Chemoreceptor sensitivity and maladaptation to high altitude in man

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.

Prior oxygenation, but not chemoreflex responsiveness, determines breath-hold duration during voluntary apnea

Central and peripheral respiratory chemoreceptors are stimulated during voluntary breath holding due to chemostimuli (i.e., hypoxia and hypercapnia) accumulating at the metabolic rate. We hypothesized that voluntary breath-hold duration (BHD) would be (a) positively related to the initial pressure of inspired oxygen prior to breath holding, and (b) negatively correlated with respiratory chemoreflex responsiveness. In 16 healthy participants, voluntary breath holds were performed under three conditions: hyperoxia (following five normal tidal breaths of 100% O2 ), normoxia (breathing room air), and hypoxia (following ~30-min of 13.5%-14% inspired O2 ). In addition, the hypoxic ventilatory response (HVR) was tested and steady-state chemoreflex drive (SS-CD) was calculated in room air and during steady-state hypoxia. We found that (a) voluntary BHD was positively related to initial oxygen status in a dose-dependent fashion, (b) the HVR was not correlated with BHD in any oxygen condition, and (c) SS-CD magnitude was not correlated with BHD in normoxia or hypoxia. Although chemoreceptors are likely stimulated during breath holding, they appear to contribute less to BHD compared to other factors such as volitional drive or lung volume.

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

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

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

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

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

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

Normo or hypobaric hypoxic tests: propositions for the determination of the individual susceptibility to altitude illnesses

Assessment of individual susceptibility to altitude illnesses and more particularly to acute mountain sickness (AMS) by means of tests performed in normobaric hypoxia (NH) or in hypobaric hypoxia (HH) is still debated. Eighteen subjects were submitted to HH and NH tests (PIO2=120 hPa, 30 min) before an expedition. Maximal and mean acute mountain sickness scores (AMSmax and mean) were determined using the self-report Lake Louise questionnaire scored daily. Cardio-ventilatory (f, V(T), PetO2 and PetCO2, HR and finger pulse oxymetry SpO2) were measured at times 5 and 30 min of the tests. Arterial (PaO2, PaCO2, pH, SaO2) and capillary haemoglobin (Hb) measurements were performed at times 30 min. Hypoxic ventilatory (HVR) and cardiac (HCR) responses, peripheral O2 blood content (CpO2) were calculated. A significant time effect is found for DeltaSpO2 (P = 0.04). Lower PaCO2 (P = 0.005), SaO2 (P = 0.07) and higher pH (P = 0.02) are observed in HH compared to NH. AMSmax varied from 3 to12 and AMSmean between 0.6 and 3.5. In NH at 30 min, AMSmax is related to PetO2 (R = 0.61, P = 0.03), CpO2 (R = -0.53, P = 0.02) and in HH to CpO2 (R = -0.57, P = 0.01). In NH, AMSmean is related to Deltaf (R = 0.46, P = 0.05), HCR (R = 0.49, P = 0.04), CpO2 (R = -0.51, P = 0.03) and, in HH at 30 min, to V(T) (R = 0.69, P = 0.01) and a tendency for CpO2 (R = -0.43, P = 0.07). We conclude that HH and NH tests are physiologically different and they must last 30 min. CpO2 is an important variable to predict AMS. For practical considerations, NH test is proposed to quantify AMS individual susceptibility using the formulas: AMSmax = 9.47 + 0.104PetO2(hPa)-0.68CpO2 (%), (R = 0.77, P = 0.001); and AMSmean = 3.91 + 0.059Deltaf + 0.438HCR-0.135CpO2 (R = 0.71, P = 0.017).

Hypoxic ventilatory response, ventilation, gas exchange, and fluid balance in acute mountain sickness

To examine whether sea-level hypoxic ventilatory responses (HVR) predict acute mountain sickness (AMS) and document temporal changes in ventilation, HVR, gas exchange, and fluid balance, we measured these parameters at low altitude (100 m) and daily during 3 days at high altitude (4559 m). At low altitude, there were no significant differences in rest or exercise isocapnic HVR, poikilocapnic HVR at rest, and hypercapnic ventilatory response between 12 subjects without significant AMS and 11 subjects who fell sick. No low altitude ventilatory responses correlated with AMS or fluid balance at high altitude. On day 1, isocapnic HVR was significantly lower in the AMS group [0.86 +/- 0.43 (SD) vs. 1.43 +/- 0.63 L/min/% Sa(O2), p < 0.05). AMS was associated with higher AaD(O2), lower Pa(O2), and Sa(O2), while Pa(CO2) was not different between subjects with and without AMS. Both groups showed equivalent reductions in urine volume, sodium output, and gain in body weight on day 1 while climbing to 4559 m, but on day 2 only subjects without AMS had diuresis, natriuresis, and weight loss. We conclude that (1) susceptibility to AMS, fluid balance, and ventilation at high altitude cannot be predicted by low altitude HVR testing and (2) that the failure to increase HVR on arrival at high altitude and impaired gas exchange, possibly due to interstitial edema, may account for the more severe hypoxemia in AMS.

Basilar artery blood flow velocity and the ventilatory response to acute hypoxia in mountaineers

Hypoxic ventilatory response is higher in successful extreme-altitude climbers than in controls. We hypothesized that these climbers have lower brainstem blood flow secondary to hypoxia which may possibly cause retention of medullary CO(2) and greater ventilatory drive. Using transcranial Doppler, basilar artery blood flow velocity (Vba) was measured at sea level in 7 extreme-altitude climbers and 10 controls in response to 10 min sequential exposures to inspired oxygen fractions (FI(O(2))) of 0.21 (baseline), 0.13, 0.11, 0.10, 0.09, 0.08 and 0.07. Sa(O(2)) was higher in climbers at FI(O(2)) of 0.11 (P<0.05), 0.08 and 0.07 (both P<0.0001). Expired ventilation (VE) increased more (n.s.), and PET(CO(2)) decreased more (n.s.) in the climbers than in controls. Vba did not significantly change in both groups at FI(O(2)) of 0.13-0.09. At FI(O(2)) of 0.08 and 0.07, Vba decreased 21% (P<0.03) and 27% (P<0.01), respectively, in climbers, and increased 29% (P<0.01) and 27% (P<0.01), respectively, in controls. The conflicting effects of hypoxia and hypocapnia on both medullary blood flow and ventilatory drive thus balance out, giving climbers a greater drive and higher Sa(O(2)), despite lower PET(CO(2)) and lower brain stem blood flow.

Assessment of high altitude tolerance in healthy individuals

The most reliable prediction of high altitude tolerance can be derived from the clinical history of previous comparable exposures. Unfortunately, there are no reliable tests for prediction prior to first-time ascents. Although susceptibility to AMS is usually associated with a low hypoxic ventilatory response (HVR), there is too much overlap with the range of normal values, which precludes measuring HVR or O(2) saturation during brief hypoxia for reliable identification of susceptibility to AMS. A low HVR and an exaggerated rise in pulmonary artery pressure with (prolonged) hypoxia, or exercise in normoxia, are markers of susceptibility to high altitude pulmonary edema (HAPE). These tests can not be recommended for routinely determining high altitude tolerance because the prevalence of susceptibility to HAPE is low and because specificity and sensitivity of these tests are not sufficiently established. On the other hand, HAPE may be avoided in susceptible individuals by ascent rates of 300 m per day above an altitude of 2000 m. Since prediction of risk of mountain sickness is difficult, it is important during the physician consultation prior to ascent to consider the altitude profile, the type of ascent, the performance capacity, the history of previous exposures, and the medical infrastructure of the area.

High altitude pulmonary edema

High altitude pulmonary edema. Med. Sci. Sports Exerc., Vol. 31, No. 1 (Suppl.), pp. S23-S27, 1999. Altitude, speed and mode of ascent, and, above all, individual susceptibility are the most important determinants for the occurrence of high altitude pulmonary edema (HAPE). This illness usually occurs only 2-5 d after acute exposure to altitudes above 2500-3000 m. Chest radiographs and CT scans show a patchy predominantly peripheral distribution of edema. Wedge pressure is normal at rest, and there is an excessive rise of pulmonary artery pressure (PAP) that precedes edema formation and appears to be a crucial pathophysiologic factor for HAPE. Additional factors such as an inflammatory response and/or a decreased fluid clearance from the lung may, however, be necessary for the development of this noncardiogenic pulmonary edema. Bronchoalveolar lavage in patients with mostly advanced HAPE shows evidence of inflammatory response with increased permeability. There are, however, no prospective data to decide whether the inflammatory response is a primary cause of HAPE or a consequence of edema formation. Supplemental oxygen is the primary treatment in areas with medical facilities whereas the treatment of choice in remote mountain areas is immediate descent. When this is impossible and supplemental oxygen is not available, treatment with nifedipine is recommended until descent is possible. Even susceptible individuals can avoid HAPE when they ascend slowly with an average gain of altitude not exceeding 300-350 m.d-1 above an altitude of 2500 m.

Catecholamines, hypoxia and high altitude

Hypoxia is a potent activator of the sympathetic nervous system by stimulating arterial chemoreceptors. However, out of 15 laboratory studies on the effects of acute and prolonged hypoxia on catecholamines, 14 failed to show any changes in plasma or urinary noradrenaline and only four studies showed significant increases in plasma or urinary adrenaline. By contrast, six out of eight studies on MSNA showed increased sympathetic nerve activity to the leg. An increased clearance of plasma catecholamines during hypoxia may be a possible explanation. Furthermore, many of the studies had limitations in a number of subjects and catecholamine assays used. Emotional aspects of the study protocols, which could contribute to the increase in adrenaline, was only assessed by sham runs in one chamber study. However, 13 out of 14 reviewed field studies on subjects staying for more than 1 week at high altitude, reported increased plasma or urinary excretion of noradrenaline which may be compatible with increased sympathetic activity. Adrenaline changed to a lesser degree. Out of seven studies on more short-term (4 h to 3 days) exposure to high altitude, only one demonstrated significantly increased plasma noradrenaline. In this study, however, several subjects had been exposed to high altitude less than 1 week before the experiment. In a new study on 12 climbers reported in this paper, a temporary reduction in plasma catecholamines was found 2 days after arrival at 4200 m. There was a steady increase towards normal levels after 1 week. Plasma vasopressin (AVP) increased suggesting a compensatory mechanism. Both plasma noradrenaline and adrenaline were positively correlated with oxygen saturation in these subjects. Thus, in previously unacclimatized subjects, short-term exposure to high altitude does not increase plasma catecholamines, rather plasma levels decreased. In addition to increased clearance, there is some evidence of reduced synthesis of catecholamines during short-term hypoxia. The oxygen sensitivity of tyrosine hydroxylase (TH) activity, may be one possible mechanism.

Acute mountain sickness relates to sea-level partial pressure of oxygen

The aim of this study was to clarify the relationships between acute mountain sickness (AMS), studied during an expedition in the Andes, and some physiological parameters determined before the expedition, i.e. biometrical characteristics of the subjects [maximal oxygen consumption (VO2max), body fat content, body mass index], functional pulmonary tests (forced vital capacity, forced expiratory volume at the first second), ventilatory or cardiac responses measured at 4,500 m [hypoxic ventilatory responses (HVR) 4,500 and hypoxic cardiac responses (HCR) 4,500, respectively), cold pressor responses. To achieve this objective, 11 subjects were firstly submitted to a hypobaric poïkilocapnic hypoxic test (589 hPa, 4,500 m) at rest and during exercise to study minute volume, respiratory frequency, end tidal partial pressure of O2 (PETO2) and CO2, HVR 4,500, HCR 4,500 and to a cold pressor test of the hand (5 min in 5 degrees C cold water) to study heart rate, blood pressure and skin temperature changes. The AMS was assessed daily by questionnaire during a 12-day expedition in the Andes following both Hackett's method and Environmental Symptoms Questionnaire (modified ESQ II). Maximal AMS-Hackett score, maximal AMS-ESQ score and mean AMS-ESQ score were defined. The quantifications of AMS following the two methods were correlated. No significant relationships were observed between mean AMS-ESQ score and the biometrical characteristics of the subjects, the functional pulmonary tests, HVR 4,500, HCR 4,500 or the cold pressor responses. However, it appeared that the mean AMS-ESQ score was correlated with PETO2 measured at rest and during exercise (50% VO2max) both in hypoxia and normoxia. A closer linear relationship was observed during the exercise in normoxia (r = -0.92, P < 0.0001). These results could suggest that AMS was related to a relative alveolar hypoventilation more in relation to breathing pattern than HVR.

Pulmonary function and hypoxic ventilatory response in subjects susceptible to high-altitude pulmonary edema

To determine if spirometric changes reflect early high-altitude pulmonary edema (HAPE) formation, we measured the FVC, FEV1, and FEF25-75 serially during the short-term period following simulated altitude exposure (4,400 m) in eight male subjects, four with a history of HAPE and four control subjects who had never experienced HAPE. Three of the four HAPE-susceptible subjects developed acute mountain sickness (AMS), based on their positive Environmental Symptom Questionnaire (AMS-C) scores. Clinical signs and symptoms of mild pulmonary edema developed in two of the three subjects with AMS after 4 h of exposure, which prompted their removal from the chamber. Their spirometry showed small decreases in FVC and greater decreases in FEV1 and FEF25-75 after arrival at high altitude in the presence of rales or wheezing on clinical examination and normal chest radiographs. One of the two subjects had desaturation (59 percent) and tachycardia during mild exercise, and excessive fatigue and inability to complete the exercise protocol developed in the other at 4 h. The six other subjects had minimal changes in spirometry and did not develop signs of lung edema. Further, we measured each subject's ventilatory response to hypoxia (HVR) prior to decompression to determine whether the HVR would predict the development of altitude illness in susceptible subjects. In contrast to anticipated results, high ventilatory responses to acute hypoxia, supported by increased ventilation during exposure to high altitude, occurred in the two subjects in whom symptoms of HAPE developed. The results confirm that HAPE can occur in susceptible individuals despite the presence of a normal or high ventilatory response to hypoxia.

Medical therapy of altitude illness

Acute mountain sickness (AMS) and high-altitude pulmonary edema (HAPE) continue to cause significant morbidity and occasional deaths among mountain recreationists and residents. Descent to lower altitude is still considered the treatment of choice, but an increased role for medical therapy is emerging. Acetazolamide is currently the drug of choice for prevention of AMS, and probably HAPE as well. Numerous studies have demonstrated the drug's effectiveness when it is started 12 to 24 hours before ascent. Suggestions for indications, dosage, and regimen vary with different authors. Lower dosage offers adequate protection with fewer side effects. Acetazolamide has still not been adequately studied for treatment of altitude illness. Oxygen effectively treats HAPE and mild AMS, but is not as useful for cerebral edema. Dexamethasone recently was found effective for treatment of AMS, including early cerebral edema, but not for advanced cerebral edema. Side effects limit its use for prophylaxis, but dexamethasone offers an alternative to acetazolamide for those with sulfa intolerance.