The crista terminalis is the most common site for right ATs. Because it is close to the sinus node, it generates sinus-like P waves (biphasic [positive-negative] in V1, inferior axis and negative in aVR) (Fig. 13-1).⁷ Occasionally, a crista tachycardia shows a positive P wave in V1 in which case, the P wave is also positive in V1 during normal sinus rhythm (NSR) (due to the anatomic position of the heart relative to V1). This can be differentiated from the RSPV tachycardia, which shows the normal biphasic (positive-negative) P wave in V1 during NSR.² Right atrial appendage and superior tricuspid annular tachycardias demonstrate a negative P wave in V1 with variable precordial progression and an inferior axis, which can be difficult to differentiate from each other because of their close anatomic proximity.⁸,⁹

Because the slow pathway (SP) of the AV node lies near the CS os, tachycardias originating from the CS os produce a similar characteristic morphology (isoelectric and then positive
or biphasic [negative-positive] P waves in V1 and inverted P waves inferiorly) (Fig. 13-1).¹⁰ CS tachycardias show precordial P-wave regression. Transition from positive to negative occurs earlier with tachycardia origin at CS os (V2) than in the body of the CS (V4).¹¹

The unusual narrow, biphasic (negative-positive) P-wave morphology in lead V1 and/or the inferior leads should raise suspicion of a septal origin: noncoronary cusp (NCC), aorto-mitral continuity (superior mitral annulus), perinodal, or interatrial septum (Fig. 13-2).⁵,¹²,¹³,¹⁴,¹⁵,¹⁶

P waves inverted in the inferior leads and negatively concordant across the precordium are characteristic of origin from the inferior tricuspid annulus (the most anterior structure of the atrium) (Fig. 13-3).⁹ Conversely, positive precordial P-wave concordance suggests origin from the pulmonary veins (the most posterior structure of the atrium and most common site for left ATs) (Fig. 13-3).¹⁷ AT arising from the left-sided pulmonary veins are wider and more commonly notched than right-sided pulmonary veins. Because of their close anatomic proximity, differentiating the left superior
pulmonary vein (LSPV) from LA appendage tachycardias can be difficult. LA appendage tachycardias might show deeper negative P waves in aVL (because of its more leftward location) and less positive concordant (because of its more anterior location).

While the response of AVNRT and ORT to entrainment from the ventricle is “AV” (or “AH”), it is “AAV” (“AAH”) for AT (see Fig. 5-23).¹⁸,¹⁹ Rarely, a macroreentrant AT can generate an “AV” response if 1) the AT circuit time > AVJ refractory period and 2) the recording atrial site is orthodromically captured during entrainment. (An antidromically captured site generates an “AAV” response.²⁰)

While both AVNRT and ORT show ventriculoatrial (VA) linking (ΔVA <10 ms) one beat after differential atrial overdrive pacing, AT does not (ΔVA >10 ms) (see Fig. 5-27).²¹,²² Apparent VA linking, however, might occur if atrial overdrive pacing terminates AT, which is then reinitiated by typical AV nodal echoes (not by pacing per se) (Fig. 13-4).

The focal site of origin of AT can be identified by 1) activation mapping, 2) pacemapping, and 3) atrial overdrive pacing.²³,²⁴,²⁵,²⁶,²⁷ Creating three-dimensional electro-anatomic reconstructions of the atrium facilitates mapping within the atrial cavity and overcomes limitations of two-dimensional fluoroscopy.