Respiratory Investigation — Written Clinical Reasoning

Model answer — Part A: three PFT panels

Patient 1 — confirmed parenchymal restriction. The TLC below the lower limit of normal confirms true restriction (spirometry alone could only suggest it), and the DLCO of 52% localises the defect to the alveolar-capillary membrane: this is the interstitial lung disease pattern. The single most discriminating result is the DLCO — it is what separates restriction within the lung from restriction outside it. The next investigation is HRCT chest to define the pattern (UIP versus non-UIP), which drives the whole subsequent pathway ^{[1] [2]}.

Patient 2 — no restriction: the obesity mimic. The low FVC with preserved ratio suggested restriction on spirometry, but the TLC of 88% refutes it — this patient has no ventilatory defect, and the normal DLCO confirms an intact membrane. The most discriminating result is the TLC, which is exactly why volumes must precede the word "restriction". The next step is not more lung testing but an explanation for dyspnoea: obesity-related mechanics, deconditioning, and clinical assessment for cardiac or vascular causes if symptoms warrant ^{[1] [3]}.

Patient 3 — neuromuscular restriction. TLC confirms restriction, but the normal DLCO with a normal-to-high KCO says the membrane is intact — the problem is the pump, not the lung. The most discriminating result is the combination of normal DLCO with a 25% sitting-to-supine FVC fall, pointing at diaphragmatic weakness. The next investigations are maximal inspiratory and expiratory pressures and a sniff nasal inspiratory pressure (with diaphragm ultrasound), and a clinical search for motor neurone disease, myopathy or phrenic nerve pathology ^{[1] [2]}.

The unifying mark-scorer sentence: restriction is a TLC diagnosis; DLCO (with KCO) then splits parenchymal from extrapulmonary causes; and obesity is the classic pseudo-restriction that only volumes can dismiss ^[1].