6.16 - Electrocardiography Analysis

Question 1

What is the basic passage of electrical conduction through the heart?

Answer 1

1. SAN - spontaneously active cells, conducts current down atria and internodal pathways –> P-wave of ECG
2. AVN - current slows down between atria and ventricles = atrial muscle contraction to expel blood into ventricles
3. Septum and His-Purkinje system - current travels through septum –> His-Purkinje system. Rapid contraction through left and right bundles to Purkinje fibres leading to ventricular contraction –> QRS complex of ECG
4. cardiac muscle relaxes and membrane potential recovers/repolarises –> T-wave of ECG

Question 2

How many electrodes are placed and how many leads are generated?

Answer 2

• 10 electrodes –> 12 lead recording
• limb leads (I, II, III aVR, aVL, aVF) - coronal view
• chest leads (V1 to V6) - axial view

Question 3

What are chest (precordial) leads?

Answer 3

• V1 - 4th intercostal space (ICS), right margin of sternum
• V2 - 4th ICS along the left margin of the sternum
• V3 - midway between V2 and V4
• V4 - 5th ICS, mid-clavicular line
• V5 - 5th ICS, anterior axillary line
• V6 - 5th ICS, mid-axillary line

Question 4

What is axis deviation?

Answer 4

• change in heart position (seen in taller people)
• OR change in position of electrical conduction
• e.g. if right bundle branch gets damaged then left branch becomes more prominent, changing the conduction –> aVF graph will show greatest change in electrical conduction
• right and left axis deviation (RAD & LAD)

Question 5

Which leads suggest which types of axis deviation?

Answer 5

• 0 to -90 degrees –> LAD
• 90 to +/- 180 degrees –> RAD
• 0 = I
• 60 = II
• 90 = aVF
• 120 = III
• -150 = aVR
• -30 = aVL

Question 6

Where does lead II go to and from?

Answer 6

• from top right to bottom left and shows the biggest QRS wave of all the leads
• if we tilt the heart slightly clockwise, the heart configuration is different and the nodes are no longer parallel to the lead II = less deflection of the QRS wave

Question 7

What would changes in atrial conduction (with examples) do to the ECG?

Answer 7

• change the P-wave
• atrial fibrillation - loss of P-wave
• atrial flutter - sawtooth pattern
• this aberrant activity is caused by other cells in heart forming pacemaker potentials
• sometimes these little fluttery currents may not be big enough to pass through AV node to cause ventricular contraction

Question 8

What would happen to the ECG if there was a barrier/problem in conduction between atria to ventricles/at AVN (e.g. cell destruction)?

Answer 8

• impacts P-R interval - longer = bigger gap between P-wave and QRS complex initiation
• first degree heart block - delay in conduction (caused by a kind of ‘traffic jam’ at AVN)
• second/third degree heart block - loss of conduction caused by a proper blockage at AVN
• we get missing QRS complexes after P-waves and no T-wave either since that is associated with ventricular repolarisation

Question 9

What do the squares represent in an ECG?

Answer 9

• 25mm/sec
• small square = 40ms (0.04s)
• big square = 200ms (0.2s)
• 5 large squares = 1 second

Question 10

How do we calculate heart rate?

Answer 10

• bpm –> 60s = 300 large squares
• heart rate = 300/large squares (in one heart beat)
• e.g. 300/3.3 = 91bpm

Question 11

What can we find out about the rhythm from an ECG?

Answer 11

• P-R interval: small squares x 40 (e.g. 5 x 40 = 200ms)
• QRS duration: small squares x 40 (e.g. 2.5 x 40 = 100ms)
• ST segment

Question 12

How do we figure out the axis?

Answer 12

• using limb leads
• e.g. here lead I is biggest suggesting it is going along lead I
• but lead III upside down = going away from lead III
• between 0 and -30 degrees –> LAD?
• look at lead I and AVF (perpendicular to each other) - if peaks same direction then normal, if lead I up and AVF down (Leaving each other) = left axis deviation, if lead I down and AVF up (towards each other) = right axis deviation.