ECG Rounds Answer

Diagnosis: Supraventricular tachycardia (SVT) with right bundle branch block (RBBB).

In our patient's ECG, an RS complex is present in the precordial leads, the RS complex has a duration of less than 80 ms, and there is no atrioventricular (AV) dissociation. Because these three findings are not sufficient to diagnose or rule out ventricular tachycardia (VT), the V₁ and V₆ morphology are important diagnostically. The presence of rSR' in lead V₁ favors a diagnosis of SVT over VT. Although the R/S ratio in V₆ is less than 1, which would indicate VT, a diagnosis of SVT was made because both leads do not demonstrate criteria for VT.

Access to a baseline ECG in a patient with bundle branch block (BBB) is another way to distinguish between SVT and VT. The baseline ECG in our patient, which was obtained during a routine physical examination several years earlier, showed a similar QRS complex, confirming the diagnosis of SVT (Figure 2).The small notch immediately following the QRS complex in V₁ is indicative of a possible P wave. Based on this finding, the differential diagnosis included atrial tachycardia, AV nodal re-entrant tachycardia, and AV re-entrant tachycardia.

Wide QRS complex has been defined as a QRS duration of more than 120 ms. In our patient's ECG, this is almost 128 ms. The differential diagnosis of wide-QRS-complex tachycardia includes (1) VT, (2) SVT with underlying BBB or tachycardia-related aberrancy, (3) pre-excited SVT with anterograde conduction across an accessory pathway, or (4) an artifact.¹

VT is commonly observed in patients who have ischemic heart disease or cardiomyopathy but can be seen in patients with structurally normal hearts. Patients with VT typically present with a heart rate of more than 100 beats/min, which can be sustained (VT longer than 30 seconds and/or requiring termination due to hemodynamic compromise in less than 30 seconds), and can be monomorphic (similar QRS shape in one lead) or polymorphic.

Brugada criteria can help clinicians differentiate between VT and SVT with reasonable probability.² These criteria include the following four findings:

1. No RS complex in the precordial leads;
2. Longest interval in any precordial lead from the beginning of the R wave to the deepest part of the S wave when an RS complex is present and exceeds 100 ms;
3. Presence of atrioventricular dissociation; and
4. QRS complex in leads V₁ and V₆ fulfilling certain morphological criteria for VT.

It is always important to compare the QRS morphology in tachycardia with that in baseline normal sinus rhythm. In patients with tachycardia and left bundle branch block, an R wave greater than 30 ms or a notched S wave in lead V₁ favors a diagnosis of VT. The presence of a Q wave in lead V₆ also favors VT, whereas the absence of a Q wave indicates SVT. In patients with tachycardia and RBBB, the presence of rSR' in lead V₁ favors SVT, but the presence of a monophasic R wave (without R' wave) favors VT. In lead V₆, a triphasic pattern or an R/S ratio of more than 1 favors SVT, whereas an R/S ratio of less than 1 or the presence of a monophasic R wave favors VT.

RBBB is a common finding on ECGs and can be seen frequently in the general population. New-onset RBBB, however, indicates an injury to the conductive system in the heart. The development of RBBB in patients with coronary artery disease is a poor prog-nosticator of cardiovascular outcomes.³ In our patient's case, RBBB was not a new finding and no further investigation was warranted.

References

1. Knight BP, Pelosi F, Michaud GF, et al. Clinical consequences of electrocardiographic artifact mimicking ventricular tachycardia. N Engl J Med. 1999;341:1270-1274.
2. Brugada P, Brugada J, Mont L, et al. A new approach to the differential diagnosis of a regular tachycardia with a wide QRS complex. Circulation. 1991;83:1649-1659.
3. Ricou F, Nicod P, Gilpin E, et al. Influence of right bundle branch block on short- and long-term survival after acute anterior myocardial infarction. J Am Coll Cardiol. 1991;17:858-863.