Lecture 9 – The Electrocardiogram

what does an ECG measure?

the summed electrical activity of the heart over time

does an ECG record APs directly?

no, it reflects the overall depolarisation and repolarisation

what does the P wave represent?

atrial depolarisation

why is the p wave small and wide?

small muscle mass + slow conduction

what does the QRS complex represent?

ventricular depolarisation

why is the QRS complex large and narrow?

large ventricular mass + rapid conduction via Purkinje fibres

what does the T wave represent?

ventricular repolarisation

why is the T wave upright?

repolarisation occurs in opposite direction to depolarisation

what does the PR interval represent?

time from atrial depolarisation to ventricular depolarisation

normal PR interval?

0.12 to 0.20 seconds

what does the QRS duration represent?

ventricular depolarisation time

normal QRS duration?

< 0.10 seconds

what does the QT internal represent?

total ventricular AP duration

the ST segment is _____ because all ventricles are depolarised

isoelectric

what’s a dipole?

pair of equal and opposite charges

in cardiac dipole, which side is depolarised tissue?

negative end

direction of electrical vector

from negative → positive

when is a positive deflection seen on ECG?

when dipole points towards the positive electrode

what does the Q wave represent?

septal depolarisation (left to right)

what does the R wave represent?

depolarisation toward apex

what does the S wave represent?

depolarisation toward base

how many leads are in a standard ECG?

12

which plane do limb leads view?

frontal plane

which plane do chest (V1 – V6) leads view?

horizontal plane

which lead is commonly used as a rhythm strip?

lead II

Einthoven’s Law: lead I + lead III = ?

lead II

what is the mean electrical axis?

average direction of ventricular depolarisation

normal QRS axis range?

-30° to +110°

what is left axis deviation (LAD)?

axis between -30° and -90°

what is right axis deviation (RAD)?

axis > +110°

what causes a widened QRS (>0.12s)?

slow ventricular conduction

what does a prolonged PR interval indicate?

AV block

what does ST elevation indicate?

myocardial infarction

what do pathological Q waves indicate?

old myocardial infarction

what does T wave inversion suggest?

myocardial ischemia

ECG hallmark of atrial fibrillation?

irregular RR intervals + absent P waves