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Cardiovascular System > ECG > Flashcards

ECG Flashcards

1
Q

What is an ECG?

A

An electrocardiogram is a graphic representation of electrical potential difference changes, against time, of the myocardium throughout the cardiac cycle.

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2
Q

How are cardiac muscle contractions visible on ECGs?

A

The myocardium has electrical currents that sequentially depolarise each individual cell and result in a change in cellular morphology that allows muscle contraction

The electro / chemical changes that cause myocardial contractions are shown by the cardiac action potentials

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3
Q

What must occur for the myocyte to contract?

A

P.d. across the cell membrane must change from negative to positive

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4
Q

How do changes in p.d. occur across the cell membrane?

A

Changes in P.d. occur through flow of ions through specialised ion channels in cell membrane and relatively freely through gap junctions

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5
Q

How is the cardiac dipole created?

A

When cardiomyocytes re/depolarise, different currents flow across the cell membrane at various points creating a p.d. between one and another part of the cell => dipole

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6
Q

How is an electrical field formed in the heart?

A

Flow of current along cell surface sets up an electrical field around the dipole

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7
Q

What happens to the dipole during the resting potential?

A

Once the cell has re/depolarised and at resting potential, there is no membrane p.d. so no electric field, despite a p.d inside and outside cell.

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8
Q

Describe the features of the electric dipole

A

The electric dipole consists of 2 equal and opposite charges =q and -q, separated by a distance, d.

The dipole is a vector, with direction as well as magnitude

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9
Q

What are the 12 leads of an ECG?

A

6 Frontal plane leads (6 limb leads)

6 Horizontal / precordial / chest leads

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10
Q

Describe the 6 frontal plane leads used

A

6 frontal plane / limb leads consist of:

  • 3 bipolar leads (I,II and III) derived from einthoven lead
  • 3 unipolar leads (aVL, aVR, aVF) from I,II and III
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11
Q

What are the 6 horizontal / precordial / chest leads?

A

V1 - 4th IC, right sternal edge
V2 - 4th IC, left sternal edge
V3 - equidistant from V2 - V4
V4 - 5th IC, midclavicular line
V5 - left anterior axillary line horizontal with V4
V6 - midaxillary line, horizontal with V4 and V5

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12
Q

Describe features of the electrical anatomy

A

Initiated by SAN, traverses through atria into AV node and His / Purkinje rapid conduction occurs through to ventricular myocardium

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13
Q

Describe the features of the ECG complex

A

components - PQRSTU
specific order in sinus rhythm
each relates to a cardiac cycle phase

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14
Q

What occurs during the P wave?

A

Depolarisation of both atria
Onset of P wave activates QRS complex
Measurement of AV conduction time
from SAN -> ventricles usually (120-200 ms)

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15
Q

What is the QRS complex?

A

Q - 1st wave of negative deflection
R - 1st wave of positive deflection
S - 2nd wave of negative deflection or 1st after positive

occurs when ventricles depolarise following traversing AVN / His / Purkinje system
Specialised conduction cells for fast conduction
From the beginning to end of QRS = 80 - 110 ms

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16
Q

What events occur during the QT interval?

A

Typically lasts 350 -420 ms
Onset of QRS to end of T wave
Ventricular depolarisation and repolarisation
Long QT syndrome causes sudden death

17
Q

What happens during ST segment / T wave?

A 
Isoelectric (flat) segment between QRS and T wave 
Repolarisation of ventricles 
Ischaemia manifests here 
- depression - ischaemia 
- elevation - myocardial infarction 
some elevation is normal
18
Q

What does an ECG tell us?

A

more tissue = higher voltage

atria - small & ventricles - large

19
Q

Outline the features of a typical ECG

A 
ECG is standardised via calibration signals 
Typical voltage scale - 10 mm = 1 mV 
1 small square = 0.1 mV 
Speed = 25 mm/s
1 small square = 0.04 s
Unit of measurement = bpm 
so more beats = faster rate
20
Q

What is Tachycardia?

A

fast heart rate

e.g. sinus tachycardia >100 bpm

21
Q

What is bradycardia?

A

slow heart rate

e.g. sinus bradycardia <50 bpm

22
Q

What components are visible on ECG from a normal sinus rhythm?

A
  • Positive P wave before each QRS
  • Normal PR interval
  • Normal QRS duration
  • Normal QT interval
23
Q

What are the effects of ventricular tachycardia?

A

Can impair CO

-> hypotension, collapse and acute cardiac failure

24
Q

What does the presence of a Y wave tell us?

A

Y complex seen due to extreme heart rates & lack of coordinated atrial contraction

25
Q

List some pathological conditions producing characteristic ECG changes

A
  • premature atrial & ventricular contractions (ectopics)
  • atrial / ventricular tachycardia
  • atrial / ventricular flutter
  • sinus arrest & Sinoatrial block
  • AV blocks
  • Bundle Branch Blocks (BBB)
  • ventricular pre-excitation
  • supraventricular arrhythmias
  • AV nodal re-entrant tachycardia
26
Q

What are the 3 arrhythmogenic mechanisms responsible for initiating tachyarrhythmia?

A
  1. Altered automaticity (normal / abnormal)
  2. triggered activity (abnormal re/depolarisation
    • ectopics)
  3. re entry
27
Q

What are arrhythmogenic mechanisms caused by?

A

alterations in ionic currents

28
Q

Describe supraventricular tachycardia effects

A
  • ST depression
  • AVNRT typically paroxysmal - occurs spontaneously or
    on provocation (exertion, caffeine, beta blockers
    (salbutamol) etc. )
  • more common in women
29
Q

How is angina and infarcts caused?

A

due to insufficient oxygen

30
Q

What is angina?

A

chest pain caused during exercise due to chronic arterial narrowing

31
Q

what is an ST depression caused by?

A

Caused by the injury potential difference

32
Q

What is the Injury current effect?

A

ischaemic myocytes have reduced membrane potentials compared to normal. The difference in p.d. between the ischaemic and healthy regions, displaces the ST segment => this is named the ‘injury current’

33
Q

Describe the ECG of an ischaemic patient

A

ST depression

+/- T wave inversion - due to altered repolarisation

34
Q

What does an injury ECG look like?

A

ST elevation
+/- elevation
loss of R wave ‘current of injury’

35
Q

Describe the ECG of an infarction

A

deep Q waves resulting from no depolarisation from necrosis

receding currents from opposite side of hear as ‘electrical window’ formed

Cardiovascular System (16 decks)

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