An electrocardiogram (EKG or ECG) is a non-invasive test that provides a detailed graph of the heart’s electrical activity. This recording captures the small electrical changes accompanying the heart’s cycle of depolarization (contraction) and repolarization (relaxation). Calculating specific measurements from the tracing, including heart rate and the duration of electrical events, allows a practitioner to evaluate the heart’s conduction system. These numerical values offer insights into the heart’s speed and the pace of electrical signal travel.
The Foundation: Interpreting the ECG Grid
The EKG tracing is recorded onto specialized paper featuring a structured grid. This grid is standardized so that the horizontal axis measures time, while the vertical axis measures voltage or amplitude. The paper typically moves at a speed of 25 millimeters per second, establishing the time scale.
The smallest unit is a single small box, measuring one millimeter on each side. Horizontally, this small box represents 0.04 seconds of time, and vertically, it represents 0.1 millivolt (mV) of electrical potential. Five small boxes form a larger box bounded by darker lines. Each large box corresponds to 0.20 seconds of time and 0.5 mV of voltage.
Step-by-Step Methods for Calculating Heart Rate
Heart rate, measured in beats per minute (BPM), depends on whether the rhythm is regular or irregular. A regular rhythm is defined by a constant distance between consecutive QRS complexes, known as the R-R interval.
Sequence Method (300 Method)
For a quick estimation of a regular heart rate, the Sequence Method is employed. Locate one R wave on a thick vertical line. The thick lines following it are assigned the values 300, 150, 100, 75, 60, and 50. The heart rate is approximated by the number corresponding to the thick line on which the next R wave falls. For instance, if the second R wave lands on the third thick line, the rate is 100 BPM.
1500 Method
A more precise calculation for a regular rhythm uses the 1500 Method. This technique relies on the fact that 1500 small boxes pass in a single minute. To use this method, count the number of small boxes between two consecutive R waves, and divide 1500 by this count. If, for example, 20 small boxes separate the R waves, the heart rate is exactly 75 BPM (1500/20).
6-Second Strip Method
When the R-R intervals are inconsistent, indicating an irregular rhythm, the 6-Second Strip Method provides the average rate. A 6-second strip of the EKG tracing corresponds to 30 large boxes, since each large box is 0.2 seconds long. To calculate the rate, count the total number of R waves visible within this 30-large-box segment. That count is then multiplied by 10 to extrapolate the number of beats that would occur over a full 60 seconds. This method provides an average rate for erratic rhythms, with a normal heart rate falling between 60 and 99 beats per minute.
Measuring Key Time Intervals and Wave Amplitudes
Beyond the rate, the duration and height of the EKG waves and segments offer information about the heart’s conduction health and muscle characteristics.
PR Interval
The PR interval measures the time required for the electrical impulse to travel from the atria to the ventricles. This interval is measured from the beginning of the P wave to the start of the QRS complex. A PR interval duration is between 0.12 and 0.20 seconds, which translates to three to five small boxes on the horizontal axis.
QRS Complex
The QRS complex represents the rapid electrical activation, or depolarization, of the ventricles. Its duration is measured from the first deflection to the point where the final deflection returns to the baseline. A normal QRS complex lasts between 0.06 and 0.10 seconds. A duration exceeding this range indicates that the electrical impulse is traveling slowly through the ventricular tissue.
QT Interval
The QT interval measures the total time required for the ventricles to depolarize and then repolarize, covering the entire electrical event of a single heartbeat. It is measured from the start of the QRS complex to the end of the T wave. Since the QT interval is influenced by the heart rate, a corrected value (QTc) is often calculated. The QTc is considered normal if it is less than 420 milliseconds.
Wave Amplitude
Wave amplitude reflects the strength of the electrical signal and is measured vertically using the grid’s voltage scale. The height of a wave is measured from the isoelectric baseline, often the PR segment, to the peak of the deflection. Since each small box vertically represents 0.1 mV, the height of the R wave or the depth of the Q wave is measured in millimeters and then converted to millivolts. Abnormal wave amplitudes, such as an excessively tall R wave, can suggest conditions like an enlarged ventricular muscle mass.