ACEM primary - physiology > ECG physiology > Flashcards

ECG physiology Flashcards

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1
Q

In a volume conductor, the sum of potentials at the points of an equilateral triangle is what at all times?

A

Zero

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2
Q

What is an equilateral triangle with the heart at the centre called?

A

Einthoven triangle

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3
Q

Where are the 3 standard limb leads placed?

A

Both arms and left leg

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4
Q

Depolarisation moving towards an active electrode causes what?

A

A positive deflection

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5
Q

Depolarisation moving away from an active electrode causes what?

A

A negative deflection

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6
Q

What produces the P wave?

A

Atrial depolarisation

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7
Q

What causes the QRS complex?

A

Ventricular depolarisation

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8
Q

What causes the T wave?

A

Ventricular repolarisation

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9
Q

U waves may be caused by what?

A

Ventricular myocytes with long action potentials

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10
Q

How long should the PR interval be?

A

0.12-0.20 seconds

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11
Q

What is the average PR interval?

A

0.18 seconds

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12
Q

What causes the PR interval?

A

Atrioventricular conduction

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13
Q

What is the average length of the QRS complex?

A

0.08 seconds

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14
Q

What should the QRS complex measure?

A

Up to 0.10 seconds

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15
Q

What is the cause of the QRS complex?

A

Ventricular depolarisation

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16
Q

What is the average length of the QT interval?

A

0.40 seconds

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17
Q

How long should the QT interval be?

A

Up to 0.43 seconds

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18
Q

What is the cause of the QT interval?

A

Ventricular action potential

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19
Q

What is the average ST interval length?

A

0.32 seconds

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20
Q

What causes the ST interval?

A

Plateau portion of the ventricular action potential

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21
Q

Where are the electrode positioned for lead I?

A

Right arm to left arm

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22
Q

Where are the electrodes positioned for lead II?

A

Right arm to left leg

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23
Q

Where are the electrodes positioned for lead III?

A

Left arm and left leg

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24
Q

Name the 3 bipolar ECG leads.

A

I
II
III

**The limb leads

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25
The bipolar limb leads record what?
Differences in potential between 2 limbs.
26
How many unipolar leads are there?
9 **6 chest, 3 limb
27
What do unipolar leads record?
The difference between an exploring electrode and an indifference electrode.
28
Which unipolar lead is aVR?
Right arm
29
Which unipolar lead is aVL?
Left arm
30
Which unipolar lead is aVF?
Left foot
31
Why are the P wave, QRS and T wave negative in aVR?
Because atrial and ventricular depolarisation and ventricular repolarisation move AWAY from the exploring electrode = negative deflection.
32
Why are the waves in aVL and aVF positive or biphasic?
Because the depolarisation and repolarisation spreads towards them.
33
Which leads do not have Q waves?
V1 and V2
34
At what degree level is an axis deviation considered a left axis deviation?
Less than (to the left) of -30 degrees
35
At what vector is an axis deviation considered a right axis deviation?
Larger (to the right) of 110 degrees.
36
What is the normal vector of a QRS complex?
-30 to +110 degrees
37
What does right axis deviation suggest?
Right ventricular hypertrophy
38
What does left axis deviation suggest?
Left ventricular hypertrophy
39
What is the normal length of the PA interval on an HBE?
27ms
40
What is the normal length of the AH interval on an HBE?
92ms
41
What is is normal length of an HV interval on an HBE?
43ms
42
What is the rhythm called when each beat originates from the SAN?
Normal sinus rhythm
43
What happens to the heart rate during inspiration?
It accelerates
44
What happens to the heart rate during expiration?
If decelerates
45
What causes sinus arrhythmia?
Fluctuations in parasympathetic output to the heart.
46
Why does inspiration cause an increased heart rate?
Impulses in the vagi from stretch receptors in the lungs inhibit the cardio-pulmonary area in the medulla oblongata. Th9s causes the tonic vagal discharge to decrease = increased HR due to decreased vagal stimulation.
47
Disease processes affecting the sinus node cause what?
Bradycardia with associated dizziness and syncope.
48
What causes third-degree (complete) heart block?
Complete interruption of the conduction from the atria to the ventricles.
49
What are 2 causes of complete heart block?
AV node disease Infranodal disease (disease of conducting system below AVN)
50
What causes an idioventicular rhythm?
When the ventricles beat at a rate independent of the atria (due to CHB)
51
What is the rate of an idioventricular rhythm generated from the AVN?
About 45 bpm
52
What is the rate of an idioventricular rhythm generated from a more peripheral location in the ventricle?
About 35 bpm
53
What is Stokes-Adams syndrome?
When cerebral ischaemia caused by long periods of asystole in CHB leads to dizziness and fainting.
54
What type of MI can cause complete heart block?
Septal MI
55
What ECG findings are there in first degree heart block?
Prolonged PR interval
56
What ECG findings are there in second degree heart block Mobitz 1?
Gradually lengthening PR intervals until a ventricular beat is dropped.
57
What ECG findings are there in second degree heart block Mobitz 2?
When a ventricular beat only follows every 2 or every 3 atrial beats.
58
The PR interval following a dropped ventricular beat is normally what?
Normal or only slightly prolonged
59
Sick sinus syndrome is an umbrella term for which 3 conditions?
Sinus bradycardia Sinus tachycardia Tachy-brady syndrome
60
What is the normal cause of sick sinus syndrome?
Non-specific scar-like degeneration of the heart's conduction system.
61
What is a common cause of sick sinus syndrome in children?
Heart surgery (namely, surgery on the upper chamber)
62
What causes right and left bundle branch blocks?
When one branch of the bundle of His is interrupted.
63
Why are QRS complexes prolonged in bundle branch blocks?
Because excitation passes normally down the intact bundle, and then sweeps back through the myocardium to activate the myocardium on the blocked side.
64
What causes fascicular blocks?
When there is a blockage in the left anterior or posterior fascicle of the left bundle branch.
65
Which deviation is caused by left anterior fascicular block?
Left axis deviation
66
Which deviation is caused by left posterior fascicular block?
Right axis deviation
67
When do you get bifascicular and trifascicular blocks?
When there is a combination of fascicular and bundle branch blocks
68
What causes premature atrial, nodal or ventricular beats (extrasystoles)?
Increased automaticity of the heart when an irritable ectopic foci discharges.
69
What happens when an ectopic focus discharged repetitively at a rate higher than that of the SA node?
Rapid, regular tachycardia occurs.
70
Which types of tachycardia can be caused by repetitive discharge of an ectopic focus at a higher rate than the SAN?
Atrial tachycardia Ventricular tachycardia Nodal paroxysmal tachycardia Atrial flutter
71
What is the most common cause of paroxysmal arrhithymias?
A defect in the conduction system that permits a wave of conduction to propagate continuously within a closed circuit.
72
What is a bundle of Kent?
An abnormal extra bundle of conducting tissue connecting the atria to the ventricles.
73
Which wave on an ECG is usually abnormal in an atrial extrasystole?
P wave
74
What normally causes atrial extrasystole?
Excitation spreading from an independently discharging focus in the atria which stimulated the AVN prematurely and is conducted to the ventricles.
75
Why is there usually a pause between an atrial extrasystole and the next normal beat?
Because the SAN needs to repolarise and then depolarise top firing level again after the extrasystole before it can initiate the next normal beat.
76
What happens to the p and t waves in atrial extrasystole?
The P wave is superimposed on a T wave = bicuspid P/T wave
77
What causes an atrial tachycardia?
Atrial focus that discharges regularly OR Reentrant activity producing atria rates up to 220bpm
78
What can be associated with an atrial tachycardia in patients on digoxin?
An AV block
79
What is the rate range for a patient in atrial flutter?
200-350bpm
80
In the most common type of atrial flutter, what is the cause?
A large counterclockwise circus movement in the right atrium
81
What causes the sawtooth pattern on an atrial flutter ECG?
Fast, repeated atrial contractions
82
What is atrial flutter almost always associated with?
A 2:1 block or greater
83
Why is atrial flutter almost always associated with a 2:1 block or greater?
Because the AVN cannot conduct more than about 230 impulses per minute.
84
What happens to the atria in atrial fibrillation?
They beat very rapidly in an irregular and disorganised fashion. **300-500bpm
85
What happens to the ventricles in atrial fibrillation?
They beat at a completely irregular rate due to the AV node discharging at irregular intervals. **Normally 80-160bpm
86
What is the cause of atrial fibrillation?
Most commonly multiple concurrently circulating reentrant excitation waves in both atria.
87
What is the most common cause of paroxysmal atrial fibrillation?
Discharges from one or more ectopic foci.
88
Why can some etopic foci that contribute to paroxysmal AF found in the pulmonary veins?
Because the right atrial muscle fibres extend along the pulmonary veins.
89
Why does cardiac output decrease in atrial tachycardias and flutter?
Because the ventricular rate can be so high that diastole is too short for adequate ventricular filling.
90
Why can carotid sinus massage or pressing on the eyeball convert atrial tachycardia or fast atrial flutter to normal sinus rhythm?
Stimulation of the vagal reflex = release of ACh at vagal nerve endings = depressed conduction in atrial musculature and AVN OR Vagal stimulation can increase the degree of AV block = abrupt lowering of ventricular rate
91
How does digoxin lower a rapid ventricular rate in atrial fibrillation?
By depressing AV conduction
92
Why do premature ventricular ectopics have strange shaped, prolonged QRS complexes?
Because of the slow spread of the impulse from the focus through the ventricular muscle to the rest of the ventricle.
93
Premature ventricular contractions are usually followed by what on the ECG?
A compensatory pause
94
What happens to the heart in ventricular fibrillation?
The ventricular muscle fibres contract in a totally irregular and ineffective way.
95
What is the cause of ventricular fibrillation?
Very rapid discharge of multiple ventricular ectopic foci or a circus movement.
96
What is the most common cause of sudden death in patients with MI?
Ventricular fibrillation
97
What is the 'vulnerable period' in the cardiac excitation cycle?
The mid-portion of the T wave when some ventricular myocardium is depolarised, some is incompletely repolarised and some is completely repolarised.
98
Give 3 causes of long QT syndrome.
Drugs Electrolyte imbalances Myocardial ischaemia Congenital
99
Mutations of how many different genes can cause long QT syndrome?
8
100
What is the anatomical difference in patients with WPW syndrome?
An additional aberrant muscular or nodal tissue connection (Bundle of Kent) between the atria and ventricles.
101
How does the Bundle of Kent cause accelerated AC conduction?
Bundle of Kent conducts more rapidly than the AVN = on ventricle is excited early.
102
How does WPW syndrome manifest on an ECG?
Short PR interval Prolonged QRS projection with slurred upstroke (delta wave)
103
Electrical activity of the heart depends on the distribution of which ions being tightly controlled across muscle cell membranes?
Sodium Potassium
104
What is the first ECG change to be seen in hyperkalaemia?
Tall, peaked T waves
105
Which ECG changes can b caused by hyperkalaemia?
Tall, peaked T waves Broad QRS complexes
106
Which arrhythmias can be caused by hyperkalaemia?
Ventricular arrhythmias
107
What happens to the resting membrane potential of the muscle fibres as extracellular potassium concentration increases?
It decreases **May cause the fibres to become unexcitable and the heart could stop in diastole.
108
Hypokalaemia causes what ECG changes?
PR prolongation Prominent U waves ST depression T wave inversion in the precordial leads (late sign)
109
What effect does hypocalcaemia have on the ST segment?
ST prolongation (and consequently QT prolongation)
110
Which drugs can cause prolongation of the St segment (and therefore QT interval)?
TCAs Phenothiazines
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