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Accident and Emergency Medicine, Ninewells Hospital, Dundee

Standard 12-Lead Electrocardiograph

Charge Nurse Jenny Fitzpatrick, Accident and Emergency Department

The standard 12-lead ECG takes 12 views of the heart’s electrical activity and records it on standardised ECG paper.

The ECG recording is used to diagnose various things, including myocardial infarction, abnormal patterns of conduction and arrhythmias. It is therefore very important that the procedure is carried out correctly.

A full explanation should be given before beginning and every attempt should be made to maintain the patient’s modesty.

1. Check that all leads are placed correctly, as follows,

·        One lead on each limb – preferably on the inner aspect of the lower part of the limb.

·        C.1 – fourth intercostal space at right sternal border.

·        C.2 – fourth intercostal space at left sternal border.

·        C.3 – midway between C.2 and C.4.

·        C.4 – fifth intercostal space at mid-clavicular line.

·        C.5 – directly lateral to C.4 at the anterior axillary line.

·        C.6 – directly lateral to C.4 at the mid-axillary line.  

2. Ensure that the wires are not touching metal as this can cause interference       

3. Ensure that the patient is as relaxed as possible. Tense muscles can also cause interference on the tracing.

4. Ask the patient to stay as still as possible while the recording is being carried out.

After the recording has been completed, the pre-jelled electrodes should be left in place for the recording of any subsequent ECG’s.

Understanding the 12-Lead ECG.

The four limb leads and six chest leads are attached to the patient as described previously.

The limb leads form an imaginary triangle, looking at the heart’s conduction from various angles. Each side of the triangle represents a lead; [1,11, and 111], as does each corner of the triangle [AVR, AVL and AVF], thus producing six different views. [insert hyperlink]

These leads plus the six chest leads give us a twelve lead ECG.

Lead 11 on the bottom of the ECG is known as the rhythm strip and is used for diagnosing the rhythm and rate of the patient’s heart. The 12-lead allows for the diagnosis of myocardial infarction and various other conditions.

ECG Findings.

On the 12-lead ECG, iscaemic but functional myocardial tissue will produce changes in the T wave, causing inversion as the electrical current is directed away from the ischaemic tissue. More seriously, ischaemic tissue will alter the ST segment causing ST depression.

With a myocardial infarction, the dead myocardium does not conduct electricity and fails to repolarise normally, resulting in ST segment elevation.

As the necrosis develops, with healing of the ischaemic rings around the necrotic area, Q waves develop. The necrotic area is an electrically inactive scar, but the ischaemic zone will reflect T wave changes as ischaemia recurs.

Initially, with a myocardial infarction, the ST elevation is accompanied by tall T waves. Hours to days later, the T waves invert. As the M.I. ages, Q waves remain, and the ST segment returns to normal.

The 12- lead ECG also identifies the location of the ischaemia or infracted tissue. By the presence of the specific lead changes, infarct area and size are determined.

ECG Patterns in Myocardial Infarctions

Inferior M.I.

Blockage – right coronary artery.

ECG changes –ST elevation in leads 11,111, and AVF.

Complications to watch for – Transient AV blocks, usually Wenckebach block. May require temporary pacing, but rarely is a permanent pacemaker needed.

Anterior M.I.

Blockage – left anterior descending artery

ECG changes – ST elevation in leads 1,AVL and C.1 – C.6.

Complications to watch for – Right Bundle Branch Block, Mobitz type 11 block [indication for pacing]. Once a block develops, if a significant amount of the anterior wall is damaged, there is a good chance that complete heart block will follow. If 50% of the ventricle is damaged, the patient can go into cardiogenic shock, with a mortality rate of greater than 85%.

Lateral M.I.

Blockage – circumflex branch of left anterior descending artery.

ECG changes – St elevation in 1,AVL, C.5 andC.6.

Complications to watch for – Transient heart blocks [may require temporary pacing].

Anteroseptal M.I.

Blockage – left anterior descending artery

ECG changes – ST elevation in C.1 – C4.

Complications to watch for – as with lateral M.I.

Anterolateral M.I.

Blockage - left anterior descending and circumflex arteries.

ECG changes – ST elevation in 1,AVL and C.4 – C6.

Complications to watch for – as with lateral M.I.

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Last updated: April 18, 2004.