In the present study, we have shown that echocardiographic cIB is a useful tool for the non-invasive, quantitative monitoring of progression and subsequent spontaneous regression of aortic valve calcifications in a rat model of diet-induced renal failure. We validated this technique by comparison with histology.
CAVD and its relationship with renal failure
CAVD accounts for 70% of all valvular heart diseases and evolves to significant aortic valve stenosis in about 10% of patients. In this population, the outcome is poor as there is a significant 5-year increased risk of progression to heart failure or death
. Hence, there is a need for medical treatment of CAVD. However, reversal of end-stage macrocalcification is deemed difficult
. To date, medical therapies have failed to demonstrate a significant effect on advanced valvular calcifications
. In animal studies, only early preventive medical interventions have shown to be of interest
Renal failure is a major risk factor for cardiovascular calcifications and CAVD due to secondary hyperparathyroidism and a disturbed mineral metabolism with increased Ca × P
. In chronic kidney disease, approximately 50% of patients will develop CAVD at an accelerated rate, leading to increased morbidity and mortality
[16, 17]. Small animal models such as rats have become increasingly important to study renal failure and CAVD
. Shuvy et al. have previously investigated a rat model of adenine and high-phosphate diet-induced renal failure and subsequent CAVD
. Interestingly, in this study discontinuation of the diet led to a progressive recovery of the renal function after 2 weeks and marked spontaneous improvement of the aortic valvular calcifications after 12 weeks
. The observed reversibility of CAVD might offer an opportunity for the development of new medical treatments.
Imaging of aortic valve calcifications
Calcification is an independent predictor of morbidity and mortality in CAVD
[2, 3]. Quantification of calcifications in CAVD with in vivo imaging tools could be helpful for the monitoring of disease progression and potential regression through medical interventions. Although the reference approach, the use of cardiac CT is limited due to irradiation exposure of patients. Repetitive evaluation by cardiac CT is thus hampered. Conventional echocardiography primarily makes use of specular reflections and Doppler ultrasound to investigate valve morphology and function
. Aortic valve calcification is rather regarded as a categorical variable with the current qualitative methodology
. However, by analyzing backscatter reflections generated by the interaction of ultrasound with small tissue structures, it is possible to quantitatively extract information related to the valve’s composition
[12, 20, 21]. IB has previously been extensively validated in the myocardium as a surrogate marker of fibrosis and for the differentiation of atherosclerotic lesions
[22, 23]. We have previously shown that with IB calcific deposits of the aortic valve can be quantitatively and reproducibly assessed in rats
[7, 8]. Others have used IB for the quantitative detection of aortic valve calcifications in vitamin D treated rabbits, which could be delayed with ramipril
[24, 25]. Preliminary clinical studies have demonstrated that higher backscatter scores are seen in sclerotic compared to non-sclerotic aortic valves, with a further increase in stenotic valves
[26, 27]. Backscatter scores of the aortic valve directly correlated with subjective scoring and with transvalvular pressure gradients in patients
Integrated backscatter analysis of aortic valve calcifications
In the present study, we have shown that cIB can detect a significant progression of CAVD after 9 weeks in rats with renal failure, compared to baseline and controls of the same age. Twelve weeks after discontinuation of the adenine diet, the cIB of the aortic valve returned to values similar to controls, suggesting a regression of the calcifications. This confirms the validity of cIB for the quantitative evaluation of aortic valve calcifications
[7, 8, 24]. Moreover, it corroborates the findings of Shuvy et al. in a similar model. In that particular study, quantitative CT was used for assessing CAVD. However, the amount of calcifications on histology and echocardiography were not provided
. In our study, there was a significant correlation between cIB and the histological Ca Area for the aortic valve. Moreover, there was a significant inter- and intra-observer reproducibility of cIB values. Global cIB values (ROIAVGLOB) and mean cIB values (ROIAVTOT) also correlated significantly. This implies that the global cIB values can be used, potentially simplifying the technique for daily practice.
Blood results showed a significant decrease of the renal function during adenine feeding in group 1 compared to controls, associated with a significant increase of P, Ca × P, AP and osteocalcin. After diet cessation in group 1, P, Ca × P and AP normalized at 21 weeks. Interestingly, in our study there was only a minor improvement of the renal function after diet cessation in group 1, in contrast to the complete recovery seen in the study of Shuvy et al.
. This could be due to a difference in susceptibility of the rat strain (Wistar versus Sprague–Dawley). Our results confirm a previous study by Okada et al., suggesting irreversible renal failure after 4 weeks of treatment with adenine (0.75%)
. As a regression of CAVD was seen in spite of the remaining elevated creatinine values, it seems that the metabolic changes induced by renal failure and secondary hyperparathyroidism may be the major mediators of CAVD, rather than renal failure per se
The elevation and subsequent regression of AP might confirm a role of increased bone turnover with mobilisation of Ca and P in secondary hyperparathyroidism-associated CAVD
. Osteocalcin levels remained high at the end of the study in group 1, even 12 weeks after diet cessation. This calcification inhibitor might be produced by differentiated osteoblast-like cells in the calcifying cardiovasculature in an attempt to further prevent and/or reduce calcifications
. In the adenine-fed group 1, there was a weight loss as previously described
, with a reduction in EF, CO and transvalvular PG on echocardiography. However, LV performance returned to normal after diet cessation in group 1.
Backscatter software is now widely available on recent echocardiography packages, which makes this tool feasible for clinical application. In daily practice, cIB may be used for the detection of early aortic valve calcifications, for the assessment of disease severity and progression as well as for evaluating the effectiveness of potential medical interventions in prospective and longitudinal trials.