In Vitro Evaluation of Membrane Oxygenation Performance of Artificial Lungs

In vitro evaluation of membrane oxygenation performance is an essential step in the design verification, quality control, and regulatory approval of artificial lungs (membrane oxygenators) used for ECMO, ECCO2R, and cardiopulmonary bypass. This article explains the testing samples, key performance parameters, standardized test methods, and typical applications of artificial lung oxygenation performance testing performed in accordance with ISO 7199 and ASTM standards.

Technical Overview

An artificial lung, commonly called a membrane oxygenator or blood-gas exchanger, is an extracorporeal device that replaces the respiratory gas exchange function of the natural lung. Blood flows over the exterior of hollow-fiber membranes—typically made of polymethylpentene (PMP) or polypropylene—while oxygen-rich sweep gas passes through the fiber lumens, allowing oxygen to diffuse into the blood and carbon dioxide to be removed. These devices are the functional core of extracorporeal membrane oxygenation (ECMO), extracorporeal carbon dioxide removal (ECCO2R), and cardiopulmonary bypass (CPB) systems.

Because the oxygenator directly contacts circulating blood and must sustain life-supporting gas exchange for hours to weeks, its performance must be rigorously characterized before preclinical and clinical use. In vitro oxygenation performance testing quantifies the device under controlled, reproducible hydraulic and hematological conditions in a mock extracorporeal circulation loop. The primary reference standards include ISO 7199 (cardiovascular implants and artificial organs—blood-gas exchangers), ASTM F1830 (selection of blood for in vitro evaluation), and ASTM F1841 (preparation and use of blood for in vitro evaluation of gas transfer in membrane oxygenators).

A complete in vitro evaluation typically addresses the following core questions:

• How much oxygen can the device transfer into venous blood at clinically relevant blood flows?
• How much carbon dioxide can the device remove under specified sweep gas conditions?
• What is the hydraulic resistance (pressure drop) and priming volume of the device?
• Does the device cause unacceptable blood trauma or activation during circulation?
• Does gas transfer performance remain stable over the intended duration of use?

Testing Samples

ECMO Membrane Oxygenators

Adult, pediatric, and neonatal hollow-fiber membrane oxygenators intended for long-term extracorporeal life support. These samples are evaluated over their full rated blood flow range, commonly from 0.5 L/min up to 7 L/min for adult devices, with endurance testing extending from several hours to several weeks depending on the intended clinical indication.

Cardiopulmonary Bypass Oxygenators

Oxygenators designed for short-term intraoperative support, frequently integrated with arterial filters, venous reservoirs, and heat exchangers. Testing covers both the gas exchange unit and the integrated heat exchange function.

ECCO2R and Respiratory Support Devices

Low-flow extracorporeal carbon dioxide removal devices, in which carbon dioxide transfer efficiency at reduced blood flows (typically 0.3 to 2.5 L/min) is the dominant performance criterion.

Prototype Modules and Membrane Bundles

Research-stage samples including membrane fiber bundles, 3D-printed housings, scaled-down modules, and novel membrane materials. These samples are tested to guide iterative design optimization before formal verification.

Testing Items

The scope of in vitro oxygenation performance testing generally includes the following items, each of which can be tailored to regulatory, developmental, or comparative study objectives:

• Oxygen transfer rate (mL O2/min) as a function of blood flow, sweep gas flow, and inlet venous conditions.
• Carbon dioxide removal rate (mL CO2/min) as a function of blood flow, sweep gas flow, and inlet PCO2.
• Rated flow determination, defined as the maximum blood flow at which the device achieves the specified outlet hemoglobin saturation (commonly 95 percent or higher) under reference venous inlet conditions.
• Pressure drop across the oxygenator over the operating flow range, evaluated with whole blood or an equivalent blood analogue.
• Priming volume, residual volume, and surface area confirmation.
• Hemolysis assessment, including plasma free hemoglobin kinetics, normalized index of hemolysis (NIH), and percentage change in plasma free hemoglobin over the test duration.
• Hemocompatibility markers such as platelet count, leukocyte count, thrombin-antithrombin complex (TAT), platelet factor 4 (PF4), complement fragments C3a and SC5b-9, and fibrinogen consumption.
• Heat exchanger performance, including thermal efficiency and heat transfer coefficient, for devices with integrated heat exchangers.
• Endurance and performance stability, documenting gas transfer decay, visible clot formation, fiber blockage, and pressure drop drift over the intended clinical use period.

Testing Methods

Test Circuit Configuration

Testing is performed in a closed mock circulation loop comprising a venous reservoir, a blood pump (roller or centrifugal), the oxygenator under test, an ancillary deoxygenator for establishing venous inlet conditions, a heat exchanger and temperature-controlled water bath for maintaining 37 ± 1 °C, ultrasonic or electromagnetic flow probes, pressure transducers upstream and downstream of the test device, and multiple sampling ports. Sweep gas (pure oxygen or a defined gas mixture) is delivered to the test oxygenator at controlled gas-to-blood flow ratios, typically from 1:1 to 3:1, with gas flow verified by calibrated mass flow controllers.

Test Fluid Preparation

For gas transfer testing, fresh whole blood—most commonly healthy donor bovine or porcine blood collected in anticoagulant citrate dextrose (ACD) solution—is used in accordance with ASTM F1830 and ASTM F1841. Typical target hematological conditions are hemoglobin of at least 12 g/dL, hematocrit of 35 to 40 percent, pH of 7.35 to 7.45, and temperature of 37 °C. Blood is conditioned with a nitrogen and carbon dioxide gas mixture across the ancillary deoxygenator to simulate mixed venous conditions: inlet hemoglobin oxygen saturation of 65 ± 5 percent and inlet PCO2 of 45 ± 5 mmHg. For purely hydraulic measurements such as pressure drop and flow visualization, glycerol-water blood analogues with matched viscosity and density may be substituted.

Gas Transfer Measurement Procedure

At each prescribed blood flow rate, the loop is operated until all monitored parameters (inlet saturation, inlet PCO2, temperature, and pressure drop) reach steady state. Paired blood samples are then drawn simultaneously from the inlet and outlet sampling ports and analyzed immediately on a calibrated blood gas analyzer and co-oximeter for pH, PO2, PCO2, hemoglobin, and oxygen saturation. Triplicate sampling is standard to support statistical reporting as mean ± standard deviation. The procedure is repeated across the full flow range and at multiple sweep gas flow rates to construct complete performance curves.

Data Reduction and Calculations

Oxygen transfer is calculated from the difference in blood oxygen content between outlet and inlet, multiplied by blood flow, using the standard oxygen content equation:

CaO2 = (1.34 × Hb × SaO2) + (0.0031 × PaO2), reported in mL O2/dL of blood.

The oxygen transfer rate is then obtained as VO2 = Q × (CaO2,outlet − CaO2,inlet) × 10, where Q is blood flow in L/min and the result is expressed in mL O2/min.

Carbon dioxide content is derived from pH, PCO2, and hemoglobin using established algorithms such as the Douglas equation, and carbon dioxide removal is computed analogously as VCO2 = Q × (CCO2,inlet − CCO2,outlet) × 10, expressed in mL CO2/min.

Hemolysis is quantified by measuring plasma free hemoglobin at fixed intervals and computing the normalized index of hemolysis, expressed as grams of free hemoglobin released per 100 liters of blood pumped, normalized to hematocrit, flow rate, and time.

Acceptance and Reporting

Results are compared against the manufacturer's performance claims and the requirements of ISO 7199, which defines minimum gas transfer performance, hemolysis limits, pressure drop limits, and labeling requirements for blood-gas exchangers. A full test report includes the blood source and hematological baseline, circuit schematic, calibration records for all sensors, raw blood gas data, calculated transfer rates with uncertainty, pressure drop curves, hemocompatibility time courses, and photographic documentation of the device after testing.

Applications

In vitro oxygenation performance testing supports the entire product lifecycle of artificial lung devices, from early research to post-market surveillance:

• Design verification and regulatory submission for market approval in major jurisdictions, including NMPA registration in China, FDA 510(k) or PMA pathways in the United States, and CE marking under the European Medical Device Regulation.
• Research and development optimization of fiber packing density, manifold geometry, blood flow distribution, and membrane surface area to maximize gas transfer while minimizing hemolysis and priming volume.
• Comparative benchmarking of commercially available oxygenators to support hospital procurement decisions and published head-to-head performance studies.
• Batch quality control and release testing for manufacturers, including verification of rated flow and pressure drop on production samples.
• Durability and failure-mode studies, such as evaluation of gas transfer decay during extended circulation, assessment of clot formation under procoagulant conditions, and screening of novel antithrombotic surface coatings.
• Academic research in extracorporeal life support, providing reproducible data for computational model validation, scale-down pediatric device development, and next-generation technologies including implantable and biohybrid artificial lungs.

FAQ

Why must venous blood conditions be simulated during testing?

Oxygenators operate in vivo on partially deoxygenated mixed venous blood. Gas transfer depends directly on the inlet saturation and PCO2 of the blood, so meaningful oxygen transfer data can only be obtained when the test blood is conditioned to simulated venous values—typically 65 ± 5 percent saturation and 45 ± 5 mmHg PCO2—before it enters the device under test.

Which standards govern in vitro oxygenation performance testing?

The principal standards are ISO 7199 for blood-gas exchangers, ASTM F1830 for the selection of blood used in in vitro evaluations, and ASTM F1841 for the preparation and use of blood in gas transfer evaluation of membrane oxygenators. Together they define test fluid quality, hematological targets, circuit requirements, and reporting criteria.

Can blood substitutes replace real blood for gas transfer testing?

No. Glycerol-water analogues are useful for hydraulic characterization such as pressure drop and flow visualization because they match viscosity and density, but they cannot transport oxygen bound to hemoglobin. Gas transfer and hemocompatibility evaluations therefore require fresh whole blood with physiologically relevant hemoglobin concentration and oxygen affinity.

What is the rated flow of an oxygenator?

Rated flow is the maximum blood flow at which the device can still raise the simulated venous blood to the specified outlet hemoglobin saturation—commonly 95 percent or higher—under standardized inlet conditions. It is the single most important summary metric of oxygen transfer capacity and appears on the device label.

How long does a typical evaluation take?

A full flow-curve gas transfer characterization with hemocompatibility sampling is generally completed within one test day using freshly collected blood, while endurance evaluations required for long-term ECMO devices may run continuously for 24 hours to several weeks, depending on the intended clinical duration claimed by the manufacturer.

Do oxygen transfer and carbon dioxide removal depend on the same operating variables?

Not equally. Oxygen transfer is primarily limited by blood flow and membrane surface area, because hemoglobin binding dominates oxygen carriage. Carbon dioxide removal is far more sensitive to sweep gas flow rate, because CO2 diffuses rapidly across the membrane and is removed efficiently once the gas side is flushed, which is the principle exploited by low-flow ECCO2R therapy.