Absolute Versus Membrane-Indexed Oxygen Transfer in Hollow-Fiber Membrane Oxygenators During Cardiopulmonary Bypass: A Systematic Review and Meta-Analysis

Cardiopulmonary bypass (CPB) depends on membrane oxygenators to maintain gas exchange while the heart and lungs are excluded from the circulation. Contemporary adult oxygenators use microporous hollow fibers, and their clinical performance is usually judged by their ability to transfer oxygen efficiently and consistently. Most clinical and technical comparisons report absolute oxygen transfer VO₂, (mL/min), which reflects the total amount of oxygen delivered across the membrane. Absolute transfer alone, however, does not describe how efficiently a device performs relative to its structural characteristics, particularly its membrane surface area. A larger oxygenator may transfer more oxygen simply because it contains more membrane, whereas a smaller oxygenator may achieve comparable transfer with less membrane. Normalizing oxygen transfer to membrane surface area (mL/min/m²) has therefore been proposed as a complementary efficiency metric: absolute oxygen transfer reflects capacity, while membrane-indexed transfer describes performance per unit of membrane. Manufacturer data are generated under standardized in vitro conditions, whereas intraoperative performance is influenced by individual metabolic demans, blood flow, hemoglobin concentration, venous saturation, sweep gas settings, temperature, pressure drop and device design. The two metrics are linked arithmetically, and this has consequences for interpretation. During steady-state clinical CPB, the oxygen transferred across the membrane matches the patient’s oxygen consumption, which is set by body size, temperature and anesthetic depth rather than by the device, as long as the oxygenator operates below its maximal capacity. If absolute VO₂ is similar between two devices, dividing by membrane area will necessarily yield a higher indexed value for the smaller device. Whether the two metrics lead to different conclusions, and why, is therefore a question that can be answered directly from published data. The aim of this systematic review and meta-analysis was to determine whether comparisons of oxygen transfer between hollow-fiber membrane oxygenators used in adult CPB reach different conclusions when expressed as absolute VO₂ versus VO₂ indexed to membrane surface area.