Electrocardiography Revisited

Question 1

What is the clinical relevance of the ECG?

Answer 1

Conduction abnormalities
Structural abnormalities
Perfusion abnormalities

Question 2

Are electrodes, cables / wires, and leads all the same thing?

Answer 2

No

Electrodes - sticky parts that stick onto the skin e.g. chest

Cables / wires - connect to the electrodes

Leads - views of the heart (i.e. the graph on the ECG)

Question 3

What is the job of the leads?

What is a vector?

Answer 3

Pick up cardiac vectors

A quantity that has both, magnitude and direction

Question 4

What are the advantages of ECGs?

Answer 4

Relatively cheap and easy to undertake
Reproducible between people & centres
Quick turnaround on results/report

Question 5

What happens if the wave of excitation travels toward the negative electrode?

Answer 5

Downward deflections are towards the anode (-)

Question 6

What happens if the wave of excitation travels toward the positive electrode?

Answer 6

Upward deflections are towards the cathode (+)

Question 7

What does the isoelectric line represent?

Answer 7

Represents no net change in voltage. i.e. vectors are perpendicular to the lead

Question 8

What does the steepness of the line relate to?

What does the width of the line relate to?

Answer 8

Steepness of line denotes the ‘velocity’ of action potential

Width of the deflection denotes the ‘duration’ of the event

Question 9

What is the P wave?

What is the QRS complex?

What is the T wave?

Answer 9

The electrical signal that stimulates contraction of the atria (atrial systole)

The electrical signal that stimulates contraction of the ventricles (ventricular systole)

The electrical signal that signifies relaxation of the ventricles

Question 10

What comprises the SAN?

Answer 10

Autorhythmic myocytes

Small amount of muscle that points more positive than negative

Question 11

What does the action of the SAN show on an ECG?

Answer 11

Deflection is wide (slow)
Not very high (thin muscle)
Positive

Question 12

What does the action of the AVN show on an ECG?

Answer 12

AVN depolarisation
Isoelectric ECG
Slow signal transduction
Protective

Question 13

What does the Bundle of His divide into?

What does the action of the bundle branches show on an ECG?

Answer 13

2 bundle branches

Sharp but small downward spike - due to thin wall of muscle

Question 14

What does the action of the purkinje fibres show on an ECG?

Answer 14

Ventricular depolarisation, followed by late ventricular depolarisation

Upward stroke, QRS peak

Question 15

What does the action of the ventricular repolarisation show on an ECG?

Answer 15

Period of isoelectric status

Slow, domed T-wave

Question 16

Where do you place the leads?

Answer 16

Lead I (one L)
Right Arm to Left Arm

Lead II (two L’s)
Right Arm to Left Leg

Lead III (three L’s)
Left Arm to Left Leg

Question 17

What is the rule of reading for the leads to figure out which are the anodes / cathodes?

Answer 17

English is read left to right and top to bottom.

Polarity does that too.
Drawn as a triangle and reading left to right and top to bottom the first electrode of each pair you reach is the anode (-ve)

Right arm anode
Left Leg cathode
Left arm both

Question 18

Where are the V1, V2, V3 and V4 electrodes placed?

Answer 18

V1 - Right sternal border
In the 4th intercostal space

V2 - Left sternal border
In the 4th intercostal space

V3 - Halfway between V2 and V4

V4 - Mid-clavicular line
In the 5th intercostal space

Question 19

Where are the V5 and V6 electrodes placed?

Answer 19

V5 - Anterior axillary line
at the level of V4

V6 - Mid-axillary line
at the level of V4

Question 20

An ECG graph paper has big squares, that are further divided up into smaller squares?

What do each of these squares represent?

Answer 20

Big square - 0.2s, 0.5mV

Little square - 0.04s, 0.1mV

Question 21

What else is seen on an ECG graph?

Answer 21

Top left = details of the patient e.g. name, sex, age

Bottom = date and printing speed

Left bottom = where the ECG was taken e.g. ICL occupational health service

Top = shows voltage

Names of the 12 leads labelled

Bottom = rhythm strip

Top = HR

Machine can also tell you whether it is a normal sinus rhythm / what the issues may be

Question 22

What view of the heart does V1 and V2 look at?

What view does V5 and V6 look at?

Answer 22

V1 and V2 = septum

V5 and V6 = lateral side of the heart

Question 23

What are the three main coronary arteris?

Answer 23

Left circumflex (LCx)
Left anterior descending (LAD)
Right coronary artery (RCA)

Question 24

Which leads correspond to which coronary arteries?

Answer 24

LCx = I, aVL, V5, V6
LAD = V1, V2
RCA = II, III, aVF, V3, V4

Question 25

Which single lead is most commonly used to identify sinus rhythm / arrhythmias?

Answer 25

Lead II

Question 26

What is the normal duration for: ``` R-R interval P wave P-R interval QRS QT interval T wave ```

Answer 26

``` R-R interval = 0.6-1.2 s P wave = 80 ms P-R interval = 120-200 ms QRS = <120 ms QT interval = 420 ms T wave = 160 ms ```

Question 27

What are the bipolar leads? Where are they found?

Answer 27

Leads I, II and III - as they all have an electrode that is positive, and an electrode that is negative ``` I = between right arm (-ve) and left arm (+ve) II = between right arm (-ve) and left leg (+ve) III = between left arm (-ve) and left (+ve) ```

Question 28

What are the augmented leads?

Answer 28

aVF, aVL, aVR - they use fixed cathodes but virtual anodes

Question 29

Where are the fixed cathodes and virtual anodes for the leads: aVF aVL aVR

Answer 29

aVF = left leg cathode, lead I anode (between right and left arms) aVL = left arm cathode, lead II anode (between right arm and left leg) aVR = right arm cathode, lead III anode (between left arm and left leg)

Question 30

Where are the right angles formed between the different leads? (aVF, aVL, aVR, lead I, lead II, lead III)

Answer 30

aVF and lead I aVL and Lead II aVR and lead III

Question 31

How can you calculate the net vector / deflection of the QRS?

Answer 31

Use the boxes on the graph, find the difference between the positive and negative deflections of the QRS complex

Question 32

Use lead II and aVL to: Calculate theta in the triangle. Then calculate the yellow angle as you need to find the angle from 0 degrees. The yellow angle shows the ventricular axis

Answer 32

tan (theta) = 0.444 theta = 24 degrees Yellow angle = 60 degrees - 24 degrees = 36 degrees This is the ventricular axis

Question 33

So how do you find the ventricular angle axis?

Answer 33

Work out the net vector of 2 leads e.g. of lead II and aVL Use those 2 net vectors to construct a triangle Figure out theta in the triangle Then figure out the angle from 0 degrees to the net vector

Question 34

What is the ECG reporting procedure?

Answer 34

Is it the correct recording? Review the signal quality and leads? - e.g. if they have muscular conditions, may give too much background noise Verify the voltage and paper speed? Review the patient background if available

Question 35

What are the 7 steps when looking at an ECG?

Answer 35

Step 1 = rate and rhythm - regular? Step 2 = P wave and PR interval - how long does it last, and does every P wave result in an R wave? Step 3 = QRS duration Step 4 = QRS axis - should be between -30 and +90 degrees Step 5 = ST segment - should be flat and at the isoelectric point Step 6 = QT interval - duration Step 7 = T wave - shape

Question 36

Can you defibrillate someone in asystole (i.e. a flatline)?

Answer 36

Not a shockable rhythm Instead good to administer drugs and start CPR (chest compressions)

Question 37

What is a sinus rhythm?

Answer 37

Sinus = signal is being generated by the SAN

Question 38

What is a normal sinus rhythm on an ECG?

Answer 38

Each P-wave is followed by a QRS wave (1:1) Rate is regular (even R-R intervals

Question 39

What is sinus bradycardia on an ECG?

Answer 39

Each P-wave is followed by a QRS wave (1:1) Rate is regular (even R-R intervals) and slow (56 bpm) Can be healthy, caused by medication or vagal stimulation

Question 40

What is sinus tachycardia on an ECG?

Answer 40

Each P-wave is followed by a QRS wave (1:1) Rate is regular (even R-R intervals) and fast (107 bpm) Often a physiological response (i.e. secondary)

Question 41

What is sinus arrhythmia on an ECG?

Answer 41

Each P-wave is followed by a QRS wave Rate is irregular (variable R-R intervals) and normal-ish (65-100 bpm) R-R interval varies with breathing cycle

Question 42

What is atrial fibrilation?

Answer 42

Oscillating baseline – atria contracting asynchronously Rhythm can be irregular and rate may be slow Turbulent flow pattern increases clot risk Atria not essential for cardiac cycle

Question 43

What is atrial flutter?

Answer 43

Regular saw-tooth pattern in baseline (II, III, aVF) Atrial to ventricular beats at a 2:1 ratio, 3:1 ratio or higher Saw-tooth not always visible in all leads

Question 44

What are the features of first degree heart block?

Answer 44

Prolonged ST segment/interval caused by slower AV conduction Regular rhythm: 1:1 ratio of P-waves to QRS complexes Most benign heart block, but a progressive disease of ageing

Question 45

What are the two types of second degree heart block?

Answer 45

Mobitz I | Mobitz II

Question 46

What are the features of Mobitz I?

Answer 46

Gradual prolongation of the PR interval until beat skipped Most P-waves followed by QRS; but some P-waves are not Regularly irregular: caused by a diseased AV node Also called Wenckebach

Question 47

What are the features of Mobtiz II?

Answer 47

P-waves are regular, but only some are followed by QRS No P-R prolongation Regularly irregular: successes to failures (e.g. 2:1) or random Can rapidly deteriorate into third degree heart block

Question 48

What are the features of third degree heart block?

Answer 48

P-waves are regular, QRS are regular, but no relationship P waves can be hidden within bigger vectors A truly non-sinus rhythm – back-up pacemaker in action

Question 49

What are the features of ventricular tachycardia?

Answer 49

P-waves hidden – dissociated atrial rhythm Rate is regular and fast (100-200 bpm) At high risk of deteriorating into fibrillation (cardiac arrest) Shockable rhythm – defibrillators widely available

Question 50

What are the features of ventricular fibrillation?

Answer 50

Heart rate irregular and 250 bpm and above Heart unable to generate an output Shockable rhythm – defibrillators widely available

Question 51

What are the features of ST elevation?

Answer 51

P waves visible and always followed by QRS Rhythm is regular and rate is normal (85 bpm) ST-segment is elevated >2mm above the isoelectric line Caused by infarction (tissue death caused by hypoperfusion)

Question 52

What are the features of ST depression?

Answer 52

P waves visible and always followed by QRS Rhythm is regular and rate is normal (95 bpm) ST-segment is depressed >2mm below the isoelectric line Caused by myocardial ischaemia (coronary insufficiency)