Description
- The coronary arteries deliver oxygen-rich blood to the myocardium.
- Epicardial coronary arteries run on the outer surface of the heart. When healthy, they are capable of autoregulation to maintain an adequate coronary blood flow (CBF) that meets oxygen demand.
- Subendocardial coronary arteries run deep within the myocardium.
- The coronary arteries are classified as "end-circulation," since they represent the only source of blood supply to the myocardium. Thus, blockage of these vessels can be critical since there is typically very little redundant blood supply (unless collateral blood flow has developed).
- Coronary blood flow (CBF) varies directly with the pressure difference across the coronary bed (coronary perfusion pressure; CPP) and inversely with the resistance of the coronary vasculature (CVR): CBF = CPP/CVR.
- The CPP equals the arterial driving pressure minus the back-pressure to flow across the coronary bed.
- for the left ventricle (LV), the driving pressure is the aortic pressure during diastole (DBP).
- Left ventricular end diastolic pressure (LVEDP) is utilized to measure the back pressure. As its name implies, it is the pressure within the left ventricle at the end of diastole; it is dependent on preload volume, ventricular compliance, and systolic function.
- Thus, CPP of left ventricle = DBP – LVEDP
- Resting CBF is 225–250 mL/min or 4–5% of the cardiac output.
- There are pronounced systolic and diastolic coronary flow variations throughout the cardiac cycle.
- The CPP equals the arterial driving pressure minus the back-pressure to flow across the coronary bed.
- Diastole. Arterial inflow primarily occurs during diastole; at least 75% of total CBF to the LV occurs during diastole.
- Systole. Arterial inflow declines as venous outflow peaks, reflecting the compression of the microcirculatory vessels. During systole, the strength of compression decreases blood flow through subendocardial arteries of the LV to almost zero. Thus, the subendocardial system of the LV is the most vulnerable site for myocardial ischemia.
- Myocardial oxygen consumption
- Myocardial oxygen extraction is near-maximal at rest (about 75% of arterial oxygen content); by far the greatest oxygen extraction of all organs.
- The major determinants of oxygen demand are heart rate, contractility, and wall stress (preload, afterload, wall thickness); heart rate is the most important among them.
- Since cardiac oxygen extraction is near-maximal under resting conditions, the primary mechanism to meet increased oxygen demand is through enhanced delivery.
- The factors responsible for regulation of CBF are not well understood; various local metabolic factors are implicated. Activation of beta-adrenoreceptors (sympathetic nervous system) also leads to coronary vasodilatation.
- The blood supply to the myocardium arises from the aorta behind the left and right aortic valve leaflets through two main coronary arteries: The left main and the right coronary artery (RCA).
The left main coronary artery (LMCA) extends for a short distance before dividing into the left anterior descending artery (LAD) and the circumflex coronary artery (LCX) (see Figure 1).
FIGURE 1. Coronary artery anatomy
- LAD
- Courses down the interventricular groove and gives rise to the septal and diagonal branches.
- The septal branches vary in number, size, and course into the interventricular septum and usually supply the anterior two thirds of the septum.
- One to three diagonal branches exist and distribute blood to the anterolateral aspect of the heart.
- The LAD continues down the interventricular groove and usually passes all the way around the apex of the LV.
- LCX
- Passes along the base of the LV within the coronary sulcus and terminates in the left posterior descending branch.
- Its main vessels are the obtuse marginal branches (one to three in number) and supply the lateral-free wall of the LV.
- RCA
- Arises from the anterior wall of the aortic root (also makes it prone to air entry on termination of cardiopulmonary bypass).
- Passes down the right atrioventricular (AV) groove.
- Usually supplies the anterior and posterior walls of the right ventricle except for the apex (supplied by LAD).
- Gives rise to the acute marginal branches that supply the anterior wall of the right ventricle.
- Also supplies the right atrium including the sinoatrial (SA) node in about 55% (in 45% SA node supplied by LCX), the AV node in about 90% (10% supplied by LCX), the posterior third of the septum, the inferior wall, and the base of the LV.
- Coronary artery dominance
- In about 80–85% of individuals, the RCA gives rise to the posterior descending artery (PDA) to supply the posterior inferior aspect of the LV; this pattern of blood supply is called a right dominant system.
- In a left dominant system the LCX supplies the posterior–inferior aspect of the LV (about 10–15%).
- In a codominant system, both the RCA and the LCX supply the posterior–inferior aspect of the LV (about 5–10%).
- Myocardial segments
- Consensus recommendations were published by the American Heart Association (AHA) on how to define 17 myocardial segments (see Figure 2) (1). The standardization of the 17 segments facilitates communication about results from the wide variety of cardiac imaging modalities currently in use for research and clinical applications.
Despite tremendous variability in the coronary artery blood supply to myocardial segments, the 17 segments were assigned to the 3 major coronary arteries.
FIGURE 2. American Heart Association consensus definitions of left ventricular segmentation.
(Reused with permission from ACC/AHA 2007 Guidelines on perioperative Cardiovascular Evaluation and Care for Noncardiac Surgery: Executive Summary. Circulation. 2007;116:1971–1996.)
- Coronary collaterals
- An extensive network of coronary capillaries are normally present from the time of birth. In normal hearts, the collaterals are not demonstrable by angiography due to their small diameter. Only enlarged collaterals (due to myocardial oxygen deprivation) are visible on angiography.
- These collaterals are not visualized on angiography in normal hearts due to their small diameter. Only enlarged collaterals (due to chronic myocardial oxygen deprivation) are visible on angiography.
- Blood supply of the papillary muscles
- Two papillary muscles tether the mitral valve to the wall of the heart. If the papillary muscles are not functioning properly, mitral regurgitation may develop.
- The anterolateral papillary muscle usually receives blood supplies from two arteries (LAD and LCX) and thus is more resistant to coronary ischemia.
- The posteromedial papillary muscle is usually only supplied by one coronary artery (PDA) and thus more susceptible to ischemia.
Physiology/Pathophysiology
- Coronary artery disease (CAD) is a condition in which a plaque builds up inside the coronary arteries. It is the leading cause of death worldwide.
- Cardiac catheterization (angiography) is the gold standard to determine the anatomy of the coronary blood supply in the individual patient. It can show whether CAD is present, but it does not necessarily assess the functional significance of a lesion and comes with its own morbidity/mortality risk (see chapter on cardiac catheterization).
- Alternatively, less-invasive imaging methods are on the rise to diagnose CAD: Computed tomography angiography, cardiac magnetic resonance imaging, and positron emission computed tomography (PET).
- Echocardiography and nuclear tests do not provide direct information about coronary arteries. Wall motion abnormalities are correlated and extrapolated to likely artery distribution.
- Patients with CAD are at increased risk for an adverse perioperative event.
- All patients should be evaluated preoperatively for the presence of cardiovascular risk factors; the decision should be made whether further diagnostic studies and/or interventions are warranted prior to surgery in accordance with the AHA/ACA guidelines (2,3) [B].
- 12-Lead EKG
- Inferior leads (II, III, and aVF): Electrical function of the inferior heart muscle. The inferior wall is usually supplied by the RCA.
- Lateral leads (aVL, I, V5, and V6): Electrical function of the lateral wall of the LV. Ischemia or infarction is typically present here, as this represents the area of greatest oxygen consumption (thick muscle, strong contractility/tension) and most tenuous supply (hypertrophy, not perfused during systole). Typically supplied by branches of the LCX.
- Anterior leads (V3 and V4): Electrical function of the anterior wall of the heart. Usually supplied by the LAD.
- Septal leads (V1 and V2): Electrical function of the intraventricular septum. Branches of the LAD usually supply the anterior two third of the septum.