Selected and shared hematological responses to apnea in elite human free-divers and northern elephant seals (Mirounga angustirostris)

Despite elite, human free-divers achieving incredible feats in competitive free-diving, there has yet to be a study that compares consummate divers, (i.e. northern elephant seals) to highly conditioned free-divers (i.e., elite, competitive free-diving humans). Herein, we compare these two diving models and suggest that hematological traits detected in seals reflect species-specific specializations, while hematological traits shared between the two species are fundamental mammalian characteristics. Arterial blood samples were analyzed in elite, human, free-divers (n=14) during a single, maximal volitional apnea and in juvenile northern elephant seals (n=3) during rest-associated apnea. Humans and elephant seals had comparable apnea durations (~6.5 mins) and end-apneic arterial PO2 (humans: 40.4±3.0mmHg (mean±SE), seals: 27.1±5.9mmHg; p=0.2). Despite similar increases in arterial PCO2 (humans: 33±5%, seals: 16.3±5%; P=0.2), only humans experienced reductions in pH from baseline (humans: 7.45±0.01, seal: 7.39±0.02) to end apnea (humans: 7.37±0.01, seals: 7.38±0.02; p<0.0001). Hemoglobin P50 was greater in humans compared to elephant seals (29.9±1.5 and 28.7±0.6mmHg, respectively; p=0.046). Elephant seals overall had higher COHb levels (5.9±2.6%) compared to humans (0.8±1.2%; p<0.0001); however, following apnea, COHb was reduced in seals (baseline: 6.1±0.3%, end-apnea: 5.6±0.3%), but was slightly elevated in humans (baseline: 0.7±0.1%, end-apnea: 0.9±0.1%; p<0.0002, both comparisons). Our data indicate that during static apnea, seals have reduced hemoglobin P50, greater pH buffering, and increased COHb levels. The differences in hemoglobin P50 is likely due to the differences in the physiological environment between the two species during apnea, whereas enhanced pH buffering and higher COHb may represent traits selected for in elephant seals.

A narrative review of periodic breathing during sleep at high altitude: From acclimatizing lowlanders to adapted highlanders

Periodic breathing during sleep at high altitude is almost universal among sojourners. Here, in the context of acclimatization and adaptation, we provide a contemporary review on periodic breathing at high altitude, and explore whether this is an adaptive or maladaptive process. The mechanism(s), prevalence and role of periodic breathing in acclimatized lowlanders at high altitude are contrasted with the available data from adapted indigenous populations (e.g. Andean and Tibetan highlanders). It is concluded that (1) periodic breathing persists with acclimatization in lowlanders and the severity is proportional to sleeping altitude; (2) periodic breathing does not seem to coalesce with poor sleep quality such that, with acclimatization, there appears to be a lengthening of cycle length and minimal impact on the average sleeping oxygen saturation; and (3) high altitude adapted highlanders appear to demonstrate a blunting of periodic breathing, compared to lowlanders, comprising a feature that withstands the negative influences of chronic mountain sickness. These observations indicate that periodic breathing persists with high altitude acclimatization with no obvious negative consequences; however, periodic breathing is attenuated with high altitude adaptation and therefore potentially reflects an adaptive trait to this environment.

Hemoglobin and cerebral hypoxic vasodilation in humans: Evidence for nitric oxide-dependent and S-nitrosothiol mediated signal transduction

Cerebral hypoxic vasodilation is poorly understood in humans, which undermines the development of therapeutics to optimize cerebral oxygen delivery. Across four investigations (total n = 195) we investigated the role of nitric oxide (NO) and hemoglobin-based S-nitrosothiol (RSNO) and nitrite (NO 2 - ) signaling in the regulation of cerebral hypoxic vasodilation. We conducted hemodilution (n = 10) and NO synthase inhibition experiments (n = 11) as well as hemoglobin oxygen desaturation protocols, wherein we measured cerebral blood flow (CBF), intra-arterial blood pressure, and in subsets of participants trans-cerebral release/uptake of RSNO and NO 2 - . Higher CBF during hypoxia was associated with greater trans-cerebral RSNO release but not NO 2 - , while NO synthase inhibition reduced cerebral hypoxic vasodilation. Hemodilution increased the magnitude of cerebral hypoxic vasodilation following acute hemodilution, while in 134 participants tested under normal conditions, hypoxic cerebral vasodilation was inversely correlated to arterial hemoglobin concentration. These studies were replicated in a sample of polycythemic high-altitude native Andeans suffering from excessive erythrocytosis (n = 40), where cerebral hypoxic vasodilation was inversely correlated to hemoglobin concentration, and improved with hemodilution (n = 6). Collectively, our data indicate that cerebral hypoxic vasodilation is partially NO-dependent, associated with trans-cerebral RSNO release, and place hemoglobin-based NO signaling as a central mechanism of cerebral hypoxic vasodilation in humans.

Manipulation of iron status on cerebral blood flow at high altitude in lowlanders and adapted highlanders

Cerebral blood flow (CBF) increases during hypoxia to counteract the reduction in arterial oxygen content. The onset of tissue hypoxemia coincides with the stabilization of hypoxia-inducible factor (HIF) and transcription of downstream HIF-mediated processes. It has yet to be determined, whether HIF down- or upregulation can modulate hypoxic vasodilation of the cerebral vasculature. Therefore, we examined whether: 1) CBF would increase with iron depletion (via chelation) and decrease with repletion (via iron infusion) at high-altitude, and 2) explore whether genotypic advantages of highlanders extend to HIF-mediated regulation of CBF. In a double-blinded and block-randomized design, CBF was assessed in 82 healthy participants (38 lowlanders, 20 Sherpas and 24 Andeans), before and after the infusion of either: iron(III)-hydroxide sucrose, desferrioxamine or saline. Across both lowlanders and highlanders, baseline iron levels contributed to the variability in cerebral hypoxic reactivity at high altitude (R2 = 0.174, P < 0.001). At 5,050 m, CBF in lowlanders and Sherpa were unaltered by desferrioxamine or iron. At 4,300 m, iron infusion led to 4 ± 10% reduction in CBF (main effect of time p = 0.043) in lowlanders and Andeans. Iron status may provide a novel, albeit subtle, influence on CBF that is potentially dependent on the severity and length-of-stay at high altitude.

Cerebral O2 and CO2 transport in isovolumic haemodilution: Compensation of cerebral delivery of O2 and maintenance of cerebrovascular reactivity to CO2

This study investigated the influence of acute reductions in arterial O₂ content (CaO₂) via isovolumic haemodilution on global cerebral blood flow (gCBF) and cerebrovascular CO₂ reactivity (CVR) in 11 healthy males (age; 28 ± 7 years: body mass index; 23 ± 2 kg/m²). Radial artery and internal jugular vein catheters provided measurement of blood pressure and gases, quantification of cerebral metabolism, cerebral CO₂ washout, and trans-cerebral nitrite exchange (ozone based chemiluminescence). Prior to and following haemodilution, the partial pressure of arterial CO₂ (PaCO₂) was elevated with dynamic end-tidal forcing while gCBF was measured with duplex ultrasound. CVR was determined as the slope of the gCBF response and PaCO₂. Replacement of ∼20% of blood volume with an equal volume of 5% human serum albumin (Alburex® 5%) reduced haemoglobin (13.8 ± 0.8 vs. 11.3 ± 0.6 g/dL; P < 0.001) and CaO₂ (18.9 ± 1.0 vs 15.0 ± 0.8 mL/dL P < 0.001), elevated gCBF (+18 ± 11%; P = 0.002), preserved cerebral oxygen delivery (P = 0.49), and elevated CO₂ washout (+11%; P = 0.01). The net cerebral uptake of nitrite (11.6 ± 14.0 nmol/min; P = 0.027) at baseline was abolished following haemodilution (-3.6 ± 17.9 nmol/min; P = 0.54), perhaps underpinning the conservation of CVR (61.7 ± 19.0 vs. 69.0 ± 19.2 mL/min/mmHg; P = 0.23). These findings demonstrate that the cerebrovascular responses to acute anaemia in healthy humans are sufficient to support the maintenance of CVR.

Pulmonary vascular reactivity to supplemental oxygen in Sherpa and lowlanders during gradual ascent to high altitude

NEW FINDINGS

What is the central question of this study? How does hypoxic pulmonary vasoconstriction and the response to supplemental oxygen change over time at high altitude? What is the main finding and its importance? Lowlanders and partially de-acclimatized Sherpa both demonstrated pulmonary vascular responsiveness to supplemental oxygen that was maintained for 12 days' exposure to progressively increasing altitude. An additional 2 weeks' acclimatization at 5050 m altitude rendered the pulmonary vasculature minimally responsive to oxygen similar to the fully acclimatized non-ascent Sherpa. Additional hypoxic exposure at that time point did not augment hypoxic pulmonary vasoconstriction.

ABSTRACT

Prolonged alveolar hypoxia leads to pulmonary vascular remodelling. We examined the time course at altitude, over which hypoxic pulmonary vasoconstriction goes from being acutely reversible to potentially irreversible. Study subjects were lowlanders (n = 20) and two Sherpa groups. All Sherpa were born and raised at altitude. One group (ascent Sherpa, n = 11) left altitude and after de-acclimatization in Kathmandu for ∼7 days re-ascended with the lowlanders over 8-10 days to 5050 m. The second Sherpa group (non-ascent Sherpa, n = 12) remained continuously at altitude. Pulmonary artery systolic pressure (PASP) and pulmonary vascular resistance (PVR) were measured while breathing ambient air and following supplemental oxygen. During ascent PASP and PVR increased in lowlanders and ascent Sherpa; however, with supplemental oxygen, lowlanders had significantly greater decrease in PASP (P = 0.02) and PVR (P = 0.02). After ∼14 days at 5050 m, PASP decreased with supplemental oxygen (mean decrease: 3.9 mmHg, 95% CI 2.1-5.7 mmHg, P < 0.001); however, PVR was unchanged (P = 0.49). In conclusion, PASP and PVR increased with gradual ascent to altitude and decreased via oxygen supplementation in both lowlanders and ascent Sherpa. Following ∼14 days at 5050 m altitude, there was no change in PVR to hypoxia or O2  supplementation in lowlanders or either Sherpa group. These data show that both duration of exposure and residential altitude influence the pulmonary vascular responses to hypoxia.

Nitric oxide contributes to cerebrovascular shear-mediated dilatation but not steady-state cerebrovascular reactivity to carbon dioxide

Cerebrovascular CO₂ reactivity (CVR) is often considered a bioassay of cerebrovascular endothelial function. We recently introduced a test of cerebral shear-mediated dilatation (cSMD) that may better reflect endothelial function. We aimed to determine the nitric oxide (NO)-dependency of CVR and cSMD. Eleven volunteers underwent a steady-state CVR test and transient CO₂ test of cSMD during intravenous infusion of the NO synthase inhibitor N^G -monomethyl-l-arginine (l-NMMA) or volume-matched saline (placebo; single-blinded and counter-balanced). We measured cerebral blood flow (CBF; duplex ultrasound), intra-arterial blood pressure and P aC O 2 . Paired arterial and jugular venous blood sampling allowed for the determination of trans-cerebral NO₂ ^- exchange (ozone-based chemiluminescence). l-NMMA reduced arterial NO₂ ^- by ∼25% versus saline (74.3 ± 39.9 vs. 98.1 ± 34.2 nM; P = 0.03). The steady-state CVR (20.1 ± 11.6 nM/min at baseline vs. 3.2 ± 16.7 nM/min at +9 mmHg P aC O 2 ; P = 0.017) and transient cSMD tests (3.4 ± 5.9 nM/min at baseline vs. -1.8 ± 8.2 nM/min at 120 s post-CO₂ ; P = 0.044) shifted trans-cerebral NO₂ ^- exchange towards a greater net release (a negative value indicates release). Although this trans-cerebral NO₂ ^- release was abolished by l-NMMA, CVR did not differ between the saline and l-NMMA trials (57.2 ± 14.6 vs. 54.1 ± 12.1 ml/min/mmHg; P = 0.49), nor did l-NMMA impact peak internal carotid artery dilatation during the steady-state CVR test (6.2 ± 4.5 vs. 6.2 ± 5.0% dilatation; P = 0.960). However, l-NMMA reduced cSMD by ∼37% compared to saline (2.91 ± 1.38 vs. 4.65 ± 2.50%; P = 0.009). Our findings indicate that NO is not an obligatory regulator of steady-state CVR. Further, our novel transient CO₂ test of cSMD is largely NO-dependent and provides an in vivo bioassay of NO-mediated cerebrovascular function in humans. KEY POINTS: Emerging evidence indicates that a transient CO₂ stimulus elicits shear-mediated dilatation of the internal carotid artery, termed cerebral shear-mediated dilatation. Whether or not cerebrovascular reactivity to a steady-state CO₂ stimulus is NO-dependent remains unclear in humans. During both a steady-state cerebrovascular reactivity test and a transient CO₂ test of cerebral shear-mediated dilatation, trans-cerebral nitrite exchange shifted towards a net release indicating cerebrovascular NO production; this response was not evident following intravenous infusion of the non-selective NO synthase inhibitor N^G -monomethyl-l-arginine. NO synthase blockade did not alter cerebrovascular reactivity in the steady-state CO₂ test; however, cerebral shear-mediated dilatation following a transient CO₂ stimulus was reduced by ∼37% following intravenous infusion of N^G -monomethyl-l-arginine. NO is not obligatory for cerebrovascular reactivity to CO₂ , but is a key contributor to cerebral shear-mediated dilatation.

GLOBAL REACH 2018: Iron infusion at high altitude reduces hypoxic pulmonary vasoconstriction equally in both lowlanders and healthy Andean highlanders

BACKGROUND

Increasing iron bioavailability attenuates hypoxic pulmonary vasoconstriction in both lowlanders and Sherpa at high altitude. In contrast, the pulmonary vasculature of Andeans suffering with chronic mountain sickness is resistant to iron administration. While pulmonary vascular remodeling and hypertension are characteristic features of chronic mountain sickness, the impact of iron administration in healthy Andeans has not been investigated. If the interplay between iron status and pulmonary vascular tone in healthy Andeans remains intact, this could provide valuable clinical insight into the role of iron regulation at high altitude.

RESEARCH QUESTION

Is the pulmonary vasculature in healthy Andeans responsive to iron infusion?

STUDY DESIGN AND METHODS

In a double-blinded, block-randomized design, 24 healthy high-altitude Andeans and 22 partially acclimatized lowlanders at 4300 m (Cerro de Pasco, Peru), received an i.v. infusion of either iron [iron (III)-hydroxide sucrose; 200mg] or saline. Markers of iron status were collected at baseline and 4 hours after infusion. Echocardiography was performed during room-air breathing (P_IO₂=∼96 mmHg) and during exaggerated hypoxia (P_IO₂=∼73 mmHg), at baseline, and at 2 and 4 hours following the infusion.

RESULTS

Iron infusion reduced pulmonary artery systolic pressure (PASP) by ∼2.5 mmHg in room air (main effect P<0.001), and by ∼7 mmHg during exaggerated hypoxia (main effect P<0.001) in both lowlanders and healthy Andean highlanders. There was no change in PASP following the infusion of saline. Iron metrics were comparable between groups, except for serum ferritin, which was 1.8-fold higher at baseline in the Andeans when compared to lowlanders [95% confidence interval (CI) 74-121 ng/ml vs. 37-70 ng/ml, respectively; P=0.003].

INTERPRETATION

The pulmonary vasculature of healthy Andeans and lowlanders remains sensitive to iron infusion and this response seems to differ from the pathological characteristics of chronic mountain sickness.

Association Between Arterial Oxygen Saturation and Lung Ultrasound B-Lines After Competitive Deep Breath-Hold Diving

Breath-hold diving (freediving) is an underwater sport that is associated with elevated hydrostatic pressure, which has a compressive effect on the lungs that can lead to the development of pulmonary edema. Pulmonary edema reduces oxygen uptake and thereby the recovery from the hypoxia developed during freediving, and increases the risk of hypoxic syncope. We aimed to examine the efficacy of SpO₂, via pulse-oximetry, as a tool to detect pulmonary edema by comparing it to lung ultrasound B-line measurements after deep diving. SpO₂ and B-lines were collected in 40 freedivers participating in an international deep freediving competition. SpO₂ was measured within 17 ± 6 min and lung B-lines using ultrasound within 44 ± 15 min after surfacing. A specific symptoms questionnaire was used during SpO₂ measurements. We found a negative correlation between B-line score and minimum SpO₂ (r_s = −0.491; p = 0.002) and mean SpO₂ (r_s = −0.335; p = 0.046). B-line scores were positively correlated with depth (r_s = 0.408; p = 0.013), confirming that extra-vascular lung water is increased with deeper dives. Compared to dives that were asymptomatic, symptomatic dives had a 27% greater B-line score, and both a lower mean and minimum SpO₂ (all p < 0.05). Indeed, a minimum SpO₂ ≤ 95% after a deep dive has a positive predictive value of 29% and a negative predictive value of 100% regarding symptoms. We concluded that elevated B-line scores are associated with reduced SpO₂ after dives, suggesting that SpO₂ via pulse oximetry could be a useful screening tool to detect increased extra-vascular lung water. The practical application is not to diagnose pulmonary edema based on SpO₂ – as pulse oximetry is inexact – rather, to utilize it as a tool to determine which divers require further evaluation before returning to deep freediving.

Alterations in arterial CO2 rather than pH affect the kinetics of neurovascular coupling in humans

KEY POINTS

We investigated the influence of arterial P C O 2 ( P aC O 2 ) with and without acute experimental metabolic alkalosis on neurovascular coupling (NVC). We assessed stepwise iso-oxic alterations in P aC O 2 prior to and following intravenous NaHCO₃ to acutely elevate arterial pH and [HCO₃ ^- ]. The NVC response was not altered following NaHCO₃ between stepwise P aC O 2 stages; therefore, NVC is acutely mediated by P aC O 2 rather than the prevailing arterial [H⁺ ]/pH. The NVC response was attenuated by 27-38% with -10 mmHg P aC O 2 and the absolute peak change was reduced by -19% with +10 mmHg P aC O 2 irrespective of acutely elevated arterial pH/[HCO₃ ^- ]. The NVC kinetics (i.e. time to peak) were markedly slower with hypercapnia versus hypocapnia (24 ± 5 vs. 7 ± 5 s, respectively) likely indicating an influence of resting cerebrovascular tone on NVC responsiveness.

ABSTRACT

Elevations in cerebral metabolism necessitate appropriate coordinated and localized increases in cerebral blood flow (i.e. neurovascular coupling; NVC). Recent pre-clinical work indicates that arterial P C O 2 ( P aC O 2 ) mediates NVC independently of arterial/extracellular pH; this has yet to be experimentally tested in humans. The goal of this study was to investigate the hypotheses that: (1) the NVC response would be unaffected by acute experimentally elevated arterial pH; rather, P aC O 2 would regulate any changes in NVC; and (2) stepwise respiratory alkalosis and acidosis would each progressively reduce the NVC response. Ten healthy males completed a standardized visual stimulus-evoked NVC test during matched stepwise iso-oxic alterations in P aC O 2 (hypocapnia: -5, -10 mmHg; hypercapnia: +5, +10 mmHg) prior to and following intravenous NaHCO₃ (8.4%, 50 mEq/50 ml) that elevated arterial pH (7.406 ± 0.019 vs. 7.457 ± 0.029; P < 0.001) and [HCO₃ ^- ] (26.2 ± 1.5 vs. 29.3 ± 0.9 mEq/l; P < 0.001). Although the NVC response was collectively attenuated by 27-38% with -10 mmHg P aC O 2 (stage post hoc: all P < 0.05), this response was unaltered following NaHCO₃ (all P > 0.05) irrespective of the higher pH (P = 0.002) at each matched stage of P aC O 2 (P = 0.417). The absolute peak change was reduced by -19 ± 41% with +10 mmHg P aC O 2 irrespective of acutely elevated arterial pH/[HCO₃ ^- ] (stage post hoc: P = 0.022). The NVC kinetics (i.e. time to peak) were markedly slower with hypercapnia versus hypocapnia (24 ± 5 vs. 7 ± 5 s, respectively; stage effect: P < 0.001). Overall, these findings indicate that temporal patterns in NVC are acutely regulated by P aC O 2 rather than arterial pH per se in the setting of acute metabolic alkalosis in humans.

Breath-Hold Diving - The Physiology of Diving Deep and Returning

Breath-hold diving involves highly integrative physiology and extreme responses to both exercise and asphyxia during progressive elevations in hydrostatic pressure. With astonishing depth records exceeding 100 m, and up to 214 m on a single breath, the human capacity for deep breath-hold diving continues to refute expectations. The physiological challenges and responses occurring during a deep dive highlight the coordinated interplay of oxygen conservation, exercise economy, and hyperbaric management. In this review, the physiology of deep diving is portrayed as it occurs across the phases of a dive: the first 20 m; passive descent; maximal depth; ascent; last 10 m, and surfacing. The acute risks of diving (i.e., pulmonary barotrauma, nitrogen narcosis, and decompression sickness) and the potential long-term medical consequences to breath-hold diving are summarized, and an emphasis on future areas of research of this unique field of physiological adaptation are provided.

Temporal changes in pulmonary gas exchange efficiency when breath-hold diving below residual volume

NEW FINDINGS

What is the central question of this study? How does deep breath-hold diving impact cardiopulmonary function, both acutely and over the subsequent 2.5 hours post-dive? What is the main finding and its importance? Breath-hold diving, to depths below residual volume, is associated with acute impairments in pulmonary gas exchange, which typically resolve within 2.5 hours. These data provide new insight into the behaviour of the lungs and pulmonary vasculature following deep diving.

ABSTRACT

Breath-hold diving involves highly integrative and extreme physiological responses to both exercise and asphyxia during progressive elevations in hydrostatic pressure. Over two diving training camps (Study 1 and 2), 25 breath-hold divers (recreational to world-champion) performed 66 dives to 57 ± 20 m (range: 18-117 m). Using the deepest dive from each diver, temporal changes in cardiopulmonary function were assessed using non-invasive pulmonary gas exchange (indexed via the O₂ deficit), ultrasound B-line scores, lung compliance and pulmonary haemodynamics at baseline and following the dive. Hydrostatically induced lung compression was quantified in Study 2, using spirometry and lung volume measurement, enabling each dive to be categorized by its residual volume (RV)-equivalent depth. From both studies, pulmonary gas exchange inefficiency - defined as an increase in O₂ deficit - was related to the depth of the dive (r²  = 0.345; P < 0.001), with dives associated with lung squeeze symptoms exhibiting the greatest deficits. In Study 1, although B-lines doubled from baseline (P = 0.027), cardiac output and pulmonary artery systolic pressure were unchanged post-dive. In Study 2, dives with lung compression to ≤RV had higher O₂ deficits at 9 min, compared to dives that did not exceed RV (24 ± 25 vs. 5 ± 8 mmHg; P = 0.021). The physiological significance of a small increase in estimated lung compliance post-dive (via decreased and increased/unaltered airway resistance and reactance, respectively) remains equivocal. Following deep dives, the current study highlights an integrated link between hydrostatically induced lung compression and transient impairments in pulmonary gas exchange efficiency.

Case Studies in Physiology: Breath-hold diving beyond 100 meters-cardiopulmonary responses in world-champion divers

In this case study, we evaluate the unique physiological profiles of two world-champion breath-hold divers. At close to current world-record depths, the extreme physiological responses to both exercise and asphyxia during progressive elevations in hydrostatic pressure are profound. As such, these professional athletes must be capable of managing such stress, to maintain performing at the forefront human capacity. In both divers, pulmonary function before and after deep dives to 102 m and 117 m in the open sea was assessed using noninvasive pulmonary gas exchange (indexed via the O₂ deficit, which is analogous to the traditional alveolar to arterial oxygen difference), ultrasound B-line scores, airway resistance, and airway reactance. Hydrostatic-induced lung compression was also quantified via spirometry. Both divers successfully performed their dives. Pulmonary gas exchange efficiency was impaired in both divers at 10 min but had mostly restored within a few hours. Mild hemoptysis was transiently evident immediately following the 117-m dive, whereas both divers experienced nitrogen narcosis. Although B-lines were only elevated in one diver postdive, reductions in airway resistance and reactance occurred in both divers, suggesting that the compressive strain on the structural characteristics of the airways can persist for up to 3.5 h. Marked echocardiographic dyssynchrony was evident in one diver after 10 m of descent, which persisted until resolving at ∼77 m during ascent. In summary, despite the enormous hydrostatic and physiological stress to diving beyond 100 m on a single breath, these data provide valuable insight into the extraordinary capacity of those at the pinnacle of apneic performance.NEW & NOTEWORTHY This study shows that world-champion breath-hold divers demonstrate incredible tolerability to extreme levels of hydrostatic-induced lung compression. Immediately following dives to >100 m, there were acute impairments in pulmonary gas exchange efficiency, mild accummulation of extravascular lung fluid, noticable intrathoracic discomfort, and evident nitrogen narcosis, however, within a few hours, these had all mostly resolved.

Influence of iron manipulation on hypoxic pulmonary vasoconstriction and pulmonary reactivity during ascent and acclimatization to 5050 m

KEY POINTS

Iron acts as a cofactor in the stabilization of the hypoxic-inducible factor family, and plays an influential role in the modulation of hypoxic pulmonary vasoconstriction. It is uncertain whether iron regulation is altered in lowlanders during either (1) ascent to high altitude, or (2) following partial acclimatization, when compared to high-altitude adapted Sherpa. During ascent to 5050 m, the rise in pulmonary artery systolic pressure (PASP) was blunted in Sherpa, compared to lowlanders; however, upon arrival to 5050 m, PASP levels were comparable in both groups, but the reduction in iron bioavailability was more prevalent in lowlanders compared to Sherpa. Following partial acclimatization to 5050 m, there were differential influences of iron status manipulation (via iron infusion or chelation) at rest and during exercise between lowlanders and Sherpa on the pulmonary vasculature.

ABSTRACT

To examine the adaptational role of iron bioavailability on the pulmonary vascular responses to acute and chronic hypobaric hypoxia, the haematological and cardiopulmonary profile of lowlanders and Sherpa were determined during: (1) a 9-day ascent to 5050 m (20 lowlanders; 12 Sherpa), and (2) following partial acclimatization (11 ± 4 days) to 5050 m (18 lowlanders; 20 Sherpa), where both groups received an i.v. infusion of either iron (iron (iii)-hydroxide sucrose) or an iron chelator (desferrioxamine). During ascent, there were reductions in iron status in both lowlanders and Sherpa; however, Sherpa appeared to demonstrate a more efficient capacity to mobilize stored iron, compared to lowlanders, when expressed as a Δhepcidin per unit change in either body iron or the soluble transferrin receptor index, between 3400-5050 m (P = 0.016 and P = 0.029, respectively). The rise in pulmonary artery systolic pressure (PASP) was blunted in Sherpa, compared to lowlanders during ascent; however, PASP was comparable in both groups upon arrival to 5050 m. Following partial acclimatization, despite Sherpa demonstrating a blunted hypoxic ventilatory response and greater resting hypoxaemia, they had similar hypoxic pulmonary vasoconstriction when compared to lowlanders at rest. Iron-infusion attenuated PASP in both groups at rest (P = 0.005), while chelation did not exaggerate PASP in either group at rest or during exaggerated hypoxaemia ( P I O 2  = 67 mmHg). During exercise at 25% peak wattage, PASP was only consistently elevated in Sherpa, which persisted following both iron infusion or chelation. These findings provide new evidence on the complex interplay of iron regulation on pulmonary vascular regulation during acclimatization and adaptation to high altitude.

Distinct contributions of skin and core temperatures to flow-mediated dilation of the brachial artery following passive heating

The primary determinant of vascular adaptations to lifestyle interventions, such as exercise and heat therapy, is repeated elevations in vascular shear stress. Whether skin or core temperatures also modulate the vascular adaptation to acute heat exposure is unknown, likely due to difficulty in dissociating the thermal and hemodynamic responses to heat. We found that skin and core temperatures modify the acute vascular responses to passive heating irrespective of the magnitude of increase in shear stress.

Spleen contraction and Hb elevation after dietary nitrate intake

Ingestion of dietary nitrate ([Formula: see text]) is associated with improved exercise tolerance and reduced oxygen (O₂) cost of exercise, ascribed to enhanced mitochondrial efficiency, muscle contractile function, or other factors. Nitrate ingestion has also been found to attenuate the reduction in arterial oxygen saturation ([Formula: see text]) during apnea and to prolong apneic duration. The spleen serves as a dynamic blood pool expelling erythrocytes into the circulation during apnea, and [Formula: see text] and nitric oxide donors may induce vasoactive effects in the mesenteric and splanchnic circulation. Our aim was to investigate the effect of ingestion of concentrated organic [Formula: see text]-rich beetroot juice (BR) on spleen volume and spleen contraction during apnea, and the resulting hemoglobin (Hb) concentration. Eight volunteers performed two apneas of submaximal and maximal duration during prone rest ∼2.5 h after ingesting 70 mL of BR (∼5 mmol [Formula: see text]) or placebo (PL; ∼0.003 mmol [Formula: see text]), on separate days in weighted order. Heart rate and [Formula: see text] were monitored continuously and spleen diameters were measured every minute for triaxial volume calculation. Capillary Hb samples were collected at baseline and after the maximal apnea. Baseline spleen volume was reduced by 66 mL after BR ingestion (22.9%; P = 0.026) and Hb was elevated (+3.0%; P = 0.015). During apneas, spleen contraction and Hb increase were similar between BR and PL conditions (NS). The study shows that dietary [Formula: see text]reduces spleen volume at rest, resulting in increased Hb. This spleen-induced Hb elevation following [Formula: see text] ingestions represents a novel mechanism that could enhance performance in conditions involving exercise, apnea, and hypoxia.NEW & NOTEWORTHY This is the first study to examine changes of spleen volume and circulating Hb following dietary [Formula: see text] supplementation. After dietary [Formula: see text] ingestion, the spleen volume at rest was reduced and Hb was elevated. The spleen contains a dynamic red blood cell reservoir, which can be mobilized and facilitate oxygen transport during various types of physiological stress. This study has revealed an additional, previously unexplored mechanism possibly contributing to the ergogenic effects of dietary [Formula: see text].

The 2018 Global Research Expedition on Altitude Related Chronic Health (Global REACH) to Cerro de Pasco, Peru: an Experimental Overview

NEW FINDINGS

What is the central question of this study? Herein, a methodological overview of our research team's (Global REACH) latest high altitude research expedition to Peru is provided. What is the main finding and its importance? The experimental objectives, expedition organization, measurements and key cohort data are discussed. The select data presented in this manuscript demonstrate the haematological differences between lowlanders and Andeans with and without excessive erythrocytosis. The data also demonstrate that exercise capacity was similar between study groups at high altitude. The forthcoming findings from our research expedition will contribute to our understanding of lowlander and indigenous highlander high altitude adaptation.

ABSTRACT

In 2016, the international research team Global Research Expedition on Altitude Related Chronic Health (Global REACH) was established and executed a high altitude research expedition to Nepal. The team consists of ∼45 students, principal investigators and physicians with the common objective of conducting experiments focused on high altitude adaptation in lowlanders and in highlanders with lifelong exposure to high altitude. In 2018, Global REACH travelled to Peru, where we performed a series of experiments in the Andean highlanders. The experimental objectives, organization and characteristics, and key cohort data from Global REACH's latest research expedition are outlined herein. Fifteen major studies are described that aimed to elucidate the physiological differences in high altitude acclimatization between lowlanders (n = 30) and Andean-born highlanders with (n = 22) and without (n = 45) excessive erythrocytosis. After baseline testing in Kelowna, BC, Canada (344 m), Global REACH travelled to Lima, Peru (∼80 m) and then ascended by automobile to Cerro de Pasco, Peru (∼4300 m), where experiments were conducted over 25 days. The core studies focused on elucidating the mechanism(s) governing cerebral and peripheral vascular function, cardiopulmonary regulation, exercise performance and autonomic control. Despite encountering serious logistical challenges, each of the proposed studies was completed at both sea level and high altitude, amounting to ∼780 study sessions and >3000 h of experimental testing. Participant demographics and data relating to acid-base balance and exercise capacity are presented. The collective findings will contribute to our understanding of how lowlanders and Andean highlanders have adapted under high altitude stress.

Qualitative identification and characterisation of self-reported symptoms arising in humans during experimental exposure to cold air

Background: Exposure to cold air is associated with increased morbidity and mortality in the general population. It is difficult to study the effects of whole-body exposure to cold air under controlled conditions in real life. Objectives: The aim of this study was to (1) explore and describe the experience of symptoms in humans during experimental and controlled exposures to cold air, by using controlled environmental chamber exposures and qualitative methodology, and to (2) categorise the symptoms. Method: The study used a randomised, double blind design, in which 34 subjects undertook rest and moderate-intensity exercise in an environmental chamber set to two or three different temperatures (0, −10, and −17°C) on separate occasions. During the chamber exposures, subjects were interviewed. Qualitative content analysis was selected as the method of analysis. Findings: Subjects reported 50 distinct symptoms during the exposures. The symptoms were grouped into ten sub-categories and two major categories; airway versus whole-body symptoms. Conclusion: We have identified a broad range of symptoms in humans undertaking rest and moderate-intensity exercise at sub-zero temperatures. The symptoms and their categories may well be used to more extensively and quantitatively map cold-induced morbidity.

Global REACH 2018: The influence of acute and chronic hypoxia on cerebral haemodynamics and related functional outcomes during cold and heat stress

KEY POINTS

Thermal and hypoxic stress commonly coexist in environmental, occupational and clinical settings, yet how the brain tolerates these multi-stressor environments is unknown Core cooling by 1.0°C reduced cerebral blood flow (CBF) by 20-30% and cerebral oxygen delivery (CDO₂ ) by 12-19% at sea level and high altitude, whereas core heating by 1.5°C did not reliably reduce CBF or CDO₂ Oxygen content in arterial blood was fully restored with acclimatisation to 4330 m, but concurrent cold stress reduced CBF and CDO₂ Gross indices of cognition were not impaired by any combination of thermal and hypoxic stress despite large reductions in CDO₂ Chronic hypoxia renders the brain susceptible to large reductions in oxygen delivery with concurrent cold stress, which might make monitoring core temperature more important in this context ABSTRACT: Real-world settings are composed of multiple environmental stressors, yet the majority of research in environmental physiology investigates these stressors in isolation. The brain is central in both behavioural and physiological responses to threatening stimuli and, given its tight metabolic and haemodynamic requirements, is particularly susceptible to environmental stress. We measured cerebral blood flow (CBF, duplex ultrasound), cerebral oxygen delivery (CDO₂ ), oesophageal temperature, and arterial blood gases during exposure to three commonly experienced environmental stressors - heat, cold and hypoxia - in isolation, and in combination. Twelve healthy male subjects (27 ± 11 years) underwent core cooling by 1.0°C and core heating by 1.5°C in randomised order at sea level; acute hypoxia ( P ET , O 2  = 50 mm Hg) was imposed at baseline and at each thermal extreme. Core cooling and heating protocols were repeated after 16 ± 4 days residing at 4330 m to investigate any interactions with high altitude acclimatisation. Cold stress decreased CBF by 20-30% and CDO₂ by 12-19% (both P < 0.01) irrespective of altitude, whereas heating did not reliably change either CBF or CDO₂ (both P > 0.08). The increases in CBF with acute hypoxia during thermal stress were appropriate to maintain CDO₂ at normothermic, normoxic values. Reaction time was faster and slower by 6-9% with heating and cooling, respectively (both P < 0.01), but central (brain) processes were not impaired by any combination of environmental stressors. These findings highlight the powerful influence of core cooling in reducing CDO₂ . Despite these large reductions in CDO₂ with cold stress, gross indices of cognition remained stable.

The Effect of an Expiratory Resistance Mask with Dead Space on Sleep, Acute Mountain Sickness, Cognition, and Ventilatory Acclimatization in Normobaric Hypoxia

We examined the hypothesis that an expiratory resistance mask containing a small amount of dead space (ER/DS) would reduce the apnea-hypopnea index (AHI) during sleep, attenuate the severity of acute mountain sickness (AMS), and offset decrements in cognitive function compared with a sham mask. In a double-blinded, randomized, sham-controlled, crossover design, 19 volunteers were exposed to two nights of normobaric hypoxia (F_IO_{2 =} 0.125), using a ER/DS mask (3.5 mm restrictive expiratory orifice; 125 mL DS volume) and sham mask (zero-flow resistance; 50 mL DS volume). Cognitive function, AMS, and ventilatory acclimatization were assessed before and after the 12-hour normobaric hypoxia exposure. Polysomnography was conducted during sleep. AHI was reduced using the ER/DS sleep mask compared with the sham (30.1 ± 23.9 events·hr^-1 vs. 58.9 ± 34.4 events·hr^-1, respectively; p = 0.01). Likewise, oxygen desaturation index and headache severity were reduced (both p < 0.05). There were also benefits on limiting the hypoxia-induced reductions in select measures of reaction speed and attention (p < 0.05). Our study indicates that a simple noninvasive and portable ER/DS mask resulted in reductions (49%) in AHI, and reduced headache severity and aspects of cognitive decline. The field applications of this ER/DS mask should be investigated before recommendations can be made to support its benefit for travel to high altitude.

UBC-Nepal Expedition: An experimental overview of the 2016 University of British Columbia Scientific Expedition to Nepal Himalaya

The University of British Columbia Nepal Expedition took place over several months in the fall of 2016 and was comprised of an international team of 37 researchers. This paper describes the objectives, study characteristics, organization and management of this expedition, and presents novel blood gas data during acclimatization in both lowlanders and Sherpa. An overview and framework for the forthcoming publications is provided. The expedition conducted 17 major studies with two principal goals—to identify physiological differences in: 1) acclimatization; and 2) responses to sustained high-altitude exposure between lowland natives and people of Tibetan descent. We performed observational cohort studies of human responses to progressive hypobaric hypoxia (during ascent), and to sustained exposure to 5050 m over 3 weeks comparing lowlander adults (n = 30) with Sherpa adults (n = 24). Sherpa were tested both with (n = 12) and without (n = 12) descent to Kathmandu. Data collected from lowlander children (n = 30) in Canada were compared with those collected from Sherpa children (n = 57; 3400–3900m). Studies were conducted in Canada (344m) and the following locations in Nepal: Kathmandu (1400m), Namche Bazaar (3440m), Kunde Hospital (3480m), Pheriche (4371m) and the Ev-K2-CNR Research Pyramid Laboratory (5050m). The core studies focused on the mechanisms of cerebral blood flow regulation, the role of iron in cardiopulmonary regulation, pulmonary pressures, intra-ocular pressures, cardiac function, neuromuscular fatigue and function, blood volume regulation, autonomic control, and micro and macro vascular function. A total of 335 study sessions were conducted over three weeks at 5050m. In addition to an overview of this expedition and arterial blood gas data from Sherpa, suggestions for scientists aiming to perform field-based altitude research are also presented. Together, these findings will contribute to our understanding of human acclimatization and adaptation to the stress of residence at high-altitude.

UBC-Nepal Expedition: acute alterations in sympathetic nervous activity do not influence brachial artery endothelial function at sea level and high altitude

Evidence indicates that increases in sympathetic nervous activity (SNA), and acclimatization to high altitude (HA), may reduce endothelial function as assessed by brachial artery flow-mediated dilatation (FMD); however, it is unclear whether such changes in FMD are due to direct vascular constraint, or consequential altered hemodynamics (e.g., shear stress) associated with increased SNA as a consequence of exposure to HA. We hypothesized that 1) at rest, SNA would be elevated and FMD would be reduced at HA compared with sea-level (SL); and 2) at SL and HA, FMD would be reduced when SNA was acutely increased, and elevated when SNA was acutely decreased. Using a novel, randomized experimental design, brachial artery FMD was assessed at SL (344 m) and HA (5,050 m) in 14 participants during mild lower-body negative pressure (LBNP; -10 mmHg) and lower-body positive pressure (LBPP; +10 mmHg). Blood pressure (finger photoplethysmography), heart rate (electrocardiogram), oxygen saturation (pulse oximetry), and brachial artery blood flow and shear rate (Duplex ultrasound) were recorded during LBNP, control, and LBPP trials. Muscle SNA was recorded (via microneurography) in a subset of participants (n = 5). Our findings were 1) at rest, SNA was elevated (P < 0.01), and absolute FMD was reduced (P = 0.024), but relative FMD remained unaltered (P = 0.061), at HA compared with SL; and 2) despite significantly altering SNA with LBNP (+60.3 ± 25.5%) and LBPP (-37.2 ± 12.7%) (P < 0.01), FMD was unaltered at SL (P = 0.448) and HA (P = 0.537). These data indicate that acute and mild changes in SNA do not directly influence brachial artery FMD at SL or HA.NEW & NOTEWORTHY The role of the sympathetic nervous system on endothelial function remains unclear. We used lower-body negative and positive pressure to manipulate sympathetic nervous activity at sea level and high altitude and measured brachial endothelial function via flow-mediated dilation. We found that acutely altering sympathetic nervous activity had no effect on endothelial function.