ECG criteria
- V1: delta wave and QRS are predominantly positive (R, Rs) — WPW type A; typically corresponds to a left-sided pathway, though the exact side and parasextal location cannot be determined from V1 alone
- V1: delta wave and QRS are predominantly negative (rS, QS), less commonly isoelectric/borderline complexes — WPW type B; more frequently corresponds to a right-sided pathway, but this sign is orientational and should be interpreted only in conjunction with other leads
- II, III, aVF: negative initial part of the QRS (pseudo-Q) — more often indicates the posteroseptal/inferior region of the annulus, but requires analysis of the complete QRS morphology in multiple leads and is not definitive proof of localisation on its own
- II, III, aVF: positive delta wave — probabilistic tendency toward a more anterior/superior pathway location, evaluated solely as part of a multi-criteria algorithm, not as an isolated rule
- I, aVL: negative delta wave — an ancillary, but not self-sufficient sign of left lateral localisation
- V1 transition zone and R/S ratio combined with the delta wave axis — refinement of parasextal localisation within the framework of the complete algorithm
- Negative delta wave simultaneously in V1 and inferior leads — increases the probability of a posteroseptal pathway when consistent with other features
What it looks like on ECG
Assessment begins with lead V1, which broadly divides pathways into two WPW types:
| Type | V1 | Probable side |
|---|---|---|
| A | R, Rs (positive delta and QRS) | usually left |
| B | rS, QS (negative delta and QRS), less frequently isoelectric/borderline complexes | usually right |
This is a historical morphological classification rather than a precise anatomical diagnosis: a positive V1 generally corresponds to a left-sided pathway, though exceptions are possible with septal pathways and varying degrees of pre-excitation; a small rS, QS or isoelectric/borderline pattern in V1 does not guarantee a right-sided localisation and can occur with septal and some left-sided pathways.
Next, the polarity of the delta wave — the initial portion of the QRS where the upstroke or downstroke is slurred or slowed compared to normal conduction — is analyzed in the limb leads together with V1, rather than in isolation. The duration of this initial portion is variable and depends on the degree of pre-excitation; there is no fixed time window for all cases:
- II, III, aVF negative (resembling a pathological Q wave) — more frequently point to the posteroseptal/inferior annular region;
- II, III, aVF positive — tend toward a more anterior/superior pathway location;
- I, aVL negative — an additional argument favoring left lateral localisation.
Each of these features individually is unreliable, especially with minimal pre-excitation and fusion of the delta wave with R; localisation relies exclusively on their aggregate assessment.
To systematize this, several surface ECG algorithms have been proposed (including the Arruda algorithm), which sequentially combine polarity in V1, inferior, and lateral limb leads, dividing the mitral and tricuspid valve annuli into anatomical segments (right free wall, right anterior/posterior/septal segment, left free wall, posteroseptal, midseptal, parahisian segment). The specific sequence of decisions, polarity threshold criteria, and correspondence tables vary among authors and are not reproduced here — for practical application of a specific algorithm, one must consult the original source. The general principle remains uniform: the accuracy of such schemes is variable and depends on the chosen algorithm, pathway location, degree of pre-excitation, ECG recording quality, and the studied population; accuracy is lower with minimal or intermittent pre-excitation, multiple pathways, and in children. Specific accuracy figures are tied to individual original publications and cannot be generalized as a universal metric for the method.
Mechanism
In manifest atrioventricular accessory pathways (Kent bundles) in WPW, an impulse from the atrium is conducted to the ventricle, bypassing the physiological delay in the AV node. Some pathways conduct only retrogradely and do not produce pre-excitation on a baseline sinus rhythm ECG; atriofascicular pathways (Mahaim fibers) possess intrinsic decremental antegrogate conduction and do not represent typical Kent bundles — the patterns described below do not apply to them. The ventricle at the insertion site of a manifest pathway is depolarized earlier than the rest of the myocardium, forming a delta wave whose direction reflects the vector from the site of entry to the rest of the ventricle. The closer the lead is to the early excitation zone, the more negative the initial part of the complex (an effect resembling an infarction Q wave), while leads 'looking away' from this zone record a positive delta wave. Therefore, delta wave polarity in different leads indirectly and with limited reliability points to the location of the pathway around the mitral and tricuspid annuli.
Causes
- WPW pattern (manifest pre-excitation on resting ECG)
- WPW syndrome — pre-excitation combined with symptomatic tachyarrhythmia
- Preparation for electrophysiological study and catheter ablation
- Multiple accessory pathways (in a subset of patients, more commonly with Ebstein's anomaly)
- Atriofascicular (less commonly nodofascicular/nodoventricular) Mahaim pathways
Differential diagnosis
- Pathological Q wave of true myocardial infarction — differentiated by context (clinical presentation, enzyme dynamics, absence of a delta wave and short PR interval) versus a stable pre-excitation pattern with a short PR interval.
- Bundle branch block — widened QRS without PR shortening and without a delta wave on the upstroke.
- Mahaim atriofascicular pathway — atrial insertion is typically right-lateral along the tricuspid annulus, distal connection to the right bundle branch / Purkinje fibers of the right ventricle, decremental antegrade conduction; in sinus rhythm, pre-excitation is often minimal or absent, typically participating in antidromic tachycardia with left bundle branch block morphology.
- Mahaim nodofascicular and nodoventricular pathways — originate in the region of the AV node, without an independent atrial insertion, and connect respectively to a conduction system fascicle or directly to the ventricular myocardium; on a surface ECG in sinus rhythm, pre-excitation is also typically minimal or absent.
- Concealed pathways without an antegrade conduction — there is no delta wave on a standard ECG, localisation via surface ECG is impossible, requiring tachycardia or EPS.
Clinical significance and management
Surface ECG localisation is an orientational rather than definitive method: it narrows the search scope prior to an invasive electrophysiological study (EPS), helps select the access route (transseptal, retrograde aortic), and assists in planning mapping. The decision for ablation is made based on clinical indications: symptomatic tachycardia, episodes of AF with pre-excitation, risk stratification in asymptomatic patients. High risk in a manifest pathway is determined primarily by EPS criteria — short antegrade refractory period of the accessory pathway, short minimum pre-excited RR interval during induced AF, inducible AV reentrant tachycardia, and/or the presence of multiple pathways. Exercise testing may be used as a non-invasive assessment of pre-excitation persistence: only sudden and complete disappearance of the delta wave during increasing heart rate, especially when well documented, argues in favor of potentially lower-risk pathway properties. This sign does not exclude a high-risk pathway — during exercise, pre-excitation can be masked by complex fusion, and partial or gradual reduction of the delta wave is uninformative. A negative test result does not replace EPS in patients with indications for invasive risk stratification. The physician should describe the WPW type (A/B), delta wave polarity in key leads, and the presumptive zone (left/right, anterior/posterior/septal), indicating that exact localisation is established via intracardiac mapping.
Common interpretation errors
- Evaluation of polarity in a complex where the delta wave is fused with R and poorly distinguishable — maximum recording speed and increased amplitude must be used.
- Applying the algorithm during intermittent pre-excitation, when the degree of pre-excitation varies from complex to complex — analyze only complexes with the maximum delta wave.
- Ignoring the possibility of multiple accessory pathways in a single patient (more common in Ebstein's anomaly).
- Attempting precise localization with minimal pre-excitation — algorithms are validated for prominent delta waves.
- Interpreting WPW type A/B in V1 as an exact indication of the side or replacing the full localization algorithm — type A/B provides only a rough, non-absolute trend regarding the side, not the annular position.
Type A — all upward in V1 (All down = left) — more often a left-sided pathway; type B — all downward in V1 — more often a right-sided pathway (Both down = right); but this is only a trend, not a precise diagnosis of the side.
1. Which WPW type in V1 corresponds to a positive delta wave and Rs complex?
A positive delta wave and dominant R in V1 are characteristic of WPW type A, which most often indicates a left-sided pathway location, although the exact side is determined only by the full algorithm.
2. A negative delta wave in II, III, aVF most often indicates:
The negative polarity of the initial QRS portion (delta wave) in the inferior leads forms a pseudo-Q wave and is more common with pathways located in the posteroinferior segments of the atrioventricular annulus, although this sign requires analysis in conjunction with other leads.
3. What is the definitive method for determining the exact position of an accessory pathway before ablation?
Surface algorithms provide only an approximate zone; precise localization for successful ablation is achieved through intracardiac electrophysiological study with mapping.
Frequently asked questions
What are WPW type A and type B?
This is a historical classification based on the polarity of the delta wave and QRS in lead V1: type A — positive (R, Rs), more commonly corresponds to a left-sided pathway; type B — negative (rS, QS), less frequently isoelectric or borderline complexes, more commonly corresponds to a right-sided pathway. This is only the first, preliminary step of localization algorithms, not the definitive diagnosis of the pathway position.
Can the pathway be accurately localized using resting ECG alone?
No, surface ECG provides only a probable zone (side, anterior/posterior/septal segment), and each individual sign is unreliable on its own. Precise localization for ablation is established during an intracardiac electrophysiological study with mapping.
Why localize the pathway on ECG at all if an EPS is required anyway?
Preliminary localization accelerates and simplifies the invasive procedure: it helps choose vascular access, anticipate procedure complexity, and prepare for the likely mapping target zone.
How do surface localization algorithms account for septal pathways?
For paraseptal and mid-septal pathways, the delta wave is often isoelectric or has intermediate polarity in multiple leads, which requires sequential analysis of V1 combined with inferior and lateral leads, rather than relying on a single sign.
How do Mahaim fibers differ from a typical Kent bundle on ECG?
The atriofasiculocventricular Mahaim pathway conducts decrementally and anterogradely to the distal right bundle branch/right ventricle, with atrial insertion usually being right-lateral; nodofascicular and nodoventricular variants originate in the AV node region without a dedicated atrial insertion. During sinus rhythm, pre-excitation in these pathways is often minimal or absent, and classic antidromic tachycardia has left bundle branch block morphology. A typical manifest Kent bundle in sinus rhythm usually yields a prominent delta wave and short PR interval.
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Related terms
More in the section «Ventricular pre-excitation»
Sources
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
- Hampton J.R. The ECG in Practice
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.