Arrhythmias and Tachycardias

In this chapter, we address the basic notions of cardiac arrhythmias. Premature ventricular and atrial contractions, also known as “extrasystoles”, are “extra” heartbeats. Atrial premature beats, describes premature beats arising from the atrium. Ectopic P' wave morphology differs from sinus beats and varies depending on the origin of the premature beat. The mechanism of the tachycardia is macro-reentry or automaticity \and the rate varies between 140 and 220 bpm. There are two types of atrial flutter, the common type I and the rarer type II. Most individuals with atrial flutter will manifest only one of these. Rarely someone may manifest both types. Flutter originates either from the right or left atrium depending on its cause. Typical atrial flutter (90% of cases) is caused by a macro re-entry in the right atrium, with a regular rate of about 300 beats per minute. Atrial fibrillation is a consequence of multiple atrial micro reentry circuits. The arrhythmia is described as irregularly irregular because of the complete disorganization of the atrial electrical activity. Characteristic findings are the absence of P waves, with unorganized electrical activity in their place (“f” waves), at a rate of 350 to 500/minute and irregularity of the R-R interval due to irregular conduction of impulses to the ventricles.

AV nodal reentrant tachycardia (AVNRT) is also called junctional tachycardia. AVNRT is usually a reentrant tachycardia using a reentry circuit located within the AV node area. In the typical form of AVNRT (> 90% of the cases), the reentry circuit uses the slow pathway antegradely and the fast pathway retrogradely. Antegrade ventricular activation occurs simultaneously with the retrograde atrial activation. The P' wave is hidden in the QRS complex, sometimes visible as a small r' wave in V1 at the end of the QRS complex. Rarely it can be seen as a small q wave in the inferior leads. Comparison of the trace in tachycardia and in sinus rhythm facilitates the diagnosis unless conduction abnormalities (BBB) are present during tachycardia. In the atypical form (5-10%of cases), the reentry circuit uses the fast pathway antegradely and the slow pathway retrogradely. The P' wave is negative in the inferior leads with a ratio P’R/ RP’< 1 as in atrial tachycardia from which it should be differentiated. In the normal heart, electrical signals use only one pathway to propagate through the heart. This is the atrio-ventricular or A-V node.

If there is an extra conduction pathway present, the electrical signal may arrive at the ventricle too soon. This condition is called Wolff-Parkinson-White syndrome (WPW). It is in a category of electrical abnormalities called “pre-excitation syndromes”. The electrical properties of this pathway, which is basically an abnormal muscular connection between the atrium and the ventricle, are different from those of the normal A-V conduction system and creates the conditions for a reentry circuit. The accessory pathway can conduct exclusively antegradely, in other words from the atrium to the ventricle, exclusively retrogradely, from the ventricle to the atrium or in a bidirectional manner. It can be located anywhere in the A-V groove but predominantely in the lateral region. Orthodromic tachycardia is the most common arrhythmia associated with accessory pathways. It is a macro-reentry circuit using the A-V node antegradely and the accesory pathway retrogradely. Passage through this accessory pathway delays the retrograde activation of the atrium. This manifests, on the ECG, as a time delay between the QRS complex and the next P' wave (> 100 ms). Less commonly, a shorter refractory period in the accessory tract may cause block of an ectopic atrial impulse in the normal A-V pathway, with antegrade conduction down the accessory tract and then retrograde conduction up the normal (A-V) pathway. This type of tachycardia produced is called antidromic tachycardia. The QRS complex is wide (> 140 ms), with an exaggeration of the delta wave seen during sinus rhythm (wide-QRS tachycardia). Atrial fibrillation is the third arrhythmia occuring in patients with accessory pathways. The depolarisation can reach the ventricles by both the normal A-V pathway and the accessory pathway. If the latter has a short refractory period and as conduction can be very fast over this accessory pathway, the ventricular response can be very high, up to 300 bpm and irregular. The QRS complexes are wide but with a variable width depending on the use of the accessory pathway by the depolarisation. Permanent junctional re-entrant tachycardia is a relatively uncommon form of re-entry tachycardia with antegrade conduction occurring through the atrioventricular node and retrograde conduction over an accessory pathway usually located in the postero-septal region. It is a macroreentry circuit using the A-V node antegradely and the accessory pathway retrogradely. The P' wave is negative in the limb leads with RP’>P’R.

Three or more beats that originate from the ventricle at a rate of more than 100 beats per minute constitute a ventricular tachycardia. If the fast rhythm self-terminates within 30 seconds, it is considered a non-sustained ventricular tachycardia. If the rhythm lasts more than 30 seconds it is known as a sustained ventricular tachycardia (even if it terminates on its own after 30 seconds). Ventricular tachycardia can be classified based on its morphology: monomorphic ventricular tachycardia means that the appearance of all the beats matches each other in each lead of a surface electrocardiogram. Polymorphic ventricular tachycardia, on the other hand, has beat-to-beat variation in morphology. The most common cause of monomorphic ventricular tachycardia is damaged or dead (scar) tissue from a previous myocardial infarction.

Ventricular fibrillation is a condition in which there is a fast un-coordinated contraction of the cardiac muscle of the ventricles in the heart. It is a chaotic dysynchronous activity of the heart without identifiable QRS complexes. If the arrhythmia continues for more than a few seconds, blood circulation will cease, and death will occur in a matter of minutes.

Keywords: Atrial extrasystoles, ventricular extrasystoles, supraventricular tachycardia, ventricular tachycardia, atrial tachycardia, atrioventriuclar tachycardia, Permanent Junctional Reentrant tachycardia, torsades de pointes, ventricular fibrillation, idioventricular accelerated rhythm, atrial tachycardia.