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"first-degree atrioventricular block" AND "systematic review"

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first degree AV block prolonged PR interval symptoms low cardiac output reliable reference

Reading File
This describes marked first-degree atrioventricular (AV) block.
  • The PR interval runs from the beginning of the P wave to the beginning of the QRS complex. It represents conduction time from atria through the AV node/His-Purkinje system to the ventricles.
  • Normal PR interval: 0.12 to 0.20 seconds.
  • Here it is 0.46 seconds (460 ms), so AV conduction is very delayed.

Why is the P wave hard to see?

With such a long delay, the atrial depolarization (P wave) occurs so early relative to the following ventricular beat that it overlaps the T wave of the preceding beat. The arrows in the ECG would point to this hidden P wave.

Why can it cause symptoms?

Normally:
  1. The atria contract first, topping up ventricular filling.
  2. The ventricles contract shortly afterward.
With an extremely long PR interval, atrial contraction can occur at an unfavorable time, sometimes while the ventricles are still contracting or the AV valves are closed. This produces AV dyssynchrony and reduces the useful “atrial kick,” so stroke volume and cardiac output may fall.
Possible symptoms:
  • Fatigue
  • Dizziness or presyncope
  • Shortness of breath
  • Exercise intolerance
  • Palpitations, sometimes neck pulsations
This symptomatic situation is sometimes called pseudo-pacemaker syndrome, despite the person not having a pacemaker.
Strictly speaking, the issue is not that atria and ventricles electrically contract at exactly the same time. It is that the very long AV delay makes their mechanical timing inefficient.
A PR interval this long merits clinical review, especially if symptoms are present or if there is medication use that slows AV conduction, such as beta-blockers, verapamil/diltiazem, digoxin, or certain antiarrhythmics. Isolated first-degree AV block is often benign, but a markedly prolonged PR interval can be clinically important. Braunwald’s Heart Disease classifies this under atrioventricular block and bradyarrhythmias (p. 1315).
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