36. High altitude physiology Flashcards
What happens to the blood when a person climbs to a high altitude?
At high altitude, the partial pressure of oxygen is reduced, leading to hypoxaemia (low oxygen in the blood)
Recognised by hypoxia detectors in the carotid bodies
Can the body compensate for the hypoxaemia at high altitude?
No.
At first it tries by causing hypoxia driven ventilation
BUT as you ascend and are blowing off CO2, the CO2 drive is depressed
We know that the main respiratory drive is CO2
SO due to the depressed CO2 drive, cannot completely compensate for this
What is the response of the body when ascending to high altitude?
Acclimatisation does occur if we ascend slowly:
Metabolic acidosis
Increased erythrocyte number - increased hematocrit
Reduced pulmonary vascular resistance
What are the heights at which acclimatisation will be successful?
Can acclimatise to 2000m rapidly
Experienced climbers can acclimatise to 2000-6000m and generally feel well
Above 7000m results in significant hypoxia - lethargy and tiredness
Above 7500m is the death zone - severe hypoxia and can remain here no longer than two/three days - this height can lead to major permanent physiological damage to e.g. the kidneys/the liver
Why and how does metabolic acidosis occur during acclimatisation ?
Initially there is a low partial pressure of O2 so hyperventilation occurs
This leads to respiratory alkalosis
SO to compensate for this - metabolic acidosis
VIA decreased renal excretion of acid i.e. of H+ and increased excretion of HCO3-
Why does the pulmonary vascular resistance fall?
Not sure why this occurs or how this happens
BUT it is useful because it means that the increased haematocrit does not lead to pulmonary hypertension
What is altitude sickness?
Occurs if the ascent is too rapid or two high
First thing is Acute mountain sickness (AMS)
If you then continue, then this can lead to HACE (high altitude cerebral oedema) or HAPE (high altitude pulmonary oedema)
What are the signs and symptoms of AMS?
Headache - essential for diagnosis Poor sleep Tiredness Loss of appetite, nausea, vomiting Dizziness
What is the treatment for AMS?
If mild, rest If severe: Descent Oxygen Acetazolamide Dexamethasone (steroid)
Why is acetazomalide used and what else can this be used for in someone who is climbing to high altitude?
Carbonic anhydrase inhibitor
SO prevents the excretion of bicarbonate in the proximal tubule of the kidney and hence allows for metabolic acidosis
Can also be used as a prophylaxis as you start your ascent
What are the symptoms of HACE?
Ataxia Nausea/vomiting Hallucination/disorientation Confusion Reduced conscious level Coma
Why does HACE occur?
Same as during shock
Brain relies on aerobic respiration so in cases of anaerobic respiration, sodium leaks into the nerve cell and pulls water in with it, causing the cells to swell
Results in raised intracranial pressure which blocks cerebral veins
Cerebral circulation fails, hypoxia gets worse and the neurones start to die
What is the treatment of HACE?
Descend immediately
Acetazolamide (reduces the formation of CSF)
Oxygen
Dexamethasone (anti-inflammatory prevents swelling)
Hyperbaric chamber
What are the signs and symptoms of HAPE?
Dyspnoea Reduced exercise tolerance Dry cough Blood stained sputum Crackles on auscultation of the chest
Why does HAPE happen?
The hypoxic pulmonary vasoconstriction that initially occurs when you ascend starts to decrease with acclimatisation
BUT if this does not occur then pulmonary arterial hypertension can develop
This raised arterial and capillary pressure can lead to fluid leaving the blood and entering the alveoli
This worsens the already compromised gas exchange and increases the hypoxia - causes constriction and vicious cycle
