1. HIGH ALTITUDE PHYSIOLOGY

2. CATEGORISATION FOR DESCRIPTIVE CONVENIENCE:
ALTITUDE TYPE
FROM SEA-LEVEL (In feet)
HIGH
8,000 – 12,000
VERY HIGH
12,000 – 18,000
EXTREMELY HIGH
Above 18,000

3. SIGNIFICANT ATMOSPHERIC PRESSURE VARIATION WITH ALTITUDE:
(FEET) ( mm of Hg) (ATMOSPHERIC UNIT)
18, /2
34, /4
48, /8
63, /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:
ALTITUDE LEVEL
INSPIRED AIR PO2
Hb-SATURATION
EFFECTS
In feet (metre)
In mm of Hg
in %
Stages (if any)
0 (i.e.sea-level)
160
~ 97 %
NIL
Upto 10,000
(3,000)
110
~ 90 %
Usually none, +/- some nocturnal visual reduction ( of indifference)
10,000 – 15,000
(3,000 – 4,500) 98
~ 80 %
Mod. Hypoxic symptoms cardiorespiratory manifestaions & early CNS involvements ( of reaction)
15,000 – 20,000
(4,500 – 6,000) 70
< 70 %
Severe hypoxic symp  aggravated CNS involvement (of disturbance)
Above 20,000 &
onwards
Further falls
below 60 %
Unconsciousness & alarming deterioration survival impossible without supplemental O (critical survival altitude)

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:
Adaptation
Gen. term-any characteristic favouring survival in specific environment
Acclimation
All fav. Responses to only single environ. Variable- alteration
Acclimatization
All fav. Physiological changes for adjusting multiple environ.variable alteration

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: D) Neurological :
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:
Air
Lungs
Blood
Tissue
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:
Hypoxia induced erythropoiesis
↑Hb Conc. & RBC count (within a few wks. stay)
Expansion of blood volume
Haemodynamics is kept within normal limit inspite of ↑ vascularity of tissues
↑Amount of circulating Hb
Inspite of ↓saturation ,O2-carrying capacity is maintained at normal limit

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.