Pericarditis

Acute pericarditis is a type of pericarditis (inflammation of the sac surrounding the heart, the pericardium) usually lasting less than 6 weeks. It is by far the most common condition affecting the pericardium.

Typical ECG changes in acute pericarditis includes:

• stage 1 - diffuse, positive, ST elevations with reciprocal ST depression in aVR and V1. Elevation of PR segment in aVR and depression of PR in other leads especially left heart V5, V6 leads indicates atrial injury.
• stage 2 - normalization of ST and PR deviations
• stage 3 - diffuse T wave inversions (may not be present in all patients)
• stage 4 - ECG becomes normal OR T waves may be indefinitely inverted

The two most common clinical conditions where ECG findings may mimic pericarditis are acute myocardial infarction (AMI) and generalized early repolarization.

Parasystole

Parasystole is a kind of arrhythmia caused by the presence and function of a secondary pacemaker in the heart, which works in parallel with the SA node. Parasystolic pacemakers are protected from depolarization by the SA node by some kind of entrance block. This block can be complete or incomplete.
Parasystolic pacemakers can exist in both the atrium or the ventricle. Atrial parasystolia are characterized by narrow QRS complexes.

There are two forms of ventricular parasystole, fixed parasystole and modulated parasystole. Fixed ventricular parasystole occurs when an ectopic pacemaker is protected by entrance block, and thus its activity is completely independent from the sinus pacemaker activity. Hence, the ectopic pacemaker is expected to fire at a fixed rate. Therefore, on ECG, the coupling intervals of the manifest ectopic beats will wander through the basic cycle of the sinus rhythm. Accordingly, the traditional electrocardiographic criteria used to recognize the fixed form of parasystole are:

• the presence of variable coupling intervals of the manifest ectopic beats;
• inter-ectopic intervals that are simple multiples of a common denominator;
• fusion beats.