The Cardiac Conduction Pathway
Key structures, speeds, and EKG correlates as the impulse travels from SA node to Purkinje fibers.
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Questions Covered in This Set
10 cards to master
What is the normal pacemaker of the heart, and where is it located?
The SA (sinoatrial) node, high in the right atrium near the entrance of the superior vena cava; it fires spontaneously 60–100 times/minute.
List the conduction pathway in order.
SA node → atrial myocardium/internodal tracts (Bachmann's bundle) → AV node → bundle of His → right and left bundle branches → Purkinje fibers.
What EKG waveform represents atrial depolarization?
The P wave (~80–100 ms), produced as the impulse spreads across both atria.
Why does the AV node conduct so slowly (~0.05 m/s)?
The ~100 ms delay lets the atria finish contracting to top off the ventricles (the 'atrial kick,' 20–30% of filling) and acts as a gatekeeper blocking excessively fast atrial rates.
What is the normal PR interval, and what does it measure?
120–200 ms (3–5 small boxes); it measures transit from SA node onset to the start of ventricular depolarization.
What does a PR interval > 200 ms indicate? What about < 120 ms?
>200 ms = first-degree AV block (slowed conduction); <120 ms suggests an accessory pathway bypassing the AV node, as in Wolff-Parkinson-White.
Which tissue is the fastest conducting in the heart?
The Purkinje fibers, at ~2–4 m/s, spraying through the subendocardium of both ventricles.
What is the core rule linking conduction speed to waveform width?
Fast, organized conduction (His-Purkinje) makes narrow waveforms; slow, cell-to-cell muscle conduction makes wide ones.
Why is the QRS wide in a bundle branch block or a PVC?
The ventricle is depolarized muscle-cell-to-muscle-cell instead of via the Purkinje web, so the QRS widens to ≥120 ms.
Why is the AV node the only normal electrical link between atria and ventricles?
A fibrous skeleton insulates the atria from the ventricles; only the AV node/His bundle penetrates it.
