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3.6 Normal ECG
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3.6 Normal ECG

There are the five waves distinguished in the record in norm – «P», «Q», «R», «S», «T» and three relatively isoelectric segments and intervals: «PR», «ST» and «QT».

The P wave is the first component of the normal ECG. The impulse that makes the P wave originates in sinoatrial node and in the atriums. It is usually upright in most leads but can be variable in leads 3 and V1, negative in aVR. It represents the depolarization of the right and left atria. Persons with the P wave more often have a normal sinus rhythm. In junctional and atrial rhythm in ECG presents pathological or abnormal P wave.

Normally, it has an amplitude not more than 2.5 small divisions. Its duration is within 0.1 seconds (2.5 small divisions).

PR interval is the distance between the beginnings of the P wave to the beginning of Q wave. It indicates the time taken for extend of the excitation wave from the sinoatrial node to the ventricles conductivity system. Some deviations in the PR interval beyond what is normal suggest a conduction delay abnormalities called AV blockage. In norm duration consists between 0.12 and 0.20 seconds.

 

Figure 14 – ECG structure

The QRS complex is a graphic representation of ventricular depolarization. Though atrial repolarization develops at this time, it is impossible to differentiate it on the ECG. If you analyze the QRS complex pay attention on its two most important characteristics: duration and configuration.

 Three distinct waves are present in normal QRS complex. These waveforms follow the directions of depolarization in ventricles. The Q wave is the first inverted deflection of the QRS complex after the P wave. Normally Q wave is slight and small in amplitude, in leads II, V1, V2 and V3 is absent in norm. In healthy persons duration of Q < 0.03 s., amplitude < 25% the amplitude of the R. Wide and deep Q may indicate a pronounced dystrophy of myocardium.

The R wave is the first upward deflection of the QRS complex and represents depolarization from the endocardium through to the epicardium across both ventricles. In normal persons in the bipolar leads the R is tallest in lead II. In the chest leads, V1 has as a rule a least amplitude R wave. The height of the R in the chest leads enlarges gradually till it becomes maximal in V4 and than progressively decreases.

The S is the next wave which goes after R wave and dips below the baseline. S is deepest in V1 lead and the depth gradually reduces till it is least in V5. The amplitude of S in V1 and in addition amplitude of R in V5 should not normally exceed 30 mm.

 The normal duration of QRS complex should be between 0.06 and 0.11 second. The usual transitional zone from S>R in the right chest leads to R>S in the left chest unipolar leads places near V3. The point that letters the finish of the QRS complex and the beginning of the ST segment is called the j-point.

The ST segment represents the beginning of ventricular repolarization. Because the cell membrane does not change its electrical potential, ECG does not record any electrical action. Normally, it lies on the baseline (like the PQ segment) because at this time, all parts of the ventricle are completely negative and there is no dissimilarity of potential between any two parts of the ventricles. The ST segment starts on the isoelectric line, except in V1 and V2 where it may be elevated (not >1 mm). The normal ST then arches gently in the direction of the T wave and should not remain exactly horizontal.

Most often ST deviation is a sign of myocardial ischemia, pericarditis, and pulmonary embolism. When a part of the myocardium is damaged the electrical impulse extend over this part is delayed or absent. This means that at no given moment, in such a ventricle, the complete ventricle is negative all the way through but the ECG continues to detect the signals. It is why elevation or depression of the ST segment from the isoelectric line appears on ECG.

The T wave characterizes the repolarization of the ventricles. The T wave should be upward direction; duration is normally about 0.1 to 0.25s. amplitude less then 7 mm, slightly asymmetrical and as a rule is larger than the P wave.

Occasionally, the U wave is registered following the T wave and before the P. This wave represents repolarization of the lowest parts of the conductive system and normally may be absent on the ECG. The U wave orient in the same direction as the T wave and, as a rule, has amplitude less then 1.5 mm. An abnormally prominent is often associated with hypokalemia, digoxin and qinidine intoxications.

The analysis of each ECG you must begin in next order: 1) evaluate the quality of ECG registration (may be presents of different hindrances); 2) check the amplitude of control milliwolt; 3) pay attention to the recording paper rate.

Then you should determinate the source of the state basic rhythm. Pay attention to the presence of P wave, shape of P wave, are the P wave similar in size and shape, are all P waves upright or all inverted, is the each P wave the same interval from its QRS complex. Healthy persons have a normal sinus rhythm. The P waves in leads I, II, aVF and V4- V6 must be upright (positive), similar in size and shape, if the rhythm is coming from the sinus node. Then it is important to determine regularity of the rhythm, direction of the electrical axis and possible presence of the abnormal ECG syndromes.

 

Figure 15 — Example of normal ECG (with sinus rhythm)

This tracing shows a normal ECG with sinus rhythm at about 75 per min. Sinus rhythm is identified as a narrow QRS rhythm with P waves preceding each QRS complexes with a fixed and normal PR interval in the range of 0.12 to 0.2 s.

P waves in sinus rhythm are positive in leads I, II and III. In abnormal atrial rhythms, P wave axis changes. For example, in low atrial or coronary sinus rhythm, the P waves are inverted in leads II, III and aVF. RS pattern in V1 and QR pattern in V6 are the usual pattern in chest leads. R and S waves are almost equal in amplitude in V2 and RS pattern is seen in V3. Here the transition from RS pattern to RS pattern occurs in V2. T waves have the same direction as the dominant QRS direction. Hence T waves are inverted in aVR and sometimes in V1 in a normal ECG.

In order to describe ECG successfully you also must remember that all waves are negative in aVR. This has to be so: aVR represents electrical activity as seen from the right shoulder. The sinus node is placed top right in the heart nearest the right shoulder and the electrical activity is moving downwards and leftwards towards the left ventricle.



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