Coronary Artery Bypass Grafting (CABG) - A Comprehensive Review

1. Introduction and Historical Background

2. Coronary Anatomy Relevant to CABG

Left Coronary System

• The left main coronary artery (LMCA) arises from the aortic root. Left main stem disease carries the worst prognosis without surgery.
• The LMCA bifurcates into:
  • Left anterior descending (LAD) artery - the most frequently diseased and most often bypassed vessel
  • Left circumflex (LCx) artery, giving off obtuse marginal (OM) branches

Right Coronary System

• The right coronary artery (RCA) runs in the atrioventricular groove to the posterior interventricular septum
• Common stenosis sites: proximal RCA or at the crux (bifurcation point)
• In right-dominant anatomy (~90%), the RCA gives the posterior descending artery (PDA)
• Left-dominant (~10%): LCx gives the PDA
• Co-dominant (~5%): both RCA and LCx supply the PDA

Coronary Dominance

3. Pathophysiology - Why CABG is Needed

• Stable angina: predictable chest pressure on exertion, relieved by rest/nitrates; stenoses typically >70% diameter
• Unstable angina/ACS: plaque rupture with thrombosis; more severe ischemia
• Hibernating myocardium: chronically underperfused, viable but non-contractile segments that downregulate function. Revascularization can restore contractility and improve survival.
• Stunned myocardium: temporary post-ischemic dysfunction after reperfusion

4. Indications for CABG

Class I (Benefit >>> Risk) - ACC/AHA 2021 Guidelines

Class IIa (CABG Reasonable)

• Left main disease with intermediate-complexity CAD
• Three-vessel disease with normal LV function when anatomy favorable
• Prior CABG with multiple saphenous vein graft stenoses (especially LAD graft)

CABG vs. PCI Decision Framework

• PCI preferred: Single- or two-vessel disease with normal LV function, anatomically suitable discrete lesions, poor surgical candidates
• CABG preferred: Left main disease, three-vessel disease, diabetes with multivessel disease, EF <50%, complex anatomy (high SYNTAX score), diffuse disease unsuitable for stenting

5. Preoperative Evaluation

History and Examination

• Document symptom severity, prior MI, prior revascularization
• Assess comorbidities: diabetes, renal disease, obesity, COPD, peripheral vascular disease
• Factors increasing perioperative risk: LV dysfunction, age ≥80 years, urgent/emergency surgery, renal failure, diabetes

Investigations

• Coronary angiography: gold standard for defining anatomy, stenosis severity, and graft targets
• Echocardiography: LV ejection fraction, wall motion, valve function
• Radionuclide scanning / PET / cardiac MRI / low-dose dobutamine echo: detect hibernating/viable myocardium in low-EF patients (viability testing determines who benefits from revascularization)
• Carotid duplex ultrasound: if carotid bruits or stroke risk (CABG + stroke risk from aortic manipulation)
• Peripheral vascular assessment: radial artery Allen's test before radial conduit harvest
• Pulmonary function testing: if significant pulmonary comorbidity

Risk Scoring

• EuroSCORE II and STS score are used to quantify operative mortality risk and guide shared decision-making

6. Surgical Technique - On-Pump CABG (Conventional)

Step 1: Anesthesia and Monitoring

• General endotracheal anesthesia
• Arterial line, central venous catheter, PA catheter or TEE for hemodynamic monitoring
• Transesophageal echocardiography (TEE) is used intraoperatively to assess wall motion, valve function, and confirm de-airing

Step 2: Conduit Harvesting

• Left internal mammary artery (LIMA): taken down as a pedicle or skeletonized from the chest wall
• Saphenous vein graft (SVG): harvested from the leg, either by open or endoscopic technique
• Radial artery (RA): harvested from the non-dominant forearm after Allen's test confirms ulnar collateral circulation

Step 3: Median Sternotomy and Cardiopulmonary Bypass Setup

• Midline sternotomy (full or occasionally mini-sternotomy)
• Pericardium opened, heart exposed
• Cannulation:
  • Arterial cannula inserted into ascending aorta
  • Venous cannulas into right atrium or separately into SVC/IVC
• Patient heparinized (ACT target >400 seconds)
• Cardiopulmonary bypass (CPB) initiated - heart-lung machine takes over systemic circulation

Step 4: Myocardial Protection (Cardioplegic Arrest)

• Aortic cross-clamp applied across ascending aorta
• Cardioplegia solution (cold crystalloid or blood-based, high potassium) infused:
  • Antegrade via aortic root
  • Retrograde via coronary sinus
• This arrests the heart in diastole, reduces metabolic demand, and provides a still, bloodless field
• Heart cooled to ~10-15°C topically or systemically

Step 5: Constructing the Anastomoses

• Coronary arteriotomy made with a #11 blade and extended with coronary scissors (typically 4-7 mm)
• End-to-side anastomosis using 7-0 or 8-0 polypropylene (Prolene) suture, running technique
• Standard graft order: LIMA to LAD, then SVG/RA to obtuse marginals and RCA branches
• Side-to-side (sequential) anastomoses can be used to maximize conduit length

• SVG and free arterial grafts anastomosed to ascending aorta with a side-biting clamp
• Alternatively as T-graft (end-to-side) from the LIMA
• The LIMA is a pedicle graft - its proximal end remains connected to the subclavian artery (no aortic anastomosis needed)

Step 6: De-airing and Weaning from CPB

• Cross-clamp released; heart reperfused
• Heart defibrillated if it does not resume sinus rhythm spontaneously
• Air vented from cardiac chambers (critical to prevent air embolism)
• CPB gradually weaned as heart resumes function
• TEE confirms wall motion, graft function, no new valve issues
• Protamine given to reverse heparin
• Chest closed with sternal wires

7. Conduit Selection - The Critical Choice

• COR 1: Radial artery preferred over SVG when feasible
• COR 2a: Right IMA may be considered as second arterial conduit
• Multi-arterial grafting (MAG) is associated with prolonged survival and is increasingly recommended
• LIMA to LAD is an STS quality measure - mandatory documentation

8. Off-Pump CABG (OPCAB)

How it works

• Standard median sternotomy
• Pericardium opened; heart stabilized with mechanical tissue stabilizers (suction-based or pressure devices)
• The cardiac apex is "verticalized" out of the pericardial well to access posterior/lateral vessels
• An elastic coronary snare reduces bleeding during arteriotomy
• A CO2 blower/mister provides a blood-free anastomosis field
• Proximal anastomoses made with a side-biting aortic clamp, or as T-grafts from the LIMA

Advantages

• Avoids CPB complications (see below)
• Lower blood product use, faster recovery, reduced cost
• Preferred in high-risk patients: severe aortic atherosclerosis, renal failure, prior stroke

Disadvantages and Challenges

• Hemodynamic instability during verticalization (impairs biventricular filling, kinks venous return)
• Technically demanding, harder to access posterolateral vessels
• Potential for incomplete revascularization
• May require conversion to on-pump surgery emergently

Evidence: On-Pump vs. Off-Pump

9. Minimally Invasive and Hybrid Approaches

MIDCAB (Minimally Invasive Direct CABG)

• Small left anterior thoracotomy (no sternotomy)
• LIMA harvested thoracoscopically; anastomosed to LAD on beating heart
• Suitable for isolated proximal LAD disease
• Avoids sternotomy morbidity; shorter recovery

TECAB (Totally Endoscopic Coronary Artery Bypass)

• Fully robotic approach via small chest port incisions
• Technically demanding; limited to select centers
• Both on-pump and off-pump versions exist
• Suitable for highly selected patients with limited disease

Hybrid Coronary Revascularization

• Combines MIDCAB (LIMA to LAD) with PCI (drug-eluting stent to non-LAD vessels)
• Best of both worlds: durability of arterial LIMA-LAD graft + minimally invasive PCI for other lesions
• Useful in patients where full sternotomy carries high risk

10. CPB Complications

11. Postoperative Management

ICU Phase

• Mechanical ventilation; extubation typically within 4-8 hours (fast-track protocol)
• Hemodynamic monitoring via arterial line, CVP ± PA catheter
• Inotropes (dopamine, milrinone, dobutamine) if low cardiac output
• AV sequential pacing for bradycardia or heart block
• Intraaortic balloon pump (IABP): for hemodynamic instability; augments diastolic coronary perfusion while reducing afterload
• Left or right ventricular assist devices (LVAD/RVAD) for refractory failure
• Target: mean arterial pressure 60-80 mmHg, adequate urine output, warm extremities
• ST-segment elevation post-op may reflect residual cardioplegia, ventricular aneurysm, or pericarditis - not necessarily acute ischemia

Ward Phase

• Chest drains removed when output <100 mL/hour
• Anticoagulation: aspirin started within 6 hours post-op; continues indefinitely (COR 1)
• Beta-blocker: started pre-op or early post-op for all CABG patients unless contraindicated
• Statins: high-intensity statin therapy for all patients
• Monitor for: atrial fibrillation (most common arrhythmia, 20-40% of patients), wound infection, pleural effusion, renal dysfunction

Discharge and Rehab

• Discharge typically day 5-7
• Cardiac rehabilitation: structured exercise program; proven to improve outcomes
• Sternal precautions for 6-8 weeks
• Return to driving: typically 4-6 weeks; return to work: 6-12 weeks

12. Complications of CABG

13. Special Populations

14. Guideline-Directed Medical Therapy After CABG

1. Aspirin (75-100 mg/day) - started within 6 hours post-op, continued indefinitely (COR 1)
2. High-intensity statin - reduces graft failure and cardiovascular events
3. Beta-blocker - reduces AF, mortality; continue indefinitely
4. ACE inhibitor/ARB - if EF <40%, hypertension, or diabetes
5. Cardiac rehabilitation - structured supervised program
6. Aggressive risk factor modification: BP control, HbA1c <7%, smoking cessation, weight management

15. Outcomes and Evidence

• Angina abolished or greatly reduced in ~90% of patients after complete revascularization (Harrison's 22E)
• Survival benefit proven for left main disease, three-vessel disease with LV dysfunction, and diabetes with multivessel disease
• FREEDOM trial: CABG + optimal medical therapy superior to PCI for type 2 diabetes with multivessel disease; reduction in nonfatal MI was the primary driver
• SYNTAX trial: CABG superior to PCI for three-vessel or left main disease at 5-year follow-up (lower MACCE rates)
• CABG remains preferred even after advances in drug-eluting stent technology for complex, multivessel disease
• Long-term patency of LIMA to LAD (~90-95% at 10 years) far exceeds SVG (~50-60% at 10 years)

Summary: CABG at a Glance

INDICATION → Multivessel CAD / LM disease / Diabetes / Failed PCI / LV dysfunction

CONDUIT HIERARCHY → LIMA-to-LAD (mandatory) > RA > RIMA > SVG

TECHNIQUE → On-pump (78%): CPB + cardioplegic arrest
             Off-pump (22%): beating heart, tissue stabilizer
             Minimally invasive/Robotic: MIDCAB, TECAB, Hybrid

KEY ANASTOMOSIS → Distal (coronary) first, then proximal (aorta) — 7-0/8-0 Prolene

MYOCARDIAL PROTECTION → Cardioplegia (antegrade ± retrograde), hypothermia

POSTOP ESSENTIALS → Aspirin within 6h, beta-blocker, statin, early extubation, rehab

MORTALITY → <1% in low-risk; higher with LV dysfunction, age ≥80, redo, diabetes