ECG - Conduction blocks

SA block 1st degree: slowed conduction (cannot be discerned from the surface ECG (because discharge of the impulse in the SA node is not noticeable on surface ECG) 2nd degree: - Type I (Wenkebach): PP interval progressively shortens prior to a pause, which is shorter than two PP intervals - Type II (Mobitz): Pause equals multiple of the P-P interval (usually 2-4 times) the preceding PP interval 3rd degree: complete conduction failure AV block 1st degree: PR > 0.2 s (5 small squares) (generally benign, possible indicator of higher degree block in the future, not in itself important, but may be a sign of CAD/acute MI, acute rheumatic carditis, digoxin toxicity or electrolyte disturbances 2nd degree: usually indicated heart disease, often seen in acute MI - Type I (Wenkebach): progressive prolongation of PR interval until a P wave is blocked, up to 10% incidence in healthy young adults, higher in endurance athletes (often nocturnal), consider pacing in older patients if evidence of symptomatic bradycardia - Type II (Mobitz): intermittent failure of AV conduction, PR intervals normal, invariably requires pacing - 2:1, 3:1, 4:1 block: may indicate a need for temporary or permanent pacing, especially if the ventricular rate is slow 3rd degree: P waves completely blocked in the AV junction, QRS complexes originate independently from below the junction, ventricles are excited by a slow 'escape mechanism', always indicates conducting tissue disease - more often fibrosis than ischaemic, consider a temporary or permanent pacemaker Incomplete bundle brunch block (BBB) Incomplete RBBB: QRS 0.10 - 0.12 s, criteria of complete RBBB, seldom of significance, normal variant Incomplete LBBB: QRS 0.10 - 0.12 s, criteria of complete LBBB, presence of LVH, R peak > 60 ms in V4, V5 and V6, absence of q wave in 1, V5 and 6 Complete bundle brunch block (BBB) Complete RBBB: QRS > 0.12 s, rsr', rsR', rSR' or notched pattern in V1 or V2 (r/R'>r/R) (lower case letters: smaller deflections (that do not exceed 3 little boxes = 3 mm)), S of greater duration than R or > 40 ms in I and V6, R peak time > 50 ms in lead V1, but normal in V5 and V6, causes: normal variant, HT, myocarditis, PE, ASD, AMI, post cardiac surgery; no specific tx Complete LBBB: QRS > 0.12s, broad notched or slurred R wave in leads I, aVL, V5 and V6, absent q waves in I, V5 and V6, but in aVL a narrow q wave may be present, R peak time > 60 ms in V5 and V6, but normal in V1-3, but normal in V5 and V6, ST and T usually opposite in direction to QRS, positive T with upright QRS may be normal (positive concordance), depressed ST segment and/or negative T in leads with negative QRS (negative concordance) abnormal; causes: HT, cardiomyopathy, AMI, (post) MI, rate related, IHD, aortic stenosis; if asymptomatic: no action is needed Fascicular block LAFB or LAHB: LAD (btw -45° and –90°), qR pattern in aVL, R peak in aVL 45 ms or more, QRS < 0.12 s LPFB or LPHB: RAD (btw 90 and 180), rS in I and aVL, qR in III and aVF, QRS < 0.12 s Bifascicular block *): RBBB + LAFB (common) or LPFB (uncommon), can be precursor for higher degrees heart block Trifasicular block*): RBBB + LAFB + LPFB, often precursor for complete heart block *) Terms not recommended by AHA/ACCF/HRS because of the great variation in anatomy and pathology producing such patterns, each conduction effect should be described separately in terms of the structure or structures involved Reference(s): Wasan BS: The Resting ECG: What is 'Abnormal'? Who to Refer? March 2017 Hampton JH: The ECG Made Easy. Eight Edition. Churchill Livingstone Elsevier, 2013 ECG Learning Center: ACC/AHA Clinical Competence in ECG Diagnoses AHA/ACC/ACCF/HRS: Recommendations for the Standarization and Interpretation of the Electrocardiogram. Part III: Intraventricular Conduction Disturbances