How to Read an Electrocardiogram

A close-up of an electrocardiogram printout showing P waves, QRS complexes, and T waves.

An electrocardiogram (ECG or EKG) is a test that measures the electrical activity of your heart to check for problems. It records the electrical signals as they travel through the heart and displays them as waves on a screen or paper.

How an ECG Works

An ECG machine detects the tiny electrical changes that occur during each heartbeat. These changes are represented as waves on the ECG tracing. The machine records three main waves: the P wave, QRS complex, and T wave.

The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. The time intervals between these waves provide information about the heart's electrical activity and rhythm.

Main Components of an ECG

An ECG tracing consists of several key components:

P wave: Represents atrial depolarization.
QRS complex: Represents ventricular depolarization.
T wave: Represents ventricular repolarization.
ST segment: The flat section between the QRS complex and T wave.
PR interval: The time between the start of the P wave and the start of the QRS complex.
QT interval: The time between the start of the QRS complex and the end of the T wave.

Common Uses of an ECG

ECGs are used to diagnose and monitor various heart conditions, such as:

Arrhythmias (irregular heartbeats)
Coronary artery disease
Heart attacks
Heart defects
Cardiomyopathy (enlarged heart)

They are also used to assess the effectiveness of treatments for heart conditions and to monitor heart health during surgery or other medical procedures.

How to Interpret an ECG

Interpreting an ECG involves analyzing the waves, intervals, and segments to identify any abnormalities. Here are some steps to follow:

Check the heart rate: Count the number of QRS complexes in a 6-second strip and multiply by 10 to get the beats per minute (bpm).
Analyze the P wave: Look for any abnormalities in shape, size, or timing.
Examine the QRS complex: Check for any widening or abnormalities in shape.
Assess the T wave: Look for any abnormalities in shape, size, or timing.
Measure the intervals: Check the PR interval, QRS duration, and QT interval for any abnormalities.
Look for ST segment changes: Check for any elevation or depression, which may indicate ischemia or infarction.

For example, a prolonged QT interval may indicate an increased risk of arrhythmias, while an elevated ST segment may indicate a heart attack.

Common Misconceptions

One common misconception is that an ECG can detect all heart problems. However, an ECG only measures the electrical activity of the heart and may not detect all types of heart disease. For example, it may not detect blockages in the coronary arteries that are not causing ischemia.

Another misconception is that an abnormal ECG always indicates a heart problem. Some people may have a normal ECG even if they have a heart condition, while others may have an abnormal ECG for reasons unrelated to their heart health.

According to the American Heart Association, ECGs are a useful tool for diagnosing and monitoring heart conditions, but they should be used in conjunction with other tests and evaluations to provide a complete picture of heart health.

I once observed a physician interpreting an ECG for a patient with chest pain. The physician carefully analyzed the waves and intervals, looking for any signs of ischemia or infarction. After reviewing the ECG and other test results, the physician was able to diagnose the patient with a heart attack and recommend appropriate treatment.

For more information on interpreting medical tests, see this guide.