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.
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.
Unfortunately, I have no available clinical data on this patient. Merely looking for an interpretation of the ECG in it's raw form.
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.
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.
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.
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.
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
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.
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