To determine no beating cardiomyocytes from ECG or video recording
A number of cardiomyocytes is lost during an infarction. Instead, fibrous tissue forms, but it only preserves structural integrity, having no contractile function [1]. Myocardial infarction is an uncommon condition in pediatrics, happening very rare, when embolization occurs due to blood coagulation affections or heart congenital defects [2].

The fibrous tissue has different properties, thus modifying:


MDE-MRI images [3]
ECG: deep Q wave [4]
echocardiography: hypoechoic and hyperdense echoes inside the fibrotic material (the latter compatible with calcium) [5]


That video record is probably about 3D Echocardiography (look here at Video 2 and figure 2).

References:


Yao Sun, Myocardial repair/remodelling following infarction: roles of local factors
Nicola Carano et al., Acute Myocardial Infarction in a Child: Possible Pathogenic Role of Patent Foramen Ovale Associated With Heritable Thrombophilia
Harikrishna Tandri et al., Noninvasive detection of myocardial fibrosis in arrhythmogenic right ventricular cardiomyopathy using delayed-enhancement magnetic resonance imaging
ECGpedia - Pathologic Q Waves
Berenzstein CS et al., Usefulness of echocardiography and doppler echocardiography in endomyocardial fibrosis