Calibration, paper speed and calculation of heart rate

Tracing
N° 1
Pathology
Patient
Young man 22 years of age, asymptomatic, with no prior history and a normal cardiac ultrasound;
Calibration, paper speed and calculation of heart rate
Calibration, paper speed and calculation of heart rate
Calibration, paper speed and calculation of heart rate
Calibration, paper speed and calculation of heart rate
Calibration, paper speed and calculation of heart rate
Comments

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.
Epigraph
Standard calibration of the ECG is signal gain of 10 mm/mV and graph paper speed of 25 mm/s.
Calibration, paper speed and calculation of heart rate