Interpreting ECG Strips – Nursing Unraveled

Interpreting ECG Strips is the foundation of this ECG course. As this course progresses we will take a look at dysrhythmias which are abnormal cardiac rhythms. To be able to identify dysrhythmias we should understand how to interpret a 6 second ECG strip.

When we print out an ECG strip to interpret, most commonly, we are printing out a 6 second strip. This means we are only seeing a small portion of cardiac function, yet we are able to get an appropriate picture of what is going on.

Below we will detail steps to interpreting a 6 second ECG strip.

1. Assess the rhythm.

First, assess if the rhythm is regular or irregular. To do this look at the R-R interval. The rhythm is considered regular if the R-R spacing is consistent, and it is considered irregular is R-R spacing is inconsistent.

2. Assess the rate.

Next, assess heart rate. Here are the most common ways to do this:

6 Second Strip Method
Big Box Method
Small Box Method

The 6 second strip method constitutes counting the R waves in a 6 second strip and then multiplying by 10 to get heart rate.

The Big Box Method consists of counting the amount of big boxes in an R-R interval and then dividing into the number of big boxes in a minute. There are 30 big blocks in a 6 second strip, so that means that 1 minute would hold 300 big blocks. So, for this method we would divide 300 by the amount of big blocks between two R waves.

The Small Box Method consists of counting the number of small boxes in an R-R interval and then dividing by the number of small boxes in a minute. There are 150 small boxes in a 6 second strip, so that means that 1 minute would hold 1500 small boxes. So, for this method we would divide 1500 by the amount of small blocks between two R waves.

3. Assess the P waves and PR interval.

Here the focus is making sure all P waves look the same and to assess the PR Interval (PRI). Focus on these questions:

Do all P waves deflect in the same direction?
Do all P waves have the same morphology (form)?
Is the PRI within standard limits? (PRI standard limits are within 0.12 seconds to 0.20 seconds, or, 3 to 5 small boxes).

4. Assess atrial and ventricular rates.

Here we are looking to see if the atria and ventricles are working together. To do this check for two things:

Are there equal numbers of P’s and QRS’s?
Does each QRS have a P wave directly before it?

5. Assess the QRS.

Now, assess the QRS complexes.

Do all QRS complexes deflect in the same and expected direction?
Do all QRS complexes have the same morphology (form)?
Is the QRS complex within standard limits? (QRS complex standard limits are within 0.06 seconds to 0.12 seconds, or, 1.5 to 3 small boxes).

6. Assess the T waves

Next, do the T Waves look normal?

Is there any ST Segment Elevation? (STEMI)
Is there any ST Segment Depression?
Are the T Waves inverted?
What is the Qt Interval? (Normal Qt interval is within 0.35 seconds to 0.45 seconds, or, 9 to 11 small boxes).

7. Assess for Ectopy.

Lastly, check for ectopy. Do you see any ectopic beats within the strip? The two forms of ectopic beats are:

Premature Atrial Contraction (PAC).
Premature Ventricular Contraction (PVC).