How to Read an ECG Strip in 5 Steps

Before you start: the patient, then the paper

The golden rule comes first: check the patient, not just the monitor. Monitors name rhythms on their own, and that name is not always right. Loose electrodes and patient movement cause artifact, and a monitor can read artifact as ventricular beats or other abnormal rhythms when nothing is wrong with the heart. A strip only means something next to the person it came from. Watch for signs that the rhythm is affecting them, such as dizziness, confusion, or passing out.

Then know your paper. The standard ECG recording speed is 25 millimeters per second. At that speed the grid becomes a clock:

• One small box (1 mm) is 0.04 seconds.
• One large box (5 small boxes) is 0.20 seconds.
• Five large boxes make one second, so 30 large boxes make a 6-second strip.

Pre-step: is the QRS narrow or wide?

Before the five steps, take one look at the QRS complexes. Are they narrow and sharp, or wide? This is not a measurement yet. It is a first sort that tells you where to pay attention.

A narrow QRS means the impulse traveled the normal path through the ventricles, so the beat started above them. A wide QRS means the beat started in the ventricles themselves, or it started above them and was slowed on the way down, as in a supraventricular rhythm with BBB/aberrancy. You will measure the QRS properly in step 5: 0.06 to 0.12 seconds is narrow, and more than 0.12 seconds is wide.

Step 1: Regularity

Question: Are the R-R intervals equal?

Measure from one R wave to the next with calipers or the edge of a piece of paper, and check across the whole strip, not just two beats. Rhythms fall into four patterns:

• Regular. Every R-R interval is the same. Normal sinus rhythm is regular.
• Irregularly irregular. No two R-R intervals match, and there is no pattern. Atrial fibrillation is the classic example.
• Regularly irregular. The intervals are unequal, but the unevenness repeats in a pattern, as some second-degree heart blocks do.
• Occasionally irregular. The intervals are equal except for an occasional beat that comes early, such as a PVC.

Regularity comes first because it decides how you count the rate in step 2.

Step 2: Rate

Question: How many beats per minute?

For adults, 60 to 100 beats per minute is the normal range taught in rhythm courses and used on this site. Below 60 is bradycardia; above 100 is tachycardia. There are three common ways to get the number:

Where do 300 and 1,500 come from? At 25 mm/s, one minute of paper holds 300 large boxes, or 1,500 small boxes. Both shortcuts assume every beat is the same distance apart, which is why they give a wrong answer on an irregular rhythm.

If P waves are present, count them the same way to get the atrial rate. The atrial and ventricular rates should match. When they do not, the P waves and QRS complexes are not moving together, and step 3 is where you find out why.

Step 3: P Wave

Question: Is there a P wave before every QRS, upright in lead II?

The P wave tells you where the beat started. Ask four things:

• Are P waves present? No P waves means the beat did not start in the SA node.
• Do they all look the same? A P wave with a different shape started somewhere else in the atria.
• Do they come at a regular rate? Measure P to P the same way you measured R to R.
• Is there one P wave for every QRS? This tells you whether the signal from the atria is reaching the ventricles every time.

In normal sinus rhythm, there is one uniform P wave before every QRS, and it is upright in lead II.

Step 4: PR Interval

Question: How long from the start of the P wave to the start of the QRS?

The PR interval is the time the impulse takes to get from the atria, through the AV node, to the ventricles. A normal PR interval is 0.12 to 0.20 seconds, or 3 to 5 small boxes.

• Longer than 0.20 seconds, with every P wave followed by a QRS, is first-degree AV block. The signal is delayed, not blocked.
• Shorter than 0.12 seconds can mean the impulse is reaching the ventricles faster than normal, as in Wolff-Parkinson-White syndrome.

Measure more than one beat. A PR that stays the same from beat to beat is one finding. A PR that gets longer before a dropped beat, or that has no fixed relationship to the QRS at all, is how the heart blocks are told apart.

Step 5: QRS Duration

Question: How wide is the QRS complex?

Measure from the start of the QRS to the point where it ends. A normal, narrow QRS is 0.06 to 0.12 seconds, up to 3 small boxes. A QRS of more than 0.12 seconds (more than 3 small boxes) is wide. Beats that start in the ventricles, such as PVCs, are wide, and so are beats slowed by a bundle branch block.

This is where the pre-step gets checked with a real measurement. If your first look said narrow and the calipers say 0.14 seconds, trust the calipers.

Normal values at a glance

Worked example: the 6-second strip

Go back to the 6-second strip near the top of the page and run the routine.

1. Pre-step: the QRS complexes are narrow and sharp.
2. Regularity: the R waves are evenly spaced across the strip. Regular.
3. Rate: there are 7 QRS complexes between the 0 and 6-second marks. 7 × 10 = 70 bpm.
4. P Wave: one upright, rounded P wave before every QRS, and they all look alike.
5. PR Interval and QRS Duration: these need calipers on a full-size strip, not a glance at a picture. The measurement drill gives you strips built for exactly this.

Regular, 70 bpm, and a matching P wave before every QRS: this is a sinus rhythm. If the PR and QRS measure normal, it is normal sinus rhythm.

Common mistakes

• Reading the monitor instead of the patient. Artifact can look like a dangerous rhythm. Look at the person first.
• Counting the rate before checking regularity. The 300 and 1500 methods give a wrong number on an irregular rhythm.
• Checking two beats instead of the whole strip. An early beat or a dropped beat can sit anywhere on the strip.
• Eyeballing intervals. A normal PR and first-degree AV block are split by one number, 0.20 seconds. A narrow and a wide QRS are split by another: anything more than 0.12 seconds is wide. Measure.
• Naming the rhythm too early. A strip that "looks like" atrial fibrillation can have P waves you have not found yet. Finish all five steps before you name it.

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