Rhythm

Rhythm analysis can be very complex, though a disciplined approach yields the appropriate rhythm in the majority of patients. The first steps in rhythm analysis after rate are to determine if the QRS complex is wide (discussed elsewhere) and if it is regular or irregular. Regular QRS complexes occur at essentially the same interval; in other words, the R-R intervals are consistent.

The next steps are to evaluate the P waves, both in their specific morphology (discussed elsewhere) and their relationship with the QRS complex (is there a P wave for every QRS and a QRS for every P wave). Sometimes, P waves are not present.

Tachycardias

Tachycardias are divided based on if the QRS complex is narrow or wide and then further divided based on the rhythm being regular or irregular.

Narrow Complex Tachycardias

Narrow complex tachycardias (NCT) will always be from a supraventricular origin (junctional tachycardia is from around the AV node). They are usually divided into regular or irregular rhythms.

The irregular rhythm NCT's are differentiated based on the P wave morphologies. Most common of these is atrial fibrillation. In atrial fibrillation, P waves will either not be present or will be present in only scattered leads with numerous P wave morphologies as atrial fibrillation results from numerous pacemaker sites in the atria depolarizing chaotically. Patients can have atrial fibrillation with a controlled ventricular rate or even a bradycardic rate if on certain medications or they have other underlying pathology. Multifocal atrial tachycardia (MAT) is defined as at least 3 P wave morphologies that will be consistently seen throughout the EKG as there are multiple atrial pacemaker sites, though usually not nearly as numerous as with atrial fibrillation. In atrial flutter, one atrial pacemaker site is firing, causing only 1 P wave morphology. Atrial flutter is irregular if there is variable conduction; that is to say that it is irregular if the conduction from the P waves variably conducts through the AV node to the ventricles.

The regular NCT's can be somewhat more difficult to differentiate at times. Sinus tachycardia will have a consistent P wave morphology that is typical of sinus node origin. Atrial tachycardia will have a consistent P wave morphology that is not of sinus node origin. Atrial flutter will have a consistent P wave morphology that will not always conduct through the AV node (differentiating it from atrial tachycardia) and may have conduction that is 2:1 or 3:1 while tachycardic (in other words, there are either 2 or 3 P waves for every QRS complex). AV reentrant tachycardias are commonly referred to as SVT and will not have P waves that cause conduction through the AV node to the ventricles as they originate around the AV node. Therefore, P waves will either not be present or they will be retrograde (conduction through the atria are from the AV node, so the P waves will be inverted compared to ones from the sinus node). An accelerated junctional rhythm is one of the more difficult ones to identify for early EKG learners as this rhythm comes from around the AV node and may also have retrograde P waves.

Wide Complex Tachycardias

The evaluation of wide complex tachycardias also starts with division by either irregular or regular. The irregular wide complex tachycardias are the simpler portion of this division with either atrial fibrillation or ventricular fibrillation. Ventricular fibrillation is defined as numerous chaotic QRS complexes without P waves as there are numerous ventricular pacemaker sites firing chaotically. Atrial fibrillation can produce wide complex tachycardias if there is aberrant conduction. This is usually either in the form of a bundle branch block, metabolic/toxicologic processes causing aberrant conduction through the AV node/ventricles, or an accessory conduction pathway (pathway to the ventricles not through the AV node).

The regular wide complex tachycardias can be very difficult to differentiate and the ability to do so will not be listed here. In general, the evaluation is to attempt to determine if the patient has ventricular tachycardia or if the rhythm is supraventricular in origin. Numerous criteria have been developed that can be complex and can be found elsewhere on this site.

Bradycardias

Bradycardias are usually much easier to evaluate and differentiated based on the evaluation of P waves and their relationship with the QRS complex. If every P wave is followed by a QRS complex, then it is a sinus bradycardia. If P waves are not always or not at all followed by a QRS complex, then there is an AV node block. If P waves are absent, then it is either a junctional escape rhythm or a ventricular escape rhythm.

Normal Rate

The evaluation of a rhythm with a normal rate depends on the regularity of QRS complexes, if the QRS complexes are wide, evaluation of the P waves, and evaluation of the relationship between the P waves and QRS complexes. Wide complex rhythms with a normal rate include accelerated idioventricular rhythm or a supraventricular rhythm with aberrant conduction (ie sinus rhythm with a bundle branch block). Again, this aberrant conduction is usually either from a bundle branch block or a tox/metabolic process (ie hyperkalemia). Narrow complex rhythms that have a normal rate are more common and include normal sinus rhythm, sinus arrhythmia, atrial fibrillation with controlled ventricular response, atrial flutter with 4:1 conduction, ectopic atrial rhythm, an accelerated junctional rhythm.