Please shoot me an E-mail at mkwong@tuftsmedicalcenter.org and let me know what electrocardiograph you are using as there are several ECG criteria and measurements.
Note that while most if not all commercial electrocardiograph products only support the standard 12-lead, the mapping I have created supports the full 16-lead system plus vectorcardiograms (XYZ) in terms of measurements. These would fall under a custom vocabulary since SNOMED does not support 16 lead nor XYZ concepts.