ECGs #2 - Some Basics

Conduction of the heart
SA Node - AV Node - Bundle of His - Right Bundle Branch & Left Bundle Branch (posterior & anterior fascicles) - Purkinje Fibres

Leads - "an electrical picture of the heart"
The ECG views the heart from two planes.

The 'standard' or limb leads view the heart vertically (e,g, from sides and feet), while the chest leads view it horizontally (e.g. straight on and left side).

It's like someone has poked a bunch of holes in the chest, and you only catch glimpses of the heart through each, and you assemble what must be going on in your head. It would be great if you had X-ray vision like Superman and could just *buzz* and see what's happening to a person's chest, but y'know - real life and all that jazz...

The different views of the heart:-
I, II and VL - Left Lateral
II, III and VF - Inferior
VR - Right Atrium
V1 and V2 - Right Ventricle
V3 and V4 - Interventricular Septum/Anterior of Left Ventricle
V5 and V6 - Anterior and Left Lateral of Left Ventricle

or another way to frame it...

I, VL, V5 & V6 = Lateral Leads
II, III and VF = Inferior Leads
V1, V2, V3 & V4 = Anterior Leads

Depolarisation

When the the muscle is depolarising towards a lead, the lead records it as the the stylus going up, and when the electrical wave moves away from a lead, the leads moves down. You might think of it like the doppler effect. As an ambulance drives towards you, its siren goes up in frequency, and as it whizzes past, the sound dies out.


Quickly calculating the rate
R-R Interval (in large squares) divided into 300

P Wave
Atrial Contraction
Usu. 3x3mm
Usu. assess in Lead II

PR Interval
Beginning of P wave to beginning of QRS complex
Conduction through SA Node - Atria - AV Node - Bundles of His - Ventricular Muscle
n. = 120 - 200ms

QRS Complex
Ventricular Conduction
n. = 120ms

Q Wave - first negative deflection after P
 - Normal in Lateral Leads and Lead III - 'septal Q waves', as the IV septum depolarises from left to right
R Wave - first positive deflection after P
S Wave - first negative deflection after R

Transition Point
As waveform progress from V1 to V6, the R waves starts small, and then get bigger, while the S wave starts big and eventually disappears. The point where the R wave = the S waves in the QRS complex is know as the 'transition point', where the IV septum should theoretically lie.
n. V3 & V4

Clockwise Rotation
If the TP shifts towards more lateral leads, then the right ventricle is taking up more space in the precordium. When the heart is seen from below this moving of the TP is clockwise, and therefore known is as 'clockwise rotation'.
This occurs in chronic lung disease.

VR tends towards having the inverse picture from lead II. This is normal. Don't panic.