7 The electrocardiogram

Question 1

Describe how excitation normally spreads through the ventricular myocardium

Answer 1

from endocardium to epicardium

repolarisation is opposite: after 280ms, repolarises from epicardium to endocardium

Question 2

Which situations will an upwards deflection be recorded on an ECG?

Answer 2

1. depolarisation towards a positive recording electrode

2. repolariation away from a positive recording electrode

Question 3

Which situations will yield a downwards deflection on an ECG?

Answer 3

1. depolarisation away from a positive electrode

2. repolarisation towards a positive recording electrode

Question 4

What are internodal tracts?

Answer 4

Bachmann bundles - how depolarisation spreads from right side to left side (from SAN)

Question 5

Describe in general terms the pattern of spread of excitation over the normal heart from the SAN to the AVN to the ventricles

Answer 5

SAN –> internodal tracts –> AVN –> bundle of His –> Left bundle branch –> right bundle branch –> Purkinje fibres (depolarise from endocardium to epicardium) contraction from apex of the heart up