Flow Physiology of Extracorporeal Circulation

Introduction

Extracorporeal circulation (ECC), used in cardiopulmonary bypass (CPB) and extracorporeal membrane oxygenation (ECMO), serves as a temporary replacement for the heart and lungs. It maintains tissue perfusion and gas exchange, making it critical in cardiac surgery and critical care. Understanding flow physiology helps optimize hemodynamics, organ perfusion, and patient outcomes (Gravlee, 2019).

1. Principles of Flow in Extracorporeal Circulation

Blood flow in extracorporeal systems follows Poiseuille’s Law, which states that flow is directly proportional to the pressure gradient and inversely proportional to resistance:

Where:

• Q = Flow rate
• ΔP = Pressure gradient
• r = Radius of tubing
• η = Blood viscosity
• L = Circuit length

In ECC, flow can be pulsatile or non-pulsatile, each with unique physiological effects:

1. Pulsatile Flow – Mimics the natural cardiac cycle, improving microcirculation and organ perfusion (Undar, 2019).
2. Non-Pulsatile Flow – Continuous, steady flow used in CPB, altering vascular reactivity and regional blood distribution (Lorusso et al., 2021).

2. Determinants of Flow in ECC

1. Pump Type and Flow Regulation

• Roller Pumps: Generate constant non-pulsatile flow, but may cause hemolysis and tubing wear (Gravlee, 2019).
• Centrifugal Pumps: Create flow via a pressure gradient, minimizing blood trauma but requiring preload-dependent operation (Ramanathan et al., 2020).

2. Cannulation and Circuit Resistance

• Arterial Cannulation: Affects systemic afterload and perfusion pressure. Narrower cannulas increase resistance, requiring higher pump pressures (Fina et al., 2022).
• Venous Drainage: Dependent on preload, cannula size, and positioning. Vacuum-assisted venous return (VAVR) optimizes drainage in ECMO and CPB (Lorusso et al., 2021).

3. Blood Viscosity and Temperature Effects

• Hemodilution (low hematocrit) reduces viscosity, improving flow but increasing the risk of tissue hypoxia (Gomez et al., 2019).
• Hypothermia increases viscosity, reducing capillary perfusion and leading to microcirculatory dysfunction (Fina et al., 2022).

3. Organ-Specific Flow Considerations

ECC affects perfusion to various organs differently:

• Brain Perfusion: Maintained through cerebral autoregulation, but non-pulsatile flow may increase the risk of cognitive dysfunction post-CPB (Lorusso et al., 2021).
• Renal Perfusion: Kidneys are sensitive to hypoperfusion; maintaining MAP > 65 mmHg is crucial for preventing acute kidney injury (AKI) (Gravlee, 2019).
• Pulmonary Circulation: Typically bypassed in VA-ECMO, but excessive circuit flow can overload the lungs, leading to pulmonary edema (Ramanathan et al., 2020).

4. Flow Monitoring and Optimization in ECC

Key Parameters for Perfusion Safety

• Cardiac Index (CI): Target >2.2 L/min/m2 for adequate organ perfusion (Undar, 2019).
• Mean Arterial Pressure (MAP): Maintain between 65-80 mmHg to prevent ischemia (Fina et al., 2022).
• Venous Oxygen Saturation (SvO?): Should remain >65%, indicating sufficient oxygen delivery (Lorusso et al., 2021).

Strategies to Optimize Flow

• Use of Pulsatile Flow: Improves microcirculatory perfusion and organ function (Undar, 2019).
• Minimizing Hemodilution: Maintains adequate oxygen-carrying capacity (Gomez et al., 2019).
• Temperature Management: Avoid excessive hypothermia to prevent viscosity-related hypoperfusion (Fina et al., 2022).

Conclusion

Extracorporeal circulation requires precise flow regulation to ensure adequate organ perfusion and oxygenation. By understanding the determinants of flow, pump selection, and perfusion monitoring, clinicians can optimize patient outcomes and minimize complications.

References

1. Gravlee, G. P. (2019). Cardiopulmonary Bypass: Principles and Practice (5th Ed.). Lippincott Williams & Wilkins.
2. Undar, A. (2019). "Pulsatile versus Non-Pulsatile Perfusion in ECC: Effects on Microcirculation and Organ Function." Annals of Thoracic Surgery, 108(5), 1421-1430.
3. Lorusso, R., et al. (2021). "Physiological Considerations of Non-Pulsatile Flow in ECMO and CPB." Perfusion Journal, 36(4), 310-322.
4. Ramanathan, K., et al. (2020). "ECMO: Physiology, Technology, and Clinical Applications." Critical Care Medicine, 48(7), 101-115.
5. Fina, D., et al. (2022). "Impact of Hemodilution on Perfusion and Organ Function During ECC." Journal of Cardiothoracic and Vascular Anesthesia, 36(1), 45-56.
6. Gomez, A., et al. (2019). "Blood Viscosity, Hematocrit, and Perfusion in Extracorporeal Circulation." Journal of Extra-Corporeal Technology, 51(2), 112-120.