An electrocardiogram (EKG or ECG) is a non-invasive test that records the electrical activity of the heart over a period of time. This recording translates the heart’s internal electrical signals into a visual tracing, allowing healthcare providers to assess the heart’s rhythm and overall function. Understanding what a normal EKG looks like is important because it establishes a baseline against which deviations, which may indicate various heart conditions, can be identified. A normal reading demonstrates an orderly, predictable sequence of electrical events, ensuring the heart pumps blood efficiently.
Mapping the Heart’s Electrical System
The heart’s consistent rhythm is orchestrated by a built-in electrical network, which generates the signals captured by the EKG. The process begins with the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the upper right atrium. The SA node spontaneously generates an electrical impulse that spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then arrives at the atrioventricular (AV) node, situated near the center of the heart, where it is momentarily delayed. This pause is important because it allows the atria to fully empty their blood into the ventricles before the next major contraction begins.
After the delay, the impulse travels rapidly down the Bundle of His, which splits into the left and right bundle branches. The electrical wave rapidly disperses through the Purkinje fibers, a network of specialized cells lining the ventricular walls. This widespread distribution causes the large muscle mass of the ventricles to contract almost simultaneously, creating the deflections recorded on the EKG tracing.
Understanding the EKG Grid and Baseline
The heart’s electrical forces are translated onto specialized graph paper that allows for precise measurement of time and voltage. The standard EKG paper is marked with a grid of small and large squares. The horizontal axis measures time, with each small square representing 0.04 seconds, and each larger square (composed of five small squares) representing 0.20 seconds.
The vertical axis represents voltage or amplitude, measuring the strength of the electrical signal. In a standard recording, one small vertical square represents 0.1 millivolt (mV), and one large square represents 0.5 mV. These standardized measurements ensure that a reading taken anywhere can be interpreted consistently.
The flat line on the EKG tracing, called the isoelectric line or baseline, indicates the absence of significant electrical activity. This line serves as the reference point from which the height and depth of the electrical waves are measured. Segments that return to this baseline, such as the PR segment, are electrically neutral and reflect a period of pause or slow conduction.
The Essential Waveforms and Complexes
A single normal heartbeat recorded on an EKG consists of three main components: the P wave, the QRS complex, and the T wave. Each component visually represents a specific phase of the electrical cycle. The P wave is the first small, rounded, and typically upright deflection, correlating with the electrical activation, or depolarization, of the atria.
The QRS complex is the heart’s most visually prominent feature, representing the rapid depolarization of the ventricles. It is composed of three potential deflections: the Q wave (first downward deflection), the R wave (first upward deflection), and the S wave (final downward deflection). The R wave is usually the tallest component of the entire EKG tracing, reflecting the large muscle mass of the ventricles.
Following the QRS complex is the T wave, which is a broader, typically asymmetrical, rounded wave. The T wave corresponds to ventricular repolarization, the electrical recovery phase where the heart muscle cells reset themselves to prepare for the next beat. Occasionally, a small, subtle U wave may follow the T wave, which can be a normal finding.
Standard Measurements of a Normal EKG
A normal EKG is defined not only by the shape of its waves but also by specific timing and amplitude measurements. The rhythm must be “sinus rhythm,” meaning the electrical impulse originates correctly from the SA node, and the heart rate falls within the range of 60 to 100 beats per minute. The rhythm should also be regular, with consistent spacing between the R waves of successive QRS complexes.
The PR interval measures the time from the start of atrial depolarization to the start of ventricular depolarization. A normal PR interval should measure between 0.12 and 0.20 seconds, which corresponds to three to five small squares on the EKG grid. This measurement reflects the time the impulse is delayed at the AV node, ensuring proper coordination between the atria and ventricles.
The QRS duration, which measures the time for ventricular depolarization, should be narrow, typically less than 0.12 seconds, or under three small squares. A wider QRS complex can suggest a delay in the electrical signal traveling through the ventricles.
The ST segment, which is the line between the end of the S wave and the beginning of the T wave, must be flat or isoelectric, lying on the same level as the baseline. The QT interval represents the total time for ventricular depolarization and repolarization, measured from the start of the QRS complex to the end of the T wave. Because this measurement changes with heart rate, the corrected QT interval (QTc) is often calculated to determine if it is within the normal range, typically less than 0.42 seconds at a heart rate of 70 beats per minute.