Left Heart Bypass (Left Atrio-Femoral Bypass)

Why It Is Needed

1. Proximal hypertension / increased LV afterload - the left ventricle suddenly ejects against a closed circuit, dramatically increasing wall stress and myocardial oxygen demand.
2. Distal ischemia - all organs below the clamp (spinal cord, kidneys, viscera, lower limbs) are cut off from perfusion.

• Miller's Anesthesia, 10e - "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."

Circuit Anatomy and Flow Direction

Left atrial-femoral bypass circuit. Left atrium and left femoral artery are cannulated; a centrifugal pump drives flow, with a heat exchanger in-line.

Simple vs. Complete LHB Circuit

FIG: (A) Simple LHB - left atrium, centrifugal pump, femoral artery. (B) Complex LHB - adds oxygenator, heat exchanger, and reservoir.

• Left atrium → centrifugal pump → femoral artery
• No oxygenator needed (lungs still functioning)
• Reduced extracorporeal surface area = less contact activation, less heparin required

• Adds an oxygenator (for patients with single-lung ventilation-related hypoxia or respiratory compromise)
• Adds a reservoir (for rapid fluid/blood product administration in haemorrhage)
• Adds a heat exchanger (active temperature control)
• Required for patients with pre-existing lung dysfunction or traumatic lung injury

• Miller's Anesthesia, 10e - "The major difference between a complete LHB circuit and the standard CPB circuit is the position of the reservoir in the circuit... With the reservoir out of the pathway of the blood, the effective surface area of the ECC is reduced; this reduction attenuates contact activation of the blood and minimizes the need for high-dose heparinization."

Anticoagulation

• The circuit uses heparin-coated tubing
• Reduced extracorporeal surface area minimises thrombus risk

Variations of the Inflow Cannulation Site

• Miller's Anesthesia, 10e - "With left atrial cannulation, the left ventricle is relieved of preload and cardiac output is reduced. Either way, proximal hypertension is controlled, the work of the ventricle is decreased, and perfusion is provided to the distal aorta."

Haemodynamic Management Goals

• Initial pump flow is typically set to ~50% of the patient's cardiac output at the time of proximal clamp application
• Flow is then titrated continuously to hit target proximal and distal pressures
• Vasodilators are rarely required at initiation
• Communication between surgeon, anaesthesiologist, and perfusionist is mandatory

Key Physiological Benefits

Organ Protection Adjuncts Used Alongside LHB

• Cerebrospinal fluid (CSF) drainage - reduces intraspinal pressure, improving spinal cord perfusion pressure (Class I, Level B in current guidelines; RCT showed paraplegia reduction from 13% to 2.6%)
• LHB during proximal anastomosis - ensures distal perfusion while the aorta is clamped proximally
• Selective visceral perfusion - balloon catheters from the LHB circuit directed into celiac and SMA orifices
• Cold renal perfusion (4°C crystalloid, ± methylprednisolone) - reduces renal ischaemic injury
• Mild permissive hypothermia (32-34°C nasopharyngeal) - slows metabolic rate of vulnerable organs
• Motor evoked potential (MEP) monitoring - guides decision to reimplant intercostal arteries (T8-L1 most important)
• Sequential aortic clamping - clamp moved progressively distally as each anastomosis is completed, restoring flow segment by segment

Surgical Approach

• Left thoracotomy provides the operative exposure; the left lung is deflated using a double-lumen endotracheal tube or bronchial blocker to expose the left atrium and thoracic aorta
• The left inferior pulmonary vein or left atrial appendage is the usual cannulation site for inflow
• The left femoral artery is exposed for return cannulation
• Moderate hypothermia (32°C) is maintained throughout bypass
• After distal anastomosis completion, pump flow is increased and the patient actively warmed to 37°C before weaning

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

• Current Surgical Therapy, 14e - "Two surgical techniques that are commonly utilized to repair DTAA and TAAAs are the 'clamp-and-sew' technique versus the use of distal aortic perfusion through a left heart bypass circuit. Each technique has been found to be efficacious and is often surgeon- and centre-dependent."

Clinical Notes and Complications

• Atrial fibrillation - occurs in ~15% of patients when hypothermia (30°C) is combined with left atrial cannulation; most revert to sinus rhythm on rewarming, but cardioversion may be required
• Single-lung ventilation hazards - patients with pre-existing lung disease or traumatic injury may desaturate; the complete LHB circuit (with oxygenator) is then preferred
• Blood loss - these are high-risk cases for haemorrhage; the complete circuit reservoir allows rapid resuscitation
• Hypothermia - large left thoracotomy exposure over prolonged procedures promotes heat loss; active warming via circuit heat exchanger is important
• Endovascular era - widespread adoption of TEVAR (thoracic endovascular aortic repair) has significantly reduced the use of open LHB procedures in contemporary practice

• Miller's Anesthesia, 10e, Chapters 50 and 52 (Left Heart Bypass; Descending TAA repair)
• Schwartz's Principles of Surgery, 11e, Chapter on thoracoabdominal aortic aneurysms (Current strategy for spinal cord and visceral protection)
• Current Surgical Therapy, 14e, Strategies for thoracic and thoracoabdominal aortic reconstruction