An electrocardiogram, commonly known as an EKG or ECG, is a quick, non-invasive diagnostic procedure used to assess the health of the heart. The test involves placing electrodes on the skin to record the tiny electrical signals generated by the heart muscle as it beats. These electrical impulses are translated into a wavy line tracing that doctors can interpret to reveal information about the heart’s function and structure. While the EKG is an extremely valuable, foundational tool in cardiac assessment, it serves as one piece of information in the larger process of diagnosing heart disease.
Understanding the EKG: What It Measures
The heart functions based on a precise electrical cycle that controls its mechanical contractions, and the EKG captures this activity. Electrodes placed across the chest and limbs detect the movement of electrical current as it spreads through the heart muscle. This recording provides a visual representation of the heart’s rate and rhythm.
A normal EKG tracing consists of three main deflections, or waves, which correspond to specific events in the cardiac cycle. The initial, small upward deflection is the P wave, which represents the electrical activation (depolarization) of the upper chambers of the heart, the atria. This electrical signal triggers the atria to contract and pump blood into the ventricles.
The largest part of the tracing is the QRS complex, signifying the rapid electrical activation of the lower pumping chambers, the ventricles. Ventricular depolarization directly precedes the powerful contraction that pushes blood out to the rest of the body. Following this action, the T wave represents the electrical recovery (repolarization) of the ventricles, preparing them for the next heartbeat.
Diagnosing Disease: What an EKG Can Identify
The primary strength of an EKG is its ability to diagnose irregularities in the heart’s electrical system, known as arrhythmias. Abnormalities in the timing or pattern of the P wave and QRS complex can signal issues like atrial fibrillation (where the upper chambers quiver) or ventricular tachycardia (a fast, abnormal rhythm originating in the lower chambers). The test can also detect heart blockages in the electrical pathway, which manifest as delays between the P wave and the QRS complex.
The EKG is also effective at identifying damage to the heart muscle caused by inadequate blood flow. When blood supply is reduced (ischemia), the EKG may show specific changes in the ST segment (the line between the QRS complex and the T wave) or inversion of the T wave. An acute myocardial infarction, or heart attack, is often marked by a significant elevation of the ST segment, indicating immediate, severe muscle injury.
For a previous heart attack that has already caused permanent damage (infarction), the EKG may show pathological Q waves, which are deeper and wider than normal. Beyond electrical and blood flow issues, the EKG can offer clues about structural changes in the heart. Prolonged conditions like high blood pressure can cause the heart chambers to enlarge (hypertrophy), which changes the overall electrical axis of the heart and increases the voltage of the QRS complex.
The EKG’s Limitations and Next Steps
Despite its broad utility, a standard resting EKG is merely a brief snapshot of the heart’s electrical activity, typically recording only ten seconds of data. A completely normal EKG tracing does not guarantee the absence of all heart disease. Conditions like partial coronary artery blockages may not cause EKG changes until the heart is placed under stress.
An EKG can easily miss intermittent problems that come and go, such as paroxysmal atrial fibrillation or sudden palpitations. If an irregular rhythm does not occur during the short recording period, the EKG will appear normal, leading to a false-negative result. Conversely, the EKG can sometimes show non-specific changes that suggest a problem where none exists, resulting in a false-positive finding.
When an EKG result is abnormal or when a patient’s symptoms suggest an issue the resting EKG did not capture, further testing is necessary. To catch intermittent rhythms, a physician might order a Holter monitor, a portable device the patient wears for 24 to 48 hours to record continuous electrical activity.
If blockages are suspected, a stress test may be performed, which records the EKG while the patient exercises on a treadmill, observing the heart’s response to increased demand. To assess structural issues suggested by the EKG, an echocardiogram uses sound waves to create moving images of the heart’s muscle and valves, providing a definitive look at the heart’s anatomy.