Electrocardiogram

Question 1

What is an ECG?

Answer 1

The ECG is a composite record of action potentials produced by all the heart muscle fibers during each heartbeat.

Question 2

How does an ECG work?

Answer 2

Electrodes are positioned on the arms and legs (limb leads) and at 6 positions on the chest (chest leads).

The electrocardiograph amplifies the heart’s electrical signals and produces 12 different tracings from different combinations of limb and chest leads.

By comparing these records with one another and with normal records,it is possible to determine if:

• the conducting pathway is abnormal
• the heart is enlarged
• certain regions of the heart aredamaged
• the cause of chest pain.

Question 3

What does the P wave represent?

Answer 3

Atrial depolarisation

Question 4

What does the QRS complex?

Answer 4

Rapid ventricular depolarisation

Question 5

What does the T wave represent?

Answer 5

Ventricular repolarisation

Question 6

What does P-Q represent?

Answer 6

Atrial systole

Question 7

What does Q-T represent?

Answer 7

Ventricular systole/atrial diastole

Question 8

What does T-P reperesent?

Answer 8

Ventricular diastole

Question 9

What does a large P wave indicate?

Answer 9

atrial enlargement

Question 10

What does a large Q wave indicate?

Answer 10

MI

Question 11

What does a large R wave indicate?

Answer 11

Enlarged ventricles

Question 12

What does a flat T wave indicate?

Answer 12

Heart isn’t receiving enough oxygen

Question 13

What does an elevated indicate?

Answer 13

Hyperkalemia

Question 14

What is a first degree block?

Answer 14

P-R interval greater than 0.2s

Question 15

What is a second degree heart block and explain the types?

Answer 15

This is where there is an intermittent absence of QRS complexes – and thus an indication that there is a blockage somewhere between the AV nodes and the ventricles.

Type 1:

• Progressive lengthening of the PR interval followed by an absence of the QRS, then a shortened PR interval and normal QRS, and the cycle begins again. The cycle is variable in length, and the R-R interval shortens with the lengthening of the PR interval

Type 2:

• This is where there is a regular rhythm, and a fairly constant PR interval, but every now and again there is an absent QRS (pictured above). basically for every QRS, there are 2 or 3 p waves.
  2: 1 and 3:1 conduction
• there is one normal cycle, then one cycle with an absent QRS (2:1) or there is one normal cycle, then two cycles without a QRS (3:1

Question 16

What is a third degree heart block?

Answer 16

This occurs when atrial contraction is normal, but no beats are conducted to the ventricles.

The ventricles are still excited by their own internal ‘ectopic pacemaker’ system! Thus the definition of complete heart block is:

P wave ~90/min (more p waves than QRS complexes)

QRS ~36/min

Variable PR intervals

No relationship between P wave and QRS complexes, but both are present.

Abnormally shaped QRS due to abnormal spread of conduction throughout ventricles

QRS will generally be broad (~160ms – as opposed to a maximum of 120ms in a normal heart – 4 little squares as opposed to 3 little squares)

Right axis deviation

Escape rhythms present (more on these later)

Question 17

What does sinus rhythm mean?

Answer 17

rhythm of the heart is being controlled by the SA node – i.e. this is the ‘normal’ way the rhythm of the heart

Question 18

What is sinus tachycardia?

Answer 18

Fast heart rate

Question 19

What is sinus bradycardia?

Answer 19

Slow heart rate