# Time limit and measurement of each wave of electrocardiogram

1. The time limit and interval of each wave of the ECG In addition to the P, QRS, T wave and ST segment, the ECG also has the interval of the time required for the conduction and spread of electrical activity through different parts of the heart. For example: PR interval, PJ interval and QT interval, etc., measuring the duration and interval of each wave of ECG is the most direct method to understand the electrical activity of the heart.

1.1 P wave duration Atrial myocardium depolarization time, the normal value is 0.11s.

1.2 QRS complex duration The time for the conduction and spread of electrical activity through the ventricular myocardium. The normal value is 0.06~0.10s (ie 2~3 small squares). The duration of the QRS complex is widened in the presence of conduction abnormalities in the ventricle.

1.3 PR interval from the beginning of the P wave to the beginning of the QRS complex. The normal PR interval value is 0.12~0.20s, which is equivalent to 3~5 cells. This interval is the time when the atrium begins to depolarize and the activation delays conduction in the atrioventricular node.

1.4 PJ interval from the beginning of the P wave to the end of the QRS wave (J point), is the time of atrial depolarization, atrioventricular nodal conduction and ventricular depolarization, the normal value is <0.26s.

1.5 The QT interval from the start of the QRS complex to the end of the T wave represents the total time for ventricular depolarization and repolarization. Since the length of the QT interval varies with heart rate, the clinical application of the corrected QT interval (QTc) to eliminate the effect of heart rate, the normal value is < 0.44s.

1. 6 PP interval from the beginning of the first P wave to the beginning of the second P wave, this interval represents the interval between 2 atrial depolarizations, and the atrial frequency can be calculated through this interval.

1.7 RR interval is the length from the beginning of the first QRS complex to the beginning of the second QRS complex. This interval represents the interval between 2 ventricular depolarizations, and the ventricular rate can be obtained by calculating this interval.

2. ECG recording and measurement In order to facilitate the understanding of cardiac electrical activity, the measurement of each wave and interval of the ECG becomes the key to the ECG. For this reason, the ECG recording uses a unified standard ECG chart, and its horizontal (length) represents time, in seconds ( s) indicates that the longitudinal (width) represents the amplitude height, expressed in millivolts (mV). The ECG chart is printed with two squares, each large square has 5 small squares, each small square has a side length of 1 mm, the time represents 0.04s, and the amplitude represents 0.1mV. And so on, the large square is 5 mm, representing a time of 0.2s and an amplitude of 0.5mV. The 5 horizontal squares represent 1s . When measuring the electrocardiogram, the time limit and amplitude are calculated according to the number of small cells occupied by each wave. The PR interval length occupies 4 small cells (0.04s ⅹ4 = 0.16s), then the PR interval of the graph is 0.16s . The heart rate can also be calculated from the ratio of the traced P wave or QRS wave to the large square. For example: when calculating the ventricular rate, a QRS wave appears every 5 large squares, that is, the RR interval is 1.0s (1000ms), and the ventricular rate is 60bpm (times/minute). A QRS complex appears every 4 major divisions, that is, the RR interval is 0.8s (800ms), the ventricular rate is 75 bpm, and so on. The formula can also be applied: Heart Rate = 60000(ms) / RR Interval (ms). Example: 60000/800 = 75bpm.