A 54-year-old woman with severe idiopathic thoracic scoliosis (Cobb angle 88°) presents with progressive dyspnea, morning headaches, and ankle edema. Spirometry shows FVC 41% predicted, FEV1 44% predicted, and FEV1/FVC ratio 0.92. The pattern marked **A** in the diagram is consistent with her findings. Which of the following findings would BEST distinguish this extrapulmonary restrictive pattern from a parenchymal restrictive pattern due to idiopathic pulmonary fibrosis?

Why DLCO/VA (KCO) elevated or normal, with reduced absolute DLCO is right

The cardinal physiological distinction between extrapulmonary restriction (chest wall, neuromuscular deformity) and parenchymal restriction (interstitial lung disease) lies in the DLCO/VA ratio, also called KCO (transfer coefficient). In kyphoscoliosis-induced extrapulmonary restriction, the underlying lung parenchyma and alveolar-capillary membrane are structurally and functionally NORMAL. The restriction occurs because the deformed chest wall and foreshortened respiratory muscles cannot fully expand the lungs—a pure mechanical problem. Consequently, absolute DLCO is reduced (because total alveolar volume is reduced), but when normalized to the reduced alveolar volume (VA), the per-unit-volume transfer factor (DLCO/VA or KCO) is ELEVATED or NORMAL. This occurs because the normal lung tissue is compressed into a smaller space, concentrating the diffusion capacity. In contrast, parenchymal restriction from ILD involves destruction or fibrosis of alveolar-capillary units, so KCO is reduced or normal. This elevated KCO is the pathognomonic finding that identifies extrapulmonary restriction and is essential for differential diagnosis (Murray and Nadel 7e Ch 96; Bergofsky Am J Med 1979).

Why each distractor is wrong

Murray and Nadel 7e Ch 96; Bergofsky Am J Med 1979; ATS Restrictive Disease Statement