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Over the last 100 years, high-altitude researchers have amassed a comprehensive understanding of the global cardiac responses to acute, prolonged and lifelong hypoxia. When lowlanders are exposed to hypoxia, the drop in arterial oxygen content demands an increase in cardiac output, which is facilitated by an elevated heart rate at the same time as ventricular volumes are maintained. As exposure is prolonged, haemoconcentration restores arterial oxygen content, whereas left ventricular filling and stroke volume are lowered as a result of a combination of reduced blood volume and hypoxic pulmonary vasoconstriction. Populations native to high-altitude, such as the Sherpa in Asia, exhibit unique lifelong or generational adaptations to hypoxia. For example, they have smaller left ventricular volumes compared to lowlanders despite having larger total blood volume. More recent investigations have begun to explore the mechanisms underlying such adaptive responses by combining novel imaging techniques with interventions that manipulate cardiac preload, afterload, and/or contractility. This work has revealed the contributions and interactions of (i) plasma volume constriction; (ii) sympathoexcitation; and (iii) hypoxic pulmonary vasoconstriction with respect to altering cardiac loading, or otherwise preserving or enhancing biventricular systolic and diastolic function even amongst high altitude natives with excessive erythrocytosis. Despite these advances, various areas of investigation remain understudied, including potential sex-related differences in response to high altitude. Collectively, the available evidence supports the conclusion that the human heart successfully adapts to hypoxia over the short- and long-term, without signs of myocardial dysfunction in healthy humans, except in very rare cases of maladaptation.

Key points Iron acts as a cofactor in the stabilization of the hypoxic-inducible factor family, and likely 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 hematological and cardiopulmonary profile of lowlanders and Sherpa were determined during: 1) a nine-day ascent to 5050m (20 lowlanders; 12 Sherpa), and 2) following partial acclimatization (11±4 days) to 5050m (18 lowlanders; 20 Sherpa), where both groups received either an i.v. infusion of 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 3400-5050m (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 5050m. Following partial acclimatization, despite Sherpa demonstrating a blunted hypoxic ventilatory response and greater resting hypoxemia, 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 hypoxemia (PI O2 = 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. This article is protected by copyright. All rights reserved.

KEY POINTS We have developed a novel porcine model of high-thoracic midline contusion spinal cord injury (SCI) at the T2 spinal level. We describe this model and the ensuing cardiovascular and neurohormonal responses, and demonstrate the model is efficacious for studying clinically relevant cardiovascular dysfunction post-SCI. We demonstrate that the high-thoracic SCI model, but not a low-thoracic SCI model, induces persistent hypotension along with a gradual reduction in plasma noradrenaline and increases in plasma aldosterone and angiotensin II. We additionally conducted a proof-of-concept long-term (12 weeks) survival study in animals with T2 contusion SCI demonstrating the potential utility of this model for not only acute experimentation but also long-term drug studies prior to translation to the clinic. ABSTRACT Cardiovascular disease is a leading cause of morbidity and mortality in the spinal cord injury (SCI) population, especially in those with high-thoracic or cervical SCI. With this in mind, we aimed to develop a large animal (porcine) model of high-thoracic (T2 level) contusion SCI and compare the haemodynamic and neurohormonal responses of this injury against a low-thoracic (T10 level) model. Ten Yorkshire pigs were randomly subjected to 20 cm weight drop contusion SCI at either the T2 or the T10 spinal level. Systolic blood pressure (SBP), mean arterial pressure (MAP) and heart rate (HR) were continuously monitored until 4 h post-SCI. Plasma noradrenaline (NA), aldosterone and angiotensin II (ANGII) were measured pre-SCI and at 30, 60, 120 and 240 min post-SCI. Additionally, two Yucatan pigs were subjected to T2-SCI and survived up to 12 weeks post-injury to demonstrate the efficacy of this model for long-term survival studies. Immediately after T2-SCI, SBP, MAP and HR increased (P

KEY POINTS The effect of combined inspiratory and expiratory muscle training on resting and reflexive cardiac function, as well as exercise capacity, in individuals with cervical spinal cord injury (SCI) is presently unknown. Six weeks of combined inspiratory and expiratory muscle training enhances both inspiratory and expiratory muscle strength in highly-trained athletes with cervical SCI with no significant effect on lung function. There was a significant decrease in left-ventricular filling and stroke volume at rest in response to 45° head-up tilt, which is irreversible by respiratory muscle training. Combined inspiratory and expiratory muscle training increased peak aerobic work rate and reduced end-expiratory lung volumes during exercise, which may have implications for left-ventricular filling during exercise. ABSTRACT To investigate the pulmonary, cardiovascular and exercise responses to combined inspiratory and expiratory respiratory muscle training (RMT) in athletes with tetraplegia, six wheelchair rugby athletes (five males and one female, aged 33 ± 5 years) completed 6 weeks of pressure threshold RMT, 2 sessions day-1 on 5 days week-1 . Resting pulmonary and cardiac function, exercise capacity, exercising lung volumes and field-based exercise performance were assessed at pre-RMT, post-RMT and after a 6-week no RMT period. RMT enhanced maximal inspiratory (pre- vs. post-RMT: -76 ± 15 to -106 ± 23 cmH2 O, P = 0.002) and expiratory (59 ± 26 to 73 ± 32 cmH2 O, P = 0.007) mouth pressures, as well as peak expiratory flow (6.74 ± 1.51 vs. 7.32 ± 1.60 L/s, P

This article was published in Journal of Physiology on 11 February 2017 (online ahead of print), available at http://dx.doi.org/10.1113/JP273368