How to Interpret ECG Rhythm: A 5-Step Process

Knowing how to interpret ECG rhythm is a core skill for clinicians who manage cardiac patients. This five-step approach reduces analysis time and supports recognition of life-threatening arrhythmias, allowing providers to focus on patient care.

Step 1: Determine if the Rhythm is Regular or Irregular

Find the first cardiac complex in the ECG and locate the QRS complex. Next, identify the R waves throughout the remainder of the ECG strip. You can make small markers above each complex for easier identification. Then take telemetry calipers to trace the R to R intervals from the first two cardiac complexes through the remainder of the ECG strip.

Some variance is expected, but if the variance in the tracing is greater than 0.12 seconds or greater than 3 small squares, the rhythm is considered irregular.

Step 2: Determine the Rate

Although there are three methods used, the goal is to determine if the rhythm is Tachycardia, over 100 bpm (fast), Bradycardia, fewer than 60 bpm (slow), or normal (60-100bpm).

These three methods help determine the rhythm:

Method #1: The 6 Second Method

Count the number of R waves on a 6 second strip and multiply by 10. This provides an approximate heart rate and is used for irregular and regular rhythms.

Method #2: The 300 Method

Count the number of large boxes between 2 successive R waves and divide by 300 to obtain heart rate. 

*Applies only for regular rhythms.

Method #3: The 1500 Method

This is the most accurate method. Count the number of small squares between 2 consecutive R waves and divide into 1500.

*Applies only for regular rhythms.

Step 3: Analyze P Waves

The P wave represents atrial depolarization. In normal sinus rhythm, the P wave is always positive in lead II and always negative in lead aVR. 

Analyze whether or not P waves are present and ask yourself:

• Are they all upright and round in appearance?
• Are some inverted, absent, or buried in the QRS complexes?

*Some rhythms, such as junctional and wandering atrial pacemakers, may have inverted or absent P waves.

Additionally, rhythms may have P waves, but are fibrillatory waves, indicating atrial fibrillation or flutter waves indicating atrial flutter. This occurs when the atria are quivering or fluttering and not effectively pumping blood. Consider the following questions:

• Are P waves present?
• Are they round and upright in appearance?
• Are they consistent or do they change in appearance throughout the strip?

Step 4: Measure the PR Interval

The PR interval (PRI) measures from the beginning of the P wave to the beginning of the QRS complex, which measures the time of depolarization of the atria to depolarization of the ventricles. It reflects a physiological delay in AV conduction imposed by the AV node. Normal range is 120 – 200 ms or 0.12 to 0.20 seconds. Keep these questions in mind:

• What is the length of the PRI?
• Is the PRI consistently the same length throughout the strip? (gradually increasing?)

Step 5: Measure the QRS Interval

The QRS complex is the next cardiac complex in the ECG. It measures ventricular depolarization and has a series of three deflections that indicate the current associated with right and left ventricular depolarization. 

By convention, the first deflection in the complex, if it is negative, is called a Q wave. The first positive deflection in the complex is called an R wave, and a negative deflection after an R wave is called an S wave.

A second positive deflection after the S wave, if there is one, is called the R wave. Some QRS complexes do not have all three deflections. Regardless of the number of waves present, they are all QRS complexes. The QRS interval is generally less than 0.10 seconds. Anything greater may be classified as ventricular beats or bundle branch blocks, classified as “wide”. Consider asking:

• What is the length of the QRS Interval? (> or = 0.10 seconds?)  
• Are there any dropped QRS complexes? (P waves present but no QRS?)

References:

Aehlert, B. (2006). ECGs made easy. St. Louis, MO: Elsevier Mosby.

Learn Rhythm Interpretation Through Code One’s ACLS Course

Code One Training Solutions offers a hybrid training course for American Heart Association® (AHA) ACLS training for healthcare providers and professionals. Our training is through HeartCode® online, followed by a self-guided hands-on skills session. If learners would like to review or clarify anything from the algorithm, they can schedule a complimentary ACLS algorithm review before the self-guided skills session.