Spirometry and paediatric pulmonary-function testing — formative SAQs

SAQ 1 — The obstructive report in suspected asthma (20 marks, ~15 minutes)

A 10-year-old girl with a two-year history of cough and wheeze performs spirometry in clinic. Two acceptable, repeatable efforts show an FEV1/FVC ratio well below the lower limit of normal for her age and height, a scooped expiratory flow-volume loop, and an FVC at the lower end of her predicted range. After an inhaled bronchodilator, her FEV1 rises substantially. [10]

Questions

1. Describe the pattern shown by her baseline spirometry and the single value that defines it. (4 marks) [2]
2. Why should her FEV1/FVC ratio be judged against a lower limit of normal rather than a fixed value of 0.70? (4 marks) [3]
3. Define a significant bronchodilator response using the current ATS/ERS interpretive standard. (4 marks) [2]
4. How does her spirometry contribute to a diagnosis of asthma, and what is the key limitation you must remember? (5 marks) [10]
5. State two features of the test you would confirm before you trusted these numbers. (3 marks) [1]

Model answer (must-hit)

1. The baseline spirometry shows an obstructive pattern. It is defined by the FEV1/FVC ratio, which is below the lower limit of normal, meaning the airways empty too slowly; the scooped (concave) expiratory limb of the flow-volume loop is the graphical signature, and the FVC may be at the lower end of normal because air is trapped behind slowly emptying airways rather than because the lungs are small. [2]
2. Children's normal FEV1/FVC ratios are higher than adults' and fall gradually with growth, so a fixed 0.70 threshold under-diagnoses obstruction in younger children and over-diagnoses it in older ones, and it ignores height, sex and ancestry. The lower limit of normal derived from the Global Lung Function Initiative equations sets the fifth percentile (a z-score of about minus 1.64) specific to the child, which is the statistically and clinically correct comparator. [3]
3. Under the 2022 ATS/ERS interpretive standard, a significant bronchodilator response is an increase in FEV1 or FVC of more than ten percent of the predicted value after an inhaled bronchodilator. This replaces the older definition of a twelve-percent rise from the child's own baseline (with an absolute volume requirement in adults), because expressing the change against predicted is fairer across body sizes. [2]
4. Her spirometry supports asthma by demonstrating variable airflow obstruction — obstruction at baseline that improves significantly after a bronchodilator — which is one of the objective criteria in the diagnostic algorithm. The key limitation is that a normal or non-reversible spirometry does not exclude asthma, because lung function is frequently normal between episodes; a normal test in a symptomatic child should prompt repeat testing on another day or a bronchial challenge rather than reassurance. [10]
5. Before trusting the numbers I would confirm the test was acceptable (a good hesitation-free start with a small back-extrapolated volume, no cough in the first second, no early termination or leak) and repeatable (the two largest FVC and FEV1 values agreeing within the accepted margin). An unacceptable effort produces low values that mimic disease. [1]

SAQ 2 — The reduced FVC: restriction or artefact? (20 marks, ~15 minutes)

An 8-year-old boy with a chest-wall deformity is tested because of exertional breathlessness. His spirometry shows a reduced FVC with an FEV1/FVC ratio that is normal, indeed at the upper end of predicted, and a small but normally shaped flow-volume loop. The technician notes his expiratory efforts were short. [1]

Questions

1. What pattern does this spirometry suggest, and why can you not yet call it restriction? (5 marks) [2]
2. List three explanations for a reduced FVC other than true restriction. (4 marks) [1]
3. What single investigation would confirm or refute true restriction, and what would it measure? (4 marks) [9]
4. In a child with progressive respiratory-muscle weakness, what additional spirometric manoeuvre adds information, and why? (4 marks) [2]
5. State why interpreting his result requires the correct reference equation. (3 marks) [3]

Model answer (must-hit)

1. The pattern suggests restriction — a reduced FVC with a preserved or high FEV1/FVC ratio and a small but normally shaped loop. I cannot yet call it restriction because spirometry cannot measure the air remaining in the chest, so it cannot measure total lung capacity; a low FVC on spirometry only raises the possibility, and here the short expiratory efforts make an artefact likely. True restriction requires a reduced total lung capacity. [2]
2. Other explanations for a reduced FVC are submaximal or short expiratory effort (as noted here), early termination of the blow before the lungs are empty, and air trapping in obstruction where the child cannot exhale it all in the time available. Poor coaching and a leak around the mouthpiece also lower the FVC. [1]
3. Measurement of static lung volumes — by body plethysmography or gas dilution — would confirm or refute true restriction. It measures the total lung capacity and residual volume: a reduced total lung capacity confirms restriction, whereas a normal or raised total lung capacity refutes it and points to submaximal effort or air trapping instead. [9]
4. Comparing the FVC sitting and lying supine adds information, because a marked fall in the supine FVC unmasks diaphragm weakness — the abdominal contents push against a weak diaphragm when lying flat. Tracking the FVC over time also monitors progressive respiratory-muscle weakness and guides the timing of respiratory support. [2]
5. His result must be read against a reference equation matched to his age, height, sex and ancestry, because the Global Lung Function Initiative predictions determine whether his FVC is genuinely reduced. Using a mismatched reference shifts the z-scores and can manufacture or hide an abnormality, turning a normal result into apparent restriction or vice versa. [3]