ECG

Question 1

What is an ECG?

Answer 1

It is a graphic representation of electrical potential difference changes against time of the myocardium throughout the cardiac cycle.
The myocardium has electrical currents that sequentially depolarise each individual cell and result in a change in cellular morphology that allows muscle contraction.

The electro/chemical changes that cause myocardial contraction are shown by the cardiac action potential.

Question 2

What needs to happen to the myocyte for it to contract, and how does it happen?

Answer 2

For the myocyte (cardiac cell) to contract, the potential difference across the cellular (semi-permeable) membrane and must change from negative to positive in relation to the inside of the cell.

Changes in potential difference occur through the flow of ions through specialised ion channels in the cellular membrane and also relatively freely through gap junctions.

Question 3

Describe the cardiac electrical field (ie. when it is and isn’t present).

Answer 3

When a cardiac cell is depolarising or repolarising, different currents flow across the cell membrane at various points and a potential difference will occur between one part of the cell and another (a dipole).

With the flow of current along the cell surface, an external electrical field is set up around the dipole.

When the cell is depolarised or repolarised and at a resting potential, there is no difference in membrane potential at different points along the cell surface, therefore, there is no electrical field, despite the potential difference between the inside and outside of the cell.

Question 4

Describe the different parts of a typical ECG wave.

Answer 4

P WAVE: atrial depolarisation
PR SEGMENT: AV node delay
QRS COMPLEX: ventricular depolarisation (atria repolarising simultaneously)
ST SEGMENT: time during which ventricles are contracting and emptying
T WAVE: ventricular repolarisation
TP INTERVAL: time during which ventricles are relaxing and filling

Question 5

List what an ECG can tell us.

Answer 5

• size of the heart muscle (eg. by taller R wave)

- heart rate

Question 6

Define tachycardia and bradycardia.

Answer 6

TACHYCARDIA: a heart rate that exceeds the normal resting rate
BRADYCARDIA: abnormally slow heart action

Question 7

What are some ways to identify a normal sinus rhythm?

Answer 7

• is there a positive P wave before every QRS complex?
• is the PR interval normal (ie. no greater than 200 ms)?
• is the QRS duration normal (ie. less than 0.11 seconds)?
• is the QT interval normal (ie. it changes, but normally 0.33 ms in women and 0.44 ms in men)?
• is there (not a perfect, but) a good start?

Question 8

What are some pathological conditions that can produce characteristic ECG changes?

Answer 8

• atrial and ventricular premature contractions
• atrial tachycardia
• atriation fibrillation/ flutter
• sinus arrest and sinus block
• ventricular escape beats
• supra-ventricular arrhythmias
• AV nodal re-entrant tachycardia
• ventricular pre-excitation
• ventricular tachycardia
• ventricular flutter