A long cycle (560 ms) followed by a short cycle (320 m) results in right bundle branch morphology aberrancy that last for 3 beats.
5, 6 On termination of the wide complex rhythm there is no compensatory pause. A long R-R interval is followed by a short R-R interval that ends with a wide QRS complex often with RBBB morphology. 4.1 Ashman's PhenomenonĪshman's phenomenon refers to aberrant conduction due to critical pre-maturity identical combinations of long and short cycle lengths may manifest as aberrant conduction (Figure 1A). There are three types of aberrant conduction. Therefore a long cycle length (R-R interval) followed by a shortened cycle length, as commonly seen in atrial fibrillation or premature atrial beats, may produce wide QRS complexes and this may continue for the next several beats until the refractory period shortens or the heart rate decreases. However, following a sudden acceleration in rate, the first few beats may still be attuned to a longer refractory period and conduction may partially or completely fail as evidenced by a wider QRS complex due to bundle branch block. Therefore the cycle length of the preceding R-R interval determines the refractory period of the following beat. Heart rate exerts an “accordion” effect on the refractory period: as the heart rate slows, the refractory period of the conduction system lengthens and as heart rate speeds up, the refractory period shortens. As a result aberrant conduction, a transient functional block, more often exhibits RBBB morphology in lead V1. The longer refractory period of the right bundle branch predisposes it to conduction slowing or failure, particularly with sudden acceleration of the heart rate as seen in atrial fibrillation. The refractory periods of the right and left bundle are different. A VT originating in the left ventricle will produce RBBB morphology and a VT originating in the right ventricle will correspondingly produce LBBB morphology but under these circumstances, the QRS morphology in V1 may not resemble that of classic right or left bundle branch block. 3 The speed of conduction will also be slower if the impulse originates in or traverses the myocardium to reach the His-Purkinje system and will therefore produce a wide QRS complex. It produces distinctive changes in lead V1 on the electrocardiogram: These electrocardiographic alterations may appear similar irrespective of the cause of slow conduction which may be functional (transient) or structural (permanent) due to scar.
4 ABERRANT CONDUCTIONĪ block or delay in conduction of supraventricular impulse in any of the bundle branches or His Purkinje system will prolong QRS duration and alter the morphology of the QRS complex resulting in aberrant conduction. WQRST is characterized by the QRS duration of more than 120 milliseconds at a heart rate of >100 bpm. As the impulse travels at a speed of three meters per second through the AV node and simultaneously down the bundle branches and His-Purkinje system, it typically produces a narrow QRS complex of 80 mseconds duration. 3 REVIEW AND ANALYSISĮlectrical impulses that arise above or within the AV node are labeled supraventricular. Only articles published in English between 19 were included. 2 METHODSĭata for this review were identified by searches of MEDLINE, PubMed, and references from relevant articles using the search term “Differential diagnosis of wide QRS complex tachycardia.” 2 The aim of this review is to provide the relevant pathophysiologic explanation for the clinical and electrocardiographic observations used in the differential diagnosis of WQRST. For example, supraventricular tachycardia (SVT) with aberrant conduction is likely to respond to atrioventricular (AV) nodal blocking agents such as verapamil whereas such therapies may be counterproductive if administered to patients with ventricular tachycardia (VT). Correct diagnosis is important for prescribing appropriate therapy and determining prognosis. 1” Sixty years later, differential diagnosis of wide QRS tachycardia (WQRST) on the electrocardiogram remains a challenging exercise. In 1960 Pick and Langendorf published, “Differentiation of supraventricular and ventricular tachycardia.
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