68 year old hispanic female with a chief complaint of weakness/syncope. PMH of type 2 diabetes mellitus and hypertension. She was brought in via EMS due to the family calling 911 as she had an episode of syncope while at the dinner table. At the time of arrival:
VS: 96/48 43 18 99% on RA 98.7
ECG obtained at arrival is shown…
Before reading on, try to come up with your own interpretation of this ECG before moving on to the final impression
- Rate: Ventricular rate ~ 48
- Rhythm: 3rd degree block with AV nodal escape rhythm
- Axis: Normal to slight RAD
- QRS: Barely wide (>0.12 msec), RBBB morphology
- ST/T Waves: ST elevation V1 – V2, hyperacute t waves in V4 – V6
- Final ECG interpretation: Likely ACS/AMI based on V1-V2 and V4-6 findings with CHB
Third-degree AV block or complete heart block occurs when the electrical impulse generated in the SA node in the atrium is not conducted to the ventricles. When the atrial impulse does not conduct to the ventricles, an accessory pacemaker (escape rhythm) in the ventricles will typically activate a ventricular contraction. Since two independent electrical impulses occur (SA node impulse & accessory pacemaker impulse), there is no apparent relationship between the P waves and QRS complexes on an ECG. This can be best characterized as:
- P waves with a regular P to P interval (Red Arrows)
- QRS complexes with a regular R to R interval (Blue Arrows)
- The PR interval will appear variable because there is no relationship between the P waves and the QRS Complexes
Example of Complete Heart Block (CHB)
Key Point: Atrioventricular (AV) dissociation is not the same thing as 3rd degree heart block (complete heart block)
- AV dissociation: Atria and ventricles beating regularly, but independent of each other
- AV dissociation: Ventricular rate is the same or faster than the atrial rate
- Complete heart block (CHB): No atrial beats are conducted to the ventricles
- AV dissociation without CHB: Some atrial beats conducted to ventricle (Look for changing QRS morphologies)
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