ECG (2) - manifestations of disease

Question 1

what information about cardiac function does the ECG provide?

Answer 1

• the electrical rhythm of the heart
• the** size of the muscle mass** of the individual chambers of the heart
• the site of any abnormal pacemaker activity
• disorders in cardiac conduction

Question 2

what is the sinus rhythm?

Answer 2

• any cardic rhythm in which depolarisation of the cardiac muscle begins in the SA node
• p wave is followed by the QRS complex

Question 3

what is an arrhythmia?

Answer 3

• any deviation from the normal sinus rhythm
• some types of arrhythmias can be serious and life threatening

Question 4

What is an example of an arrhythmia that is not of clinical concern?

Answer 4

• respiratory sinus arrhythmia
• characterised by faster heart rate during inhalation and slower heart rate during exhalation

Question 5

what is sinus bradycardia vs sinus tachycardia?

Answer 5

• brady - slow- ie slower heart rate
• tachy - fast - rapid heart rate - associated with exercise, stress etc

Question 6

how is the heart rate calculated in terms of the ECG?

Answer 6

• calculated from the time between peaks of successive R waves

Question 7

what is the normal range for heart rate?

Answer 7

• 60-100 beats per minute

Question 8

what are the different intervals of the ECG?

Answer 8

• P-R interval - varies with heart rate, but it is the time it takes for the AP to travel through the atria and the AV node
  * QRS interval- time taken for complete depolarisation of the ventricles
• QT interval - time taken for the complete depolarisation and repolarisation of the ventricles

Question 9

what is the electrical axis of the heart?

Answer 9

• the normal electrical axis is -30 degrees to 90 degrees
• it is the average direction of the ventricular depolarisation during ventricular contraction

Question 10

what disease is involved with a reduced PR interval?

Answer 10

• wolff-parkinson white syndrome
• the excitation of ventricles is not delayed by the AV node
• causes a faster heart rate

Question 11

What is ‘heart block’?

Answer 11

• refers to a defect in the conduction of the electrical activity of any point in the pathway betwen the SA node and the venrtcular muscle

Question 12

what are problems with conduction from the atria to the ventricles called?

Answer 12

• atrioventricular block

Question 13

what are problems with conduction through the bundle of His called?

Answer 13

Bundle block

Question 14

How are heart blocks classified?

Answer 14

based on their severity
* first degree
* second degree
* third degree

Question 15

What occurs in first degree heart block ?

Answer 15

• each wave of depolarisation starting in the SA node is conducted to the ventricles but there is an** abnormally long PR interval** - **long delay at the AV node **
• not normally a problem but may indicate other diseases eg coronary heart disease

Question 16

what occurs in second degree heart block?

Answer 16

• there is a failure of excitation passing to the ventricles
• PR interval becomes longer until one **P wave fails to initate the QRS complex **

Question 17

What occurs during third degree heart block?

Answer 17

• also called complete heart block
• the wave of excitation fails to excite the ventricles (there is an atrioventricular block)
• contains an abnormally shaped QRS complex, not associated with P waves- due to the ventricles showing a slow rhythm of their own

Question 18

what is cardiac ischaemia?

Answer 18

• lack of blood supply to the heart which is not sufficient to meet the metabolic requirements of cardiac tissues

Question 19

what is cardiac infarction?

Answer 19

• the death of cardiac tissue resulting from a failure of blood supply to the tissue.. ie arises from ischaemia
• also called a heart attack

Question 20

how does cardiac ischaemia and cardiac infarction influence the ECG?

Answer 20

• the damaged myocardio cells undergo depolarisation
• injury currents form
• they affect the ST segments
• during cardiac ischaemia, the ST segment is below the isoelectric axis - which indicates that the cells that repolarising earlier

Question 21

What is atrial fibrillation?

Answer 21

• abnormal heart rhythm that is caused by rapid contraction of the atria
• the ECG trace shows no P waves and the base line is very irregular

Question 22

What is ventricular fibrillation?

Answer 22

• ventricular fibers fail to contract in a normal way
  * very irregular - no waves are able to be detected
• the ventricles are** unable to pump blood around the body**
• can proceed to cardiac arrest ( stopping of the hearts function/ stops beating)