Learn About Reading ECG Results

What an ECG Is and Why Doctors Use It

An electrocardiogram, commonly called an ECG or EKG, is a test that records the electrical activity of your heart. Your heart beats because of electrical signals that travel through the heart muscle, telling it when to contract and pump blood. An ECG machine detects these signals and creates a visual record of them on paper or a computer screen. Think of it like a graph that shows what your heart's electrical system is doing moment by moment.

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Doctors have used ECGs since the early 1900s, making it one of the longest-standing medical tests. The procedure itself is quick—usually taking 5 to 10 minutes—and completely painless. There are no needles, no injections, and no radiation involved. The machine simply reads signals that your body naturally produces.

Doctors order ECGs for many reasons. They use them to check for irregular heartbeats, also called arrhythmias. They use them to see if the heart muscle has been damaged by a heart attack. An ECG can show if someone has heart disease, high blood pressure effects on the heart, or electrolyte imbalances that affect heart function. Doctors also use ECGs as a baseline test before surgery or to monitor people taking certain medications that affect heart rhythm.

According to the American Heart Association, millions of ECGs are performed each year in the United States. In hospital emergency rooms, an ECG is often one of the first tests done when someone complains of chest pain or shortness of breath. Primary care doctors may order routine ECGs for patients over 65 or for those with risk factors like diabetes or a family history of heart disease.

Practical takeaway: Understanding that an ECG is a simple, painless electrical recording of your heart helps you know what to expect if a doctor recommends one. There is no preparation needed, and you can return to normal activities immediately after.

How the ECG Test Is Performed and What Happens

When you arrive for an ECG, you will be asked to remove clothing from the waist up and lie on your back on an examination table. The technician will clean small areas of your skin on your chest, wrists, and ankles with a gel or alcohol pad. This cleaning helps the electrodes stick better and makes electrical contact easier.

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Next, the technician places small metal discs called electrodes on your skin at specific locations. A standard 12-lead ECG uses 10 electrodes placed on your chest, arms, and legs. Despite being called a "12-lead" test, it uses only 10 electrodes because each electrode can record signals in different directions, creating 12 different views of your heart's electrical activity. The electrodes are held in place with adhesive and connected to wires that run to the ECG machine.

Once all electrodes are attached, you will be asked to relax and remain still. The machine will begin recording for about 10 seconds. You may hear or see the paper moving or the machine beeping softly. Some machines are digital and display the recording on a screen instead. During this brief recording period, the machine captures multiple heartbeats and all the electrical signals they produce.

The entire process, from when you enter the room to when you can get dressed again, typically takes 10 to 15 minutes. Most of this time is setup. The actual recording takes only seconds. You will not feel any sensation while the machine is recording—there is no shock, no pain, and no discomfort. Some people notice mild itching when the electrodes are removed because the adhesive pulls slightly on skin, but this is minor and temporary.

After the test, the electrodes are removed, and the gel is wiped away. You can immediately put your clothes back on. There are no lasting marks or residue. You can eat, drink, take medications, and do all your normal activities right away. If you were taking a stress ECG (where you exercise on a treadmill while wearing electrodes), you will need a few minutes to cool down and return to normal breathing.

Practical takeaway: Knowing the simple steps of an ECG—electrode placement, a brief recording, and quick removal—can reduce anxiety about the test. The procedure requires no fasting, no special preparation, and no recovery time.

Reading the Basic Components of an ECG Printout

An ECG printout shows a series of waves and lines on graph paper. Understanding what you are looking at starts with knowing the basic components. The horizontal axis (left to right) represents time, and the vertical axis (up and down) represents the voltage or strength of the electrical signal. Each small square on the paper represents a specific amount of time and voltage, which allows doctors to measure and analyze the heart's activity precisely.

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The main features visible on an ECG are labeled with letters: P, Q, R, S, T, and sometimes U. The P wave is the first bump on the ECG. It represents the electrical activity that causes the upper chambers of your heart (called atria) to contract. The P wave should be small and rounded, usually less than 0.12 seconds wide and less than 2.5 millimeters tall.

The QRS complex is the tallest and most distinctive feature on the ECG. It consists of three parts: the Q wave (a small dip down), the R wave (a large spike up), and the S wave (another dip down). This complex represents the electrical activity that causes the lower chambers of your heart (called ventricles) to contract and pump blood out to your body. The entire QRS complex should take less than 0.12 seconds. The R wave is usually the tallest point on the entire ECG strip.

The T wave comes after the QRS complex and represents the electrical activity as the ventricles are recovering and preparing for the next heartbeat. The T wave is usually smaller than the R wave and has a rounded shape. In a normal ECG, the T wave points in the same general direction as the QRS complex.

The baseline is the flat line that appears between heartbeats and waves. A normal baseline indicates that no electrical activity is occurring at that moment. Any bumps or waves in the baseline suggest electrical noise or problems. The ST segment is the flat section between the end of the QRS complex and the beginning of the T wave. Changes in the ST segment can indicate a heart attack or other heart problems.

Practical takeaway: Familiarizing yourself with the letters P, QRS, T, and the concept of baseline helps you understand basic ECG reports. When a doctor mentions these features, you will have a mental picture of what they are referring to on the paper.

Common Patterns That Indicate Normal Heart Rhythm

A normal heart rhythm is called sinus rhythm because it originates from the sinoatrial node, a natural pacemaker in the right upper chamber of the heart. In a normal ECG, you will see a regular pattern that repeats with each heartbeat. The pattern follows the sequence: P wave, QRS complex, T wave, baseline, then the pattern repeats.

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In normal sinus rhythm, the heart rate is between 60 and 100 beats per minute. You can calculate the heart rate from an ECG by counting the number of QRS complexes in a six-second strip and multiplying by 10, or by using a calculation method based on the spacing between complexes. The distance between each QRS complex should be the same, showing that the heartbeats are evenly spaced in time.

The intervals between waves should fall within normal ranges. The PR interval (from the start of the P wave to the start of the QRS) measures the time it takes for the electrical signal to travel from the upper chambers to the lower chambers. This should be between 0.12 and 0.20 seconds. If this interval is too long, it may indicate a delay in electrical conduction. If it is too short, it might suggest an abnormal electrical pathway.

The QRS duration should not exceed 0.12 seconds, as mentioned earlier. This narrow width indicates that the electrical signal is traveling normally through the ventricles. A wider QRS complex might suggest that the signal is taking a slower or abnormal path through the heart muscle.

The axis of the heart is another normal measurement. This refers to the overall direction of the heart's electrical activity. Doctors determine this by looking at the QRS complexes in different leads and calculating an angle. A normal axis typically falls between -30 and 90 degrees. An abnormal axis might suggest a change in heart position or structure or certain heart