Reading a normal ECG

However, the 12-lead ECG must be looked at carefully and in a systematic way and this often takes many years to master. The ECG should always be used along with the patient’s history.

Each month an ECG will be presented with a short patient history for the reader to analyze. In this first edition, a system-atic approach to analysing ECGs is pre-sented along with a normal 12-lead ECG (Figure 1) so that the reader can practice applying the framework to the ECG. The framework uses ten rules that can be applied to any ECG.

The ten rulesA starting framework for the systematic approach to the 12-lead ECG. For posi-tioning of the leads see Figure 2 and for the view of the limb leads see Figure 3.w All waves are negative in aVR. This has

to be so: aVR represents electrical

activity as seen from the right shoul-der. 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.w The ST segment starts on the isoelec-

tric line, except in V1 and V2 where it may be elevated (not >1 mm). The normal ST then curves gently in the direction of the T wave and should not remain exactly horizontalw The PR interval should be 0.12–0.2

seconds. A longer PR implies AV block, a shorter PR may indicate a vulnerabil-ity to supraventricular arrhythmiasw The QRS complex should not exceed

0.11–0.12 seconds. A wider QRS is sometimes seen in healthy people but may represent an abnormality of intra-ventricular conductionw The QRS and T waves tend to have the

This month we revisit Mark Whitbread’s tool for reading a 12-lead ECG, followed by this month’s ECG.

Mark Whitbread is the Clinical Practice Manager for the London Ambulance Service

Key wordsw ECG w Patient history w 10 rules w Isoelectric line Accepted for publication 19 January 2006

The 12-lead electrocardiogram (ECG) remains one of the most useful clinical tools in the evaluation

of the cardiac patient. Its use is widespread and can be of use as part of the assessment process in many presentations such as: w Chest painw Shortness of breathw Blackoutsw Palpitations w Syncope w And many others…

V3

V2

V1

V6

V5

V4I

II

III

aVR

aVL

aVF

Figure 1. The normal electrocardiogram.

Practical Procedures

58 British Journal of Cardiac Nursing February 2007 Vol 2 No 2

British Journal of Cardiac Nursing February 2007 Vol 2 No 2 59

Practical Procedures

Figure 2. Positioning of chest leads

3 Limb leads: I II III

3 Augmented (modified) limb leads: aVL (augmented view left) aVR (augmented view right) aVF augmented view foot/left leg

6 Chest leads: V1 V2 V3 V4 V5 V6

From: Adam and Osborne, 2003.

Table 1. Definition of

electrocardiogram leads

British Journal of Cardiac Nursing February 2007 Vol 2 No 2 59

4th intercostal space

V1 V1R

V2 V2RV3 V3R

V4 V4R

V5 V5R

V6 V6R

A. Standard chest lead placement

B. Right sided chest lead placement

LL

IRA LA

II III

Limb leads

same general direction in the standard (limb) leads. For example, if the QRS in aVL is dominantly positive than the T wave in that lead should also be positive. Slight disparities are likely to be normalw The R wave in the precordial (chest)

leads grows from V1 to at least V4 where it may or may not decline again. A spurious abnormality frequently occurs in R wave size or growth because of faulty placement of precordial leadsw The QRS is mainly upright in I and II.

Otherwise there is axis deviationw The P wave is upright in I II and V2 to

V6. By implication they may be flat or negative in other leadsw There is no Q wave or only a small q (<

0.04second in width) in I, II and V2 to V6. A narrow q is expected in V6 and represents the early septal activation.w The T wave is upright in I II and V2 to

V6. The end of the T wave should not dip below the baseline. This is some-times seen in unstable angina.

Adam SK, Osborne S (2003) Critical Care Nursing: Science and Practice. Oxford University Press, OxfordFigure 3. View of the limb leads

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