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Physiology 1 > Introduction to the electrocardiogram (ECG) > Flashcards

Introduction to the electrocardiogram (ECG) Flashcards

1
Q

What must happen to the potential difference for a myocyte to contract?

A

The potential difference across the membrane must change from negative to positive in relation to the inside of the cell.

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2
Q

How do changes in potential difference occur?

A

Through the flow of ions through specialised ion channels and through gap junctions.

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3
Q

How is a potential difference (a dipole) generated in a myocyte?

A

Different currents flow across the cell membrane at various points when a cardiac cell is depolarising or repolarising. This causes a potential difference to occur between one part of the cell and another

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4
Q

What does the flow of current along the cardiac cell surface that occurs during depolarisation/repolarisation cause?

A

Causes an external electrical field to be set up

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5
Q

Why is there no electrical filed set up when the heart is at rest?

A

Because there’s no difference in the membrane potential at different points along the cell surface

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6
Q

What leads make up the six frontal limb leads?

A

Three bipolar limb leads : Lead I, Lead II and Lead III

Three unipolar limb leads: aVF, aVL and aVR

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7
Q

What are the different combinations in which the bipolar limb leads can be connected?

A 
Left arm (+ terminal) to right arm (-terminal) = Lead I
Left leg (+ terminal) to right arm (- terminal) = Lead II 
Left leg (+ terminal) to left arm (- terminal) = Lead III
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8
Q

How is Einthoven’s triangle formed by the 3 bipolar limb leads?

A

Each limb serves as an electrical conductor meaning the limb leads can record the potential difference from each shoulder and the pelvis.
This means the limb leads form a “sensing triangle” around the heart called Einthoven’s triangle

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9
Q

What are the different viewing angles of the heart that the bipolar limb leads provide?

A

Lead I – angle of view 0°
Lead II - angle of view 60° (relative to lead I)
Lead III - angle of view 120° (relative to Lead I)

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10
Q

Explain how the 3 unipolar limb leads are derived from the same leads as the bipolar leads

A

Produce a central terminal by feeding the inputs of the 2 other limb leads that are not being viewed into the negative terminal of the voltmeter.
This central terminal is used as the negative pole while the reading from the limb lead being viewed is used as the positive pole

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11
Q

What are the different combinations in which the unipolar limb leads can be connected?

A

aVF (augmented Vector Foot) – foot (+ terminal) to left and right arm (make up - terminal)

aVR (augmented Vector Right) – right arm (+ terminal) to foot and left arm (make up – terminal)

aVL (augmented Vector Left) – left arm (+ terminal) to foot and right arm (make up – negative terminal)

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12
Q

What are the viewing angles of the heart that the unipolar limb leads provide?

A

aVF – angle of view +90° (relative to lead I)
aVR – angle of view -150° (relative to lead I)
aVL – angle of view - 30° (relative to lead I)

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13
Q

Where are the 6 precordial leads placed on the body?

A

V1 – 4th Intercostal space (immediately right of sternum)
V2 – 4th Intercostal space (immediately left of sternum)
V3 – between V2 and V4
V4 – 5th Intercostal space (midclavicular line)
V5 – left anterior axillary line (in horizontal line with V4)
V6 – mid axillary line (horizontal with V4 and V5)

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14
Q

What is the difference in the way the precordial leads and the limb leads view the heart?

A

Precordial leads “look” transversely at the heart which is in contrast with the limb leads which record the frontal plane.

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15
Q

Why is it useful for the precordial leads to view the heart from a different perspective from the limb leads?

A

Because depolarisation spreads in 3 dimensions throughout the heart so by looking at different dimensions you get a better idea at how depolarisation spreads through the heart.

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16
Q

What are the different areas of the heart that the 12 leads of the 12 lead ECG look at?

A

Inferior surface – aVF, Lead II, Lead III
Septum – V1 and V2
Anterior surface – V3 and V4
Lateral surface – V5, V6, Lead I and aVL

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17
Q

On an ECG what does the P wave represent?

A

P wave represents depolarisation of both atria

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18
Q

What is the PR interval?

A

PR interval is the time from the onset of P wave to onset of QRS complex

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19
Q

What does the PR interval represent?

A

The time taken for depolarisation to spread from SA node to ventricle

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20
Q

What does the QRS complex represent?

A

QRS complex represents depolarisation of ventricles

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21
Q

Within the QRS complex what does the Q wave represent?

A

Q wave - The 1st negative deflection

22
Q

What does the R wave represent within the QRS complex?

A

R wave - The 1st positive deflection

23
Q

What does the S wave represent within the QRS complex?

A

S wave - The 2nd negative deflection or if a negative deflection follows a positive deflection

24
Q

Within the QRS complex of an ECG do all 3 waves have to be present?

A

No QRS complex can just be RS or just QR for example

25
Q

What is the QT interval?

A

Interval between onset of QRS and end of T wave

26
Q

What does the QT interval represent?

A

Represents Ventricular depolarisation and repolarisation

27
Q

What is Long QT syndrome?

A

Condition which affects repolarisation of the heart after a heartbeat. This results in an increased risk of an irregular heartbeat which can result in palpitations, fainting, drowning, or sudden death.

28
Q

What is the ST segement and what does it represent?

A

Segment between the end of QRS and start of T wave

Represents ending of ventricular contraction and rapid ventricular ejection

29
Q

Why is the ST segement isoelectric?

A

Represents time between ventricular depolarisation and ventricular repolarisation so There’s no extracellular current flowing meaning no potential difference.

30
Q

How fast does the heart rate have to be for it to be considered a sinus tachycardia?

A

> 100 bpm

31
Q

How slow does the heart rate have to be for it to be considered a sinus bradycardia?

A

<50 bpm

32
Q

What is sinus rhythm?

A

The normal cardiac rhythm which starts at and is defined by the sinus node

33
Q

Give some examples of normal variations to the sinus rhythm

A
  1. When we sleep the parasympathetic nervous system begins to take over and slows down our heart rate resulting in sinus bradycardia
  2. Newborn babies may have a heart rate as fast as 180 bpm but as they grow up their heart rates slow down due to the maturation of the Vagus nerve
  3. Respiratory Sinus arrhythmia.
34
Q

What is respiratory sinus arrhythmia?

A

A naturally occurring variation in heart rate that occurs during each breathing cycle. Specifically, heart rate increases during inspiration and decreases during expiration.

35
Q

What is Respiratory sinus arrhythmia caused by?

A

Caused by an increase in Vagal nerve activity during expiration and a decrease during inspiration

36
Q

What are the 2 different categories of arrhythmia?

A
  1. Conduction abnormalities, e.g. blocks

2. Abnormal impulse initiation e.g. ectopic rhythyms

37
Q

What is Sinoatrial block and how can you identify it on an ECG trace?

A

When SA node fails to produce action potential. Results in no P wave/QRS complex on ECG trace.

38
Q

What is 3rd degree AV Block / complete AV block

A

When an action potential generated in the SA node in the atrium of the heart cannot propagate to the ventricles.

39
Q

Describe how 3rd degree AV block can be treated

A

Immediate treatment for 3rd degree AV Block includes giving the patient atropine. This is because atropine (Muscarinic acetylcholine receptor antagonist) binds to Muscarinic receptors preventing activation of the Vagus nerve via acetylcholine. This prevents heart rate from being slowed down via Vagus nerve.

40
Q

Describe a more permanent treatment for 3rd degree AV block

A

Dual-chamber artificial pacemaker. Works by listening for pulse from the SA node via lead in the right atrium and then sending pulse via a lead to the right ventricle causing both the right and left ventricles to contract.

41
Q

What is an ectopic rhythm?

A

An irregular heart rhythm due to a premature heartbeat.

42
Q

What can an ectopic rhythm lead to?

A

Tachycardia, e.g. Ventricular tachycardia

43
Q

What is Ventricular tachycardia?

A

An arrhythmia (an irregular heartbeat) that results from the Ventricles contracting very quickly

44
Q

How can you identify Ventricular tachycardia on an ECG trace?

A

ECG shows a characteristic regular broad QRS complex - longer than 0.125 seconds and faster than 100 bpm

45
Q

What are the 3 main mechanisms responsible for initiating tachycardia?

A
  1. Altered automaticity
  2. Triggered activity - where normal action potential suddenly swings positive again allowing another depolarisation to occur abnormally.
  3. Re-entry
46
Q

What are the 2 types of triggered activity?

A 
Early afterdepolarisation (EAD)
Delayed afterdepolarisation (DAD)
47
Q

How does EAD lead to tachycardia?

A

Defined as a slowing or reversal of normal repolarisation during phase 2 or phase 3 of the action potential. This reversal of depolarisation can eventually lead to an afterdepolarisation which can reach threshold and trigger an action potential.

48
Q

How does DAD lead to tachycardia?

A

Occurs due to elevated cytosolic calcium concentrations. The overload of the sarcoplasmic reticulum may cause spontaneous Ca2+ release after repolarisation, causing the released Ca2+ to exit the cell through the 3Na+/Ca2+exchanger. The net positive charge transferred into cell via this mechanism causes an afterpolarisation which can reach threshold and trigger an cation potential.

49
Q

How does re-entry lead to tachycardia?

A

This is when an action potential doesn’t complete the normal circuit, but instead completes an alternative circuit looping back upon itself. This eventually leads to the same action potential contracting the ventricles more than once.

50
Q

What is atrial fibrillation?

A

Most common pathological arrhythmia caused by action potentials causing the atria to contract very quickly

51
Q

What type of blood vessel is thought to have a role in atrial fibrillation and how does it initiate it?

A

Pulmonary veins can play a part in atrial fibrillation as they can cause ectopic beats and they can also act as a site of re-entry for the action potential.

52
Q

What is atrial flutter?

A

Re-entry tachycardia around the right atrium in which an action potential completes a re-entry circuit that causes the same action potential to contract the right atrium more than once.

Physiology 1 (31 decks)

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