Peripheral chemoreceptor responsiveness and hypoxic pulmonary vasoconstriction in humans

Symposium review: high altitude travel with pulmonary vascular disease

Pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension are the main precapillary forms of pulmonary hypertension (PH) summarized as pulmonary vascular diseases (PVD). PVDs are characterized by exertional dyspnoea and oxygen desaturation, and reduced quality of life and survival. Medical therapies improve life expectancy and physical performance of PVD patients, of whom many wish to participate in professional work and recreational activities including traveling to high altitude. The exposure to the hypobaric hypoxic environment of mountain regions incurs the risk of high altitude adverse events (AEHA) due to severe hypoxaemia exacerbating symptoms and further increase in pulmonary artery pressure, which may lead to right heart decompensation. Recent prospective and randomized trials show that altitude-induced hypoxaemia, pulmonary haemodynamic changes and impairment of exercise performance in PVD patients are in the range found in healthy people. The vast majority of optimally treated stable PVD patients who do not require long-term oxygen therapy at low altitude can tolerate short-term exposure to moderate altitudes up to 2500 m. PVD patients that reveal persistent severe resting hypoxaemia (S p O 2 ${{S}_{{\mathrm{p}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$  <80% for >30 min) at 2500 m respond well to supplemental oxygen therapy. Although there are no accurate predictors for AEHA, PVD patients with unfavourable risk profiles at low altitude, such as higher WHO functional class, lower exercise capacity with more pronounced exercise-induced desaturation and more severely impaired haemodynamics, are at increased risk of AEHA. Therefore, doctors with experience in PVD and high-altitude medicine should counsel PVD patients before any high-altitude sojourn. This review aims to summarize recent literature and clinical recommendations about PVD patients travelling to high altitude.

Pulmonary vascular diseases at high altitude - is it safe to live in the mountains?

PURPOSE OF REVIEW

This review addresses the concern of the health effects associated with high-altitude living and chronic hypoxia with a focus on pulmonary hypertension. With an increasing global population residing at high altitudes, understanding these effects is crucial for public health interventions and clinical management.

RECENT FINDINGS

Recent literature on the long-term effects of high-altitude residence and chronic hypoxia is comprehensively summarized. Key themes include the mechanisms of hypoxic pulmonary vasoconstriction, the development of pulmonary hypertension, and challenges in distinguishing altitude-related pulmonary hypertension and classical pulmonary vascular diseases, as found at a low altitude.

SUMMARY

The findings emphasize the need for research in high-altitude communities to unravel the risks of pulmonary hypertension and pulmonary vascular diseases. Clinically, early and tailored management for symptomatic individuals residing at high altitudes are crucial, as well as access to advanced therapies as proposed by guidelines for pulmonary vascular disease. Moreover, identifying gaps in knowledge underscores the necessity for continued research to improve understanding and clinical outcomes in high-altitude pulmonary vascular diseases.

The normal distribution of the hypoxic ventilatory response and methodological impacts: a meta-analysis and computational investigation

The hypoxic ventilatory response (HVR) is the increase in breathing in response to reduced arterial oxygen pressure. Over several decades, studies have revealed substantial population-level differences in the magnitude of the HVR as well as significant inter-individual variation. In particular, low HVRs occur frequently in Andean high-altitude native populations. However, our group conducted hundreds of HVR measures over several years and commonly observed low responses in sea-level populations as well. As a result, we aimed to determine the normal HVR distribution, whether low responses were common, and to what extent variation in study protocols influence these findings. We conducted a comprehensive search of the literature and examined the distributions of HVR values across 78 studies that utilized step-down/steady-state or progressive hypoxia methods in untreated, healthy human subjects. Several studies included multiple datasets across different populations or experimental conditions. In the final analysis, 72 datasets reported mean HVR values and 60 datasets provided raw HVR datasets. Of the 60 datasets reporting raw HVR values, 35 (58.3%) were at least moderately positively skewed (skew > 0.5), and 21 (35%) were significantly positively skewed (skew > 1), indicating that lower HVR values are common. The skewness of HVR distributions does not appear to be an artifact of methodology or the unit with which the HVR is reported. Further analysis demonstrated that the use of step-down hypoxia versus progressive hypoxia methods did not have a significant impact on average HVR values, but that isocapnic protocols produced higher HVRs than poikilocapnic protocols. This work provides a reference for expected HVR values and illustrates substantial inter-individual variation in this key reflex. Finally, the prevalence of low HVRs in the general population provides insight into our understanding of blunted HVRs in high-altitude adapted groups. KEY POINTS: The hypoxic ventilatory response (HVR) plays a crucial role in determining an individual's predisposition to hypoxia-related pathologies. There is notable variability in HVR sensitivity across individuals as well as significant population-level differences. We report that the normal distribution of the HVR is positively skewed, with a significant prevalence of low HVR values amongst the general healthy population. We also find no significant impact of the experimental protocol used to induce hypoxia, although HVR is greater with isocapnic versus poikilocapnic methods. These results provide insight into the normal distribution of the HVR, which could be useful in clinical decisions of diseases related to hypoxaemia. Additionally, the low HVR values found within the general population provide insight into the genetic adaptations found in populations residing in high altitudes.

Seven days of ischemic preconditioning augments hypoxic exercise ventilation and muscle oxygenation in recreationally trained males

This investigation sought to assess whether single or repeated bouts of ischemic preconditioning (IPC) could improve oxyhemoglobin saturation ([Formula: see text]) and/or attenuate reductions in muscle tissue saturation index (TSI) during submaximal hypoxic exercise. Fifteen healthy young men completed submaximal graded exercise under four experimental conditions: 1) normoxia (NORM), 2) hypoxia (HYP) [oxygen fraction of inspired air ([Formula: see text]) = 0.14, ∼3,200 m], 3) hypoxia preceded by a single session of IPC (IPC1-HYP), and 4) hypoxia preceded by seven sessions of IPC, one a day for 7 consecutive days (IPC7-HYP). IPC7-HYP heightened minute ventilation (V̇e) at 80% HYP peak cycling power output (Wpeak) (+10.47 ± 3.35 L·min-1, P = 0.006), compared with HYP, as a function of increased breathing frequency. Both IPC1-HYP (+0.17 ± 0.04 L·min-1, P < 0.001) and IPC7-HYP (+0.16 ± 0.04 L·min-1, P < 0.001) elicited greater oxygen consumption (V̇o2) across exercise intensities compared with NORM, whereas V̇o2 was unchanged with HYP alone. [Formula: see text] was unchanged by either IPC condition at any exercise intensity, yet the reduction of muscle TSI during resting hypoxic exposure was attenuated by IPC7-HYP (+9.9 ± 3.6%, P = 0.040) compared with HYP, likely as a function of reduced local oxygen extraction. Considering all exercise intensities, IPC7-HYP attenuated reductions of TSI with HYP (+6.4 ± 1.8%, P = 0.001). Seven days of IPC heightens ventilation, posing a threat to ventilatory efficiency, during high-intensity submaximal hypoxic exercise and attenuates reductions in hypoxic resting and exercise muscle oxygenation in healthy young men. A single session of IPC may be capable of modulating hypoxic ventilation; however, our present population was unable to demonstrate this with certainty.

Respiratory responses to hypoxia during rest and exercise in individuals born pre-term: a state-of-the-art review

The pre-term birth survival rate has increased considerably in recent decades, and research investigating the long-term effects of premature birth is growing. Moreover, altitude sojourns are increasing in popularity and are often accompanied by various levels of physical activity. Individuals born pre-term appear to exhibit altered acute ventilatory responses to hypoxia, potentially predisposing them to high-altitude illness. These impairments are likely due to the use of perinatal hyperoxia stunting the maturation of carotid body chemoreceptors, but may also be attributed to limited lung diffusion capacity and/or gas exchange inefficiency. Aerobic exercise capacity also appears to be reduced in this population. This may relate to the aforementioned respiratory impairments, or could be due to physiological limitations in pulmonary blood flow or at the exercising muscle (e.g. mitochondrial efficiency). However, surprisingly, the debilitative effects of exercise when performed at altitude do not seem to be exacerbated by premature birth. In fact, it is reasonable to speculate that pre-term birth could protect against the consequences of exercise combined with hypoxia. The mechanisms that underlie this assertion might relate to differences in oxidative stress responses or in cardiopulmonary morphology in pre-term individuals, compared to their full-term counterparts. Further research is required to elucidate the independent effects of neonatal treatment, sex differences and chronic lung disease, and to establish causality in some of the proposed mechanisms that could underlie the differences discussed throughout this review. A more in-depth understanding of the acclimatisation responses to chronic altitude exposures would also help to inform appropriate interventions in this clinical population.

Early hours in the development of high-altitude pulmonary edema: time course and mechanisms

Clinically evident high-altitude pulmonary edema (HAPE) is characterized by severe cyanosis, dyspnea, cough, and difficulty with physical exertion. This usually occurs within 1-2 days of ascent often with the additional stresses of any exercise and hypoventilation of sleep. The earliest events in evolving HAPE progress through clinically silent and then minimally recognized problems. The most important of these events involves an exaggerated elevation of pulmonary artery (PA) pressure in response to the ambient hypoxia. Hypoxic pulmonary vasoconstriction (HPV) is a rapid response with several phases. The first phase in both resistance arterioles and venules occurs within 5-10 min. This is followed by a second phase that further raises PA pressure by another 100% over the next 2-8 h. Combined with vasoconstriction and likely an unevenness in the regional strength of HPV, pressures in some microvascular regions with lesser arterial constriction rise to a level that initiates greater filtration of fluid into the interstitium. As pressures continue to rise local lymphatic clearance rates are exceeded and interstitial fluid begins to accumulate. Beyond elevation of transmural pressure gradients there is a dynamic noninjurious relaxation of microvascular and epithelial cell-cell contacts and an increase in transcellular vesicular transport which accelerate leakage. At some point with further pressure elevation, damage occurs with breaks of the barrier and bleeding into the alveolar space, a late-stage situation termed capillary stress failure. Earlier before there is fluid accumulation, alveolar hypoxia and hyperventilation-induced hypocapnia reduce the capacity of the alveolar epithelium to reabsorb sodium and water back into the interstitial space. More modest ascent which slows the rate of rise in PA pressure and allows for adaptive remodeling of the microvasculature, drugs which lower PA pressure, and those that can enhance fluid reabsorption will all forestall the deleterious early rise of microvascular pressures and diminished active alveolar fluid reabsorption that precede and underlie the development of HAPE.

Systemic Oxygen Transport with Rest, Exercise, and Hypoxia: A Comparison of Humans, Rats, and Mice

The objective of this article is to compare and contrast the known characteristics of the systemic O₂ transport of humans, rats, and mice at rest and during exercise in normoxia and hypoxia. This analysis should help understand when rodent O₂ transport findings can-and cannot-be applied to human responses to similar conditions. The O₂ -transport system was analyzed as composed of four linked conductances: ventilation, alveolo-capillary diffusion, circulatory convection, and tissue capillary-cell diffusion. While the mechanisms of O₂ transport are similar in the three species, the quantitative differences are naturally large. There are abundant data on total O₂ consumption and on ventilatory and pulmonary diffusive conductances under resting conditions in the three species; however, there is much less available information on pulmonary gas exchange, circulatory O₂ convection, and tissue O₂ diffusion in mice. The scarcity of data largely derives from the difficulty of obtaining blood samples in these small animals and highlights the need for additional research in this area. In spite of the large quantitative differences in absolute and mass-specific O₂ flux, available evidence indicates that resting alveolar and arterial and venous blood PO₂ values under normoxia are similar in the three species. Additionally, at least in rats, alveolar and arterial blood PO₂ under hypoxia and exercise remain closer to the resting values than those observed in humans. This is achieved by a greater ventilatory response, coupled with a closer value of arterial to alveolar PO₂ , suggesting a greater efficacy of gas exchange in the rats. © 2018 American Physiological Society. Compr Physiol 8:1537-1573, 2018.

The Influence of CO2 and Exercise on Hypobaric Hypoxia Induced Pulmonary Edema in Rats

Introduction: Individuals with a known susceptibility to high altitude pulmonary edema (HAPE) demonstrate a reduced ventilation response and increased pulmonary vasoconstriction when exposed to hypoxia. It is unknown whether reduced sensitivity to hypercapnia is correlated with increased incidence and/or severity of HAPE, and while acute exercise at altitude is known to exacerbate symptoms the effect of exercise training on HAPE susceptibility is unclear.

Purpose: To determine if chronic intermittent hypercapnia and exercise increases the incidence of HAPE in rats.

Methods: Male Wistar rats were randomized to sedentary (sed-air), CO₂ (sed-CO₂,) exercise (ex-air), or exercise + CO₂ (ex-CO₂) groups. CO₂ (3.5%) and treadmill exercise (15 m/min, 10% grade) were conducted on a metabolic treadmill, 1 h/day for 4 weeks. Vascular reactivity to CO₂ was assessed after the training period by rheoencephalography (REG). Following the training period, animals were exposed to hypobaric hypoxia (HH) equivalent to 25,000 ft for 24 h. Pulmonary injury was assessed by wet/dry weight ratio, lung vascular permeability, bronchoalveolar lavage (BAL), and histology.

Results: HH increased lung wet/dry ratio (HH 5.51 ± 0.29 vs. sham 4.80 ± 0.11, P < 0.05), lung permeability (556 ± 84 u/L vs. 192 ± 29 u/L, P < 0.001), and BAL protein (221 ± 33 μg/ml vs. 114 ± 13 μg/ml, P < 0.001), white blood cell (1.16 ± 0.26 vs. 0.66 ± 0.06, P < 0.05), and platelet (16.4 ± 2.3, vs. 6.0 ± 0.5, P < 0.001) counts in comparison to normobaric normoxia. Vascular reactivity was suppressed by exercise (−53% vs. sham, P < 0.05) and exercise+CO₂ (−71% vs. sham, P < 0.05). However, neither exercise nor intermittent hypercapnia altered HH-induced changes in lung wet/dry weight, BAL protein and cellular infiltration, or pulmonary histology.

Conclusion: Exercise training attenuates vascular reactivity to CO₂ in rats but neither exercise training nor chronic intermittent hypercapnia affect HH- induced pulmonary edema.

Acute oxygen sensing by the carotid body: from mitochondria to plasma membrane

Maintaining oxygen homeostasis is crucial to the survival of animals. Mammals respond acutely to changes in blood oxygen levels by modulating cardiopulmonary function. The major sensor of blood oxygen that regulates breathing is the carotid body (CB), a small chemosensory organ located at the carotid bifurcation. When arterial blood oxygen levels drop in hypoxia, neuroendocrine cells in the CB called glomus cells are activated to signal to afferent nerves that project to the brain stem. The mechanism by which hypoxia stimulates CB sensory activity has been the subject of many studies over the past 90 years. Two discrete models emerged that argue for the seat of oxygen sensing to lie either in the plasma membrane or mitochondria of CB cells. Recent studies are bridging the gap between these models by identifying hypoxic signals generated by changes in mitochondrial function in the CB that can be sensed by plasma membrane proteins on glomus cells. The CB is important for physiological adaptation to hypoxia, and its dysfunction contributes to sympathetic hyperactivity in common conditions such as sleep-disordered breathing, chronic heart failure, and insulin resistance. Understanding the basic mechanism of oxygen sensing in the CB could allow us to develop strategies to target this organ for therapy. In this short review, I will describe two historical models of CB oxygen sensing and new findings that are integrating these models.

Acute high-altitude sickness

At any point 1–5 days following ascent to altitudes ≥2500 m, individuals are at risk of developing one of three forms of acute altitude illness: acute mountain sickness, a syndrome of nonspecific symptoms including headache, lassitude, dizziness and nausea; high-altitude cerebral oedema, a potentially fatal illness characterised by ataxia, decreased consciousness and characteristic changes on magnetic resonance imaging; and high-altitude pulmonary oedema, a noncardiogenic form of pulmonary oedema resulting from excessive hypoxic pulmonary vasoconstriction which can be fatal if not recognised and treated promptly. This review provides detailed information about each of these important clinical entities. After reviewing the clinical features, epidemiology and current understanding of the pathophysiology of each disorder, we describe the current pharmacological and nonpharmacological approaches to the prevention and treatment of these diseases.

Lack of acclimatisation is the main risk factor for acute altitude illness; descent is the optimal treatmenthttp://ow.ly/45d2305JyZ0

High-Altitude Pulmonary Vascular Diseases

More than 140 million people permanently reside in high-altitude regions of Asia, South America, North America, and Africa. Another 40 million people travel to these places annually for occupational and recreational reasons, and are thus exposed to the low ambient partial pressure of oxygen. This review will focus on the pulmonary circulatory responses to acute and chronic high-altitude hypoxia, and the various expressions of maladaptation and disease arising from acute pulmonary vasoconstriction and subsequent remodeling of the vasculature when the hypoxic exposure continues. These unique conditions include high-altitude pulmonary edema, high-altitude pulmonary hypertension, subacute mountain sickness, and chronic mountain sickness.

Determinants of ventilation and pulmonary artery pressure during early acclimatization to hypoxia in humans

Pulmonary ventilation and pulmonary arterial pressure both rise progressively during the first few hours of human acclimatization to hypoxia. These responses are highly variable between individuals, but the origin of this variability is unknown. Here, we sought to determine whether the variabilities between different measures of response to sustained hypoxia were related, which would suggest a common source of variability. Eighty volunteers individually underwent an 8-h isocapnic exposure to hypoxia (end-tidal P(O2)=55 Torr) in a purpose-built chamber. Measurements of ventilation and pulmonary artery systolic pressure (PASP) assessed by Doppler echocardiography were made during the exposure. Before and after the exposure, measurements were made of the ventilatory sensitivities to acute isocapnic hypoxia (G(pO2)) and hyperoxic hypercapnia, the latter divided into peripheral (G(pCO2)) and central (G(cCO2)) components. Substantial acclimatization was observed in both ventilation and PASP, the latter being 40% greater in women than men. No correlation was found between the magnitudes of pulmonary ventilatory and pulmonary vascular responses. For G(pO2), G(pCO2) and G(cC O2), but not the sensitivity of PASP to acute hypoxia, the magnitude of the increase during acclimatization was proportional to the pre-acclimatization value. Additionally, the change in G(pO2) during acclimatization to hypoxia correlated well with most other measures of ventilatory acclimatization. Of the initial measurements prior to sustained hypoxia, only G(pCO2) predicted the subsequent rise in ventilation and change in G(pO2) during acclimatization. We conclude that the magnitudes of the ventilatory and pulmonary vascular responses to sustained hypoxia are predominantly determined by different factors and that the initial G(pCO2) is a modest predictor of ventilatory acclimatization.

Hypoxia, not pulmonary vascular pressure, induces blood flow through intrapulmonary arteriovenous anastomoses

KEY POINTS

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased by acute hypoxia during rest by unknown mechanisms. Oral administration of acetazolamide blunts the pulmonary vascular pressure response to acute hypoxia, thus permitting the observation of IPAVA blood flow with minimal pulmonary pressure change. Hypoxic pulmonary vasoconstriction was attenuated in humans following acetazolamide administration and partially restored with bicarbonate infusion, indicating that the effects of acetazolamide on hypoxic pulmonary vasoconstriction may involve an interaction between arterial pH and PCO2. We observed that IPAVA blood flow during hypoxia was similar before and after acetazolamide administration, even after acid-base status correction, indicating that pulmonary pressure, pH and PCO2 are unlikely regulators of IPAVA blood flow.

ABSTRACT

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased with exposure to acute hypoxia and has been associated with pulmonary artery systolic pressure (PASP). We aimed to determine the direct relationship between blood flow through IPAVA and PASP in 10 participants with no detectable intracardiac shunt by comparing: (1) isocapnic hypoxia (control); (2) isocapnic hypoxia with oral administration of acetazolamide (AZ; 250 mg, three times a day for 48 h) to prevent increases in PASP; and (3) isocapnic hypoxia with AZ and 8.4% NaHCO3 infusion (AZ + HCO3 (-) ) to control for AZ-induced acidosis. Isocapnic hypoxia (20 min) was maintained by end-tidal forcing, blood flow through IPAVA was determined by agitated saline contrast echocardiography and PASP was estimated by Doppler ultrasound. Arterial blood samples were collected at rest before each isocapnic-hypoxia condition to determine pH, [HCO3(-)] and Pa,CO2. AZ decreased pH (-0.08 ± 0.01), [HCO3(-)] (-7.1 ± 0.7 mmol l(-1)) and Pa,CO2 (-4.5 ± 1.4 mmHg; P < 0.01), while intravenous NaHCO3 restored arterial blood gas parameters to control levels. Although PASP increased from baseline in all three hypoxic conditions (P < 0.05), a main effect of condition expressed an 11 ± 2% reduction in PASP from control (P < 0.001) following AZ administration while intravenous NaHCO3 partially restored the PASP response to isocapnic hypoxia. Blood flow through IPAVA increased during exposure to isocapnic hypoxia (P < 0.01) and was unrelated to PASP, cardiac output and pulmonary vascular resistance for all conditions. In conclusion, isocapnic hypoxia induces blood flow through IPAVA independent of changes in PASP and the influence of AZ on the PASP response to isocapnic hypoxia is dependent upon the H(+) concentration or Pa,CO2.

Ischemic preconditioning improves oxygen saturation and attenuates hypoxic pulmonary vasoconstriction at high altitude

Exposure to hypoxic environments is associated with decreased arterial oxygen saturation and increased pulmonary artery pressures. Ischemic preconditioning of an extremity (IPC) is a procedure that stimulates vasoactive and inflammatory pathways that protect remote organ systems from ongoing or future ischemic injury. To test the effects of IPC on oxygen saturation and pulmonary artery pressures at high altitude, 12 healthy adult volunteers were evaluated in a randomized cross-over trial. IPC was administered utilizing a standardized protocol. IPC or placebo was administered daily for 5 days prior to ascent to altitude. All participants were evaluated twice at 4342 m altitude (placebo and IPC conditions separated by 4 weeks, randomized). The pulmonary artery systolic pressure (PASP) at 4342 m was significantly lower in the IPC condition than the placebo condition (36 ± 6.0 mmHg vs. 38.1 ± 7.6 mmHg, respectively, p = 0.035). Oxygen saturation at 4342 m was significantly higher with IPC compared to placebo (80.3 ± 8.7% vs. 75.3 ± 9.6%, respectively, p = 0.003). Prophylactic IPC treatment is associated with improved oxygen saturation and attenuation of the normal hypoxic increase in pulmonary artery pressures following ascent to high altitude.