All recordings of ECGs are made with appropriate calibration lines on the recording paper. Either these calibration lines are already ruled on the paper, as is the case when a pen recorder is used, or they are recorded on the paper at the same time that the ECG is recorded, which is the case with the photographic types of electrocardiographs.
As shown in Figure 1, the horizontal calibration lines are arranged so that 10 of the small line divisions upward or downward in the standard ECG represent 1 millivolt, with positivity in the upward direction and negativity in the downward direction.
Fig1. Normal electrocardiogram.
The vertical lines on the ECG are time calibration lines. A typical ECG is run at a paper speed of 25 millimeters per second, although faster speeds are sometimes used. Therefore, each 25 millimeters in the horizontal direction is 1 second, and each 5-millimeter segment, indicated by the dark vertical lines, represents 0.20 second. The 0.20-second intervals are then broken into five smaller intervals by thin lines, each of which represents 0.04 second.
Normal Voltages in the Electrocardiogram. The recorded voltages of the waves in the normal ECG depend on the manner in which the electrodes are applied to the surface of the body and how close the electrodes are to the heart. When one electrode is placed directly over the ventricles and a second electrode is placed elsewhere on the body remote from the heart, the voltage of the QRS complex may be as great as 3 to 4 millivolts. Even this voltage is small in comparison with the monophasic action potential of 110 millivolts recorded directly at the heart muscle membrane. When ECGs are recorded from electrodes on the two arms or on one arm and one leg, the voltage of the QRS complex usually is 1.0 to 1.5 millivolts from the top of the R wave to the bottom of the S wave, the voltage of the P wave is between 0.1 and 0.3 millivolts, and the voltage of the T wave is between 0.2 and 0.3 millivolts.
P-Q or P-R Interval. The time between the beginning of the P wave and the beginning of the QRS complex is the interval between the beginning of electrical excitation of the atria and the beginning of excitation of the ventricles. This period is called the P-Q interval. The normal P-Q interval is about 0.16 second. (Often this interval is called the P-R interval because the Q wave is likely to be absent.)
Q-T Interval. Contraction of the ventricle lasts almost from the beginning of the Q wave (or R wave, if the Q wave is absent) to the end of the T wave. This interval is called the Q-T interval and ordinarily is about 0.35 second.
Rate of Heartbeat as Determined from the Electrocardiogram. The rate of the heartbeat can be determined easily from an ECG because the heart rate is the reciprocal of the time interval between two successive heartbeats. If the interval between two beats as determined from the time calibration lines is 1 second, the heart rate is 60 beats per minute. The normal interval between two successive QRS complexes in an adult is about 0.83 second, which is a heart rate of 60/0.83 times per minute, or 72 beats/min.
