12 lead ecg recording delegate notes€¦ · t wave – this deflection shows the repolarisation of...

12 Lead ECG Recording

Delegate Notes

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Reviewed November 2019

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© 2019 ECG – All rights reserved LessonPlans/ECG12Lead/Pack&Kit/R&I/Handout November 2019

2

Anatomy and Physiology of the Heart

The Cardiac Cycle

o Veins bring blood into the heart which collects in the atria.

o The atria contract and blood moves to the ventricles through valves which prevent the

backflow of blood (step 1 below).

o Once the ventricles are full of blood, they contract to pump blood out through valves

into arteries which take blood away from the heart (step 2 below).

o Blood circulates around this system, taking up oxygen in the lungs and giving it up to

the tissues and organs of the body.

o The heart has four chambers – the right atrium, the

left atrium, the right ventricle and the left ventricle.

o The right side of the heart pumps deoxygenated

blood to the lungs.

o The left side of the heart pumps oxygenated blood

to the body.

o Veins bring blood towards the heart, while arteries

take blood away from the heart.

A tip to remember this:

VeIN – IN to the heart

Arteries – take blood Away

Right

Atrium

Left

Atrium

Right

Ventricle

Left

Ventricle

Step 1 Step 2


3

The Structure of the Heart

The Electrical Conduction System of the Heart

Contraction of the heart muscle is associated with electrical activity. The electrical conduction

system of the heart is made up of specialised cells. These specialist cells can initiate a heartbeat

and transmit the impulses through the heart in an organised manner. The change in electrical

charge of a cell is known as depolarization.

1) The impulse is initiated from the

Sino-Atrial (SA) node, which is the

hearts natural pacemaker and

beats between approximately

60-100 times per minute at

rest.

2) The wave of depolarisation travels

across the atria to the

Atrioventricular (AV) node.

3) The depolarisation continues down

through the Bundle of His which

branches off into the left and right

bundle branches.

4) Purkinje fibres emerge from these

branches and innervate the

myocardial cells.

5) After depolarisation the cells return

to their original electrical state

known as repolarisation (which

causes relaxation of the muscle).

The heart consists of three layers:

o Pericardium – a thin outer lining that

protects and surrounds your heart.

o Myocardium – a thick muscular middle

layer that contracts and to squeeze blood

out of your heart.

o Endocardium – The innermost layer of the

heart.


4

How an ECG complex is formed

The depolarisation of cardiac cells is detected by electrodes on the skin when we perform an

ECG. A standard ECG complex consists of three main components.

A wave is a deflection from the baseline that represents a cardiac event.

P wave – this deflection shows the depolarisation of the atria, which causes atrial contraction.

QRS complex – this deflection shows the depolarisation of the ventricles, which causes the

ventricles to contract.

T wave – this deflection shows the repolarisation of the ventricles, which causes the ventricles

to relax.

A specific portion of a complex is described as a segment, for example the segment between

the end of the P wave and the beginning of the QRS complex is known as the PR segment.

The distance occurring between two cardiac events measured as time is known as the interval.

The time interval between the beginning of the P wave and the beginning of the QRS complex

is known as the PR interval (note there is a PR interval as well as a PR segment).


5

The ECG Paper

The standard calibration settings should be set to:

Paper speed: 25mm/s Voltage: 10mm/mV

The ECG paper runs at a rate of 25mm per second. Each 1mm square is therefore is 1/25th of

a second or 0.04 seconds. Each large box is made up of 5 smaller boxes, it represents 5 X

0.04 seconds = 0.2 seconds. Therefore 5 large boxes make 1 second.

The ECG should print 10mm of height per 1mV of electrical activity detected. When talking

about the height of a wave we use millimetres.

The paper is also broken down into four strips, the top 3 strips are made up of the 12 leads

which are appropriately labelled for easy identification. The 4th, a continual strip found at the

bottom of the page is a rhythm strip (lead II). The complete ECG is 10 seconds long.

See the example ECG below.

1 second = 5 large

squares

5mm

=

0.5m

V

0.2 sec.

0.04

sec

1mm


6

How does the ECG ‘look’ at the heart?

This is helpful to understand the term ’12 lead ECG’. This is more advanced information for

your reference.

To make sense of an ECG we need to understand the concept of the ‘lead’, this term does not

refer to the wires that connect the patient to the machine, but the different viewpoints of the

heart’s electrical activity.

An ECG machine uses the information it collects via its four limb electrodes and six chest

electrodes to compile a comprehensive picture of the electrical activity in the heart as

observed from 12 different viewpoints (hence the name ’12 Lead ECG’).

Each lead is given a name. I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5 & V6.

Leads I, II and III are bipolar (measure electrical potentials between a negative and a positive

electrode). All other leads are unipolar using a nominal centre point of the heart (use a single

positive electrode and use a combination of all other electrodes to act as a negative

electrode).

The measured electrical potentials from the four limb electrodes are used by the ECG

machine to create the six limb viewpoints (Leads I, II, III, aVR, aVL & aVF – see below).

V1, V2, V3, V4, V5, V6 correspond to each of the six chest electrodes. This is useful to

know when dealing with artefact on the ECG.

The different sections on the ECG represent the different regions of the heart -

lateral being the left side of the heart, anterior being the front of the heart and the inferior

being the lower area of the heart.


7

Preparing the Patient

Prior to undertaking the procedure, the following should be checked:

• That the ECG machine is safe and ready to use (date & time settings are correct)

• The patient area is clean and tidy

• There is enough paper, electrodes, razors and skin preparation equipment

• The identity of the patient should be confirmed and cross-checked with the request

• Room to be warm and private to maintain dignity.

Once the electrodes are positioned and the connecting wires are appropriately attached, the

patient should be covered with a gown to preserve his/her dignity during the procedure.

Patients may feel uncomfortable about being touched on their upper torso. The ECG

procedure requires sensitivity. Operators must take every effort to respect the sensitivities of

patients and minimise any discomfort. Operators must adhere to the organisation’s chaperone

policy and ensure that patients are made aware of the policy.

Skin preparation:

Skin preparation is required to help produce an artefact-free and accurate ECG. Various

methods are available, all of which are designed to minimise the skin-to-electrode problems.

Considerations include:

The removal of chest hair may be required to ensure adequate contact with the skin. Verbal

consent should be obtained from the patient and a clean razor used which should be disposed

of in a sharps bin immediately afterwards.

Exfoliation may be required and should be undertaken with very light abrasion using either a

paper towel, gauze swab or proprietary abrasive tape designed specifically for this purpose.

On occasions the skin may require cleansing. A variety of methods exist ranging from

washing with mild soap to cleaning with an alcohol wipe. However, care must be taken in

patients with sensitive or broken skin.

Check the wires and connection areas as occasionally gel can accumulate on the clips which

can affect contact. Clean with alcohol wipes.


8

Chest Electrode Placement

Chest electrodes should be accurately placed as according to the SCST Guidelines (2017).

Step one:

Find the Angle of Louis by feeling for the ‘bump’ on the

sternum. Direct your finger down and leftwards until you feel

the space between two ribs. This is an accurate way to locate

the second intercostal (rib) space. From this position, count

down two more rib spaces and place an electrode in the fourth

intercostal space at the right sternal edge (V1).

Repeat on the patients left side (moving your finger to the

right from the Angle of Louis) and place an electrode on in the

fourth intercostal space at the left sternal edge (V2).

Step two:

Look at the patient’s clavicle (collar bone) and estimate the

midway distance across this bone. Follow a straight line down

from this point by eye. Place the electrode in the fifth

intercostal space by counting one space lower and following

across to the level of the mid-clavicle (V4). Where there is

breast tissue, the electrode should be placed at the level of the

mid-clavicle underneath the breast.

Step three:

Place an electrode at the mid-point between the second and

fourth electrodes (V3) (as shown).

Step four:

Place an electrode at the same horizontal level as V4, on the

anatomical ‘mid-axillary line’ – this is usually where you would

find the line of the seam of a tshirt. From here you can place

an electrode between V4 and V6 – known as the enterior

axillary line.


9

Recording an ECG

It is vital to get the electrode placement, wire connections and overall recording procedure

correct as an inaccurate ECG could result in inappropriate diagnosis and treatment.

Electrode Position

V1

V2

V3

V4

V5

V6

Fourth intercostal space at the right sternal edge

Fourth intercostal space at the left sternal edge

Midway between V2 and V4

Fifth intercostal space in the mid-clavicular line

Left anterior axillary line at same horizontal level as V4

Left mid-axillary line at same horizontal level as V4 & V5


10

Recording an ECG

Colours on the wires are useful, but if you are new to recording ECGs or do so infrequently,

we recommend you read the numbers labelled 1-6 on the wires which should be placed from

left to right in order across the chest electrodes.

Limb electrodes:

• Right arm limb lead (RA, red) - right forearm, proximal to wrist

• Left arm limb (LA, yellow) - left forearm, proximal to wrist

• Left leg limb lead (LL, green) - left lower leg, proximal to ankle

• Right leg limb lead (N, black) - right lower leg, proximal to ankle

This may help you remember

the order of limb positions:

Red, Yellow, Green, Black

‘Ride Your Green Bike’


11

Recording an ECG

• Begin by asking the patient to lie down (at an angle of 45 degrees – halfway between flat and upright) and relax, this reduces artefact and ensures consistency between ECGs.

• Attach the chest and limb electrodes as described and connect wires.

• Check that paper is loaded and ensure the date and time is correct.

• Enter patient’s details – name, DOB, gender.

• Press start and allow the machine to follow its process until printing is complete. • Inform the necessary clinician that the ECG has been done. Any changes on the ECG that

might require urgent medical attention should be identified and advice sought from a senior member of staff if necessary.

• If the patient has any cardiac symptoms at the time of recording, such as chest pain or

palpitations then this should be noted on the tracing and brought to the immediate attention of a senior member of staff.

• It is imperative that any changes from the standard procedure of recording are noted on

the ECG to prevent misinterpretation (e.g. patient sat upright in wheelchair).

Once the ECG has been printed you should check the following:

• Calibration – calibration markers are printed on the ECG alongside a rectangular box

that measures 10 small squares (10mm) in height. The paper speed is usually printed

on the bottom left corner of the ECG. Standard setting is 25mm/second. Any alterations

to this will alter the analysis. The vertical axis of the ECG measures the amplitude (size)

of the waveforms. The standard calibration is 10mm/mv. Alterations to the amplitude

settings can alter the size of the waveforms and lead to incorrect analysis of the ECG.

• Quality of the trace – the ECG should be clear with no artefact, wandering baseline,

electrical or muscle interference or missing leads.

• aVR should always be negative –if aVR is positive, it is usually because the right and

left arm wires have accidently been switched.

• The relevant patient details including name, date and time of the ECG need to be on

the printout. It is also useful to make a note of any symptoms that the patient was

experiencing e.g. chest pain when the ECG was being recorded.


12

Examples of poor-quality ECGs

Wandering baseline

Caused by poor skin to electrode contact.

Muscle interference

Caused by patient movement/muscle tension.

Electrical interference

Caused by electrical devices/sources of AC artefact. Internal neurostimulators (e.g. DBS).


13

Other Types of ECG Recording

There are other ways that an ECG may be recorded. Although you might not be involved in

these, your patient may be asked to have further testing because of the initial ECG. The 12

lead ECG may not pick up on abnormalities occurring less frequently.

Other tests include:

Exercise stress test – this is usually performed to see if there are any changes to the

heart that occur during exercise. The test may be performed with the individual on an

exercise bike or walking on a treadmill. They are also connected to an ECG and this is

monitored as the intensity increases to look for heart rhythm abnormalities or signs of

ischaemia.

Holter monitor (also known as ambulatory monitoring) – a small device is connected to

the individual via 3 or 6 electrodes which are worn for 24-72 hours. The patient is advised

not to shower or bath while wearing the device. They would usually be asked to keep a

diary of activities during this time so that this can be compared to the recording.

7-day Holter monitor – this is the same device as above however the device can be

removed when the patient wishes to bath or shower.

Cardiac event recorder – this is useful if the patient’s symptoms are infrequent. There

are no wires or electrodes. The patient would hold this to the chest when they are having

symptoms.

Implantable loop recorder – this is a small device (approximately the size of a

computer memory stick). It is implanted under the skin in the upper left chest area. The

battery can last up to three years. When the patient experiences symptoms they hold a

hand-held activator over the loop recorder and press a button to record the activity.

For further information regarding any of these investigations, please see the British Heart

Foundation website: www.bhf.org.uk/heart-health/tests


14

References

12-Lead ECG. The Art of Interpretation. Garcia & Holtz.2001.

Making Sense of the ECG. A Hands-On Guide. Houghton & Gray 2003.

Cardiology. Lowe et al 1997.

Advanced Life Support manual. (Fifth Edition) Resuscitation Council UK 2006.

Some pictures have been reproduced thanks to Nigel Barraclough. First on Scene Training Ltd

Campbell B, Richley D, Ross C, Eggett CJ. Clinical Guidelines by Consensus: Recording a

standard 12-lead electrocardiogram. An approved method by the Society for Cardiological

Science and Technology (SCST) 2017. Available at:

http://www.scst.org.uk/resources/SCST_ECG_Recording_Guidelines_2017

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