Low Venous Drainage After Aortic Cannula Repositioning
A 68-year-old man, 84 kg, undergoes elective coronary bypass on cardiopulmonary bypass (CPB). The circuit has a centrifugal pump, a hollow-fiber oxygenator, and a 3/8-inch venous line with a 29/37 French two-stage cannula placed via the right atrium. Target flow is 2.4 L/min/m². After aortic cross-clamping, the surgeon retracts the heart for a distal anastomosis. The perfusionist observes the venous reservoir level falling steadily, venous line pressure becoming progressively more negative, and pump flow dropping from 5.1 to 3.6 L/min despite the arterial pressure remaining near 65 mmHg. The patient had a central venous pressure of 6 mmHg before the maneuver. Sweep gas is unchanged, and arterial saturation is 99%.
1. Identify the most likely cause of the falling flow and describe the immediate sequence of corrective actions you would take before asking the surgeon to change anything.
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The pattern is inadequate venous drainage, not pump failure: reservoir level falls, venous pressure becomes more negative, and flow is limited by what returns to the pump. The most likely cause is mechanical obstruction of the venous cannula, such as the cannula abutting the atrial wall or being kinked during retraction, or a positional reduction in venous return. Immediate actions: (1) Confirm the problem is venous by checking the venous line for kinks, clamps, and connector obstruction, and verify the reservoir level trend. (2) Reduce pump flow to match venous return so the reservoir does not empty, and avoid chasing the target flow with a suction-starved circuit. (3) Ensure adequate anticoagulation and check the activated clotting time if not already documented. (4) Ask the surgeon to reposition the venous cannula or release retraction, since the timing of the change coincides with heart retraction. (5) Check that the patient is not hypovolemic by reviewing recent fluid administration and central venous pressure. Only after these steps, and if flow remains inadequate, consider adding vacuum-assisted venous drainage or a second venous cannula. Throughout, maintain mean arterial pressure and monitor for air entrainment at the venous line.
2. If the reservoir continues to fall despite these measures, what are the risks of simply increasing pump speed, and how would you decide whether to convert to a different drainage strategy?
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Increasing pump speed when venous return is limited does not create more drainage; it increases negative pressure in the venous line and the right atrium. The consequences include cavitation and hemolysis, air entrainment through cannula side holes or the purse-string site, and a risk of venous collapse that further reduces drainage. A sudden release of obstruction can then deliver a bolus of air or a surge of flow. The decision to change strategy should be based on whether the reservoir level can be maintained above a safe minimum, whether venous pressure remains excessively negative, and whether the patient is stable. If repositioning and volume administration do not restore adequate drainage, adding a second venous cannula or converting to vacuum-assisted drainage with a controlled negative pressure is reasonable. The perfusionist should communicate the trend to the surgeon, document the interventions, and keep flow matched to return rather than to the target number until drainage is corrected.