Selected contribution: Ventilatory response to CO2 in high-altitude natives and patients with chronic mountain sickness

High-Altitude Erythrocytosis: Mechanisms of Adaptive and Maladaptive Responses

Erythrocytosis, or increased production of red blood cells, is one of the most well-documented physiological traits that varies within and among in high-altitude populations. Although a modest increase in blood O₂-carrying capacity may be beneficial for life in highland environments, erythrocytosis can also become excessive and lead to maladaptive syndromes such as chronic mountain sickness (CMS).

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.

Hypercapnic ventilatory response is decreased in a mouse model of excessive erythrocytosis

The impact of cerebral erythropoietin (Epo) in the regulation of the hypercapnic ventilatory response (HcVR) is controversial. While we reported that cerebral Epo does not affect the central chemosensitivity in C57Bl6 mice receiving an intracisternal injection of sEpoR (the endogenous antagonist of Epo), a recent study in transgenic mice with constitutive high levels of human Epo in brain and circulation (Tg6) and in brain only (Tg21), showed that Epo blunts the HcVR, maybe by interacting with central and peripheral chemoreceptors. High Epo serum levels in Tg6 mice lead to excessive erythrocytosis (hematocrit ~80-90%), the main symptom of chronic mountain sickness (CMS). These latter results support the hypothesis that reduced central chemosensitivity accounts for the hypoventilation observed in CMS patients. To solve this intriguing divergence, we reevaluate HcVR in Tg6 and Tg21 mouse lines, by assessing the metabolic rate [O consumption (V̇) and CO production (V̇)], a key factor modulating ventilation, the effect of which was not considered in the previous study. Our results showed that the decreased HcVR observed in Tg6 mice (~70% reduction; < 0.01) was due to a significant decrease in the metabolism (~40%; < 0.0001) rather than Epo's effect on CO chemosensitivity. Additional analysis in Tg21 mice did not reveal differences of HcVR or metabolism. We concluded that cerebral Epo does not modulate the central chemosensitivity system, and that a metabolic effect upon CO inhalation is responsible for decreased HcVR observed in Tg6 animals. As CMS patients also show decreased HcVR, our findings might help to better understand respiratory disorders at high altitude.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

Sleep-Disordered Breathing and Vascular Function in Patients With Chronic Mountain Sickness and Healthy High-Altitude Dwellers

BACKGROUND

Chronic mountain sickness (CMS) is often associated with vascular dysfunction, but the underlying mechanism is unknown. Sleep-disordered breathing (SDB) frequently occurs at high altitude. At low altitude, SDB causes vascular dysfunction. Moreover, in SDB, transient elevations of right-sided cardiac pressure may cause right-to-left shunting in the presence of a patent foramen ovale (PFO) and, in turn, further aggravate hypoxemia and pulmonary hypertension. We speculated that SDB and nocturnal hypoxemia are more pronounced in patients with CMS compared with healthy high-altitude dwellers, and are related to vascular dysfunction.

METHODS

We performed overnight sleep recordings, and measured systemic and pulmonary artery pressure in 23 patients with CMS (mean ± SD age, 52.8 ± 9.8 y) and 12 healthy control subjects (47.8 ± 7.8 y) at 3,600 m. In a subgroup of 15 subjects with SDB, we assessed the presence of a PFO with transesophageal echocardiography.

RESULTS

The major new findings were that in patients with CMS, (1) SDB and nocturnal hypoxemia was more severe (P < .01) than in control subjects (apnea-hypopnea index [AHI], 38.9 ± 25.5 vs 14.3 ± 7.8 number of events per hour [nb/h]; arterial oxygen saturation, 80.2% ± 3.6% vs 86.8% ± 1.7%, CMS vs control group), and (2) AHI was directly correlated with systemic blood pressure (r = 0.5216; P = .001) and pulmonary artery pressure (r = 0.4497; P = .024). PFO was associated with more severe SDB (AHI, 48.8 ± 24.7 vs 14.8 ± 7.3 nb/h; P = .013, PFO vs no PFO) and hypoxemia.

CONCLUSIONS

SDB and nocturnal hypoxemia are more severe in patients with CMS than in control subjects and are associated with systemic and pulmonary vascular dysfunction. The presence of a PFO appeared to further aggravate SDB. Closure of the PFO may improve SDB, hypoxemia, and vascular dysfunction in patients with CMS.

TRIAL REGISTRY

ClinicalTrials.gov; No.: NCT01182792; URL: www.clinicaltrials.gov.

A computerized integrated system for the assessment of central and peripheral chemoreflex sensitivity

The assessment of chemoreflex sensitivity (CRS) is of major importance in studies investigating the adaptation of ventilation to the needs of human body. Increased sensitivity of chemoreceptors to both hypoxia and hypercapnia has recently been shown to be a powerful and independent prognosticator in heart failure (HF) patients, thus highlighting the importance of the assessment of CRS also in the clinical setting. In spite of this, the measurement of CRS is currently limited to the research setting. One possible reason might be the lack of suitable commercial equipments. On the basis of these considerations, we designed a system to carry out a comprehensive assessment of CRS, including both central and peripheral chemoreceptors. The system is based on the integration of different commercial devices and is entirely managed by a custom software written in Matlab language. The main features of our system are: (1) the implementation of standard methods (the Read's rebreathing test, the CO2 single breath test and the transient hypoxia test) suitable for both pathological and healthy subjects, (2) data quality assurance and reduction of subjective judgment in the analysis through advanced analysis procedures and statistical outliers rejection, and (3) full interactive control of every step of the recording and analysis procedures. The system is currently used in our Institution in the assessment of CRS in HF patients, chronic obstructive pulmonary disease patients and healthy subjects. It has proven to be very effective and easy to use even by clinical personnel without a specific background in respiratory function assessment.

Sleep-disordered breathing and oxidative stress in preclinical chronic mountain sickness (excessive erythrocytosis)

Chronic mountain sickness (CMS) is considered to be a loss of ventilatory acclimatization to high altitude (>2500m) resulting in marked arterial hypoxemia and polycythemia. This case-control study explores the possibility that sleep-disordered breathing (SDB) and associated oxidative stress contribute to the etiology of CMS. Nocturnal respiratory and [Formula: see text] patterns were measured using standard polysomnography techniques and compared between male high-altitude residents (aged 18-25) with preclinical CMS (excessive erythrocytosis (EE), n=20) and controls (n=19). Measures of oxidative stress and antioxidant status included isoprostanes (8-iso-PGF2alpha), superoxide dismutase and ascorbic acid. EE cases had a greater apnea-hypopnea index, a higher frequency of apneas (central and obstructive) and hypopneas during REM sleep, and lower nocturnal [Formula: see text] compared to controls. 8-iso-PGF2alpha was greater in EE than controls, negatively associated with nocturnal [Formula: see text] , and positively associated with hemoglobin concentration. Mild sleep-disordered breathing and oxidative stress are evident in preclinical CMS, suggesting that the resolution of nocturnal hypoxemia or antioxidant treatment may prevent disease progression.

Chronic mountain sickness and the heart

Chronic Mountain Sickness (CMS) is an important high-altitude (HA) pathology in most mountainous regions of the world. Although its most characteristic sign is excessive erytrocytosis (EE), in the more severe stages of the disease, high-altitude pulmonary hypertension (HAPH), with remodeling of pulmonary arterioles and right ventricular enlargement is commonly found. The degree of ventricular hypertrophy depends on the vasoconstrictor pulmonary response, the intensity of vascular resistance and the level of altitude, and therefore on the degree of hypoxemia. This chapter briefly summarizes the existing data regarding the clinical and pathophysiological features of the cardiopulmonary system in CMS, with emphasis in findings from research in the Andes. The literature shows variability in cardiac output values in CMS, which might be related to the degree of EE. Recent findings have shown that cardiac output (l/min) is lower in CMS when compared with sea-level (SL) dwellers. Mean pulmonary acceleration time (ms) is significantly lower in CMS subjects than in SL and HA natives, and pulmonary vascular resistance index (Wood units) is higher in CMS and HA natives when compared with SL dwellers. Systemic blood pressure has similar values in CMS patients and healthy HA natives, but some differences arise in its control mechanisms. Although CMS individuals have a less effective vasoconstrictor reflex, their tolerance to orthostatic stress is similar to that of healthy HA natives which might be explained in terms of the larger blood volume present in CMS subjects. At present research is directed to design strategies on pharmacological intervention for CMS treatment. Recently, a clinical trial with acetazolamide, in patients with CMS has proven to be effective in increasing mean pulmonary acceleration time and decreasing pulmonary vascular resistance index, which might be indirectly due the reduction of hematocrit.

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.

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).

Differences in the control of breathing between Himalayan and sea-level residents

We compared the control of breathing of 12 male Himalayan highlanders with that of 21 male sea-level Caucasian lowlanders using isoxic hyperoxic ( = 150 mmHg) and hypoxic ( = 50 mmHg) Duffin's rebreathing tests. Highlanders had lower mean +/- s.e.m. ventilatory sensitivities to CO(2) than lowlanders at both isoxic tensions (hyperoxic: 2.3 +/- 0.3 vs. 4.2 +/- 0.3 l min(1) mmHg(1), P = 0.021; hypoxic: 2.8 +/- 0.3 vs. 7.1 +/- 0.6 l min(1) mmHg(1), P < 0.001), and the usual increase in ventilatory sensitivity to CO(2) induced by hypoxia in lowlanders was absent in highlanders (P = 0.361). Furthermore, the ventilatory recruitment threshold (VRT) CO(2) tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 +/- 0.9 vs. 48.9 +/- 0.7 mmHg, P < 0.001; hypoxic: 31.2 +/- 1.1 vs. 44.7 +/- 0.7 mmHg, P < 0.001). Both groups had reduced ventilatory recruitment thresholds with hypoxia (P < 0.001) and there were no differences in the sub-threshold ventilations (non-chemoreflex drives to breathe) between lowlanders and highlanders at both isoxic tensions (P = 0.982), with a trend for higher basal ventilation during hypoxia (P = 0.052). We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO(2). Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO(2)-independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H(+)] at the central chemoreceptor. In some highlanders, the decrease in VRT was accompanied by an increase in sensitivity to CO(2), while in others VRT remained unchanged and their sub-threshold ventilations increased, although these were not statistically significant.

Respiratory control during air-breathing exercise in humans following an 8 h exposure to hypoxia

Hypoxic exposure lasting a few hours results in an elevation of ventilation and a lowering of end-tidal PCO2(PETCO2) that persists on return to breathing air. We sought to determine whether this increment in ventilation is fixed (hypothesis 1), or whether it increases in proportion to the rise in metabolic rate associated with exercise (hypothesis 2). Ten subjects were studied on two separate days. On 1 day, subjects were exposed to 8 h of isocapnic hypoxia (end-tidal PO2 55 Torr) and on the other day to 8 h of euoxia as a control. Before and 30 min after each exposure, subjects undertook an incremental exercise test. The best fit of a model for the variation in PETCO2 with metabolic rate gave a residual squared error that was ∼20-fold less for hypothesis 2 than for hypothesis 1 (p < 0.005, F-ratio test). We conclude that the alterations in respiratory control induced during early ventilatory acclimatization to hypoxia better reflect those associated with hypothesis 2 rather than hypothesis 1.

Population genetic aspects and phenotypic plasticity of ventilatory responses in high altitude natives

Highland natives show unique breathing patterns and ventilatory responses at altitude, both at rest and during exercise. For many ventilatory traits, there is also significant variation between highland native groups, including indigenous populations in the Andes and Himalaya, and more recent altitude arrivals in places like Colorado. This review summarizes the literature in this area with some focus on partitioning putative population genetic differences from differences acquired through lifelong exposure to hypoxia. Current studies suggest that Tibetans have high resting ventilation (V (E)), and a high hypoxic ventilatory response (HVR), similar to altitude acclimatized lowlanders. Andeans, in contrast, show low resting V (E) and a low or "blunted" HVR, with little evidence that these traits are acquired via lifelong exposure. Resting V (E) of non-indigenous altitude natives is not well documented, but lifelong hypoxic exposure almost certainly blunts HVR in these groups through decreased chemosensitivity to hypoxia in a process known as hypoxic desensitization (HD). Together, these studies suggest that the time course of ventilatory response, and in particular the origin or absence of HD, depends on population genetic background i.e., the allele or haplotype frequencies that characterize a particular population. During exercise, altitude natives have lower V (E) compared to acclimatized lowland controls. Altitude natives also have smaller alveolar-arterial partial pressure differences P(AO2) - P(aO2) during exercise suggesting differences in gas exchange efficiency. Small P(AO2) - P(aO2) in highland natives of Colorado underscores the likely importance of developmental adaptation to hypoxia affecting structural/functional aspects of gas exchange with resultant changes in breathing pattern. However, in Andeans, at least, there is also evidence that low exercise V (E) is determined by genetic background affecting ventilatory control independent of gas exchange. Additional studies are needed to elucidate the effects of gene, environment, and gene-environment interaction on these traits, and these effects are likely to differ widely between altitude native populations.

Respiratory control in residents at high altitude: physiology and pathophysiology

Highland population (HA) from the Andes, living above 3000 m, have a blunted ventilatory response to increasing hypoxia, breathe less compared to acclimatized newcomers, but more, compared to sea-level natives at sea level. Subjects with chronic mountain sickness (CMS) breathe like sea-level natives and have excessive erythrocytosis (EE). The respiratory stimulation that arises through the peripheral chemoreflex is modestly less in the CMS group when compared with the HA group at the same P(ET(O2)). With regard to CO(2) sensitivity, CMS subjects seem to have reset their central CO(2) chemoreceptors to operate around the sea-level resting P(ET(CO2)). Acetazolamide, an acidifying drug that increases the chemosensitivity of regions in the brain stem that contain CO(2)/H(+) sensitive neurons, partially reverses this phenomenon, thus, providing CMS subjects with the possibility to have high CO(2) changes, despite small changes in ventilation. However, the same type of adjustments of the breathing pattern established for Andeans has not been found necessarily in Asian humans and/or domestic animals nor in the various high altitude species studied. The differing time frames of exposure to hypoxia among the populations, as well as the reversibility of the different components of the respiratory process at sea level, provide key concepts concerning the importance of time at high altitude in the evolution of an appropriate breathing pattern.

Hypoxic adaptation during development: relation to pattern of neurological presentation and cognitive disability

Children with acute hypoxic-ischaemic events (e.g. stroke) and chronic neurological conditions associated with hypoxia frequently present to paediatric neurologists. Failure to adapt to hypoxia may be a common pathophysiological pathway linking a number of other conditions of childhood with cognitive deficit. There is evidence that congenital cardiac disease, asthma and sleep disordered breathing, for example, are associated with cognitive deficit, but little is known about the mechanism and whether there is any structural change. This review describes what is known about how the brain reacts and adapts to hypoxia, focusing on epilepsy and sickle cell disease (SCD). We prospectively recorded overnight oxyhaemoglobin saturation (SpO2) in 18 children with intractable epilepsy, six of whom were currently or recently in minor status (MS). Children with MS were more likely to have an abnormal sleep study defined as either mean baseline SpO2 <94% or >4 dips of >4% in SpO2/hour (p = .04). In our series of prospectively followed patients with SCD who subsequently developed acute neurological symptoms and signs, mean overnight SpO2 was lower in those with cerebrovascular disease on magnetic resonance angiography (Mann-Whitney, p = .01). Acute, intermittent and chronic hypoxia may have detrimental effects on the brain, the clinical manifestations perhaps depending on rapidity of presentation and prior exposure.

The respiratory response to carbon dioxide in humans with unilateral and bilateral resections of the carotid bodies

The acute hypercapnic ventilatory response (AHCVR) arises from both peripheral and central chemoreflexes. In humans, one technique for identifying the separate contributions of these chemoreflexes to AHCVR has been to associate the rapid component of AHCVR with the peripheral chemoreflex and the slow component with the central chemoreflex. Our first aim was to validate this technique further by determining whether a single slow component was sufficient to describe AHCVR in patients with bilateral carotid body resections (BR) for glomus cell tumours. Our second aim was to determine whether the slow component of AHCVR was diminished following carotid body resection as has been suggested by studies in experimental animals. Seven BR subjects were studied together with seven subjects with unilateral resections (UR) and seven healthy controls. A multifrequency binary sequence in end-tidal PCO2 was employed to stimulate ventilation dynamically under conditions of both euoxia and mild hypoxia. Both two- and one-compartment models of AHCVR were fitted to the data. For BR subjects, the two-compartment model fitted significantly better on 1 out of 13 occasions compared with 22 out of 28 occasions for the other subjects. Average values for the chemoreflex sensitivity of the slow component of AHCVR differed significantly (P < 0.05) between the groups and were 0.95, 1.38 and 1.50 l min-1 Torr-1 for BR, UR and control subjects, respectively. We conclude that, without the peripheral chemoreflex, AHCVR is adequately described by a single slow component and that BR subjects have sensitivities for the slow component that are lower than those of control subjects.