Physiology of Physiology of High AltitudeHigh Altitude

by : by : Jagjit KhoslaJagjit Khosla

High Altitude• A height above 10,000 feet (3000 m) above the sea level is

defined as High Altitude• According to Dalton’s law, total pressure of air is equal to

sum of partial pressures of gases it contains.P = pO2 + pCO2 + pN2 + pH2O

• pH2O and pCO2 doesn’t depend upon altitude.

• pO2 and pN2 decrease with increase in height.

• At High Altitude, low partial pressure of O2 leads to Hypoxia.

Effects of High Altitude on O2-Hb Saturation

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Oxygen -Hb Saturation Curve

• Above 10,000 feet, the arterial oxygen saturation falls rapidly.

• It is due to increased 2,3- DPG concentration in RBCs

• It is slightly less than 70% at 20,000 feet and much less at higher altitudes.

Effects of High Altitude on Human body

Level ofAltitude (ft)

Common effects

5,000 No effects10,000 No hypoxia upto pO2 60 mmHg

Rapid ascent upto 10,000 ft is Safe zoneof ascent.

15,000 Moderate hypoxia with CVS andrespiratory symptoms

18,000 Severe hypoxia with involvement of CNS

20,000 Hypoxia aggravates. Unconsciousnesswhen Hb saturation falls below 60%Critical Survival Altitude

30,000 Severe hypoxia even with oxygentherapy

Effects of High Altitude

• Effect of Hypoxia

• Effect of expansion of gases

• Effect of fall in atmospheric

temperature

• Effect of light rays

Effect of Hypoxia• On Blood -

> Increased RBC count> Due to increased secretion of erythropoietin from JGA of kidney

• On CVS -> Increased Heart rate, Force of contraction of heart, Cardiac output, Blood pressure> Due to Reflex stimulation of Cardiac and vasomotor centers

• On Respiration - > Increased pulmonary ventilation> Due to chemoreceptor reflex following increased pCO2

Effect of Hypoxia• On Digestive system -

> Loss of appetite, nausea, vomiting• On Kidney -

> Alkaline urine due to increased erythropoietin secretion• On CNS -

> Depression, Apathy with general loss of self control> Talkative, quarrelsome, ill-tempered, rude> Disorientation, Loss of power of judgement> Memory impaired, fatigue, lack of coordination> In acute/ severe hypoxia, sudden loss of consciousness

Effect of Expansion of gasesAccording to Boyle’s law, as the atmospheric pressure decreases with altitude, volume of gases increases proportionally.

• In GIT -> Painful distention of stomach and intestine

• In Lungs-> Expansion of gases may destroy alveoli

• Rapid ascent may cause-> Decompression sickness

Effect of Fall in Atmospheric Temperature

The atmospheric temperature falls by 2o C for every 1000 ft increase in altitude above sea level.

• Effects -> Widespread cutaneous vasoconstriction.

> Prolonged and severe vasoconstriction may lead to

tissue damage known as Frost-bite.

Effect of Light raysUltraviolet (UV) rays at high altitude also cause many hazardous effects such as skin irritation.

Acute Mountain SicknessIt refers to the symptom complex which occurs in an individual residing at sea level, when he ascends to high altitude over a period of 1-2 days for the first time. The symptoms develop 8 to 24 hours after arrival at high altitude and last for 4 to 8 days.

Acute Mountain Sickness• Symptoms -

> Digestive system - Loss of appetite, nausea, vomiting> Respiratory system - Breathlessness, High Altitude Pulmonary Edema (HAPE)> Nervous System - Irritability, Insomnia, headache, depression, High Altitude Cerebral Edema (HACE)

• Treatment -> Decreasing Cerebral edema - administration of large

doses of Glucocorticoids>Decreasing alkalosis -administration of Acetazolamide. Acetazolamide decreases H+ excretion through kidneys

by inhibiting the enzyme carbonic anhydrase.

Chronic Mountain Sickness• Also called Monge’s disease• It occurs in some long-term residents of high altitude• Effects -

> Extreme polycythemia> Pulmonary arterial pressure becomes elevated> Right side of heart becomes greatly enlarged> Peripheral arterial pressure begins to fall > Congestive heart failure> Death follows if left untreated

• These individuals must be removed to a lower altitude to prevent rapid development of fatal pulmonary edema.

High Altitude Pulmonary Edema• Occurs due to -

> Rapid Ascent> Doing heavy physical work during first 3-4 days after rapid ascent to high altitude.

• Mechanism -> Increased sympathetic activity> produces vasoconstriction which leads to increase in Pulmonary capillary hydrostatic pressure. > drives the fluid out of the pulmonary capillaries producing pulmonary edema.

• Treatment -> O2 therapy> Administration of Ca2+ channels blocker e.g. Nifedipine

High Altitude Cerebral Edema• Mechanism -

> Low pO2 cause arteriolar dilation which is normally compensated by cerebral autoregulation. > Once the limit of cerebral circulation autoregulatory mechanism is reached, there occurs an increase in capillary pressure.> It favours increased transudation of fluid into brain tissue.

• Treatment -> Administeration of large doses of Glucocorticoids

Acclimatization• Definition -

It refers to changes in body tissues in response to long-term exposure to high altitude hypoxia, such as when a person living at sea level goes and stays at high altitude for a long time.

• Changes in the body -> Blood- Increased haematocrit to 60%- Increased Hb conc. to about 20g%- due to increased erythropoietin secretion

Acclimatization> Respiration- Increased Pulmonary ventilation and blood flow- Increased diffusing capacity of lungs- Due to stimulation of chemoreceptors> CVS- Increased Heart rate, cardiac output and BP- Increased blood flow to vital organs like heart, brain, muscles, etc.> Tissues- Increased quantities of oxidative enzymes involved in metabolism present in cells- Increased number of mitochondria in cells- Increased myoglobin content

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