1- ECG - theory & practice

Question 1

what is the simplified physics of the ECG?

Answer 1

action potential through conducting system & muscles of the heart causes separation of charge/differences in potential between cardiac regions

• charges that are separated constitute an electrical dipole which is a vector with components pf magnitude & direction e.g. from atria to ventricles (these potential differences are what electrodes detect)

Question 2

what is being recorded in an ECG?

Answer 2

recording of potential differences (potential differences in the order of a millivolt (mV] between distant sites on the body surface - detected by electrodes placed on the skin coupled to a sensitive recording device) against time

Question 3

how many electrodes are placed on skin to record 12 lead ECG?

Answer 3

10 electrodes

Question 4

How do potentials arise?

Answer 4

at the body surface, arise from currents that flow when membrane potential of myocardial tissue is changing (depolarisation or repolarisation)

• only large masses of cardiac tissue (i.e. atrial & ventricular muscle) generate sufficient current to be detected at body surface as potential changes

Question 5

what is an ECG lead?

Answer 5

an imaginary line, the lead axis, between 2 or more electrodes (it is NOT the wire that connects electrode to recording device)

• in an ECG lead 1 electrode acts as a recording (positive) electrode (think like a seeing electrode)
• and when depolarisation moves towards recording electrode an upward deflection created and when depolarisation moves away from recording electrode, it generates a downward deflection on ECG

Question 6

what is isopotential?

Answer 6

when no movement towards or away from the recording electrode and therefore no deflection on the ECG

Question 7

what is each recording electrode detecting?

Answer 7

lead I = RA negative to left atrium positive

lead II = RA negative to LL positive

lead III = LA negative to LL positive

Question 8

what is interpretation of lead I detection?

Answer 8

Atrial depolarisation spreads from SA node inferiorly and to the left - depolarisation is moving towards the recording electrode in lead II producing a (normally) upward deflection in this lead - the P wave represents atrial depolarisation

(from RA negative to LL positive)

Question 9

what does RA, LA, LL stand for?

Answer 9

right arm, left arm, left leg

Question 10

what is the interpretation of lead II?

Answer 10

all about QRS complex = ventricular depolarisation

• downward deflection preceding R wave is Q wave
• deflection upwards (+ve) is R wave (irrespective is preceded by Q wave)
• downward negative deflection following R wave is S wave

Question 11

how long does QRS complex last?

Answer 11

0.1 seconds or less

Question 12

what causes QRS complex?

Answer 12

• ventricular depolarisation starts in the interventricular septum and spreads from left to right causing the small & narrow Q wave
• subsequently the main free walls (walls not connected to middle) of the ventricles depolarise causing a tall & narrow R wave
• finally, the ventricles at the base of the heart depolarise, producing a small & narrows S wave

Question 13

what is interpretation of lead III?

Answer 13

about T wave = represents ventricular repolarisation

• it is an upward (positive) reflection because the wave of repolarisation is spreading away from the recording electrode (think of it as negative charge moving away from recording electrode which is equivalent of positive charge moving towards it)

Question 14

for each wave - is depolarisation moving towards or away from recording electrode?

Answer 14

P wave - towards (atrial depolarisation)
Q wave = away (left to right depolarisation of interventricular septum)
R wave = towards (depolarisation of main ventricular mass)
S wave = away (depolarisation at base of heart)
T wave = repolarisation away from recording electrode but like opposite of depolarisation so like positive moving towards it so positive inflection

Question 15

what is the PR interval?

Answer 15

from the start of the P wave to start of the QRS complex

= reflects the time for SA node impulse to reach the ventricles (normally 0.12 - 0.2 seconds)

• very strongly influenced by the delay in conduction through the AV node

Question 16

what is the ST segment?

Answer 16

from the end of the QRS complex to the start of the T wave - normally isoelectric
= ventricles contract (systole)

Question 17

what is the QT interval?

Answer 17

from start of QRS complex to end of T wave
- primarily reflects the time for ventricular depolarisation & repolarisation
- normally 0.36 - 0.44s at heart rate of 60bpm (but needs to be corrected for heart rate)

Question 18

what does 12 lead ECG comprise of?

Answer 18

3 standard limb leads (I, II, III) - termed bipolar

3 augmented voltage (aV) leads (aVR, aVL, aVF) - termed unipolar

6 chest leads (V1-6) - aka precordial leads

Question 19

what do the 12 leads do?

Answer 19

provide a comprehensive picture of the heart in different directions & planes

vertical (frontal/coronal) = leads I, II, III, aVR, aVL, aVF

horizontal (transverse) = leads V1-6

Question 20

what does looking at leads from different directions allow you to do?

Answer 20

• determine the axis of the heart in thorax
• look for any ST segment or T wave changes in relation to specific regions of the heart. This is crucial e.g. in diagnosing ischaemic heart disease
• look for any voltage criteria changes. This is crucial e.g. in diagnosing chamber hypertrophy

Question 21

what is the hexaxial reference system?

Answer 21

6 views - of the heart in frontal plane provided by standard & augmented limb leads

Question 22

what are lateral leads?

Answer 22

leads I and aVL - each has recording electrode on left arm and views from the left

Question 23

what are the inferior leads?

Answer 23

leads I, III and aVF - each have recording electrode on left foot and views the heart from inferior direction

Question 24

what are the V1-6 leads?

Answer 24

the 6 chest leads - recording electrodes placed on chest walls at defined positions

• the electrodes of the standard limb leads are all linked together to effectively provide a reference electrode in the centre of the heart

Question 25

where do you place V1 - V6 chest electrode?

Answer 25

V1 - 4th intercostal space immediately right of sternum
V2 = 4th intercostal space immediately left of sternum
V4 = 5th intercostal space midclavicular line
V3 = midway between V2 & V4
V5 = same horizontal level as V4, anterior axillary line
V6 = same horizontal level as V4, mid-axillary line

Question 26

what chest leads look from what angle?

Answer 26

• V1&2 coming from right - looking at interventricular septum
• V3&4 looking at anterior
• V5&6 looking at lateral aspect (LV of heart)

Question 27

what lead best shows ECG waves?

Answer 27

lead 2

Question 28

what does TP segment?

Answer 28

ventricles relax (diastole)

Question 29

what is PR interval?

Answer 29

largely AV nodal delay (0.12-0.2 s)
from start of P wave to start of R wave

Question 30

what is QT interval?

Answer 30

depolarisation & repolarisation of ventricles (0.36-0.44 s at heart rate 60) = from start of Q wave to end of T wave

Question 31

how to calculate heart rate of ECG trace (paper speed 25mm/sec) ?

Answer 31

heart rate = 300/number of large squares between beats (for regular rhythm)

heart rate = 300/number of large squares between R-R interval

Question 32

what is ECG strip?

Answer 32

prolonged recording of 1 lead (best is lead 2)
= allows you to determine heart rate & identify cardiac rhythm

Question 33

what is the practical approach to analysing ECG?

Answer 33

1. verify patient details - name & DOB
2. check date & time ECG taken
3. check calibration of ECG paper
4. determine the axis (if possible - don’t want to focus on this now - will revisit in wk3)
5. workout heart rate & rhythm - use following rhythm strip ask yourself following questions
   
   1. is electrical activity present?
   2. is rhythm regular or irregular?
   3. what is heart rate? (300 / n.o large squares)
   4. P- waves present? tells you there is atria activity, makes you think if heart getting excited like normal
   5. what is the PR interval? should be 0.12 - 0.2 seconds
   6. is each P-wave followed by QRS complex?
   7. Is the QRS duration normal? no more than 0.1 seconds (100 ms)
6. Look at individual leads for voltage criteria changes OR any ST or T-wave changes

Question 34

what does a normal resting ECG not exclude?

Answer 34

• myocardial infarction (heart attack) - may or may not cause characteristic EG changes e.g. ST elevation
• intermittent rhythm disturbance! If suspected do ambulatory ECG recording for 24hrs or 7 days
• stable angina (form of ischaemic heart disease) if suspected do exercise ECG. Look for ST changes during/after exercise. usually absent at rest