An EKG (electrocardiogram) tells you how well your heart’s electrical system is working. It can reveal your heart rate, heart rhythm, signs of a current or past heart attack, thickening of the heart walls, and problems with how electrical signals travel through your heart. The test takes about 10 minutes and is one of the most common first steps in evaluating heart health.
How Your Heart’s Electrical System Creates the Signal
Your heart beats because of a built-in electrical system. A cluster of cells in the upper right chamber acts as your heart’s natural pacemaker, firing a signal that tells the upper chambers to squeeze and push blood downward. That signal then pauses briefly at a relay point between the upper and lower chambers, giving the upper chambers time to empty completely. From there, the signal races down a bundle of specialized fibers that split into left and right branches, triggering the lower chambers to contract and pump blood out to your lungs and body.
An EKG picks up this electrical activity through 10 small sticky sensors placed on your chest and limbs. Those 10 sensors generate 12 different “views” of the heart’s electricity from different angles. Six sensors go across the chest in specific positions along the breastbone and left side, and four go on your arms and legs. Each view highlights a different region of the heart, so together they create a surprisingly detailed picture of how electrical signals move through the entire organ.
What the Waves on the Tracing Mean
The jagged line on an EKG printout isn’t random. Each bump and dip corresponds to a specific event inside your heart.
- P wave: This small initial bump represents your upper chambers contracting. The first half comes from the right upper chamber, and the second half from the left.
- QRS complex: The tall, sharp spike in the middle represents your lower chambers contracting. This is the main pumping action that sends blood to your body. It happens fast, normally lasting only 80 to 100 milliseconds.
- T wave: The final rounded bump shows your lower chambers resetting their electrical charge, preparing for the next beat.
The spaces between these waves matter just as much as the waves themselves. The gap between the P wave and the QRS complex (called the PR interval) should be 120 to 200 milliseconds. If it’s too long, the electrical signal is being delayed on its way from the upper to lower chambers. The total time from the start of the QRS to the end of the T wave (the QT interval) should be about 420 milliseconds or less at a resting heart rate of 60 beats per minute. A QT interval that’s too long can signal a risk for dangerous rhythm problems.
Heart Rhythm Problems
One of the most valuable things an EKG reveals is whether your heart is beating in a normal, steady rhythm. In a healthy heart, each beat follows the same electrical sequence with consistent timing. When that pattern breaks down, the EKG captures exactly how and where the disruption occurs.
Atrial fibrillation, one of the most common rhythm disorders, shows up as an absence of clear P waves and an irregular spacing between beats. The upper chambers are firing chaotically instead of contracting in an organized way. Heart block, where the electrical signal is delayed or completely interrupted between the upper and lower chambers, appears as an abnormally long PR interval or as P waves that aren’t followed by QRS complexes at all. Bundle branch blocks, where one of the two pathways into the lower chambers is damaged, show up as a widened QRS complex because the signal has to take a detour.
Signs of a Heart Attack
An EKG is one of the first tests performed when a heart attack is suspected because it can show changes within minutes of blood flow being cut off to part of the heart muscle. The most critical finding is elevation of the segment between the QRS complex and the T wave (the ST segment). When this segment rises above its normal baseline, it strongly suggests that a coronary artery is completely blocked and heart muscle is actively being damaged.
Not all heart attacks produce this classic pattern, though. Between 25% and 34% of heart attacks classified as “non-ST-elevation” types still involve a fully blocked artery. An EKG can also show evidence of an old heart attack you may not have known about. Dead heart tissue doesn’t conduct electricity, so it leaves permanent changes in the waveform, particularly abnormal Q waves in certain leads.
Thickened or Enlarged Heart Chambers
When the heart muscle thickens (a condition called hypertrophy, often caused by long-standing high blood pressure), the electrical signals it produces are larger than normal. An EKG picks this up as taller-than-expected waves in specific leads. Left ventricular hypertrophy, the most common form, produces unusually tall spikes in the leads that face the left side of the heart and unusually deep dips in leads facing the right. The thresholds differ slightly between men and women.
The EKG is not the most sensitive tool for detecting hypertrophy. It catches roughly half of cases, meaning it misses the other half. But when it does flag hypertrophy, it’s right more than 90% of the time. An echocardiogram (ultrasound of the heart) is far better at measuring wall thickness directly, which is why doctors often order one as a follow-up if hypertrophy is suspected.
Heart Rate Measurement
An EKG provides the most accurate measurement of heart rate available. It calculates your rate directly from the electrical signals rather than estimating it from blood flow in a fingertip or arm, the way pulse oximeters and blood pressure cuffs do. A study of over 1,000 EKG recordings in hospitalized patients found that pulse oximeters showed an average error of about 1.4 beats per minute compared to EKG, but individual readings could be off by more than 20 beats in either direction. More critically, both pulse oximeters and blood pressure cuffs failed to identify more than half the cases where heart rate was dangerously high or low.
What an EKG Can Miss
A standard resting EKG captures only about 10 seconds of your heart’s activity. If you have a rhythm problem that comes and goes, a normal EKG doesn’t rule it out. It simply means the problem wasn’t happening during those 10 seconds. For intermittent issues, doctors use longer-term monitors that record your heart for 24 hours, two weeks, or even longer.
A resting EKG also cannot reliably detect partially blocked arteries. Blockages typically need to narrow an artery by 70% or more before they cause changes visible on a stress test (an EKG done while you exercise), and even stress tests can miss significant disease. More importantly, heart attacks often result from smaller blockages that rupture suddenly and form a clot. A completely normal EKG and even a normal stress test cannot guarantee that won’t happen.
An EKG also doesn’t show the mechanical function of your heart, meaning how well it actually pumps. It tells you about the electrical side. You can have normal electrical activity but poor pumping strength, or vice versa. Imaging tests like echocardiograms fill that gap.
What to Expect During the Test
The test itself is painless and quick. You’ll lie on a table while a technician places 10 adhesive electrode patches on your chest and limbs. If you have chest hair, the technician may need to shave small patches where the electrodes go. Your skin may be lightly sanded and wiped with alcohol to remove oils and dead skin cells, which helps the sensors pick up a clean signal. Lotions or oily skin can interfere with the reading.
Once the electrodes are attached, you’ll need to lie still and breathe normally for about 10 seconds while the machine records. Moving or tensing your muscles can create electrical noise that makes the tracing harder to read. The entire process from walking in to walking out typically takes under 10 minutes. Results are often available immediately, since the tracing is interpreted in real time by the ordering clinician or by the machine’s built-in algorithm (though doctors always review the automated reading themselves).