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Clinical Mnemonics · Cardiology & ECG

First-Degree AV Block — "If the R is Far From P, It's a First Degree"

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  1. In 30 seconds
  2. The college version
  3. Eli explains
  4. Study tools

In 30 seconds

Recognize first-degree AV block: a prolonged but constant PR interval (> 0.20 sec) with every P wave still followed by a QRS — the impulse is slow but always gets through.

The college version

Recognize first-degree AV block: a prolonged but constant PR interval (> 0.20 sec) with every P wave still followed by a QRS — the impulse is slow but always gets through.

R — R wave (QRS complex)

The ventricle's beat — "the wife" in the husband/wife analogy

The QRS complex reflects ventricular depolarization. In first-degree AV block a QRS follows every P wave — no beats are dropped. Its width depends on where the delay is; with a delay confined to the AV node the QRS is usually narrow.

Physiology. A QRS appears every time the depolarization wave successfully reaches the ventricles. In first-degree block conduction is delayed (not blocked), so the QRS always follows — just later.

Safety. Confirm on the monitor that every P wave is followed by a QRS; the relationship must be constant. If QRS complexes start dropping, the rhythm has progressed to a higher-degree block.

P — P wave

The atrium's beat — "the husband" firing from the SA node

The P wave reflects atrial depolarization originating in the SA node. In first-degree AV block the SA node and atria are normal; the problem is entirely downstream at the AV node, where the impulse is delayed before reaching the ventricles.

Physiology. The SA node is the heart's natural pacemaker (60–100 bpm). In first-degree block it keeps firing normally and the atria respond normally — the rhythm's only abnormality is the delay between P and QRS.

Safety. A normal, regular P wave with a prolonged PR tells you the atria are fine — look downstream (AV node, medications, ischemia) for the cause of the delay.

Far — Prolonged PR interval

A consistent but stretched-out delay between the P wave and the R wave

The PR interval (normal 0.12–0.20 sec, or 3–5 small boxes) is the time from the start of atrial depolarization to the start of ventricular depolarization — essentially AV conduction time. In first-degree block the PR interval is constantly greater than 0.20 sec but fixed.

Physiology. The AV node normally delays the impulse ~0.1 sec so the atria finish emptying before the ventricles contract. In first-degree block that delay is pathologically lengthened (disease, vagal tone, or drugs), but conduction still completes on every beat.

Safety. First-degree block is usually benign by itself, but a new, sudden PR prolongation can be the first sign of digoxin/beta-blocker/calcium-channel-blocker toxicity or ischemia — investigate the cause rather than dismissing it.

Memory aids

  • R = R wave (QRS complex)
  • P = P wave
  • = Prolonged PR interval

Quick review

  • R = R wave (QRS complex) — The ventricle's beat — "the wife" in the husband/wife analogy
  • P = P wave — The atrium's beat — "the husband" firing from the SA node
  • Far = Prolonged PR interval — A consistent but stretched-out delay between the P wave and the R wave
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

It's the mildest traffic jam in the heart's electrical road. The signal from the top rooms to the bottom rooms is slow, but every signal still gets through — nothing is blocked, just delayed.

R — R wave (QRS complex). The "R" is the tall spike on the ECG that shows the bottom rooms (ventricles) squeezing. In the story, the R wave is the wife who does the heavy lifting of pumping blood out. In first-degree block she always shows up for work — just a little late, every single time.

P — P wave. The "P" is the little bump before the spike that shows the top rooms (atria) squeezing. It is the husband in the story. In first-degree block the husband always calls home — the SA node fires normally and the atria contract on time, every time.

Far — Prolonged PR interval. "Far" means the husband and wife stand a little farther apart than usual — the time between the P bump and the R spike is stretched out. The distance is always the same stretched length, and they always still connect.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

R
R wave (QRS complex)
P
P wave
Far
Prolonged PR interval

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