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AV Block

Second-Degree AV Block, Type I

Sat, 06/13/2015 - 22:07 -- Dawn

This ECG is from an 80-year-old woman who had an acute inferior wall M.I. with a second-degree AV block.
 
Some people incorrectly call ALL second-degree AV blocks that are conducting 2:1 "Type II".  This is incorrect, as Mobitz Type I can also conduct with a 2:1 ratio.  The progressive prolongation of the PR interval will not be seen with a 2:1 conduction ratio, because there are not two PR intervals in a row.

ECG BASICS: Second-degree AV Block, Type II

Sat, 05/25/2013 - 13:12 -- Dawn

Today's basic rhythm strip illustrates second-degree AV block, Type II.  Even though there is fine baseline artifact present, it is easy to measure the P-to-P interval, and your students will be able to see that every third P wave falls in the T wave.  The PR intervals are constant and the atrial rate is about 110/min.  The ventricular rate results from a 3:1 conduction ratio, and is less than 30/min.

Second-degree AV Block, Type I

Sat, 02/16/2013 - 18:52 -- Dawn

This 67 year old man is noted to have a slightly irregular pulse.  At the beginning of this ECG, he appears to be in NSR with a first-degree AV block.  Twice, P waves are non-conducted.  Careful measurement of the P to P interval shows that it is regular, there are no PACs noted.  The PR interval changes very subtly by lengthening just before the non-conducted P waves.  A hint when non-conducted P waves are noted, first check for non-conducted PACs.  If the sinus rhythm is regular, check the PR interval before the non-conducted beat, and the PR interv

Inferior Wall M.I. With Right Ventricular M.I.

Sat, 01/12/2013 - 20:49 -- Dawn

This week's ECG of the Week is from an elderly woman who suffered an acute occlusion of the right coronary artery.  The ECG clearly shows ST elevation in leads II, III, and aVF, indicating inferior wall injury.  In this case, this ECG was obtained in the field by paramedics, and was the second ECG done on this patient. For this tracing, the paramedics obtained V3 and V4 on the right side to better view the right ventricle.

Ask The Expert

Tue, 11/06/2012 - 11:03 -- Dawn

 QUESTION: How do you explain the difference between "AV block" and "AV dissociation" to your students?

Our Expert today is Christopher Watford, BSc, NREMT-P 

Christopher began in EMS as an EMT on a volunteer industrial fire brigade at GE's Global Nuclear Fuels facility in Wilmington, North Carolina. He has worked there as a Lead Software Engineer since 2001 and currently is a Captain on the fire brigade. Outside of his day job, he volunteers as a Paramedic and Field Training Officer for Leland Volunteer Fire/Rescue where he also serves on the board of directors.Through Cape Fear and Brunswick Community Colleges heteaches continuing educat ation for all levels of providers. He also is an associate editor for the EMS 12-Lead Blog and Podcast, presenting electrocardiography case studies for pre-hospital personnel. 

Christopher's excellent blog can be found at My Variables Have Only Six Letters.  His contributions to EMS 12-Lead can be found at this link.

 

Answer:

I think the first step in understanding the difference between an  atrioventricular block and atrioventricular dissociation is to have a  firm understanding of physiological and pathological conduction.  The most common example of this is a non-conducted premature atrial  contraction (PAC). If an atrial stimulus arrives early enough at the  atrioventricular node (AVN), while it is still refractory, forward  conduction will be blocked. Likewise in atrial flutter, you typically  see one ventricular activation for every two F-waves, due to the  physiological rate limiting by the AVN. However, as this is due to the physiological function of the AVN we would not consider this a block!

 In higher degree AV blocks, we encounter a pathological decrease in  conduction and so we label non-conducted stimuli as "blocked". Type I  and Type II AV blocks provide visual confirmation of pathological conduction as you have examples of both conducted and non-conducted stimuli.  However, in the case of a presumed complete AV block, it is important  that you look at whether the atrial impulses were blocked or simply not conducted. With monomorphic ventricular tachycardia you may see  uncoordinated atrial and ventricular impulses on the ECG. In this case  the ventricular rhythm and the atrial rhythm "compete" for access to  the AV nodal tissue. There is no "AV block" present, instead we say they are "dissociated" from the ventricular rhythm. More specifically,  we say that the atrial rhythm is dissociated from the ventricular rhythm due to usurpation. Best illustrating the competitive nature of two rhythms during dissociation are capture or fusion beats.

 Therefore when classifying dyssynchrony between the atria and ventricles, students should look to see whether conduction blocked due to pathological processes or because the AV node is appropriately refractory.

 

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