AIMS: Experimental autoimmune myocarditis (EAM) is a widely used murine model, in which cellular myocardial infiltration resembles human viral myocarditis. Although myocarditis can be readily assessed on histology, heart failure has not been fully characterized, as there are limitations in available markers and difficulties in hemodynamic measurements, especially on small rodents. We investigated whether intravital microscopy of the microcirculation can be used to characterize heart failure in EAM.

METHODS: BALB/c mice (n= 10 versus n= 5 controls) were immunized with alpha myosin heavy chain peptide and myocarditis was confirmed on hematoxylin-eosin (HE) histology on day 21. Echocardiography assessment included ejection fraction (EF), fractional shortening (FS), mitral valve doppler, left-ventricular end-diastolic diameter (LVEDd) and diastolic intra-ventricular septum thickness (IVSd). Microcirculatory analysis was performed using a sidestream dark field (SDF) microcirculation camera. The proportion of perfused vessels (PPV) and perfused vessel density (PVD) were recorded on the intestinal mucosa of the anaesthesized mice.

RESULTS: Immunized mice developed EAM with typical cellular infiltration (p< 0.003), left-ventricular hypertrophy (IVSd, p= 0.027) and diastolic dysfunction (E/A, p= 0.028) without significant EF reduction (p= 0.845) or LV dilation (p= 0.854). SDF recording consisted mainly of venules, as capillaries were too small. PPV and PVD were significantly increased in EAM mice (p 0.001 and 0.01 respectively) and correlated significantly with the histological myocarditis severity score (r= 0.557, p= 0.03 and r= 0.57, p= 0.025 respectively), whereas PPV but not PVD correlated with IVSd (r= 0.588, p= 0.02) and E/A ratio (r= 0.703, p= 0.003).

CONCLUSIONS: Intravital microscopy can be used to characterize post-capillary intestinal perfusion of EAM mice. Thus we show a congestion of intestinal venules in EAM which correlates to the severity of myocarditis.