Discussion
The present study is the first to report the association between cIB and myocardial fibrosis in patients without extensive fibrosis. In contrast to previous studies of patients with dilated or hypertrophic cardiomyopathy and mean myocardial fibrosis of 15–34%,1–5 we found that cIB was weakly but inversely associated with myocardial fibrosis of 0.7–4%, as assessed by picrosirius red staining. However, cIB was not significantly associated with other histological parameters, including immunostaining for collagens I and III, CML and RAGE. In addition, cIB was weakly associated with plasma levels of NT-proBNP, creatinine and eGFR, and more strongly associated with plasma sVEGFR-1 and sRAGE levels. We previously demonstrated less myocardial fibrosis in women than in men undergoing CABG surgery,18 and increased myocardial interstitial fibrosis in patients with aortic stenosis than in patients with CABG surgery without aortic stenosis.21 The present findings suggest that higher cIB values cannot be used as a surrogate measure of greater myocardial fibrosis in patients without dilated or hypertrophic cardiomyopathy and extensive fibrosis, and that potential surrogate markers of myocardial fibrosis need to be validated in different patient populations.
Although cIB is a measure of reflectance of the ultrasound signal by the myocardium, there is uncertainty about the determinants of cIB. The cIB values we measured were similar to those reported by Jellis et al17 in a group of patients with early diabetic cardiomyopathy; their finding of only a borderline correlation between T1 values and cIB (p=0.053) is in agreement with our finding that higher cIB values were not a marker of greater fibrosis in patients without extensive fibrosis. Taken together with previous reports,1–5 our data suggest that the contribution of fibrosis to cIB depends on the extent of fibrosis and other factors play a greater role in determining cIB when fibrosis is less extensive. This may result in a threshold level of fibrosis below which cIB is not positively related to fibrosis, but above which it may be a useful measure. It is also possible that in previous studies, the observed relationship between cIB and fibrosis was, in fact, mediated by another property, and that this relationship was lost with less-extensive fibrosis. Our finding that cIB was associated with plasma NT-proBNP, creatinine and eGFR suggests that cIB is influenced by fluid status, and this is in agreement with previous reports that cIB is correlated with serum creatinine in patients with chronic renal failure12 and is reduced by increased hours/week of dialysis over 12 months.13
The mechanism of the associations between cIB and plasma levels of sVEGFR-1 and sRAGE is not known. Increased plasma levels of sVEGFR-1 are associated with disease severity and adverse outcomes in chronic heart failure,22 and plasma sRAGE levels are associated with coronary events and cardiovascular mortality in diabetes.23 ,24 Thus, the basis of the association of plasma sVEGFR-1 and sRAGE levels with cIB may be similar to the association of acute myocardial ischaemia and acute cardiac allograft rejection with cIB.9 ,10 sVEGFR-1 functions as a VEGF-A antagonist by preventing VEGF-A binding to VEGFR-2, the main VEGFR-mediating angiogenesis, and may thereby inhibit angiogenesis. It is also possible that the association of plasma levels of sVEGFR-1 and sRAGE with cIB has a similar basis to the reported association of plasma IL-13 levels with cIB in patients with idiopathic dilated cardiomyopathy.11 IL-13 causes anti-inflammatory activities,25 and angiogenic and antiangiogenic activities have been reported for this cytokine.26 ,27 However, the failure of either sVEGFR-1 or sRAGE to correlate with myocardial capillary length density suggests that these markers represent mechanisms of myocardial reflectivity that are unrelated to angiogenesis.
There is uncertainty about the relevance of circulating collagen-derived peptides to cardiac fibrosis because these result from collagen turnover in many different tissues other than the heart.28 It is, therefore, not surprising that we found no correlation between PIP, PINP and PIIINP levels and either cIB or myocardial total or interstitial fibrosis. Jellis et al17 reported that PIIINP, but not PINP or PIP, was correlated with cIB; however, none were correlated with the postcontrast T1 value.
Study limitations
This was necessarily a cross-sectional study that provided limited information about the mechanisms of myocardial reflectivity because of the impossibility of obtaining serial myocardial biopsies, and our study was limited to 40 participants given the difficulty of obtaining myocardial biopsies in patients who were well characterised clinically, biochemically, haemodynamically and with cardiac imaging. This number of patients is, however, greater than the number studied in previous studies,1–3 ,5 except for the study of dilated cardiomyopathy by Mizuno et al.4 Moreover, we studied biopsies from the same subepicardial region of the left ventricular myocardium in all patients and we do not know if the histology of these biopsies was representative of the regions used to estimate cIB. Although the patients had extensive coronary artery disease, we biopsied a region of myocardium that was free of ischaemia, without wall motion abnormalities; the histology of the biopsies showed no evidence of ischaemia or replacement fibrosis. We cannot exclude the possibility that myocardial interstitial and perivascular fibrosis were inhomogeneous, but our data clearly provide no support for a positive relationship between cIB and myocardial fibrosis in this patient population. Future studies are necessary to directly compare myocardial histology with alternative non-invasive methods of evaluation myocardial fibrosis, such as T1 mapping by cardiac MRI,17 and to investigate the relationships between cIB and plasma levels of NT-proBNP, creatinine, eGFR, sVEGFR-1 and sRAGE.