CORONARY VESSELS, ANATOMICAL CONSIRATIONS, RELEVANT INVESTIGATIONS & HYPOTHESES Part - I

A detailed knowledge of the coronary vasculature is needed before planning circulatory resumption for effective myocardial perfusion in established lesions. Separate arterial and venous systems are there for supply of oxygenated blood to the tissue and subsequent return to the heart. The constant beating of the heart makes the myocardium one of the most reactive tissues in the body with maximal oxygen consumption. Thus the venous blood returning after tissue perfusion of the myocardium is maximally de-oxygenated. The heart also has a system of minute valveless small veins which drain directly into the chambers of the heart where reversal of the circulation occurs in times of need.

Anatomically there are two coronary arteries originating from the lowest regions of the dilatation of sinuses of Valsalva of the ascending aorta and these are accordingly named the right and left sinuses. Thus there is the right and left main (RCA & LMCA) coronary arteries..Tthese are mostly involved in the supply of the more active ventricular musculature. The right coronary artery is single and  just after originating from the right sinus of Valsalva runs a bit superiorly for a short distance, and then courses  horizontally and rightwards initially in a groove (the coronary sulcus or the AV groove) in the epicardial fat externally visible anteriorly between the right atrium and the right ventricle. The main trunk of RCA then runs in the AV groove downwards for a distance, it reaches the ’crux’ and shortly after reaching the 'crux' or the angulation with the inferior surface where the IVC enters the RA, the trunk of the RCA bends and deviates for a short distance to bifurcate into the terminal posterior  descending (PDA) and posterior left ventricular (PLV) branches.

The RCA trunk can again be demarcated by three divisions --

●     The proximal segment : extending from the origin to the middle of the trunk in the AV groove,

●     The mid-segment :  the rest of the trunk in the AV groove to the acute margin,

●     The distal segment : from the acute margin onwards.

Mention is important of the fact that the first part of RCA after origin is obscured by the main pulmonary artery and the right atrial appendage.

The significant RCA branch worth mentioning is the artery to the sino-atrial node. In about 60% of the cases, this branch arises from the right superior aspect of the trunk, and courses along the postero-superior aspect of the interatrial septum to run centrally surrounded by the nodal Purkinje cells. Variations in the course and origin are common and in nearly 40% of cases the origin is from the left circumflex system. This branch can even directly arise from the left coronary sinus or the adjacent portion of descending thoracic aorta. The risk of interruption to SA nodal supply should be kept in mind specially in the superior septal approach for access in mitral valve operations. The conus branch, though inconsistent both in size and ramification, forms the Vieusssens ring in the upper anterior aspect of the RV outflow tract while anastomosing with a branch of the left sided circumflex artery. This may be an important supply and caution is justified in RV outflow incisions. The major right atrial circumflex branch originates from the mid RCA trunk, while in the AV groove proper the RV branches and the right marginal arteries originate on the left side and mainly supply the RV in a transverse manner. The distal RCA terminates by dividing into a PDA and PLV branch. The AV nodal branch then arises from the PDA in majority and sometimes from the PLV.

The left main coronary artery (LMCA) originates from the dilated left coronary sinus of Valsalva of the ascending aorta. The ostial opening is a bit posterior and the trunk of the artery is initially single for a varying length of 2 mm to 4 cm (depending upon size, gender, and ethnicity of the patient). Usually there are no branches of the left main coronary artery and after running a variable distance behind the main pulmonary artery, LMCA divides typically into the left anterior descending artery (LAD) and the circumflex artery (LCx). In approximately 30% of individuals, the LMCA trifurcates and a large middle branch, or the ramus intermedius, originates. This really is a variant or early misplaced branch from the 1st obtuse marginal division of LCx

The left interventricular branch or the anterior descending artery (LAD) supply about 45-55% of the heart. Supply is to the anterior and antero-lateral myocardium, apical region, and anterior 2/3rd of the interventricular septum. This LAD is considered as the most important artery for myocardial supply and any organic interruption will lead to a massive area of myocardial infarction which eventually, if not immediately, causes death. Hence the colloquial term ' widow-makers artery'. Anatomically, the LAD has two distinct set of branches --

1.   The septal branches arise at right angles with the main trunk to supply the majority of the interventricular septum (IVS). The large first septal artery that supplies the upper part  of IVS is, however, not a branch of the LAD, but arises from the LCx.

2.   The important right-sided diagonals. These supply the lateral wall of the left ventricle and the antero-lateral papillary muscle and experience in real revascularization cases and angiography show that higher diagonals are larger.

Depending upon the  length of the LAD, the LAD is classified into four types:-

●      Type-I –falls short of the left ventricular (LV) apex,

●      Type-2 – supplies part of the  apex, the rest being supplied by the right coronary both,

●      Type3 –  supplies the entire apical area, and

●      Type-4 – extends up to the apex and proceeds to supply >25% of  the inferior wall ("wrap around LAD").

The circumflex branch of the LMCA, or the LCx enters and runs along almost the whole of the atrioventricular groove between the left atrium and the left ventricle. This artery almost encircles the heart, running in the left part of  the coronary sulcus, first to the left and then to the right,  reaching nearly as far as the posterior longitudinal sulcus. The proximal segment of the artery is usually covered by the left atrium and left atrial  appendage. On reaching the left cardiac margin,  it wraps around the left side of the heart, emerging on its  diaphragmatic surface. Commonly, the circumflex artery terminates  before reaching the crux of the heart in the region on the diaphragmatic surface where the interatrial and interventricular septum converge.

In addition to the atrial branches supplying the left atrium, the LCx supplies mainly the postero-lateral portion of the left ventricle. The obtuse marginal arteries, so named because these seem to sprout from the LCx trunk at an obtuse angle and run towards the left margin of the heart, actually supply the major portion of posterolateral left ventricular myocardium. The LCx continues as a gradually tapering posterior left ventricular artery or the posterolateral artery.

When function and exertional load is considered, the left ventricle is found thicker than the right and the distribution of coronary flow is proportional. A strict relationship between the distribution and size of the major  epicardial coronary arteries and the extent of their dependent  myocardium. In cross-section, the left ventricle is thick and muscular with a crescentic cavity. The right ventricle, on the  other hand, is comparatively thin with a coarse interior and the cavity is more or less triangular in shape.

Dominance of coronary flow is simply defined as the artery from which the PDA and a posterolateral branch originates, determining the  coronary dominance. Mostly it is the RCA, the left circumflex is next and the apex "wraparound" LAD with a LCx contribution is another. So there can be three situations:

  1. Right-dominance  (approximately 70% of the cases) (supply from the RCA),

 2. left-dominance  (10%) (supply from the LCX or both from LCx and LAD),                                 

                        and

 3. Codominance (20%), (PDA and posterolateral branches arise from both rt. and lt. systems).

4.In addition, the right coronary system may be superdominant with large PDA and PLV extending into the supply area of the LCx. Such situations are commonly associated with attenuated or absent LCx systems.

Thus we see that the thicker left ventricle, which has to pump against the higher vascular resistance of the large systemic circulation, has the preferential (> 50%) flow, and left dominance is potentially dangerous as the situation will be devoid of a retrograde flow to the left system beyond a block.

The named coronary arteries supply extensively the right and left ventricles and both share the supply of the interventricular septum (IVS). The left anterior descending branch (LAD) of the left coronary artery supplies the anterior portion of the IVS, while the terminal posterior descending branch (PDA) of the  right coronary artery supplies the posterior portion. These arteries ensure that the IVS receives adequate oxygenated blood to support  its function. Around 5-10 % of the coronary arterial blood supply the conducting system of the heart which includes the sinoatrial (SA) node,  atrioventricular (AV) node, bundle of His, and Purkinje fibers,  receives its arterial blood supply from branches of the coronary  arteries. Specifically, the SA node is primarily supplied by the right  coronary artery (RCA) in most individuals, while the AV node is usually  supplied by the right coronary artery or the left circumflex artery  (LCx). The bundle of His and Purkinje fibers receive blood supply from  various branches of the left and right coronary arteries. This arterial  supply ensures that the conducting system has sufficient oxygen and  nutrients to facilitate the electrical conduction and coordination of  the heart's contractions. The sinoatrial (SA) node, which is responsible for initiating the electrical impulses that regulate the heart's rhythm, typically receives its blood supply from the coronary arteries. The SA node can receive blood supply from both the right and left coronary arteries, but the extent to which it is supplied by each artery can vary among individuals. In most individuals, the SA node is predominantly supplied by the right coronary artery (RCA). The RCA gives rise to a branch called the "SA nodal artery" or "nodal artery," which carries oxygenated blood to the SA node. The nodal artery can arise from different locations on the RCA. However,  in some individuals, particularly those with specific variations in coronary artery anatomy, the SA node may receive some blood supply from branches of the left coronary artery (LCA), such as the circumflex artery. This dual blood supply provides redundancy to ensure that the SA node receives adequate oxygen and nutrients, regardless of individual anatomical variations. The precise percentage of blood supply to the SA node or AV node from the RCA and LCA can vary among individuals, and it is not always straightforward to quantify. It depends upon the site of origin, course,  and dominance. Additionally, there can be variations in the coronary artery supply to the SA node among different people.r, in some individuals, particularly those with specific variations in coronary artery anatomy, the SA node may receive some blood supply from branches of the left coronary artery (LCA), such as the circumflex artery. This dual blood supply provides redundancy to ensure that the SA node receives adequate oxygen and nutrients, regardless of individual anatomical variations.  A specific percentage can not be universally applied. In summary, the SA node can receive blood supply from both the right and left coronary arteries, but the exact distribution varies among individuals. The majority of individuals primarily receive SA nodal blood supply from the right coronary artery.

In most individuals, the AV node is primarily supplied by the right coronary artery (RCA). The RCA gives rise to a branch called the "atrioventricular nodal artery" or "nodal artery," which carries oxygenated blood to the AV node. This nodal artery supplies the AV node in the majority of cases. In some individuals, especially those with variations in coronary artery anatomy, the AV node may receive some blood supply from branches of the left coronary artery (LCA), such as the left circumflex artery. The percentage of blood supply to the AV node from the RCA and LCA can vary among individuals and is not always easy to quantify precisely. The specific origin and course of the nodal artery and individual coronary artery dominance play a role in determining the distribution of blood supply to the AV node. Additionally, there can be variations in the coronary artery supply to the AV node among different people, similar to the SA node. As with the SA node, there isn't a specific percentage that can be universally applied for the distribution of blood supply to the AV node. In general, the RCA predominantly supplies the AV node, but the extent of contribution from the LCA can vary.

The coronary artery supply to both the SA node and AV node is critical for ensuring the electrical activity of the heart. A well-preserved anastomosis between the left and right coronary systems and identifiable presence of the Kugel's artery and its branches in supplying the nodes have been observed. Anatomical studies in atrial fibrillation and other atrial arrhythmias show that supply to the SA node have revealed conflicting results when dominance, gender, race, and race are taken into consideration. Both the electrical nodes are preferentially supplied by the right arterial system (60% for the SA node). The atrioventricular nodal branch sees significant variation in origin:

●     proximal posterolateral branch from the right coronary artery in around 77%.

●     distal posterolateral branch from the right coronary artery in around 2%

●     distal right coronary artery in around 10%.

●     right posterior interventricular artery in around 7%.

●     distal circumflex branch of left coronary artery in around 4%.

Therefore, the right coronary artery supplies the atrioventricular node in around 90% of people. In approximately 2% of people, the vascular supply to the  atrioventricular node arises from both the right coronary artery and the left circumflex branch.

When evolutionary development is considered, it was found the coronary vessels embryonically originated from different entities and a later collation was necessary for the development of the complete pattern. Initially sinusoids as extension of the trabeculae of the spongiosa into the  developing myocardium, formed in the sub-epicardial space as the primitive sites of metabolic  exchange between the blood in the cardiac cavities and the cardiac  mesenchyme. This happened in the first week of cardiac development. After the bulbus cordis stage, development of valves with separation of the developing heart into distinct chambers and the formation of aorta and pulmonary artery as outflow channels occurred. Buds arose from the nadir of different aortic sinuses and with the rotation of the heart these buds joined the major sinusoidal systems to form the coronary arterial network supplying the myocardium. A major role was played in the development by the angiogenic progenitor cells and some of the sinuses remained as intramyocardial trabeculae. A considerable part of the coronary arteries are visible epicardially. On reaching a particular point, the arteries dip down into the myocardium, and are transformed into the smaller intramyocardial arterioles. There was extensive branching of these intramyocardial arterioles into a capillary system so as to encircle all the myocytes of the endocardium. The epicardial coronary arteries appear as tapering end arteries at systole due to the contractile squeeze, still in normal hearts circulation is more than adequate.

Coronary circulation and O2 extraction by myocardial cells is different from the flow to other tissue.  In coronary flow, the importance of head pressure (known as the coronary perfusion pressure, {CPP}, that equals the subtraction of the left ventricular end diastolic pressure from the aortic diastolic pressure) only initiates a perfusion pressure. Across a narrowed segment, the character of flow changes according to the degree of obstruction and vasodilatation by autoregulation ensures endocardial blood flow. Thus considering the energy required for expenditure, the beating heart is a highly vascular organ. There is increased blood flow through the coronary system during diastole and the endocardium, (which happens to be the most vulnerable region of the myocardium) boasts a greater circulation (factor of 1.2:1) which is required for an increased metabolic activity for initiation of contraction and exposure to wall stress. This is not impeded by the systolic compression of intramyocardial blood flow but  the vulnerability of the endocardium to ischemia. Oxygen extraction by the myocytes is maximum from the circulating blood and this is exemplified by the low oxygen content of the returning blood. Autoregulation occurs proportionately in the demand-supply model and explain regional wall motion defects with zonal ischemia to some effect but still extensive studies with the ionic channels, increased amounts of adenosine and carbon dioxide, neurohumoral agents, etc., can not explain the adequate diastolic blood flow to the endocardium. Present studies show that coronary perfusion pressure (CPP)) and autoregulation play important roles till the basic arterial architecture is intact and the typical ‘windkessel’ effect and the vascular waterfall phenomenon of the general circulation is not there. Thus, to maintain a flow in normal conditions where 70-80% of circulatory oxygen can be extracted, CPP in epicardial coronary arteries is the main driving force while the vasodilation and vasoconstriction of zonal autoregulation is responsible for the maintenance of the paradoxical diastolic flow to the vulnerable endocardium.

Revascularization beyond a stenotic or obstructed epicardial coronary lesion has been practiced since time immemorial and justification is proved by relief from the angina, recruitment of shocked and hibernating regional myocardium, and correction of the wall motion abnormalities with an increase in the ejection fraction. With time, studies are coming up with new investigations that are providing data about several unknown facets of the condition. But, still there is a dispute within the cardiology community about the suitability of an investigation able to provide the most useful data informing about the need for revascularization. That angina pectoris had an intimate relation with a defect or dysfunction of the heat was recognized quite early by astute clinical methods. A mis-match in the circulation of a region of the heart was surmised and histopathology helped in the corroboration. Morbid histopathology not only helped in demarcating a regional area of discrepancy in circulatory demand and ratio, but also provided an idea of the thickness of the heart muscle involved, and it was found to correlate fairly accurately with the regional wall motion abnormalities found with the later developed non-invasive echocardiography.

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