Understanding ECGs

Question 1

P Waves

Answer 1

Atrial depolarisation

Before QRS complex

Question 2

PR Interval

Answer 2

Start of P wave –> start of Q wave

Time taken for electrical activity to move between atria and ventricle

Question 3

QRS Complex

Answer 3

Ventricular depolarisation

3 closely related waves (QRS)

Question 4

ST Segment

Answer 4

End of S wave –> start of T wave
Isoelectric line
Time between depolarisation and repolarisation of ventricles (vent contraction)

Question 5

T Waves

Answer 5

Ventricular Repolarisation

Small wave after QRS

Question 6

RR Interval

Answer 6

Peak of one R wave –> peak of next R wave

Time between 2 QRS complexes

Question 7

QT Interval

Answer 7

Start of QRS complex –> end of T wave

Time taken for ventricles to depolarise then repolarise

Question 8

Characteristics of ECG Paper

Answer 8

Small square - 0.04s
Large square - 0.2s
(300 small or 5 large = 1 min)

Question 9

Chest Electrodes

Answer 9

V1-V6

Question 10

Position of V1-V3

Answer 10

V1: 4th intercostal space at the right sternal edge
V2: 4th intercostal space at the left sternal edge
V3: midway between the V2 and V4 electrodes

Question 11

Position of V4-V6

Answer 11

V4: 5th intercostal space in the midclavicular line
V5: left anterior axillary line at the same horizontal level as V4
V6: left mid-axillary line at the same horizontal level as V4 and V5

Question 12

Limb Electrodes

Answer 12

Red (RA)
Yellow (LA)
Green (LL)
Black (RL)

Question 13

Position of Limb Electrodes

Answer 13

Red (RA): on the ulnar styloid process of the right arm
Yellow (LA): on the ulnar styloid process of the left arm
Green (LL): on the medial or lateral malleolus of the left leg
Black (RL): on the medial or lateral malleolus of the right leg

Question 14

View of Heart from Chest Leads

Answer 14

V1-V2: septal view
V3-V4: anterior view
V5-V6: lateral view

Question 15

View of Heart from Other Leads

Answer 15

Lead I: lateral view
Lead II/III: inferior view
aVR/aVL: lateral view
aVF: inferior view

Question 16

Shape of ECG Waveform

Answer 16

When elec activity travels towards a lead - positive deflection
(vice versa)

Question 17

Deflection on ECG

Answer 17

Average direction of electrical travel
Height represents amount of elec activity flowing in that direction
Lead with most deflection = direction heart’s elec activity is travelling

Question 18

Size of R/S Waves and Direction of Depolarisation

Answer 18

R>S: depol moving towards lead
S>R: depol moving away from lead
R=S: depol exactly 90deg to lead

Question 19

Right Axis Deviation

Answer 19

Between 90 and 180deg

Question 20

Causes of Right Axis Deviation

Answer 20

Right vent hypertrophy (deflection in lead I is negative and deflection in lead aVF/III more positive)
Pulm HT (as it causes RV hypertrophy)
May be normal finding in extremely tall people

Question 21

Left Axis Deviation

Answer 21

Between -30 and -90deg

Deflection of lead III becomes negative (only significant if the deflection of lead II also becomes negative)

Question 22

Causes of Left Axis Deviation

Answer 22

Conduction abnormalities