Table 2: Useful Baseline ECG Signs to Localise Posteroseptal Accessory Pathways.

	Right Endocardial Posteroseptal Favoured	Left-sided Posteroseptal (Including Subepicardial CS) Favoured	Caveats and Nuances
Negative δ wave in V1, defined as either: End of δ wave at 40 ms below the isoelectric line Negative δ wave (first 20 ms) Negative δ wave (first 20 ms) combined with a negative δ wave in lead aVF Negative δ wave in both the first and second half of the first 40 ms	+ + ++ ++		– – More specific More specific
QRS polarity: R/S ratio in V1 ≥1		+	More specific in the case of substantial degree of preexcitation (QRS > 130 ms?)
The ‘double transition’ pattern Q wave in V1 or predominantly negative wide QRS in V1 (QRS > 130 ms) + Proportion of the positive precordial QRS component in V1 < V2 > V3	+++		QRS polarity in V2 enables further refinement of AP localisation: If positive: right posteroseptal AP If negative or isoelectric: AP localised more laterally on the tricuspid annulus
Negative δ wave (first 20 ms) in lead I or aVLç		++	Rarely observed (more frequent in left posterior or posterolateral APs)

+ = mild association; ++ = strong association; +++ = highly specific association; AP = accessory pathway; CS = coronary sinus.