ECG rhythm strip interpretation: practice problems with step-by-step solutions

1ECG paper and the waves

A rhythm strip is usually lead II, printed at a standard speed of 25 mm/s with 10 mm per mV. That fixes what every box means:

• One small box (1 mm) is 0.04 s.
• One large box (5 mm, five small boxes) is 0.20 s, and five large boxes are one second.
• A 6-second strip is 30 large boxes; marks at the top of the paper often show every 3 seconds.

The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation. In lead II a normal P wave and QRS point upwards.

2Heart rate: three methods

For a regular rhythm, measure the R–R interval between two neighbouring R waves:

Here the R waves are 4 large boxes apart: 300 ÷ 4 = 75 per minute. The 1500 method is more exact when the R waves do not fall on thick lines: 19 small boxes give 1500 ÷ 19 ≈ 79 per minute.

For an irregular rhythm one R–R interval means little. Count the QRS complexes in a 6-second strip and multiply by 10: 8 complexes give about 80 per minute.

3Intervals: PR, QRS, QT and QTc

• PR interval: from the start of the P wave to the start of the QRS. Normal 0.12–0.20 s (3–5 small boxes).
• QRS duration: from the first deflection of the QRS to the J point, where the ST segment starts. Normal below 0.12 s (under 3 small boxes).
• QT interval: from the start of the QRS to the end of the T wave. It shortens when the heart rate rises.

Because QT depends on the rate, it is corrected with the R–R interval in seconds. Bazett’s formula is the one most textbooks use; Fridericia’s is preferred at fast rates:

Reference limits vary. A QTc above about 0.45 s in men or 0.46 s in women is usually called prolonged, and above 0.50 s markedly prolonged.

4The five steps and the common rhythms

Read every strip in the same order before you name it:

1. Rate.
2. Rhythm: are the R–R intervals equal?
3. P waves: present, upright in lead II, one before every QRS, all alike?
4. PR interval: 0.12–0.20 s and constant?
5. QRS duration: below 0.12 s?

• Sinus rhythms pass steps 2–5. Normal sinus rhythm is 60–100 per minute, sinus bradycardia below 60, sinus tachycardia above 100.
• Atrial fibrillation: no P waves, a wavy fibrillatory baseline and an irregularly irregular R–R.
• Atrial flutter: sawtooth flutter waves at about 250–350 per minute, conducted at a fixed ratio such as 2:1 or 4:1.
• Junctional rhythm: narrow QRS at 40–60 per minute, P waves absent or inverted, either just before the QRS (PR below 0.12 s) or after it.
• PVC: an early, wide (0.12 s or more), bizarre QRS without a P wave, the T wave pointing the other way, often followed by a full compensatory pause. In bigeminy every second beat is a PVC.

5AV blocks and ventricular rhythms

For a block, march out the P waves and check which ones are followed by a QRS:

• First-degree: every P conducted, PR constant and longer than 0.20 s.
• Second-degree type I (Mobitz I, Wenckebach): the PR lengthens from beat to beat until one P wave is not conducted; then the cycle repeats, so the QRS complexes come in groups.
• Second-degree type II (Mobitz II): the PR of the conducted beats is constant, and a P wave is suddenly not conducted. The QRS is often wide.
• Third-degree (complete) block: P waves and QRS complexes are each regular but independent. The PR varies, and the atrial rate is faster than the escape rate (junctional 40–60, ventricular 20–40 per minute).

Ventricular tachycardia is a run of wide complexes at more than 100 per minute without related P waves. Ventricular fibrillation is a chaotic waveform without P, QRS or T. Asystole is an almost flat line; a flat line is always checked against the patient, the leads and a second lead.

6ACLS-style rhythms: SVT, torsades, escape, pauses and pacing

Rhythm-strip practice for resuscitation courses adds a few rhythms to the list. Use the same five steps for each:

• Supraventricular tachycardia (SVT): regular, narrow QRS, about 150–250 per minute, and no visible P waves because they hide in the QRS or the T wave. Sinus tachycardia is slower and shows a P wave before every QRS.
• Torsades de pointes: a polymorphic VT. The wide complexes grow and shrink in spindles and twist around the baseline, pointing up and then down. It is linked to a prolonged QT interval.
• Idioventricular rhythm: a ventricular escape rhythm. Regular, wide QRS, no P waves, 20–40 per minute (40–100 per minute is called accelerated idioventricular rhythm).
• Sinus arrest: the sinus node stops for a moment, so a pause appears with no P wave and no QRS. Its length is not a whole number of P–P cycles; in type II block, by contrast, the P wave in the pause is still there.
• Paced rhythms: a pacing spike is a thin vertical line. A spike before a P wave paces the atria; a spike before a wide QRS paces the ventricles; a dual-chamber pacemaker shows both. A spike with nothing after it is failure to capture.

Pulseless electrical activity (PEA) is not a strip diagnosis: the monitor shows an organised rhythm, but the patient has no pulse. You only know it by checking the patient. In resuscitation guidelines PEA and asystole are the non-shockable rhythms, and VF and pulseless VT the shockable ones.

A 2:1 AV block cannot be called type I or type II from one strip, because there are never two conducted P waves in a row.

7Common mistakes and exam questions

• Using 300 with small boxes or 1500 with large boxes.
• Using the 300 or 1500 method on an irregular rhythm instead of the 6-second count.
• Measuring the PR to the peak of the R wave instead of the start of the QRS.
• Including the ST segment in the QRS duration, or measuring the QT to the peak of the T wave.
• Putting R–R into the QTc formula in small boxes or milliseconds instead of seconds.
• Missing a P wave hidden in a T wave, which makes a block look like a sinus rhythm.
• Calling a 2:1 block type II for certain, or naming PEA from the strip alone without the pulse check.

How the exam questions look at each level:

• Easy: rate and intervals. You find the heart rate with the 300, 1500 or 6-second method, measure the PR interval and the QRS duration, and decide whether the rhythm is regular.
• Medium: QT and rhythm recognition. You measure the QT, calculate the QTc with Bazett or Fridericia, work with flutter rates and identify sinus rhythms, atrial fibrillation, flutter, first-degree block, junctional rhythm, PVCs, SVT and sinus arrest, and measure a pause.
• Hard: heart blocks and ventricular rhythms. You tell the AV blocks apart, recognise VT, VF and asystole, compare atrial and ventricular rates, measure the Wenckebach PR intervals and find the first abnormal step of the five.
• Hard, ACLS-style: you recognise torsades de pointes, idioventricular rhythm and paced rhythms (including failure to capture), discuss 2:1 block and answer PEA questions.