Altitude physiology Flashcards
Are the physiological effects of ascending to altitude due to hypoxia or hypercapnia
Hypoxia
When is the hypoxic drive from carotid body significant?
When oxygen levels are at or below 60mmHg
Why doesn’t the low partial pressure of oxygen at high altitudes lead to increased ventilation by hypoxia detectors in carotid body
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
What is meant by acclimatisation to high altitude
Body’s physiological response to high altitude stops (pulmonary arterial hyperventilation and hypoxia disappears)
How long does acclimatisation take up to 2000m
1-2 days
How long does acclimitisation take between 2000-7000m
Weeks
What happens to the body at altitudes above 7000m
Significant hypoxia, tiredness and lethargy, continuous exercise impossible
What are altitudes aboves 7500m called
Death zone
What happens to the body at altitudes above 7500
Severe hypoxia after a few days
Severe physiological damage
Name the 3 mechanisms of acclimatisation to high altitudes
- Metabolic acidosis
- Raised erythropoeitin
- Reduced pulmonary vascular resistance
Describe the process by which metabolic acidosis allow for acclimatisation to high altitudes
- 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)
Describe the process by which raised erythropoeitin production allows for acclimatisation to high altitudes
- Interstitial cells in kidney produce more erythropoeitin
- Causes increased haematocrit and increases oxygen carrying capacity in blood
Why can’t erythropoeitin and therefore haematocrit be raised indefinitly
Increases viscocity therefore increasing pulmonary vascular resistance
This can lead to arterial hypertension and right heart failure
How is a reduced pulmonary vascular resistance achieved
Reduced hypoxic vasoconstriction partly due to collateral circulations
Mediated by increased NO synthesis
What is the name of type 1 mountain sickness
Acute mountain sickness
Symptoms of acute mountain sickness
Headache, poor sleep, nausea, dizziness, loss of appetite
What % of people get mild illness at >2500m altitude
20
At what height does everybody become temporarily unwell
5000m and above
How should mild acute mountain sickness be treated
Simply by resting
How should more severe acute mountain sickness be treated
- Descend
- Oxygen
- Acetazolamide at 250mg 3 times a day
- Dexamethasone 4mg 4 times a day
State the mechanism of acetazolamide
Carbonic anhydrase inhibitor
What is the role of carbonic anhydrase
Renal HCO3- reabsorption
What is the effect of inhibiting carbonic anhydrase
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
What is the effect of acetazolamide in high doses
Inhibits carbonic anhydrase in red cells so blocks transports of CO2 from tissues to lungs
What is the name of type 2 altitude sickness
High altitude cerebral oedema
Symptoms of high altitude cerebral oedema
Ataxia Nausea and vomiting Confusion Hallucination Coma
Why does high altitude cerebral oedema occur
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
Treatment of HACE
1) Descend
2) Acetazolamide
3) Oxygen
4) Dexamethasone
5) Hyperbaric chambers
How does acetazolamide help treat HACE
Reduces CSF formation so reduces intracranial pressure
What dosage of dexamethosone should be used to treat HACE
8mg then 4mg
What is the name of type 3 altitude sickness
HIgh altitude pulmonary oedema
Signs and symptoms of HAPE
Dyspnoea, reduced execise tolerance, dry cough, blood stained sputum, crackles
When does HAPE happen
If body doesn’t acclimatise well and hypoxic pulmonary vasoconstriction doesn’t decrease
Pulmonary arterial hypertension develops
How does fluid move in HAPE
Out of the body and into alveoli
Treatment outline for HAPE?
- Descend
- Sit upright
- Oxygen
- Nifedipine (CCB) 20mg orally
- Hyperbaric chamber
- Sildenafil
What is the purpose of nifedipine
Treats HAPE by blocking pulmonary arterial constriction (calcium channel blocker)
