ECG Interpretation Theory

8/14/2019 ECG Interpretation Theory

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ECG Interpretation Theory

The Waveform

Figure 1: PQRST Complex

The electrocardiograph is a recording of the electrical activity of the heart. The normal ECGis composed of:

P wave representing atrial depolarisation QRS complex which represents ventricular depolarisation T wave, representing ventricular repolarisation Occasionally a U wave can be seen If present, it will be seen after the T wave.

The direction of flow of electrical conduction or impulse in the heart is called a vector.Impulses travel from a negative pole to a positive pole. Impulses traveling toward a positiveelectrode will be seen as a positive impulse (above the isoelectric line). Impulses travelingaway from a positive pole will be seen as a negative impulse (below the isoelectric line).Impulses traveling at right angles (90 degree angle) to the axis of the lead may be seen asisoelectric or biphasic.

Areas within a 12 Lead reading:


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ECG Interpretation can be done in 8 steps:

1. Rate and Rhythm

BPM Described as Abbreviation forEscribe

60-100 Sinus Rhythm /sr100 Tachycardia /st

Regular or Irregular? Are the R waves the same distance apart?

2. P waveRepresents the electrical activity associated with the initiation of the impulse from the SAnode and the passage through the atria (atrial depolarisation). If the p wave is present,

upright and of normal morphology (shape) and size, it can be assumed that the impulseoriginated in the sinus node.

The p wave should be:

Rounded in shape 0.12 seconds 2.5 mV (2.5 mm) in height

Normally upright in left sided and inferior leads (II, III, aVF, V4 to V6, I and aVL) and invertedin aVR. There should be one p wave per QRS complex.

3. Check Intervals

PR interval: measured from the beginning of p wave to the beginning of the QRS, it isisoelectric and represents the time the impulse is held in the AV node (delay). It is designedto slow down the impulse to allow for atrial kick (the last 30% of blood ejected from the atria)and protects ventricles from very fast rates.

The PR interval should be:

0.12-0.20 seconds in duration If it is greater than 0.20 seconds, AV Block is noted

First Degree AV Block: Note prolonged PR interval over 0.2 seconds from prolongedtransmission of impulse through the AV junction.

Second Degree AV Block Type 1 (Wenckebach): ECG has a lengthening PR interval.

Second Degree AV Block 2:1 : Every second P wave results in a dropped beat.

Second Degree AV Block Type II (Mobitz II): Fixed PR and one or more QRS complexesis dropped.

Third Degree AV Block : Full AV block with no obvious association or teamwork betweenthe p waves and QRS complex.


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QRS complex : represents ventricular depolarisation, that is, the time it takes for the impulseto travel down the bundle of His, bundle branches and Purkinje fibres.

If the QRS interval is

below 0.10 seconds, this is normal if it is between 0.10-0.12, minor IVCD is noted If it is above 0.12 seconds, IVCD or it can assume the morphology of Left or Right

Bundle Branch BlockTo differentiate between the two, William Morrow is a helpful mnemonic.

V1 V6

WiLLiaM

MoRRoW

Within LBBB, a W can be seen in the QRS complex in V1 and an M in V6.Within RBBB , a M can be seen in the QRS complex in V 1 and an W in V6.

Determine the Axis

5. Q(q) wave

First downward (negative) deflection May be written as Q or q depending on the size of the waveform: if it is greater than a

quarter of the R wave it is usually written as a Q Normal Q waves in leads V5, V6, I and aVL

Abnormal Q waves if a quarter of the height of R wave and less than 0.04 seconds induration

Septal Q wave (Seen in V1 or V2)

6. R wave Progression


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First upward deflection Can be a R or r wave depending on height Should become taller (and appear biphasic) than the S wave in either V3/V4, and R

wave height may decrease slightly in lateral leads.

Premature R wave progression is noted if the QRS is positive before V3. A R wave isPoor if the QRS does not become positive until after V5.

7. ST segmentST segment starts at the end of the QRS complex and terminates at the beginning of the Twave. J point is the junction where the S wave and ST segment meet and is the indicator forischaemia. The J point and ST segment should be on the baseline or isoelectric. Is normal ifit is within 1mm of the isoelectric line, and abnormal if above 1mm of the isoelectric line,indicates ischaemia or infarction. Note on interpretation if there is an abnormality ordownwards deflection of the ST segment.

8. T Wave

T wave represents ventricular repolarisation and is normally upright and rounded. It can beflattened, inverted, biphasic (above and below the baseline or distorted by ST elevation ordepression).

Peaked Inverted

Normal if T wave is in the same direction as the QRS complex, and Normal T wave height is

usually less than two thirds of the R wave height. T wave inversion in V5/V6 is alwaysabnormal. If present, it is noted as Widespread/Insert area T wave abnormality.

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ECG Interpretation Theory