Altitude physiology

Question 1

Are the physiological effects of ascending to altitude due to hypoxia or hypercapnia

Answer 1

Hypoxia

Question 2

When is the hypoxic drive from carotid body significant?

Answer 2

When oxygen levels are at or below 60mmHg

Question 3

Why doesn’t the low partial pressure of oxygen at high altitudes lead to increased ventilation by hypoxia detectors in carotid body

Answer 3

Because hypoxia driven hyperventilation response is antagonised by depression of ventilation powered by excess CO2 blow off
Receptors are alkalosed
Unable to increase resp drive

Question 4

What is meant by acclimatisation to high altitude

Answer 4

Body’s physiological response to high altitude stops (pulmonary arterial hyperventilation and hypoxia disappears)

Question 5

How long does acclimatisation take up to 2000m

Answer 5

1-2 days

Question 6

How long does acclimitisation take between 2000-7000m

Answer 6

Weeks

Question 7

What happens to the body at altitudes above 7000m

Answer 7

Significant hypoxia, tiredness and lethargy, continuous exercise impossible

Question 8

What are altitudes aboves 7500m called

Answer 8

Death zone

Question 9

What happens to the body at altitudes above 7500

Answer 9

Severe hypoxia after a few days

Severe physiological damage

Question 10

Name the 3 mechanisms of acclimatisation to high altitudes

Answer 10

• Metabolic acidosis
• Raised erythropoeitin
• Reduced pulmonary vascular resistance

Question 11

Describe the process by which metabolic acidosis allow for acclimatisation to high altitudes

Answer 11

• Low oxygen leads to increased rate and depth breathing
• Blowing off excess Co2 (resp alkalosis)
• Inhibits central chemoreceptors (hypoxaemia)
• Kidneys respond by lowering ATPase in tubules
• Decreases renal excretion, increased bicarb excretion
• Metabolic acidosis (restore ph, central chemoreceptor drive restored)

Question 12

Describe the process by which raised erythropoeitin production allows for acclimatisation to high altitudes

Answer 12

• Interstitial cells in kidney produce more erythropoeitin

- Causes increased haematocrit and increases oxygen carrying capacity in blood

Question 13

Why can’t erythropoeitin and therefore haematocrit be raised indefinitly

Answer 13

Increases viscocity therefore increasing pulmonary vascular resistance
This can lead to arterial hypertension and right heart failure

Question 14

How is a reduced pulmonary vascular resistance achieved

Answer 14

Reduced hypoxic vasoconstriction partly due to collateral circulations
Mediated by increased NO synthesis

Question 15

What is the name of type 1 mountain sickness

Answer 15

Acute mountain sickness

Question 16

Symptoms of acute mountain sickness

Answer 16

Headache, poor sleep, nausea, dizziness, loss of appetite

Question 17

What % of people get mild illness at >2500m altitude

Answer 17

20

Question 18

At what height does everybody become temporarily unwell

Answer 18

5000m and above

Question 19

How should mild acute mountain sickness be treated

Answer 19

Simply by resting

Question 20

How should more severe acute mountain sickness be treated

Answer 20

• Descend
• Oxygen
• Acetazolamide at 250mg 3 times a day
• Dexamethasone 4mg 4 times a day

Question 21

State the mechanism of acetazolamide

Answer 21

Carbonic anhydrase inhibitor

Question 22

What is the role of carbonic anhydrase

Answer 22

Renal HCO3- reabsorption

Question 23

What is the effect of inhibiting carbonic anhydrase

Answer 23

Increased HCO3- excretion leading to metabolic acidosis to compensate for resp alkalosis
Blocks conversion of HCO3 to CO2 so can’t diffuse back into tubular cells

Question 24

What is the effect of acetazolamide in high doses

Answer 24

Inhibits carbonic anhydrase in red cells so blocks transports of CO2 from tissues to lungs

Question 25

What is the name of type 2 altitude sickness

Answer 25

High altitude cerebral oedema

Question 26

Symptoms of high altitude cerebral oedema

Answer 26

Ataxia
Nausea and vomiting
Confusion
Hallucination
Coma

Question 27

Why does high altitude cerebral oedema occur

Answer 27

Because brain relies on aerobic respiration to pump sodium out and maintain osmotic equlibrium
With hypoxaemia, ATP supply decreases and sodium pumps run down
Sodium moves into brain bringing water with it leading to swelling
Increased intracranial pressure blocking cerebral veins so hypoxia worsens and neurones begin to die

Question 28

Treatment of HACE

Answer 28

1) Descend
2) Acetazolamide
3) Oxygen
4) Dexamethasone
5) Hyperbaric chambers

Question 29

How does acetazolamide help treat HACE

Answer 29

Reduces CSF formation so reduces intracranial pressure

Question 30

What dosage of dexamethosone should be used to treat HACE

Answer 30

8mg then 4mg

Question 31

What is the name of type 3 altitude sickness

Answer 31

HIgh altitude pulmonary oedema

Question 32

Signs and symptoms of HAPE

Answer 32

Dyspnoea, reduced execise tolerance, dry cough, blood stained sputum, crackles

Question 33

When does HAPE happen

Answer 33

If body doesn’t acclimatise well and hypoxic pulmonary vasoconstriction doesn’t decrease
Pulmonary arterial hypertension develops

Question 34

How does fluid move in HAPE

Answer 34

Out of the body and into alveoli

Question 35

Treatment outline for HAPE?

Answer 35

• Descend
• Sit upright
• Oxygen
• Nifedipine (CCB) 20mg orally
• Hyperbaric chamber
• Sildenafil

Question 36

What is the purpose of nifedipine

Answer 36

Treats HAPE by blocking pulmonary arterial constriction (calcium channel blocker)