The basics of reading an ECG

The ECG:

Lead placement:

Limb leads:
- all in a coronal plane & thus can be used to measure the mean frontal axis
Chest leads:
- V1: right sternal edge, 4ICS
- V2: left sternal edge, 4ICS
- V3: half-way between V2 & V4
- V4: pt's apex beat
- V5: AAL
- V6: MAL
- additional leads:
  - V7: PAL
- NB. all leads subsequent to V4 are horizontal to V4

Paper speed:

usually 25mm/sec which results in:
each “big” square = 0.2secs
each “small” square = 0.04secs
can speed paper up to 50mm/sec esp. when interpreting tachycardias looking for P waves, etc.
NB. modern ECG machines, record ECG in 5 discrete, sequential time periods so pt's ECG status may have altered during the performance of the ECG and this will be reflected as differences between the ECG sectors:
- I, II & III are recorded together
- avR, avL & avF are recorded together
- V1-3 are recorder together
- V4-6 are recorded together
- finally, the rhythm strip is recorded which is usually lead II

Systematic ECG interpretation:

What is the rate?

count number of large squares in one RR interval and divide into 300:
- 1.5 squares = 200 bpm
- 2 squares = 150 bpm (could this be atrial flutter with 2:1 block)
- 3 squares = 100 bpm (faster than this is tachycardia)

What is the rhythm & how has the depolarisation been initiated?

regular:
- tachycardias:
  - narrow complex:
    - sinus tachycardia
    - AV nodal reentrant tachycardia
    - atrial tachycardia
    - atrial flutter with fixed AV conduction
  - wide complex (see wide complex tachycardia):
    - VT
    - Torsade de Pointes
    - SVT with BBB or aberrancy
    - atrial flutter with WPW conduction
    - pre-excited (circus movement) tachycardia (WPW anterograde conduction)
- bradycardias:
  - sinus bradycardia
  - AV block
irregularly irregular:
- AF:
  - narrow complex
  - broad complex if WPW anterograde conduction or intraventricular conduction defect
- atrial flutter with variable block
- VF
regularly irregular:
- multifocal atrial tachycardia
- tachycardia

what is the mean frontal QRS axis?

the mean frontal axis is the sum of all the ventricular forces during ventricular depolarisation
determined by analysing the 6 limb leads which are in a coronal plane
axis directions of the limb leads (NB. 0 degrees is taken here as being the East position of a compass):
- aVL = -30deg
- I = 0 deg
- II = 60deg
- aVF = 90deg
- III = 120deg
- aVR = -150deg (ie. 210deg)
normal axis is -30deg to + 120deg
left axis deviation (LAD):
- more negative than -30deg
- see left axis deviation (LAD)
- aetiology includes:
  - Left Bundle Branch Block (LBBB) or left ventricular hypertrophy (LVH)
  - LAHB
right axis deviation (RAD):
- more positive than +120deg
- see right axis deviation (RAD)
- aetiology includes:
  - Right Bundle Branch Block (RBBB) or right ventricular hypertrophy (RVH)
rules of thumb:
- if there is an isoelectric complex in the limb leads then the axis is 90deg to that lead, check which way by looking at direction in other leads
- if predominantly +ve in both I & II then it is normal
- if predominantly -ve in all I, II & III then it is in “no man's land” ie NW quadrant & +ve in aVR:
  - ventricular focus highly likely if leads are correctly placed
mathematical determination:
- create a right angle triangle from 2 resultant vectors:
  - horizontal vector side determined by net sum of lead I complex:
    - length = net sum in mm (ie. subtract height of negatively directed components from height of positively directed components)
    - direction of vector: to right if net sum is +ve
  - vertical side determined by net sum aVF is attached to arrow end of horizontal vector:
    - length = net sum in mm
    - directed downwards if net sum +ve
- the axis is the angle between the horizontal vector & the hypotenuse:
  - ie. Inverse Tan (net sum aVF / net sum I) (nb. ensure + or - included here)

P wave shape:

P waves are best seen in leads II & V1
if inverted in lead I and upright in aVR then arm leads are reversed or dextrocardia is present, but in dextrocardia there is loss of R waves in V4-6
P mitrale:
- wide P waves >= 0.12secs
- notching of wide P wave in lead II with distance b/n peaks > 0.04sec
- biphasic P wave in V1 - if P-terminal force (depth of inversion x width) > 0.04
P pulmonale:
- tall, peaked P waves:
- height in lead III > lead I
- >= 3mm in an inferior lead

P-R interval:

normally 0.12-0.21sec
shortened PR interval:
- WPW
- normal variant
- low atrial or upper AV junctional rhythms
- accelerated AV conduction
- phaeochromocytoma
- glycogen storage disease
- Fabry's disease
prolonged PR interval:
- no dropped beats ⇒ 1st degree heart block
dropped beats:
- dropped beat after successive increasingly long PR interval ⇒ Wenckebach (Mobitz type I)
- at least 2 regular, consecutive impulses conducted with same PR ⇒ Mobitz type II
- no conduction ⇒ complete HB

QRS width:

normally < 0.12sec

QRS height:

QRS morphology:

Tall R waves in V1:

thin chest wall or normal variant
RBBB - NB. slurred S wave in leads I, V5 & V6
RVH
WPW
true posterior infarction - assoc. inf. AMI & no slurred S in V5/6, T wave upright in V1-2
HOCM
Duchenne's muscular dystrophy
low placement of leads V1-2

RSR' or RSr' pattern in V1 or V2 (ie. M-shape):

QRS > 0.10sec and slurred S in leads I, V5/6:
- RBBB:
  - aetiology:
    - normal variant
    - IHD
    - HT
    - RhHD
    - CHD esp. ASD, VSD, Fallot's tetralogy
    - pericarditis, myocarditis
    - PE
    - cor pulmonale
    - cardiomyopathy
    - Chaga's disease
    - endomyocardial fibrosis
    - other conditions
  - unlike LBBB, usually does not interfere with ECG Dx of AMI
  - see RBBB
QRS 0.09-0.10sec and slurred S in leads I, V5 or 6:
- incomplete RBBB:
  - < 1% of normal individuals
  - secundum ASD (90% of these pts have incomplete RBBB)
QRS < 0.08sec and no slurred S in leads I, V5 or 6:
- RSr' variant:
  - aetiology:
    - 5% of normal individuals
    - recording artifacts
    - chest deformities - straight back syndrome, pectus excavatum
    - CHD eg. ASD & rarely VSD or coarctation aorta
    - acquired heart disease - eg. MS
    - RVH
    - RV diastolic or volume overload
    - cor pulmonale
    - pulm. embolism
    - WPW
    - AVNRT - should have pseudo-S wave in leads II, III & avF
    - Duchenne's muscular dystrophy
    - late activation of outflow tract of RV eg. crista supraventricularis

LBBB:

ECG:
- QRS duration > 0.11sec
- M shape in leads I, V5 or V6
- QS complex usually present in leads facing RV ie. V1
  - but <50% have small r wave in V1
- ST elevation common in V1-4 with ST-T waves opposite in direction to terminal QRS direction
aetiology:
- normal hearts but extremely rare in children
- IHD - these pts have high incidence of LV dysfunction & CCF
- cardiomyopathy
- degenerative diseases
if present, cannot make diagnosis of LVH & Dx of AMI is difficult - see ECG Diagnosis of AMI
see LBBB

Pathologic Q waves:

> 25% of succeeding R waves
> 0.04secs wide
see ECG Diagnosis of AMI

Q-T interval:

the QT interval on the ECG is measured from the start of QRS complex to end of the T wave
QTc refers to a QT interval “corrected” for the effect of differing heart rate to give a QT value as if the heart rate is 60bpm
if rate 60-100bpm, then QT should be less than half the RR interval
Bazett's formula: QTc = QT / square root (RR interval)

prolonged QTc:

prolonged QTc represents delayed repolarisation & predisposes to early after-depolarisations, re-entrant tachycardias, especially potentially fatal torsade de pointes VT which is most likely if bradycardia is present too.
see prolonged QTc

shortened QTc:

not of significance
aetiology:
- hypercalcaemia
- hyperkalaemia
- digoxin intoxication

ST segment:

ST elevation:
- see also:
  - an approach to the ECG with ST elevation
  - ECG Diagnosis of AMI
- aetiology:
  - AMI, variant angina
  - benign early repolarisation
  - pericarditis
  - hyperkalaemia
  - Left Bundle Branch Block (LBBB) or left ventricular hypertrophy (LVH) (right precordial leads)
  - mechanical (pericardiocentesis needle striking ventricle)
  - electrical artefacts (artificial pacemaker, monitor)
  - hypothermia
  - ventricular aneurysm

T wave:

normal T wave is asymmetric with steeper descending limb than ascending limb
inverted T waves:
- normal in V1-3 in prepubertal children
- V1-3 in older patients may represent Wellens syndrome
- aVL T inversion may represent a mid segment LAD stenosis
- Tinv in ant and inf leads with SOB is most likely due to PE
- Tinv in many leads with prolonged QTc suggests SAH
- myocardial ischaemia
- digoxin
- myocardial strain
  - eg PE
- very deep inverted T waves with prolonged QTc suggests raised intracranial pressure (ICP) such as subarachnoid haemorrhage (SAH)
  - see https://lifeinthefastlane.com/ecg-library/100-ecgs/quiz-ecg-012/
peaked T waves:
- aetiology:
  - acute myocardial ischaemia (the hyperacute T wave) - symmetric wave form esp. if > 5-7mm height
  - benign early repolarisation
  - Left Bundle Branch Block (LBBB) or left ventricular hypertrophy (LVH) (right precordial leads)
  - hypertrophic cardiomyopathy
  - hyperkalaemia
  - subarachnoid haemorrhage (SAH)
  - acute haemopericardium
flattened T waves:
- aetiology:
  - normal newborn infants
  - digoxin
  - hypothyroidism
  - hypokalaemia
  - pericarditis
  - myocarditis
  - myocardial ischaemia
biphasic T waves:
- consider life threatening Wellen's syndrome
- long sinus like inverted T waves with prominent U wave suggests life threatening severe hypokalaemia
De Winter's waves
- combination of ST depression with rocket-shaped T waves in the precordial leads V1-6
- becoming increasingly recognised as an anterior STEMI equivalent (~2% of LAD occlusions)
- see https://lifeinthefastlane.com/ecg-library/100-ecgs/quiz-ecg-019/

U wave:

may be prominent in hypokalaemia but may be present in normal pts

Table of Contents

The basics of reading an ECG

The ECG:

Lead placement:

Paper speed:

Systematic ECG interpretation:

What is the rate?

What is the rhythm & how has the depolarisation been initiated?

what is the mean frontal QRS axis?

P wave shape:

P-R interval:

QRS width:

QRS height:

QRS morphology:

Tall R waves in V1:

RSR' or RSr' pattern in V1 or V2 (ie. M-shape):

LBBB:

Pathologic Q waves:

Q-T interval:

prolonged QTc:

shortened QTc:

ST segment:

T wave:

U wave: