Shepherd, John R.A.

New Findings What is the central question of this study? Do humans with high‐affinity haemoglobin (HAH) demonstrate attenuated skeletal muscle deoxygenation during normoxic and hypoxic exercise? What is the main finding and its importance? Examination of near‐infrared spectroscopy‐derived muscle oxygenation profiles suggests that fractional oxygen extraction is blunted during hypoxic exercise in humans with HAH compared with control subjects. However, muscle tissue oxygen saturation levels were higher in humans with HAH during exercise in normoxia compared with control subjects. These alterations in fractional oxygen extraction in humans with HAH might influence blood flow regulation and exercise capacity during hypoxia. AbstractRecently, researchers in our laboratory have shown that humans with genetic mutations resulting in high‐affinity haemoglobin (HAH) demonstrate better maintained aerobic capacity and peak power output during hypoxic exercise versus normoxic exercise in comparison to humans with normal‐affinity haemoglobin. However, the influence of HAH on tissue oxygenation within exercising muscle during normoxia and hypoxia is unknown. Therefore, we examined near‐infrared spectroscopy‐derived oxygenation profiles of the vastus lateralis during graded cycling exercise in normoxia and hypoxia among humans with HAH (n = 5) and control subjects with normal‐affinity haemoglobin (n = 12). The HAH group elicited a blunted increase of deoxygenated haemoglobin + myoglobin during hypoxic exercise compared with the control group (P = 0.03), suggesting reduced fractional oxygen extraction in the HAH group. In addition, the HAH group maintained a higher level of muscle tissue oxygen saturation during normoxic exercise (HAH, 75 ± 4% vs. controls, 65 ± 3%, P = 0.049) and there were no differences between groups in muscle tissue oxygen saturation during hypoxic exercise (HAH, 68 ± 3% vs. controls, 68 ± 2%, P = 0.943). Overall, our results suggest that humans with HAH might demonstrate divergent patterns of fractional oxygen extraction during hypoxic exercise and elevated muscle tissue oxygenation during normoxic exercise compared with control subjects.

Abstract Elite performing men continue to record faster record times in running events compared to women. These sex‐based differences in running speed and endurance in humans are expected based on sexual dimorphisms that contribute to differences in the determinants of aerobic performance. Comparatively, the sexual dimorphisms contributing to sex‐based differences in elite aerobic performance are not ubiquitous across other species that compete in running events. The purpose of this review is to offer a framework and model for ongoing discussions of the physiological determinants and ultimately limits of physical performance. The records for average running speed of champion athletes were delineated by sex for thoroughbred horses, greyhound dogs, and humans. Male and female performances within each of these species are being optimized by training, nutrition, and financial incentives, and are approaching a performance maximum. For horses and greyhounds breeding also plays a role. Analysis of athletic records shows that there is a sex‐related difference of ~10% or more in elite athletic performance for humans; however, the upper limit of performance does not appear to be different between sexes for thoroughbred horses and greyhound dogs. In the context of the nil sex differences in running performance in thoroughbreds and greyhounds, we discuss the physiological role of sexual dimorphisms on sex‐specific limits to running performance. We highlight that studies on both human and animal performance in athletic events stimulate critical physiological questions and drive novel research.

Background In humans, maximal exercise capacity is reduced during hypoxic environmental conditions. In response to hypoxia, the hemoglobin binding affinity is decreased, promoting oxygen off‐loading at the tissue level and causing a right‐shift of the oxyhemoglobin dissociation curve (ODC). However, comparative biology demonstrates many species (e.g. Bar‐headed geese), who are adapted to hypoxia instead have high‐affinity hemoglobin (left‐shifted ODC). Therefore we studied unique human participants with hemoglobinopathies that result in high affinity hemoglobin (HAH) to investigate the role of oxygen affinity in exercise performance and tissue oxygenation during hypoxic conditions. Methods HAH participants (n=5, 1M/4F, Age= 34±11 yr, P 50 =16±1 mmHg) and matched control participants (n=12, 5M/7F, age=39±12 yr, and P 50 =26±1 mmHg) completed two incremental exercise tests to exhaustion on a cycle ergometer under two conditions: 1) room air (F i O 2 = 0.21, NORM), and 2) normobaric hypoxia gas mixture (F i O 2 = 0.15, HYP), separated by a 45 minute break. The vastus lateralis (VL) muscle was studied using frequency‐domain near infrared spectroscopy (NIRS) to measure concentrations of oxygenated ([OxyHb+Mb]), deoxygenated ([DeoxyHb+Mb]) hemoglobin and myoglobin, and tissue saturation (S t O 2 ). Data were analyzed using a three‐way (Inspirate [HYP, NORM], Hb type [Normal, HAH], exercise intensity [Rest, 40, 60, 80, 100% normoxic PPO]) univariate analysis of variance. Results HAH had an attenuated decline in exercise tolerance (peak power output at maximal exertion) in HYP compared to controls (HAH: 2.9±6.4% decline vs CTL: 11.7±5.4% decline, P<0.05). There was a significant effect of Hb type on [DeoxyHb+Mb] (P=0.048) suggesting less O 2 offloading in HAH compared to controls in the VL at rest and throughout graded exercise in both NORM and HYP. There were no significant effects of inspirate or exercise intensity on [DeoxyHb+Mb], and no significant differences in [OxyHb+Mb] between HAH and controls. There was a significant effect of Hb type on S t O 2 in NORM controls (P<0.01) However, during hypoxic exercise S t O 2 was the same in both HAH and controls except at end exercise (HAH 64.1±6.4% vs CTL: 62.1±4.2%, P<0.05). Conclusion HAH individuals have less oxygen off‐loading at rest and during exercise but higher tissue saturation during exercise in NORM compared to controls. Our results suggest that the similar levels of exercise capacity in hypoxia for HAH are due, at least in part, to maintenance of microvascular extraction similar to that during normoxic exercise. Future studies utilizing direct measurements of skeletal muscle blood flow during exercise are needed to understand integrative exercise in HAH patients. Support or Funding Information This work was funded by NIH R‐35‐HL139854 and the Mayo Foundation. Muscle oxygenation during graded exercise tests in normoxia and hypoxia. Data are presented as means ± standard error. Abbreviations: [OxyHb+Mb], concentration of oxygenated hemoglobin+myoglobin; [DeoxyHb+MB], concentration of deoxygenated hemoglobin+ myoglobin; PPO, peak power output. † denotes significant main effect of Hb types across all workloads. * denotes significant difference from CTL at same %PPO. Figure 1

Background Lower body negative pressure (LBNP) simulates hemorrhage, evoking compensatory cardiovascular mechanisms of increased heart rate (HR) while stroke volume (SV) and cardiac output (CO) progressively decline, ultimately leading to presyncope. Previous work has shown that circulating levels of plasma epinephrine are elevated during LBNP; however, the role of early administration of epinephrine in compensatory cardiovascular mechanisms to simulated hemorrhage is not well understood. Purpose We hypothesized that during simulated hemorrhage, a low‐dose exogenous infusion of epinephrine in humans would: 1) augment HR response, 2) attenuate the decline of SV and CO, and 3) prolong time to tolerance. Methods Nine healthy subjects (5M/4F, 35±9yrs, BMI 25±1 kg·m −2 ) participated in LBNP on two separate study days. Protocols on each day consisted of a 10‐minute pre‐infusion baseline, 15‐minute infusion period (randomized to saline or epinephrine, dose = 0.1 μg·kg −1 ·min −1 , delivered through a peripherally inserted central catheter), 10‐minute post‐infusion baseline, and 5 minute stages of progressive LBNP with stepwise increases of 10–15 mmHg negative pressure until tolerance (systolic blood pressure less than 80 mmHg, bradycardia, or voluntary termination due to presyncopal symptoms). Heart rate (HR; electrocardiogram) and blood pressure (BP; radial arterial catheter) were recorded continuously. Cardiovascular function (HR, SV, CO, and systemic vascular resistance (SVR)) was determined using arterial waveform analysis. Data were analyzed via percent tolerance using repeated measures ANOVA with student method for pairwise comparisons. Results Infusion of exogenous epinephrine lowered MAP (p<0.01) and diastolic blood pressure (DBP; p<0.001), but raised systolic blood pressure (SBP; p=0.019) throughout all stages of LBNP. Epinephrine compared to saline augmented mean HR, SV and CO to the penultimate stage of LBNP (p≤0.01 for all cardiac measures) and concurrently reduced SVR (p=0.002). However, time to tolerance was not significantly different between epinephrine and saline conditions (1415 ± 79 vs. 1475 ± 150s, respectively; p=0.69). Discussion Our findings suggest that during simulated hemorrhage exogenous epinephrine can augment cardiac function without increasing tolerance to progressive central hypovolemia. During the epinephrine infusion, benefits to cardiac function may have been limited by reduced vascular resistance. Support or Funding Information The United States Department of Defense W81XWH‐13‐2‐0038

Background Maintaining baroreflex sensitivity is integral to ensuring blood pressure maintenance and end‐organ perfusion during hemorrhage. This is especially true under conditions of hypoxemia, where decreased tolerance to central hypovolemia is commonly observed. Hypoxia activates peripheral chemoreceptors and reduces baroreceptor‐vascular resistance reflex sensitivity. However, a direct role for the peripheral chemoreceptors in the hemodynamic response to hemorrhage in hypoxia remains unclear. Objective We tested the hypothesis that attenuation of peripheral chemoreceptor activity (low‐dose dopamine; 1–2 μg/kg/min) during acute hypoxia would promote maintenance of mean arterial pressure, pulse pressure, and other hemodynamic markers [heart rate (HR), stroke volume (SV), cardiac output (CO), total peripheral resistance (TPR)] during simulated hemorrhage. Methods Ten healthy men (35±3 years, BMI 25±1 kg/m 2 ) participated in a randomized, double‐blind, placebo controlled study. Participants completed two experimental visits (saline and low‐dose dopamine) at which lower body negative pressure (LBNP) was conducted during hypoxia (SpO 2 ~85%; PaO2 ~52 mmHg) delivered via a non‐rebreather mask. Ventilation [V̇ (turbine transducer)], arterial blood pressure (brachial artery catheter), HR (ECG), and central venous pressure (peripherally inserted central catheter) were measured continuously and SV, CO, and TPR were calculated. Following a 5‐min rest, participants underwent LBNP for 5‐min at 3 intervals (−15, −30, and −45 mmHg). Outcome values were analyzed and compared using a mixed linear model (SAS PROC MIXED) with a direct product AR(1) covariance structure using indicator variables for saline vs. dopamine. Data from the mixed linear model are reported as an estimated change per 15 mmHg of LBNP. Results Dopamine had no effect on hypoxia P a O 2 , P a CO 2 , V̇ or respiratory rate (all P >0.05) and this relationship persisted throughout all LBNP stages. The mixed linear model showed no effect of dopamine vs. saline on changes in: central venous pressure (−1.1±0.4 vs −1.3±0.3 mmHg, P =0.15), mean arterial pressure (−4.7±2.3 vs −5.3±1.4 mmHg, P =0.25), pulse pressure (−6.7±1.5 vs −8.3±1.4 mmHg, P =0.50), HR (7.8±1.5 vs 6.1±1.5 bpm, P =0.41), SV (−10.9±1.9 vs −11.6±3.3 mL, P =0.41), CO (−0.3±0.2 vs −0.4±0.4 L · min −1 , P =0.31) or TPR (0.2±0.4 vs 0.7±0.6 mmHg · L · min −1 P =0.27). Discussion Intravenous infusion of low‐dose dopamine designed to attenuate peripheral chemoreflex activity during acute hypoxia did not alter the integrated hemodynamic response to simulated hemorrhage. Our data suggests that peripheral chemoreceptors have a limited role during hypoxic central hypovolemia. More research is needed to understand the sensitivity and cross‐talk between chemoreceptors and baroreceptors during hemorrhage. Support or Funding Information Funding: W81XWH‐13‐2‐0038 (DOD), DK90541 (NIH), UL1 TR000135 (NIH).

Objective Bronchopulmonary dysplasia (BPD) is a condition of neonatal chronic lung disease due to disruption or dysregulation of pulmonary development. It is associated with decreased septation, alveolar hypoplasia, and abnormal disruption of pulmonary alveolar capillaries resulting in increased pulmonary resistance. However, the pathophysiology of BPD in the large conducting airways is not fully understood. The objective of our study was to determine if the luminal area of the central airways, measured by imaging, are altered in patients with a history of BPD. We hypothesized that compared to age‐ and sex‐matched controls, BPD patients would have a decreased luminal area of the central conducting airways and lung volume. Methods Fourteen BPD patients (8M/6F; Age=5.7±5.5 yrs) who had undergone a chest computed tomography (CT) scan were retrospectively identified. Age and sex matched controls (8M/6F, Age=5.6±5.3 yrs) who had undergone a CT scan for trauma were used for comparison. Measurement and analysis were performed by a single investigator using commercially available software (Aquarius, TeraRecon Inc.) that reconstructs the airways into 3D. Measurements of luminal airway area were conducted at three points based on anatomic bifurcations (corresponding to the beginning, middle and end of the airway) for each of the following structures: trachea, left main bronchus, left upper lobe, left lower lobe, right main bronchus, intermediate bronchus, and right upper lobe. Each airway’s luminal area was calculated based on the averages of the three measures obtained for each anatomic airway structure. Independent Student’s t‐test was used for statistical analysis. Results Airway luminal area was not different between BPD patients and matched controls (trachea: 89.8±57.1 vs 87.4±58.6 mm 2 ; right main bronchus: 58.8±40.8 vs 64.0±44.5 mm 2 ; intermediate bronchus: 33.6±21.2 vs 41.0±28.9 mm 2 ; left main bronchus: 39.3±27.1 vs 46.6±36.5 mm 2 ; left lower lobe: 22.7±14.6 vs 29.9±18.9 mm 2 ; p>0.05 for all comparisons; BPD and controls, respectively). Total lung volume detected in the CT scans was not different between BPD patients and matched controls (3069±1073 vs 3215±984 cm 3 , p>0.05). Conclusions Our results suggest the luminal areas of the large conducting airways in patients with BPD are not different from matched controls. Given the degree of respiratory complications and sequelae associated with BPD, our findings suggest future research should focus on alveolar and vascular impaired physiological and anatomic developments versus the large conducting airways. Support or Funding Information National Institutes of Health Grants R35‐HL139854 (MJJ), F32 HL‐131151 (SEB), T32 DK‐007352‐39 (CCW) and Mayo Clinic Internal Funding and Natural Science and Engineering Council of Canada RGPIN‐2019‐04615 (PBD)

Background Despite improvements in military aircraft safety, many aviation incidents are attributed to hypoxia. The correlation between moderate hypoxia and cognitive function are well studied but results remain controversial. Comparatively, the effects of severe hypoxia are still not well understood. In the present study, we examined the executive cognitive function and physiological indices under both moderate and severe hypoxia conditions. Purpose To examine the relationship between executive function and physiological indices during acute hypoxia exposure. Methods Ten young healthy participants (4M/6F, 29±4 years) completed two different levels of hypoxia: 1) 10 minute moderate hypoxia (MH; F i O 2 = 0.118) and 2) 3 minute severe hypoxia (SH; F i O 2 = 0.077) following 3 minute baseline (BL; F i O 2 =0.209). Reaction time and errors of commission (defined as a response to a non‐target stimulus) were evaluated continuously using a custom Go/No‐go test at baseline and throughout hypoxia exposure. In this Go/No‐go test, participants were required to push a key when upper case “A” or “@” appeared but not when lower case “a” appeared on a computer monitor as quick and as accurately as possible. The stimuli were presented one at a time at any one of nine differing locations across the screen. Cerebral oxygenation (rSO 2 ) was continuously monitored with near‐infrared spectroscopy (NIRS) positioned above the left supraorbital ridge. Finger oxygen saturation (S p O 2 ) was continuously assessed using a pulse oximetry. Data were analyzed in every 30 seconds and are presented as mean±SE. Two‐way ANOVA and Bonferroni test for multiple comparisons was used to evaluate significant differences. Results Both S p O 2 and rSO 2 were reduced with SH (S p O 2 %, BL: 99.3±0.1% vs. SH: 70.7±1.5%, P<0.01; rSO 2 %, BL: 70±4% vs. SH: 58±4%, P<0.01). In SH, correlations of S p O 2 and rSO 2 were significant to the number of errors during the cognitive performance task (S p O 2 ; R= −0.65, rSO 2 ; R= −0.58, Both P<0.01). SH significantly increased errors of commission of the last 30 seconds in 3 minute hypoxic exposure. (BL: 0.6±0.2 errors vs. SH: 1.7±0.5 errors, P<0.01). SH significantly decreased reaction time over hypoxic exposure (BL: 0.43±0.01 sec vs. SH: 0.41±0.01 sec, P<0.01). Comparatively, MH did not elicit an increase in the number of errors of commission nor a decrease in reaction time (Both, P>0.05). Conclusion Executive function deteriorated during severe hypoxia. Oxygenation of the prefrontal lobe which controls inhibition may be related to the increase in commission errors. Devices that monitor physiological indices such as S p O 2 or rSO 2 might be beneficial for a model to predict executive deterioration. Support or Funding Information This project was funded by the Office of Naval Research (ONR), grant number N00014‐18‐D‐7001‐TO1.