An electrocardiogram (EKG or ECG) is a simple, non-invasive diagnostic tool used to measure and record the electrical activity of the heart. Electrodes placed on the skin detect the tiny electrical changes resulting from cardiac muscle cells depolarizing and repolarizing during each cycle. This test provides a graphic representation of the electrical impulses that trigger the heartbeat, allowing assessment of the heart’s rhythm and function. Understanding the basic wave patterns offers foundational knowledge, but it is not a substitute for professional medical interpretation.
Understanding the EKG Graph Paper
The EKG tracing is printed on specialized graph paper marked for precise time and voltage measurements. The grid consists of small squares (1mm x 1mm) and larger squares (5mm x 5mm), defined by heavier lines.
The horizontal axis measures time, with the standard paper speed set to 25 millimeters per second. At this speed, one small square represents \(0.04\) seconds, and one large square represents \(0.20\) seconds.
The vertical axis measures amplitude, or voltage, typically in millivolts (mV). Standard calibration ensures that 10 millimeters in height equals 1 millivolt. Therefore, one small square represents \(0.1\) mV, and one large square represents \(0.5\) mV.
Interpreting the Basic Waveform Components
The electrical activity of a single heartbeat is represented by a sequence of characteristic deflections known as waves and complexes. The process begins with the P wave, the first small, positive deflection, which represents atrial depolarization. This electrical activation causes the atria, the heart’s upper chambers, to contract.
Following the P wave is the QRS complex, a rapid, larger set of deflections reflecting the depolarization of the ventricles. The Q wave is the first negative deflection, the R wave is the first positive deflection, and the S wave is the second negative deflection; however, not all three waves are always present. This complex is larger than the P wave because the ventricular muscle mass is significantly greater, leading to a higher voltage signal.
The final component is the T wave, which follows the QRS complex and represents ventricular repolarization, the electrical recovery phase. The T wave is typically slightly asymmetric and normally follows the same direction as the preceding QRS complex. Atrial repolarization occurs but is obscured on the tracing by the larger QRS complex.
The timing between these components, known as intervals, provides insight into the speed of electrical conduction. The PR interval measures the time from the start of the P wave to the start of the QRS complex. This indicates the duration of impulse travel from the atria through the atrioventricular (AV) node to the ventricles. A normal PR interval ranges between \(0.12\) and \(0.20\) seconds (three to five small squares). The QT interval measures the total time required for the ventricles to depolarize and fully repolarize, from the beginning of the QRS complex to the end of the T wave.
Calculating Heart Rate and Basic Rhythm
Calculating the heart rate and assessing rhythm regularity involves measuring the distance between consecutive R waves, known as the R-R interval. The heart rate, measured in beats per minute (bpm), is calculated using different methods depending on whether the rhythm is regular or irregular.
For regular rhythms, the “300-method” offers a quick approximation. Locate an R wave on a heavy line and count the number of large squares until the next R wave. Divide 300 by this count to estimate the rate. For example, if there are four large squares between two R waves, the heart rate is approximately 75 bpm (300/4).
For irregular rhythms, the “6-second strip method” is more reliable. Since 30 large squares equal a 6-second duration, count the number of R waves within 30 large squares. Multiply this number by 10 to yield the average heart rate in beats per minute.
Normal Sinus Rhythm (NSR) is the baseline against which other tracings are measured. NSR is characterized by a regular rhythm and a heart rate between 60 and 100 bpm. For a rhythm to be classified as NSR, the following criteria must be met:
- Every QRS complex must be preceded by a P wave.
- P waves must have a consistent shape, confirming the impulse originates correctly from the sinoatrial node.
- The PR interval and QRS complex duration must fall within their normal time limits.
What Common EKG Results Mean
Deviations from Normal Sinus Rhythm are often categorized by changes in heart rate, providing the simplest initial interpretations. A heart rate consistently below 60 beats per minute in a resting adult is classified as bradycardia, or a slow heart rate. While this can be normal for athletes or during sleep, it may also indicate issues with the heart’s pacemaker or conduction system.
Conversely, a resting heart rate consistently faster than 100 beats per minute is classified as tachycardia, or a fast heart rate. Tachycardia can occur in response to exercise or stress, but sustained rates may signal a problem with the heart’s electrical signaling. Understanding these rate deviations provides a basic framework for interpreting EKG results.