Synchrotron infrared microspectroscopic analysis of collagens I, III, and elastin on the shoulders of human thin-cap fibroatheromas

Of the many people who experience a sudden cardiac event (acute coronary syndromes and/or sudden cardiac death), a large portion has no prior symptoms. One potential in vivo spectroscopic technique for diagnosis of pathological conditions that underlie these sudden cardiac events involves the use of a near-infrared spectrometric catheter with moderate in vivo spatial resolution. To justify the time and expense of such an in vivo protocol, the putative vulnerable narrow region at the shoulder of the thin cap fibroatheroma is chemically characterized by high spatial resolution mid-infrared microspectroscopy. The sharp peaks of the mid-infrared and the previous band assignments that are readily available are useful in establishing the basis needed to support the development and validity of future in vivo NIR probing. The spatial resolution of in vivo NIR spectrometric catheters is limited by light scattering from blood and by the motion of the catheter and blood vessel wall, making it difficult to characterize a fibrous cap in the rupture zone. However, the spatial resolution of in vitro synchrotron IR microspectroscopy is high and probably sufficient to characterize chemically the actual area of disruption. A thin-cap fibroatheroma is a rupture-prone plaque. The shoulder of the cap (where the cap meets the vessel wall) is most vulnerable to rupture because mechanical stress at this point weakens the collagen and elastin fibers. It is hypothesized that the breakdown of elastin is highest in this target zone, followed by collagen III. The analysis of collagen I, collagen III, and elastin concentration in the small (ca.10 μm) interface zone, between the intimal wall of the artery and the fibrous cap, is of concern because it is the shoulder where the protein degradation is expected to be the highest. (A similar degradation occurs on a larger scale in the vessel wall in abdominal aortic aneurysm.) For this reason, if confirmed, testing at this location would presumably offer the highest sensitivity and provide the earliest possible warning of rupture-prone plaque. In the current study, post-mortem human tissue was used. Future experiments will be performed on animal models where in vivo NIR catheterization is followed by post-mortem mid-infrared microspectroscopy on the same animal. Subsequently it may be possible to develop in vivo near-infrared spectrometric catheter techniques suitable for use with human subjects in a clinical setting.