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CVS > Session 7 > Flashcards

Session 7 Flashcards

1
Q

What do electrodes outside of cells record?

A

Changes in membrane potential and the spread over the myocardium. The ECG is explained by a combination of the effect of depolarisation/repolarisation and their spread over the heart.

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2
Q

In what direction does the myocardium depolarise and repolarise?

A

Depolarises from endocardium to epicardium

Repolarises from epicardium to endocardium

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3
Q

How many electrodes are there on an ECG and what are they used for?

A

10 electrodes are converted into 12 leads (an electrical view of the heart). These inspect the heart in a frontal and horizontal plane.

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4
Q

What are them two different types of lead?

A
  • Unipolar - chest leads V1-V6 and the augmented leads aVR, aVL and aVF. Utilises several other electrodes as negative.
  • Bipolar - I, II and III. Uses one positive and one negative electrode from standard limb leads
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5
Q

What factors affect the amplitude of a signal on an ECG?

A

How much the muscle is depolarising and how directly towards the electrode the excitation is moving.

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6
Q

Outline what each of the waves on an ECG trace show

A 
P wave - atrial depolarisation
Q wave - septal depolarisation
R wave - main ventricular depolarisation
S wave - end ventricular depolarisation
T wave - ventricular repolarisation
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7
Q

Describe what causes the different parts of the ECG as viewing from lead II

A

Atrial depolarisation will produce a small upwards deflection (P wave) because there is little muscle and it is moving towards the electrode.
Conduction from atrium to ventricle via AV node is slow. Seen as flat line (isoelectric)
Spread in the IV septum is faster in the left bundle branch than the right. The septum therefore depolarises from left to right, producing a small downward deflection (Q wave)
Spread through the ventricle produces a large upwards deflection (QRS complex) as there is lots of muscle and it is moving directly towards the electrode.
Depolarisation finally spreads towards the base of ventricles, producing a small downwards deflection (S wave) because it moves obliquely away.
Ventricular depolarisation spreads the opposite way to depolarisation (moving away) to produce a medium upward deflection (T wave).

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8
Q

What are possible confounders when conducting an ECG?

A

Lead misplacement, muscle contraction (movement, shivering, talking, coughing), interference (e.g.alternating current) and poor electrode contact (sweat, hair).

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9
Q

How do amplifiers used to record the ECG work?

A

They take the signal coming in on their negative electrode, invert it and add it to the signal coming in on their positive electrode. The sum is multiplied by a factor known as the gain before outputting.

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10
Q

Why is lead II often used for a rhythm strip?

A

Best for looking at P waves

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11
Q

What speed to all ECG machines run at?

A

25mm/s
5 large squares per second
1 large square = 0.2s
1 small square = 0.04s

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12
Q

How is the heart rate calculated from the rhythm strip?

A

If regular - 300 divided by the number of large squares in the R-R interval
If irregular - count the number of QRS complex in 30 large squares (6 seconds) then multiply by 10

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13
Q

When assessing an ECG what needs to be checked?

A

Assess P waves - present, absent or abnormal. Indicates atrial fibrillation
Calculate PR interval - estimates conduction in AV node and bundle of His
Relationship between P waves and QRS complexes
Width of QRS complex - if broad rhythm could be originating in ventricle or bundle block can be present
Calculate rate

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15
Q

How is the PR interval, QRS width and QT interval measured?

A

PR - from start of P to start of Q
QRS - from start of Q to end of S
QT - from start of Q to end of T

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16
Q

Describe the placement of the limb electrodes

A 
Red - right arm
Yellow - left arm
Green - left leg
Blue - right leg
Ride Your Green Bike
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17
Q

Describe the placement of the chest electrodes

A

V1 - 4th intercostal space to the right of the sternum
V2 - 4th intercostal space to the left of the sternum
V3 - directly between V2 and V4
V4 - 5th intercostal space at mid clavicular level
V5 - directly between V4 and V6
V6 - 5th intercostal space mid axillary line

18
Q

Describe where the leads in the vertical plane view the heart

A

Leads II, III and aVF view the inferior surface of the ventricles.
Leads I and aVL view the lateral surface of the left ventricle.
Lead aVR views the atrial and ventricular cavities.

19
Q

Describe where the leads in the horizontal plane view the heart

A

Leads V1 and V2 view the right ventricle.
Leads V3 and V4 view the interventricular septum.
Leads V5 and V6 view the left ventricle.

20
Q

What is seen in an ECG showing sinus rhythm?

A

Rate 60 to 100 bpm
P is upright in leads I and II
PR interval 3-5 small boxes (0.12-0.20 seconds)
Every P wave is followed by a QRS complex
Every QRS complex is preceded by a P wave
QRS width less than 3 small squares (

24
Q

What is atrial fibrillation and what is shown on an ECG?

A

AF occurs when there is multiple abnormal atrial pacemaker cells discharging randomly. Atrial depolarisation is chaotic and leads to a loss of normal atrial contraction.
The impulses are conducted irregularly to the ventricles but ventricular depolarisation and contraction is normal (narrow QRS). Pulse and heart rate is irregularly irregular. P wave absent.

25
Q

What is seen in first degree heart block?

A

Elongated PR interval (>200ms) due to slow conduction in the AV node and bundle of His. Could be due to ischaemia or degenerative changes. QRS normal.

26
Q

What is seen in type 1 second degree heart block?

A

Progressive lengthening of the PR interval until one P wave is not conducted (no QRS) to allow time for the AV node to recover before the cycle starts again.

27
Q

What is seen in type 2 second degree heart block?

A

Sudden lack of conduction of a beat (dropped QRS) with a normal PR interval. Not all atrial contractions are followed by ventricular contraction. There is a high risk of progression to complete heart block and a pacemaker is inserted.

28
Q

What is seen in third degree heart block?

A

Atrial contractions are normal but no electrical conduction is conveyed to the ventricles. The ventricular pacemaker takes over but the rate is very slow, usually with wide QRS complexes. Adequate blood pressure is not maintained and urgent pacemaker insertion is performed.

29
Q

What are ectopic foci and what is seen when they are located in the ventricles?

A

Abnormal pacemaker cells in the heart muscle that display automaticity.
When the ectopic focus in in the ventricles, depolarisation does not spread through the fast Purkinje system so there is a wide and abnormally shaped QRS. The ventricular ectopic beats occur at regular intervals around or between normal beats.

30
Q

What is ventricular tachycardia?

A

A run of three or more ventricular ectopics

31
Q

What is ventricular fibrillation?

A

Abnormal, chaotic, fast ventricular depolarisation originating from impulses arising in numerous ectopic sites in the ventricle. There is no coordinated contraction leasing to cardiac arrest.

32
Q

What is seen in an ECG lead facing a fully evolved myocardial infarction?

A

Pathological Q waves (>0.04s wide and >2mm deep) due to myocardial necrosis.
ST segment elevation due to subepicardial injury.
T wave inversion due to ischaemia

33
Q

What is the cardiac axis and what is normal/abnormal?

A

The average spread of the ventricular depolarisation.
Normal: -30 to +90
Right axis deviation: >+90
Left axis deviation:

34
Q

What can cause left axis deviation and what is seen?

A

Left ventricular hypertrophy or conduction blocks in the left bundle branch.
QRS is upright in lead I and inverted in lead II.

35
Q

What can cause right axis deviation and what is seen?

A

Right ventricular hypertrophy.

QRS is inverted in lead I and upright in lead III

CVS (13 decks)

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