20%, p = 0.004). Since changes in venous outflow may affect cerebral arterial circulation, the findings of the present study suggest that orthostatic stress induced by different techniques modulates cerebral blood flow regulation through its effect on venous outflow.

Interactive effects of apneic and baroreflex stress on neural coding strategies in human muscle sympathetic nerve activity

The sympathetic nervous system exhibits patterns of action potential (AP) discharge in human muscle sympathetic nerve activity that suggest coding strategies express reflex specificity. This study explored the interactive effects of baroreceptor unloading using lower body negative pressure (LBNP) and volitional end-expiratory apnea (APN) on sympathetic postganglionic neuronal discharge patterns inferred from the firing patterns of differently sized sympathetic AP clusters. Seven individuals were studied using multiunit microneurography (fibular) and a continuous wavelet approach to quantify AP discharge probability, recruitment, and latency during APN performed under ambient conditions, -10, and -40 mmHg LBNP. Compared with the ambient condition, LBNP increased AP discharge rate at -10 and -40 mmHg and recruited larger previously silent sympathetic neurons at -40 mmHg. Compared with spontaneous breathing, APN increased AP discharge when performed during the ambient condition (Δ351 ± 132 AP/min), -10 mmHg (Δ423 ± 184 AP/min), and -40 mmHg (Δ355 ± 278 AP/min; main effect APN: P < 0.01; LBNP-by-APN interaction: P = 0.55). APN recruited larger previously silent AP clusters during the ambient condition (Δ4 ± 3; P < 0.02) and -10 mmHg (Δ4 ± 3; P < 0.01), but not -40 mmHg (Δ0 ± 2; P = 0.53; LBNP-by-APN: P < 0.01). LBNP did not affect AP latency. However, APN reduced AP latency similarly during all conditions (ambient pressure: Δ-0.04 ± 0.04s, -10 mmHg: Δ-0.03 ± 0.03s, -40 mmHg: Δ-0.03 ± 0.04s; main effect APN: P < 0.01; LBNP-by-APN: P = 0.48). These data indicate that apneic and baroreflex mechanisms appear to additively modify the axonal discharge rate of previously active sympathetic postganglionic neurons and interact to affect recruitment of previously silent sympathetic neurons. Reductions in AP latency due to apneic stress were not impacted by baroreflex unloading.NEW & NOTEWORTHY Discrete physiological stressors differentially affect sympathetic postganglionic neuronal rate-, population-, and temporal-coding strategies. When performing end-expiratory apnea (APN) during graded baroreflex unloading via lower body negative pressure (LBNP), we found: 1) augmented sympathetic axonal firing probability, 2) recruitment of larger and previously silent sympathetic postganglionic neurons at ambient and -10 mmHg, but not -40 mmHg LBNP, and 3) APN reduced axonal discharge latency similarly across all conditions, independent of the level of baroreflex unloading.

Cerebrovascular Effects of Lower Body Negative Pressure at 3T MRI : Implications for Long‐Duration Space Travel

Optic disc edema develops in most astronauts during long-duration spaceflight. It is hypothesized to result from weightlessness-induced venous congestion of the head and neck and is an unresolved health risk of space travel. Determine if short-term application of lower body negative pressure (LBNP) could reduce internal jugular vein (IJV) expansion associated with the supine posture without negatively impacting cerebral perfusion or causing IJV flow stasis. Prospective. Nine healthy volunteers (six women). 3T/cine two-dimensional phase-contrast gradient echo; pseudo-continuous arterial spin labeling single-shot gradient echo echo-planar. The study was performed with two sequential conditions in randomized order: supine posture and supine posture with 25 mmHg LBNP (LBNP25 ). LBNP was achieved by enclosing the lower extremities in a semi-airtight acrylic chamber connected to a vacuum. Heart rate, bulk cerebrovasculature flow, IJV cross-sectional area, fractional IJV outflow relative to arterial inflow, and cerebral perfusion were assessed in each condition. Paired t-tests were used to compare measurement means across conditions. Significance was defined as P < 0.05. LBNP25 significantly increased heart rate from 64 ± 9 to 71 ± 8 beats per minute and significantly decreased IJV cross-sectional area, IJV outflow fraction, cerebral arterial flow rate, and cerebral arterial stroke volume from 1.28 ± 0.64 to 0.56 ± 0.31 cm2 , 0.75 ± 0.20 to 0.66 ± 0.28, 780 ± 154 to 708 ± 137 mL/min and 12.2 ± 2.8 to 9.7 ± 1.7mL/cycle, respectively. During LBNP25 , there was no significant change in gray or white matter cerebral perfusion (P=0.26 and P=0.24 respectively) and IJV absolute mean peak flow velocity remained ≥4cm/sec in all subjects. Short-term application of LBNP25 reduced IJV expansion without decreasing cerebral perfusion or inducing IJV flow stasis. 1 TECHNICAL EFFICACY STAGE: 1.

B-PO05-138 CATHETER ABLATION OF ATRIAL FIBRILLATION IS NOT ASSOCIATED WITH DESTRUCTION OF CARDIAC AFFERENTS DRIVING VOLUME-REGULATING AUTONOMIC REFLEXES

We have previously shown abnormal cardiac volume-sensitive reflexes (whose receptors are co-located in veno-atrial tissue) in AF patients. Whether unintended nerve disruption from Pulmonary Vein Isolation (PVI) could also occur is unknown. To evaluate whether PVI disrupts afferent volume-sensitive reflexes. We consecutively studied paroxysmal/persistent AF patients pre-PVI (n =18) and recontacted them for a post-PVI visit. We excluded those with AF recurrence/procedural complications, allowing redo procedures if AF-free >6 months. A cohort of 9 patients consented to both visits. We measured beat-to-beat mean arterial pressure (MAP) and heart rate (HR) continuously while testing reflex vasoconstriction from Lower Body Negative Pressure (LBNP), which offloads veno-atrial volume receptors, using forearm blood flow (FBF∝1/vascular resistance), at 0, -20 & -40 mmHg LBNP by venous occlusion plethysmography. We withheld anti-hypertensives/arrhythmics (5 half-lives). Mean age was 64±3 years; 78% male; BMI 28±1 kg/m2; LA size 37±2 ml/m2; and left ventricular function 65±3 %. We performed single ring radiofrequency PVI (PV and posterior wall, n=7); cryoablation (n=1) and wide antral PVI (n=1). Redo in n=2. During LBNP, MAP decreased slightly both pre (-1.6±3%) and post-PVI (-2.8±1.8%); P=0.7. HR increased similarly (P=0.7) pre (10.6±6.4%) and post-PVI (7.2±1.5%). FBF was unchanged (P=0.8), Figure. The reflex response to decreased cardiac volume is not altered by PVI. More work to assess other lesion sets or ganglionated plexi ablation is needed to ensure absence of adverse autonomic remodelling due to catheter ablation for AF.

Internal Jugular Vein Volume Pulsatility during Head‐Down Tilt and Lower Body Negative Pressure

The objective of this study was to investigate the impact of internal jugular vein (IJV) distension on blood volume pulsatility. We hypothesized that pulsatility would be enhanced during cephalad fluid shifts and attenuated during caudal fluid shifts. Thirteen young healthy participants (8/5 male/female) were assessed during supine rest (repeated); 3 and 6 degrees of head-down tilt (HDT); and -20, -30, -40 mmHg of lower body negative pressure (LBNP) after at least 5 minutes in each condition. The order of the HDT and LBNP protocols was randomized. Right-side IJV blood volume was estimated using widefield near-infrared spectroscopy and quantified as jugular venous optical attenuation (JVA). Central venous pressure (CVP) was measured using a transducer attached to a catheter inserted into an antecubital vein in the right arm and corrected to the level of the heart. Participants were rightward tilted throughout to facilitate venous pressure measurements. Continuous signals were acquired for 30s in each condition and time-synced at 60Hz. Volume and pressure pulsatility were calculated as the differences between peak and nadir points along the cardiac cycle in JVA and CVP, respectively. Pressure-volume loops were created after ensemble averaging waveforms normalized to the cardiac cycle. Area of the pressure-volume loops and slope of the pressure-volume relationship during atrial filling (i.e., venous drainage) were assessed. Group mean JVA and CVP were highest during 6° HDT and lowest during -40 mmHg LBNP (see figure). Compared to baseline, JVA pulsatility was attenuated during -30 mmHg LBNP (P=0.048) and -40 mmHg LBNP (P=0.004) and was unchanged during each level of HDT (P>0.9). In contrast, CVP pulsatility was attenuated compared to baseline during -20, -30, and -40 mmHg LBNP (P<0.001) and was unchanged during each level of HDT (P>0.68). Area (P=0.088) and slope (P=0.269) of the pressure-volume loop were unchanged across all conditions. In multiple regression analysis, loop area was associated directly with pressure amplitude (P<0.001) but not volume amplitude (P=0.130); model r2=0.69. These data suggest that, while volume pulsatility did decrease when venous distension was reduced by higher levels of LBNP, this was primarily a function of changes in pressure pulsatilty. IJV compliance appeared to be unchanged across a wide range of venous distension. This work has relevance to states where venous pressure and distension are altered, including heart failure and hemorrhagic shock.

Cerebral Blood Flow Pulsatility Index is Unchanged during Superimposed Lower‐Body Negative Pressure in Head‐Up Tilt in Anterior and Posterior Cerebral Circulations

Lower body negative pressure (LBNP) chambers can be utilized to experimentally-elicit reductions in blood pressure, cerebral blood flow (CBF) and associated symptoms of presyncope. There is high between-individual variability in tolerance to LBNP, however the underlying physiological responses affecting tolerance remains unclear. Pulsatility in CBF may affect LBNP tolerance, as more pulsatile flow may protect the delivery of oxygen to cerebral tissues in hypotension. We aimed to assess the pulsatility index (PI) in anterior and posterior cerebral circulations during LBNP in superimposed head-up tilt (HUT), where gravitational-induced blood volume re-distribution augments volume unloading during LBNP. We recruited and included 12 healthy male participants, and instrumented them in a custom-build integrated 45° HUT-LBNP chamber. Participants were instrumented for heart rate (HR; ECG), end-tidal CO2 (PETCO2; mouthpiece and gas analyzer),beat-by-beat mean arterial pressure (MAP; finometer), and middle and posterior cerebral artery velocity (MCAv, PCAv; transcranial Doppler ultrasound). All measures were recorded during baseline (BL) and during -50 mmHg LBNP exposure up to a maximum of 10-minutes (600s). Presyncope was defined as a 30% reduction in systolic blood pressure (i.e., investigator stop) or onset subjective symptoms (i.e., participant stop). We quantified all variables as a 30-sec mean bin during BL and the final 30-sec of LBNP prior to presyncope. MAP, MCAv and PCAv PI was calculated as systolic-diastolic/mean. LBNP tolerance time was 480.9±134.3 sec. HR increased from 72.3±10.3 (BL) to 103.0±18.2 bpm (P<0.01) during LBNP, suggesting a baroreflex response, and PETCO2 was reduced from 32.0±2.9 (BL) to 28.7±2.9 Torr (P<0.01) during LBNP, suggesting mild hyperventilation. MAP decreased from 84.8±11.3 (BL) to 76.2±14.7 mmHg (P<0.01) during LBNP, with associated reductions in mean MCAv from 52.1±15.0 to 44.3±14.5 cm/s (P<0.01), and mean PCAv from 35.5±12.6 to 29.0±9.9 cm/s (P<0.01) during LBNP. MAP PI was reduced from 0.76±0.2 to 0.59±0.02 (P<0.01) during LBNP. However, MCAv and PCAv PI were unchanged during LBNP (P=0.6 and P=0.9, respectively). Our results suggest that although MAP and MAP PI were both reduced during LBNP at pre-syncope, with associated reductions in mean MCAv and PCAv, MCAv and PCAv PI remained stable with LBNP in HUT. These findings suggest that PI in CBF variables at the cardiac frequency are not related to LBNP tolerance in healthy men.

The effect of hypercapnia on regional cerebral blood flow regulation during progressive lower-body negative pressure.

Previous work indicates that dynamic cerebral blood flow (CBF) regulation is impaired during hypercapnia; however, less is known about the impact of resting hypercapnia on regional CBF regulation during hypovolemia. Furthermore, there is disparity within the literature on whether differences between anterior and posterior CBF regulation exist during physiological stressors. We hypothesized: (a) lower-body negative pressure (LBNP)-induced reductions in cerebral blood velocity (surrogate for CBF) would be more pronounced during hypercapnia, indicating impaired CBF regulation; and (b) the anterior and posterior cerebral circulations will exhibit similar responses to LBNP. In 12 healthy participants (6 females), heart rate (electrocardiogram), mean arterial pressure (MAP; finger photoplethosmography), partial pressure of end-tidal carbon dioxide (PETCO2), middle cerebral artery blood velocity (MCAv) and posterior cerebral artery blood velocity (PCAv; transcranial Doppler ultrasound) were measured. Cerebrovascular conductance (CVC) was calculated as MCAv or PCAv indexed to MAP. Two randomized incremental LBNP protocols were conducted (-20, -40, -60 and -80mmHg; three-minute stages), during coached normocapnia (i.e., room air), and inspired 5% hypercapnia (~ + 7mmHg PETCO2 in normoxia). The main findings were: (a) static CBF regulation in the MCA and PCA was similar during normocapnic and hypercapnic LBNP trials, (b) MCA and PCA CBV and CVC responded similarly to LBNP during normocapnia, but (c) PCAv and PCA CVC were reduced to a greater extent at -60mmHg LBNP (P = 0.029; P < 0.001) during hypercapnia. CBF regulation during hypovolemia was preserved in hypercapnia, and regional differences in cerebrovascular control may exist during superimposed hypovolemia and hypercapnia.

Enhanced Blood Supply Through Lower Body Negative Pressure During Slow-Paced, High Load Leg Press Exercise Alters the Response of Muscle AMPK and Circulating Angiogenic Factors

Lower body negative pressure (LBNP) is an established method of simulating the gravitational effects of orthostasis on the cardiovascular system during space flight or at supine body position on Earth. We hypothesized that LBNP added onto leg press exercise would promote leg muscle perfusion, stimulate oxygen consumption, and modify acute molecular responses. Eighteen subjects performed fifteen slow-paced concentric (4 s) and eccentric contractions (4 s) without or with 40 mmHg LBNP. Force corresponding to 6% of the one-repetition maximum (1-RM) at knee flexion gradually increased to 60% 1-RM within the first half of the range of motion, thereafter remaining constant. AMPK and P-AMPK protein expression was determined in biopsies of vastus lateralis. Venous blood samples were used to measure angiogenic factors. Physiological responses to LBNP included an elevated EMG amplitude, higher heart rate and doubling of the cardiac output compared to control (p < 0.001). Muscle total hemoglobin was increased by around 20 μmol/l vs. control (p < 0.001), accompanied by decreasing tissue oxygen saturation and elevated oxygen uptake (p < 0.05). MMP-2 levels were reduced, and the ratio of P-AMPK to AMPK elevated after exercise with LBNP (p < 0.05). MMP-9 similarly increased in both groups, whereas endostatin was only elevated in the control group (p < 0.05). Our results indicate facilitated peripheral blood supply and higher oxygen exploitation leading to activation of the energy sensor AMPK and differential regulation of angiogenic factors involved in muscle tissue remodeling and capillary growth. Simulating orthostasis with LBNP might promote beneficial structural adaptations of skeletal muscles during resistance exercise and contribute to future exercise countermeasures achieving increased muscle strength and endurance during space flight.

MNX (Medium Duration Nutrition and Resistance-Vibration Exercise) Bed-Rest: Effect of Resistance Vibration Exercise Alone or Combined With Whey Protein Supplementation on Cardiovascular System in 21-Day Head-Down Bed Rest.

Current inflight countermeasures do not completely prevent bone and cardiovascular changes induced by microgravity. High load Resistance Exercise combined with whole body Vibration (RVE) demonstrated benefits on bone and cardiovascular system during previous Head-Down Bed Rest (HDBR) studies. We examined the effectiveness of RVE alone or combined with a nutritional supplementation of Whey protein (NeX) on cardiovascular deconditioning. Eight male subjects (age 34 ± 8 years) in a crossover design completed three 21-day HDBR campaigns (Control-CON, RVE, and NeX). Pre and post HDBR Orthostatic Tolerance (OT) was evaluated by a 15-min head-up tilt test followed by increasing levels of Lower Body Negative Pressure (LBNP). Heart rate (HR), blood pressure (BP), and Sympathetic Index (ΣI) through spectral analysis were measured during OT test. Plasma Volume (PV), and Maximal Oxygen Uptake (VO2max) were measured before and after each campaign. Left ventricular mass, left ventricular end diastolic (LVEDV), end systolic (LVESV), stroke (SV) volumes, and circumferential deformation at rest and during an orthostatic stress simulated by a 30 mmHg LBNP were measured by cardiac MRI. RVE failed to prevent any change in these variables and NeX did not have any additional effect over exercise alone. In the 3 groups, (1) OT time dropped similarly (bed rest p < 0.001), (2) HR and ΣI were increased at rest at the end of HDBR and HR increased markedly during LBNP-tilt test, with inability to increase further the ΣI, (3) PV dropped (bed rest p < 0.001), along with LVEDV, LVESV and SV (p = 0.08, p < 0.001, and p = 0.045, respectively), (4) Left ventricle mass did not change significantly, (5) Deformation of the heart assessed by global circumferential strain was preserved and early diastolic circumferential strain rate was increased during orthostatic stress at the end of HDBR, illustrating preserved systolic and diastolic function respectively, without any difference between groups. Despite the drop in PV and LV volumes, RVE and NeX tended to alleviate the decrease in VO2max. In conclusion, RVE and NeX failed to prevent the cardiovascular deconditioning induced by a 21 day-HDBR.