On an electrocardiogram tracing, a delta wave is usually an unexpected finding. In the QRS complex, the Delta wave takes the place of the Q wave. In the graphic below you can see the difference between the tracing from normal sinus rhythm and a heartbeat tracing that includes a Delta wave.
You may also notice that the PR segment (the portion of the tracing along the baseline, between the depolarization of the atria (P wave) and the beginning of the depolarization of the ventricles (QRS complex)) is missing. This happens because the electrical signal from the atria is being conducted to the ventricles immediately after atrial contraction, carried along an anomalous accessory pathway called the Bundle of Kent instead of through the AV node, which delays signal conduction to allow the ventricles to fill with blood before contraction. This produces Wolff Parkinson White (WPW) syndrome, which may be asymptomatic for a person’s entire life, but it also has the potential for initiating dangerous, rapid ventricular rhythms.
The good news is, the Delta wave is easy to recognize, and the condition that wave conveys (WPW) is one that’s successfully treated through catheter ablation of the Bundle of Kent.