A Novel FiO2 Titration Protocol for Quantifying Pulmonary Oxygen Reserve Capacity : Dynamic Assessment Framework for Infection-Associated Respiratory Dysfunction

2BackgroundProgressive decline in pulmonary oxygen reserve capacity (ORC) is a hallmark of infection-associated respiratory dysfunction. Current tools (PaO2/FiO2 ratio, cardiopulmonary exercise testing [CPET], computed tomography [CT]) are limited in dynamic monitoring due to delayed responsiveness, operational complexity, or radiation risks, and other constraints. MethodsThe ORC testing methodology integrates the dynamic load-incrementation logic of cardiopulmonary exercise testing (CPET) with the oxygenation quantification framework of PaO2/FiO2. Its operational paradigm comprises three phases: {sscircled1}Testing Protocol Conducted under ventilation-locked conditions, a stepwise FiO2 titration protocol is applied, with termination triggered when SpO2 {sscircled2}Parameter Definition The minimum FiO2 required to maintain SpO2 [≥]90% (FiO2-MIN) is recorded, and the oxygen reserve capacity is calculated as ORC = 0.21 - FiO2-MIN. {sscircled3}Dynamic Modeling Through continuous monitoring throughout the entire disease course, ORC time-series data are acquired. A time-ORC curve is then fitted, and based on differential calculus ({beta} = {Delta}ORC/{Delta}t, {gamma} = {Delta}{beta}/{Delta}t), they collectively establish a quantitative respiratory compensation dynamics model in conjunction with the time-ORC curve. ResultsThe ORC test provides a novel non-invasive tool for dynamic quantification of respiratory reserve. Early warning of ARDS transformation during acute infection and quantitative dynamic tracking of lung dysfunction of long-COVID syndrome are its potential application scenarios. Its clinical utility requires prospective validation through multicenter trials integrated with CPET and CT quantitative analysis.