This is a series of short answer questions (SAQs) based on the following clinical vignette surrounding respiratory failure.

You are treating a 60 year old male who has presented with shortness of breath, fever and a productive cough. His chest x-ray shows consolidation in his left lung but a clear right lung. You suspect pneumonia.

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What is the difference between a consolidation and an effusion, and how would they appear on an x-ray?

Effusion: fluid accumulating in the pleural cavity – would appear as a dense white shadow at the base of the lung

Consolidation: fluid accumulating in the lungs – cloudy appearance in the lung fields.

What difference would you expect to see in the V/Q rations of the left and right lungs and why?

The V/Q ratio for the right lung will be greater than the V/Q ratio for the left lung as the consolidation in the left lung reduces the amount of ventilation.

The saturation of haemoglobin with oxygen in the left lung is likely to be reduced. Is this an example of perfusion-limited or diffusion-limited oxygen saturation?

Diffusion limited. There is nothing limiting the blood supply to the lung; the consolidation is preventing the amount of oxygen able to diffuse into the blood stream.

What is the name of the physiological process that responds to a lack of oxygenation in one lung?


Pneumonia is often associated with another condition in which part of the lung collapses due to a blockage, such as a mucous plug, preventing gas being able to reach the bronchioles distal to the blockage. What is this condition called?


The patient’s condition deteriorates and you suspect he is going into respiratory failure. What is respiratory failure and list four symptoms.

A failure of the lungs to meet the oxygen demands of the body. 

These are all signs of lack of oxygen and/or increased carbon dioxide: increased heart rate*, increased resp rate*, increased blood pressure*, increased lactate, decreased blood pH, confusion, cyanosis, inability to complete a sentence (either from confusion or shortness of breath), asterixis (CO2 flap), flared nostrils, use of accessory muscles

* These can all be reduced in extreme cases

You take an ABG and get the following results:

pH – 7.22         (7.36 – 7.44)

pCO2 – 5.87       (4.7 – 6)

pO2 – 5.91        (11 – 13)

HCO3 – 11         (22 – 26)

What type of respiratory failure is the patient in?

Type I respiratory failure – oxygen levels are below 8kPa but CO2 levels are still below 6kPa

Is the patient acidotic or alkalotic? Is it metabolic or respiratory?

Metabolic acidosis. pH is below 7.36 so they are acidotic. Bicarbonate is below 22. Something must be producing excess acid so we have more hydrogen ions, so the following equation is shifted to the left, reducing the amount of bicarbonate in the blood.

CO2 + H2O <– –> H+ + HCO3

For more on ABGs see Geeky Medics: https://geekymedics.com/abg-interpretation/

What is the likely physiological cause of this acidosis/alkalosis?

Lactic acid/lactate build up. In type I respiratory failure you have a reduction in oxygen in the blood so tissues will start to respire anaerobically producing lactic acid. 

In respiratory failure, increased carbon dioxide (due to less being excreted due to hypoventilation or problems with diffusion across the membrane) can also contribute to acidosis, but as the patient isn’t hypoventilating, lactic acid would probably be the correct answer for this scenario.

What is the risk of treating type 2 respiratory failure with oxygen?

Blood oxygen levels have some control over respiratory rate. Increasing blood oxygen levels could cause a reduction in respiratory rate making it harder to exhale the increased carbon dioxide in the blood.

Name a neurological condition that could cause respiratory failure and explain where it acts

Several to choose from and there are probably a few others that I’ve missed:

  • ALS – can affect the motor tracts and the anterior horns of the spinal cord preventing contraction of intercostal muscles
  • Polio – causes damage to the anterior horns in the spinal cord, which could also prevent contraction of intercostal muscles
  • Guillain-Barre syndrome – reduced myelination of LMN
  • Myasthenia gravis – loss of nicotonic acetylcholine receptors at the neuromuscular junction

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