Intro to ECG

05/02/23 © Clinical Skills Resource Centre, University of Liverpool, UK 1

Basic electrocardiogram (ECG)


Basic electrophysiology of conduction of electrical impulse - sequence of events

Sino-atrial node depolarisation

Atrial depolarisation Atrio-ventricular node

depolarisation Bundle of His Right and left bundles Ventricular depolarisation Ventricular repolarisation


The HeartR

As a result of Atrial & Ventricular depolarisation a visual representation is produced on the 12 lead ECG or on a cardiac monitor.

This is one complete cardiac cycle.


Relationship of electrical events to ECG

SA node Atrial depolarisation (P

wave) AV node (main component

of PR interval) Bundles of His and

ventricular depolarisation (QRS)

Ventricular repolarisation (T wave)


The Iso-electrical Line

This represents the resting potential of the heart. The electrical events of the cardiac cycle will be represented by deflections away from this line.


Sino-atrial node depolarisation

The events of the cardiac cycle are initiated by depolarisation of the sino-atrial node


Atrial Depolarisation (P Wave) The wave of electrical

depolarisation is conducted through the cardiac muscle of both atria


Atrial Contraction (P Wave)The depolarising

wave causes contraction of the atria, pushing blood into the ventricles


AVN depolarisation (PR Interval) The wave of depolarisation

reaches the atrio-ventricular node which depolarises and conducts, but slows the wave


Ventricular depolarisation (QRS Complex)

The AVN conducts the depolarisation to the Bundle of His

The wave of depolarisation quickly moves through the specialised conducting tissue


Ventricular contraction (QRS Complex)

The co-ordinated, synchronised depolarisation is depicted below

This produces an effective contraction of both ventricles


Ventricular Repolarisation (T Wave)

After depolarisation and contraction the ventricles repolarise, returning to the resting potential.


Taking a 12 lead ECG


12 Lead ECG12 views of the heart

6 chest leads

6 limb leads

Only 10 wires


The limb leadsPositioning the limb

leads

AVR AVL

RL AVF

Black Green

Red

Yellow

Position of the electrodes for limb leads are just above:

Right wrist AVR Left wrist AVL Left ankle AVF Right ankle (earth)

Right Left

Please note if placed elsewhere this must be clearly documented on the ECG to avoid potential misinterpretation of the recording


The chest leads V1 - 4th ICS RSE V2 - 4th ICS LSE V3 - Midway between V2 & V4 V4 - 5th ICS MCL V5 – Horizontal with V4 AAL V6 – Horizontal with V4 MAL

V1 V2 V3 V4 V5 V6

Sternomanubrial joint - Angle of Louis

ICS = Intercostal spaceSE = Sternal edgeCL = Clavicular LineAL = Axillary Line

The patient


Wash your hands, introduction yourself and check patient identity

Explain the procedure, warn the patient they will need to expose their chest (including removing any underwear) as well as their ankles and wrist.

Gain consent, consider a chaperone.

Patient should lie supine (if unable this should be recorded on the ECG as it may alter the appearance of the trace.

Skin Preparation


You may need to remove chest hair to ensue adequate contact with the skin, remember to seek consent and follow Trust policy.

If the electrodes will not fix to the skin, then light exfoliation with a paper towel, gauze swab or tape designed for the purpose.

Sometimes cleaning the skin helps remove any oils or creams applied to the skin, please follow Trust policy (ranges from soap and water to alcohol swabs). Avoid cleaning broken or dry skin.

Once the electrodes are in place, cover the patient with a gown to maintain dignity.

Recording an ECG


The patient should be as relaxed as possible, with arms at the side of them and supported by the bed

Many machines require you to enter the patients details electronically before recording, alternatively they must be written on the trace immediately after.

The patient should be encouraged to stay as still as possible

Press to record (usually start or auto)

Lots of interference record? Perform a second recording with the filter button selected, if filter selected this must be recorded on the ECG recording.


Relationship of limb and chest leads The chest leads look at the

heart across the horizontal plane

The limb leads look at the heart in a vertical plane

Leads aVR, aVL and aVF look from three separate directions

The bipolar leads of I, II and III are summation of potential differences between limb leads

I

II III

V1

V2V3

V4

V6

V5

aVR aVL

aVF

Bipolar leads view when myocardial conduction is normal

Lead I is the sum of the potentials from the left arm and right arm electrodes and looks at the left lateral surface of the heart

Lead II is the sum of the potentials between the AVR and AVF and also looks at the left lateral surface (and inferior surface)

Lead III is the sum of AVL and AVF look at the inferior surface


I

IIIII


12 lead ECG please see full size ECG at the back of the study guide


Positive / Negative Deflections

Positive deflections above the Iso Electrical line mean the electricity is flowing towards that lead

Negative deflections below the Iso Electrical line mean the electricity is flowing away from that lead


ECG Changes in Relation to LeadLEAD AVR AVR

Lead AVR is the view from the right superior aspect of the heart. The electrical impulse’s is moving away from the electrode and therefore the deflections are away from the isoelectric line (and look upside down). This is normal.


ECG Changes in Relation to Lead

LEAD AVF

AVF

Lead AVF is the veiw from the inferior aspect of the heart. The electrical impulse is moving directly to the electrode and therefore the deflections are above the isoelectrical line


ECG Changes in Relation to Leads

Look at the chest leads V1 – V6 .The electrical impulse in V1 is moving away

from the electrode, the resulting complex is below the isoelectric line.

V2 is less negative as the impulse is moving more towards the electrode than V1.

This continues across the chest leads.Therefore V6 is the most positive.


ECG paper ECG paper runs at a

standard speed of 25 mm/second

Standard calibrated paper is used:

each large square is equivalent to 0.2 seconds

each small square is equivalent to 0.04 seconds

the vertical scale is standardised at 1 millivolt per cm


12 Lead ECG (normal) PR Interval ( 3-5 small

squares 0.12 – 0.20 secs) QRS Complex (2-3 small

squares 0.08 – 0.12 secs) ST Segment < 3 small

squares deflection from Iso electrical line in health

Occasionally a small negative deflection is seen after the T wave in health and (no significance)

R

12 Lead ECG

QRS Complex

RR Interval

STSegment

TWave

QRS

QT

PR

PWave


Basic rhythm assessment


How to read a rhythm strip1. How is the patient?

Always treat the patient not the monitor.2. Is there any electrical activity?

If you can see deflections above and/or below the isoelectric line then there is electrical activity.

3. What is the ventricular (QRS) rate?4. Is the QRS rhythm regular or irregular?

Measure 2 consecutive R waves and then transpose that measure onto the next 2 R waves and see if they are the same. If they are the same the then rhythm is regular.


What is the ventricular rate? Normal 60 -100 per minute <60 = bradycardia >100 = tachycardia To calculate rate

Count number of QRS complexes in a given number of seconds (e.g. 5 sec. = 25 large squares), then calculate rate per min (multiply by 12 for a 5 second period), or

Count number of large squares between consecutive R waves and divide into 300, or

Count number of small squares between consecutive R waves and divide into 1500

NB last two methods apply if rhythm is regular


RR interval

Paper speed = 25 mm/second

Each small square = 0.04 seconds(= 1/25 sec)

Five small squares = 0.2 seconds(= 1/5 sec)

Five large squares = 1 sec

Rate = 300/RR interval (in large squares) or= 1500/RR interval (in small squares)

ECG paper timings

PP interval


How to read a rhythm strip5. Is the QRS width normal or prolonged?

The QRS complex should measure < 3 small squares <0.12 seconds.

6. Is atrial activity present? (If so, is there a normal P wave or some other atrial activity) A normal P wave is rounded.

7. How is atrial activity related to ventricular activity? Is there a P wave for every QRS complex. Is the P-R interval within normal limits.


Basic electrocardiogram (ECG) interpretation

ECG - common rhythm abnormalities


BradycardiaRate <60May be

sinus (normal PR interval) heart block

first second third


Heart block1st degree

retains 1:1 relationship (P:QRS) slowed AVN conduction prolonged PR interval (>0.2s)

2nd degree loss of 1:1 relationship dropped QRS complexes

3rd degree complete dissociation of P and QRS waves idio-ventricular rate (~40-50/min)


2nd degree heart blockMobitz type 1

PR interval progressively lengthens until a QRS complex is dropped

Mobitz type 2 PR interval constant, but a QRS complex is

periodically dropped dropped QRSs may occur in runs


TachycardiaRate >100May be

Narrow complex (impulses are initiated above the ventricles and follow normal conductive pathway)

Broad complex (impulses are initiated at the ventricles or are aberrantly (abnormally) conducted through the ventricles)


Narrow complex tachycardia

Atrial Rate = 140 Ventricular Rate = 140 Rhythm = Regular QRS complex = 0.08 (2 small squares) P-R interval = 0.16 (4 small squares)


Atrial fibrillation

Atrial Rate = unable to determine Ventricular Rate = approx 100 Rhythm = Irregular QRS complex = 0.08 (2 small squares) P-R interval = unable to see P waves therefore no P-R interval


Broad complex tachycardia

Atrial Rate = No P waves Ventricular Rate = 220 Rhythm = Regular QRS complex = Wide (0.20 second, 4 small squares) P-R interval = No P waves therefore no P-R interval


1st degree heart block

Atrial Rate = 60 Ventricular Rate = 60 Rhythm = Regular QRS complex = Normal 0.06 (1.5 small squares) P-R interval = Prolonged 0.28 seconds (7 small squares)


2nd degree Heart Block Mobitz I

Atrial Rate = 80 Ventricular Rate = 60 Rhythm = irregular QRS complex = normal (0.08 second, 2 small squares) P-R interval = Progressively getting longer until QRS complex is dropped.


2nd degree Heart Block Mobitz II

Atrial Rate = 80 Ventricular Rate = 60 Rhythm = irregular QRS complex = normal (0.08 seconds, 2 small squares) P-R interval = constant P-R interval with intermittently dropped

QRS complexes.


3rd degree heart block

Atrial Rate = 80 Ventricular Rate = 40 Rhythm = P waves are regular, QRS complexes are regular QRS complex = Wide (0.20 second, 4 small squares) P-R interval = No measurable P-R interval, The atria and ventricles are

producing impulses independently.


Ventricular fibrillation

Atrial Rate = unable to determine Ventricular Rate = Unable to determine Rhythm = irregular (erratic) QRS complex = Wide bizarre P-R interval = none


Ventricular asystole

Atrial Rate = None Ventricular Rate = None Rhythm = No rhythm QRS complex = None P-R interval = None

Intro to ECG

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