An electrocardiogram (ECG or EKG) is essentially a snapshot of the heart’s electrical life, recorded onto specialized graph paper. This non-invasive test captures the small electrical changes that occur as the heart muscle contracts and relaxes with each beat. By placing electrodes on the body, the machine traces a series of waves and lines that represent the synchronized firing of the heart’s natural pacemaker system. Understanding the basic components of this tracing allows one to grasp how the heart is electrically performing, providing a foundational insight into its rhythm and function.
The Language of the ECG Grid
The ECG is printed on specialized graph paper that serves as a standardized ruler, translating electrical activity into measurable units of time and voltage. The horizontal axis of the grid measures time, while the vertical axis measures the amplitude, or voltage, of the electrical signal.
The machine is typically calibrated to run the paper at a standard speed of 25 millimeters per second. The smallest squares are one millimeter by one millimeter (1 mm x 1 mm), and horizontally, each small box represents a duration of 0.04 seconds.
Five small squares together form a larger, darker-lined box. Horizontally, this large box represents five times the small box duration, equaling 0.20 seconds.
The vertical axis uses these small boxes to measure the strength of the electrical signal, recorded in millivolts (mV). At standard calibration, ten small boxes, or two large boxes, equate to 1.0 mV. Therefore, each small box vertically represents 0.1 mV of electrical voltage.
Decoding the Major Waves (P, QRS, T)
The waves on the ECG tracing are labeled sequentially with the letters P, Q, R, S, and T. The first recognizable deflection is the P wave, which is typically a small, rounded bump that signals the electrical activation of the heart’s upper chambers, the atria. This event, called atrial depolarization, triggers the atria to contract.
Following the P wave is the QRS complex, a sharp, taller series of deflections representing the electrical activation of the lower, more muscular chambers, the ventricles. This ventricular depolarization is the powerful electrical signal that initiates the main pumping action of the heart. The Q wave is the first downward deflection, the R wave is the large upward spike, and the S wave is the final downward deflection of this complex.
The final major component is the T wave, a broader, rounded wave that follows the QRS complex. The T wave signifies ventricular repolarization, which is the heart muscle electrically resetting and preparing for the next beat. Atrial repolarization is also occurring, but its electrical signal is usually masked by the much larger QRS complex.
Measuring the Heart’s Timing (Intervals and Segments)
Intervals and segments are the distances and flat lines between waves, measuring the time taken for the electrical signal to travel through the heart. These measurements rely directly on the grid scale, converting the number of small boxes into seconds. The PR interval measures the time from the start of the P wave to the beginning of the QRS complex.
This interval reflects the time it takes for the electrical impulse to travel from the atria, through the connecting junction, and into the ventricles. A normal PR interval should measure between 0.12 and 0.20 seconds, or the equivalent of three to five small boxes on the grid. If this measurement falls outside the normal limits, it suggests a delay or an abnormally fast passage of the signal between the upper and lower chambers.
The ST segment is the flat line that follows the QRS complex and continues to the start of the T wave. This segment represents the period between ventricular depolarization and the start of ventricular repolarization. In a healthy heart, it should be flat, or isoelectric, meaning it aligns with the baseline. Deviations in the level of the ST segment can be significant, as they may indicate a lack of blood flow to the heart muscle.
The QT interval measures the total duration of ventricular electrical activity, from the start of the QRS complex to the end of the T wave. The normal duration of the QT interval is influenced by heart rate, but it should generally be less than half the distance between consecutive R waves.
A simple way to approximate the heart rate, particularly for a regular rhythm, involves counting the number of large boxes between two consecutive R waves. By dividing 300 by that number, one can quickly estimate the beats per minute. For instance, if two R waves are separated by four large boxes, the heart rate is approximately 75 beats per minute.
Identifying Normal Sinus Rhythm (The Baseline Check)
Normal Sinus Rhythm (NSR) is the standard baseline rhythm against which all other heart rhythms are compared. NSR indicates that the heart’s electrical impulses originate from the sinoatrial (SA) node and follow the correct conduction pathway. Identifying NSR requires checking three fundamental criteria:
- Rate: The heart rate must fall within the normal range of 60 to 100 beats per minute for an adult. Rates faster than 100 bpm are called tachycardia, and rates slower than 60 bpm are known as bradycardia.
- Rhythm: The rhythm must be regular, meaning the time interval between consecutive R waves is consistent. This consistency shows that the heart’s pacemaker is firing at a steady tempo.
- Relationship: Every QRS complex must be preceded by a P wave, and the PR interval must be within the normal time limit of 0.12 to 0.20 seconds. This 1:1 relationship confirms successful conduction from the atria to the ventricles.