An electrocardiogram (ECG or EKG) is a non-invasive test that captures the electrical activity generated by the heart. This activity is translated into a graphical tracing, displaying the heart’s rhythm and rate. The ECG records the electrical changes that occur as the heart muscle depolarizes and repolarizes with each beat. Analyzing this tracing provides insight into the function of the heart’s electrical conduction system and overall health.
The Purpose and Procedure of an Electrocardiogram
A healthcare provider may order an ECG primarily to check the heart’s rhythm and rate. The test is routinely used when patients experience symptoms such as chest pain, shortness of breath, dizziness, or palpitations. It can also help diagnose a previous heart attack or signs of reduced blood flow to the heart muscle. Additionally, an ECG allows doctors to monitor the effectiveness of heart medications or check the function of an implanted pacemaker.
The procedure is quick, simple, and painless, requiring only a few minutes. Small, sticky patches called electrodes are placed on the skin of the chest, arms, and legs. These electrodes connect by wires to the ECG machine, which records the heart’s electrical signals. No electricity is sent into the body; the machine only records the natural electrical impulses the heart produces.
Components of a Healthy Heartbeat Tracing
The wavy line produced by the ECG is a graphic representation of the heart’s electrical events, with each wave corresponding to a specific action. The cycle begins with the P wave, a small, rounded upward deflection. This wave signifies atrial depolarization, the electrical activation and subsequent contraction of the heart’s upper chambers (atria).
Following the P wave is the QRS complex, a sharp, larger set of deflections representing ventricular depolarization. This electrical signal causes the powerful contraction of the ventricles, which pump blood throughout the body. The QRS complex is typically the most prominent feature on the tracing, reflecting the large muscle mass of the ventricles.
The final component is the T wave, a broader, dome-shaped wave after the QRS complex. The T wave represents ventricular repolarization, the electrical recovery period when the ventricles relax and reset. Although atrial repolarization occurs, its small signal is usually hidden within the much larger QRS complex.
Defining Normal ECG Measurements
A normal ECG tracing adheres to precise timing and amplitude measurements reflecting a healthy electrical system. The heart rate for a resting adult should fall within the range of 60 to 100 beats per minute (bpm). The rhythm must be a normal sinus rhythm, meaning the impulse originates correctly in the sinoatrial (SA) node and follows a predictable pathway.
The PR interval measures the time for the electrical impulse to travel from the atria to the ventricles. In a healthy heart, this interval is between 120 and 200 milliseconds (ms), or 0.12 to 0.20 seconds. A shorter or longer PR interval suggests a problem with conduction speed between the chambers.
The duration of the QRS complex represents the speed of ventricular activation. A normal QRS duration is less than 100 milliseconds (0.10 seconds). If the complex is widened beyond 120 milliseconds, it suggests a delay in the electrical signal traveling through the ventricles.
The QT interval measures the total time required for the ventricles to depolarize and repolarize. Because this interval changes with heart rate, it is often expressed as a corrected value (QTc). The normal upper limit for QTc is around 450 ms for men and 460 ms for women, with deviations indicating a potential risk for serious rhythm disturbances.
Interpreting Deviations from Normal
When an ECG measurement falls outside the established normal ranges, it signals a possible underlying issue. Deviations often manifest as problems with the rate, such as tachycardia (over 100 bpm) or bradycardia (under 60 bpm). These rate abnormalities indicate that the heart’s natural pacemaker is firing incorrectly or that another part of the heart has taken over.
Other deviations involve the rhythm, where the pattern of beats becomes irregular, suggesting an arrhythmia. Timing errors, such as a prolonged PR or QRS interval, point toward issues with the heart’s electrical conduction system. For example, a widened QRS can signify a block in the pathways distributing the signal to the ventricles.
Changes in the shape of the waves or the electrical axis can indicate structural problems, like thickened heart muscle or tissue damage. Any result outside of the normal parameters serves as an important diagnostic clue, prompting further investigation.