39 - High Altitude Physiology Flashcards
What is the effect on PaO2 at high altitudes?
Less paO2
Blood through lungs is less saturated
Pulmonary hypoxia and hypoxaemia
Where are hypoxia detectors
Carotid bodies
What causes decrease in ventilation at high altitudes
Excess blow of CO2
Causes alkalosis at central chemoreceptors
inhibits the increase in respiratory drive
When is the hypoxic drive from carotid bodies significant?
PO2 below 60mmHg
How is hypoxia worsened with rapid ascent to high altitude?
PaO2 in alveoli is low - pulmonary circulation VASOCONSTRICTS in hypoxia so worsens
= pulmonary artery hypertension
When is acclimatisation rapid and how long does it take?
to 2000m - usually within a day a or two
What happens up to 6000m
Fully acclimatised
well, reasonable appetite, normal sleep
What happens above 7000m
Hypoxia present
Tired
lethargy
Hard to walk
What happens above 7500m
Death Zone
Severe hypoxia
Physiological damage
What are the 3 mechanisms in acclimatisation
1 - metabolic acidosis
2 - Increase in erythrocyte number
3 - Reduced pulmonary vascular resistance
What causes the metabolic acidosis
Retention of acid
Increased excretion of bicarbonate in the kidney
What causes respiratory alkalosis at high altitude
low PaO2 increased breathing rate respiratory alkalosis High pH inhibits central chemoreceptors Breathing decreases Hypoxaemia
How does the body acclimatise to oppose the respiratory alkalosis
Kidneys decrease proton excreting ATP-ase in the kidney tubules so kidneys decrease renal excretion of acid and increase excretion of bicarbonate
- restores PH to normal
How is EPO production increased in acclimatisation? What is the result of this?
Hypoxaemia stimulates interstitial cells in the kidney
raise epo production
= increases the haematocrit
= increase the oxygen carrying capacity of the blood
Why is there a limit to maximum haematocrit?
Increase in haematocrit increases blood viscosity
Increases the pulmonary vascular resistance
can cause pulmonary arterial hypertension and RHF
Why does pulmonary vascular resistance fall?
Reduced hypoxic vasoconstriction
collateral circulations open up between pulmonary arteries and vens
What mediates the decrease in hypoxic vasoconstriction?
Increased synthesis of nitric oxide in the pulmonary endothelium
What is AMS
Acute Mountain Sickness
= The first sign that something is wrong
HACE
High Altitude Cerebral Edema. Increased IC pressure. Can follow on from AMS if not treated. A serious neurological condition; fatal if not treated
HAPE
High Altitude Pulmonary Edema. Equally serious pulmonary condition which can follow on from AMS.
Signs and symptoms of AMS
Headache Poor Sleep Tiredness Loss of appetite, nausea, vomiting Dizziness
How do you diagnose AMS
The signs and symptoms are scored 0-3 and a score of more than 3 is needed for diagnosis
Would you get AMS between 1500-2000m
Unlikely
may get mild illness
Would you get AMS at 2500m
1 in 5 people have some symptoms if they ascend from sea level within a day - most will acclimatise in a day
Would you get AMS at 5000m
Everyone will be temporary ill if they ascend within a few hours to 5000m
acclimatisation occurs but takes several days or more
Treatment for AMS
IF mild then rest and no further ascent
IF severe then descent, oxygen, acetazolamide 250mg tds 3 times a day
Dexamethasone 4mg qds 4 times a day
How do you prevent AMS
Slow ascent (<300m per day over 3000m) Avoid unnecessary exercise Acetazolamide 250g
What does acetazolamide do
Carbonic anhydrase inhibitor - increased bicarbonate excretion producing metabolic acidosis
compensates for the alkalosis caused by hyperventilation
What is the effect of acetazolamide
less acid is secreted in the urine. The blood thus becomes acidified and respiration is stimulated.
= Speeds up acclimatization process
(read over the paragraph on this in notes, make sure u understand I cba to make qs about it)
Why does High altitude cerebral oedema occur?
ATP decreases, Na pumps not working so Na leaks into nerve cells – pulls water = cell swelling
Symptoms of brain swelling
Ataxia, nausea/hallucination, confusion, coma
Treatment for HACE
Descend immediately, acetazolamide, oxygen, dexamethasone, hyperbaric chamber
How does acetazolamide treat HACE
Reduces formation of CFS so reduces intracranial pressure
How does high altitude pulmonary oedema occur
Pulmonary arterial hypertension - raised arterial and capillary pressure - fluid leaving blood and entering alveoli - worsens gas exchange
Signs and symptoms of HAPO
Dysponea, reduced exercise tolerance, blood stained sputum, crackles on auscultation of chest
Treatment of HAPO
Descend immediately Sit patient upright Oxygen Nifedipine Hyperbaric chamber Viagra - sildenafil
What is the action of nifedipine
Calcium channel blocker
Prevent the pulmonary arteries contracting
What is the action of the hyperbaric chamber
increases the PaO2 of oxygen to improve oxygenation of blood - reduces hypoxic vasoconstriction
What is the action of viagra - sildenafil
Pulmonary hypoxic vasoconstriction due to lack of NO - slows down the breakdown of cyclic GMP (vasodilator) - increases cyclin GMP levels
viagra relaxes pulmonary arteries and stops PAH and improves oxygenation of blood
