Calibration, paper speed and calculation of heart rate
The recording of an electrocardiogram is carried out on graph paper which moves at a constant speed. The graph paper is covered by large grid-like squares measuring 5 mm x 5 mm, subdivided into smaller squares of 1 mm x 1 mm. The lines defining the large squares are thicker than those defining the small squares.
The standard calibration of the signal amplitude gain is 1 mV equals 10 mm (1 mm on the vertical axis corresponds to 0.1 mV; a 15 mm R wave corresponds to 1.5 mV) and the graph paper speed is 25 mm/s: 1 mm on the horizontal axis represents 40 ms (small square) while 5 mm represents 200 ms (large square).
Both the amplitude gain and chart speed can be changed:
- When the deflections are too large (e.g. ventricular hypertrophy) and cannot be fully registered on the graph paper at a calibration of 10 mm/mV, it is possible to program an amplification gain of 5 mm/mV; conversely, a calibration of 20 mm/mV can be proposed when the recorded deflections are of low amplitude (low voltage); the interpretation of a tracing must be performed while taking into account the calibration, especially during evaluation of deflection voltages to diagnose atrial enlargement, ventricular hypertrophy, etc.
- To allow an accurate measurement of time intervals (PR-interval, QT-interval) and duration of deflections (P-wave, QRS-complex), the chart speed can be accelerated (50 mm/s or 100 mm/s) which results in broader deflections for easierer analysis of detail and improved measurement. Conversely, a slower paper speed (12.5 mm/s) allows the recording over longer periods and increases the probability of recording a fleeting abnormality.
There are various means to measure heart rate:
- use of a graded ruler (ECG ruler);
- when the intervals between the QRS-complexes are stable (regular rhythm): measurement of the time interval between 2 QRS-complexes (RR interval); heart rate = 60/RR interval (measured in seconds) or 60 000/RR interval (measured in milliseconds); if the interval between two QRS-complexes is 1 second (1000 ms), the heart rate is equal to 60/1 or 60000/1000 = 60 beats/minute (bpm); if the interval between 2 QRS-complexes is 0.5 sec (500 ms), the heart rate is equal to 60/0.5 or 60000/500 = 120 beats/minute (bpm);
- when the intervals between the QRS-complexes are stable (regular rhythm): measurement of the number of large and small squares (more accurate) between 2 QRS-complexes; heart rate = 300/number of large squares or = 1500/number of small squares; if the rhythm is very rapid and there is a large square (200 ms, 5 small squares) between 2 QRS-complexes, the heart rate is 300/1 or 1500/5 = 300 bpm; if there are 2 large squares: 300/2 or 1500/10 = 150 bpm; if there are 3 large squares: 300/3 or 1500/15 = 100 bpm; if there are 4 large squares: 300/4 or 1500/20 = 75 bpm... if there are 10 large squares : 300/10 or 1500/50 = 30 bpm...;
- When the intervals are inconsistent (irregular rhythm), measuring the heart rate on a single RR cycle leads to wrongful estimation; it is therefore necessary to measure the number of QRS-complexes over a prolonged period; the longer the duration, the greater the accuracy; for example, one can count the number of QRS-complexes over 6 seconds (30 large squares) and multiply by 10, or over 10 seconds (50 large squares, total duration of the tracing) and multiply by 6 to obtain the heart rate.