Probing the intravascular and interstitial compartments of remodeled myocardium in heart failure patients with preserved and reduced ejection fraction: a CMR study

Heart failure with preserved ejection fraction (HFpEF) is a common condition with increasing prevalence and high morbidity and mortality [1, 2] HFpEF occurs in association with advanced age, cardiovascular and metabolic comorbidities as well as with a pro-inflammatory state [2]. It is characterized by impaired left ventricular (LV) relaxation and increased diastolic stiffness [2, 3]. Despite intensive research, the myocardial abnormalities subtending HFpEF are incompletely understood. Studies of endomyocardial biopsy or of autopsy of selected HFpEF patients reported myocyte hypertrophy, diffuse interstitial fibrosis and signs of systemic and myocardial inflammation [4, 5]. In this setting microvasculature rarefaction was also observed, and it was inversely correlated to myocardial fibrosis [6, 7]. Accordingly, the detection and quantification of microvascular rarefaction might be of great value for assessing the pathophysiological abnormalities occurring in patients with HFpEF and for establishing its diagnosis. However, the non-invasive measurement of myocardial intravascular compartment in humans is challenging.

Based on these premises, we developed a cardiac magnetic resonance (CMR)-based approach for the quantification of the myocardial intravascular volume (IVV) consisting in the use of an intravascular gadolinium-based contrast medium (CM) combined with the quantitative measurement of myocardial T₁ by currently available mapping technique [8, 9]. To this end, we administered an intravascular and extravascular CM 1). in healthy controls, 2). in HFpEF patients with LV hypertrophy (LVH) and/or proven interstitial myocardial fibrosis, and 3). in patients with chronic myocardial infarctions (MI). Post-MI patients were included to study the remote non-infarcted myocardium, which typically undergoes remodelling processes with eccentric hypertrophy [10]. In addition, in post-MI patients, we tested the hypothesis that the intravascular CM allows to quantify the reduced IVV, which is a typical feature of post-ischemic infarct scar [11].

The main findings of the current study can be summarized as follows. First, in patients with HFpEF, there was no reduction of the myocardial intravascular compartment as derived by T₁-mapping after gadofosveset administration, while the extracellular compartment was expanded as compared to controls. Second, in the scar tissue of post-MI patients the intravascular compartment was significantly reduced, while the extracellular compartment was massively increased. Third, myocardial edema was not detected in the remodeled myocardium of HFpEF patients as evidenced by the normal T₂ values.

By assuming that the vascular endothelium is not permeable to albumin under normal conditions, gadofosveset, which binds reversibly to albumin, behaves preferentially as an intravascular CM after intravenous bolus injection. Using T₁-mapping, we calculated the relaxivity of myocardium and blood pool immediately, i.e. at 3 min, after gadofosveset bolus injection, and these measurements, in conjunction with the hematocrit, were used to derive the myocardial IVV by adopting the previously published formula [9, 11]. The IVV was significantly decreased in the myocardial scar of post-MI patients as compared to normal myocardium of healthy controls, indicating that this approach is able to detect a reduced intravascular space, which is a key feature of dense fibrotic scar [11]. When estimating IVV by the administration of an intravascular CM, it is important to recognize that T₁-mapping measures the effect of CM on proton relaxation, not reflecting directly the concentration of CM in the intravascular compartment. Although gadofosveset is a prevalently intravascular CM, it affects the extravascular compartment by proton (i.e. water) exchange across the vascular wall [19]. Several studies suggested a slow exchange regime for the intra – extravascular water [19]. Simulations and in-vivo measurements by Donahue and co-workers also demonstrated an increasing overestimation of IVV with increasing intravascular CM dosages (i.e. with an increase in pre-contrast to post-contrast difference of blood relaxivity ΔR₁). For the slow exchange model, simulations found an overestimation of IVV by approximately 10–30% at a ΔR₁ of 9, which is matching the ΔR₁ found in our study (at 3 min after gadofosveset injection) [19]. After IV injection, ~ 70% of circulating gadofosveset binds reversibly to albumin resulting in an increase in relaxivity by 5- to 10-fold (r₁ = 25 mmol^.L^-1.s^− 1) as compared with the free fraction (r₁ = 5 mmol^.L⁻ 1^.s^− 1) [20]. The unbound fraction of gadofosveset behaves as an extra-vascular extra-cellular CM, thereby increasing tissue relaxivity in the extravascular space and thus, overestimating IVV. Although we acknowledge that our protocol may lead to an overestimation of IVV by means of either mechanisms mentioned above, it should be kept in mind that overestimation is expected to occur systematically across the diverse study groups and should therefore not affect the trend of the observed pathophysiological abnormalities (e.g., increase or decrease of IVV). Taking into account that the unbound fraction of gadofosveset behaves as an extracellular extravascular CM, the overestimation of IVV is expected to be larger in those pathological conditions with an expanded extracellular compartment (i.e. larger ECV). Nevertheless, our approach showed an important decrease of IVV in the ischemic scar for which we measured a substantial expansion of the extracellular compartment in the order of 70% using an extracellular CM (ECV of 0.698 ± 0.106). Our results are in line with a study by Prech and co-workers [11], who quantified capillary density in autopsy hearts. In that study, in post-MI patients, capillary density was reduced in post-ischemic non-reperfused scar tissue by approximately 40% versus normal. In infarcted myocardium, we found a reduction of IVV of about 25% versus normal, i.e. less than that reported by Prech et al. [11]. This difference matches the expected slight overestimation of 10–30% by our method as discussed above. With this reasoning, it follows that the unbound fraction of gadofosveset is very unlikely to contribute significantly to IVV overestimation in HFpEF patients, who had only slightly increased ECV as compared to healthy volunteers. In summary, we set out a non-invasive CMR approach to estimate IVV by measuring T₁-maps before and at 3 min after intravascular CM bolus injection, which was demonstrated in ischemic scar versus normal myocardium of healthy controls. Reduced IVV of chronic ischemic scar is well established in the literature, and our approach was capable to correctly gauge this key feature of dense chronic fibrotic scar.

When applying the IVV measurement to non-infarcted myocardium, IVV was not reduced in HFpEF myocardium or in non-infarcted myocardium of post-MI patients. Hence, our results do not confirm some of the previous studies in HFpEF [6, 7]. In particular, in an autopsy cohort of 124 patients who died of HFpEF, Mohammed et al. reported lower coronary microvascular density and more severe fibrosis as compared to control subjects [6]. These authors also found an inverse relationship between microvascular density and fibrosis. However, in their study the HFpEF cohort was somewhat heterogeneous being constituted by 37% of patients with a previous history of MI, and as many as 74% of patients showed LV dilatation and hypertrophy. On the other hand, in our study none of the HFpEF patients had a history of coronary artery disease, evidence of ischemia, or ischemic scar, and none was in advanced stage of heart failure. Conversely, our results are in line with the study by Prech and co-workers [11], who quantified capillary density in autopsy hearts of post-MI patients. In that study, capillary density in remote non-infarcted myocardium was not different from normal myocardium of control hearts.

While slow water exchange was addressed as a potential confounder for measuring IVV in chronic fibrotic post-ischemic scar, additional confounders should be considered when interpreting our IVV results for non-infarcted remodeled myocardium. The concept of using an intravascular CM to quantify IVV is strictly based on the assumption that the intravascular CM does not leak into the interstitial space. In the situation of an inflammatory state of the microcirculation of remodeled myocardium as proposed in the literature [2, 21], leaking intravascular CM would cause overestimation of IVV, in particular, if one considers the higher relaxivity of the albumin-bound fraction of Gadofosveset. To minimize this effect, in the current study T₁ was measured at 3 min. Post CM injection, as Pedersen et al. [20] showed maximum effect of CM leakage occurring at 30 min post-injection in an animal model. Elevated myocardial native, i.e. pre-contrast, T₁ values result in elevated IVV according the formula we used (page 5). Puntmann et al. found elevated native T₁ of viable myocardium of post-MI patients [22] and hypertrophied myocardium [23], which correlated with outcome [22]. In agreement with these studies, we also found elevated native T₁ values of non-infarcted remodeled myocardium compared to controls (see Table 2), and this elevated native T₁ may contribute to an increase in IVV. Furthermore, the type of the T₁ mapping pulse sequence may also influence the calculated T₁ and thus, distribution volumes. With the MOLLI sequence we used, we were able to measure reduced IVV in scar tissue (i.e. even in tissue with massively expanded extracellular space) and in a recent study, this MOLLI sequence performed best in measuring ECV among a set of different pulse sequences in comparison to histology [24].

While we did not observe differences in IVV between healthy controls and HFpEF patients, ECV was expanded in the later, which is very likely related to an accumulation of extracellular matrix in the cardiac interstitium, since myocardial edema was excluded by normal myocardial T₂ values. Accordingly, our data on ECV, i.e. on interstitial fibrosis, are in line with other studies, which reported an expanded ECV compared to healthy controls and which correlated well with markers of HFpEF severity [15]. Thus, this study is in line with others in quantifying ECV as a measure of collagen accumulation, while the current non-invasive approach to measure IVV could not detect a rarefaction of the microvasculature of hypertrophied/remodeled myocardium.

The proposed technique exploiting T₁-mapping in combination with the administration of an intravascular CM is able to detect a reduction of the intravascular compartment in post-infarction scar tissue. However, our study findings do not support the hypothesis of microcirculatory rarefication as a patho-mechanism in the myocardium of HFpEF patients.