Critical illness due to high altitude

Question 27 from the second paper of 2019 explored high altitude pulmonary oedema (HAPE) and high altitude cerebral oedema (HACE). As there was nowhere else to put this topic, it went to Misc. The unprepared trainees, confronted with HACE and HAPE, generally did very poorly (12.3% pass rate). Given that it could not possibly be viewed as a critically important knowledge territory, one can expect that it will never appear in the papers again. In case it does, the trainees will be prepared.

The single best (free) article covering this territory is probably Mehta et al (2011), from the Medical Journal of the Armed Forces of India. 

How high is high altitude?

The definition relies on the physiological changes which occur at an altitude of around 2700 meters above mean sea level. These changes are mainly related to the changes in alveolar partial oxygen pressure (about 60mmHg at this altitude), which is in turn related to atmospheric pressure (around 540 mmHg). As one ascends higher, the alveolar oxygen tension decreases. The "death zone" is said to be above 8,000m, where atmospheric pressure is around 260, and prolonged survival without supplemental oxygen is impossible.

High altitude pulmonary oedema

Clinical features

• Occurs within the first four days of exposure, but usually on the second or third day
• Dry cough
• Decreased exercise tolerance
• Pink frothy sputum
• Respiratory distress 
• Orthopnoea
• ECG: sinus tachycardia, RV strain, right axis deviation, RBBB and P wave abnormalities.
• ABG: usually hypoxia and respiratory alkalosis

Pathophysiology

• Hypoxia at altitude leads to hypoxic pulmonary vasoconstriction
• Hypoxic pulmonary vasoconstriction leads to raised pulmonary arterial pressure
• High pulmonary artery pressure leads to:
  • disruption of the alveolar endothelial barrier
  • endothelial dysfunction
  • Extravasation of high-protein fluid into the alveoli
  • Inflammatory changes due to this extravasation
• Susceptible individuals have:
  • greater heterogeneity of pulmonary blood flow than resistant subjects
  • Low production of endogenous vasodilators such as nitric oxide
  • High sympathetic activity and increased cardiac output

Management

• Correct hypoxia
  • Supplement oxygen
  • Retrieve the affected person to a lower altiitude
• Decrease pulmonary artery pressure
  • Decrease cardiac output
    • Bed rest
    • β-blockers
    • CPAP
  • Decrease pulmonary vascular resistance
    • Sildenafil or tadalafil
    • Nifedipine

High altitude cerebral oedema

Clinical features

• A change in mental status
• Ataxia
• Features of acute mountain sickness:
  • Headache
  • Fatigue
  • Dizziness
  • Poor sleep
  • Peripheral oedema
• Cranial nerve findings:
  • Papilloedema
  • Retinal haemorrhages
  • Cranial nerve palsy
• Imaging:
  • MRI: increased signal intensity in corpus callosum and splenium

Pathophysiology

• Four simultaneous problems:
  • Hypoxia mediated cerebral vasodilatation
  • Impairment of the autoregulation of cerebral blood flow
  • Disruption of the integrity of the blood brain barrier
  • Higher ratio of brain mass to CSF volume resulting in impaired ability to buffer a rise in intracranial pressure
• As the consequence of these, vasogenic cerebral oedema develops
• Additionaly, sympathetic nervous system stimulation increases salt and water retention
  • This exacerbates any existing oedema

Management

• Correct hypoxia
  • Supplement oxygen
  • Retrieve the affected person to a lower altitude
  • If possible, repressurise the person to 760mmHg (or even more if  the cerebral oedema is severe)
• Decrease vasogenic oedema
  • Dexamethasone 8mg, followed by 4mg qid
  • Acetazolamide 250mg bd
  • Osmotherapy