12 lead ecg delegate notes

12 Lead ECG Delegate Notes

Reviewed April 2019

© 2019 ECG – All rights reserved LessonPlans/ECG12Lead/Pack&Kit/R&I/Handout April 2019 2

Anatomy and Physiology of the Heart

o The heart consists of four chambers,

two atria and two ventricles.

o Blood empties from the atria into the

ventricles

o The left ventricle empties into the

systemic circulatory system

o The right ventricle empties into the

pulmonary system

o Veins bring blood to the heart, while

arteries take blood away from the heart

o Blood circulates around this system,

taking up oxygen in the lungs and giving

it up to the peripheral tissues

o Think of the heart as a pump!

The Heart consists of 3 layers:

o Pericardium – a thin outer

lining that protects and

surrounds your heart.

o Myocardium – a thick

muscular middle layer that

contracts and squeezes

blood out of your heart.

o Endocardium – a thin

inner lining. Inside the

heart there are four

chambers – two on the left

and two on the right.

Structure of the Heart


A normal heartbeat lasts

approximately 0.8 seconds and is

broken up into 3 parts.

0.1 secs- Atria Depolarising

0.3 secs- Ventricles Depolarising

0.4 secs- Ventricles Repolarising

This can be seen on an ECG known

as a Sinus Beat.

0.1

0.3 0.4

Atria Depolarisation

Ventricular Depolarisation

Ventricular Repolarisation

Conduction of the Heart

The electrical conduction system of the heart is made up of specialised cells, some of which are

specialised in pace making functions and some for the transmission of the impulse through the

heart. The main function is to create an electrical impulse and transmit it in an organised manner

throughout the heart. This process creates electrical energy that can be picked up by electrodes

when we perform an ECG.

1) The impulse is initiated from

the Sino-Atrial (SA) node, which

is the hearts natural pacemaker

and beats between

approximately 60-99 times per

minute.

2) The impulse travels down

through to the Atrioventricular

(AV) node which can beat as a

backup if the SA node fails at

around 40-60 bpm.

3) The impulse continues down

through The Bundle of His (>40

bpm)

4) The bundle of His branches off

into the left and right bundle

Branches

5) The Purkinje fibres then

innervate the myocardial cells.

Cardiac Cycle


How an ECG complex is formed

This is essential to understand when you are performing basic rhythm

interpretation. Have a look at the picture on the conduction page to compare again!

◼ Contraction of the heart is associated with electrical activity.

◼ Normally the hearts “pacemaker” (SA node) causes the atria to contract causing a P

wave.

◼ Electricity passes via the AV node down into the ventricles via the Bundle of His and

into the two bundle branches causing contraction (QRS).

◼ Ventricles then “repolarise” causing the T wave.

A wave is a deflection from the baseline that represents a cardiac event, for instance, the P

wave represents atrial depolarisation. 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).


The ECG Paper

This is useful when making accurate ECG diagnosis and deciding on treatment,

which you may or may not be involved in!

The ECG paper runs at a rate of 25mm per second, each little box 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

= 0.20 seconds. So, 5 large boxes make 1 second.

Each lead is represented for 2.5 seconds and the complete ECG is 10 seconds long.

The paper is also broken down into either 3 or 4 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).

When talking about the vertical height of a wave or segment we use millimetres, for example:

a wave that is 5 small boxes high would be 5 mm high. This is important when it comes to

deciding on appropriate treatment, especially for the ST segment. After treatment, the size of

the ST elevation will determine the success of the treatment! You may not be involved in this.

1 second = 5 large

squares5m

m =

0.5m

V

0.2 sec.

0.04

sec

1mm


How does the ECG ‘look’ at the heart?

This is helpful to understand the term ’12 lead ECG’

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

refer to the wires (electrodes) that connect the patient to the machine but are 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).

• Each limb electrode looks at the heart from the frontal plane and the angle is dependent on

which electrode it is.

For example, lead aVR looks at the heart from the viewpoint of approximately the patient’s right

shoulder. aVL looks from the left shoulder and leads aVF looks directly upwards from the feet.


Preparing the Patient

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

• That electrocardiograph 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.

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

their embarrassment. 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.

For example:

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 crocodile clips as occasionally gel gets onto the clips and this can affect contact.

Clean with alcohol wipes.


Recording an ECG

It is so important to get the recording right as poor recording could equal

inappropriate diagnosis and treatment!

• Begin by asking the patient to lie down and relax, this reduces electrical interference form

skeletal muscle.

• Attach the limb electrode tabs & electrodes first. (They are usually labelled and/or colour

coded.) To ensure consistency between recordings it is recommended that the electrodes

are attached to both arms and legs, slightly proximal to the wrist and ankle.

• It is imperative that recordings from other sites are labelled accordingly so that the results

are not confused with those obtained from standard sites.

• Best results are obtained over bony prominences as muscular areas produce tremor and

fatty tissue such as breast tissue distorts results (ask the patient to lift their breast).

• 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 (RL, 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’


Then attach the chest tabs and electrodes

(these need to be accurately positioned)

Electrode Position

V1 (C1)

V2 (C2)

V3 (C3)

V4 (C4)

V5 (C5)

V6 (C6)

Fourth intercostals space at the right sternal edge

Fourth intercostals space at the left sternal edge

Midway between V2 and V4

Fifth intercostals 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

• 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.


Once the ECG has been recorded and printed there are a few things that you need to check to

ensure that it is a good quality ECG.

• 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 – aVR looks at the heart from the right shoulder. The

electrical conduction of the heart travels away from this viewpoint which should lead to

a negative complex (the waveform is upside down). If aVR is positive, it often indicates

that the limb electrodes have been put on incorrectly.

• 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.

Examples of poor-quality ECGs

1. Wandering baseline

2. Muscle interference

3. Electrical interference


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. As the 12

lead ECG only captures a moment in time it may not pick up on abnormalities for example, the

patient may be having palpitations but unless this happens during the recording of an ECG,

the ECG may appear normal.

Other tests include:

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

heart that may 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 24-48 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 like the description 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


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

The Society for Cardiological Science & Technology. Clinical Guidelines by Consensus.

Recording a standard 12-lead electrocardiogram. February 2010

Emergency Care Gateway

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Alston Drive

Milton Keynes

MK13 9AP

Tel: 0845 423 8993

www.ecgtraining.co.uk

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