ECG interpretation

V1 and V2

Septal view of the heart

V3 and V4

Anterior view of the heart

V5 and V6

Lateral view of the heart

Lead I

lateral view

lead II and III

inferior view

aVL

lateral view

aVF

inferior view

Standard limb leads

Bipolar

Leads I, II and III

Form Einhoven’s triangle

Augmented limb leads

Unipolar

aVR, aVL and aVF

Chest leads

V1-6

unipolar

Methods for calculating HR

R-R interval —> for regular rhythm

QRS complexes —> for irregular rhythm

R-R interval method

count number of large squares within one R-R interval

divide 300 by this number

QRS complex method

count number of QRS complexes in 10 secs

multiply by 6

Normal cardiac axis

represents the overall direction of electrical spread within the heart

In a normal person it is expected to lie between -30 (aVL) and 90 (aVF) degrees

positive deflection in lead I and II - indicates normal cardiac axis

(most positive in lead II - most closely aligned to overall electrical activity of the heart)

negative reflection in AVR (looks at heart in opposite direction)

Right axis deviation

depolarisation distorted to the right (90 to 180 degrees)

most commonly seen in RV hypertrophy eg. PHTN

or normal finding in tall individuals

lead I negative deflection and lead III more positive

Left axis deviation

depolarisation distorted to the left (-30 to -90)

normally due to LVH or conduction abnormalities

lead I = positive

lead II and III = negative deflection

Determining the cardiac axis

estimate using lead I and aVF

Lead I and aVF positive

normal cardiac axis (0 to 90 degrees)

lead I positive aVF negative

left axis deviation

lead I negative aVF positive

right axis deviation

lead I and aVF negative

extreme axis deviation

‘northwest axis’

atrial flutter

sawtooth baseline

atrial fibrillation

chaotic baseline

PR interval

should be between 120 and 200ms

prolonged PR interval suggests presence of AV delay/AV block

shortened PR suggests accessory pathway

Wolf-parkinson-white syndrome (WPW)

an accessory pathway leads to stimulation of the ventricles

allows electrical impulses to bypass the AV node

results in premature excitation of the proximal ventricles

coupled with normal conduction results in double excitation of the ventricles

shortened PR interval, widened QRS, delta wave

QRS complexes

normal <0.12s

broad complex = ventricular ectopic, BBB

Bundle branch blocks

unaffected ventricle contracts first

then electrical impulses move through the myocardium to the other side

results in slow and delayed depolarisation of the affected ventricle

loss in ventricle synchrony

therefore broader QRS

Right BBB

septal depolarisation from left to right

V1 - tall often notched R wave (rabbit ear pattern)

V6 - wide S wave

Left BBB

septal depolarisation right to left

V1 - deep S wave with little to no R wave

V6 - broad notched M-shaped R wave

distorted ST segment

ST segment

should be an isoelectric line

abnormalities indicate ischaemia or infarction

STEMI

significant when ST elevation >1mm in 2 or more contiguous limb leads

or >2mm in 2 or more chest leads

caused by acute full thickness MI

NSTEMI

ST depression

significant when equal to or more than 0.5mm in 2 or more continuous leads

ischaemia without complete artery blockage

can also see ST depression as a mirror image opposite territory with ST elevation

inferior wall

leads II, III and aVF

lateral wall

leads I, aVL, V5 and V6

anterior and septal wall

leads V1-4

Posterior wall

not directly seen on a standard 12 lead

requires additional leads V7, V8 and V9

Tall T waves

indicate hyperkalaemia or hyper acute STEMI

T wave inversion

normal variation (aVR, V1, III, V2 and V3)

new T wave inversion should be considered abnormal

non specific sign in pathological illness or specific for MI or myocarditis

U wave

not a common finding

>0.5mm deflection after the T wave

may indicate electrolyte abnormalities, hypothermia and anti-arrhythmia therapy

Things to think about when looking at ECG

rate

rhythm

axis

intervals

waveform

morphology

ST segments

ECG artifacts

muscle tremor (typically seen in limb leads not chest)

wandering baseline (breathing or poor electrode contact)

electrical interference (fuzzy baseline)