A 22-year-old male climbed to a height of 3574 m above sea level. On arrival at this altitude he complained of chest tightness, breathlessness, tiredness and had an altered sensorium. He was evacuated to a nearby medical facility which was situated at an altitude of 700 m. His ECG was unremarkable and chest X-ray showed bilateral infiltrates.

The following arterial blood gas was taken at the medical facility:

Parameter

Patient Value

Adult Normal Range

Barometric pressure

701 mmHg (94 kPa)

FiO2

0.21

pH

7.30*

7.35 – 7.45

pO2

57.0 mmHg (7.6 kPa)

pCO2

32.0 mmHg (4.3 kPa)*

35.0 – 45.0 (4.6 – 6.0)

SpO2

85%

Bicarbonate

15.0 mmol/L*

22.0 – 26.0

Lactate

6.0 mmol/L*

0.5 – 1.6

Sodium

140 mmol/L

135 – 145

Potassium

4.1 mmol/L

3.5 – 5.0

Chloride

102 mmol/L

95 – 105

Glucose

5.6 mmol/L

3.5 – 6.0

a) Interpret the blood gas. (20% marks)

b) What is the most likely diagnosis? (20% marks)

c) What treatment would you institute in this patient? (60% marks)

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College answer

  1. Interpret the blood gas. (2 marks)
    • High anion gap metabolic acidosis with respiratory compensation
    • Elevated Aa gradient 40mmHg (5.3kPa)
  2. What is the most likely diagnosis? (2 marks)
    • HAPE = High altitude pulmonary oedema
    • HACE = High altitude cerebral oedema
  1. What is the treatment would you institute in this patient? (6 marks)

General (2)

    • Supplemental oxygen
    • Descend to lower altitude

High altitude pulmonary oedema (2)

    • Prompt reduction of pulmonary artery (PA) pressure :
      • Limit physical exertion and cold exposure,
      • Non-invasive ventilation (CPAP)
      • Pharmacological therapies to decrease pulmonary artery pressures: Nifedipine, Sildenafil/Tadalafil (Phosphodiesterase inhibitors)
    • Diuretic therapy, nitrates, and morphine are no longer recommended and could be harmful

High altitude cerebral oedema (2)

    • Dexamethasone
    • Consider hyperbaric therapy
    • ICP management – MAP maintained, minimise venous hypertension, adequate sedation and osmotic therapy

Discussion

First, let's go though the ABG:

  1. The A-a gradient is raised: 1 (0.21 x 654) - (32 x 1.25) - 57.0 = 40.34 mmHg. This answer would not have penalised those who did not notice the lower barometric pressure, because they would have ended up with an A-a gradient of 52.7 mmHg (i.e. still raised).
  2. There is mild acidaemia
  3. The CO2 is appropriately decreased
  4. There is no base excess given and the bicarbonate is 15; therefore there is probably a metabolic acidosis afoot. 
  5. The assessment of compensation by Winter's rule gives the expected PaCO2 as  (15 × 1.5) + 8 = 30.5,  i.e. approximately the same as the CO2 offered in the ABG. 
  6. The anion gap is raised: (140) - (102 + 15) = 23, or 27.1 when calculated with potassium.
  7. The delta ratio, without using potassium and assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (23 - 12) / (24 - 15) = 1.2.

In short, this is a straightforward high anion gap metabolic acidosis with adequate respiratory compensation.

Now, for 2 marks, "what is the most likely diagnosis?"

The clinical features are:

  • chest tightness
  • breathlessness
  • tiredness
  • an altered sensorium
  • bilateral infiltrates on CXR

Combine this with the story of high altitute, and HACE/HAPE become the inevitable conclusions. The college does not give any extensive explanations of what these are, and the trainees were not expected to expand on their pathophysiology or produce a list of differentials

Management:

  • For HAPE:
    • 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
  • For HACE:
    • 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

References

References

Mehta, S. R., A. Chawla, and A. S. Kashyap. "Acute mountain sickness, high altitude cerebral oedema, high altitude pulmonary oedema: The current concepts." Medical journal, Armed Forces India 64.2 (2008): 149.

Basnyat, Buddha, and David R. Murdoch. "High-altitude illness." The Lancet 361.9373 (2003): 1967-1974.

Hackett, Peter H., and Robert C. Roach. "High-altitude illness." New England journal of medicine 345.2 (2001): 107-114.

Bhagi, Shuchi, Swati Srivastava, and Shashi Bala Singh. "High-altitude pulmonary edema." Journal of occupational health (2014): 13-0256.

Basnyat, Buddha. "High altitude cerebral and pulmonary edema." Travel medicine and infectious disease 3.4 (2005): 199-211.

Stuber, Thomas, and Yves Allemann. "High altitude illness-pathogenesis and treatment." SCHWEIZERISCHE ZEITSCHRIFT FUR SPORTMEDIZIN UND SPORTTRAUMATOLOGIE 53.2 (2005): 88.