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AV dissociation

Complete AV Block With Junctional Escape Rhythm

Thu, 10/27/2016 - 14:29 -- Dawn

This ECG is from a 78-year-old woman.  We do not know any clinical details.

 We break from our usual habit of removing the ECG machine’s interpretation of the ECG to serve as a reminder that the computer interpretation can be wrong.  ECGs should ALWAYS be interpreted by a knowledgeable person.  The machine interpretation can serve as a reminder, but should not take the place of human interpretation. 

Here is what we DO see:  There is a normal sinus rhythm present, as evidenced by the regular P waves that do not change their morphology.  Some of the P waves are “buried” behind QRS or T waves.  The atrial rate is 95 bpm. 

The ventricular rhythm, at 40 bpm, is also regular, but is separate from the atrial rhythm.  Even though some of the P waves LOOK like they have conducted to produce QRS complexes, they have not.  The PRIs are not all the same.  Neither do they “progressively prolong”.  There is no irregularity of the QRS rhythm or variation in QRS morphology.  We see the classic “AV DISSOCIATION” of complete heart block. 

When there is a third-degree AV block with a narrow-QRS escape rhythm, we can assume the block is in the AV node.  The junction is the escape focus, producing a narrow-complex rhythm between approximately 40-60 bpm.  In this case, the QRS is slightly wide at 112 ms (.11 sec), and the QRS complexes in several leads are fragmented.  Some might argue that there is an idioventricular escape mechanism.  But, with a normal frontal plane axis, borderline width,  and no T wave inversions, the rhythm looks more supraventricular.  The R wave progresson on the precordial leads shows a persistently negative QRS with late transition in V5.  The QRS complexes in V1 and V2 appear to have pathological Q waves.  When R wave progression is not normal, we should also consider electrode misplacement. 

Complete AV Block

Tue, 09/16/2014 - 14:26 -- Dawn

This ECG is from an 84-year-old man who experienced dizziness and a fall.  He was not injured in the fall.  In this ECG, we can clearly see regular P waves at about 110 per minute.  We also see wide QRS complexes at about 52 per minute.  There is AV  dissociation - there are no regular PR intervals, or even progressively-prolonging PR intervals.  The atrial and the ventricles are beating to separate rhythms.  What is interesting about this rhythm is the origin of the escape rhythm.  The wide complex suggests a ventricular focus and the rate suggests supraventricular origin.  Near the end of the ECG, the escape rhythm either fails or slows significantly.  To see the next 12-lead ECG for this patient, go to this LINK.

The second ECG makes it more clear that this is an idioventricular escape rhythm, but the morphology of the QRS complexes suggested that, even in the first ECG when the rate was faster.  There are several clues that this is probably ventricular, including a very "backward" axis with aVR being upright and II, III, and aVF all being negative.  Also, V6 is negative, and there is nearly precordial concordance:  all except V1 are negative.  The morphology of the QRS does not fit a diagnosis of either left bundle branch block OR right bundle branch block.  The evidence points to a ventricular origin for this escape rhythm, and the patient quickly goes on to slow down severely.  Ventricular escape rhythm strongly suggests a sub-Hisian location for the block, and they tend to be more life-threatening than supra-Hisian blocks.

The take-home clinical lesson here is to BE PREPARED for worsening of the rate whenever AV block is present, especially high-grade AV block or sub-Hisian block.  This ECG is a very good one for teaching students to "march out" P waves, and find "hidden" P waves.  We have included a marked copy of this ECG to indicate those P waves.

Thanks to Sebastian Garay for donating these ECGs.

 

ECG Basics: Sinus Rhythm With Complete AV Block and Ventricular Escape Rhythm

Sun, 07/28/2013 - 13:36 -- Dawn

This rhythm strip shows a good example of complete (third-degree) AV block with ventricular escape rhythm.  It will be easy for your basic students to "march out" the P waves.  They are regular at a rate of about 88/min., and they are either visible, or are "hiding" in the QRS complex.  The ventricular rhythm is wide and very slow, and completely dissociated from the sinus rhythm.

For your more advanced students, you may want to discuss the likely origin or "level" of the block.  Blocks above the Bundle of His can have JUNCTIONAL escape rhythms, while blocks that occur below the Bundle of His generally have ventricular escape rhythms.  Ask your students which type of CHB they would prefer to have:  suprahisian or subhisian - and why?

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. V3 and V4 right clearly show ST elevation as well, indicating RVMI.  The slight coving and elevation observable in V1 is also an indication of RV involvement, and the ST depression in V2 indicates posterior wall injury.  All of this results from a proximal lesion of the RCA in this patient.  Such a lesion carries a high morbidity and mortality.

Taking the time to obtain a right ventricular lead is controversial in some settings.  Some believe the patient's hemodynamic condition should be treated, regardless of the presence or absence of ST elevation in right chest leads.  Others find it very helpful to know that the right ventricle is affected.  In this case, paramedics in this community have a protocol to avoid the use of nitroglycerine in RVMI patients, even when the BP is adequate.  So, for them, it is important to have the information gathered from V Right leads.

The rhythm here is interesting, as well, and not at all uncommon for IWMI patients.  The baseline artifact makes it a bit difficult to march out all the P waves, but it appears they are sinus P waves that are slightly irregular at a rate of 52 to 54.  The PR intervals appear to be progressively prolonging, but there is no "grouped beating" observable on this short strip.  A lack of a concurrent Lead II rhythm strip also makes it difficult to determine the rhythm, as Lead II does have very visible P waves, and would be helpful.  The regularity of the narrow complex bradycardia points to a junctional escape rhythm, which would make this a third-degree AVB at the AV node level, which is very common with IWMI.  What do you think?

This month's strip from Jason Roediger's ECG Challenge blogpost is a nice complement to the strip presented here.  His is much clearer, and has a Lead II rhythm strip.  Do you think these ECGs show the same rhythm, or two different rhythms?

For an excellent discussion of "AV Dissociation" vs. "AV Block", go to Christopher Watford's Ask the Expert post.  Thanks very much to ECG Guru Sebastian Garay for this interesting ECG.

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.

 

Third-degree AV Block (Complete Heart Block)

Wed, 08/22/2012 - 16:37 -- Dawn

This 84-year-old man called 911 because he felt dizzy and fell.  He was not injured in the fall, but the paramedics noted a slow pulse. He denied significant medical history. The initial ECG showed sinus rhythm at about 80 bpm and AV dissociation with an apparent acellerated idioventricular rhythm at about 40 bpm.  Less than one minute later, he has developed a complete heart block with an idioventricular escape rhythm less than 30 bpm.  The escape rhythm speeds slightly toward the end of the strip.  He retained stable vital signs and adequate perfusion during transport.  It is presumed that he was scheduled for an implanted pacemaker.  It is interesting to note the machine's interpretation, and it reminds us to always interpret the ECG ourselves.   Thanks to ECG Guru member, Sebmedic, for his contribution of this ECG. 

jer5150's picture

Jason’s blog: ECG Challenge of the Week for June 24 – July 1, 2012. Pinpoint the primary disturbance.

This patient was seen by his primary care provider (PCP) on an outpatient basis.  The PCP decided to send her patient over to me to perform a routine ECG and establish a baseline, hince the computer's statement below of "No previous ECGs available".  I printed out the above 12-lead ECG and became slightly concerned with the rhythm I was seeing.  Consequently, I also recorded six full pages of continuous rhythm (not shown here).  I don’t ordinarily resort to doing this

Inferior Wall M.I. with Right Ventricular M.I.

Sat, 02/11/2012 - 17:39 -- Dawn

These two ECGs are from a 57 year old man with chest pain. The initial ECG shows ST elevation in Leads II, III, and aVF - inferior wall STEMI. Reciprocal changes are as expected in I and aVL. Reciprocal ST depression also seen in V1 and V2 indicate probable posterior wall involvement. Not surprising since the inferior wall is simply the lower part of the posterior wall. The first ECG also shows the patient in sinus brady with junctional escape: AV dissociation. The sinus node is often affected in IWMI that is caused by right coronary artery occlusion. The second ECG shows a slight increase in the sinus rate, and a sinus bradycardia. A V4 right lead has been performed, clearly showing ST elevation, and indicating right ventricular M.I.

 

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