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Electrical Conduction System and ECG Basics - Anatomy and Physiology

  1. Which electrical event does the T wave represent?
    Ventricular repolarization.
  2. Why is atrial repolarization not usually visible on an ECG?
    It occurs during the QRS complex and is hidden by the much larger signal of ventricular depolarization.
  3. An ECG shows a PR interval of 0.28 seconds, and every P wave is followed by a QRS complex. What is the most likely finding?
    First-degree AV block: conduction through the AV node is slowed, but every impulse still reaches the ventricles.
  4. What is the order of the cardiac conduction pathway after the SA node?
    Atrial myocardium, AV node, AV bundle (bundle of His), right and left bundle branches, then Purkinje fibers.
  5. If the SA node fails, which structure usually takes over as pacemaker, and at about what rate?
    The AV node (junctional rhythm), at about 40 to 60 beats per minute.
  6. Influx of which ion causes the depolarizing upstroke of an SA nodal action potential?
    Ca2+, through voltage-gated L-type calcium channels. Nodal cells lack the fast Na+ upstroke of ventricular muscle cells.
  7. An ECG shows a regular R-R interval of 0.75 seconds. What is the heart rate?
    80 beats per minute. Heart rate equals 60 divided by the R-R interval in seconds, so 60/0.75 = 80.
  8. Which electrical event does the P wave represent?
    Atrial depolarization.
  9. What is the purpose of the conduction delay at the AV node?
    It gives the atria time to finish contracting and the ventricles time to fill before ventricular contraction begins.
  10. Which structure normally sets the heart rate, and what resting rate does normal sinus rhythm produce?
    The sinoatrial (SA) node, producing a resting rate of about 60 to 100 beats per minute.
  11. Which electrical event does the QRS complex represent?
    Ventricular depolarization.
  12. What is the normal duration of the PR interval, and what does it mainly reflect?
    About 0.12 to 0.20 seconds. It runs from the start of atrial depolarization to the start of ventricular depolarization, mainly reflecting AV nodal delay.

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