What is the process of cannulation in cardiac bypass surgery for adults with a history of cardiovascular disease?

Cannulation in Cardiac Bypass Surgery

Overview of Cannulation Process

Cannulation for cardiopulmonary bypass involves establishing arterial inflow and venous outflow connections to the CPB circuit, with the distal ascending aorta serving as the primary arterial site and central venous access providing blood return to the oxygenator. 1

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Arterial Cannulation Strategy

Primary Site and Preparation

• The distal ascending aorta is the most common arterial cannulation site, used in the majority of routine cardiac surgery cases 1
• Systolic blood pressure must be lowered below 100 mmHg before cannulation to reduce the risk of aortic dissection, which occurs in 0.06-0.23% of cases 1

Alternative Arterial Sites

The choice of alternative sites depends on specific clinical scenarios:

• Femoral artery: Primary choice for rapid emergency cannulation in cardiac arrest, aortic dissection, severe bleeding, or reoperative/minimally invasive surgery 1
• Axillary-subclavian artery: Increasingly preferred for aortic dissection cases because it permits antegrade flow in arch vessels, is typically free from atherosclerosis, and has rich collateral circulation 1
• Other sites (innominate artery, iliac artery, left ventricular apex): Rarely used 1

Critical Safety Measures for Stroke Prevention

Epiaortic ultrasonography (EAS) is the preferred screening method for detecting atherosclerosis before cannulation, as both manual palpation and transesophageal echocardiography provide inadequate views of the ascending aorta 1, 2

• EAS should be performed in patients over 50-60 years or those with history of transient ischemic attacks, strokes, peripheral arterial disease, or visible calcifications 1, 2
• EAS leads to cannula relocation and operative approach changes in 29% of CABG patients 1
• Porcelain aorta is found in 0.7-4.3% of cardiac surgery patients, requiring alternative cannulation strategies 1, 2

Mechanism of Stroke Risk

Dislodgement of atheromatous debris occurs through three mechanisms during arterial cannulation 1, 3:

• Direct manipulation of the aortic wall
• Cross-clamping trauma
• "Sand-blasting" effect from the cannula jet stream

Special cannulas designed to minimize jet effects should be used to reduce embolic complications 1

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Venous Cannulation Approaches

Central Venous Cannulation Options

Three primary techniques are available 1:

• Bicaval cannulation: Separate cannulas in superior and inferior vena cava
• Single atrial cannulation: Direct right atrial access
• Cavoatrial (2-stage) cannulation: Entry through right atrial appendage with cannula tip positioned in upper inferior vena cava

Cannula Selection Criteria

Cannula size must be selected based on 1:

• Patient size and weight
• Anticipated CPB flow rate (target >2.5 L/min/m² based on body surface area) 2
• Catheter flow characteristics and resistance
• Vessel diameter being cannulated

Technical Considerations

• Venous cannulas are made of flexible plastic, often reinforced to prevent kinking 1
• Cannulas may have bent or straight ends depending on anatomical requirements 1
• Negative pressure (vacuum-assisted drainage) can be applied using roller pumps, centrifugal pumps, or regulated vacuum on closed hard-shell reservoirs 1
• Vacuum assistance permits smaller-diameter cannulas and helps with long cannula lengths, but increases risk of air aspiration 1

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Special Consideration: Severe Aortic Regurgitation

Critical Pathophysiology

Severe aortic regurgitation creates a life-threatening situation during CPB initiation because regurgitant flow causes rapid ventricular distension, leading to subendocardial ischemia, potential ventricular rupture, inadequate systemic perfusion, and irreversible myocardial damage 2

Mandatory Venting Protocol

Immediate left ventricular venting must be established before or simultaneously with CPB flow initiation in severe aortic regurgitation 2

Venting options in order of preference 2:

1. Left ventricular vent through right superior pulmonary vein (most direct decompression)
2. Aortic root vent
3. Transapical left ventricular vent

Critical Pitfall to Avoid

Never initiate CPB flow in severe aortic regurgitation without functional left ventricular venting in place, as this causes immediate ventricular distension, myocardial ischemia, and inability to wean from bypass 2

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Preoperative Planning Requirements

Team Coordination

The perfusionist and surgeon must reach preoperative agreement on cannula sizes, types, and venting strategy specific to the patient's pathology (Class I recommendation, Level C evidence) 2

Flow Rate Planning

Anticipated CPB flow requirements should be calculated based on body surface area, targeting flow rates greater than 2.5 L/min/m² 2

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Common Pitfalls and How to Avoid Them

Embolic Complications

• Avoid femoral cannulation as first choice unless emergency access is needed, as retrograde flow may worsen cerebral embolic risk in patients with aortic atherosclerosis 2
• Do not rely on aortic root venting alone after cross-clamp application in aortic regurgitation, as the incompetent valve allows continued regurgitation until cardioplegia arrests the heart 2

Cannulation Site Selection

• Always perform EAS before cannulation in high-risk patients (age >50-60 years, vascular disease history) to detect atherosclerotic plaque and modify surgical approach 2, 3
• Consider alternative cannulation sites when EAS reveals significant ascending aortic disease 1

Equipment Issues

• Ensure proper cannula positioning to avoid kinking or partial occlusion against vessel walls, which significantly reduces flow rates 4
• Monitor for cannula malposition, which can cause dissection or gas embolism 4