An electrocardiogram (EKG or ECG) is a simple, non-invasive test that measures the electrical activity of the heart. Every heartbeat is triggered by an electrical signal traveling through the heart muscle. The EKG machine records these tiny voltage changes on the skin’s surface, providing a graphic representation of the heart’s electrical rhythm and function. Healthcare providers analyze the resulting wave patterns to screen for conditions affecting heart health and electrical stability.
The Purpose and Basic Process of an EKG
A healthcare provider may order an EKG to investigate symptoms like chest pain, dizziness, or an irregular heartbeat. The test screens for electrical abnormalities, assesses heart rate, and checks for signs of previous heart damage or reduced blood flow. It is often the first diagnostic step taken to evaluate the heart’s electrical system.
The process of capturing this electrical activity is straightforward and takes only a few minutes. Small, sticky patches called electrodes are placed on specific locations across the chest, arms, and legs. These electrodes connect by wires to the EKG machine, which records the signals onto paper or a screen. The machine listens only to the heart’s natural electricity; it does not send any electricity into the body.
Decoding the Standard EKG Waveform
The wavy line produced by the EKG is a precise map of the heart’s electrical sequence, where each deflection corresponds to a specific event. The cycle begins with the P wave, a small upward deflection representing the electrical activation (depolarization) of the heart’s upper chambers, the atria. Depolarization is the signal necessary for muscle cells to contract and push blood into the lower chambers.
Following the P wave is the larger and sharper QRS complex, which displays the electrical activation of the main pumping chambers, the ventricles. Since the ventricles have far more muscle mass than the atria, their depolarization signal is stronger, resulting in the tall, distinct spike of the QRS complex. This complex overshadows the electrical recovery of the atria, meaning that event is not visible on the tracing.
The final component is the T wave, a rounded upward curve reflecting ventricular repolarization. Repolarization is the electrical recovery phase that allows the ventricular muscle cells to relax and prepare for the next beat. A normal EKG confirms a healthy electrical pathway based on the correct timing, shape, and sequence of these three main waves: P, QRS, and T.
Defining Normal Sinus Rhythm and Rate
The definition of a normal EKG result begins with “Normal Sinus Rhythm” (NSR), the benchmark for healthy heart function. The term “sinus” specifies that the electrical impulse originates correctly in the Sinoatrial (SA) node, the heart’s natural pacemaker located in the upper right chamber. In NSR, the electrical signal travels along the expected pathway, resulting in a consistent and regular beat-to-beat pattern.
The rate component refers to the frequency of these beats, which for a resting adult falls between 60 and 100 beats per minute (bpm). A rate below 60 bpm is bradycardia, while a rate above 100 bpm is tachycardia. The EKG measures this rate by analyzing the space between successive QRS complexes, which must be equal to confirm a regular rhythm.
To qualify as NSR, every QRS complex must be preceded by a P wave, and the P waves must appear uniform in shape. This confirms that the atria activated before the ventricles, ensuring the chambers contract in the proper sequence to effectively pump blood. A regular rhythm originating from the SA node, paired with a rate in the normal range, establishes the normalcy of the heart’s electrical function.
Essential Timing and Interval Measurements
EKG analysis involves measuring specific time intervals to ensure electrical signals travel at the correct speed. These measurements are expressed in seconds or milliseconds and must fall within established normal ranges. The PR interval is the first timing measurement, starting at the beginning of the P wave and ending at the start of the QRS complex.
The PR interval represents the time for the electrical impulse to travel from the atria, through connecting tissues, and into the ventricles. The normal range is between 0.12 and 0.20 seconds. An interval that is too long suggests a delay in conduction, while one that is too short may indicate an abnormal bypass pathway.
The QRS duration measures the time it takes for the electrical signal to spread across the ventricles, which should be brief, between 0.06 and 0.10 seconds. A short QRS duration confirms the impulse spread rapidly through the specialized conduction fibers. Finally, the QT interval measures the total time required for the ventricles to complete both depolarization (contraction) and repolarization (relaxation). A corrected QT interval is normal if it is 0.45 seconds or less for men and 0.46 seconds or less for women.