Think back to your elementary school days, and I’m sure you will easily be able to recall Schoolhouse Rock. The classic Jack Sheldon favorites helped me understand everything from grammar through history and government. Today, let’s begin breaking down the AV junction structures and purpose – Schoolhouse Rock style.
The area between the atria and ventricle contains the fibrotic skeleton. This not only provides structure to the heart but acts as an insulator to prevent impulse from the atria to travel to the ventricles and for the ventricles to the atria.
But… the ventricles need to depolarize in order to contract and eject blood out to the body. So, how does the impulse travel to the ventricle?
Through the bundle of His. This bundle travels through the fibrotic ring to bring impulses from the atria to the ventricles. In a normal heart, this is the only pathway from the A to V and reverse from the V to A.
In this AV junction, there is the AV node and the bundle of His. The AV node’s primary function is to delay the impulses traveling to the ventricle. But why???
This delay allows for ventricular filling… more filling = larger ejection fraction (EF) or the amount of blood ejected from the heart with each contraction. This delay is seen on the ECG during the PR interval. On the intracardiac electrogram, it is the AH interval. The A is seen on the His catheter when the impulse enters the AV node and the H is observed when it exits.
Just as importantly, the AV node acts as a gatekeeper to the ventricles. It slows down the impulses to prevent the ventricles from fast heart rates. Notice when you are pacing the atrium, the faster you pace the more the AV node delays the impulse. This is seen in every EP study when we are performing decremental pacing to observing Wenckebach. The faster the pacing the more the AV node delays which is seen as a progressively lengthening PR interval and AH interval.
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Why is this important? When do we really need to slow down the rate of conduction to the ventricles?
During fast atrial rhythms. If the patient is in atrial fibrillation and each impulse was sent to the ventricles then the ventricular rate would be way too fast for hemodynamic stability. The AF could lead to VF! This may be seen in other atrial rhythms as well such as atrial flutter or a fast atrial tachycardia. The AV node protects the ventricles.
Remember your schoolhouse rock moto…. Knowledge is Power!
Every day, expand your knowledge but putting into practice what you have learned. Whether that is recognizing waveforms or interpreting pacing maneuvers. No matter what level you are at, challenge yourself to continue to grow.
For more explanations on all things EP and hundreds of examples of electrograms for interpretation, check out our “Understanding EP – A Comprehensive Guide” book and workbook.
This week’s EP question:
Which catheter placement will display three waveforms?
- HRA
- HIS
- CS
- RV
Answer
HIS
The His catheter is positioned across the tricuspid valve. Since it is partially in the atrium and ventricle it will display both an A and V. In is positioned near the anterior septum, which is the location of the AV node and bundle of His. Therefore a discrete H will be observed in-between the A and V.
