Generation and reading of the

12 lead ECG

AWC Chow

The 12 lead ECG

Advantages

• Common clinical tool

• Independent marker of cardiac disease

• Non-invasive

• Rapid information acquisition

• Cheap

• Gold standard for arrhythmia management

The 12 lead ECG

Disadvantages

• Average of potentials

• Limited resolution

• Snapshot of activity

• Electrical and not haemodynamic data

Left bundle

Non-specialised atrial tissue

Anterior superior fascicle

Posterior inferior fascicle

Right bundle

+/-+

-

History of the ECG

• 1842 -Carlo Matteucci shows that an electric current accompanies each heart beat.

• 1874 - Sanderson and Page record the heart's electrical current with a capillary electrometer

• 1887 - British physiologist Augustus D. Waller publishes the first human electrocardiogram.

• 1901 - Einthoven develops the string galvometer

• 1910 – Eithoven’s triangle

Theoretical consideration

• Myocytes have a resting potential

• Transmembrane flux create voltage difference

- activation

• Cellular coupling cause rapid deploarisation

• Ionic flux seen ECG deflections

Theoretical considerations

• Resting state - no potential/field change

• Depolarisation - boundary potential change

• Represented as a dipole/vector

• Restitution of polarity: repolarisation

Theoretical considerations

• Greater muscle mass – Larger potential change

– Larger voltage changes of ECG

• Direction of activation dependent on– Site of initiation

– Specialised conduction system distribution

– Anatomical considerations

» Barriers (scar, valves)

» Muscle mass

I

IIIII

LARA

RL LL

aVL -30

I 0

II +60aVF +90

III +120

aVR +210

P

QRS

T

PR

QT

Diagnostic criteria for LVH

There are many different criteria for LVH.• Sokolow + Lyon (Am Heart J, 1949;37:161)

S V1+ R V5 or V6 > 35 mm

• Cornell criteria (Circulation, 1987;3: 565-72)

SV3 + R avl > 28 mm in men

SV3 + R avl > 20 mm in women

• Framingham criteria (Circulation,1990; 81:815-820)

R avl > 11mm, R V4-6 > 25mm

S V1-3 > 25 mm, S V1 or V2 +

R V5 or V6 > 35 mm, R I + S III > 25 mm

• Romhilt + Estes (Am Heart J, 1986:75:752-58)

Point score system

Causes of RBBB

• normal finding in children and tall thin adults

• right ventricular hypertrophy

• chronic lung disease even without pulmonary hypertension

• anterolateral myocardial infarction

• left posterior hemiblock

• pulmonary embolus

• Wolff-Parkinson-White syndrome - left sided accessory pathway

• atrial septal defect

• ventricular septal defect

Causes of LBBB

• left anterior hemiblock • Q waves of inferior myocardial

infarction • artificial cardiac pacing • emphysema • hyperkalaemia • Wolff-Parkinson-White syndrome - right

sided accessory pathway • tricuspid atresia • ostium primum ASD

ECG Analysis

• Rate 60-100b/min

• Rhythm SR

• PR <200ms

• QRS <120ms

• Axis -30 to +120

• QT interval <500ms

• ST segment

ECG