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Cardiovascular system > ECG 1 > Flashcards

ECG 1 Flashcards

1
Q

What is the significance of a flat line on an ECG

A

Non-shockable rhythm- DNR

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2
Q

Describe the roles of the electrodes, cables and leads

A

The electrodes are attached to cables and the leads give a view of the heart ( a perspective of electrical activity).

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3
Q

What is meant by a vector and how are they represented

A

A vector is ‘a quantity that has both magnitude and direction’
Typically represented by an arrow in the net direction of movement, whose size reflects the magnitude

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4
Q

What does the isoelectric line denote

A

The isoelectric line represents no net change in voltage. i.e. vectors are perpendicular to the lead

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5
Q

What does an upward deflection denote

A

Upward deflections are towards the cathode (+)

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6
Q

What does a downward deflection denote

A

Downward deflections are towards the anode (-)

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7
Q

What does the steepness of the line denote

A

Steepness of line denotes the ‘velocity’ of action potential

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8
Q

In reality, what will each wave be composed of

A

Each wave is composed of both the up- and downstrokes

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9
Q

What does the P wave signify

A

The electrical signal that stimulates contraction of the atria (atrial systole)

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10
Q

What does the QRS complex signify

A

The electrical signal that stimulates contraction of the ventricles (ventricular systole)

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11
Q

What does the T wave signify

A

The electrical signal that signifies relaxation of the ventricles

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12
Q

What does the deflection width denote

A

The duration of the event

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13
Q

Describe the PR segment

A

AVN depolarisation
Isoelectric ECG
Slow signal transduction
Protective (delays impulse to allow ventricles to fill fully before they contract)

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14
Q

What causes the isoelectric line at Q

A

bundle of His rapidly conducts wave of depolarisation down septum

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15
Q

What causes the downward deflection at Q (Q wave)

A

septum depolarises away from cathode

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16
Q

What is the R wave

A

ventricular depolarisation due to the purkinje fibres, with wave spreading towards the cathode

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17
Q

What is the S wave

A

purkinje fibres carry wave up the myocardium for late ventricular depolarisation, moving away from the cathode

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18
Q

Describe the ST segment

A

Fully depolarised ventricles- isoelectric ECG.

Coincides with plateau phase of ventricular action potential.

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19
Q

Describe the T wave

A

Ventricular repolarisation- moving towards the cathode.

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20
Q

Why is the T wave in the same direction as the R wave

A

After travelling through the atria, the wave of depolarisation travels from the AV node down to the apex of the heart- causing the R wave.
if repolarisation took place in the same direction, then the T wave would be in the same direction as the R wave
Repolarisation occurs from the apex towards the top of the septum- due to differences in the duration of action potentials (shorter at the apex). Thus the wave of repolarisation occurs in the opposite direction as the wave of repolarisation- double negative (repolarisation is negative depolarisation)- and it occurs in a negative direction and so appears positive.

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21
Q

What is important to remember about the QRS complex

A

Any negative wave following a P wave is the Q wave
If a positive deflection follows a P wave then it is an R wave
Any negative deflection following this is an S wave- regardless if the Q wave was present or not
R is always present- but the same cannot be said for Q.
Abnormally large Q waves have an additional pathological significance and indicate a previous myocardial infarction.

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22
Q

Why do we not see atrial repolarisation on the ECG

A

Happens at the same time as the QRS complex and so is hidden.

23
Q

Why is the Q wave quite small

A

Bundle branches- not much muscle- fast.

24
Q

Does atrial systole occur at the same time as SA depolarisation

A

No- the electrical and mechanical events do not occur simultaneously.

25
Q

Why is it useful to record ECGs over time

A

To see changes over time- the temporal nature is important- an abnormal ECG may be normal for that patient and not pathological- changes are usually pathological.

26
Q

How many ECG leads are there

A

12

27
Q

Describe the positioning of the bipolar leads

A

Lead 1- Right arm to left arm- LA cathode
Lead 2- Right arm to left leg- LL cathode
Lead 3- Left arm to left leg- LL cathode.

28
Q

What view of the heart do these bipolar leads give

A

These bipolar limb leads view the heart in the coronal plane- measure electrical activity away from the heart.
These 3 leads make up Einthoven’s triangle around the heart.

29
Q

What causes the deflections in the bipolar limb leads

A

I, II, III - measuring potential difference between the limbs; for a given complex the net deflection is equal to the sum of the net deflections in the other two leads.
Bipolar as they have physical anodes and cathodes.

30
Q

Describe the different views of the 3 bipolar leads

A

1- Lateral

2 and 3- inferior

31
Q

Describe the unipolar limb leads

A

Unipolar leads measure any positive potential difference directed towards their solitary positive electrode from an estimate of zero potential (centre of triangle). They include aVL, aVR and aVF. They also view the heart in the coronal plane.
These leads are augmented and refer to the coronary arteries.

32
Q

Describe Einthoven’s triangle

A
  • An imaginary formation formed by the 3 limb leads.

* The heart is at the centre and represents the zero potential when the voltages are summed up.

33
Q

Describe the aVR

A

Measure flow of electricity from centre of heart towards right arm.
Anode is 0.5(LA+LF) LF is left leg.
Gives no view of the heart
Displays as negative on an ECG as the measured current towards the positive electrode flows against depolarisation current flow of heart.

34
Q

Describe the aVL lead

A

Measures flow of electricity from the centre of the heart to the left arm
* Mid-point of right arm and left leg (negative electrode) and flows to the positive left arm electrode.
* Displays as net-positive on an ECG as the measured current towards the positive electrode flows a little with the depolarisation current flow of heart (not perfect so has some negative).
Lateral view of the heart

35
Q

Describe the aVF lead

A

Measures flow of electricity from the centre of the heart
* Mid-point of right arm and left arm (negative electrode) and flows to the positive left leg electrode.
* Displays as net-positive on an ECG as the measured current towards the positive electrode flows a little with the depolarisation current flow of heart.
Inferior view of the heart.

36
Q

Describe the chest leads

A

Six chest electrodes (the cathodes) labelled V1 to V6 measure any potential changes in the transverse plane and they are arranged around the left side of the chest. They are also unipolar leads.

37
Q

What is the anode for the chest leads

A

1/3 (RA+LA+LL)
* The negative pole is Wilson’s Central Terminal
o A composite pole of right arm, left arm and left leg = the average potential across the body.

38
Q

Which views of the heart do each chest lead give

A

V1,V2- Septal
V3,V4- Anterior
V5,V6- Lateral

39
Q

Describe the positioning of the chest leads

A

V1: 4th intercostal, right sternal margin
V2: 4th intercostal, left sternal margin
V3: in-between V2 and V4 (on top of 5th rib)
V4: 5th intercostal, mid-clavicular line
V5: 5th intercostal, anterior axillary line (usually half-way between V4/V6)
V6: 5th intercostal, mid-axillary line

40
Q

Describe and explain the deflections seen on each chest lead

A

The wave of depolarisation in the ventricles starts in the septum and then spreads into the right and left ventricles. Because the left ventricle is usually larger than the right, the average depolarisation will head towards the left ventricle. This means that V1 and V2 will have a predominant S wave and a small R wave, while V5 and V6 will have a predominant R wave with a small S wave. The interventricular septum lies where there are equal positive and negative deflections ( R and S waves) and is usually at V3 or V4.

41
Q

What is meant by R wave progression

A

The changing height of the R wave from V1 to V6 . If this is normal; termed to be good R wave progression.

42
Q

Describe cardiac axis

A

The average direction of the wave of depolarisation of the heart (coronal axis) is referred to as the cardiac axis.
Does not use precordial (chest leads).
Usually lies closest to lead 2.

43
Q

How can you interpret whether the wave of depolarisation is moving towards or away from a lead

A

When the depolarisation wave in the ventricles is moving towards a lead, then the R wave will be larger than the S wave.
If it is moving away form the lead- then the S wave will be larger than the R wave
Perpendicular to the lead- equal deflections.

44
Q

How can you easily assess left axis or right axis deviation

A

Left axis- due to left ventricular hypertrophy- lead 1 positive and lead 2 negative
Right- lead 1 negative and lead 2 positive.
Both negative- check lead!
Both positive- normal.

45
Q

What are the lead axes

A 
Lead axes: Centre of Einthoven's triangle is the centre, so the leads have the following axes: 
Lead I+: 0 O 
Lead II+: 60 O 
Lead III+: 120 O 
aVL+: -30 0 
aVF+: 90 O 
aVR+:-150 O
46
Q

Describe how you calculate the axis

A

Select two leads at 90O
Work out net QRS depolarisation on each lead
Form a triangle by drawing x mm along the numerical lead e.g. II, and y mm perpendicular from the lead (where x is the numerical lead net depolarisation and y is the augmented lead net depolarisation)
Theta = tan-1(aug depol/num depol)
Cardiac Axis = Numerical Lead Axis - theta

47
Q

What is the difference between the orientation of the heart and the cardiac axis

A

The cardiac axis is an electrical entity and is the angle at which the maximum amplitude R wave is generated.
Cardiac axis gives the net vector of the R-wave, which is usually the largest electrical event as it is the biggest amount of muscle – normal range is -30 to +90 degrees

48
Q

Describe the ECG graph itself

A

Small squares: 0.04s width, 0.1mV height
Large squares: 0.2s width, 0.5mV height

25mm/sec: speed ECG run at
10mV spike: 2 big squares, representing what 1mV should look like

x-axis=time (s)
y-axis=amplitude (mV)

49
Q

What are the normal ECG parameters

A

PR Interval: 120-200ms
QRS Interval: 80-120ms (usually no more than 100ms)
ST Interval: 320ms
QT Interval: 350-450ms

P Wave: 80ms 
Q Wave: \ 
R Wave:  QRS Complex: 80-120ms 
S Wave:  / 
T Wave: 160ms
50
Q

Describe the conduction of the heart

A

The SA node spontaneously depolarises to trigger cardiac cycle
Specialist conductive pathways conduct through atria and to AV node
AV node slows transmission
Bundle of His & branches (composed of Purkinje fibres) insulate signal and carry to bottom of ventricles
Purkinje fibres then spread through myocardium (from endocardium to epicardium) to apex then up to base of ventricles.

51
Q

Describe the bundle of his

A

Bifurcates into right and left bundles
Insulated- to take the message all the way to the bottom
Left- does not go all the way to the septum.

52
Q

Why do we normally use lead 2

A

Lines up with heart

Major direction of impulses- big and clear deflections.

53
Q

How do you place the electrodes

A

Spray with conductive gel

Connect cables to electrodes before placing the electrodes on the body

54
Q

What happens as you hold your breath

A

Spike in ECG- resting heart rate controlled by breathing and parasympathetic nerve- less inhibition when you hold your breath- heart rate therefore increases.

Cardiovascular system (19 decks)

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