An electrocardiogram, commonly known as an ECG or EKG, is a medical test that records the electrical activity of the heart. This non-invasive procedure provides a visual representation of the heart’s electrical signals, including its rate and rhythm. The primary purpose of an ECG is to evaluate heart function and detect various cardiac conditions.
The Heart’s Electrical Rhythms
The heart’s rhythmic beating is orchestrated by a specialized electrical system. Within the heart, pacemaker cells generate electrical impulses that spread through the heart muscle, causing contraction. This electrical journey typically begins in the sinoatrial (SA) node, often referred to as the heart’s natural pacemaker.
These impulses then travel through the atria before reaching the atrioventricular (AV) node. The AV node briefly delays the signal. Following this pause, the impulses continue down through the bundle of His and Purkinje fibers, leading to the contraction of the ventricles.
From Electrical Signal to ECG Tracing
Recording the heart’s electrical activity involves placing electrodes on the skin. These electrodes detect electrical changes as impulses move through the heart. The electrical signals generated by the heart are quite small.
Once detected, these signals are sent to the ECG machine, which amplifies them. The machine then converts these signals into a visual tracing. This tracing can be displayed on a digital screen or printed onto specialized graph paper, creating a record of the heart’s electrical events over time.
The ECG Grid: Visualizing Time
An ECG is typically printed on graph paper with a grid of small and large squares. This grid provides a standardized measurement of both time and voltage. The horizontal axis of the ECG paper measures time, while the vertical axis measures the amplitude of the electrical signals.
A standard ECG machine operates at a paper speed of 25 millimeters per second (mm/s). This consistent speed allows for precise time measurements. Each small square on the grid measures 1 millimeter, representing 0.04 seconds. Five small squares make up one larger square, representing 0.20 seconds horizontally. This standardized grid and paper speed ensure that medical professionals can accurately measure the duration of various electrical events within the heart.
Interpreting Time Intervals for Heart Health
The accurate recording of time on an ECG tracing is fundamental for assessing heart health. Medical professionals measure specific time intervals between different waves and segments on the ECG to gain insights into the heart’s electrical conduction system. For example, the P-R interval reflects the time it takes for an impulse to travel from the atria to the ventricles, with a normal range between 0.12 and 0.20 seconds.
The QRS duration measures the time for ventricular depolarization, less than 0.10 seconds. The Q-T interval indicates the total time for ventricular depolarization and repolarization. Analyzing the duration of these intervals helps identify potential abnormalities in the heart’s electrical function.
The precise timing captured on the ECG also allows for the calculation of heart rate. By counting the number of QRS complexes within a specific time frame, such as a six-second strip, the heart rate in beats per minute can be determined. This ability to accurately measure time is central to the diagnostic utility of the ECG.