Teaching Visual: How to Interpret an Electrocardiogram

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Chapter 12
Teaching Visual: How to Interpret an Electrocardiogram

Jessica L. Israel MD


images Develop a systematic approach to ECG interpretation.

images Create a narrative description of ECG findings.

images Apply this systematic approach and narrative description techniques to the interpretation of three commonly encountered ECGs and two rhythm strips.


The ECG is one of the hallmarks of cardiac diagnostic testing. Interestingly, many hospitalized patients will have ECGs recorded upon admission, and many will have “baseline” ECGs filed in their outpatient charts. However, the US Preventive Services Task Force recommends against screening for coronary heart disease in low-risk adults. There is also no clear evidence that screening helps predict the progression of disease in high-risk adults (www.uspreventiveservicestaskforce.org/).

In general, ECGs should be done for patients with cardiac complaints or in those who present with related complaints secondary to other cardiovascular or pulmonary disease. Outpatient ECGs are helpful to document a known ECG abnormality, which may serve as a useful comparison in the future, and for preoperative screening in the appropriate patient.


For most internists, ECG interpretation is a common task in patient care. However, even the most experienced internists may need consultation with a colleague to analyze a complicated tracing. Developing a systematic approach to ECG interpretation helps to ensure that you look at all of the aspects of the tracing (not just the obvious ones); even if you are unsure what the underlying problem may be, you will still be able to systematically describe your findings to a colleague in consultation. This overview is a guide to reading ECGs and is by no means exhaustive. However, you can learn by simply applying this rubric over and over again. In moments that are not so busy during your medicine rotation, one useful exercise is to pull multiple ECG tracings from the charts of patients and review them, perhaps even with the help of a resident. The more ECGs you read, the more comfortable you will be interpreting them.

P waves, T waves, and QRS complexes appear differently in each of the 12 standard ECG leads. The P waves, T waves, and QRS complexes have been labeled in lead II in Figure 12-1. Label them for leads aVR and V6.


Figure 12-1 (From Goldberger AL. Clinical electrocardiography: a simplified approach. 7th ed. Philadelphia: Mosby; 2006, p. 44, question 1.)

A systematic stepwise approach includes examination in five major areas of the ECG tracing in the following order:

Step 1: Rate

Step 2: Rhythm

Step 3: Axis

Step 4: Hypertrophy/chamber size

Step 5: Ischemic changes/infarction

Step 1: Determining the Rate

To calculate the rate, find an R wave that is lined up with a heavy border of a background grid box. After this, you simply count down until your next R wave appears. The counting is specific: the distance to the next grid line is 300 bpm, to the second grid line is 150 bpm, the third is 100 bpm, the fourth is 75 bpm, the fifth is 60 bpm, and the sixth is 50 bpm. Often the next R wave will appear between two major grid lines, and in these cases you simply estimate the rate. For example, if your rate determination falls exactly between the 75 and the 60 line, you can perhaps estimate the heart rate at 68 bpm. Note: to be precise the distance between two heavy grid lines is 1/300 of a minute. Therefore, the distance between two heavy grid lines is 2/300, and three grid lines is 3/300, which can be simplified to 1/150 and 1/100, respectively. That’s where these numbers come from, so committing them to memory can be helpful.

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