Question 27

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

Interpret the blood gas. (2 marks)
- High anion gap metabolic acidosis with respiratory compensation
- Elevated Aa gradient 40mmHg (5.3kPa)
What is the most likely diagnosis? (2 marks)

- HAPE = High altitude pulmonary oedema
- HACE = High altitude cerebral oedema

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:

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).
There is mild acidaemia
The CO₂ is appropriately decreased
There is no base excess given and the bicarbonate is 15; therefore there is probably a metabolic acidosis afoot.
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.
The anion gap is raised: (140) - (102 + 15) = 23, or 27.1 when calculated with potassium.
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

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.

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