Calculating pulmonary shunt fraction using standard clinical measurements

This single-case pedagogical tutorial features a respiratory therapist who underwent a physiological shunt study for departmental education. Four formulas were used to estimate the shunt. A standardized 100% oxygen shunt test in a healthy male adult (51 years old, 185 cm, 84 kg) was performed to demonstrate how equation choice and assumptions affected the results. After 20 min breathing FiO₂ = 1.0 at barometric pressure 752 mmHg, routine blood gas variables (PaO₂ 587 mmHg, PaCO₂ 38 mmHg, SaO₂ 0.993, Hb 15.2 g/dL) were used to compute shunt by four approaches: the classic content equation, Chiang's arterial approximation, a P_AO₂-PaO₂ rule-of-thumb, and a simplified saturation method. Across methods, estimates clustered between ~2.8% and 5.2%, illustrating close agreement in health yet revealing how dissolved oxygen, estimation of Sv̄O₂, and assumed arterial-venous content shift results. The protocol and equations are presented as a teaching template for bedside calculation, with reporting of units, devices/supplies, and timing. We emphasize the disclosure of assumptions (RQ, nitrogen washout, capillary saturation), performing at least two methods when venous data are limited, and interpreting shunt results alongside clinical context. This practical, single-case tutorial supports respiratory care/internal medicine education and strengthens confidence in the 100% oxygen shunt test.