Left Heart Bypass

Left Heart Bypass: An Overview

Introduction Left Heart Bypass (LHB) is a surgical technique used to temporarily divert blood from the left side of the heart to maintain distal perfusion while allowing surgical intervention on the descending thoracic aorta or other structures. Unlike cardiopulmonary bypass (CPB), LHB does not require complete circulatory arrest, making it a preferred technique for select aortic and cardiovascular procedures (Estrera et al., 2018).

Indications for Left Heart Bypass LHB is primarily used in:

• Thoracic aortic aneurysm repair
• Descending aortic dissection surgery
• Aortic coarctation repair in adults
• Left ventricular assist device (LVAD) implantation
• Certain pulmonary and vascular surgeries requiring distal organ perfusion (Kazui et al., 2015).

Components of Left Heart Bypass System The LHB system includes:

1. Venous Drainage – Blood is typically drained from the left atrium or pulmonary veins using a left atrial cannula or a femoral venous cannula.
2. Pump Mechanism – A centrifugal pump is used to maintain flow (Kouchoukos et al., 2019).
3. Oxygenator (Optional) – Although LHB usually avoids oxygenators to reduce hemodilution, an oxygenator may be incorporated in cases of poor oxygenation.
4. Arterial Return – Blood is returned to the femoral artery, descending aorta, or left subclavian artery to maintain distal perfusion.
5. Heparinization and Anticoagulation Monitoring – Partial anticoagulation is maintained to prevent clotting without excessive bleeding risk.

Cannulation Strategy in Left Heart Bypass Cannulation sites are carefully selected based on surgical needs and patient-specific factors:

• Venous Cannulation: A left atrial cannula is commonly inserted through the left atrium via a pulmonary vein or directly into the left atrial appendage. Alternatively, femoral vein cannulation can be used in select cases.
• Arterial Cannulation: The most frequent sites include the femoral artery, descending thoracic aorta, or left subclavian artery. The choice depends on the requirement to maintain adequate distal perfusion while avoiding complications like embolism or ischemia.
• De-airing Measures: Proper de-airing techniques are critical to prevent air embolism, particularly when inserting or removing cannulas.
• Monitoring Cannulation Flow: Ensuring optimal blood flow through cannulation sites is crucial to prevent complications such as vessel injury, thrombosis, or inadequate perfusion (Crawford et al., 2016).

Cannula Sizing Based on Flow Requirements Appropriate cannula selection is essential to achieve the required flow rates:

• Venous Cannula: 24-28 Fr for a flow rate of 1.5-2.5 L/min 28-32 Fr for a flow rate of 2.5-3.5 L/min Larger cannulas (32-36 Fr) may be used in high-flow situations (Griepp et al., 2020).
• Arterial Cannula: 14-16 Fr for a flow rate of 1.5-2.5 L/min 16-18 Fr for a flow rate of 2.5-3.5 L/min 18-20 Fr for a flow rate above 3.5 L/min (Griepp et al., 2020).

Brain Perfusion Strategy in Left Heart Bypass Maintaining cerebral perfusion during LHB is critical to prevent neurological complications. Strategies include:

• Adequate Arterial Pressure: Maintaining a mean arterial pressure (MAP) of at least 60 mmHg ensures sufficient cerebral blood flow.
• Cannulation Considerations: Selecting the left subclavian artery or axillary artery for arterial return can improve cerebral perfusion.
• Perfusion Monitoring: Near-infrared spectroscopy (NIRS) and transcranial Doppler ultrasonography are used to assess cerebral oxygenation and blood flow.
• Temperature Management: Mild hypothermia (32-34°C) can provide neuroprotection by reducing metabolic demand.
• CO2 Control: Avoiding hypocapnia by maintaining normocapnia or mild hypercapnia helps prevent cerebral vasoconstriction and ensures adequate blood supply to the brain (Svensson et al., 2017).

Temperature Management in Left Heart Bypass Temperature control is essential in LHB to protect organs and optimize metabolic conditions:

• Mild Hypothermia (32-34°C): Provides neuroprotection by reducing cerebral metabolic demand.
• Normothermia (35-37°C): Preferred in procedures where hypothermia is unnecessary to maintain normal physiological function.
• Active Cooling and Rewarming: External cooling blankets or heat exchangers in the bypass circuit help regulate body temperature as needed.
• Organ-Specific Temperature Monitoring: Continuous temperature monitoring of the brain, core, and extremities ensures optimal management and reduces ischemic risk (Svensson et al., 2017).

Anticoagulation Protocol for Left Heart Bypass Anticoagulation is a crucial aspect of LHB, particularly when arterial return is directed to the left subclavian artery for cerebral perfusion:

• Initial Heparin Dose: 100-150 U/kg administered intravenously before cannulation.
• Activated Clotting Time (ACT) Monitoring: Target ACT is maintained between 180-250 seconds to balance bleeding risk and thrombotic protection.
• Heparin Reversal: Protamine sulfate is used post-procedure to reverse heparin effects as needed.
• Special Considerations for Cerebral Perfusion: Careful monitoring of anticoagulation is essential when the left subclavian artery is used for return to minimize the risk of cerebral embolism and ensure optimal perfusion (Svensson et al., 2017).

Flow Regulation in Left Heart Bypass Maintaining optimal blood flow during LHB is crucial for organ protection. Flow regulation depends on:

• Pump Speed: Centrifugal pumps are adjusted to provide adequate cardiac output while avoiding excessive afterload.
• Pressure Monitoring: Continuous arterial pressure monitoring ensures that perfusion pressures remain within safe limits (50-70 mmHg).
• Venous Drainage Control: Adjustments in venous cannula positioning and suction levels prevent over-drainage or insufficient return flow.
• Dynamic Flow Adjustments: Flow is adjusted based on metabolic demands, oxygen saturation, and end-organ perfusion assessments (Crawford et al., 2016).

Conclusion Left Heart Bypass is a valuable technique in cardiovascular and aortic surgery, offering a safer alternative to complete cardiopulmonary bypass in select procedures. Proper patient selection, circuit management, and hemodynamic monitoring are crucial for optimal outcomes.

References

• Estrera, A.L., et al. (2018). “Thoracic Aortic Surgery and Left Heart Bypass.” Annals of Thoracic Surgery.
• Kazui, T., et al. (2015). “Aortic Arch and Descending Aorta Surgery.” Journal of Cardiothoracic Surgery.
• Kouchoukos, N.T., et al. (2019). “Cardiovascular Surgery Principles.” Elsevier.
• Crawford, E.S., et al. (2016). “Techniques in Thoracic Aortic Repair.” Circulation.
• Svensson, L.G., et al. (2017). “Minimally Invasive Aortic Surgery and Organ Protection.” JACC Cardiovascular Interventions.
• Griepp, R.B., et al. (2020). “Surgical Techniques in Aortic Bypass and Cannulation.” European Journal of Cardiothoracic Surgery.