Rush Center for Congenital
and Structural Heart Disease

 

Normal ECG

 

The ECG above represents normal ECG tracings.  A proper interpretation would include the following:
Normal standard, normal speed.
Similar atrial and ventricular rates = 65-70 beats per minute (bpm): Normal sinus rhythm.
Durations:
  Normal AV conduction (PR interval 0.12 sec).
  Normal ventricular conduction (QRS duration less than 0.08 sec).
  Normal ventricular repolarization (QTc interval 0.39 sec).
Axis: 
  P-wave axis=60 degrees
  QRS axis=60 degrees
No evidence of atrial enlargement.
No evidence of ventricular hypertrophy.
Normal ST segments.
Normal T waves.


ECG provides numerous and diverse information.  It is important to use a consistent methodology when reading any ECG, if this methodology is not followed, a single abnormality may overshadow other abnormalities.  However, if the same steps are always followed in reading an ECG, thorough reading can be achieved.

The typical sequence of reading ECG is as listed above, details in these steps and examples of potential abnormalities will be provided below.

 

Terminology

Each small square in an ECG grid represents time in the horizontal axis and voltage in the vertical axis.  Therefore, each small square is 0.04 sec (or 40 msec) long and 1 mm (or 0.1 millivolt) high.  Typically in pediatric cardiology ECG wave's height and depth are referred to in millimeters, not millivolt.

Intervals:

PR interval represents the time atrial depolarization takes place as well as conduction through the AV conduction system till the ventricles initiate their depolarization.  Prolonged PR interval indicates damage to the AV conduction system (heart block), while short PR interval indicates the existence of a bypass track.
QRS duration represents the time in which ventricular depolarization takes place.  A normal QRS duration should be less than 0.08 sec (2 small squares wide).  Prolonged QRS duration indicates abnormal conduction through the His-Purkinje system leading to longer period of depolarization.
QT interval varies by heart rate, therefore, corrected QT interval (QTc) is used instead.  A normal QTc should be less than 0.42 sec in men and less than 0.45 sec in women.

Axis

P wave axis indicates if atrial depolarization originated from the sinus node, or from an ectopic atrial focus.  A normal P wave axis is around 60 degrees.
QRS axis indicates the propagation pattern of ventricular depolarization.  A normal QRS axis varies by age (see table).  Abnormal QRS axis may indicate abnormal ventricular depolarization due to abnormality in His-Purkinje system or due to right or left ventricular hypertrophy, shifting the heart rightwards or leftwards respectively.

 

 

 

First Step

Was the ECG recording performed in a standard fashion.
 

A full standard ECG is when the standardization marker is 10 mm high.
If the standardization marker is half that tall (5 mm), then all waves (up and down) must be multiplied by 2.
On the other hand, if the standardization marker is full, then half the normal height (10/5 mm), then waves in the chest lead only must be multiplied by 2.
ECG machines are designed to go into either half or full/half mode if the ECG waves are touching or superimposed over each other.  Reducing the size of superimposed waves to half allow better visualization and interpretation.  However, the magnitude of each wave must be corrected to correctly diagnose chamber hypertrophy.

 

Next,

How old is the patient, in pediatrics ECG values vary by age, see table below:

 

Age

HR

bpm

QRS

axis

degrees

PR

interval

seconds

QRS

interval

seconds

R

in V1

mm

S

in V1

mm

R

in V6

mm

S

in V6

mm

1st week

90-160

60-180

0.08-0.15

0.03-0.08

5-26

0-23

0-12

0-10

1-3wks

100-180

45-160

0.08-0.15

0.03-0.08

3-21

0-16

2-16

0-10

1-2 mo

120-180

30-135

0.08-0.15

0.03-0.08

3-18

0-15

5-21

0-10

3-5 mo

105-185

0-135

0.08-0.15

0.03-0.08

3-20

0-15

6-22

0-10

6-11 mo

110-170

0-135

0.07-0.16

0.03-0.08

2-20

0.5-20

6-23

0-7

1-2 yr

90-165

0-110

0.08-0.16

0.03-0.08

2-18

0.5-21

6-23

0-7

3-4 yr

70-140

0-110

0.09-0.17

0.04-0.08

1-18

0.5-21

4-24

0-5

5-7 yr

65-140

0-110

0.09-0.17

0.04-0.08

0.5-14

0.5-24

4-26

0-4

8-11 yr

60-130

-15-110

0.09-0.17

0.04-0.09

0-14

0.5-25

4-25

0-4

12-15 yr

65-130

-15-110

0.09-0.18

0.04-0.09

0-14

0.5-21

4-25

0-4

> 16 yr

50-120

-15-110

0.12-0.20

0.05-0.10

0-14

0.5-23

4-21

0-4

 

 
Then,
 
determine from the rhythm strip (typically lead II) at the bottom of the 12 lead ECG if atrial depolarization (P waves) are consistently followed by ventricular depolarization (QRS complex of waves) and that the duration of the PR interval is constant.  A consistent pattern of P waves, followed by QRS with a constant and normal PR interval indicate that the AV conduction is normal.  In normal ECG, the atrial (P) and ventricular (QRS) rates are equal.
 

To calculate the heart rate (atrial and ventricular rates if each p is followed by a QRS) calculate the number of large squares between 2 consecutive QRS complexes, divide 300 by that number, that will lead the heart rate (HR) in beats per minute (bpm).

In the example below, there are 22 small squares between 2 consecutive QRS complexes.
Therefore, HR = 300/4.5 = 66 bpm

 

Then,

Measure intervals: PR, QRS and QTc and compare to normal values for age.
Prolonged PR interval indicates first degree heart block.

Short PR interval indicates bypass track (pre-excitation syndrome, such as WPW)

Wide QRS indicates bundle branch block.  This could be right bundle branch block (RBBB) or left bundle branch block (LBBB).
RBBB is identified through the RSR (M-shaped) pattern QRS in the right chest leads (V1 and V2).

LBBB is identified through the RSR (M-shaped) pattern QRS in the left chest leads (V5 and V6).

QT interval should be compared to the heart rate to determine if it is normal or prolonged.  This could be done by referencing the measured QT interval to normal QT interval range for each heart rate, as obtained from normal population.  Alternatively, the QT interval could be corrected to the heart rate using the Bazett formula:

QTc = QT / square root of RR interval

The table below provides the QTc for each listed QT intervals at various heart rates.

To determine the QTc, measure the QT interval from the onset of the QRS complex to the end of the T wave.  Then determine the heart rate and find from the table below the QTc.

 
    QT

HR

0.20 0.25 0.30 0.35 0.40 0.45 0.50
50 0.18 0.23 0.27 0.32 0.37 0.41 0.46
52 0.19 0.23 0.28 0.32 0.37 0.42 0.46
54 0.19 0.23 0.28 0.33 0.38 0.42 0.47
56 0.19 0.24 0.29 0.34 0.38 0.43 0.48
58 0.20 0.24 0.29 0.34 0.39 0.44 0.49
60 0.20 0.25 0.30 0.35 0.40 0.45 0.50
63 0.21 0.25 0.31 0.36 0.41 0.46 0.51
66 0.21 0.26 0.31 0.36 0.42 0.47 0.52
68 0.22 0.26 0.32 0.37 0.43 0.48 0.53
71 0.22 0.27 0.33 0.38 0.44 0.49 0.55
75 0.23 0.27 0.34 0.39 0.45 0.51 0.56
79 0.24 0.28 0.34 0.40 0.46 0.52 0.57
83 0.24 0.29 0.35 0.41 0.47 0.53 0.69
88 0.25 0.29 0.36 0.43 0.49 0.55 0.61
94 0.26 0.30 0.38 0.44 0.50 0.56 0.63
100 0.27 0.31 0.39 0.45 0.52 0.58 0.65
107 0.28 0.32 0.40 0.47 0.53 0.60 0.67
115 0.28 0.35 0.42 0.49 0.55 0.63 0.69
125 0.29 0.36 0.43 0.51 0.58 0.65 0.72
136 0.30 0.38 0.45 0.53 0.60 0.68 0.75
150 0.32 0.40 0.47 0.56 0.63 0.71 0.79
 

 

Then,

Calculate the P and QRS axis.  Use leads I and aVF, which are perpendicular to each other (I is zero degrees, while aVF is 90 degrees) to make this determination.

P wave axis helps determine if the origin of the impulse is from the sinus node.  A normal P wave axis, as in tracing below is normal (60 degrees)

 

In the example below the QRS is slightly negative in lead I and positive in aVF.  The QRS axis in this ECG is 150 degrees:

 

Heart Rhythm

A normal heart rhythm is when the origin of the impulse is from the sinus node (sinus rhythm) with normal AV conduction.  Therefore, a normal heart rhythm is when the P wave axis is normal + Normal AV conduction (PR interval) + Normal heart rate for age.

Please go to Heart Rhythm link for further details on detecting abnormal heart rhythms.

 

Chamber Hypertrophy

A search for hypertrophy of each cardiac chamber must be conducted when reading an ECG.
P waves determine atrial hypertrophy, while QRS determine ventricular hypertrophy.
Lack of atrial hypertrophy is when the P wave is less than 3 mm tall and 3 small squares wide.
Lack of ventricular hypertrophy is when QRS in leads V1 and V6 do not exceed normal range for age (see table) and without RVH patterns of QRS in V1,2 (see below).

 

Right Atrial Enlargement (RAE)

RAE is diagnosed when the P waves (in any lead, although best seen in lead II) is > 3 mm tall in older children and adults and > 2 mm in younger children.

 

Left Atrial Enlargement (LAE)

LAE is diagnosed when P waves are wider than 3 mm tall in older children and adults and wider than 2 mm in younger children.

 

Right Ventricular hypertrophy (RVH)

Is diagnosed when any of the following criteria are met:

R in V1 and S in V6 are taller and deeper than normal range for age.

 

QRS pattern in V1,2: rSR' without widening of QRS duration

 

QRS pattern in V1,2: pure R wave

 

QRS pattern in V1,2: pure qR wave

 

Left Ventricular hypertrophy (LVH)

R in V6 and S in V1 are taller and deeper than normal range for age.