Question 28.1

College answer

Aim: To explore the candidate understanding of common measurements of respiratory function and interpretation.
Key sources include: Paper 2011.1 Q9.1 repeated 2014.1 Q21.2., 2018 26.1, CanMEDS Medical Expert.
Discussion: Common non acute, non-ICU investigations that might influence ICU practice are considered appropriate topics for inclusion in the examination,

28.1 - Many candidates answered parts a and b well. A restrictive defect (reduced lung capacity) with a normal lung diffusion capacity was correctly interpreted and successful candidates were able to provide differentials for this. These included but were not limited to a chest wall abnormality and morbid obesity.
Part c was answered poorly. Better answers referred to the clinical context provided in the question, understood the meaning behind the PFTs, and could make clinical deductions based on the provided history and investigations. For example: The history provided of a drowsy hypoxic post operative patient is likely to have an ABG showing
a respiratory acidosis with hypercapnia and decreased pO2. In this patient an elevated A-a gradient may be present IF there is a primary respiratory issue (e.g., aspiration
post op, or pulmonary embolus) as the prior pulmonary function tests state the diffusion gradient is normal.
There would be no elevated A-a gradient if the hypoxia was hypoventilation only.
Understanding and recognition of the two potential blood gas results has the ability to influence management. (i.e., requirement for a CTPA).
 

Discussion

The college refer to some past SAQs which asked the candidates to interpret lung function tests:

• Question 21.2 from the first  paper of 2014
• Question 9.1 from the first  paper of 2011
• Question 26.1 (and 26.2, and 26.3) from the second paper of 2018

a) asked to merely explain the results:

• The FVC is reduced, suggesting restrictive lung disease
• The FEV1 is 63% of what is predicted, and reverses minimally with bronchodilators, suggesting that bronchoconstriction is not the main issue, but rather some kind of mechanical or muscular problem with the respiratory system
• The FEV1/FVC is 102% of predicted, again supporting the assertion that this is not obstructive lung disease
• The TLC is greatly reduced, also suggesting restrictive lung disease
• The DLCO is basically normal, which suggests that the lung parenchyma is blameless
• The KCO is completely normal (in fact supranormal!), i.e. there is definitely no diffusion defect.

In short, this patient has a restrictive defect with normal lungs. The alveoli are fewer, but those that remain seem to be doing a fine job at exchanging gases.

So: what could this be?

b) asked to list the possible causes. for 20% of the marks. The number of expected causes was not mentioned but we can generally assume that four or so differentials would have satisfied the examiners on the basis of the 20% weight. Plausible suggestions are listed below:

• Mechanical problems with the lungs or chest wall
  • Morbid obesity
  • Severe kyphosis
  • Pectus excavatum
  • Large pleural effusions
  • Increased intraabdominal pressure, eg. ascites
  • Phrenic nerve palsy subsequent to the surgery
  • Reduced lung volume due to lobectomy or pneumonectomy
• Mechanical problems with respiratory effort (any causes of weakness go here)
  • Myasthenia gravis
  • Muscular dystrophy
  • Amyotrophic lateral sclerosis
  • Polio (seriously)
  • Guillain-Barre

c) asked the candidates to use their imagination to predict the ABG results for such a patient. From what is suggested by the stem, we might expect:

• Acidaemia
• Hypoxia (the stem says so)
• A normal A-a gradient may be seen, as diffusion is pretty reasonable on the PFTs
• Hypercapnia (because drowsy)
• Some degree of SBE/bicarbonate elevation, considering that some of this may be chronic

Pellegrino, Riccardo, et al. "Interpretative strategies for lung function tests."European Respiratory Journal 26.5 (2005): 948-968.

The American Thoracic Society has a page which features an excellent bibliography of the articles which support their interpretation standards.