ECGs

Question 1

what is an ECG?

Answer 1

An electrocardiogram is a representation of the electrical events of the cardiac cycle.

Each event has a distinctive waveform and the study of waveform can lead to greater insight into a patient’s cardiac pathophysiology.

Question 2

what produces an upright positive deflection?

Answer 2

Electrical impulse that travels towards the electrode

Question 3

what can ECGs identify?

Answer 3

-Arrhythmias
-Myocardial ischaemia and infarction
-Pericarditis
-Chamber hypertrophy
-Electrolyte disturbances (hyper/hypokalaemia)
-Drug toxicity

Question 4

describe SAN

Answer 4

– dominant pacemaker with an intrinsic rate of 60-100bpm

Question 5

describe AVN

Answer 5

back up pacemaker with an intrinsic rate of 40-60 bpm

Question 6

describe ventricular cells

Answer 6

back up pacemaker with an intrinsic rate of 20-45 bpm

Question 7

what are the three pacemakers of the heart?

Answer 7

1. SAN
2. AVN
3. Ventricular cells

Question 8

what is the standard calibration?

Answer 8

25mm/s
0.1mV/mm

Question 9

what are the impulse conduction points in an ECG?

Answer 9

1. SAN
2. AVN
3. Bundle of His
4. Bundle branches
5. Purkinje fibres

Question 10

describe p waves

Answer 10

atrial depolarisation

Question 11

describe the QRS complex

Answer 11

ventricular depolarisation

Question 12

describe the t waves

Answer 12

ventricular repolarisation

Question 13

describe the PR interval

Answer 13

atrial depolarisation and delay in AV junction (delay allows time for the atria to contract before the ventricles contract)

Question 14

what do the ECG leads measure?

Answer 14

The leads measure the difference in electrical potential between two points

Question 15

what are the two leads called?

Answer 15

1. bipolar leads
2. unipolar leads

Question 16

describe bipolar leads

Answer 16

two different points on the body (positive and negative)

Question 17

describe unipolar leads

Answer 17

one point on the body and a virtual reference point with zero electrical potential, located in the centre of the heart (only require a positive electrode for monitoring)

Question 18

how many electrodes, precordial, limbs are used?

Answer 18

There are 12 leads calculated using only 10 electrodes: 6 precordial, 4 limbs (1 neutral).

Question 19

what do the chest electrodes do?

Answer 19

The 6 chest electrodes give 6 chest leads which provide information about the heart’s horizontal plane

Question 20

what do the 4 limb electrode do?

Answer 20

The 4 limb electrodes give 6 frontal leads that provide information about the heart’s vertical plane. The placement can be explained as Einthoven’s triangle.

Question 21

name the 6 leads

Answer 21

Lead I
Lead II
Lead III
Augmented vector right (aVR)
Augmented vector left (aVL)
Augmented vector foot (aVF)

Question 22

which leads are bipolar

Answer 22

Leads I, II, and III require bipolar leads

Question 23

which leads are unipolar?

Answer 23

all the augmented leads are unipolar.

Question 24

what are common p wave abnormalities?

Answer 24

Right atrial enlargement – tall >2.5mm – P pulmonale

Left atrial enlargement – notched (M-shaped) – P mitrale

Question 25

what are common QRS complex abnormalities?

Answer 25

Depth of the S wave should not excess 30mm Pathological Q wave: >2mm deep and >1mm wide >25% amplitude of the subsequent R wave

Question 26

what does the QRS complex represent?

Answer 26

This represents the overall direction of the heart’s electrical activity, and abnormalities of the QRS axis hint at ventricular enlargement or conduction blocks.

Question 27

what are common ST segment abnormalities?

Answer 27

ST segment is usually flat (isoelectric), elevation or depression of ST segment by 1mm or more can be pathological.

Question 28

what are common T wave abnormalities?

Answer 28

Should be at least 1/8 but less than 2/3 of the amplitude of R Abnormal T waves are symmetrical, tall, peaked, biphasic or inverted T wave amplitude rarely exceeds 10mm

Question 29

what are common U wave abnormalities?

Answer 29

Small, round, symmetrical and positive in lead II, with amplitude <2mm (regular) U wave should be same direction as T wave

Question 30

what are common QT interval abnormalities?

Answer 30

QT interval decreases when heart rate increases Regular interval is 0.35-0.45s Should not be more than half of the interval between adjacent R waves

Question 31

how do you calculate heart rate from a ECG?

Answer 31

Rule of 300/1500 – regular rhythms Count the number of big/small boxes between two QRS complexes and divide this into 300/1500 for regular rhythms. 10 second rule – irregular rhythms Count the number of beats present on the ECG and multiply by 6.

Question 32

describe the quadrant approach for the QRS axis

Answer 32

1. QRS complex in leads I and aVF 2. Determine if they are predominantly positive or negative 3. The combination should place the axis into one of the 4 quadrants below LAD = left axis deviation RAD = right axis deviation