ECG Conduction Disturbances

Definition

Intraventricular conduction disturbances (IVCDs) are abnormalities in intraventricular propagation of supraventricular impulses producing changes in QRS shape and/or duration. They may be fixed or intermittent (rate-dependent), structural (necrosis/fibrosis/infiltration/calcification/ischemia) or functional (aberrant conduction during relative refractory period), or due to abnormal AV connections (preexcitation). The 2009 AHA/ACCF/HRS consensus criteria represent the current standard definitions used in automated ECG systems and clinical practice.

Key Concepts

Normal QRS Duration

• Adults (>16 years): ≤110 ms; >110 ms = abnormal
• Adult males: range 74–114 ms, average 95 ms; up to 110 ms considered normal
• Children 4–16 years: ≥100 ms = prolonged
• Children <4 years: ≥90 ms = prolonged
• Global measurement (earliest onset to latest offset across all leads) is the preferred standard — longer than single-lead measurements; QRS wider in precordial than limb leads
• (sources/ecg-bbb-aha-2009, rating: high)

Frontal Plane Axis — Normal Limits by Age

• Adults: moderate LAD −30° to −45°; marked LAD −45° to −90° (often LAFB); moderate RAD +90°–+120°; marked RAD +120°–+180° (often LPFB)
• Indeterminate axis when QRS is equiphasic (no dominant deflection)
• (sources/ecg-bbb-aha-2009, rating: high)

Complete RBBB Criteria

All three of criteria 1–3 must be met; criterion 4 required when pure dominant R wave (±notch) is in V1:

1. QRS ≥120 ms (adults); >100 ms (4–16 yr); >90 ms (<4 yr)
2. rsr', rsR', or rSR' in V1 or V2 — R'/r' usually wider than initial R; wide notched R in V1/V2 in minority of cases
3. S wave duration > R wave, or >40 ms, in leads I and V6
4. R-peak time normal in V5/V6 but >50 ms in V1

• Incomplete RBBB: QRS 110–120 ms (adults); same morphology; rsr' in V1/V2 with normal QRS is a normal variant in children
• "rsr'" and "normal rsr'" are not recommended terms

RBBB Clinical Context

• Prevalence: 2–3% general population; strongly age/sex-dependent (1% at age 50 → 18% at age 80 in men); incomplete RBBB ~3× more common and less age-associated
• RBBB can occur as a normal variant in structurally normal hearts (unlike LBBB, which rarely occurs in a normal heart)
• Initial 30–40 ms of QRS are normal in RBBB → MI co-diagnosis using Q/R wave criteria is preserved (unlike LBBB); QSR' in V1–V2 with RBBB = anteroseptal MI
• Concordant T wave (same direction as terminal QRS) in RBBB = abnormal → ischemia or MI
• Pseudo-RBBB differential — Brugada syndrome: Brugada ECG = pseudo-RBBB (NOT true RBBB); key distinction: wide S wave in I/V6 is absent in Brugada (present in true RBBB); ST changes are dynamic and spontaneous; Brugada patients occasionally have true RBBB → Brugada pattern may be concealed; Chiale maneuver (right apical pacing with timed AV intervals) can unmask the concealed Brugada ECG
• Isolated RBBB: benign prognosis; no treatment needed; RBBB + CVD = independent predictor of all-cause mortality (multiple prospective studies and meta-analysis)
• (sources/rbbb-ccr-2021, rating: medium); see concepts/RBBB for full clinical detail

Complete LBBB Criteria

1. QRS ≥120 ms (adults); >100 ms (4–16 yr); >90 ms (<4 yr)
2. Broad notched or slurred R wave in I, aVL, V5, V6 — occasional RS in V5/V6
3. Absent q waves in I, V5, V6 — narrow q in aVL acceptable without pathology
4. R-peak time >60 ms in V5 and V6; normal in V1/V2/V3 when small initial r discernible
5. ST and T waves usually opposite to QRS (secondary changes)
6. Positive T in leads with upright QRS = acceptable (positive concordance)
7. Depressed ST or negative T in leads with negative QRS = ABNORMAL (negative concordance) — this is the AHA 2009 formal endorsement of Sgarbossa concordant criteria; see concepts/Sgarbossa-Criteria
8. LBBB may shift frontal axis to the right, left, or superior (may be rate-dependent)

• Incomplete LBBB: QRS 110–119 ms + LVH pattern + R-peak time >60 ms in V4/V5/V6 + absent q in I/V5/V6

Strauss' Strict LBBB Criteria — "True LBBB"

• QRS ≥140 ms (men) or ≥130 ms (women), plus QS or rS in V1–V2, plus mid-QRS notching or slurring in ≥2 contiguous leads of V1, V2, V5, V6, I, aVL (Strauss 2011)
• ~1/3 of conventionally-defined LBBB may be pseudo-LBBB (LVH + LAFB combination); these patients are CRT non-responders
• Initial r wave ≥1 mm in V1 suggests intact left-to-right septal activation — proposed as exclusion criterion for CLBBB; present in ~28% of conventional LBBB
• Vectorcardiographic (VCG) confirmation: mid + end conduction delay in the QRS loop is pathognomonic of true LBBB; its absence distinguishes pseudo-LBBB (LVH) from true LBBB
• Stricter criteria controversy: Mascioli 2012 and Garcia-Seara 2018 support better CRT response with true LBBB; Bertaglia 2017 found no improvement with strict criteria vs AHA definition
• (sources/lbbb-evg-ane-2019, rating: high); see concepts/LBBB-Criteria for full detail

Concordant vs Discordant LBBB

• Discordant LBBB ("appropriate discordance"): ST/T in opposite direction to main QRS — present in ~68–70% of CLBBB; associated with lower LVEF, larger LV, higher BNP, worse prognosis, greater CRT benefit
• Concordant LBBB: T-wave in same direction as QRS (positive T in ≥2 of I, V5, V6) — present in ~28–32%; milder disease, higher LVEF (~51%), better prognosis
• Discordant LBBB ≠ concordant LBBB of Sgarbossa (which is negative concordance indicating ischemia); these are distinct concepts — see concepts/Sgarbossa-Criteria
• (sources/lbbb-evg-ane-2019, rating: high)

Nonspecific IVCD

• QRS >110 ms (adults) without RBBB or LBBB morphologic criteria
• Also applicable when RBBB criteria in precordial leads coexist with LBBB criteria in limb leads, or vice versa

Left Anterior Fascicular Block (LAFB)

1. Frontal axis −45° to −90°
2. qR pattern in aVL
3. R-peak time in aVL ≥45 ms
4. QRS <120 ms (no complete BBB)

• Not applicable to CHD patients with congenital LAD in infancy
• Additional Elizari/Rosenbaum criteria: initial 20ms vector inferior-rightward → small Q in I/aVL, small R in II/III/aVF; S III deeper than S II (key sign); QRS widening ≤20ms in pure LAFB; VCG: wide-open CCW loop in frontal plane is pathognomonic; CCW = diagnostic; CW = excludes LAFB
• If S II is deeper than S III → LAFB is very unlikely
• LAFB masking/simulating MI: (1) conceals inferior MI (negative T in II/III/aVF despite LAFB = strong sign of inferior ischemia); (2) can simulate anteroseptal MI (small Q in V2–V3 from initial rightward forces); (3) conceals RBBB ("masquerading RBBB" — S waves of RBBB disappear from I and V5–V6)
• Epidemiology: 2.77% in healthy population; 4.58% in hospital cardiology patients; 62% of cases in patients <40 years; main causes: CAD, hypertension, cardiomyopathy, Lev/Lenègre disease; spontaneous VSD closure in young patients
• Isolated LAFB in healthy patients: benign ECG finding; prognosis determined by associated pathology
• (sources/hemiblock-circ-2007, high); (sources/ecg-bbb-aha-2009, high); see concepts/Fascicular-Blocks for full detail

Left Posterior Fascicular Block (LPFB)

1. Frontal axis +90° to +180° (adults); children: only when distinct rightward axis change documented
2. rS pattern in leads I and aVL
3. qR pattern in leads III and aVF
4. QRS <120 ms

• Additional criteria: S1Q3 pattern in limb leads; requires exclusion of RVH, vertical heart in slender subject, and large lateral infarction; VCG: wide-open CW loop in frontal plane (exact mirror image of LAFB); LPFB VCG distinguishes from LAFB mimics
• Isolated LPFB is extremely rare; posterior division has dual blood supply (LAD + PDA) and is the most protected fascicle — its isolated block implies extensive conduction system disease
• LPFB almost always associated with RBBB → near-trifascicular block: RBBB + LPFB in acute MI = mortality 80–87%; 42% progression to complete AV block; Adams-Stokes in 58.6% of Elizari cohort
• Transient LPH can conceal inferior MI — the inferior-rightward QRS forces mask Q waves; subtle ST changes may be the only sign
• (sources/hemiblock-circ-2007, high); (sources/ecg-bbb-aha-2009, high); see concepts/Fascicular-Blocks for full detail

WPW Ventricular Preexcitation

Four criteria suggesting full preexcitation (degree of preexcitation cannot be determined from surface ECG):

1. PR <120 ms (adults); <90 ms (children) — assuming no intra-atrial conduction block
2. Delta wave — slurring of initial QRS, interrupting P wave or immediately following its termination
3. QRS >120 ms (adults); >90 ms (children)
4. Secondary ST and T-wave changes

Additional Clinical Terms (AHA 2009)

• "Possible peri-infarction block": Abnormal Q wave from MI in inferior/lateral leads + wide terminal QRS deflection directed opposite to Q (QR complex in inferior/lateral leads)
• "Peri-ischemic block": Transient QRS duration increase accompanying ST-segment deviation in acute ischemic injury — reflects reversible His-Purkinje ischemia

Deprecated Terminology (AHA 2009)

• (sources/ecg-bbb-aha-2009, rating: high)

Clinical Significance of LBBB and BBB — ACC/AHA/HRS 2018 Guideline Perspective

• LBBB on ECG markedly increases the likelihood of underlying structural heart disease and LV systolic dysfunction → echocardiography is the most appropriate initial screening test (sources/bradycardia-acc-aha-hrs-2018, very high)
• Isolated RBBB or fascicular block: patients are often asymptomatic; RBBB alone does not require pacemaker without symptoms or other indications
• Alternating bundle branch block (QRS complexes alternating between LBBB and RBBB morphologies): implies unstable bilateral infranodal disease and high risk of sudden complete AV block → permanent pacing is recommended (Class I, C-LD) (sources/bradycardia-acc-aha-hrs-2018, very high)
• Syncope + BBB + HV ≥70 ms or infranodal block at EPS: permanent pacing recommended (Class I, C-LD) (sources/bradycardia-acc-aha-hrs-2018, very high)
• AV block definitions adopted from 2009 AHA/ACCF/HRS recommendations (same document source); first-degree AV "block" more accurately termed AV delay

Contradictions / Open Questions

• "Bifascicular/trifascicular" term persistence: AHA 2009 recommends against these terms; however they remain in widespread clinical use in guidelines, textbooks, and ACS/HF management literature — a longstanding terminology disconnect
• "Brugada pattern" automated labeling: AHA 2009 recommends against automated reporting of "Brugada pattern" given the 3 ST subtypes and lack of specificity; however most commercial ECG systems continue to flag type 1 Brugada patterns; clinical practice does not follow this recommendation
• LAFB threshold debate: The −45° axis threshold for LAFB is somewhat arbitrary; some authors use −30° as the lower limit, which would classify many more ECGs as LAFB; AHA 2009 adheres to the stricter −45° threshold
• Rate-dependent LBBB: LBBB may shift frontal axis in a rate-dependent manner, making the axis unreliable for fascicular block co-diagnosis in the presence of LBBB
• Global vs single-lead QRS measurement: Global measurement (preferred) systematically yields longer QRS durations than single-lead measurements used in most clinical trials establishing treatment thresholds (e.g., CRT QRS ≥150 ms) — measurement method creates implicit discrepancy. AHA 2007 Part I explicitly states that leads perpendicular to the heart vector during onset/offset record isoelectric segments, causing single-lead measurements to miss earliest onset and latest offset; all existing QRS duration criteria require recalibration for global digital measurement. This recalibration has not occurred in published CRT/LBBB criteria (sources/ecg-technology-aha-2007, rating: high)

Connections

• Related to concepts/Sgarbossa-Criteria — LBBB criterion 7 (negative concordance = abnormal) is AHA 2009 formal basis
• Related to concepts/ST-T-Changes — secondary ST-T changes in LBBB; concordance principles
• Related to concepts/ECG-Ventricular-Hypertrophy — incomplete LBBB requires LVH pattern; LAFB alters LVH voltage criteria
• Related to concepts/ECG-Lead-Standards — global QRS measurement technical basis; AHA 2007 Part I rationale for why single-lead criteria require recalibration
• Related to concepts/Conduction-Disorders-in-Young-Adults — clinical disease context for IVCD
• Related to concepts/Conduction-System-Pacing — QRS duration and morphology define CRT/CSP eligibility
• Related to entities/Brugada-Syndrome — "Brugada pattern" terminology guidance
• Related to concepts/Cardiac-Repolarization — secondary ST-T mechanism in bundle-branch blocks
• Related to concepts/LBBB-Criteria — Strauss strict criteria, concordant/discordant LBBB, VCG differentiation, CRT selection implications
• Related to concepts/RBBB — RBBB clinical overview: epidemiology, causes, prognosis, Brugada pseudo-RBBB differential, masquerading RBBB (LAH concealing RBBB), CRT implications
• Related to concepts/Fascicular-Blocks — full clinical detail for LAFB and LPFB: VCG criteria, MI masking/simulation patterns, masquerading RBBB, epidemiology, LPH prognosis, Lev/Lenègre disease

Sources

• sources/ecg-bbb-aha-2009
• sources/ecg-technology-aha-2007
• sources/bradycardia-acc-aha-hrs-2018 — clinical significance of LBBB, alternating BBB, AV block classification
• sources/lbbb-evg-ane-2019 — Strauss strict criteria, concordant/discordant LBBB, VCG differentiation of true vs pseudo-LBBB
• sources/rbbb-ccr-2021 — RBBB epidemiology, MI co-diagnosis, Brugada pseudo-RBBB differential, Chiale maneuver, prognosis
• sources/hemiblock-circ-2007 — LAFB and LPFB: VCG criteria, MI masking/simulation, masquerading RBBB, epidemiology, RBBB+LPH prognosis, Lev/Lenègre disease