An Electrocardiogram (EKG or ECG) is a simple, non-invasive test that measures the electrical activity of the heart. Electrodes placed on the skin detect the electrical changes that occur as the heart muscle depolarizes and repolarizes during each heartbeat. This process generates a visual tracing that provides information about the heart’s rate, rhythm, and the timing of its electrical impulses. The EKG allows healthcare professionals to screen for heart disease, diagnose irregular heartbeats, and check for signs of poor blood flow or previous heart damage.
The Basics of EKG Tracings
The EKG output is typically printed on specialized grid paper, which standardizes the measurements of the heart’s electrical events. The grid lines define the two fundamental dimensions of the electrical signal. The horizontal axis represents time, where each small square measures 0.04 seconds, and each large square (five small squares) represents 0.20 seconds.
The vertical axis measures voltage, corresponding to the strength of the electrical signal. Each small vertical square typically represents 0.1 millivolts (mV) of electrical activity, while large squares represent 0.5 mV.
The flat line observed when no electrical activity is recorded is called the isoelectric line or baseline. Any deflection, upward (positive) or downward (negative), indicates an electrical current moving through the heart muscle. These deflections reflect the direction of the electrical wave relative to the recording electrodes.
Decoding the Waves and Intervals
The EKG tracing consists of distinct waves, intervals, and segments, each representing a specific physiological event in the cardiac cycle. The first small, rounded upward deflection is the P wave, which signifies atrial depolarization, the electrical activation of the heart’s upper chambers. The P wave duration is typically less than 0.12 seconds.
Following the P wave is the QRS complex, a sharp, larger deflection representing the rapid depolarization of the ventricles. The QRS complex is usually narrow, lasting between 0.06 and 0.10 seconds, reflecting the speed of electrical conduction. The final major wave is the T wave, a broader, gentler upward curve that represents ventricular repolarization, the electrical resting phase of the ventricles.
Time measurements connecting these waves assess conduction speed. The PR interval measures the time from the start of the P wave to the start of the QRS complex, reflecting impulse travel time from the atria through the atrioventricular node. A normal PR interval ranges from 0.12 to 0.20 seconds. The ST segment is the flat line between the end of the QRS complex and the beginning of the T wave, representing the period when the ventricular muscle is fully depolarized before repolarization begins.
Determining Heart Rate and Rhythm
Calculating the heart rate from an EKG tracing relies on the standardized grid paper and the R wave, which is the tallest peak in the QRS complex and marks a ventricular contraction.
Calculating Rate for Regular Rhythms
For a regular rhythm, one common method involves counting the number of large squares between two consecutive R waves and dividing 300 by that number. For example, if there are three large squares between R waves, the estimated heart rate is 100 beats per minute (bpm). A quick visual approximation uses a sequence of numbers corresponding to the large squares between R waves: 300, 150, 100, 75, 60, and so on.
Calculating Rate for Irregular Rhythms
If the rhythm is irregular, a different approach is necessary to determine an average heart rate. This involves counting the number of R waves that appear within a six-second strip of the EKG tracing. Since a six-second strip equals 30 large squares on the paper, the number of R waves counted is then multiplied by 10 to calculate the beats per minute.
A heart rate is considered a normal sinus rhythm if it falls between 60 and 100 bpm, with every QRS complex preceded by a P wave. Rates outside this range indicate abnormalities:
- Bradycardia: A rate below 60 bpm.
- Tachycardia: A rate exceeding 100 bpm.
Recognizing Common Abnormalities
Changes in the shape or timing of the waves and segments can indicate various heart conditions.
Ischemia and Injury
The ST segment is an indicator of myocardial ischemia, or reduced blood flow to the heart muscle. Elevation or depression of the ST segment suggests injury or strain to the heart muscle. ST segment elevation often suggests an acute blockage, while horizontal or downsloping ST depression can indicate ischemia.
Rhythm Abnormalities (Arrhythmias)
Arrhythmias are identified by analyzing the regularity and relationship between the waves. Atrial fibrillation, a common sustained arrhythmia, is characterized by an irregularly irregular rhythm and the absence of distinct P waves. The tracing may instead show fine, chaotic electrical activity known as fibrillatory waves.
Structural and Conduction Issues
Abnormalities in heart chamber size, such as ventricular hypertrophy (thickening of the muscle wall), are suggested by abnormally large QRS complexes. Conduction issues, known as heart blocks, are identified by a prolonged PR interval, showing a delay in the electrical signal traveling from the atria to the ventricles. Advanced blocks can cause “dropped beats,” where a P wave is not followed by a QRS complex.