Aviation, High-Altitude and Space Physiology

1. Aviation, High-Altitude and Space Physiology

2. High altitude = stressful environment for the lungs • At extreme altitudes : lung = primary and essential organ for human function and survival

3. CO2 and Water Vapor Decrease Alveolar O2 • Water vapor pressure in the alveoli remains 47 mmHg as long as the body temperature is normal • During exposure to very high altitudes, the alveolar PCO2 falls from the sea-level value of 40 mmHg to lower values • In the acclimatized person, who increases his/her ventilation about five fold, the PCO2 falls to about 7mmHg because of increased respiration

4. Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas concentrations and Arterial Oxygen Saturation *Numbers in parentheses are acclimatized values.

5. Saturation of Hemoglobin with O2 at Different Altitudes

6. Acute Effects of Hypoxia • Drowsiness, lassitude, mental and muscle fatigue • Headache, occasionally nausea and sometimes euphoria • These effects progress to a stage of twitchings and seizures above 18,000 ft • One of the most important effects of hypoxia is decreased mental proficiency

7. ACCLIMATIZATION • Process by which people gradually adjust to high altitude • Determines survival and performance at high altitude • Series of physiological changes • O2 delivery • hypoxic tolerance +++ • Acclimatization depends on • severity of the high-altitude hypoxic stress • rate of onset of the hypoxia • individual’s physiological response to hypoxia

8. Acclimatization to Low PO2 • The principal means by which acclimatization comes about: • 1) A great increase in pulmonary ventilation • Role of arterial chemoreceptors • 2) Increased numbers of red blood cells • 3) Increased diffusing capacity of the lungs • Perfusion of upper parts of the lungs • 4) Increased vascularity of the peripheral tissues • 5) Increased ability of the tissue cells to use O2 despite low PO2

9. Natural Acclimatization • Natural acclimatization of native human beings living at high altitudes • Reduced work capacity at high altitudes and positive effect of acclimatization

10. HOW HIGH IS HIGH-ALTITUDE ? • High altitude: 1500-3000m above sea level • Very high altitude: 3000-5000m • Extreme altitude: above 5000m Tibetan plateau & Himalayan valleys (8848m) Andes (6962m) • For sea level visitors, 4600-4900m = highest acceptable level for permanent habitation • For high altitude residents, 5800-6000m = highest so far recorded Ethiopian highlands (4620m)

11. Decreased PCO2 VENTILATORY ACCLIMATIZATION • Hypoxic ventilatory response =  VE • Starts within the 1st few hours of exposure  1500m • Mechanism Ascent to altitude Hypoxia Carotid body stimulation Respiratory centres stimulation Increased ventilation Improved hypoxia CO2 + H2O H2CO3HCO3- + H+

12. LUNG DIFFUSION • Definition Process by which O2 moves from the alveolar gas into the pulmonary capillary blood, and CO2 moves in the reverse direction • High altitude   O2 diffusion, because of • a lower driving pressure for O2 from the air to the blood • a lower affinity of Hb for O2 on the steep portion of the O2/Hb curve •  and inadequate time for equilibration

13. VA/Q HETEROGENEITY • At rest • At high altitude • interstitial edema  heterogeneity +++ O2 - Inhaled air is not evenly distributed to alveoli - Composition of gases is not uniform throughout lungs - Different areas of the lungs have different perfusion - Differences are less in recumbent position

14. VA/Q HETEROGENEITY • Varies from zero to infinity • Zero : perfusion but no ventilation • O2 and CO2 tensions in arterial blood, equal those of mixed venous blood because there is no gas exchange in the capillaries • Infinity:ventilation but no perfusion • no modification of inspired air takes place due to over-ventilation or under-perfusion

15. WHEN ACCLIMATIZATION FAILS • Altitude syndromes • Acute mountain sickness (AMS): the least-threatening and most common • High altitude pulmonary edema • High altitude cerebral edema • All these syndromes have • several features in common • respond to descent or oxygen • potentially lethal form of AMS

16. ACUTE MOUNTAIN SICKNESS • Major symptoms • Headache • Fatigue • Dizziness • Anorexia • Dyspnea • Incidence and severity depend on • Rate of ascent • Altitude attained • Length of time at altitude • Degree of physical exertion • Individual’s physiological susceptibility • Treatment hardly needed

17. HIGH ALTITUDE PULMONARY EDEMA (HAPE) • Noticed only after 24-48hr and occurs after the 2d night • Occurs in otherwise healthy people without known cardiac or pulmonary disease • Occurs when people go rapidly to high altitude • Extravasation of fluid from the intra- to extravascular space in the lung

18. WHY DOES HAPE OCCUR ? • Hypothesis 1. Pulmonary hypertension • Hypothesis 2. Pulmonary endothelium barrier fragility • Hypothesis 3. Perturbation of alveolar fluid clearance

19. TREATMENT OF HAPE • Get the patient down in altitude as fast and as low as possible • Give O2 or hyperbaria • Give antibiotics • Apply expiratory positive airways pressure • With a respiratory valve device • Or by pursed lips breathing