Physiology and Impact of Coronary Collateral Vessels

The ability of collateral circulation to limit myocardial ischaemia and prevent infarction has been long established. However, the influence of collaterals on the outcome following percutaneous coronary revascularisation (PCI) remains the subject of much debate. While some studies have shown that the presence of collaterals reduces the likelihood of clinical events on long term follow up, there is also evidence suggesting an association between collaterals and the risk of restenosis. Much of this controversy may be attributed to the qualitative and potentially inadequate methods previously used to assess collateral flow. Using a pressure-sensor tipped coronary guidewire of 0.014” diameter, a quantitative method of documenting collateral flow can be determined, based on the simultaneous measurement of mean arterial, coronary wedge and central venous pressures under conditions of maximum coronary vasodilatation ( Figure 1 ).

Figure 1: Invasive quantification of coronary collateral flow. Distal coronary pressure (P d , green trace), aortic pressure (P a , red trace) and central venous pressure (P v , not shown) are measured simultaneously during maximal hyperaemia. During coronary occlusion at the time of angioplasty, the Collateral Flow Index (CFI) is calculated as (P d -P v )/(P a -P v ).

 

Our aims are, firstly to determine the short and long term plasticity of collaterals, which is integral to our understanding of the role of collaterals following PCI; and secondly to demonstrate the relationship between collaterals and restenosis. This is being achieved by measuring collateral flow at the time of PCI, 18-24 hours later and repeating these measurements at 6 months follow-up. In addition, by using intravascular ultrasound, the volume of neo-intimal tissue within the stent can be determined as an accurate measure of restenosis ( Figure 2 ). Preliminary results suggest little change in recruitable collaterals in the early post-PCI period.

 
Figure 2: Intracoronary Ultrasound is used to define the vessel border (external elastic lamina, red contour), stent (green contour) and vessel lumen (yellow contour). Integration of serial contours along the length of the vessel segment allows 3-D quantification of restenosis (neointimal volume) and delineates the remodelling process.

 

These detailed measures of collateral flow and restenosis, in the setting of altered myocardial ischaemic burden, also provide an opportunity to investigate circulating endothelial progenitor cells (EPCs). The purpose of these investigations is to determine whether these cells are related to collateral plasticity, restenosis and myocardial ischaemia by examining their phenotype in vitro and their expression of surface antigens.