ALTITUDE PHYSIOLOGY
- High altitude has “Low atmospheric pressure”.
- Air composition unchanged with altitude.
– PO2 forms about 21% of atmospheric pressure at any altitude.
– Hence, PO2 is low at high altitudes.
– PO2 of inspired air (PiO2) is low.
- Adequate oxygen delivery to tissues happens only at pressure close to sea level pressure.
- If PO2 falls much below sea level,
– PO2 tissue oxygenation suffers (hypoxic hypoxia)
- Consequently, physiological functions are deranged.
EFFECTS OF HIGH ALTITUDE:
- Earliest response to fall in PO2 –
– Hyperventilation – At altitude 3000 meters above sea level.
- Hypoxic symptoms – At altitude of 4000 meters.
– Becomes severe – at 5000 meters.
- Loss of consciousness – At 6000 meters.
TYPES OF RESPONSE:
- Acute response/Acute Transient Response/Acute Mountain Sickness.
- Acclimatization.
- Chronic Mountain Sickness.
- Earliest response – Hyperventilation.
– Followed by increased cardiac output.
- Responses include – Breathlessness & palpitation
– Hence, referred “Acute Transient Response”/”Transient Mountain Sickness”.
- “Acute Mountain Sickness” –
– Develop after a lag of 6 hours to 4 days
- Usually develops within a day & lasts for a week.
Characterized by hypoxic symptoms –
- Breathlessness.
- Weakness.
- Headache
- Dizziness
- Palpitation
- Tachycardia.
- Sweating
- Dimness of vision
- Partial sleeplessness.
- Nausea.
May cause syndromes –
- High Altitude Pulmonary Edema (HAPE).
- High Altitude Cerebral Edema (HACE).
1a. HIGH ALTITUDE PULMONARY EDEMA (HAPE):
- Hypoxia causes pulmonary capillaries vasoconstriction causing,
– Pulmonary hypertension → In turn, raises pulmonary capillary hydrostatic pressure.
– Results in,
- Distrupion of weaker capillaries walls.
- Exudation of protein-rich fluid in lung tissues.
1b. HIGH ALTITUDE CEREBRAL EDEMA (HACE):
- Low PO2 causes arteriolar dilation.
– If cerebral autoregulation does not compensate,
– Results in increased cerebral capillary pressure → Favoring increased fluid transudation into brain tissue.
- For rapid ascent to sleeping altitude above 2750 mts.
- Effective if started early in course of acute mountain sickness (AMS).
- “Development of compensatory mechanisms to ward off ill-effects of low barometric pressure”.
- Due to continued stay at high altitude.
EFFECTS:
- Hyperventilation – Most fundamental response to hypoxia.
- Low arterial PO2 (hypoxemia) stimulates carotid body peripheral chemoreceptors.
– Causing hyperventilation.
- Leading to,
– Arterial PCO2 fall.
– Fall in PCO depresses respiration.
– Via central chemoreceptor mechanism.
- Respiratory alkalosis (increased pH).
– Compensated by increased renal excretion of bicarbonate.
– Thus blood carbonic acid/bicarbonate ratio & pH maintained at normal level.
- Peripheral chemoreceptors are not affected by pH.
- An acclimatized person maintain slight acidic CSF pH.
– Because HCO3– pump is located at blood-brain barrier
– Pumping out HCO3-.
– Consequently, CSF pH is restored to normal.
- Since central chemoreceptors are affected primarily by CSF pH
– No longer inhibit respiration despite CO2 washout.
- Increased respiratory chemoreceptor sensitivity causing hypoxia & CO2.
- Eventual effect is “Increased minute volume”, primarily a result of increased tidal volume.
- Increases DPG.
- Causing “Rightward” shift of oxygen-Hb dissociation curve.
- Resulting in increased tissue O2 delivery.
- Hypoxia stimulates “Erythropoietin” release.
– Stimulating erythropoiesis → Increased RBC count & hemoglobin concentration.
– Hence, absolute polycythemia with increased red cell mass.
5. OTHER COMPENSATORY MECHANISM:
- Increases renal alkali (HCO3–) excretion.
- Increased cardiac output immediately.
– Returns normal within few weeks.
– Due to increased blood hematocrit.
- Increased lung diffusion capacity.
- Increased vascularity of tissues.
- Increased capillary density.
- Enhancement of oxidative metabolism.Increased myoglobin.
– By increased mitochondrial count & cytochrome oxidase content.
- Work capacity of skeletal & cardiac muscle –
– Decreased in unacclimatized person
– Acclimatization improves work capacity gradually.
- Exercise performed at high altitude hastens acclimatization process.
– Exercise increases oxygen requirement despite low avaialability.
– Thus, increasing duration of exercise increases work capacity.
- Work capacity at high altitude can be increased by,
– Increasing duration of exercise.
– Also by reducing/maintaining workload at an optimum level corresponding to maximum oxygen uptake rate.
- Thus all available oxygen utilized.
III. CHRONIC MOUNTAIN SICKNESS/ MONGE’S DISEASE:
- Due to insensitive peripheral chemoreceptors to hypoxia.
– Causes severe hypoxic symptoms.
- Causes widespread pulmonary vasoconstriction → Eventually right ventricular failure (cor pulmonale).
Exam Important
- PO2 is low at high altitudes.
- PO2 of inspired air (PiO2) is low.
- Earliest response to fall in PO2 – Hyperventilation
- Acute mountain sickness is characterized by hypoxia-like symptoms Tachycardia.
- High altitude pulmonary edema causes raise pulmonary capillary hydrostatic pressure.
- Acetazolamide is DOC prevention of acute mountain sickness.
- During acclimatization, Hyperventilation is most fundamental response to hypoxia.
- Low arterial PO2 (hypoxemia) stimulates carotid body peripheral chemoreceptors → Causing hyperventilation.
– Leading to “Arterial PCO2 fall” & “Respiratory alkalosis (increased pH).
- Respiratory alkalosis compensated by increased renal excretion of bicarbonate.
– Thus blood carbonic acid/bicarbonate ratio & pH maintained at normal level.
– Peripheral chemoreceptors are not affected by pH.
- “Increase in sensitivity of respiratory chemoreceptor mechanisms to hypoxia & CO2.
- Eventual effect is “Increased minute volume”, as a result of increased tidal volume.
- Acclimatization increases DPG.
– Causing “Rightward” shift of oxygen-Hb dissociation curve.
– Resulting in increased tissue O2 delivery.
- Hypoxia stimulating erythropoiesis → Increased RBC count & hemoglobin concentration.
– Hence, absolute polycythemia with increased red cell mass.
- Other compensatory mechanisms – Increases renal alkali (HCO3-) excretion.
- Exercise performed at high altitude hastens acclimatization process.
– Increasing duration of exercise increases work capacity.
- Work capacity at high altitude can be increased by,
– Increasing duration of exercise.
– Also by reducing/maintaining workload at an optimum level corresponding to maximum rate of oxygen uptake.
– All available oxygen can be utilized.
- Chronic mountain sickness/Monge’s disease can lead toright ventricular failure (cor pulmonale).
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