Question 28.2
A 63-year-old patient with a history of smoking underwent outpatient spirometry for investigation of chronic cough and breathlessness. The flow-volume loop obtained is shown. The expected curve is shown in grey, with the patient’s result in red.
a) List with examples, three possible mechanisms for the pattern seen above. (10%)
b) List three possible causes of a reduced DLCO on pulmonary function tests. For each condition listed, briefly outline the expected findings on lung spirometry (FEV1, FVC and FEV1/FVC). (10% marks for each condition listed, 30% total)
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,
28.2 Answers which addressed the potential mechanisms which included chronic obstructive airways disease, variable intrathoracic airways obstruction and listed potential causes scored well. Three possible causes of reduced DLCO included but were not limited to interstitial lung disease, severe emphysema, pulmonary embolism, and pneumonectomy. Most candidates were successfully able to give the expected findings of these pathologies on lung spirometry.
Discussion
Without attempting any sort of respiratory flow-volume pun, one sighs with exasperation at any unlabelled set of axis coordinates, and especially at any loop where inspiration and expiration are not clearly indicated. By convention, ventilator manufacturers for some reason seem to flip their loops, and have inspiratory flow on top, whereas spirometer manufacturers (and online resources discussing spirometry) have their inspiratory flow on the bottom.
The reader will agree that the result of this confusion is a completely different interpretation of the loop. If the sharp triangular section of the loop was a positive pressure mechanical breath, the patient would appear to have some reduced airflow during inspiration. However the question specifically states that the loop was measured during outpatient spirometry, and therefore the sharp triangular section of the breath actually represents spontaneous patient effort. The "scooped out" flow pattern during this forced expiratory manoeuvre suggests that the flow restriction is expiratory, which is what you would usually expect in:
- COPD, or asthma
- Poor effort (i.e. the patient is fine, just didn't blow hard enough)
- Variable intrathoracic obstruction, eg. tumour, lymph nodes, retrosternal goitre, thoracic aortic aneurysm, tracheomalacia, etc etc.
- Weakness, eg. myastnenia gravis
b) is better answered with a table:
| Cause | FEV1 | FVC | FEV1/FVC |
| COPD | Reduced | Normal | Reduced |
| Pulmonary fibrosis | Reduced | Reduced | Normal |
| Pulmonary hypertension | Normal | Normal | Normal |
| Anaemia | Normal | Normal | Normal |
| Pulmonary embolism | Normal | Normal | Normal |
References
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.
Correger, E., et al. "Interpretation of ventilator curves in patients with acute respiratory failure." Medicina Intensiva (English Edition) 36.4 (2012): 294-306.
Frank Rittner, Martin Doring. Curves and loops in mechanical ventilation. not sure what year; published by Drager.
R Scott Harris, Pressure-Volume Curves of the Respiratory System Respir Care 2005;50(1):78–98. © 2005
Valta, Paivi, et al. "Detection of expiratory flow limitation during mechanical ventilation." American journal of respiratory and critical care medicine 150.5 (1994): 1311-1317.