1 Introduction

Coronary collateral circulation is the small vessel connections formed between one epicardial coronary artery and another. Coronary collaterals can exist in the presence or absence of coronary artery disease (CAD) [1,2,3]. They can be recruited when a territory is jeopardized by the occlusion of an epicardial coronary artery, potentially providing an alternative source of blood supply to the myocardium. The presence of coronary collaterals is associated with preservation of left ventricular function and improved prognosis [4,5,6,7,8,9,10]. Beyond their protective role, recent interest has focused on the utility of collaterals during chronic total occlusion (CTO) percutaneous coronary intervention (PCI). In addition to providing visualization of the distal bed of an occluded coronary artery, collaterals can potentially be crossed retrogradely via their donor vessel, allowing access to the occluded artery and facilitating recanalization of the occlusive segment.

2 Historical Knowledge of Coronary Collateral Circulation

The debate around the possible existence of human coronary collateral circulation began in the mid-seventeenth century. Anastomoses between the right and left coronary arteries were first documented by Richard Lower in Amsterdam using postmortem imaging [11]. Two centuries later, Cohnheim conducted canine studies and pronounced that coronary arteries were in fact end arteries. It was not until 1963 in Glasgow that William Fulton unequivocally demonstrated the existence of coronary anastomoses using a patho-anatomical imaging technique developed to differentiate between vascular overlay and true connections between adjacent parts of the circulation [1] (◘ Fig. 31.1). Fulton discovered the presence of coronary collaterals in human hearts, even in the absence of CAD. These coronary anastomoses, which can be identified in autopsy specimens, are usually too small to be visible angiographically in the absence of significant CAD and impaired coronary perfusion. The collateral vessels identified ranged in size from <200 μm in the absence of CAD to 800 μm in the presence of CAD [1].

Fig. 31.1
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Filling of coronary collaterals in a normal postmortem human heart by immersion radiography (Reprinted from Fulton [46], with permission)

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In his landmark paper in 1974, Levin described the anatomical distribution of coronary collaterals in a small number of subjects with both stenotic and occluded vessels [12] (◘ Fig. 31.2a–c). In 1985 Rentrop devised his collateral score as an index of collateral function [13]. This assessment of the extent of retrograde filling was originally performed using balloon occlusion of the contralateral coronary artery. While the balloon occlusion method is now rarely employed, the Rentrop score is commonly used in research and the clinical setting to grade collateral perfusion from the donor vessel to the occluded vessel (◘ Table 31.1).

Fig. 31.2
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Collateral pathways in a right coronary artery (RCA) obstruction, b left anterior descending artery (LAD) obstruction, and c circumflex artery (Cx) obstruction (Reprinted from Levin [12], with permission)

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Table 31.1 Rentrop classification of coronary collateral circulation [13]
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3 Development of Coronary Collateral Circulation

The development of functional coronary collateral circulation is driven by the process of arteriogenesis. This involves recruitment and remodeling of pre-existing collateral vessels [14, 15]. While angiogenesis (development of new vessels) is driven by recurrent and severe myocardial ischemia [16], arteriogenesis does not require an ischemic stimulus but instead is induced by the pressure gradient created by a hemodynamically significant obstruction of a major coronary artery and the resulting increase in shear stress [17, 18]. The decrease in pressure distal to the obstruction causes redistribution of blood flow through pre-existing collateral vessels which, as a consequence, now connect a high-pressure system to a low-pressure system. The increased flow velocity leads to increased shear stress, which activates expression of endothelial chemokines, adhesion molecules, and growth factors. This process causes proliferation of endothelial and smooth muscle cells in the collateral vessels, resulting in vascular remodeling and development of angiographically visible collateral vessels within weeks following a coronary artery occlusion [19,20,21]. As collateral vessels grow in size, their numbers decrease, reflecting a reduction in vascular resistance. This process, called «pruning,» efficiently improves collateral flow through a smaller number of larger vessels [22].

Several clinical and angiographic variables correlate with the degree of collateralization. Variables that are associated with more extensive collateralization in patients without CAD include hypertension and a lower resting heart rate [23], while in patients with CAD they include severity of coronary stenosis (for angiographically visible collaterals to develop, severe narrowing (>90%) or complete obstruction is necessary), longer duration of angina, longer duration of vessel occlusion, and proximal lesion location [24,25,26,27].

4 Physiological Function of Coronary Collateral Circulation

While collateral function can be assessed using the Rentrop score, which angiographically grades the degree of perfusion from the donor vessel to the recipient vessel, this method is semiquantitative and can be influenced by the blood pressure, force of contrast injection, and duration of angiographic acquisition. In 2003, on demonstrating a poor correlation between the Rentrop score and invasive measures of collateral function, Werner proposed a visual grading of the size of collateral connections [28] (◘ Table 31.2). With the collateral connection (CC) classification he was able to discriminate collaterals with different functional capacity and predict collateral adequacy to preserve left ventricular (LV) function [28].

Table 31.2 Werner collateral connection (CC) classification of coronary collateral circulation [36]
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The gold standard assessment of collateral function is the invasively measured collateral flow index (CFI) [29, 30]. The CFI utilizes simultaneous coronary pressure measurements to quantify coronary flow through the collateral circulation. The amount of flow via collaterals to the vascular region of interest is expressed as a fraction of the flow via a patent vessel. Taking into account the central venous pressure (CVP), and assuming that coronary pressure directly reflects coronary flow, the CFI is the ratio between the pressure proximal to and distal to the occlusion. The resulting equation is CFI = (P occl − CVP)/(P ao − CVP), where P occl is the distal coronary occlusive or wedge pressure and P ao is the aortic pressure.

Severe stenoses are associated with increased collateral function and a higher CFI [2]. Patients with a CTO have a higher CFI in the occluded vascular territory than patients with nonocclusive CAD [31]. Collateral function can therefore be considered both an adverse marker (as a direct indicator of CAD severity) and a favorable marker (of the degree of protection provided to the jeopardized myocardium). The net effect is determined by the balance between CAD severity and the presence and extent of coronary collateral circulation [32].

5 Clinical Significance of Coronary Collateral Circulation

The benefit of well-developed coronary collateral circulation is the potential to preserve myocardial function, with associated improved survival. This is evident in patients with preserved LV function without regional wall motion abnormalities, despite the presence of a totally occluded coronary artery [33]. While this preservation of function is frequently seen in patients with CTO, with an acute coronary occlusion the collateral circulation is usually inadequate to prevent myocardial ischemia and injury without emergency recanalization and reperfusion [34]. In fact, only a quarter of patients with normal coronary arteries, and a third of those with CAD, have sufficient recruitable collaterals to prevent ischemia during an acute occlusion [3, 10, 35, 36]. In patients with an acute myocardial infarction without detectable collaterals on presentation, several weeks or months are required to develop collaterals to a functional capacity similar to that seen in patients with pre-existing collateral circulation [19,20,21]. In patients with pre-existing collaterals at the time of an acute occlusion, the presence of collaterals has been shown to preserve myocardial function, limit infarct size, and reduce postinfarct ventricular dilatation, aneurysm formation, and the risk of cardiogenic shock [6, 10, 37, 38].

However, despite this protective role, there is no evidence that the extent of collateral supply is an indicator of myocardial viability [39, 40]. Well-developed collaterals can be present in the absence of viable myocardium, as the development of collaterals is the result of a pressure gradient across the circulation and is independent of ischemia [40].

Collateral function, quantified by the CFI, is an independent predictor of prognosis [41]. A meta-analysis assessing the impact of coronary collateral circulation on mortality in patients with stable or acute CAD found that a high degree compared with a low degree of collateralization was associated with improved survival, with the association being stronger in patients with stable CAD [9].

6 Coronary Collateral Pathways

Coronary collateral pathways, first described by Levin in 1974, were recently comprehensively described and illustrated in a large cohort of patients with CTOs [12, 42]. CTOs most commonly occur in the right coronary artery (RCA), followed by the left anterior descending artery (LAD), and least frequently the circumflex artery (Cx) [42]. The most prevalent collateral pathways to an RCA CTO are LAD septal collaterals to the right posterior descending artery (RPDA), followed by collaterals from the Cx to the right postero-lateral ventricular branch (RPLV). The commonest collateral pathways to an LAD CTO are RPDA septal collaterals to LAD, followed by right atrial (RA) or right ventricular (RV) collaterals to LAD. The collateral pathways to Cx CTOs are less prevalent with the most common, diagonal (D) to obtuse marginal (OM) collaterals, followed by RPLV to Cx collaterals (◘ Fig. 31.3a–c).

Fig. 31.3
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Common collateral patterns in a right coronary artery (RCA) chronic total occlusions (CTOs), b left anterior descending artery (LAD) CTOs, and c circumflex artery (Cx) CTOs (Reprinted from McEntegart et al. [42], with permission)

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7 Role of Coronary Collateral Circulation in Percutaneous Coronary Intervention

CTO PCI can be performed antegradely, working forward through the target vessel, or retrogradely, working backward via a donor artery and collateral that connect with the distal vessel beyond the occlusive segment. The approach is determined by the specific anatomy and characteristics of the occlusion and the potential utility of the collaterals. Therefore, collateral circulation has two key roles in CTO PCI: to provide visualization of the distal coronary bed beyond an occlusion; and to provide retrograde access to the occluded vessel to facilitate crossing and recanalization of the occlusive segment.

A collateral channel is defined as «interventional» if it is considered suitable to deliver equipment from the donor vessel to the target vessel beyond the occlusion [42]. The specific collateral pathways available have implications for the feasibility and safety of obtaining retrograde access to the target artery [42] (◘ Table 31.3). In RCA CTOs, while the LAD septal collaterals, when present, are considered to be «interventional» in over 75% of patients, the Cx to RPLV collaterals can be used in fewer than 20% of cases. In LAD CTOs, when RPDA septal collaterals are present they can be used in 80% of patients, and while LAD septal to septal autocollaterals are infrequent, when present they can be used in over 60% of cases. In Cx CTOs, when a posterolateral ventricular branch (PLV) to Cx or OM collateral is present it can be used in 50% of cases. In a left-dominant Cx, LAD septal to left posterior descending artery (LPDA) collaterals, when present, can be used in two thirds of cases.

Table 31.3 Common chronic total occlusion (CTO) collateral pathways and interventional suitability for the right coronary artery (RCA), left anterior descending artery (LAD), and circumflex artery (Cx) (Reprinted from McEntegart et al. [42], with permission)
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The most common reason for failed retrograde CTO PCI is an inability to cross the collateral connections. It is therefore important to identify for each individual case which of the available collaterals are anatomically suitable for retrograde access. The collateral characteristics that should be considered are listed in ◘ Table 31.4 [42]. By simplifying these criteria to the four characteristics of collateral channel size, collateral exit from the donor vessel, collateral tortuosity, and collateral entry into the target vessel, a collateral score can be devised to identify the most favorable channels for retrograde crossing (◘ Fig. 31.4a–d).

Table 31.4 Collateral characteristics that determine favorability for retrograde crossing [42]
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Fig. 31.4
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a Collateral channel size affecting feasibility of collateral crossing. b Collateral exit from donor vessel. c Collateral tortuosity. d Collateral entry into target vessel

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Performing a detailed assessment of the available collateral pathways and their anatomical characteristics during procedural planning can improve CTO PCI procedural efficiency and success.

8 Therapeutic Options

Several experimental studies and early clinical studies have demonstrated that it is possible to promote arteriogenesis with the growth factors granulocyte–macrophage colony-stimulating factor (GM-CSF) or granulocyte colony-stimulating factor (G-CSF), or with external counterpulsation [2, 43,44,45]. However, these studies were too small to evaluate whether these findings would translate into improved clinical outcomes.

9 Summary

While coronary collateral circulation can be seen in patients with unobstructed coronary arteries, it is of particular importance in the presence of coronary artery disease. It has an important protective role in acute and chronic ischemia and is associated with preservation of left ventricular function and improved survival. Beyond this physiological function, it plays two key roles in percutaneous coronary intervention for chronic total occlusions: allowing visualization of the distal coronary bed beyond an occlusion; and providing access routes to the distal vessel to facilitate retrograde recanalization of the occlusive segment. Recognizing the protective and interventional importance of collateral circulation, there are ongoing trials of biological therapies aiming to enhance development of coronary collaterals and their capacity for myocardial perfusion.