An electrocardiogram, commonly known as an EKG or ECG, is a non-invasive test that records the electrical activity of the heart. This electrical activity drives the heart’s contractions, pumping blood throughout the body. The EKG provides a visual snapshot of these electrical impulses, allowing healthcare professionals to assess heart function. Small electrode patches placed on the chest, arms, and legs detect these signals.
The Basic Components of a Normal EKG Waveform
A normal EKG tracing displays a repeating pattern of distinct waves, each representing specific electrical events within the heart. The P wave, QRS complex, and T wave are the primary components observed. These waves reflect the coordinated electrical activation (depolarization) and recovery (repolarization) of the heart’s chambers.
The P wave is typically the first small, rounded, and upright deflection on the EKG. It signifies the electrical activation, or depolarization, of the atria, the heart’s upper chambers. The P wave is usually positive in lead II and often biphasic in lead V1. Its duration should be less than 0.12 seconds, and its amplitude typically less than 2.5 millimeters in the limb leads.
Following the P wave is the QRS complex, which is a larger and sharper deflection. This complex represents the rapid electrical activation, or depolarization, of the ventricles, the heart’s lower pumping chambers. The QRS complex consists of a downward Q wave, an upward R wave, and another downward S wave, though not all three waves may be visible in every lead.
The T wave is a rounded, usually upright deflection that follows the QRS complex. It indicates the electrical recovery, or repolarization, of the ventricles. A normal T wave is generally asymmetrical, with a slower upslope and a steeper downslope. It is typically upright in most leads, except for lead aVR and sometimes V1, where it can be inverted.
Occasionally, a small, upright U wave may be seen after the T wave. Its exact origin is debated, but it is thought to represent the repolarization of specific ventricular fibers. A normal U wave has the same direction as the T wave and is usually less than one-third the amplitude of the T wave. U waves are more commonly observed at slower heart rates.
Key Measurements on a Normal EKG
Beyond individual waves, specific time intervals and segments on an EKG provide important information about the heart’s electrical conduction. The EKG paper is standardized so that each small square represents 0.04 seconds and each large square (five small squares) represents 0.20 seconds.
The PR interval measures the time from the beginning of the P wave to the start of the QRS complex. This interval reflects the time for the electrical impulse to travel from the atria through the atrioventricular (AV) node to the ventricles. A normal PR interval in adults ranges from 0.12 to 0.20 seconds.
The QRS duration measures the time it takes for the ventricles to depolarize. A normal QRS duration is typically between 0.08 and 0.10 seconds, and generally less than 0.12 seconds.
The ST segment is the flat line that follows the QRS complex and precedes the T wave. This segment represents the period when the ventricles are fully depolarized before they begin to repolarize. In a normal EKG, the ST segment should be isoelectric, meaning it lies on the baseline.
The QT interval measures the total time for ventricular depolarization and repolarization, extending from the beginning of the QRS complex to the end of the T wave. This interval varies with heart rate, so a corrected QT (QTc) interval is often calculated to standardize the measurement. A normal QTc interval is generally less than 450 milliseconds (0.45 seconds) for men and less than 460 milliseconds (0.46 seconds) for women. The normal QT interval should be less than half the preceding R-R interval.
Heart rate can be calculated from an EKG tracing by counting the number of R waves within a specific time frame. For a regular rhythm, one method involves counting the number of large squares between two consecutive R waves and dividing 300 by that number. For irregular rhythms, counting the R waves in a 6-second strip (30 large squares) and multiplying by 10 provides an estimated heart rate. A normal resting heart rate for adults typically ranges from 60 to 100 beats per minute.
Putting It All Together: Recognizing a Normal EKG
Recognizing a normal EKG involves synthesizing the appearance of individual waves and the measurements of intervals and segments. A normal EKG tracing demonstrates a consistent and orderly pattern, known as normal sinus rhythm.
In normal sinus rhythm, the heart rate is regular. Each P wave is consistently followed by a QRS complex and then a T wave, indicating the electrical impulse originates in the heart’s natural pacemaker and conducts predictably. The P wave morphology remains consistent, and it is typically upright in leads I and II.
The PR interval, QRS duration, and QT interval all fall within their normal ranges, indicating proper timing of electrical conduction. The ST segment remains at the baseline, reflecting appropriate ventricular repolarization. Variations from these established norms can suggest underlying heart conditions, prompting further evaluation by a healthcare professional.