CVS 11 + 12 + 13 - ECG

Question 1

How is a depolarisation represented on an ECG? a repolarisation?

Answer 1

DEPOLARISATION:
Positive deflection= moving towards cathode (+ve)
Downward deflection= moving away from the cathode (+ve)
REPOLARISATION:
Downward deflection= moving towards cathode (+ve)
Positive deflection= moving away from the cathode (+ve)

–> Repolarising current is OPPOSITE polarity to depolarising current

Question 2

What do P, QRS, T, Q, R, S waves represent?

Answer 2

P wave= atria contraction
QRS complex= ventricle contraction
T wave= ventricle repolarisation

Q wave= depolarisation of interventricular septum (towards –ve electrode)
R wave= ventricular depolarisation (towards +ve electrode)
S wave= depolarisation towards –ve electrode

Repolarisation of atria ‘buried’ in QRS of ventricle depolarisation

Question 3

What is the Einthoven Triangle?

Answer 3

Triangle formed between lead I, II and III.
Lead I: Right arm to Left arm (one ‘L’ so lead I)
Lead II: Right arm to Left Leg (2 ‘L’)
Lead III: Left arm to Left Leg (3’L’)
Neutral: Right leg

Always ends on the Left side

Question 4

Describe lead I

Answer 4

RA (-ve) to LA (+ve)
Gives a good view of depolarisation from right to left
QRS positive

Question 5

Describe lead II

Answer 5

RA (-ve) to LL (+ve)

Depolarisation of heart is in this direction, so QRS nearly always positive

Question 6

Describe lead III

Answer 6

LA (-ve) to LL (+ve)

Very little positive deflection as heart does not depolarize in this direction

Question 7

How are augmented leads obtained?

Answer 7

Obtained by using the average voltage of two electrodes as the negative pole, and reading from the remaining electrode as the positive pole
aVR has right arm as positive electrode
aVL has left arm
aVF has left leg

Question 8

What is a normal physiological value of the QRS axis?

Answer 8

+90 to -30 degrees

Question 9

How do you work out the QRS axis?

Answer 9

Sum 2 vectors that are at right angles like Lead I and aVF.
Look at net positive deflection for QRS.
QRS = I + aVF (in vectors)
then calculate angle that the vectors form

Question 10

What heart rate is considered bradycardia and tachycardia?

Answer 10

Bradycardia = < 60 bpm
Tachycardia = > 100 bpm

Question 11

What is the difference between segments and intervals?

Answer 11

Segments are isoelectric regions between two waveforms.

Interval is the time between the start of one wave and the start of the next.

Question 12

What is the sweep speed of ECG?

Answer 12

25 mm/s

Question 13

How wide is a small square and a large square and what time interval does that represent?

Answer 13

Small Square = 0.04 s (1 mm)

Large Square = 0.2 s (5 mm)

Question 14

What’s the duration and amplitude of a normal P wave?

Answer 14

Duration = < 0.11 s
Amplitude = < 2.5 mm

Question 15

What is the duration of a normal PR interval?

Answer 15

0.12 - 0.2 s (=one large square max)

NOTE
PR interval has no clinical significance

Question 16

What is the duration and amplitude of a normal QRS complex?

Answer 16

Duration = < 0.12 s
Amplitude = < 25 mm

Question 17

What is the duration and amplitude of a normal Q wave?

Answer 17

Duration = < 0.04 s 
Amplitude = < 25% of the total QRS complex

Question 18

What is the duration of a normal QT interval?

Answer 18

0.38-0.42s

NOTE
Important as this is the start of the ventricles depolarising.
Long QT syndrome - predispose to arrythmias and sudden cardiac death.

Question 19

What does a QRS complex with a large amplitude indicate?

Answer 19

Ventricular Hypertrophy

Question 20

What are the ECG features of sinus tachycardia?

Answer 20

Normal waveforms - normal P wave, precedes each QRS complex.
Abnormally fast resting heart rate
Atrial and Ventricular Rate = 200 bpm

Question 21

What are the ECG features of atrial fibrillation? Include atrial rate and ventricular rate in your answer.

Answer 21

ABSENT P WAVES (may get an oscillating baseline)
Irregular ventricular rhythm (duration between QRS varies) - irregularly irregular - v imp.
Could be high or normal ventricular rate
QRS complexes are normal
Atrial Rate = 350-600 bpm
Ventricular rate = 100-180 bpm

Question 22

What are the ECG features of atrial flutter?

Answer 22

SAW-TOOTHED BASE LINE - ‘ flutter’
No isoelectric line - shows constant atrial activity
Regular ventricular rhythm - one in every few atrial depolarisations will get conducted down to the ventricles
QRS normal + regular ventricular rhythm
Atrial Rate = 250-350 bpm
Ventricular Rate = 150 bpm (with 2:1)
4:1 is also common

Question 23

How is atrial fibrillation different to atrial flutter?

Answer 23

Atrial flutter has a more regular ventricular rhythm

Question 24

What is atrioventricular nodal reentrant tachycardia?

Answer 24

When a local circuit is created within the AV node

Question 25

What is atrioventricular reentrant tachycardia?

Answer 25

Local circuit is within the atria and the ventricles

Question 26

What are the ECG features of AVNRT and AVRT?

Answer 26

Lots of QRS complexes
No clear P wave
QRS complexes are RAPID and IRREGULAR
You get simultaneous depolarisation of the atria and ventricles so you get instantaneous P wave and QRS complexes - P waves often burried into QRS or just after.

Question 27

What happens in AVNRT?

Answer 27

Depolarisation is rotating within the AV node

Then it re-enters and causes simultaneous atrial and ventricular contraction

Question 28

What is preexcitation syndrome and what is a defining feature of the ECG?

Answer 28

Defining Feature = DELTA WAVES aka slurred ventricular contraction (big).
Some people are born with a congenital connection between the atria and the ventricles called an ACCESSORY PATHWAY.
This allows early depolarisation of the ventricles leading to slurring of the QRS complexes - ventricular pre excitation.
This gives an abnormally short PR interval

Question 29

What syndrome causes preexcitation syndrome?

Answer 29

Wolff-Parkinson-White Syndrome

Question 30

What is the treatment to remove the accessory pathway?

Answer 30

Radio frequency ablation

Question 31

What are the three types of atrioventricular nodal block and how do they vary?

Broad QRS complexes that are VOID OF ANY PATTERN

Answer 31

1st degree = prolonged PR interval 
2nd degree (Mobitz type 1 and type 2) = some conduction gets there but it's slow 
3rd degree = complete heart block

Question 32

What is an ECG feature of grade 1 AVN block?

Answer 32

Prolonged PR interval but still 1 to 1 relationship between P wave and QRS complex. but in between nothing happens.

Question 33

What is the difference between Mobitz type 1 and Mobitz type 2 atrioventricular nodal block?

Answer 33

2nd degree block = some of the beats from the atria do NOT reach the ventricles
Mobitz type I = gradual prolongation of the PR interval culminating in a dropped beat
Mobitz type II = fixed PR interval and then a dropped beat (you do NOT see gradually prolonging of the PR interval)

Question 34

What is the ECG feature of 3rd degree atrioventricular nodal block?

Answer 34

There is NO conduction from atria to ventricles .
ECG shows COMPLETE DISSOCIATION between QRS complexes and P waves .
Ventricles fire on their own as a protective mechanism

Question 35

What is the main ECG feature of bundle branch blocks?

Answer 35

QRS complex WIDENS and morphology changes.

It takes longer to depolarise the ventricles

Question 36

How do you distinguish between RBBB and LBBB?

Answer 36

WilliaM MarroW
RBBB = V1 + V2 = rabbit ears
LBBB = V1 + V2 = deep S waves

Question 37

What are the ECG features of ventricular tachyarrhythmia?

Answer 37

Rapid, regular, broad QRS complex pattern

Question 38

What are the ECG features of ventricular fibrillation?

Answer 38

Broad QRS complexes that are VOID OF ANY PATTERN

Question 39

How should the ST segment be?

Answer 39

‘isoelectric’

As elevated it may indicate myocardial infarction.