Rising Oxygenator Pressure Drop and Falling Oxygen Transfer
A 45-year-old man, 100 kg, is on CPB for a complex redo operation. After four hours, the oxygenator pressure drop has risen from 40 to 90 mmHg at a flow of 6 L/min. The arterial oxygen saturation from the oxygenator has fallen from 99% to 93% despite an FiO2 of 1.0, and the sweep gas has been increased to 6 L/min. The venous saturation is 60%, and the hematocrit is 26%. The activated clotting time is above 480 seconds. The patient is being rewarmed, and the nasopharyngeal temperature is rising from 32°C to 36°C.
1. What are the likely causes of the rising pressure drop and falling oxygen transfer, and how would you evaluate them?
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A rising pressure drop across the oxygenator with falling oxygen transfer suggests progressive obstruction or deterioration of the oxygenator. Causes include clot formation in the fibers, fibrin deposition, or water condensation and plasma leakage. The long duration, the high flow, and the rewarming phase are risk factors. Evaluate by checking the anticoagulation status, inspecting the oxygenator for visible clot or discoloration, reviewing the pressure drop trend, and assessing the gas exchange. A falling arterial saturation despite maximal FiO2 and high sweep gas indicates a true oxygen transfer problem rather than a sweep gas setting issue. Also consider that rewarming increases oxygen consumption and that the venous saturation of 60% is acceptable but the arterial saturation is not. Confirm the blood gas analyzer is functioning and that the sample is from the post-oxygenator port.
2. At what point would you decide to change the oxygenator, and how would you plan the change?
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The decision to change the oxygenator should be based on the trend and the patient's stability, not a single number. If the pressure drop continues to rise, if arterial saturation cannot be maintained despite maximal FiO2 and sweep gas, or if there is evidence of clot or plasma leakage, change the oxygenator. Plan the change by ensuring adequate anticoagulation, preparing a replacement oxygenator and circuit, and coordinating with the surgeon. Notify the team, ensure the patient is stable, and have blood products available. During the change, maintain flow and oxygenation as much as possible, and monitor for air embolism and hypotension. After the change, reassess gas exchange and pressure drop, and document the event. The key judgment is to act before gas exchange fails completely rather than waiting for a crisis.