Introduction
Coronary artery disease (CAD), or ischaemic heart disease, is the leading cause of death worldwide. Current prevalence is approximately 6% in men and women aged 40-59, rising to approximately 27% after 80 years of age. More worryingly, there has been a substantial increase in the prevalence of CAD in recent years, especially in higher income countries.
Figure 1 – Arterial Supply of the Heart (A) Anterior view (B) Posterior view
Pathophysiology
Myocardial tissue is perfused by the coronary arteries. Luminal Narrowing or occlusion of these arteries result in reduced blood flow (ischaemia) to the myocardium (Fig. 2), thereby reducing the capability to match myocardial metabolic demand.
The most common pathophysiologic mechanism responsible for CAD is the formation of atheromatous plaques on the walls of the coronary arteries, termed atherosclerosis.These plaques significantly reduce the luminal area of the vessels and precipitate the formation of plaque thrombi, which can acutely occlude the vessel and result in a myocardial infarction.
Figure 2 – Illustration of the heart with normal and partially-blocked coronary vessels
Risk Factors
The main risk factors for CAD are smoking, hypertension, hypercholesterolaemia, and diabetes mellitus. Other factors include male gender, obesity, increasing age and a strong family history(especially in first-degree relatives).
Clinical Features
Patients with CAD typically present initially with angina pectoris, which describes as a chest pain which is central, heavy, and gripping in nature. Importantly, in cases of stable angina, this typically occurs during exertion, but is relieved with rest.
Other clinical manifestations of CAD includes heart failure and acute coronary syndrome. Acute coronary syndrome (ACS) is a group of conditions comprising of unstable angina (worsening angina not relieved at rest), non-ST elevation myocardial infarction (NSTEMI), and ST elevation myocardial infarction (STEMI), that all arise from acute ischaemia to the myocardium. The investigation and management of heart failure and ACS is beyond the scope of this article.
CCS Classification for Angina
The Canadian Cardiovascular Society (CCS) classification for angina can be used to quantify symptoms, especially useful in the evaluation of a candidate for coronary artery bypass grafting (CABG):
- Class I – Presence of angina during strenuous, rapid, or prolonged ordinary activity (walking or climbing the stairs)
- Class II – Slight limitation of ordinary activities when they are performed rapidly, after meals, in cold, in wind, under emotional stress, or during the first few hours after waking up
- Class III – Having difficulties walking one or two blocks or climbing one flight of stairs at normal pace and conditions
- Class IV – No exertion needed to trigger angina
Investigations
Investigations for CAD include electrocardiogram, cardiac enzymes and coronary angiography (gold standard), as discussed below
Electrocardiogram
An electrocardiogram (ECG) is often the earliest test for CAD, as it can be performed by paramedics or bedside in hospital. Typical signs of CAD on ECG include:
- ST segment changes in 2 or more contiguous leads:
- Elevation of 1mm above baseline, or >2mm specifically in leads V2 and V3 = STEMI
- No change or depression = NSTEMI or unstable angina
- T wave changes
- Early MI (minutes to hours) = Hyperacute T waves
- Late MI (hours) = T wave inversion (may persist even after revascularisation)
- Development of Q waves, which are not present in normal ECGs.
Figure 3 – An ECG demonstrating a ST-elevation myocardial infarction (STEMI)
Cardiac Enzymes
Blood tests can also help with diagnosing CAD, using enzymes that are released from dying myocardium into the blood. Therefore, elevated levels of these enzymes can indicate CAD. Typical enzymes used include Troponin and Creatine Kinase MB (CK-MB).
Importantly, elevated cardiac enzymes mark the difference between unstable angina and a myocardial infarction (either NSTEMI or STEMI). In unstable angina, there is reduced blood flow, but the myocardium has not started dying, and therefore cardiac enzymes may not be elevated, whilst an MI involves myocardial death and therefore elevated enzymes.
Coronary Angiography
This interventional procedure involves contrast being injected into the coronary arteries and serial plain film radiographs or CT images (Fig. 4) taken in sync with the patient’s heartbeat; the site and degree of stenosis within the coronary arteries can then be identified. The choice of treatment is often largely influenced by angiography findings. This is often performed by cardiologists.
An echocardiogram (ECHO) should also be requested for any potential CABG candidates, as it provides a measure of left ventricular ejection fraction (LVEF), a strong prognostic factor in CABG, as well as allowing for concurrent review of any additional valvular disease.
Figure 4 – Coronary angiography of a patient with a partial occlusion of left circumflex coronary artery, before (A) and after (B) undergoing PCI
Management
Patients with coronary artery disease should typically be started on an anti-platelet agent, a beta-blocker, a statin, and a calcium channel blockers, alongside a short-acting nitrate (typically a glyceryl tri-nitrate (GTN)) spray, as first-line treatment for symptomatic relief of angina.
However, the mainstay, definitive treatment for CAD is with revascularisation therapy. This is true in both acute and routine presentations, either via primary percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG) surgery.
Of note, fibrinolysis can also be used if the patient presents within 12hrs since start of symptoms and PCI is not possible within 2hrs of when fibrinolysis could have been given.
Percutaneous Coronary Intervention
Percutaneous coronary intervention (PCI) involves a guidewire introduced via a puncture into the radial or femoral artery, passing up to the coronary arteries under radiological guidance. A catheter containing a balloon is then passed over the guidewire and aligned with the lesion, for the balloon to then be inflated to restore the normal width of the lumen and re-establish blood flow through the artery (Fig. 5).
This process can be repeated to treat multiple lesions in the same procedure. A stent, commonly made of a wired mesh, can be permanently deployed across a lesion(s), reducing the rate of post-PCI re-stenosis. Stents are now almost always indicated in patients undergoing PCI.
Figure 5 – Urgent PCI of stenosis in the proximal LAD, before and after images
Coronary Artery Bypass Grafting
A CABG facilitates the circumvention of blood around a lesion within the coronary artery by connecting a graft to the coronary artery at a site distal to the lesion. This procedure can either be performed with cardiopulmonary bypass (on-pump) or without (off-pump).
Broadly speaking, CABG is preferred to PCI in the presence of complex coronary artery disease. Common indications for a CABG include three-vessel disease, complex two-vessel disease, significant left main stem stenosis, significant proximal LAD disease, the presence of diabetes mellitus, or left ventricular dysfunction or mitral regurgitation.
The great saphenous vein or radial artery are prime candidates for conduits used in a CABG*. The left internal mammary artery (LIMA) is often the conduit of choice for an affected left anterior descending artery (LAD). The LIMA is only disconnected from the chest wall at the distal end, which is then attached to the distal part of the coronary artery. The proximal end is not usually released and remains left as a branch of the subclavian artery. Blood flow from the subclavian passes through the LIMA to the distal LAD, bypassing any LAD blockage.
*Arterial grafts consistently achieve longer patency periods than their venous counterparts.
Risk Prediction in Cardiac Surgery
The EuroSCORE is useful in estimating the percentage risk of cardiac procedures. It is influenced by patient-related (e.g., age, co-morbidities), cardiac- (e.g., NYHA score, MI history) and surgery-related (e.g., emergency vs routine) factors.
The main risks for CABG are:
- Death
- Pre-, intra-, or postoperative MI
- Stroke
Other operative risks such as bleeding, infections and venous thromboembolism also apply.
Procedure
The CABG procedure varies with anatomical variations, disease complexity and site, and surgeon preference. Commonly, a median sternotomy is performed and the conduits harvested. The cardiopulmonary bypass is commenced, allowing the heart to arrest (using cardioplegia*) whilst also maintaining systemic perfusion.
The grafts are anastomosed, end-to-side onto coronary arteriotomies via a ‘parachuting’ technique (Fig. 6), at the identified targeted region (both through pre-operative imaging and intra-operative assessment)
The proximal ends of the venous and radial artery grafts are anastomosed to the ascending aorta. This allows blood to pass directly from the aorta to the distal coronary artery, bypassing the blockage. This step is not required in a LIMA graft because the proximal end is not usually released, and is directly supplied by its origin, the left subclavian artery.
The insertion of pacing wires facilitates the control of post-operative arrythmias particularly heart block. Chest drains are inserted and cardiopulmonary bypass is stopped, with restoration of cardiac contraction prior to closure.
*Cardioplegia lowers myocardial metabolic demand, which prevents myocardial ischaemia during the temporary suspension of coronary blood supply during the procedure
Figure 6 – Illustration depicting single, double, triple, and quadruple bypass anatomy
Key Points
- Coronary artery disease is the leading cause of death worldwide
- The main risk factors are hypertension, hypercholesterolaemia, and diabetes mellitus
- The gold standard of investigation for patients with suspected coronary artery disease is coronary angiography
- The mainstay of definitive treatment for coronary artery disease is with revascularisation therapy, either via percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG) surgery
Figure 3 - An ECG demonstrating a ST-elevation myocardial infarction (STEMI)[/caption]