Pneumothorax Size (Rhea / Collins / BTS / ACCP)

Pneumothorax size guides the choice between observation, needle aspiration, and tube drainage. The Collins method (AJR 1995) regressed three erect-PA interpleural distances — at the apex (A), the midpoint of the upper half of the collapsed lung (B), and the midpoint of the lower half (C) — against helical-CT volumetry: size % = 4.2 + 4.7 × (A + B + C), r = 0.98 (a summed distance of about 3.5 cm corresponds to ~20% and 12 cm to ~60% of the hemithorax). The Rhea method averages the same three distances (AID) and converts the average to a percentage with a nomogram; this implementation uses the published digitized regression of that nomogram (Choi et al., 1998): size % = 4.95 + 8.8 × AID for erect PA films and 9 + 10 × AID for supine AP films. Rhea is accurate for smaller pneumothoraces and underestimates larger ones relative to Collins/CT. Guideline thresholds are depth-based: BTS 2010 calls a pneumothorax large when the rim between lung edge and chest wall at the level of the hilum is 2 cm or more (large or breathless PSP → needle aspiration first line, drain if aspiration fails; small asymptomatic PSP → discharge/observation), while ACCP 2001 uses an apex-to-cupola distance of 3 cm or more (large PSP → tube thoracostomy; stable small PSP → observe and discharge). Secondary spontaneous pneumothorax is managed more aggressively: admission, oxygen, and a low threshold for tube drainage.