The electrocardiogram - ECG (CVS6)

Question 1

how does the heart generate surface potentials upon the skin

Answer 1

electrical activity within the cardiac muscle causes current flow within the heart and within surrounding tissues distant to the heart

Question 2

what is caused by electrical activity within cardiac muscle cells

Answer 2

• current flow within the heart
• current flow within the surrounding tissues
• potential differences between distant sites on the body surface measurable by electrodes placed on the skin coupled to a sensitive recording device, the electrocardiograph

Question 3

what is an electrocardiogram

Answer 3

recording of potential differences in current flow between distant sites on the body surface measurable by electrodes placed on the skin coupled to a sensitive recording device, the electrocardiograph

Question 4

3 standard limb leads used when recording an ECG

Answer 4

• lead I: RA/right arm(-ve) - LA/left arm(+ve)
• lead II:RA/right arm(-ve) - LL/left leg (+ve)
• lead III: LA/left arm(-ve) - LL/left leg (+ve)

Question 5

einthoven’s triangle

Answer 5

imaginary equilateral triangle with the heart at its center, formed by the axes of the three bipolar limb leads

Question 6

why is lead II (between RA - LL) used when recording ECG

Answer 6

-lead II goes along the direction of the heart of the heart and so picks up electrical potentials better

Question 7

why are the limb leads described as bipolar

Answer 7

• the heart is like a big generator
• so bipolar limb leads have a positive and negative end to them so that the maximum deflection in the voltage can be detected (depolarization moving towards the electrode gives an upward deflection)

Question 8

why is right leg not included in ECG

Answer 8

• right leg is the earth leg

- but you don’t need to see that area of the heart so its irrelevant

Question 9

how does the ECG work (calum sreenans explanation)

Answer 9

• there are no actual leads, the leads are vectors created between the two points that the pads are put on the person
• so as the electricity passes across the heart and then back up it, it causes deflections in the voltage across different directions, and you can work out where it is from the direction the deflection on the graph goes
• and you can work out where heart problems are occurring by which leads are affected

Question 10

P wave on ECG

Answer 10

atrial depolarization (0.8-0.10seconds)

Question 11

QRS complex on ECG

Answer 11

-ventricular depolarization (

Question 12

T wave in ECG

Answer 12

ventricular repolarization (why an upward deflection?)

Question 13

why does ventricular repolarization cause an upward deflection (T wave on ECG)

Answer 13

?

Question 14

PR interval on ECG

Answer 14

largely due to AV node delay (0.12-0.20 seconds)

Question 15

ST segment on ECG

Answer 15

ventricular systole

Question 16

TP interval on ECG

Answer 16

diastole

Question 17

unipolar limb leads

Answer 17

• The unipolar leads
• augmented leads and their respective limb electrodes are:
• aVR lead: right arm
• aVL lead: left arm
• aVF lead: left leg
• one +ve electrode, two others linked as -ve eg. if looking at aVR, one lead will be +ve and the other 2 leads will be negative
• > augmented limb leads are used to look at the heard in other directions/look at the heart in 3D

Question 18

aVR

Answer 18

augmented voltage right

Question 19

limb lead I

Answer 19

RA (-ve) to LA (+ve)

Question 20

limb lead II

Answer 20

RA (-ve) to LL (+ve)

Question 21

limb lead III

Answer 21

LA (-ve) - LL (+ve)

Question 22

P wave

Answer 22

atrial

Question 23

aVR

Answer 23

• augmented voltage right

- RA (+ve) to [LA and LF] (-ve)

Question 24

aVL

Answer 24

• augmented voltage left

- LA (+ve) to [RA and LF] (-ve)

Question 25

aVF

Answer 25

• augmented voltage foot

- LF (+ve) to [RA and LA] (-ve)

Question 26

how to calculate heart rate from ECG

Answer 26

• 300/number of large squares between beats

- 300/number of large squares between R-R interval

Question 27

what is the ECG rhythm strip

Answer 27

• prolonged reading of one lead
• usually standard limb lead II
• allows you to detect rhythm disturbance

Question 28

why are 12 leads needed if one lead (lead II) is enough to show/monitor cardiac rhythm?

Answer 28

• the 12 leads allow you to look at the heart from different directions
• this allows you to:
• > determine the axis of the heart in the thorax
• > look for any ST segment or T wave changes in relation to specific regions of the heart (eg. in diagnosing ischaemic heart disease)
• > look for any voltage criteria changes (eg. when diagnosing chamber hypertrophy)

Question 29

eg.’s that a normal resting ECG doesnt exclude

Answer 29

• normal resting ECG doesnt exclude significant heart disease:
• > myocardial infarction/heart attack may or may not cause characteristic ECG changes (eg. ST elevation)
• > intermittent rhythm disturbance (if susbected do amulatory ECG for 24 hours or 7 days)
• > stable angina/form of ischaemic heart disease, if suspected do exercise ECG (look for changes during/after exercise - usually absent during rest)