ECG (Electrocardiogram)                                                                   

An ECG (electrocardiogram) is a common investigation recording the electrical activity of the heart.

Most children will be cooperative for an ECG although it can try one’s patience obtaining it in a toddler!

The result is printed out on paper and gives information about the heart rhythm, morphology and function.

Interpretation

Check the name and age of patient and test date. Whilst the machine provides a computer interpretation of the result this may sometimes be misleading as the ECG in children can be difficult to interpret and in particular normal values vary with age.

Rate & Rhythm

Rate is calculated by dividing the big squares (each has 5 little ones) into 300. In the example the R-R interval is 4 large squares so the rate is 75 bpm. To more accurately determine the rate remember each little square is 0.04 of a second in duration. Measure the R-R interval in seconds and divide into 60 to determine the beats per minute.

Each QRS complex is preceded by a p wave unless an abnormal rhythm is present. Small Q waves are common in children are provided their duration is less than 40ms (1 small square) and amplitude less than mm they can be ignored.

P wave

The normal p wave is less than 2.5mm in amplitude. It may be notched. A biphasic wave may be found in V₁ but the negative portion should be less 1 small square in amplitude & duration.

The PR interval is measured from the onset of the p wave to the onset of the QRS wave. The longest PR interval in lead II is used. It lengthens with age from the neonatal maximum of 140ms to 180 ms at any age.

Q waves

With the exception of lead aVR Q waves are commonly seen. To be considered normal their duration must be less than 30ms (as shown on left compared to the pathological Q wave on the right). The amplitude varies with both lead and age but is generally less than 5 mm.

QRS Complex

The QRS duration increases slightly with increased muscle mass and hence with age. Its maximum in a neonate is 70ms increasing to 90ms by adulthood. It is abnormally prolonged in bundle branch block which is a frequent occurrence following cardiac surgery.

QRS Axis

Subtract the number of little squares below the baseline in lead 1 from the number above the line (+5 mm in the example) & plot horizontally towards the lead. Repeat for lead AVF (+11 in the example) &  plot vertically towards the lead. If the sum is negative then plot the number of squares away from the lead. Join the dots to find the axis.

Lead I is 0°, aVF +90°. Opposite Lead I is 180° etc. This example is +60°.

Usually if the axis is abnormal aVF is predominantly negative. The same system can be used to find the P wave axis. Inverted p waves in I, II, III or AVF imply an abnormal p wave axis. Left axis (superior axis) is seen in atrioventricular septal defects, tricuspid atresia, inlet VSD, single ventricles, WPW syndrome and occasionally as a normal variant.

R & S Waves

The V leads assess the heart in the horizontal plane. The RV forces are directed anteriorly and to the right - this is reflected by the height of the R wave in V₁ and the depth of the S wave in V_6. The converse is true for the LV. LV hypertrophy is also often assessed by the sum of  the SV₁ and RV_6.  V_2-5 are the transitional leads and so the sum of RV₄ and SV₄ (normal maximum value 50 mm) is used to assess whether both ventricles are hypertrophied. All values should be plotted against the normal values for age but in general terms the maximum values are:

• RV₁ 26 mm (neonate) decreasing to 10 mm as adult

• SV₆ 10 mm (neonate) decreasing to 4 mm as adult

• RV₆ 11 mm (neonate) increasing to 23 mm as adult

• SV₁ 23 mm (neonate) decreasing to 11 mm by 1 month and rising again to 21 mm as adult

• SV₁ + RV₆ 28mm (neonate) decreasing to 21 mm by 1 month and increasing to 41 mm as adult

The minimum value is also important as it is often reduced in myocarditis. Normal values for the sum of the R and S waves are greater than 5 mm in the limb leads and 8 mm in the ventricular  leads.

ST Segment

The ST segment should be compared to the PR baseline. Values more than 1 mm above or 0.5 mm below are abnormal. 

QT Interval

A rare cause of collapse, loss of consciousness or fits is the prolonged QT syndrome. It is treatable. The QT interval should be corrected for the heart rate according to Bazet’s formula:

QTc = QT interval
           √RR interval

Measurements are made in seconds (1 small square = 0.04s). The longest QT interval is measured and the preceding RR interval. The normal value is < 0.44 secs.

T wave Morphology

The T waves are upright for the 1st week of life, then become inverted in V_1-3 until 10 yrs age when become upright again to achieve the adult pattern by age 15 years. An inverted T wave in V₁ is a normal variant in adults. The amplitude in children is very variable.

U wave

This follows the T wave and is of the same polarity. It is often not seen due to a low amplitude and at heart rates above 90 bpm it merges with the T wave. If the amplitude is greater than 50% of the T wave it is abnormal.

Atrial Hypertrophy

Right atrial hypertrophy is diagnosed at any age when the amplitude is greater than 2.5 mm in any lead.

Left atrial hypertrophy is indicated by a late negative deflection of the p wave in V₁ occurring more than 40ms after the onset of the p wave. The LA component of the p wave occurs after the RA component and so biatrial hypertrophy exits when both criteria are met. The trace on the right shows pure left atrial hypertrophy.

Ventricular Hypertrophy

There is no one specific ECG change of ventricular hypertrophy thus the more signs of hypertrophy present the more confident one can be that an abnormality is truly present.

Right Ventricular Hypertrophy (RVH)

1. QR pattern in right chest leads is a reliable sign of RVH and implies a markedly elevated RV pressure

2. T wave changes are a good sign of RVH

3. R wave amplitude in V₁ greater than normal value - this is specific but not very sensitive - significant RVH may be present with a normal amplitude

4. S wave amplitude in V₆ greater than normal value and again is specific but not sensitive for RVH

5. RSR' in V₁ with a normal QRS duration is quite sensitive (but less specific) for the mild RVH - especially that found in atrial septal defects. If pathological the R' is usually large.

6. Right Axis Deviation correlates well with RVH in children.

Left Ventricular Hypertrophy (LVH)

1. Q waves in leads II, III, AVF, V_5-6
    
2. T wave inversion in V_5-6 are reflective of significant LVH
    
3. R wave amplitude in V₆ greater than normal value
    
4. S wave amplitude in V₁ greater than normal value

note that on both the ECGs on the right the voltages are halved to enable the complexes to fit on the paper - the true size of the deflections are double that seen. Always therefore look for the scale (blue arrow).

Biventricular Hypertrophy (BVH)

If one ventricle is hypertrophied the ECG signs reflecting the other ventricle are usually diminished thus if the ECG shows mean or increased values for the other ventricle then BVH is present. Another criterion is abnormal summated voltages in V₄ - usually greater than 60 mm.

Bundle Branch Block

Right Bundle Branch Block

Partial RBBB may be a normal feature of the ECG but is associated with RVH found in atrial septal defects. The QRS duration is normal but an RsR' pattern found in V_1-2

Complete RBBB is most common following cardiac surgery but occasionally is familial. It is diagnosed when the QRS duration is abnormally prolonged for age (100 ms over 4 years of age, 120ms in an adult) with a RsR' or RR' pattern. The delay in RV depolarisation also delays the repolarisation and T wave inversion is commonly seen. These ECG changes precludes the diagnosis of ventricular hypertrophy in CRBB. Very wide QRS duration (> 180ms) may be a predictor of arrhythmias post surgery. In the ECG on the right the QRS duration is 184ms.

Left Bundle Branch Block

This is rare in children occurring in myocarditis, ischaemia and hypertrophic cardiomyopathy. The QRS duration is abnormally prolonged with an RsR' pattern in V_5-6. In addition there is loss of the normal initial R wave in V₁ and the Q wave in V₆.

Bifascicular Block     

Occasionally post surgery (especially tetralogy of Fallot) both the right and either the anterior (more usual) or posterior fascicles of the left bundle are damaged. Left bundle anterior hemiblocks are associated with left axis deviation and so in the common bifascicular block post surgery the ECG pattern demonstrates an RsR' pattern found in V_1-2 in association with left axis deviation. There was initial concern that these patients were more likely to go into complete heart block but this is extremely uncommon.

Myocardial Injury

This is usually reflected in the ST & T wave segments and may be due to myocarditis, pericarditis or ischaemia.  These diseases are relatively uncommon in children and ST & T wave changes are usually either part of normal (in which case they usually vary in time between ECGs), manifestations of ventricular hypertrophy (which should therefore be excluded before such changes are attributable to ischaemia) or due to electrolyte disturbances (eg hyperkalaemia). The clinical situation usually helps in narrowing the differential diagnosis.

Myocarditis

The T waves in the lateral chest leads may become flattened or inverted and in general the amplitude of the QRS complexes are reduced.

Pericarditis

This is most commonly seen postoperatively and ST elevation is seen in numerous leads. It usually evolves with flattening of the segment, T wave inversion and finally resolution to normal.

Ischaemia

This is very uncommon in pediatric practice but may occur in Kawasaki disease, anomalous coronary arteries and asphyxiated neonates. Initially there is ST elevation in a few leads followed by T wave inversion and the development of Q waves overlying the area of infarction. 

This page was last edited 16/2/2004

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