The Role of Sweep Gas in ECMO
Sweep gas flow through the membrane oxygenator is the primary determinant of carbon dioxide removal in ECMO, while having minimal impact on oxygenation. 1, 2
Primary Function: CO2 Removal
Sweep gas (typically oxygen or air) flows through the membrane oxygenator on the opposite side of blood, creating a concentration gradient that drives CO2 elimination from the blood. 1
The rate of sweep gas flow is the main determinant of decarboxylation, independent of blood flow rate or FiO2 settings in the circuit. 2
In venovenous ECMO, PaCO2 remains stable even when ECMO blood flow is reduced to <2.5 L/min or FiO2 is decreased to 40%, as long as sweep gas flow is maintained. 2
Clinical Management of Sweep Gas
Titration Strategy
Sweep gas flow should be adjusted based on arterial blood gas measurements to target a PaCO2 between 35-45 mmHg, while avoiding rapid changes (>20 mmHg within 24 hours). 3, 1
Regulating sweep gas flow to achieve normal or slightly alkalotic pH is common practice following ECPR, particularly when patients present with combined respiratory and metabolic acidosis. 3
Critical Pitfall: Avoiding Rapid CO2 Correction
A large drop in PaCO2 (>20 mmHg) within 24 hours of cannulation is associated with acute brain injury, intracranial hemorrhage, and poorer survival in ECPR patients. 3
Mild hypercarbia in the peri-cannulation period may be protective by potentiating cerebral vasodilation and increased blood flow, reducing serum biomarkers of acute brain injury. 3
Moderate-to-high hypercarbia should be avoided as it may increase intracranial pressure through excessive vasodilation, which is catastrophic in patients with existing brain injury. 3
Sweep Gas Impact by ECMO Type
VV-ECMO and ECCO2R
In ECCO2R systems operating at low blood flow rates (200-1,500 mL/min), sweep gas provides adequate CO2 removal but allows only minimal blood oxygenation. 3, 1
The influence of sweep gas on CO2 removal capacity depends predominantly on blood flow rate and membrane lung surface area—considerable CO2 removal requires membrane surface ≥0.8m² and blood flow ≥900 mL/min. 4
Increasing sweep gas from 2 to 8 L/min at 900 mL/min blood flow increases normalized CO2 elimination from 35±5 to 41±6 mL/min with smaller membranes (0.4m²), but the effect plateaus at 4 L/min with lower blood flows. 4
VA-ECMO Considerations
In VA-ECMO, sweep gas flow remains the primary determinant of patient PaCO2, while oxygenation is determined by ECMO blood flow and FiO2 settings. 5
Mechanical ventilation should be maintained during VA-ECMO to prevent Harlequin syndrome, with FiO2 titrated to maintain arterial O2 saturation >92%. 3
Distinction from Oxygenation
Blood oxygenation in ECMO is determined by ECMO blood flow rate and FiO2 in the circuit, NOT by sweep gas flow. 2
An ECMO flow/cardiac output ratio >60% consistently achieves adequate blood oxygenation (SaO2 >90%) in VV-ECMO patients. 2
Decreasing ECMO blood flow from maximum (5.8±0.8 L/min) to 40% less (2.4±0.3 L/min) significantly decreases PaO2 (88±24 to 45±9 mmHg) and SaO2 (97±2% to 82±10%), while PaCO2 remains unaffected if sweep gas is maintained. 2