To help remember the various types of AV block, it is useful to know the anatomical location of the block within the conducting system. Tips for remembering types of heart block Narrow-complex escape rhythms (QRS complexes of 0.12 seconds duration) originate from below the bifurcation of the bundle of His. Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.Ĭardiac function is maintained by a junctional or ventricular pacemaker. Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction. Second-degree AV block (Mobitz type 2 AV block) Third-degree heart block (complete heart block) The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave. Typical ECG findings in Mobitz type 2 AV block include a consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction.
Second-degree AV block (type 2) is also known as Mobitz type 2 AV block. Second-degree AV block (Mobitz Type 1 – Wenckebach) Second-degree heart block (type 2) Typical ECG findings in Mobitz type 1 AV block include progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped.ĪV nodal conduction resumes with the next beat, and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself. Second-degree AV block (type 1) is also known as Mobitz type 1 AV block or Wenckebach phenomenon. First-degree heart block (AV block) Second-degree heart block (type 1) First-degree heart block (AV block)įirst-degree heart block involves a fixed prolonged PR interval (>200 ms). Prolonged PR interval (>0.2 seconds)Ī prolonged PR interval suggests the presence of atrioventricular delay (AV block). The PR interval should be between 120-200 ms (3-5 small squares). If P waves are absent and there is an irregular rhythm, it may suggest a diagnosis of atrial fibrillation. If P waves are absent, is there any atrial activity? Do the P waves look normal? – check duration, direction and shapeĤ. If so, is each P wave followed by a QRS complex?ģ. The next step is to look at the P waves and answer the following questions:Ģ. Left axis deviation is associated with heart conduction abnormalities.Lead I has the most positive deflection.Typical ECG findings for left axis deviation: Right Axis Deviation (RAD) Left axis deviation Right axis deviation is associated with right ventricular hypertrophy.Lead III has the most positive deflection, and lead I should be negative.Typical ECG findings for right axis deviation: Lead II has the most positive deflection compared to leads I and III.Typical ECG findings for normal cardiac axis: Read our cardiac axis guide to learn more.
To determine the cardiac axis, you must look at leads I, II and III. In a healthy individual, the axis should spread from 11 o’clock to 5 o’clock. Cardiac axisĬardiac axis describes the overall direction of electrical spread within the heart.
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