Principles of Left Heart Bypass (Left Atrial-to-Femoral Bypass)

Definition and Rationale

"The application of vascular clamps to this major vessel acutely increases the afterload of the heart and produces global ischemia in all parts of the body distal to the clamp." - Miller's Anesthesia, 10e

Circuit Description

Components

Why No Oxygenator in Simple LHB?

Physiological Goals

1. Proximal arterial pressure (above clamp): Controlled to prevent left ventricular overload, hypertension, and myocardial strain
2. Distal arterial pressure (below clamp): Maintained above 60 mmHg to perfuse the spinal cord, kidneys, and viscera

"The management goal is to keep arterial blood pressure higher than 60 mm Hg both proximal and distal to the aortic cross-clamp throughout the procedure." - Miller's Anesthesia, 10e

Mechanism of Cardiac Unloading

Anticoagulation

• Standard LHB uses heparin-bonded (coated) tubing - full-dose systemic heparin is not required
• Typical heparin dose: 100 units/kg (rather than the 300-400 units/kg used for full CPB)
• ACT targets: 180-220 seconds (compared to >400 seconds for CPB)
• This reduced anticoagulation is important in trauma patients and those at bleeding risk

Circuit Variants

1. Simple LHB

• Left atrium → centrifugal pump → femoral artery
• No oxygenator, no reservoir
• Suitable for most elective DTAA/TAAA repairs

2. Complete LHB (Complex Circuit)

• Adds: oxygenator + heat exchanger + reservoir
• Indicated when the patient also has:
  • Hypoxia (single-lung ventilation failure)
  • Hypothermia risk
  • Anticipated massive hemorrhage
• The reservoir is positioned out of the main blood pathway (unlike CPB), reducing the effective extracorporeal surface area and minimizing contact activation of blood

"The major difference between a complete LHB circuit and the standard CPB circuit is the position of the reservoir in the circuit." - Miller's Anesthesia, 10e

Operative Management During LHB

Hemodynamic Management

1. Initial pump flow is set to approximately 50% of cardiac output when the proximal clamp is applied
2. Flow is then titrated to achieve target proximal and distal pressures
3. Vasodilators are rarely needed at initial clamping
4. Careful coordination between surgeon, anesthesiologist, and perfusionist is mandatory

Sequential Aortic Clamping

• As repair proceeds from proximal to distal, the clamp is moved stepwise down the graft
• This restores perfusion to each newly anastomosed branch (intercostal/lumbar arteries) progressively
• Intercostal artery reimplantation can be done with minimal pump flow adjustments

Visceral and Renal Phase

• During reimplantation of visceral and renal vessels, pump flow is reduced significantly
• Distal perfusion is only to the lower limbs during this phase
• Balloon perfusion cannulas from the LHB circuit can deliver blood directly into the celiac axis and superior mesenteric artery orifices during reattachment - reducing bowel and hepatic ischemia

Protection of the Spinal Cord (Key Exam Topic)

For All Extents of TAAA Repair:

• Permissive mild hypothermia (32-34°C nasopharyngeal)
• Moderate heparinization (1 mg/kg)
• Aggressive reattachment of segmental arteries, especially T8-L1 (region of the artery of Adamkiewicz)
• Sequential aortic clamping when possible
• Perfusion of renal arteries with cold 4°C crystalloid

For Crawford Extent I and II (Highest Risk):

• Cerebrospinal fluid (CSF) drainage - reduces CSF pressure, improves spinal perfusion pressure (Class I, Level B evidence from RCT; reduced paraplegia from 13% to 2.6%)
• Left heart bypass during proximal anastomosis
• Selective perfusion of celiac axis and SMA during intercostal/visceral anastomoses
• Motor evoked potential (MEP) monitoring

Comparison: LHB vs. Clamp-and-Sew vs. Full CPB

Indications

• Repair of descending thoracic aortic aneurysm (DTAA)
• Repair of thoracoabdominal aortic aneurysm (TAAA) - all extents when cardiac reserve permits
• Open repair of acute aortic dissection (Type B / DeBakey III)
• Traumatic aortic injury (open repair)
• Patients with poor cardiac reserve - LHB unloads the LV which is particularly beneficial

Monitoring Requirements

• Two arterial lines: one in the right radial artery (above clamp) and one in the femoral artery (below clamp) - to continuously monitor both proximal and distal pressures
• Neuromonitoring: MEPs and/or SSEPs for spinal cord function
• NIRS cerebral and flank saturation probes

Complications Specific to LHB

Summary

Principles of Left Heart Bypass (Left Atrial-to-Femoral Bypass)

Complete 10-Mark Answer

1. Definition and Context

2. The Problem It Solves: Effects of Aortic Cross-Clamping

3. Circuit Setup and Components

4. Anticoagulation

• Full CPB requires: heparin 300-400 U/kg, ACT >400 seconds
• LHB requires: heparin 100 U/kg, ACT 180-220 seconds
• Heparin-bonded tubing reduces the extracorporeal surface area that activates the clotting cascade, allowing this reduced dose
• In trauma cases, the surgeon may choose to proceed without heparinization

5. Physiological Principles: Cardiac Unloading

6. Hemodynamic Management During LHB

• Mandatory: two arterial lines - one right radial (proximal, above clamp) and one femoral (distal, below clamp)
• Target distal mean pressure: >60 mmHg throughout clamping
• Initial pump flow: ~50% of cardiac output at time of clamp application, then titrated
• Vasodilators are rarely needed at onset of clamping
• Continuous coordination between surgeon, anesthesiologist, and perfusionist is essential

• Increasing pump flow → lowers proximal pressure, raises distal pressure
• Decreasing pump flow → raises proximal pressure, lowers distal pressure

7. Surgical Conduct: Sequential Clamping Strategy

1. Proximal anastomosis phase: Clamp applied; LHB at ~50% CO; surgeon performs proximal graft anastomosis
2. Intercostal reimplantation phase: Clamp moved sequentially downward as repair progresses; each segment reperfused progressively; intercostal arteries reimplanted with minimal flow adjustments
3. Visceral/renal anastomosis phase: Pump flow is reduced significantly (distal perfusion now only to lower limbs). Balloon perfusion cannulas from the LHB circuit are placed directly into the celiac axis and superior mesenteric artery orifices to deliver oxygenated blood during their reattachment - reducing hepatic and bowel ischemia
4. Renal protection: Kidneys perfused with cold 4°C crystalloid via separate catheters placed into renal orifices (independently confirmed by RCT to preserve renal function)
5. Completion: Distal anastomosis completed, pump flow increased, patient actively warmed to 37°C

8. Spinal Cord Protection - The Critical Complication

Incidence of Paraplegia (without adjuncts):

• Coarctation repair: 0.5-1.5%
• DTAA repair: 0-10%
• TAAA repair: 10-20%
• Extensive (Crawford Extent II) dissecting TAAA: up to 40%
• With adjuncts (including LHB): reduced to ~7.2%

Anatomy of Spinal Cord Blood Supply:

• Anterior spinal artery (supplies 75% - motor tracts): formed from vertebral arteries, fed by anterior radicular arteries including the artery of Adamkiewicz (great anterior radicular artery)
• Artery of Adamkiewicz - usually branches from the aorta at T9-T12 (range T7-L2), predominantly on the left side - this is the most vulnerable vessel
• Posterior spinal arteries (25% - sensory tracts): from posterior cerebellar and vertebral arteries

How LHB Protects the Spinal Cord:

• Maintains perfusion pressure to the collateral network (intercostal/lumbar arteries feeding the anterior spinal artery) while segmental perfusion is interrupted
• Aortic cross-clamping raises CSF pressure by 10-15 mmHg, compressing spinal cord perfusion pressure
• Spinal cord perfusion pressure = Distal mean aortic pressure - CSF pressure (or CVP, whichever is greater)
• LHB keeps distal aortic pressure >60 mmHg, directly protecting this equation

Reimplantation of Intercostal Arteries:

• Ligation of T9-T12 intercostals significantly increases neurologic deficit risk
• Patent intercostals in this region should be reimplanted as one large patch, separate buttons, or with interposition grafts
• Decision guided by intraoperative MEP loss and/or preoperative identification of the artery of Adamkiewicz on CT/MRA or DSA angiography

9. Multi-Modal Organ Protection Strategy (Schwartz's, 11e)

For ALL TAAA Extents:

• Permissive mild hypothermia (32-34°C nasopharyngeal)
• Moderate heparinization (1 mg/kg)
• Aggressive reattachment of segmental arteries, especially T8-L1
• Sequential aortic clamping when possible
• Cold (4°C) crystalloid renal perfusion

Additional for Crawford Extent I and II (Highest Risk):

• CSF drainage - target CSF pressure ≤10 mmHg
  • Prospective RCT (Coselli et al.): paraplegia rate reduced from 13% to 2.6% (80% relative risk reduction)
  • Meta-analysis shows 10% absolute risk reduction
• Left heart bypass during proximal anastomosis
• Selective celiac/SMA perfusion via balloon cannulas during visceral anastomoses
• MEP monitoring intraoperatively

10. Simple vs. Complete LHB vs. Full CPB

11. Complications of LHB

12. Indications and Surgical Approach

• DTAA repair (especially if cross-clamp time >30 minutes)
• TAAA repair (Crawford Extent I-IV)
• Type B / DeBakey III aortic dissection - open repair
• Traumatic aortic transection (open repair)
• Poor cardiac reserve (LHB's LV unloading is especially valuable)

• Left posterolateral thoracotomy (level depends on extent: 4th-5th interspace for extent I-II; 6th-7th for extent III; 8th for extent IV)
• Double-lumen ETT or bronchial blocker for single-lung ventilation
• Patient in right lateral decubitus, pelvis rotated 30° for femoral access
• Diaphragm divided circumferentially to preserve phrenic nerve branches

Summary (Exam Closing Points)