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In contrast to bradycardia, if the ventricular rate is fast, the regularity and duration of the QRS complexes should be assessed. Regularity is divided into two categories: mostly or completely regular and chaotic (i.e., “irregularly irregular” without any pattern). Chaotic rhythms are usually due to atrial fibrillation, irrespective of the appearance of the baseline or QRS duration. Other less common causes include multifocal atrial tachycardia and frequent extrasystoles (i.e., atrial, ventricular, or junctional).

To simplify the process of measuring duration and assessing regularity, EMS personnel should obtain an ECG strip. From this, they or the medical oversight physician can measure in “small boxes” how wide the QRS duration is and look for irregularity. Each small box represents 0.04 seconds at normal paper speed. Having EMS clinicians seek out “How many small boxes wide is the QRS complex?” will limit mathematic or conversion errors. Similarly, evaluating printed strips also helps detect irregularity that may be difficult to appreciate on a monitor screen if the ventricular rate is greater than 150/minute. In these cases, close tracking on a 6‐second ECG strip may help detect chaos and identify atrial fibrillation.

Those rhythms with a QRS duration of less than three small boxes (0.12 seconds) are narrow‐complex dysrhythmias. Conversely, any rhythm with a QRS duration of greater than three small boxes is a wide‐complex dysrhythmia. Nearly all narrow‐complex rhythms originate from atrial or nodal (i.e., supraventricular) sources. Wide‐complex rhythms can originate from a ventricular or supraventricular source. In the latter situation, some abnormality in ventricular conduction is responsible for the prolonged QRS duration. In the field, attempts to separate the myriad causes of wide‐complex tachydysrhythmias rarely alter therapy and are unnecessary. Treatment should be based on the clinical stability of the patient, basic history, and the simple ECG characteristics previously defined.