Rhythm Strip Analysis: A Structured ECG Story

When faced with a rhythm strip, treat it like a mystery and follow your evidence-based trail. A structured approach is your best ally in separating the benign from the deadly.

Rhythm – It’s called a “Rhythm strip”, so start by asking: Is the rhythm regular or irregular? Regular rhythms keep time like a metronome. Irregular ones wobble, sometimes predictably (as in heart block), other times chaotically (think atrial fibrillation).

Rate – What’s the heart rate? If the rhythm’s regular, count the number of large squares between R waves and divide 300 by that number. For example, 4 large squares = 75 bpm.

P-Waves – Are there P waves before every QRS? This tells you if the atria are leading the charge. In sinus rhythm, the SA node fires first, so you’ll see a visible P wave before every QRS. But if P waves are absent, don’t jump straight to AF… If the rhythm is regular and P waves are hidden or inverted, consider a junctional rhythm. The AV node may be pacing the heart, and P waves might be retrograde, lost in or after the QRS.

P-R Interval (The Love Story) – A normal range is 120–200 ms (3–5 small squares). And this is where it gets interesting, especially with heart blocks.

In First Degree AV block, The PR interval is delayed by greater than 200ms. Its a benign condition and is often only detected when an ECG is carried out.

In Second Degree AV block (Mobitz 1 or Wenckebach), the PR interval gradually lengthens until a beat is dropped, then the cycle repeats. It’s affectionately known as the “auctioneer’s rhythm”: the P wave calls, the QRS shows up later… and later… and then suddenly doesn’t show up at all — “Going… going… gone!”
Or think of it as a dysfunctional couple. The P wave is the reliable partner. The QRS is increasingly late, until, one night, they just don’t come home.

In Second Degree AV block (Mobitz 2), some electrical signals are blocked without the gradual PR interval prolongation seen in Type I. The PR interval remains constant before the signal is blocked, leading to a dropped QRS complex. This is frequently fixed in a 2:1 or 2:2 ratio. This form is more concerning because it is associated with a higher risk of progressing to complete (third-degree) AV block. The P wave is consistent and dependable. But the QRS replies inconsistently, vanishing unpredictably, like a partner who ghosts you for no clear reason.

In Third Degree AV block (complete heart block), the couple has broken up entirely. The P wave and QRS complex are still living in the same house, but they’re not speaking. Each is on their own schedule. The atria and ventricles are depolarising independently – Complete AV dissociation.

QRS Width. Narrow QRS complexes (<120 ms) suggest normal, swift conduction through the His–Purkinje system. Broad complexes may arise from ventricular origin or delayed conduction, a red flag for VT or a bundle branch block.
Finally, name the rhythm.

Common rhythms to identify:

Sinus Rhythm: P before every QRS, regular, normal intervals, a well-rehearsed routine.

Junctional Rhythm: Regular rhythm with absent or inverted P waves, often buried in or following the QRS. Like a quiet understudy stepping in when the lead doesn’t show, keeping the show going from backstage.

Atrial Fibrillation: Irregularly irregular, no P waves, chaotic baseline. Like a jazz band with no conductor.

SVT: Fast, regular, narrow. P waves may be hidden. A rapid trick from above the ventricles.

AV Blocks: (The Dysfunctional Couple)

Type 1 AV Block: The P wave is the reliable partner. The QRS is increasingly late, until, one night, they just don’t come home.

Type 2-1st Degree AV block (2:1 pattern), the P wave and QRS are drifting apart. Only every second P wave gets a response. It’s an on/off relationship, half the time, QRS isn’t picking up the phone.

Type 2-2nd Degree AV block (2:2 pattern), the P wave is consistent and dependable. But the QRS replies inconsistently, vanishing unpredictably, like a partner who ghosts you for no clear reason.

Type 3 AV block (complete heart block), the couple has broken up entirely. The P wave and QRS complex are still living in the same house, but they’re not speaking. Each is on their own schedule.

Ventricular Tachycardia: Broad, fast and regular. A dangerous solo act from the ventricles. Stand by with a defibrillator!