ECG

Question 1

how does electrical conduction occur in the heart?

Answer 1

• in the sinus rhythm, the SA node generates action potentials that automatically conduct
• rapidly through the atria - causing contraction
• slowly through the AV node
• rapidly through the bundle of his and left and right bundle branches
• rapidly through the purkinje fibres causing ventricular contraction

Question 2

what is an ECG?

Answer 2

• recording of potential changes at the body surface which arise from currents that flow when the membrane potentials of myocardial tissue is changing

Question 3

what is the purpose of an ECG?

Answer 3

• provides information about cardiac rate and rhythm, chamber size, the electrical axis of the heart
• main test to assess for myocardial ischaemia and infarction

Question 4

what is an electrical dipole?

Answer 4

• a vector with components of magnitude and direction eg from atria to ventricles
• this allows the electrical axis of the heart to be estimated

Question 5

what is magnitude determined by?

Answer 5

the mass of cardiac muscle that is involved in the generation of the signal - this atria and ventricles dominates

Question 6

what is direction determined by?

Answer 6

the overall activity of the heart at any instant time and varies during the cardiac cycle

Question 7

what is an ECG lead?

Answer 7

• the imaginary line, the . lead axis between two or more electrodes
• not the wire

Question 8

what happens when depolarisation moves towards/away from the recording electrode (positive) ?

Answer 8

• towards = generates an upward deflection on the ECG

- away from = generates a downwards . deflection on the ECG

Question 9

what is the 12 ECG composed of?

Answer 9

• 3 standard limb leads = bipolar
• 3 augmenteed voltage leads = unipolar
• 6 chest leads = precordial leads

Question 10

where are the standard leads placed?

Answer 10

1. RA - LA
2. RA - LL
3. LA - LL
   (all negative to positive so there is an upwards deflection)

Question 11

why is the P-wave upright?

Answer 11

• atrial depolarisation spread from SA to AV, inferiorly and to the left
• depolarisation is moving towards to recording electrode
• so an upward deflection is formed

Question 12

what does the QRS look like it does?

Answer 12

Q = ventricular depolarisation starts in the interventricular septum and spreads from left to right causing the small and narrow Q wave
R = the main free walls of the ventricles depolarise causing a tall and narrow R wave
S = the ventricles at the base of the heart depolarise, producing a small and narrow S wave

Question 13

what is the T wave an upright?

Answer 13

• it represents ventricular repolarisation but it is moving away from the recording electrode
• eg a negative signal moving away from a positive = positive signal

Question 14

what is the isoelctric line?

Answer 14

• regions where there are no potential changes

- the straight line

Question 15

what is the PR interval?

Answer 15

• dominated by the delay in conduction through the AV node

Question 16

what is the QT interval?

Answer 16

• reflects the time for ventricular depolarisation and repolarisation, normall in males 0.44 and 0.46 in females
• prolongation predisposes to disturbances of cardiac rhythm

Question 17

what do the augmented leads do?

Answer 17

• see heart from different angles

- one is positive, two are negative

Question 18

what do the precordial leads do?

Answer 18

• look at the heart from horizontal directions

Question 19

how do you place V1-V6?

Answer 19

1 = 4th intercostal space immediately right of sternum
2 = 4th intercostal space left of sternum
3 = midway between 2 and 4
4 = 5th space, mid clavicular line
5 = same horizontal line as 4, anterior axillary line
6 = same horizontal level as v4, mix axillary line

Question 20

how fast must standard calibration paper move?

Answer 20

25mm/sec

Question 21

what is ACG rhythm strip?

Answer 21

• prolonged reading of one lead
• usually 2 lead
• allows you to determine heart rate and identify the cardiac rhythm

Question 22

why are 12 leads required?

Answer 22

allows you to

• determine the axis of the heart in the thorax
• to diagnose ischaemic HD
• to diagnose chamber hypertrophy

Question 23

how do you calculate the rate of an ECG?

Answer 23

300 / number of large squares

Question 24

what are the signs of atrial fibrillation?

Answer 24

• no P waves

- irregularly irregular QRS

Question 25

what are the signs of an atrial flutter?

Answer 25

saw-toothed baseline

Question 26

what are the signs of a nodal rhythm?

Answer 26

regular QRS but no P wave

Question 27

what would cause a P wave to be absent?

Answer 27

AF, SAN block, nodal rhythm

Question 28

what would cause there to be dissociated P waves?

Answer 28

complete heart block

Question 29

what would cause P mitrile?

Answer 29

LA hypertrophy

Question 30

what would cause P pulmonale?

Answer 30

RA hypertrophy

Question 31

what would cause a wide QRS? (>120ms)

Answer 31

• ventricular initaiton
• condction defect
• WPW

Question 32

what would cause a long PR interval?

Answer 32

heart block

Question 33

what would cause a short PR interval?

Answer 33

• accessory conduction eg WPW
• nodal rhythm
• HPCM

Question 34

what would cause a depressed PR interval?

Answer 34

pericarditis

Question 35

what would cause an elevated ST?

Answer 35

• acute MI
• prinxmentals angina
• pericardiits - saddle shaped
• aneurysm - ventrciular

Question 36

what would cause a depressed ST?

Answer 36

ischaemia: flat
digoxin: down-sloping

Question 37

what are the features of a right BBB on an ECG?

Answer 37

wide QRS