Coronary Artery Bypass Grafting (CABG)

Coronary Artery Revascularization

Percutaneous Coronary Intervention (PCI) and Coronary Artery Bypass Grafting (CABG) are the main procedures  for coronary artery revascularization. Percutaneous coronary intervention ncludes meths such as Balloon angioplasty, Bare metallic stents and putting Drug-eluting stents. For all acute conditions, PCI is performed. For example in Acute STEMI  Level 1 Coronary artery revascularization is done according to the American College of Cardiology (ACC)/ American Heart Association (AHA) guidelines. In all other conditions, CABG provides superior evidence and is considered a gold standard procedure.

Indications for Coronary Revascularization

1. Unprotected left main artery disease - CABG has level 1 evidence.
   • Level 1 evidence – Benefit is much more than risk so it should be performed. 
   • PCI has an level IIa evidence.
   • Benefit is more than risk.
2. Three vessel disease with or without proximal LAD artery disease
   • Level 1 evidence for CABG.
   • PCI has an IIb evidence i.e., Benefit ≥ Risk.
3. Two vessel disease with proximal LAD artery disease
   • Level 1 evidence for CABG.
   • PCI has an IIb evidence i.e., Benefit ≥ Risk.
4. Two vessel disease without proximal LAD artery disease
   • CABG shows Level IIa evidence with extensive ischemia.
   •  PCI has an IIb evidence i.e., Benefit ≥ Risk.
5. One vessel disease with proximal LAD artery disease
   • CABG shows Level IIa evidence if left internal mammary artery graft is done.
   • PCI has an IIb evidence i.e., Benefit ≥ Risk.
6. CABG and PCI are harmful to one vessel disease without proximal LAD artery involvement.
   • Both CABG and PCI have level III evidence i.e., there may or may not be any benefit.
7. LV dysfunction
   • CABG has level IIa evidence when LVEF is 35%-50%,
   • PCI shows insufficient data.
8. Survivors of sudden cardiac death with presumed ischemia
   • Both CABG and PCI, show level I evidence. 

Coronary Artery Bypass Grafting(CABG) vs. Balloon Angioplasty/ BMS (PCI)

• There are multiple randomized controlled trials that state where to prefer CABG over PCI
• Advantages of CABG
  • CABG provides more effective relief of angina at 1 and 5 years.
  • Repeat revascularisation is less with CABG-1Y and 5Y after index procedure.
  • CABG leads to more complete revascularisation.
• The risk of procedural stroke is higher in CABG as compared to PCI.

Indication for Coronary artery Bypass Grafting(CABG)

• When the patient has > 50% stenosis of the left main stem disease.
• > 50% stenosis of the proximal LAD
• Three main coronary arteries disease (triple vessel disease)
• Two vessel diseases, including the proximal LAD. 

In all these cases, the level of recommendation for CABG has level I evidence. 

Acute Conditions where CABG is Preferred

• Unstable patients with left main stem/ multivessel disease and failed/ unsuitable PCI.
• When the patient has complex anatomy.
• Acute STEMI with mechanical complications.

Risk Assessment for Coronary artery Bypass Grafting(CABG)

The Expected benefits should be more than the negative consequences of the procedure. Risk assessment is to decide suitability of surgery. We use objective scoring method of risk assessment by the Society of Thoracic Surgeons (STS) score and EURO Score II is preferred.

In left main coronary artery involvement, without surgery the prognosis is poor. The Most common artery involved in CAD is  Left anterior descending artery.

Preoperative Assessment  for Coronary artery Bypass Grafting(CABG)

The Clinical assessment is done by checking the Severity and stability of ischemic heart disease (IHD). the Gold standard  for this is  Coronary angiography. During coronary angiography the interventional radiologist looks at coronary anatomy. Special attention is given to the Degree and number of stenosis and the Distal target arteries.

Left ventricular function and risk of  valvular disease is checked  through ECHO.

Patients whi have a Previous history of CVA also have a High risk for Carotid artery disease. Evaluation of comorbidities such as diabetes, Renal/liver dysfunction (Do USG, LFT, KFTs), Assessment for peripheral vascular disease (PVD), Ankle brachial index < 0.9, there is increased risk of cardiovascular disease associated, Assess respiratory functions, Coagulation profile, evaluation of drugs (antiplatelet drugs, ACE inhibitors, β-blockers), Evaluation of conduit.

Evaluate lower limb for any evidence of varicose veins. The upper limb vascularity is checked as well. 

Choice of Graft in Coronary artery Bypass Grafting(CABG)

Arterial graft

The Left internal mammary artery is graft of choice for LAD.  Arterial grafts are more resistant to atherosclerosis. The Graft survival rate is 95% with a 10-year graft survival. The Right internal mammary artery or B/L mammary artery can also be used. If the radial artery is used Radial artery then we must Check for pre-operative patency by Allen’s test. Gastroepiploic artery can also be used.

Venous grafts

The Most commonly used venous grafts are from great saphenous vein (GSV). Cephalic vein is an alternative for GSV.  It is also advantageous than arterial grafts as it is Easy to harvest, Easy to handle and Long grafts can be taken. 

Arterial vs Venous graft

• Arterial grafts are better graft for survival.
• High incidence of vein grafts atherosclerosis due to pulsatile blood flow. 
• For venous grafts there is need for early post op use of lipid lowering agents and antiplatelet agents. 
• For GSV 90% chance for 1-year survival and 60-70% chances for 10-year survival.

In triple artery vessel disease, For LAD,  Left internal mammary artery can be used and for For other coronaries use venous grafts such as great saphenous vein.

Total Arterial Revascularisation

In 90% og CABG procedures single arterial graft is used. EACTS (European Association of cardiothoracic surgeon) says total arterial vascularization without using venous conduits improves the long-term results of coronary surgery and decreases the revascularization procedures. The Disadvantages of total artey revascularization

• Radial artery is prone to spasm so CCBs are given.
• Inadequate length issues. So, it is difficult to use LIMA for posterior descending artery. 
• There is a risk of competitive flow across the native vessel in the arterial graft. Used only when there is sever stenosis. In RCA it is used only when there is > 90% stenosis. 
• Post operative - There can compromised sternal blood supply with B/L internal mammary artery which can lead to high risk of sternal malunion and poor sternal healing.
• It takes longer operative time.

CABG Procedure

It starts with the establishment of lines (central, intra-arterial) for intraoperative monitoring. Nasopharyngeal probe is used for the temperature monitoring. Plan for harvesting of conduit. 

• Great saphenous vein from lower limbs.
• For left internal mammary artery go for median sternotomy.

CABG is done by median sternotomy. The patient is placed on cardiopulmonary bypass and start heparinisation. Hypothermia is induced and cross clamping of the aorta is done. Myocardial arrest and myocardial protection are given. Identification target vessel and distal anastomoses is done. The Suture of choice for anastomoses is polypropylene (7-O/8-O suture).

The Integrity is checked through flushing technique (through cold blood or cardioplegic solution). the  Last anastomoses done where the left internal mammary artery (LIMA) is anastomosed to left anterior descending artery (LAD) to prevent avulsion. 

In Sequential anastomosis a Single graft is used for 2 targets. It is done when vein segment is short or target vessels are small.

After this is done, the cross clamps are removed and the cardiac electromechanical activity  restore of of the heart. Proximal anastomoses is done only when the venous graft is used. Patient is slowly Weaned from cardiopulmonary bypass. The adjuncts are evaluated. 

There is Reversal of anticoagulation with protamine in ratio 1:1 and establishment of haemostasisafter that the doctor Evaluates surgical sites and establishment of surgical drainage 

The final step is Closure of sternotomy.

Post-operative complications to Coronary artery Bypass Grafting(CABG)

Most common Post operative complications includes significant amount of bleeding in 2-3% patients which may need re-exploration.the causes for bleeding are surgery, conduit/anastomoses, cannulation/mammary bed, thymic vein, pericardial edge, sternal wire site.  There can be platelet dysfunction, inadequate protamine reversal and hypothermia. If drain output is > 500 ml in first hour or > 200 ml per hour in 4 hours, there is large hemithorax and pericardial tamponade, this needs reexploration.

There is also a risk of Arrhythmias.  The most common arrhythmias after CABG - sinus tachycardia and the 2^nd most common arrhythmia is Atrial fibrillation (30-60%). The Treatment of arrhythmia is Potassium correction, amiodarone, digoxin, beta blockers, cardioversion in hemodynamic unstable patients.

Bradycardias are rare and can be treated by cardiac pacing

The patient can also get Ischemic reperfusion injury. Neuronal dysfunction occurs in 2% of the patients. Most common cause for it is embolization from aortic arch or cardiac chambers. Risk factors include old age more than 70 years, diabetes or carotid artery stenosis.

Wound infections lead to 40% of the mortality.

Treatment of the post operative CABG

There could be presence of features of hypovolemia which present themselves as Cold extremities and Hypotension . The main Treatment Principles for it are:

• Treat Underlying cause 
• Adequate oxygenation
• Preload optimisation
• Afterload reduction
• Rhythm correction
• Improving cardiac contractility
• If low cardiac output persists go for pharmacological support. 

Pharmacological support

Inotropes like Dobutamine, epinephrine and norepinephrine are used as a pharmacological support for the complications. If still low CO persist then we go for mechanical support.

Mechanical support

Intra-aortic balloon pump (IABP) for severe myocardial dysfunction not responding to volume resuscitation, pharmacological support. It is silastic balloon with capacity of 40-60ml. It is Inserted percutaneously through CFA into descending thoracic aorta. Proximal ends lie below the level of distal arch branches especially below subclavian artery.  Distal end is placed proximal to visceral branches of aorta. 

Frequently Asked Questions

 Q: Which is the most commonly affected artery in CAD?

Answer: Left anterior descending artery.

Q: Who is the Father of CABG?

Answer: R. Favoloro

Q: What is the most common arrhythmia seen post operative after a CABG?

Answer: Tachycardia

Q: Which is the most commonly used venous grafts taken from?

Answer:  Great saphenous vein (GSV)

Also Read: Knowing Venous Diseases and How to Treat Them

Hope you found this blog helpful for your Cardiothoracic and Vascular Surgery preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs.