1: Coronary circulation

Published on 02/03/2015 by admin

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Last modified 22/04/2025

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Topic 1 Coronary circulation

Topic Contents

Coronary blood flow 1
Coronary anatomy 2

Right coronary artery (RCA) 2
Left coronary artery 3
Coronary venous anatomy 4
Coronary artery anomalies 4

Clinically important anomalies 5
Anomalies associated with congenital heart disease 5

Coronary blood flow

Oxygen consumption of myocardium at rest = 8–10 ml/min/100 g.

The heart receives ~5% of total cardiac output.

Myocardial oxygen extraction is high (~75%), and therefore there is little extraction reserve. Furthermore, the myocardium does not have great capacity for anaerobic glycolysis.

During exercise myocardial oxygen consumption can increase to > 40 ml/min/100 g.

Increased demand is met by increasing coronary blood flow.

Coronary blood flow is subject to autoregulation which is closely coupled to and driven by myocardial oxygen consumption (mVO2). Autoregulation is lost when coronary perfusion pressure drops below 60 mmHg.

There is local metabolic control of coronary blood flow, thought to be largely mediated by adenosine and nitric oxide. There is additionally sympathetic innervation of alpha and beta adrenoceptors.

Coronary blood flow varies within the cardiac cycle and most flow occurs during diastole. During systole, extravascular compression reduces intramyocardial flow. The greatest resistance to perfusion is the subendocardial layer where the extravascular compressive forces are greatest and vascular pressure is reduced. Compressive forces are lower in the right ventricle and the drop in flow during systole is less pronounced.

Wave intensity analysis, using pressure- and flow-sensitive wires in human coronary arteries, has identified six predominant waves influencing phasic flow. It is postulated that a dominant backward-propagating ‘suction’ wave, generated by a fall in resistance of the coronary microvasculature with myocardial relaxation, is largely responsible for diastolic flow.

Coronary anatomy

Right coronary artery (RCA) (see Figure 1.1)

Arises from the right coronary sinus.

image

Figure 1.1 LAO view of right coronary artery.

1. Right coronary artery
2. Conus branch artery
3. Sinoatrial nodal artery
4. Right ventricular artery
5. Acute marginal artery
6. Posterolateral artery
7. Posterior descending artery

The first branch is the conus (infundibular) branch, which passes anteriorly to supply the right ventricular outflow tract. The conus branch may instead arise from the aorta.

In 55% of people, the proximal RCA gives off a small branch to the sinoatrial node. In 45% of cases it is supplied by the left circumflex vessel (LCx).

The RCA then passes along the right atrioventricular (AV) groove and gives off acute marginal branches to the free wall of the right ventricle.

At the crux (junction of AV groove and posterior interventricular sulcus) it supplies the inferior left ventricular (LV) wall.

The posterior descending branch supplies blood to the posterior third of the interventricular septum.

The posterolateral branch supplies the basal posterolateral LV wall.

Left coronary artery (see Figure 1.2)

Arises from the left coronary sinus as the left main stem and bifurcates into the left anterior descending artery (LAD) and circumflex artery.

image

Figure 1.2 Left coronary artery anatomy. Left – RAO view, Right – LAO caudal (‘spider’) view.

1. Left main stem coronary artery
2. Left anterior descending coronary artery
3. Circumflex coronary artery
4. AV circumflex coronary artery
5. Obtuse marginal coronary arteries
6. Septal coronary arteries
7. Diagonal coronary artery

The LAD passes along the anterior interventricular groove. It gives off septal branches to the anterior 2/3 of the interventricular septum and diagonal branches which supply the anterolateral free LV wall. Terminal branches supply the apex.

The circumflex passes in the left AV groove and supplies the lateral LV. It gives off branches to the left atrium and obtuse marginal branches to supply the posterolateral LV.

• In some cases the left main stem trifurcates into LAD, circumflex and ramus intermedius branches. The ramus intermedius arises between the other two. It supplies the anterior LV free wall.
• In some cases the left main stem is absent and the LAD and LCx arise from separate ostia in the left coronary sinus.
• In some cases the circumflex artery arises from the right coronary sinus.
• Left or right dominance: The dominant vessel supplies the AV node and gives off the posterior descending artery (PDA) which supplies the posterior third of the interventricular septum. The RCA is dominant in 85%, the LCx is dominant in 10% and codominance is present in 5%.

Coronary venous anatomy

Most venous drainage of the heart passes into the right atrium via the coronary sinus, the ostium of which lies in the posteroinferior interatrial septum. It receives blood from the middle cardiac vein (which runs in the posterior interventricular groove, alongside the posterior descending artery) and it is in continuity with the great cardiac vein (which runs parallel to the left circumflex artery). The anterior interventricular vein runs with the left anterior descending artery and drains into the great cardiac vein. The great cardiac vein also receives tributaries from the left marginal vein and left posterior vein. The small cardiac vein receives tributaries draining the right ventricle and also drains into the coronary sinus. The anterior cardiac vein returns blood separately to the right atrium and there are additional small veins that open directly into the cardiac chambers. The coronary venous anatomy when viewed in the LAO and RAO configurations is displayed in Figure 1.3.

image

Figure 1.3 Normal coronary venous anatomy. (A) RAO view. (B) LAO view.

Coronary artery anomalies

Congenital coronary artery anomalies include:

1. Anomalous origin/course.
2. Intramyocardial (myocardial bridging). Part of the coronary artery takes an intramyocardial course.
3. Fistulae. The majority drain into the right heart (right atrium, coronary sinus, right ventricle, pulmonary artery).
4. Structural abnormalities. Stenosis, hypoplasia, atresia.
5. Duplications or single coronary vessel.

Clinically important anomalies

Anomalies associated with adverse events include:

1. Anomalous coronary artery from the opposite sinus (ACAOS). The coronary artery arises from the incorrect (contralateral) sinus of Valsalva with an anomalous course between the aorta and pulmonary trunk. Origin of the left main coronary artery from the right sinus of Valsalva with a course between the great arteries is associated with myocardial ischaemia and sudden death, particularly after exertion. Anomalous origin of the right coronary artery from the left sinus of Valsalva is also associated with myocardial ischaemia and sudden death. The mechanism is uncertain but may be related to abnormal vessel takeoff and direct compression by the aorta.
2. Anomalous left coronary artery origin from the pulmonary artery (ALCAPA). This usually presents in infancy with heart failure. Survival to adulthood requires adequate collateralisation from the right coronary circulation. Adults can present with angina, mitral regurgitation from papillary muscle ischaemia or occasionally sudden death.
3. Congenital structural abnormalities: stenosis/atresia/hypoplasia. May be present in early life with myocardial ischaemia or heart failure.
4. ‘High takeoff’ – The coronary artery arises from the aorta, above the sinotubular junction. Postulated association with myocardial ischaemia and sudden death.
5. Myocardial bridging – Uncertain pathophysiologic significance but has been linked to myocardial ischaemia and sudden death. Controversial.
6. Large coronary artery fistulae may cause a significant left-to-right shunt with volume overload and steal phenomenon.

Anomalies associated with congenital heart disease

Important coronary artery anomalies associated with congenital heart defects include:

Tetralogy of Fallot. The most commonly described anomaly is the LAD arising from the RCA and crossing the right ventricular outflow tract. This has important surgical implications as the vessel can be damaged during right ventriculotomy.

Transposition of the great arteries. Occasionally a coronary artery takes an intramural course within the wall of the aorta and can cause difficulties during the arterial switch procedure.

Pulmonary atresia with intact ventricular septum. There may be RV-coronary artery fistulous connections, coronary stenoses or interruption and coronary ectasia. In some patients, coronary supply is dependent on retrograde filling from the right ventricle.

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