Electrocardiogram (ECG/EKG): What the Test Shows

What Is an ECG?

An electrocardiogram (ECG or EKG) is a quick, simple, and noninvasive test that shows how the heart is working. Small, sticky sensors called electrodes are placed on the skin of the chest, arms, and legs. These electrodes record the heart’s electrical signals as it beats. The test does not send electricity into the body - it only measures what the heart naturally produces.

What an ECG Can Show

An ECG gives doctors important information about how the heart is working. It can reveal:

• Heart rate and rhythm - helps detect irregular heartbeats (arrhythmias).
• Evidence of heart attack or reduced blood flow (ischemia) - shows if damage has happened in the past or is occurring now.
• Electrical conduction or chamber abnormalities - identifies issues with how signals travel through the heart or if parts of the heart are enlarged.

Types of ECGs

Different kinds of ECGs are used depending on what your doctor needs to check:

• Resting 12-lead ECG - done in a clinic or hospital while you sit or lie still.
• Ambulatory monitors - worn during daily life:
  • Holter monitor - records continuously for 24-48 hours.
  • Event monitor - worn up to ~30 days, records only when you trigger it or when an irregular rhythm is detected.
  • Loop recorder - implanted under the skin for long-term monitoring.
• Stress ECG - measures heart activity during exercise (treadmill/bike) or after medicine that simulates exercise.
• Consumer wearables - smartwatches and fitness devices can record a single-lead ECG, but results are limited; always review with a clinician.

When Doctors Order an ECG

Doctors may order an ECG in different situations, including:

• Symptoms - such as chest pain, palpitations, dizziness or fainting, shortness of breath (SOB), or unexplained fatigue.
• Baseline checks - before a procedure, after cardiac treatment, or to monitor how medicines or implanted devices affect the heart.
• Preventive use - sometimes even without symptoms, depending on your age, health history, or specific risk factors.

What Happens During an Electrocardiogram

The test is quick and simple. A technician places about 10-12 small electrodes on your chest and limbs. Once everything is connected, you’ll be asked to lie still while the machine records your heart’s electrical activity. The entire process usually takes just a few minutes.

There is no pain during the test. At most, you may feel slight discomfort when the adhesive pads are removed.

What Numbers Does an ECG Show?

An ECG doesn’t just make wave patterns - it also gives doctors important numbers to understand how your heart is working. Some of the key ones are:

• Heart rate - how many times your heart beats per minute (BPM).
• PR interval (~120-200 ms) – how long it takes for the signal to move from the upper chambers (atria) to the lower chambers (ventricles).
• QRS duration (<120 ms) - how long it takes the ventricles to contract.
• QT interval / QTc (~350-440 ms) - how long the ventricles take to contract and then relax.
• Electrical axis - the overall direction of your heart’s electrical activity, shown in degrees.
• Voltage (amplitude) – measured in millivolts (mV), showing the strength of the electrical signals.

These values help doctors spot rhythm problems, blockages, or other heart conditions, even when the ECG waves look normal at first glance.

Getting Your Results & Next Steps

Once your ECG is complete, your doctor will review the tracing and compare it to what’s expected for your age, health, and symptoms.

Normal Results

Normal results indicate that your heart rate, rhythm, and electrical patterns are within a healthy range. In this case, you may only need routine monitoring or follow-up appointments.

Abnormal Results

Abnormal results could suggest issues such as arrhythmias, signs of a prior or current heart attack, or problems with the heart’s electrical conduction. Depending on what the ECG shows, your doctor might recommend further tests like blood work, an echocardiogram, a stress test, ambulatory monitoring, or a referral to a cardiologist.

Remember: ECG results are interpreted in the context of your overall health and medical history, so your doctor can provide the most accurate guidance.

Risks and Limitations

ECG is generally very safe, with only minor risks such as temporary skin irritation from the electrodes.

However, a single ECG has some limitations. It may miss intermittent heart problms, and factors like body habitus (physical characteristics), movement during the test, electrolyte imbalances, or certain medications can affect the results. In such cases, your doctor may recommend additional testing, such as an ambulatory monitor (Holter/event monitor) or a stress ECG, to get a more complete picture of your heart’s activity.

ECG vs Other Heart Tests

• ECG (Electrocardiogram): Records the heart’s electrical activity to detect arrhythmias, conduction issues, or signs of a heart attack.
• Echocardiogram: Uses ultrasound to visualize heart structures and function, including valves and pumping ability.
• Cardiac Enzymes (Blood Tests): Detect markers of heart muscle damage, typically used to confirm a heart attack.