HIGH ALTITUDE PHYSIOLOGY

1. HIGH ALTITUDE PHYSIOLOGY

2. CATEGORISATION FOR DESCRIPTIVE CONVENIENCE:

3. SIGNIFICANT ATMOSPHERIC PRESSURE VARIATION WITH ALTITUDE: PRESSURE ALTITUDE (FEET) ( mm of Hg) (ATMOSPHERIC UNIT) 0 760 1 18,000 380 1/2 34,000 190 1/4 48,000 95 1/8 63,000 47 1/16

4. BASIC CONCEPT: Human body is specifically designed in such a way that it delivers adequate O2 to the tissues only when oxygen is supplied at a pressure close to the sea-level (P = 760 mm Hg  PO2 =159 mm Hg) So, at high altitude there is hypoxic hypoxia  tissue oxygenation suffers physiological derangements. “connecting a 24 volt motor to a 6 volt battery”—perfect comparison by J.S.Milledge.

5. COMMON HYPOXIC EFFECTS WITH DIFFERENT ALTITUDES:

6. ALTITUDES & HEIGHTS • Normally most people on earth stay at around the Mean Sea Level (MSL) • However, groups of people stay at higher than normal attitudes. • The Sherpas, a mountain tribe of Himalayas live at around 5500 Meters above MSL, normally. • When people living around MSL go up to higher altitudes, changes occur in their physiology, especially in Respiration.

7. BAROMETRIC PRESSURE CHANGES IN HIGH ALTITUDE • As the altitude increases above the sea level, the corresponding atmospheric pressure decreases. • The partial pressure of Oxygen also decreases. • The arterial Oxygen saturation levels also decrease with increase in the altitude.

8. PARTIAL PRESSURES OF OXYGEN

10. ACUTE EFFECTS OF HYPOXIA • As the altitude increases, the barometric pressure decreases. • This causes a handicap which may be: • Appreciable • Considerable • Serious • Causing Imminent Collapse

11. ACCLIMATIZATION • “Getting used to…” • People remaining at high altitudes for days, weeks or years become more and more acclimatized to low PO2. • This causes the hypoxia to cause fewer deleterious effects on their bodies. • They can thus work harder at higher altitudes without hypoxic effects.

12. How does acclimatization occur? • Increased : • Pulmonary ventilation. • Number of RBCs ( Hypoxia causes excess erythropoiesis). • Diffusing capacity of the lungs. • Vascularity of the peripheral tissues • Ability of the tissue cells to use Oxygen despite low PO2.

13. NATURAL ACCLIMATIZATION • This occurs in people living from their birth at high altitudes. • Those living in the Andes & Himalayan mountains, for instance. • Acclimatization begins in them in infancy. • The chest size is greatly increased. • Their hearts are considerably larger than those of lowlanders

14. ACUTE MOUNTAIN SICKNESS • This is also called as “High Altitude Pulmonary Edema.” • This occurs in a small number of lowlanders who ascend rapidly to high altitudes. • Begins from a few hours up to 2 days after their ascent. • Is serious and results in their death unless they are given Oxygen or taken to a low altitude.

15. ACUTE MOUNTAIN SICKNESS : SYMPTOMS & SIGNS • Acute Cerebral Edema: • Hypoxia causes cerebral vasodilatation • Increases capillary pressure • Causes fluid to leak out into the tissues • This leads to cerebral edema causing: • Severe disorientation • Other cerebral dysfunctions

16. ACUTE MOUNTAIN SICKNESS : SYMPTOMS & SIGNS • Acute Pulmonary Edema: • Severe hypoxia causes Pulmonary arteriolar constriction. • In some areas it is more and causes edema. • This can extend progressively to other areas of the lungs. • Can be reversed within hours on Oxygen therapy

17. CHRONIC MOUNTAIN SICKNESS • Seen in people who reside for long at high altitudes. • Red cell number and mass increases exceptionally. • Pulmonary arterial pressure becomes very high. • The heart becomes enlarged in the right side. • The peripheral arterial pressure begins to fall • Congestive Cardiac failure & death follows • They need to be taken to low altitudes as soon as possible. • They recover in low altitudes within days or weeks.

18. PHYSIOLOGICAL RESPONSES TO HIGH ALTITUDE HYPOXIA: • Arbitrarily Divided into following two--- • Acute responses (aka accommodation) • Long term responses ( aka acclimatization) • “Arbitrary” because ---- • Acute are also beneficial for long-term coping up. ii) Acute are modified steadily & imperceptibly in such a way that after 2-3 days are considered as beginninng of acclimatization . iii) Sharpness of division depends on rate of ascent .

19. IMP. CONCEPTS IN ENVIRONMENTAL PHYSIOLOGY:

20. ACCOMMODATION AT HIGH ALTITUDE: • immediate reflex responses of the body to acute hypoxic exposure. • Hyperventilation: arterial PO2  stimulation of peripheral chemoreceptors  increased rate & depth of breathing B) Tachycardia: Also d/t peripheral chemo. Response  CO  oxygen delivery to the tissues

21. Contd….. • Increased 2,3-DPG conc. in RBC: within hours, ↑deoxy-Hb conc.  locally ↑pH  ↑2,3-DPG  ↓oxygen affinity of Hb  tissue O2 tension maintained at higher than normal level D) Neurological : • Considered as “warning signs” • Depression of CNS  feels lazy, sleepy ,headache • ‘Release Phenomena’ like effect of alcohol • At further height  cognitive impairment, twitching, convulsion & finally unconsciousness

22. ACCLIMATIZATION AT HIGH ALTITUDE: • Delivery of atmospheric O2 to the tissues normally involve 3 stages---with a drop in PO2 at each stage. • When the starting PO2 is lower than normal, body undergoes acclimatization so as to— • ↓ pressure drop during transfer • ↑ oxygen carrying capacity of blood • ↑ ability of tissues to utilize O2 • With longer stay at high altitude ,body is able to adjust by certain physiological adaptations..

23. A)Sustained Hyperventilation: • Prolonged hyperventilation  CO2 wash-out  respiratory alkalosis renal compensation alkaline urine normalization of pH of blood & CSF withdrawal of central chemo-mediated respiratory depression  net result is ↑resting pulmonary ventilation (by ~5 folds to 60L/min),primarily d/t ↑ in TV (upto 50% of VC) • Such powerful ventilatory drive is also possible as- • ↑sensitivity of chemo- mechs to PO2 & PCO2 • Somewhat ↓ in work of breathing  make easy & less tiring

24. B) Other Respiratory Changes: • ↑ TLC :esp in high-landers(natives for generations) evidenced by relatively enlarged (barrel-shaped) chest l/t ↑ventilatory capacity in relation to body mass. • ↑ Diffusing capacity of lungs: d/t hypoxic pulmonary vasoconstriction  Pul. Hypertension  ↑ no. of pulmonary capillaries →existence of this effect is still debatable!!!

25. C)↑Vascularity of the Tissues: More capillaries open up in tissues than at sea-level (normal ~25 % at rest—remaining as ‘reserve’). This combined with systemic vasodilatation(also a hypoxic response) more O2 delivery to tissues. D) Cellular level changes: ↑ intracellular mitochondrial density ↑ conc. of cellular oxidative enzymes ↑ synthesis of Mb( O2-storing pigment) →all aimed to improve O2 utilization.

26. E) Physiological Polycythemia:

27. Cheyne-Stokes Respirations: • Above 10,000 ft (3,000 m) most people experience a periodic breathing during sleep. The pattern begins with a few shallow breathsincreases to deep sighing respirations falls off rapidly. • Respirations may cease entirely for a few secs& then shallow breaths begin again. During period of breathing-arrest, person often becomes restless & may wake with a sudden feeling of suffocation. • Can disturb sleeping patterns exhausting the climber. Acetazolamide is helpful in relieving this. Not considered abnormal at high altitudes. But if occurs first during an illness (other than Altitude illnesses) or after an injury (particularly a head injury)  may be a sign of a serious disorder.