Augmented hypoxic ventilatory response in men at altitude

High Altitude

With ascent to high altitude, barometric pressure declines, leading to a reduction in the partial pressure of oxygen at every point along the oxygen transport chain from the ambient air to tissue mitochondria. This leads, in turn, to a series of changes over varying time frames across multiple organ systems that serve to maintain tissue oxygen delivery at levels sufficient to prevent acute altitude illness and preserve cognitive and locomotor function. This review focuses primarily on the physiological adjustments and acclimatization processes that occur in the lungs of healthy individuals, including alterations in control of breathing, ventilation, gas exchange, lung mechanics and dynamics, and pulmonary vascular physiology. Because other organ systems, including the cardiovascular, hematologic and renal systems, contribute to acclimatization, the responses seen in these systems, as well as changes in common activities such as sleep and exercise, are also addressed. While the pattern of the responses highlighted in this review are similar across individuals, the magnitude of such responses often demonstrates significant interindividual variability which accounts for subsequent differences in tolerance of the low oxygen conditions in this environment.

Do cardiopulmonary exercise tests predict summit success and acute mountain sickness? A prospective observational field study at extreme altitude

OBJECTIVE

During a high-altitude expedition, the association of cardiopulmonary exercise testing (CPET) parameters with the risk of developing acute mountain sickness (AMS) and the chance of reaching the summit were investigated.

METHODS

Thirty-nine subjects underwent maximal CPET at lowlands and during ascent to Mount Himlung Himal (7126 m) at 4844 m, before and after 12 days of acclimatisation, and at 6022 m. Daily records of Lake-Louise-Score (LLS) determined AMS. Participants were categorised as AMS+ if moderate to severe AMS occurred.

RESULTS

Maximal oxygen uptake (V̇O_2max) decreased by 40.5%±13.7% at 6022 m and improved after acclimatisation (all p<0.001). Ventilation at maximal exercise (VE_max) was reduced at 6022 m, but higher VE_max was related to summit success (p=0.031). In the 23 AMS+ subjects (mean LLS 7.4±2.4), a pronounced exercise-induced oxygen desaturation (ΔSpO_2exercise) was found after arrival at 4844 m (p=0.005). ΔSpO_2exercise >-14.0% identified 74% of participants correctly with a sensitivity of 70% and specificity of 81% for predicting moderate to severe AMS. All 15 summiteers showed higher V̇O_2max (p<0.001), and a higher risk of AMS in non-summiteers was suggested but did not reach statistical significance (OR: 3.64 (95% CI: 0.78 to 17.58), p=0.057). V̇O_2max ≥49.0 mL/min/kg at lowlands and ≥35.0 mL/min/kg at 4844 m predicted summit success with a sensitivity of 46.7% and 53.3%, and specificity of 83.3% and 91.3%, respectively.

CONCLUSION

Summiteers were able to sustain higher VE_max throughout the expedition. Baseline V̇O_2max below 49.0 mL/min/kg was associated with a high chance of 83.3% for summit failure, when climbing without supplemental oxygen. A pronounced drop of SpO_2exercise at 4844 m may identify climbers at higher risk of AMS.

Steady-state chemoreflex drive captures ventilatory acclimatization during incremental ascent to high altitude: Effect of acetazolamide

Ventilatory acclimatization (VA) is important to maintain adequate oxygenation with ascent to high altitude (HA). Transient hypoxic ventilatory response tests lack feasibility and fail to capture the integrated steady-state responses to chronic hypoxic exposure in HA fieldwork. We recently characterized a novel index of steady-state respiratory chemoreflex drive (SSCD), accounting for integrated contributions from central and peripheral respiratory chemoreceptors during steady-state breathing at prevailing chemostimuli. Acetazolamide is often utilized during ascent for prevention or treatment of altitude-related illnesses, eliciting metabolic acidosis and stimulating respiratory chemoreceptors. To determine if SSCD reflects VA during ascent to HA, we characterized SSCD in 25 lowlanders during incremental ascent to 4240 m over 7 days. We subsequently compared two separate subgroups: no acetazolamide (NAz; n = 14) and those taking an oral prophylactic dose of acetazolamide (Az; 125 mg BID; n = 11). At 1130/1400 m (day zero) and 4240 m (day seven), steady-state measurements of resting ventilation (V̇_I ; L/min), pressure of end-tidal (P_ET )CO₂ (Torr), and peripheral oxygen saturation (SpO₂ ; %) were measured. A stimulus index (SI; P_ET CO₂ /SpO₂ ) was calculated, and SSCD was calculated by indexing V̇_I against SI. We found that (a) both V̇_I and SSCD increased with ascent to 4240 m (day seven; V̇_I : +39%, p < 0.0001, Hedges' g = 1.52; SSCD: +56.%, p < 0.0001, Hedges' g = 1.65), (b) and these responses were larger in the Az versus NAz subgroup (V̇_I : p = 0.02, Hedges' g = 1.04; SSCD: p = 0.02, Hedges' g = 1.05). The SSCD metric may have utility in assessing VA during prolonged stays at altitude, providing a feasible alternative to transient chemoreflex tests.

Effects of Acute Exposure and Acclimatization to High-Altitude on Oxygen Saturation and Related Cardiorespiratory Fitness in Health and Disease

Maximal values of aerobic power (VO₂max) and peripheral oxygen saturation (SpO₂max) decline in parallel with gain in altitude. Whereas this relationship has been well investigated when acutely exposed to high altitude, potential benefits of acclimatization on SpO₂ and related VO₂max in healthy and diseased individuals have been much less considered. Therefore, this narrative review was primarily aimed to identify relevant literature reporting altitude-dependent changes in determinants, in particular SpO₂, of VO₂max and effects of acclimatization in athletes, healthy non-athletes, and patients suffering from cardiovascular, respiratory and/or metabolic diseases. Moreover, focus was set on potential differences with regard to baseline exercise performance, age and sex. Main findings of this review emphasize the close association between individual SpO₂ and VO₂max, and demonstrate similar altitude effects (acute and during acclimatization) in healthy people and those suffering from cardiovascular and metabolic diseases. However, in patients with ventilatory constrains, i.e., chronic obstructive pulmonary disease, steep decline in SpO₂ and V̇O₂max and reduced potential to acclimatize stress the already low exercise performance. Finally, implications for prevention and therapy are briefly discussed.

Cardiopulmonary and cerebrovascular acclimatization in children and adults at 3800 m

Maturational differences exist in cardiopulmonary and cerebrovascular function at sea-level, but the impact of maturation on acclimatization responses to high altitude is unknown. Ten children (9.8 ± 2.5 years) and 10 adults (34.7 ± 7.1 years) were assessed at sea-level (BL), 3000 m and twice over 4 days at 3800 m (B1, B4). Measurements included minute ventilation ( V ̇ E ${\dot{V}}_{\rm{E}}$ ), end-tidal partial pressures of oxygen ( P ETO 2 ${P}_{{\rm{ETO}}_{\rm{2}}}$ ) and carbon dioxide, echocardiographic assessment of pulmonary artery systolic pressure (PASP) and stroke volume (SV) and ultrasound assessment of blood flow through the internal carotid and vertebral arteries was performed to calculate global cerebral blood flow (gCBF). At 3000 m, V ̇ E ${\dot{V}}_{\rm{E}}$ was increased from BL by 19.6 ± 19.1% (P = 0.031) in children, but not in adults (P = 0.835); SV was reduced in children (-11 ± 13%, P = 0.020) but not adults (P = 0.827), which was compensated for by a larger increase in heart rate in children (+26 beats min^-1 vs. +13 beats min^-1 , P = 0.019). Between B1 and B4, adults increased V ̇ E ${\dot{V}}_{\rm{E}}$ by 38.5 ± 34.7% (P = 0.006), while V ̇ E ${\dot{V}}_{\rm{E}}$ did not increase further in children. The rise in PASP was not different between groups; however, ∆PASP from BL was related to ∆ P ETO 2 ${P}_{{\rm{ETO}}_{\rm{2}}}$ in adults (R²  = 0.288, P = 0.022), but not children. At BL, gCBF was 43% higher in children than adults (P = 0.017), and this difference was maintained at high altitude, with a similar pattern and magnitude of change in gCBF between groups (P = 0.845). Despite V ̇ E ${\dot{V}}_{\rm{E}}$ increasing in children but not adults at a lower altitude, the pulmonary vascular and cerebrovascular responses to prolonged hypoxia are similar between children and adults. KEY POINTS: Children have different ventilatory and metabolic requirements from adults, which may present differently in the pulmonary and cerebral vasculature upon ascent to high altitude. Children (ages 7-14) and adults (ages 23-44) were brought from sea level to high altitude (3000 to 3800 m) and changes in ventilation, pulmonary artery systolic pressure (PASP) and cerebral blood flow (CBF) were assessed over 1 week. Significant increases in ventilation and decreases in left ventricle stroke volume were observed at a lower altitude in children than adults. PASP and CBF increased by a similar relative amount between children and adults at 3800 m. These results help us better understand age-related differences in compensatory responses to prolonged hypoxia in children, despite similar changes in pulmonary artery pressure and CBF between children and adults.

Genetic variation in HIF-2α attenuates ventilatory sensitivity and carotid body growth in chronic hypoxia in high-altitude deer mice

KEY POINTS

High-altitude natives of many species have experienced natural selection on the gene encoding HIF-2α, Epas1, including high-altitude populations of deer mice. HIF-2α regulates ventilation and carotid body growth in hypoxia, so the genetic variants in Epas1 in high-altitude natives may underlie evolved changes in control of breathing. Deer mice from controlled crosses between high- and low-altitude populations were used to examine the effects of Epas1 genotype on an admixed genomic background. The high-altitude variant was associated with reduced ventilatory chemosensitivity and carotid body growth in chronic hypoxia, but had no effects on haematology. The results help us better understand the genetic basis for the unique physiological phenotype of high-altitude natives.

ABSTRACT

The gene encoding HIF-2α, Epas1, has experienced a history of natural selection in many high-altitude taxa, but the functional role of mutations in this gene are still poorly understood. We investigated the influence of the high-altitude variant of Epas1 in North American deer mice (Peromyscus maniculatus) on control of breathing and carotid body growth during chronic hypoxia. We created hybrids between high- and low-altitude populations of deer mice to disrupt linkages between genetic loci so physiological effects of Epas1 alleles (Epas1^H and Epas1^L , respectively) could be examined on an admixed genomic background. In general, chronic hypoxia (4 weeks at 12 kPa O₂ ) enhanced ventilatory chemosensitivity (assessed as the acute ventilatory response to hypoxia), increased total ventilation and arterial O₂ saturation during progressive poikilocapnic hypoxia, and increased haematocrit and blood haemoglobin content across genotypes. However, effects of chronic hypoxia on ventilatory chemosensitivity were attenuated in mice that were homozygous for the high-altitude Epas1 allele (Epas1^H/H ). Carotid body growth and glomus cell hyperplasia, which was strongly induced in Epas1^L/L mice in chronic hypoxia, was not observed in Epas1^H/H mice. Epas1 genotype also modulated the effects of chronic hypoxia on metabolism and body temperature depression in hypoxia, but had no effects on haematological traits. These findings confirm the important role of HIF-2α in modulating ventilatory sensitivity and carotid body growth in chronic hypoxia, and show that genetic variation in Epas1 is responsible for evolved changes in the control of breathing and metabolism in high-altitude deer mice. Abstract figure legend ventilation and carotid body growth in hypoxia, so we investigated the role genetic variants in Epas1 in highaltitude deer mice on the control of breathing. In the lab, hybrids between high- and lowaltitude populations of deer mice were created to disrupt linkages between genetic loci so physiological effects of Epas1 alleles (Epas1H and Epas1L, respectively) could be examined on an admixed genomic background. The high-altitude variant was associated with reduced ventilatory chemosensitivity and carotid body growth after 4 weeks of chronic hypoxia, compared to mice homozygous for the low-altitude allele (Epas1LL). These results help us better understand the genetic basis for the unique physiological phenotype of high-altitude natives. This article is protected by copyright. All rights reserved.

Respiratory and heart rate dynamics during peripheral chemoreceptor deactivation compared to targeted sympathetic and sympathetic/parasympathetic (co-)activation

BACKGROUND

The importance of peripheral chemoreceptors for cardiorespiratory neural control is known for decades. Pure oxygen inhalation deactivates chemoreceptors and increases parasympathetic outflow. However, the relationship between autonomic nervous system (ANS) activation and resulting respiratory as well as heart rate (HR) dynamics is still not fully understood.

METHODS

In young adults the impact of (1) 100 % pure oxygen inhalation (hyperoxic cardiac chemoreflex sensitivity (CHRS) testing), (2) the cold face test (CFT) and (3) the cold pressor test (CPT) on heart rate variability (HRV), hemodynamics and respiratory rate was investigated in randomized order. Baseline ANS outflow was determined assessing respiratory sinus arrhythmia via deep breathing, baroreflex sensitivity and HRV.

RESULTS

Baseline ANS outflow was normal in all participants (23 ± 1 years, 7 females, 3 males). Hyperoxic CHRS testing decreased HR (after 60 ± 3 vs before 63 ± 3 min^-1, p = 0.004), while increasing total peripheral resistance (1053 ± 87 vs 988 ± 76 dyne*s + m²/cm⁵, p = 0.02) and mean arterial blood pressure (93 ± 4 vs 91 ± 4 mm Hg, p = 0.02). HRV indicated increased parasympathetic outflow after hyperoxic CHRS testing accompanied by a decrease in respiratory rate (15 ± 1vs 19 ± 1 min^-1, p = 0.001). In contrast, neither CFT nor CPT altered the respiratory rate (18 ± 1 vs 18 ± 2 min^-1, p = 0.38 and 18 ± 1 vs 18 ± 1 min^-1, p = 0.84, respectively).

CONCLUSION

Changes in HR characteristics during deactivation of peripheral chemoreceptors but not during the CFT and CPT are related with a decrease in respiratory rate. This highlights the need of respiratory rate assessment when evaluating adaptations of cardiorespiratory chemoreceptor control.

Neuromuscular fatigability at high altitude: Lowlanders with acute and chronic exposure, and native highlanders

Ascent to high altitude is accompanied by a reduction in partial pressure of inspired oxygen, which leads to interconnected adjustments within the neuromuscular system. This review describes the unique challenge that such an environment poses to neuromuscular fatigability (peripheral, central and supraspinal) for individuals who normally reside near to sea level (SL) (<1000 m; ie, lowlanders) and for native highlanders, who represent the manifestation of high altitude-related heritable adaptations across millennia. Firstly, the effect of acute exposure to high altitude-related hypoxia on neuromuscular fatigability will be examined. Under these conditions, both supraspinal and peripheral fatigability are increased compared with SL. The specific mechanisms contributing to impaired performance are dependent on the exercise paradigm and amount of muscle mass involved. Next, the effect of chronic exposure to high altitude (ie, acclimatization of ~7-28 days) will be considered. With acclimatization, supraspinal fatigability is restored to SL values, regardless of the amount of muscle mass involved, whereas peripheral fatigability remains greater than SL except when exercise involves a small amount of muscle mass (eg, knee extensors). Indeed, when whole-body exercise is involved, peripheral fatigability is not different to acute high-altitude exposure, due to competing positive (haematological and muscle metabolic) and negative (respiratory-mediated) effects of acclimatization on neuromuscular performance. In the final section, we consider evolutionary adaptations of native highlanders (primarily Himalayans of Tibet and Nepal) that may account for their superior performance at altitude and lesser degree of neuromuscular fatigability compared with acclimatized lowlanders, for both single-joint and whole-body exercise.

A comparative analysis of lung function and spirometry parameters in genotype-controlled natives living at low and high altitude

Background

The reference values for lung function are associated to anatomical and lung morphology parameters, but anthropometry it is not the only influencing factor: altitude and genetics are two important agents affecting respiratory physiology. Altitude and its influence on respiratory function has been studied independently of genetics, considering early and long-term acclimatization.

Objective

The objective of this study is to evaluate lung function through a spirometry study in autochthonous Kichwas permanently living at low and high-altitude.

Methodology

A cross-sectional study of spirometry differences between genetically matched lowland Kichwas from Limoncocha (230 m) at Amazonian basin and high-altitude Kichwas from Oyacachi (3180 m) in Andean highlands. The sample size estimates permitted to recruited 118 patients (40 men and 78 women) from Limoncocha and 95 (39 men and 56 women) from Oyacachi. Chi-square method was used to analyze association or independence of categorical variables, while Student’s t test was applied to comparison of means within quantitative variables. ANOVA, or in the case that the variables didn’t meet the criteria of normality, Kruskal Wallis test were used to compare more than two groups.

Results

The FVC and the FEV₁ were significantly greater among highlanders than lowlanders (p value < 0.001), with a proportion difference of 15.2% for men and 8.5% for women. The FEV₁/FVC was significantly higher among lowlanders than highlanders for men and women. A restrictive pattern was found in 12.9% of the participants.

Conclusion

Residents of Oyacachi had greater FVC and FEV₁ than their peers from Limoncocha, a finding physiologically plausible according to published literature. Lung size and greater ventilatory capacities could be an adaptive mechanism developed by the highlander in response to hypoxia. Our results support the fact that this difference in FVC and FEV₁ is a compensatory mechanism towards lower barometric and alveolar partial pressure of oxygen pressure.

Prolonged Sojourn at Very High Altitude Decreases Sea-Level Anaerobic Performance, Anaerobic Threshold, and Fat Mass

Background: The influence of high altitude on an organism’s physiology depends on the length and the level of hypoxic exposure it experiences. This study aimed to determine the effect of a prolonged sojourn at very high altitudes (above 3,500m) on subsequent sea-level physical performance, body weight, body composition, and hematological parameters.

Materials and Methods: Ten alpinists, nine males and one female, with a mean age of 27±4years, participated in the study. All had been on mountaineering expeditions to 7,000m peaks, where they spent 30±1days above 3,500m with their average sojourn at 4,900±60m. Their aerobic and anaerobic performance, body weight, body composition, and hematological parameters were examined at an altitude of 100m within 7days before the expeditions and 7days after they descended below 3,500m.

Results: We found a significant (p<0.01) decrease in maximal anaerobic power (MAP_WAnT) from 9.9±1.3 to 9.2±1.3W·kg⁻¹, total anaerobic work from 248.1±23.8 to 228.1±20.1J·kg⁻¹, anaerobic threshold from 39.3±8.0 to 27.8±5.6 mlO₂·kg⁻¹·min⁻¹, body fat mass from 14.0±3.1 to 11.5±3.3%, and a significant increase (p<0.05) in maximal tidal volume from 3.2 [3.0–3.2] to 3.5 [3.3–3.9] L after their sojourn at very high attitude. We found no significant changes in maximal aerobic power, maximal oxygen uptake, body weight, fat-free mass, total body water, hemoglobin, and hematocrit.

Conclusion: A month-long exposure to very high altitude led to impaired sea-level anaerobic performance and anaerobic threshold, increased maximal tidal volume, and depleted body fat mass, but had no effect on maximal aerobic power, maximal oxygen uptake, or hemoglobin and hematocrit levels.

Integrated respiratory chemoreflex-mediated regulation of cerebral blood flow in hypoxia: Implications for oxygen delivery and acute mountain sickness

NEW FINDINGS

What is the central question of this study? To what extent do hypoxia-induced changes in the peripheral and central respiratory chemoreflex modulate anterior and posterior cerebral oxygen delivery, with corresponding implications for susceptibility to acute mountain sickness? What is the main finding and its importance? We provide evidence for site-specific regulation of cerebral blood flow in hypoxia that preserves oxygen delivery in the posterior but not the anterior cerebral circulation, with minimal contribution from the central respiratory chemoreflex. External carotid artery vasodilatation might prove to be an alternative haemodynamic risk factor that predisposes to acute mountain sickness.

ABSTRACT

The aim of the present study was to determine the extent to which hypoxia-induced changes in the peripheral and central respiratory chemoreflex modulate anterior and posterior cerebral blood flow (CBF) and oxygen delivery (CDO₂ ), with corresponding implications for the pathophysiology of the neurological syndrome, acute mountain sickness (AMS). Eight healthy men were randomly assigned single blind to 7 h of passive exposure to both normoxia (21% O₂ ) and hypoxia (12% O₂ ). The peripheral and central respiratory chemoreflex, internal carotid artery, external carotid artery (ECA) and vertebral artery blood flow (duplex ultrasound) and AMS scores (questionnaires) were measured throughout. A reduction in internal carotid artery CDO₂ was observed during hypoxia despite a compensatory elevation in perfusion. In contrast, vertebral artery and ECA CDO₂ were preserved, and the former was attributable to a more marked increase in perfusion. Hypoxia was associated with progressive activation of the peripheral respiratory chemoreflex (P < 0.001), whereas the central respiratory chemoreflex remained unchanged (P > 0.05). Symptom severity in participants who developed clinical AMS was positively related to ECA blood flow (Lake Louise score, r = 0.546-0.709, P = 0.004-0.043; Environmental Symptoms Questionnaires-Cerebral symptoms score, r = 0.587-0.771, P = 0.001-0.027, n = 4). Collectively, these findings highlight the site-specific regulation of CBF in hypoxia that maintains CDO₂ selectively in the posterior but not the anterior cerebral circulation, with minimal contribution from the central respiratory chemoreflex. Furthermore, ECA vasodilatation might represent a hitherto unexplored haemodynamic risk factor implicated in the pathophysiology of AMS.

Return to High Altitude After Recovery from Coronavirus Disease 2019

Luks, Andrew M. and Colin K. Grissom. Return to high altitude after recovery from coronavirus disease 2019. High Alt Med Biol. 22: 119-127, 2021.-With the increasing availability of coronavirus disease 2019 (COVID-19) vaccines and the eventual decline in the burden of the disease, it is anticipated that all forms of tourism, including travel to high altitude, will rebound in the near future. Given the physiologic challenges posed by hypobaric hypoxia at high altitude, it is useful to consider whether high-altitude travel will pose risks to those previously infected with severe acute respiratory syndrome coronavirus 2, particularly those with persistent symptoms after resolution of their infection. Although no studies have specifically examined this question as of yet, available data on the cardiopulmonary sequelae of COVID-19 provide some sense of the problems people may face at high altitude and who warrants evaluation before such endeavors. On average, most individuals who have recovered from COVID-19 have normal or near normal gas exchange, pulmonary function testing, cardiovascular function, and exercise capacity, although a subset of individuals have persistent functional deficits in some or all of these domains when examined up to 5 months after infection. Evaluation is warranted before planned high-altitude travel in individuals with persistent symptoms at least 2 weeks after a positive test or hospital discharge as well as in those who required care in an intensive care unit or suffered from myocarditis or arterial or venous thromboembolism. Depending on the results of this testing, planned high-altitude travel may need to be modified or even deferred pending resolution of the identified abnormalities. As more people travel to high altitude after the pandemic and further studies are conducted, additional data should become available to provide further guidance on these issues.

Ureteral calculi associated with high-altitude polycythemia: A case report

RATIONALE

High-altitude polycythemia (HAPC) is a common disease in high-altitude areas characterized by excessive erythrocyte proliferation and severe hypoxemia. Recently, the incidence of ureteral calculi has risen. However, cases of ureteral calculi associated with HAPC have not been reported.

PATIENT CONCERNS

We present the cases of 2 patients (26-year-old female, Case 1; 31-year-old male, Case 2) with HAPC who were born in the lowlands and worked in areas of high altitudes. Both patients were admitted to the hospital with acute severe pain in the ureter as the first symptom.

DIAGNOSES

Urological examinations confirmed the presence of a ureteral stone. Interestingly, the biochemical tests showed elevated serum uric acid levels, and the calculous component analysis suggested anhydrous uric acid.

INTERVENTIONS

In the first case, the patient underwent extracorporeal shock wave lithotripsy. In the second case, the patient underwent right ureteroscopy and right ureteral stenting. The patient received postoperative anti-inflammatory, hemostatic, and rehydration therapy.

OUTCOMES

Both patients recovered well with no recurrences observed upon regular re-examinations.

LESSONS

Recently, extensive research has demonstrated a significant correlation between hyperuricemia and HAPC. Therefore, we speculated that the occurrence of ureteral calculi among immigrants to the plateau might be related to hyperuricemia associated with HAPC. This case report and literature review highlights that the prevention of ureteral calculi in patients with polycythemia who immigrate to the plateaus from high-altitude areas should be considered. Additionally, the serum uric acid levels and urine pH should be monitored regularly.

Short-term hypoxia does not promote arrhythmia during voluntary apnea

The presence of bradycardic arrhythmias during volitional apnea at altitude may be caused by chemoreflex activation/sensitization. We investigated whether bradyarrhythmic episodes became prevalent in apnea following short‐term hypoxia exposure. Electrocardiograms (ECG; lead II) were collected from 22 low‐altitude residents (F = 12; age=25 ± 5 years) at 671 m. Participants were exposed to normobaric hypoxia (Spo₂ ~79 ± 3%) over a 5‐h period. ECG rhythms were assessed during both free‐breathing and maximal volitional end‐expiratory and end‐inspiratory apnea at baseline during normoxia and hypoxia exposure (20 min [AHX]; 5 h [HX5]). Free‐breathing HR became elevated at AHX (78 ± 10 bpm; p < 0.0001) and HX5 (80 ± 12 bpm; p < 0.0001) compared to normoxia (68 ± 10 bpm), whereas apnea caused significant bradycardia at AHX (nadir end‐expiratory −17 ± 14 bpm; p < 0.001) and HX5 (nadir end‐expiratory −19 ± 15 bpm; p < 0.001), but not during normoxia (nadir end‐expiratory −4 ± 13 bpm), with no difference in bradycardia responses between apneas at AHX and HX5. Conduction abnormalities were noted in five participants during normoxia (Premature Ventricular Contraction, Sinus Pause, Junctional Rhythm, Atrial Foci), which remained unchanged during apnea at AHX and HX5 (Premature Ventricular Contraction, Premature Atrial Contraction, Sinus Pause). End‐inspiratory apneas were overall longer across conditions (normoxia p < 0.05; AHX p < 0.01; HX5 p < 0.001), with comparable HR responses to end‐expiratory and fewer occurrences of arrhythmia. While short‐term hypoxia is sufficient to elicit bradycardia during apnea, the occurrence of arrhythmias in response to apnea was not affected. These findings indicate that previously observed bradyarrhythmic events in untrained individuals at altitude only become prevalent following chronic hypoxia specificlly.

Relationships Between Chemoreflex Responses, Sleep Quality, and Hematocrit in Andean Men and Women

Andean highlanders are challenged by chronic hypoxia and many exhibit elevated hematocrit (Hct) and blunted ventilation compared to other high-altitude populations. While many Andeans develop Chronic Mountain Sickness (CMS) and excessive erythrocytosis, Hct varies markedly within Andean men and women and may be driven by individual differences in ventilatory control and/or sleep events which exacerbate hypoxemia. To test this hypothesis, we quantified relationships between resting ventilation and ventilatory chemoreflexes, sleep desaturation, breathing disturbance, and Hct in Andean men and women. Ventilatory measures were made in 109 individuals (n = 63 men; n = 46 women), and sleep measures in 45 of these participants (n = 22 men; n = 23 women). In both men and women, high Hct was associated with low daytime SpO₂ (p < 0.001 and p < 0.002, respectively) and decreased sleep SpO₂ (mean, nadir, and time <80%; all p < 0.02). In men, high Hct was also associated with increased end-tidal P_CO2 (p < 0.009). While ventilatory responses to hypoxia and hypercapnia did not predict Hct, decreased hypoxic ventilatory responses were associated with lower daytime SpO₂ in men (p < 0.01) and women (p < 0.009) and with lower nadir sleep SpO₂ in women (p < 0.02). Decreased ventilatory responses to CO₂ were associated with more time below 80% SpO₂ during sleep in men (p < 0.05). The obstructive apnea index and apnea-hypopnea index also predicted Hct and CMS scores in men after accounting for age, BMI, and SpO₂ during sleep. Finally, heart rate response to hypoxia was lower in men with higher Hct (p < 0.0001). These data support the idea that hypoventilation and decreased ventilatory sensitivity to hypoxia are associated with decreased day time and nighttime SpO₂ levels that may exacerbate the stimulus for erythropoiesis in Andean men and women. However, interventional and longitudinal studies are required to establish the causal relationships between these associations.

Interaction between the respiratory system and cerebral blood flow regulation

This review summarizes the interaction between the regulatory system of respiration and cerebral vasculature. Some clinical reports provide evidence for the association between these two physiological regulatory systems. Physiologically, arterial carbon dioxide concentration is mainly regulated by two feedback control systems: respiration and cerebral blood flow. In other words, both of these systems are sensitive to the same mediator, i.e., carbon dioxide, at a set point. In addition, respiratory dysfunction alters various physiological factors that affect the cerebral vasculature. Therefore, it is physiologically plausible that these systems are closely linked. The regulation of arterial carbon dioxide concentration affected by respiration and cerebral blood flow may be a key factor for a rise in the risk of brain disease in the patients with respiratory dysfunction. For example, the management of respiratory disease (e.g., patients with chronic obstructive pulmonary disease) and the use of prophylactic therapy are essential to reduce the risk of stroke.

What Is the Point of the Peak? Assessing Steady-State Respiratory Chemoreflex Drive in High Altitude Field Studies

Measurements of central and peripheral respiratory chemoreflexes are important in the context of high altitude as indices of ventilatory acclimatization. However, respiratory chemoreflex tests have many caveats in the field, including considerations of safety, portability and consistency. This overview will (a) outline commonly utilized tests of the hypoxic ventilatory response (HVR) in humans, (b) outline the caveats associated with a variety of peak response HVR tests in the laboratory and in high altitude fieldwork contexts, and (c) advance a novel index of steady-state chemoreflex drive (SS-CD) that addresses the many limitations of other chemoreflex tests. The SS-CD takes into account the contribution of central and peripheral respiratory chemoreceptors, and eliminates the need for complex equipment and transient respiratory gas perturbation tests. To quantify the SS-CD, steady-state measurements of the pressure of end-tidal (P_ET)CO₂ (Torr) and peripheral oxygen saturation (SpO₂; %) are used to quantify a stimulus index (SI; P_ETCO₂/SpO₂). The SS-CD is then calculated by indexing resting ventilation (L/min) against the SI. SS-CD data are subsequently reported from 13 participants during incremental ascent to high altitude (5160 m) in the Nepal Himalaya. The mean SS-CD magnitude increased approximately 96% over 10 days of incremental exposure to hypobaric hypoxia, suggesting that the SS-CD tracks ventilatory acclimatization. This novel SS-CD may have future utility in fieldwork studies assessing ventilatory acclimatization during incremental or prolonged stays at altitude, and may replace the use of complex and potentially confounded transient peak response tests of the HVR in humans.

Precise Control of End-tidal Carbon Dioxide Levels Using Sequential Rebreathing Circuits

Anaesthesiologists have traditionally been consulted to help design breathing circuits to attain and maintain target end-tidal carbon dioxide (PETCO2). The methodology has recently been simplified by breathing circuits that sequentially deliver fresh gas (not containing carbon dioxide (CO2)) and reserve gas (containing CO2). Our aim was to determine the roles of fresh gas flow, reserve gas PCO2 and minute ventilation in the determination of PETCO2. We first used a computer model of a non-rebreathing sequential breathing circuit to determine these relationships. We then tested our model by monitoring PETCO2 in human volunteers who increased their minute ventilation from resting to five times resting levels. The optimal settings to maintain PETCO2 independently of minute ventilation are 1) fresh gas flow equal to minute ventilation minus anatomical deadspace ventilation, and 2) reserve gas PCO2 equal to alveolar PCO2. We provide an equation to assist in identifying gas settings to attain a target PCO2. The ability to precisely attain and maintain a target PCO2 (isocapnia) using a sequential gas delivery circuit has multiple therapeutic and scientific applications.

Preparation for Endurance Competitions at Altitude: Physiological, Psychological, Dietary and Coaching Aspects. A Narrative Review

It was the Summer Olympic Games 1968 held in Mexico City (2,300 m) that required scientists and coaches to cope with the expected decline of performance in endurance athletes and to establish optimal preparation programs for competing at altitude. From that period until now many different recommendations for altitude acclimatization in advance of an altitude competition were proposed, ranging from several hours to several weeks. Those recommendations are mostly based on the separate consideration of the physiology of acclimatization, psychological issues, performance changes, logistical or individual aspects, but there is no review considering all these aspects in their entirety. Therefore, the present work primarily focusses on the period of altitude sojourn prior to the competition at altitude based on physiological and psychological aspects complemented by nutritional and sports practical considerations.

Ventilatory and cerebrovascular regulation and integration at high-altitude

Ascent to high-altitude elicits compensatory physiological adaptations in order to improve oxygenation throughout the body. The brain is particularly vulnerable to the hypoxemia of terrestrial altitude exposure. Herein we review the ventilatory and cerebrovascular changes at altitude and how they are both implicated in the maintenance of oxygen delivery to the brain. Further, the interdependence of ventilation and cerebral blood flow at altitude is discussed. Following the acute hypoxic ventilatory response, acclimatization leads to progressive increases in ventilation, and a partial mitigation of hypoxemia. Simultaneously, cerebral blood flow increases during initial exposure to altitude when hypoxemia is the greatest. Following ventilatory acclimatization to altitude, and an increase in hemoglobin concentration-which both underscore improvements in arterial oxygen content over time at altitude-cerebral blood flow progressively decreases back to sea-level values. The complimentary nature of these responses (ventilatory, hematological and cerebral) lead to a tightly maintained cerebral oxygen delivery while at altitude. Despite this general maintenance of global cerebral oxygen delivery, the manner in which this occurs reflects integration of these physiological responses. Indeed, ventilation directly influences cerebral blood flow by determining the prevailing blood gas and acid/base stimuli at altitude, but cerebral blood flow may also influence ventilation by altering central chemoreceptor stimulation via central CO₂ washout. The causes and consequences of the integration of ventilatory and cerebral blood flow regulation at high altitude are outlined.

Assessing chemoreflexes and oxygenation in the context of acute hypoxia: Implications for field studies

Carotid chemoreceptors detect changes in PO₂ and elicit a peripheral respiratory chemoreflex (PCR). The PCR can be tested through a transient hypoxic ventilatory response test (TT-HVR), which may not be safe nor feasible at altitude. We characterized a transient hyperoxic ventilatory withdrawal test in the setting of steady-state normobaric hypoxia (13.5-14% F_IO₂) and compared it to a TT-HVR and a steady-state poikilocapnic hypoxia test, within-individuals. No PCR test magnitude was correlated with any other test, nor was any test magnitude correlated with oxygenation while in steady-state hypoxia. Due to the heterogeneity between the different PCR test procedures and magnitudes, and the confounding effects of alterations in CO₂ acting on both central and peripheral chemoreceptors, we developed a novel method to assess prevailing steady-state chemoreflex drive in the context of hypoxia. Quantifying peak hypoxic/hyperoxic responses at low altitude may have minimal utility in predicting oxygenation during ascent to altitude, and here we advance a novel index of chemoreflex drive.

Evidence from high-altitude acclimatization for an integrated cerebrovascular and ventilatory hypercapnic response but different responses to hypoxia

Ventilation and cerebral blood flow (CBF) are both sensitive to hypoxia and hypercapnia. To compare chemosensitivity in these two systems, we made simultaneous measurements of ventilatory and cerebrovascular responses to hypoxia and hypercapnia in 35 normal human subjects before and after acclimatization to hypoxia. Ventilation and CBF were measured during stepwise changes in isocapnic hypoxia and iso-oxic hypercapnia. We used MRI to quantify actual cerebral perfusion. Measurements were repeated after 2 days of acclimatization to hypoxia at 3,800 m altitude (partial pressure of inspired O₂ = 90 Torr) to compare plasticity in the chemosensitivity of these two systems. Potential effects of hypoxic and hypercapnic responses on acute mountain sickness (AMS) were assessed also. The pattern of CBF and ventilatory responses to hypercapnia were almost identical. CO₂ responses were augmented to a similar degree in both systems by concomitant acute hypoxia or acclimatization to sustained hypoxia. Conversely, the pattern of CBF and ventilatory responses to hypoxia were markedly different. Ventilation showed the well-known increase with acute hypoxia and a progressive decline in absolute value over 25 min of sustained hypoxia. With acclimatization to hypoxia for 2 days, the absolute values of ventilation and O₂ sensitivity increased. By contrast, O₂ sensitivity of CBF or its absolute value did not change during sustained hypoxia for up to 2 days. The results suggest a common or integrated control mechanism for CBF and ventilation by CO₂ but different mechanisms of O₂ sensitivity and plasticity between the systems. Ventilatory and cerebrovascular responses were the same for all subjects irrespective of AMS symptoms. NEW & NOTEWORTHY Ventilatory and cerebrovascular hypercapnic response patterns show similar plasticity in CO₂ sensitivity following hypoxic acclimatization, suggesting an integrated control mechanism. Conversely, ventilatory and cerebrovascular hypoxic responses differ. Ventilation initially increases but adapts with prolonged hypoxia (hypoxic ventilatory decline), and ventilatory sensitivity increases following acclimatization. In contrast, cerebral blood flow hypoxic sensitivity remains constant over a range of hypoxic stimuli, with no cerebrovascular acclimatization to sustained hypoxia, suggesting different mechanisms for O₂ sensitivity in the two systems.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Ventilatory acclimatisation is beneficial for high-intensity exercise at altitude in elite cyclists

AIM

The aim of this study was to examine the relationship between ventilatory adaptation and performance during altitude training at 2700 m.

METHODS

Seven elite cyclists (age: 21.2 ± 1.1 yr, body mass: 69.9 ± 5.6 kg, height 176.3 ± 4.9 cm) participated in this study. A hypoxic ventilatory response (HVR) test and a submaximal exercise test were performed at sea level prior to the training camp and again after 15 d at altitude (ALT15). Ventilation (VE), end-tidal carbon-dioxide partial pressure (PETCO2) and oxyhaemoglobin saturation via pulse oximetry (SpO2) were measured at rest and during submaximal cycling at 250 W. A hill climb (HC) performance test was conducted at sea level and after 14 d at altitude (ALT14) using a road of similar length (5.5-6 km) and gradient (4.8-5.3%). Power output was measured using SRM cranks. Average HC power at ALT14 was normalised to sea level power (HC%). Multiple regression was used to identify significant predictors of performance at altitude.

RESULTS

At ALT15, there was a significant increase in resting VE (10.3 ± 1.9 vs. 12.2 ± 2.4 L·min(-1)) and HVR (0.34 ± 0.24 vs. 0.71 ± 0.49 L·min(-1)·%(-1)), while PETCO2 (38.4 ± 2.3 vs. 32.1 ± 3.3 mmHg) and SpO2 (97.9 ± 0.7 vs. 94.0 ± 1.7%) were reduced (P < .05). Multiple regression revealed ΔHVR and exercise VE at altitude as significant predictors of HC% (adjusted r(2) = 0.913; P = 0.003).

CONCLUSIONS

Ventilatory acclimatisation occurred during a 2 wk altitude training camp in elite cyclists and a higher HVR was associated with better performance at altitude, relative to sea level. These results suggest that ventilatory acclimatisation is beneficial for cycling performance at altitude.

Ibuprofen Blunts Ventilatory Acclimatization to Sustained Hypoxia in Humans

Ventilatory acclimatization to hypoxia is a time-dependent increase in ventilation and the hypoxic ventilatory response (HVR) that involves neural plasticity in both carotid body chemoreceptors and brainstem respiratory centers. The mechanisms of such plasticity are not completely understood but recent animal studies show it can be blocked by administering ibuprofen, a nonsteroidal anti-inflammatory drug, during chronic hypoxia. We tested the hypothesis that ibuprofen would also block the increase in HVR with chronic hypoxia in humans in 15 healthy men and women using a double-blind, placebo controlled, cross-over trial. The isocapnic HVR was measured with standard methods in subjects treated with ibuprofen (400mg every 8 hrs) or placebo for 48 hours at sea level and 48 hours at high altitude (3,800 m). Subjects returned to sea level for at least 30 days prior to repeating the protocol with the opposite treatment. Ibuprofen significantly decreased the HVR after acclimatization to high altitude compared to placebo but it did not affect ventilation or arterial O₂ saturation breathing ambient air at high altitude. Hence, compensatory responses prevent hypoventilation with decreased isocapnic ventilatory O₂-sensitivity from ibuprofen at this altitude. The effect of ibuprofen to decrease the HVR in humans provides the first experimental evidence that a signaling mechanism described for ventilatory acclimatization to hypoxia in animal models also occurs in people. This establishes a foundation for the future experiments to test the potential role of different mechanisms for neural plasticity and ventilatory acclimatization in humans with chronic hypoxemia from lung disease.

HIGH LIFE: High altitude fatalities led to pulse oximetry

In 1875, Paul Bert linked high altitude danger to the low partial pressure of oxygen when 2 of 3 French balloonists died euphorically at about 8,600 m altitude. World War I fatal crashes of high altitude fighter pilots led to a century of efforts to use oximetry to warn pilots. The carotid body, discovered in 1932 to be the hypoxia detector, led to most current physiologic understanding of the body's respiratory responses to hypoxia and CO2. The author describes some of his UCSF group's work: In 1963, we reported both the brain's ventral medullary near-surface CO2 (and pH) chemosensors and the role of cerebrospinal fluid in acclimatization to altitude. In 1966, we reported the effect of altitude on cerebral blood flow and later the changes of carotid body sensitivity at altitude and the differences in natives of high altitude. In 1973, pulse oximetry was invented when Japanese biophysicist Takuo Aoyagi read and applied to pulses a largely forgotten 35-year-old discovery by English medical student J. R. Squire of a method of computing oxygen saturation from red and infrared light passing through both perfused and blanched tissue.

End tidal-to-arterial CO2 and O2 gas gradients at low- and high-altitude during dynamic end-tidal forcing

We sought to characterize and quantify the performance of a portable dynamic end-tidal forcing (DEF) system in controlling the partial pressure of arterial CO2 (PaCO2) and O2 (PaO2) at low (LA; 344 m) and high altitude (HA; 5,050 m) during an isooxic CO2 test and an isocapnic O2 test, which is commonly used to measure ventilatory and vascular reactivity in humans ( n = 9). The isooxic CO2 tests involved step changes in the partial pressure of end-tidal CO2 (PetCO2) of −10, −5, 0, +5, and +10 mmHg from baseline. The isocapnic O2 test consisted of a 10-min hypoxic step (PetO2 = 47 mmHg) from baseline at LA and a 5-min euoxic step (PetO2 = 100 mmHg) from baseline at HA. At both altitudes, PetO2 and PetCO2 were controlled within narrow limits (<1 mmHg from target) during each protocol. During the isooxic CO2 test at LA, PetCO2 consistently overestimated PaCO2 ( P < 0.01) at both baseline (2.1 ± 0.5 mmHg) and hypercapnia (+5 mmHg: 2.1 ± 0.7 mmHg; +10 mmHg: 1.9 ± 0.5 mmHg). This Pa-PetCO2 gradient was approximately twofold greater at HA ( P < 0.05). At baseline at both altitudes, PetO2 overestimated PaO2 by a similar extent (LA: 6.9 ± 2.1 mmHg; HA: 4.5 ± 0.9 mmHg; both P < 0.001). This overestimation persisted during isocapnic hypoxia at LA (6.9 ± 0.6 mmHg) and during isocapnic euoxia at HA (3.8 ± 1.2 mmHg). Step-wise multiple regression analysis, on the basis of the collected data, revealed that it may be possible to predict an individual's arterial blood gases during DEF. Future research is needed to validate these prediction algorithms and determine the implications of end-tidal-to-arterial gradients in the assessment of ventilatory and/or vascular reactivity.

Indomethacin-induced impairment of regional cerebrovascular reactivity: implications for respiratory control

Cerebrovascular reactivity impacts CO₂-[H(+)] washout at the central chemoreceptors and hence has marked influence on the control of ventilation. To date, the integration of cerebral blood flow (CBF) and ventilation has been investigated exclusively with measures of anterior CBF, which has a differential reactivity from the vertebrobasilar system and perfuses the brainstem. We hypothesized that: (1) posterior versus anterior CBF would have a stronger relationship to central chemoreflex magnitude during hypercapnia, and (2) that higher posterior reactivity would lead to a greater hypoxic ventilatory decline (HVD). End-tidal forcing was used to induce steady-state hyperoxic (300 mmHg P ET ,O₂) hypercapnia (+3, +6 and +9 mmHg P ET ,CO₂) and isocapnic hypoxia (45 mmHg P ET ,O₂) before and following pharmacological blunting (indomethacin; INDO; 1.45 ± 0.17 mg kg(-1)) of resting CBF and reactivity. In 22 young healthy volunteers, ventilation, intra-cranial arterial blood velocities and extra-cranial blood flows were measured during these challenges. INDO-induced blunting of cerebrovascular flow responsiveness (CVR) to CO₂ was unrelated to variability in ventilatory sensitivity during hyperoxic hypercapnia. Further results in a sub-group of volunteers (n = 9) revealed that elevations of P ET,CO₂ via end-tidal forcing reduce arterial-jugular venous gradients, attenuating the effect of CBF on chemoreflex responses. During isocapnic hypoxia, vertebral artery CVR was related to the magnitude of HVD (R(2) = 0.27; P < 0.04; n = 16), suggesting that CO₂-[H(+)] washout from central chemoreceptors modulates hypoxic ventilatory dynamics. No relationships were apparent with anterior CVR. As higher posterior, but not anterior, CVR was linked to HVD, our study highlights the importance of measuring flow in posterior vessels to investigate CBF and ventilatory integration.

Physiology in Medicine: A physiologic approach to prevention and treatment of acute high-altitude illnesses

With the growing interest in adventure travel and the increasing ease and affordability of air, rail, and road-based transportation, increasing numbers of individuals are traveling to high altitude. The decline in barometric pressure and ambient oxygen tensions in this environment trigger a series of physiologic responses across organ systems and over a varying time frame that help the individual acclimatize to the low oxygen conditions but occasionally lead to maladaptive responses and one or several forms of acute altitude illness. The goal of this Physiology in Medicine article is to provide information that providers can use when counseling patients who present to primary care or travel medicine clinics seeking advice about how to prevent these problems. After discussing the primary physiologic responses to acute hypoxia from the organ to the molecular level in normal individuals, the review describes the main forms of acute altitude illness--acute mountain sickness, high-altitude cerebral edema, and high-altitude pulmonary edema--and the basic approaches to their prevention and treatment of these problems, with an emphasis throughout on the physiologic basis for the development of these illnesses and their management.

Cerebral hemodynamic and ventilatory responses to hypoxia, hypercapnia, and hypocapnia during 5 days at 4,350 m

This study investigated the changes in cerebral near-infrared spectroscopy (NIRS) signals, cerebrovascular and ventilatory responses to hypoxia and CO2 during altitude exposure. At sea level (SL), after 24 hours and 5 days at 4,350 m, 11 healthy subjects were exposed to normoxia, isocapnic hypoxia, hypercapnia, and hypocapnia. The following parameters were measured: prefrontal tissue oxygenation index (TOI), oxy- (HbO2), deoxy- and total hemoglobin (HbTot) concentrations with NIRS, blood velocity in the middle cerebral artery (MCAv) with transcranial Doppler and ventilation. Smaller prefrontal deoxygenation and larger ΔHbTot in response to hypoxia were observed at altitude compared with SL (day 5: ΔHbO2-0.6±1.1 versus -1.8±1.3 μmol/cmper mm Hg and ΔHbTot 1.4±1.3 versus 0.7±1.1 μmol/cm per mm Hg). The hypoxic MCAv and ventilatory responses were enhanced at altitude. Prefrontal oxygenation increased less in response to hypercapnia at altitude compared with SL (day 5: ΔTOI 0.3±0.2 versus 0.5±0.3% mm Hg). The hypercapnic MCAv and ventilatory responses were decreased and increased, respectively, at altitude. Hemodynamic responses to hypocapnia did not change at altitude. Short-term altitude exposure improves cerebral oxygenation in response to hypoxia but decreases it during hypercapnia. Although these changes may be relevant for conditions such as exercise or sleep at altitude, they were not associated with symptoms of acute mountain sickness.

The von Hippel-Lindau Chuvash mutation in mice causes carotid-body hyperplasia and enhanced ventilatory sensitivity to hypoxia

The hypoxia-inducible factor (HIF) family of transcription factors coordinates diverse cellular and systemic responses to hypoxia. Chuvash polycythemia (CP) is an autosomal recessive disorder in humans in which there is impaired oxygen-dependent degradation of HIF, resulting in long-term systemic elevation of HIF levels at normal oxygen tensions. CP patients demonstrate the characteristic features of ventilatory acclimatization to hypoxia, namely, an elevated baseline ventilation and enhanced acute hypoxic ventilatory response (AHVR). We investigated the ventilatory and carotid-body phenotype of a mouse model of CP, using whole-body plethysmography, immunohistochemistry, and electron microscopy. In keeping with studies in humans, CP mice had elevated ventilation in euoxia and a significantly exaggerated AHVR when exposed to 10% oxygen, with or without the addition of 3% carbon dioxide. Carotid-body immunohistochemistry demonstrated marked hyperplasia of the oxygen-sensing type I cells, and the cells themselves appeared enlarged with more prominent nuclei. This hypertrophy was confirmed by electron microscopy, which also revealed that the type I cells contained an increased number of mitochondria, enlarged dense-cored vesicles, and markedly expanded rough endoplasmic reticulum. The morphological and ultrastructural changes seen in the CP mouse carotid body are strikingly similar to those observed in animals exposed to chronic hypoxia. Our study demonstrates that the HIF pathway plays a major role, not only in regulating both euoxic ventilatory control and the sensitivity of the response to hypoxia, but also in determining the morphology of the carotid body.

Acute mountain sickness, chemosensitivity, and cardiorespiratory responses in humans exposed to hypobaric and normobaric hypoxia

We examined the control of breathing, cardiorespiratory effects, and the incidence of acute mountain sickness (AMS) in humans exposed to hypobaric hypoxia (HH) and normobaric hypoxia (NH), and under two control conditions [hypobaric normoxia (HN) and normobaric normoxia (NN)]. Exposures were 6 h in duration, and separated by 2 wk between hypoxic exposures and 1 wk between normoxic exposures. Before and after exposures, subjects (n = 11) underwent hyperoxic and hypoxic Duffin CO2 rebreathing tests and a hypoxic ventilatory response test (HVR). Inside the environmental chamber, minute ventilation (V(E)), tidal volume (V(T)), frequency of breathing (fB), blood oxygenation, heart rate, and blood pressure were measured at 5 and 30 min and hourly until exit. Symptoms of AMS were evaluated using the Lake Louise score (LLS). Both the hyperoxic and hypoxic CO2 thresholds were lower after HH and NH, whereas CO2 sensitivity was increased after HH and NH in the hypoxic test and after NH in the hyperoxic test. Values for HVR were similar across the four exposures. No major differences were observed for Ve or any other cardiorespiratory variables between NH and HH. The LLS was greater in AMS-susceptible than in AMS-resistant subjects; however, LLS was alike between HH and NH. In AMS-susceptible subjects, fB correlated positively and Vt negatively with the LLS. We conclude that 6 h of hypoxic exposure is sufficient to lower the peripheral and central CO2 threshold but does not induce differences in cardiorespiratory variables or AMS incidence between HH and NH.

Intermittent hypoxia, respiratory plasticity and sleep apnea in humans: present knowledge and future investigations

This review examines the role that respiratory plasticity has in the maintenance of breathing stability during sleep in individuals with sleep apnea. The initial portion of the review considers the manner in which repetitive breathing events may be initiated in individuals with sleep apnea. Thereafter, the role that two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of upper airway and respiratory muscle activity, might have in modifying breathing events in humans is examined. In this context, present knowledge regarding the initiation of respiratory plasticity in humans during wakefulness and sleep is addressed. Also, published findings which reveal that exposure to intermittent hypoxia promotes breathing instability, at least in part, because of progressive augmentation of the hypoxic ventilatory response and the absence of long-term facilitation, are considered. Next, future directions are presented and are focused on the manner in which forms of plasticity that stabilize breathing might be promoted while diminishing destabilizing forms, concurrently. These future directions will consider the potential role of circadian rhythms in the promotion of respiratory plasticity and the role of respiratory plasticity in enhancing established treatments for sleep apnea.

Adaptive and maladaptive cardiorespiratory responses to continuous and intermittent hypoxia mediated by hypoxia-inducible factors 1 and 2

Hypoxia is a fundamental stimulus that impacts cells, tissues, organs, and physiological systems. The discovery of hypoxia-inducible factor-1 (HIF-1) and subsequent identification of other members of the HIF family of transcriptional activators has provided insight into the molecular underpinnings of oxygen homeostasis. This review focuses on the mechanisms of HIF activation and their roles in physiological and pathophysiological responses to hypoxia, with an emphasis on the cardiorespiratory systems. HIFs are heterodimers comprised of an O(2)-regulated HIF-1α or HIF-2α subunit and a constitutively expressed HIF-1β subunit. Induction of HIF activity under conditions of reduced O(2) availability requires stabilization of HIF-1α and HIF-2α due to reduced prolyl hydroxylation, dimerization with HIF-1β, and interaction with coactivators due to decreased asparaginyl hydroxylation. Stimuli other than hypoxia, such as nitric oxide and reactive oxygen species, can also activate HIFs. HIF-1 and HIF-2 are essential for acute O(2) sensing by the carotid body, and their coordinated transcriptional activation is critical for physiological adaptations to chronic hypoxia including erythropoiesis, vascularization, metabolic reprogramming, and ventilatory acclimatization. In contrast, intermittent hypoxia, which occurs in association with sleep-disordered breathing, results in an imbalance between HIF-1α and HIF-2α that causes oxidative stress, leading to cardiorespiratory pathology.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Heart rate variability during two sequential mountaineering expeditions

This study was undertaken to assess the duration of altitude acclimatization retention in individuals after initial exposure to a maximum altitude of 5360 m during a mountaineering expedition. Spectral heart rate variability analysis accompanied by an assessment of acute mountain sickness using the Lake Louise Scoring System was performed during two sequential mountaineering expeditions to altitudes of 5360 m and 5642 m, with a period of 30 days between each expedition. Subjects displayed varying degrees of alterations in heart rate variability during the initial expedition, which indicated differing degrees of dysadaptation and stress development. Their Lake Louise Scores accounted for the presence of acute mountain sickness throughout the trip. During the subsequent expedition, the subjects' heart rate variability measures were within the normal range, and there were no signs of acute mountain sickness. All three subjects who underwent step-by-step exposure to altitudes of 5360 m displayed differing degrees of alterations in heart rate variability in conjunction with differing degrees of acute mountain sickness. All subjects also developed acclimatization to hypoxic conditions at this altitude, which was preserved for 30 days, and was sufficient to prevent them from showing any significant alterations in heart rate variability when re-exposed to the same altitude.

Altitude preexposure recommendations for inducing acclimatization

For many low-altitude (<1500 m) residents, their travel itineraries may cause them to ascend rapidly to high (>2400 m) altitudes without having the time to develop an adequate degree of altitude acclimatization. Prior to departing on these trips, low-altitude residents can induce some degree of altitude acclimatization by ascending to moderate (>1500 m) or high altitudes during either continuous or intermittent altitude preexposures. Generally, the degree of altitude acclimatization developed is proportional to the altitude attained and the duration of exposure. The available evidence suggests that continuous residence at 2200 m or higher for 1 to 2 days or daily 1.5- to 4-h exposures to >4000 m induce ventilatory acclimatization. Six days at 2200 m substantially decreases acute mountain sickness (AMS) and improves work performance after rapid ascent to 4300 m. There is evidence that 5 or more days above 3000 m within the last 2 months will significantly decrease AMS during a subsequent rapid ascent to 4500 m. Exercise training during the altitude preexposures may augment improvement in physical performance. The persistence of altitude acclimatization after return to low altitude appears to be proportional to the degree of acclimatization developed. The subsequent ascent to high altitude should be scheduled as soon as possible after the last altitude preexposure.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Iron homeostasis and its interaction with prolyl hydroxylases

The ability of iron to accept or donate electrons, coupled with the ability of oxygen to act as an electron acceptor, renders both elements essential to normal cellular biology. However, these same chemical properties allow free iron in solution to generate toxic free radicals, particularly in combination with oxygen. Thus, closely interwoven homeostatic mechanisms have evolved to regulate both iron and oxygen concentrations at the systemic and the cellular levels. Systemically, iron levels are regulated through hepcidin-mediated uptake of iron in the duodenum, whereas intracellular free-iron levels are controlled through iron-regulatory proteins (IRPs). Cardiorespiratory changes increase systemic oxygen delivery, whereas at a cellular level, many responses to altered oxygen levels are coordinated by hypoxia-inducible factor (HIF). However, the mechanisms of iron homeostasis also are regulated by oxygen availability, with alterations in both hepcidin and IRP activity. In addition, many genes involved in iron homeostasis are direct targets of HIF. Furthermore, HIF activation is modulated by intracellular iron, through regulation of hydroxylase activity, which requires iron as a cofactor. In addition, HIF-2alpha translation is controlled by IRP activity, providing another level of interdependence between iron and oxygen homeostasis.

Differences in the control of breathing between Andean highlanders and lowlanders after 10 days acclimatization at 3850 m

We used Duffin's isoxic hyperoxic ( mmHg) and hypoxic ( mmHg) rebreathing tests to compare the control of breathing in eight (7 male) Andean highlanders and six (4 male) acclimatizing Caucasian lowlanders after 10 days at 3850 m. Compared to lowlanders, highlanders had an increased non-chemoreflex drive to breathe, characterized by higher basal ventilation at both hyperoxia (10.5 +/- 0.7 vs. 4.9 +/- 0.5 l min(1), P = 0.002) and hypoxia (13.8 +/- 1.4 vs. 5.7 +/- 0.9 l min(1), P < 0.001). Highlanders had a single ventilatory sensitivity to CO(2) that was lower than that of the lowlanders (P < 0.001), whose response was characterized by two ventilatory sensitivities (VeS1 and VeS2) separated by a patterning threshold. There was no difference in ventilatory recruitment thresholds (VRTs) between populations (P = 0.209). Hypoxia decreased VRT within both populations (highlanders: 36.4 +/- 1.3 to 31.7 +/- 0.7 mmHg, P < 0.001; lowlanders: 35.3 +/- 1.3 to 28.8 +/- 0.9 mmHg, P < 0.001), but it had no effect on basal ventilation (P = 0.12) or on ventilatory sensitivities in either population (P = 0.684). Within lowlanders, VeS2 was substantially greater than VeS1 at both isoxic tensions (hyperoxic: 9.9 +/- 1.7 vs. 2.8 +/- 0.2, P = 0.005; hypoxic: 13.2 +/- 1.9 vs. 2.8 +/- 0.5, P < 0.001), although hypoxia had no effect on either of the sensitivities (P = 0.192). We conclude that the control of breathing in Andean highlanders is different from that in acclimatizing lowlanders, although there are some similarities. Specifically, acclimatizing lowlanders have relatively lower non-chemoreflex drives to breathe, increased ventilatory sensitivities to CO(2), and an altered pattern of ventilatory response to CO(2) with two ventilatory sensitivities separated by a patterning threshold. Similar to highlanders and unlike lowlanders at sea-level, acclimatizing lowlanders respond to hypobaric hypoxia by decreasing their VRT instead of changing their ventilatory sensitivity to CO(2).

NMDA receptor-mediated processes in the Parabrachial/Kölliker fuse complex influence respiratory responses directly and indirectly via changes in cortical activation state

We tested the hypothesis that glutamate, acting via NMDA-type receptors (NMDAr) in the Parabrachial/Kölliker fuse (PBrKF) nucleus of the pons, is involved both directly and indirectly (via changes in cortical activation state) in modulating breathing and ventilatory responses to hypoxia. To this end we examined the effects of MK-801, injected either systemically or directly into the PBrKF, on the breathing patterns of urethane-anaesthetized rats breathing air or an hypoxic gas mixture as electroencephalographic (EEG) activity alternated between State I (awake-like) and State III (NREM sleep-like) EEG patterns. Regardless of EEG state, systemic MK-801 reduced ventilation primarily by reducing tidal volume while microinjection of MK-801 into the PBrKF reduced ventilation by reducing breathing frequency. With both injections, EEG pattern changed from State I to III mimicking the change from wakefulness to NREM sleep that occurs in unanaesthetized rats given MK-801 systemically. Systemic injection of MK-801 delayed and reduced the response to hypoxia while microinjection of MK-801 into the PBrKF did not reduce the HVR but sustained the hypoxic increase in tidal volume well into the post-hypoxic recovery period. Thus, while NMDAr in the PBrKF complex of the pons play a role in modulating sleep/wake-like states as well as changes in breathing pattern associated with changes in cortical activation state, they are neither involved in the hypoxic ventilatory response nor in the change in hypoxic sensitivity associated with the changes in cortical activation state.