(1) P wave on the electrocardiogram:  it is blunt and round, and may have slight notches. The P wave width does not exceed 0.11 seconds, and the amplitude does not exceed 0.25 millivolts. The direction of P wave is upright in leads Ⅰ, Ⅱ, aVF, V4-6, and inverted in lead aVR. In Ⅲ, aVL, V1-3 leads can be upright, inverted or bidirectional. P wave amplitude and width beyond the above range is abnormal, often indicating atrial hypertrophy. The P wave is upright in lead aVR, and the inverted P wave in leads II, III, and aVF is called retrograde P wave, which means that the excitation is conducted retrogradely from the atrioventricular junction to the atrium, which is common in atrioventricular junction rhythm, which is a kind of ectopic heart rate.

(2) PR interval: that is, the time between the starting point of the P wave and the starting point of the QRS complex. The general adult P-R interval is 0.12 to 0.20 seconds. The PR interval varies with heart rate and age. The older the person or the slower the heart rate, the longer the PR interval. Prolonged P-R interval often means that the time for excitation to pass through the atrioventricular junction is prolonged, indicating that there is an atrioventricular conduction disorder, which is common in atrioventricular block.

(3) QRS wave complex on the electrocardiogram:  representing the potential and time changes of the depolarization and the earliest repolarization process of the two ventricles.

①QRS wave group time: normal adult is 0.06-0.10 seconds, children is 0.04-0.08 seconds.

The wall activation time of leads V1 and V2 is less than 0.03 seconds, and the wall activation time of V5 and V6 is less than 0.05 seconds. Prolonged QRS complex time or wall activation time is common in ventricular hypertrophy or intraventricular conduction block.

② QRS wave group amplitude on electrocardiogram: pressurized unipolar limb lead R wave does not exceed 1.2 millivolts in lead aVL, and R wave in lead aVF does not exceed 2.0 millivolts. If this value is exceeded, it may be left ventricular hypertrophy. The R wave in lead aVR should not exceed 0.5 millivolts. If it exceeds this value, it may be right ventricular hypertrophy. If each QRS complex voltage (arithmetic sum of R+S or Q+R) of the six limb leads is less than 0.5 millivolts or the arithmetic sum of QRS voltages of each precardiac lead does not exceed 0.8 millivolts, it is called low voltage , found in emphysema, pericardial effusion, generalized edema, myxedema, myocardial damage, but also in a very small number of normal people. Individual lead QRS complex amplitude is very small, meaningless.
Precordial leads: V1 and V2 leads are rS type, R/S1. In lead V3, the R wave is approximately equal in amplitude to the S wave. In normal people, from V1 to V5, the R wave gradually increases, and the S wave gradually decreases.

(4) Q wave on the electrocardiogram: Except for the aVR lead which can be QS or Qr, the amplitude of the Q wave in other leads should not exceed 1/4 of the R wave in the same lead, the duration should not exceed 0.04 seconds, and there should be no notch. Normal V1, V2 lead should not have Q wave, but can show QS wave pattern. Q waves that exceed the normal range are called abnormal Q waves, which are common in myocardial infarction.

(5) S-T segment: A horizontal line from the end point (J point) of the QRS complex to the starting point of the T wave is called the S-T segment. Normally, any lead S-T downward shift should not exceed 0.05 millivolts. S-T segment depression beyond the normal range is common in myocardial ischemia or strain. The normal upward shift of ST segment should not exceed 0.1 millivolts in limb leads and precardiac leads V4-6, and no more than 0.3 millivolts in precardiac leads V1-3. S-T upward shifts exceeding the normal range are more common in acute myocardial infarction, Acute pericarditis, etc.

(6) T wave: T wave is blunt and round, takes a long time, rises slowly from the baseline, and then falls rapidly, forming a waveform with longer forelimbs and shorter hindlimbs. The direction of the T wave is often consistent with the direction of the main wave of the QRS complex. Upright in leads Ⅰ, Ⅱ, V4-6, and inverted in lead aVR. Other leads can be upright, bidirectional or inverted. If V1 is upright, V3 cannot be inverted. In the leading lead with R wave, the amplitude of T wave should not be lower than 1/10 of the R wave in the same lead, and the T wave in the precordial lead can be as high as 1.2-1.5 millivolts. In the leads where the main wave of the QRS complex is upward, the T wave is flat or inverted, which is common in myocardial ischemia and hypokalemia.

(7) Q-T interval: Q-T interval is closely related to heart rate. The faster the heart rate, the shorter the Q-T interval; otherwise, the longer it is. Generally, when the heart rate is about 70 beats/min, the Q-T interval is about 0.40 seconds. Generally check the table. Where the Q-T interval exceeds the normal maximum value by more than 0.03 seconds, it is said to be significantly prolonged, and if it is less than 0.03 seconds, it is said to be slightly prolonged.
Prolonged Q-T interval is seen in bradycardia, myocardial damage, cardiac hypertrophy, heart failure, hypocalcemia, hypokalemia, coronary heart disease, prolonged Q-T interval syndrome, drug effects, etc. Shortening of the Q-T interval is seen in hypercalcemia, the effect of digitalis, and the application of epinephrine.

(8)U wave: the amplitude is very small, and it is clearer in the precardiac lead, especially V3, which can be as high as 0.2-0.3 millivolts. Significantly increased U wave is common in hypokalemia, taking quinidine and so on. U wave inversion is seen in coronary heart disease or exercise tests; U wave enlargement is often accompanied by increased ventricular muscle irritability, which can easily induce ventricular arrhythmia.