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ECG Analysis

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Tutorial from Queens University Toronto

Introduction: see credits below

The electrocardiogram (ECG) is one of the most useful diagnostic tests in emergency medicine. It is an easy and inexpensive test that is used routinely in the assessment of patients with chest pain.
The ECG is the cornerstone for making the diagnosis of cardiac ischaemia and is used for making decisions about eligibility for thrombolytic therapy.
To avoid misinterpreting the ECG, the clinician must have a systematic approach. This module is designed to guide the learner through a stepwise approach to ECG interpretation.
Specific examples of a variety of abnormal ECG's are included at the end along with a brief quiz.

Objectives

By the end of this learning module you should have a systematic approach to interpreting the ECG and be able to identify common ECG abnormalities.

The 12 lead ECG

The 12 lead ECG is made up of the three standard limb leads (I, II and III), the augmented limb leads (aVR, aVL and aVF) and the six precordial leads (V1, V2, V3, V4, V5 and V6)

Waves and complexes

Intervals and segments>

PR Interval:	From the start of the P wave to the start of the QRS complex
PR Segment:	From the end of the P wave to the start of the QRS complex
J Point:	The junction between the QRS complex and the ST segment
QT Interval:	From the start of the QRS complex to the end of the T wave
QRS Interval:	From the start to the end of the QRS complex
ST Segment:	From the end of the QRS complex (J point) to the start of the T wave

Normal Values

Heart rate			60 - 100 bpm
PR interval			0.12 - 0.20 s
QRS interval			= 0.12 s
QT interval			< half RR interval (males < 0.40 s; females < 0.44 s)
P wave amplitude (in lead II)			= 3 mV (mm)
P wave terminal negative deflection (in lead V1)			= 1 mV (mm)
Q wave			< 0.04 s (1 mm) and < 1/3 of R wave amplitude in the same lead

Approach to the ECG

Developing a systematic approach to the interpretation of the ECG is a critical skill for all clinicians. The following outlines one such approach.

Step 1

There are a number of strategies for determining the heart rate. A simple, quick technique is to find a QRS complex that falls on a major vertical grid-line (1), then count the number of large squares to the next QRS complex (2). Dividing this number into 300 gives you the heart rate. In the ECG below, there are 2 large squares between QRS complexes. 300/2 gives a heart rate of 150 beats per minute.

Step 2: Measure important intervals

The measurement of important electrocardiographic intervals usually includes the PR interval, the QRS interval and the QT interval. At a standard paper speed of 25 mm/second, the width of each small square (1mm) represents 0.04 seconds. One large square (5mm) represents 0.2 seconds.

Evaluate the Rhythm

If the rhythm is regular, the RR interval should be constant throughout the ECG. This can be checked using calipers, or more simply by marking on a piece of paper the distance between two R waves, and comparing this distance between pairs of QRS complexes on the ECG. Next, check to see if a P wave is present before each of the QRS complexes.

Inspect P waves for atrial enlargement

The P waves in leads I, II, III and V1 should be inspected for evidence of right or left atrial enlargement. Usually, lead II will have the clearest P wave.

P wave amplitude should not exceed 3 small squares (3 mm or 0.3mV). If it does, this represents right atrial enlargement.
In lead V1, the terminal negative deflection of the P wave represents left atrial depolarization and should not exceed 1 mm (0.1mV). If it does, this is indicative of left atrial enlargement.

Inspect QRS complexes for ventricular hypertrophy or low voltage

In the setting of Left Ventricular Hypertrophy (LVH), the left ventricle enlarges and so the leads oriented to the left ventricle (V5, V6, aVL) will "see" more electrical activity moving towards them. As well, the leads oriented away from the left ventricle (V1, V2) will "see" more activity moving away from them. In LVH therefore, leads V5, V6 and aVL will have tall R waves, while leads V1 and V2 will have deep S waves. (The arrow in the diagram on the right shows the direction of the net electrical activity in LVH.)

V1 or V2

V5, V6 or aVL

The voltage criteria for LVH are satisfied if the sum of the amplitude of the deepest S wave in V1 or V2, and the amplitude of the tallest R wave in V5 or V6, is equal to or greater than 35 mm (3.5 mV). The voltage criteria are also satisfied if the amplitude of the R wave in lead aVL is equal to or greater than 12 mm (1.2mV).

Assess Q waves and determine significance

The Q waves should be assessed and their significance determined, particularly in regard to the diagnosis of myocardial infarction. Small Q waves are commonly a normal finding in the inferior leads III and aVF, and in the anterolateral leads aVL, I, V5 and V6. Q waves of 0.04 seconds (1 mm) duration and greater than one third the R wave's amplitude in the same lead may be pathological.

The pathological Q waves are seen in V1 - V6 indicate that this patient has had an anterior MI in the past. This patient also has evidence of an acute inferior MI as shown by the ST-segment elevation in leads III and aVF.

This ECG is from a male patient with familial prolonged QT syndrome. The QTI corrected is approximately 0.52 seconds. Normal QTI corrected: 0.40 seconds for males; 0.44 seconds for females.

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