Positive and negative ECG waveforms for students and new nurses
The graphic above illustrates one of the first basic rules of ECG interpretation.
That is, a positive impulse (or wave of depolarization) traveling toward a positive electrode should produce a positive (upright; above the isoelectric baseline) deflection. Cardiac pacemaker cells and cardiomyocytes maintain a relatively negative inner charge while at “rest”. When the cells of the heart DE-polarize, ions are exchanged and the net result is a more relatively positive charge of the cell.
This alerts neighboring cells to do the same and this wave of cellular depolarization is what we’re referring to when we say “a positive impulse that’s travelling toward…” (a surface electrode on the skin; positive electrode). The excitement and depolarization stimulates the cardiomycytes to contract and produce cardiac output.
The inverse is also true. A positive impulse travelling away from a positive electrode is expected to produce a negative (downward) deflection on the ECG (upper right-hand box in the illustration). While the direction of impulse travel may not necessarily be changing (although it can; i.e. in Torsades de pointes and other arrhythmias) the angle of the lead viewing this waveform may differ.
Lead V1 for example, sits just over the right atrium where depolarization typically begins at the SA node (see below). It would make sense in this case, that most of the depolarization waveform would be traveling AWAY from the V1 electrode in the normal healthy heart. And so, we expect most of the waveforms to appear negative in this lead for this reason.
As the precordial (chest) leads are positioned closer and closer to the apex of the heart from V1 to V6 (where depolarization is headed from top to bottom), the R wave should become more positive and larger. This is referred to as “R wave progression” and is appreciated on the 12-lead ECG.
Hope this helps with conceptualizing why some deflections are up, some are down, and why this can be completely normal depending upon the lead being viewed!
*For illustrative purposes only. Exceptions may exist.
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