The ECG

Question 1

What is an ECG measuring?

Answer 1

It is a measure of the currents generated in the EXTRACELLULAR FLUID by the changes occurring
simultaneously in many cardiac cells

Question 2

What is Ohm’s Law?

Answer 2

V = IR
V: Voltage
I: Current
R: Resistance

Question 3

What are the Phases of myocyte action
potential?

Answer 3

Phase 0 - rapid depolarisation. Peak Summits to Sodium in
Phase 1 - partial depolarisation. Peak Plummets so potassium out
Phase 2 - plateau curve Continues so Calcium influx leading to Contraction
Phase 3 - repolarisation. peak Plummets so K+ open and influx of potassium as calcium channels closed
Phase 4 - Resting membrane potential reached. Heart is in diastole so no action potential needed to lead to contraction

Question 4

What is the P wave on the ECG representing?

Answer 4

The atrial depolarisation

Question 5

What is the QRS complex on the ECG representing?

Answer 5

Ventricular depolarisation

Question 6

What is the T wave on the ECG representing?

Answer 6

Ventricular repolarisation

Question 7

What is the normal PR interval values?

Answer 7

120-200 ms (3-5 small squares)

Question 8

What is the normal QRS complex width?

Answer 8

Less than 120 ms (3 small squares)
0.06-0.1s

Question 9

What is the normal value for the QT interval?

Answer 9

• Measure of time of ventricular repolarisation
  Men: 350-440 ms
  Women: 350-460 ms

Question 10

What is Tachycardia?

Answer 10

increased heart rate (>100 bpm)

Question 11

What is Dextrocardia?

Answer 11

heart on right side of chest instead of left

Question 12

What is Bradycardia?

Answer 12

decreased heart rate (<60 bpm)

Question 13

What are the 2 types of ECG can we have?

Answer 13

A 12 lead and a 3 lead

Question 14

How do electrical impulses in the heart move?

Answer 14

• Electrical impulses in the heart move in 3 dimensions
• ECG only measure voltage in 1 dimension

Question 15

Why does an impulse look big or small?

Answer 15

• If an impulse is towards the electrode it looks big
• If an impulse is away from the electrode it looks small or even negative

Question 16

What is the difference between an ECG electrode and lead?

Answer 16

Electrode:
- Physical connection to patient in order to measure potential at that point
- 10 electrodes to record a 12 lead ECG
Lead:
- Graphical representation of electrical activity
- Calculated by the machine from electrode signals
- 12 leads for a 12 lead ECG

Question 17

What is a bipolar vs unipolar lead?

Answer 17

Bipolar:
- Measures potential difference (voltage) between 2 electrodes
- One electrode designated positive and the other negative
Unipolar:
- Measures potential difference between an electrode (+ve) and a combined reference electrode (-ve)
- Sometimes known as augmented leads

Question 18

Where do the standard limb leads go?

Answer 18

• form a triangle between electrodes on the wrists and left leg (right leg is a ground electrode)
• the negative poles are REFERENCE electrodes and the positive poles are RECORDING electrodes

Question 19

Where do augmented leads go?

Answer 19

• Augmented leads (aVR, aVL & aVF) bisect the angles of the triangle by combining two electrodes as reference e.g. for lead aVL, the right wrist & foot are combined as the negative pole, thus creating a reference point along the line between them, pointing toward the recording electrode on the left wrist

Question 20

Where do the precordial leads go?

Answer 20

recording electrodes placed on the chest. 6 leads

Question 21

When reading an ECG what does each square represent?

Answer 21

When reading an ECG, the graph shows changes in voltage over time, each small square across represents 40ms & each big square across represents 0.2s

Question 22

In a normal ECG the P and T waves should be?

Answer 22

• In a normal ECG the p waves are POSITIVE in EVERY LEAD (apart from the aVR)
• T waves are POSITIVE in EVERY LEAD (apart from the aVR & sometimes the V1 and V2 depending on trace)

Question 23

How many of each leads go where in an ECG summary?

Answer 23

3 unipolar - avR, avL, avF
3 bipolar - I, II, III (4 sticky pad)
6 chest (6 sticky pad)