ECMO and oxygen Delivery

Oxygen Delivery on ECMO

Oxygen delivery (DO₂) on ECMO is a critical factor in maintaining adequate tissue perfusion and preventing hypoxia. The efficiency of oxygenation depends on several factors related to circuit dynamics, patient physiology, and ECMO settings.

1. Determinants of Oxygen Delivery (DO₂)

Oxygen delivery on ECMO is influenced by:

1. Cardiac Output (CO) (in VA ECMO) or Native Lung Function (in VV ECMO)

2. Oxygen Content of Blood (CaO₂), which depends on:

• Hemoglobin concentration (Hb)

• Arterial oxygen saturation (SaO₂)

• Partial pressure of oxygen in arterial blood (PaO₂)

3. ECMO Flow Rate

• Higher flow rates provide greater oxygen delivery.

Formula for Oxygen Delivery (DO₂)

Where:

• CaO₂ (Arterial Oxygen Content) = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)

• Cardiac Output (CO) is patient-dependent in VA ECMO but less relevant in VV ECMO, where ECMO flow primarily determines DO₂.

2. Oxygen Delivery in VA vs. VV ECMO

A. VA ECMO (Veno-Arterial)

• DO₂ is dependent on both the ECMO circuit and native cardiac output.

• High ECMO flow can reduce pulmonary blood flow, limiting oxygenation through the lungs.

• North-South Syndrome (Harlequin Syndrome): Occurs when native cardiac output increases but poorly oxygenated blood from the lungs competes with highly oxygenated ECMO flow.

• Oxygen Saturation (SaO₂) Monitoring:

• Right upper extremity (pre-ductal) SaO₂ is the best indicator of cerebral oxygenation.

• Mixed venous oxygen saturation (SvO₂) helps assess systemic perfusion.

B. VV ECMO (Veno-Venous)

• Oxygenation depends almost entirely on ECMO flow and sweep gas settings.

• Higher ECMO flow increases oxygen saturation by replacing native venous blood with highly oxygenated blood.

• Recirculation (oxygenated blood returning to the ECMO circuit instead of systemic circulation) can reduce efficiency.

• Monitoring Oxygenation:

• Post-oxygenator blood gas (PaO₂ should be 150-450 mmHg)

• Pre-oxygenator venous blood gas (SvO₂ should be > 60%)

• Patient’s systemic arterial saturation (SpO₂ target > 88-92%)

3. Factors Affecting Oxygen Delivery

Circuit Factors

1. ECMO Flow Rate – Higher flow improves oxygenation but can reduce native lung contribution (in VA ECMO).

2. Sweep Gas Flow (VV ECMO) – Higher sweep gas flow increases CO₂ removal, indirectly improving oxygenation.

3. Membrane Oxygenator Function – A failing oxygenator reduces oxygen transfer efficiency.

Patient Factors

1. Hemoglobin (Hb) – Low Hb reduces oxygen-carrying capacity. Maintaining Hb ≥ 7-10 g/dL may be necessary in high oxygen demand states.

2. Cardiac Function (VA ECMO) – Poor cardiac function may limit systemic oxygen delivery even if ECMO flow is high.

3. Lung Function (VV ECMO) – Improved native lung function allows for weaning off ECMO support.

4. Optimizing Oxygen Delivery

• Increase ECMO Flow: Enhances oxygenation but may reduce native cardiac contribution (VA ECMO).

• Optimize Hemoglobin Levels: Transfuse PRBCs if needed to maintain adequate CaO₂.

• Adjust Sweep Gas Flow (VV ECMO): Controls CO₂ removal and indirectly impacts oxygenation.

• Optimize Native Lung Function (VV ECMO): Maintain lung recruitment to enhance oxygenation.

• Monitor Recirculation (VV ECMO): Adjust cannula positioning if needed.

Conclusion

Oxygen delivery on ECMO is a balance between circuit settings, patient physiology, and oxygen consumption. In VA ECMO, systemic oxygenation depends on ECMO flow and native cardiac output, while in VV ECMO, ECMO flow and recirculation determine oxygen delivery. Careful monitoring and adjustments are essential to optimize oxygenation and prevent complications.