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Medicine 2 - 04 Atrial Fibrillation > 2 The Electrocardiogram > Flashcards

2 The Electrocardiogram Flashcards

1
Q

What is an electrocardiogram?

A
  • It is a recording of electrical activity of the heart (potentially other muscles)
  • Electrodes are placed on arms, legs, and chest wall to detect electrical activity
  • A graph of voltage against time is produced

These are extracellular recordings

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2
Q

Describe the normal path for conduction of electrical activity through the heart:

A
  • SAN undergoes spontaneous discharge
  • Electrical activity spreads across into R atrium and across into L atrium
    > insulating membrane stops AP going straight down into ventricles
  • There is a slight pause in the discharge of the Atrioventricular (AV) node
    > This is to allow the ventricle to fill completely with blood
  • When the AV node discharges, this activates the bundle of His, and Purkinje fibre system, ensuring near-simultaneous depolarisation of the muscle mass in both ventricles
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3
Q

How many electrodes are placed around the body?

(and what are they connected to?)

and how many on each place?

A

There are 10 electrodes placed around the body, which are connected with cables to ECG machine (Galvanometer)

4 are placed on limbs
(2 on legs, 2 on arms)

6 are placed on the chest

These 10 electrodes work together to produce a 12 lead ECG

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4
Q

What are the positions of the electrodes on the limbs?

A

2 on L+R arm (wrist)
2 on L+R leg (ankle)

but if amputated, place it on the end of the stump, equal distance from the heart

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5
Q

What are the positions of the electrodes on the chest (precordial)?

A

V1 - 4th Right ICS at Sternal Edge
V2 - 4th Left ICS at Sternal Edge
V3 - equidistant between V2 and V4
V4 - 5th Left ICS at the midclavicular line (apex of the heart)
V5 - horizontally left of V4 at Ant. Axillary line
V6 - more horizontal of V4, V5 at Mid Axillary line

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6
Q

Explain the concept of ECG leads, and how the 12 lead ECG is made up

A

ECG leads

  • In order to record a voltage (i.e. a difference in electrical potential)
  • The potential at one electrode has to be compared to that at another electrode (or combination of electrodes)
  • There are 3 active limb electrodes
    (R leg electrode is an earth electrode)
    AND
    6 chest electrodes

BUT
- 6 limb leads (I, II, III, aVR, aVL, aVF) and
6 chest leads (V1 - V6)

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7
Q

Name the two different types of limb leads

A
  • Bipolar leads (leads I, II, III)

- Unipolar (augmented leads) - aVR, aVL, aVF

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8
Q

Describe Limb lead I

A

Lead I

  • Left-arm (LA) electrode: +ve
  • Right arm (RA) eletrode: -ve

Compares LA electrode to RA

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9
Q

Describe Limb lead II

A

Lead II

  • L Led (LL): +ve
  • RA electrode: -ve
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10
Q

Describe Limb lead III

A

Lead III

  • L arm (LA): +ve
  • L Leg (LL): -ve
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11
Q

Describe aVR lead

A

aVR

- Compares RA with an avg. of LA + LL

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12
Q

Describe aVL lead

A

aVL

- Compares LA with an avg. of RA + LL

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13
Q

Describe aVF lead

A

aVF

- Compares LL with an avg. of RA + LA

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14
Q

Describe the 6 chest leads

A

These are simply the electrodes in their positions, measuring in relation to different parts of the heart

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15
Q

Describe what the 12-lead ECG is made up of

A

Together, the 6 chest leads and 6 limb leads come together to form and produce a 12 lead ECG

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16
Q

Give the structures of a 12 lead ECG

A
  • 4 lines: bottom line is an electrical signal from lead II - known as rhythm strip (10s)

The remaining 3 lines are split into 4 sections, each showing the electrical signals of a different lead
- Each first AP of each lead represents the same heartbeat (same electrical signal = in sync)

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17
Q

Discuss leads and dipoles, and their relation to the ECG

A
  • ECG machines use a galvanometer, as an instrument used to detect, measure + determines the direction of small electric current
  • Each ECG lead has a positive and negative pole
  • Leads record the electrical current associated with a dipole (i.e. when there is a difference in charge between one bit of a membrane and another)
  • Thus, when all of the muscle is polarised (i.e. rest), there is no dipole
  • When part of the muscle is depolarised, a dipole exists
  • When all of the muscle is depolarised (i.e. contracting), there is no dipole
18
Q

Explain why isoelectric line presence does not equal inactivity

A

If an isoelectric line is shown, it’s not indicating inactivity

An isoelectric can come about if a muscle is relaxed, polarised, and hence has no dipole

A partially depolarised muscle (partially contracted) will have a dipole, hence producing a positive deflection

A fully contracted, fully depolarised muscle will have no dipole and hence will still produce an isoelectric line (no p.d. measure/low)

19
Q

What effects do depolarising events have on the ECG?

A

Depolarisation moving towards:
- Unipolar electrode
- +ve pole of bipolar lead
> means = positive (upwards) deflection

Depolarisation moving away:
- From a unipolar electrode
- +ve pole of bipolar lead
> means = negative (downward) deflection

20
Q

What effects do repolarising effects have on the ECG?

A

Repolarisation moving towards:
- Unipolar electroke
- +ve pole of bipolar lead
> means = negative (downward) deflection

Repolarisation moving away:
- From a unipolar electrode
- +ve pole of bipolar lead
> means = positive (upward) deflection

21
Q

Describe the typical shape of one AP in lead II of a normal ECG

A
  • P wave - atrial depolarisation
  • QRS complex - ventricular depolarisation
    > Q wave is septal depolarisation
  • T wave is ventricular repolarisation
22
Q

Explain how a T wave can be positive (in most leads) when it is a repolarising event?

A
  • The sub-endocardial myocytes start their AP slightly before those in the sub-epicardium
  • In addition, the AP in sub-endocardial myocytes are slightly larger
  • Hence, the cells that depolarise first also repolarise last (and vice versa)

In effect, this means ventricular repolarisation travels in the opposite direction to ventricular depolarisation, giving a positive T wave in the ECG

23
Q

Describe a P wave on a typical ECG

A

P wave

  • Depolarisation of the atria
  • Right atrial activation begins first
  • Relatively little muscle, therefore small in amplitude
  • Normal waves have a slight notch (bifid)
24
Q

Describe the PR interval on a typical ECG

A
  • Time for conduction through AV node, Bundle of His, Purkinje fibers
  • Time from onset of atrial depolarization to the onset of ventricular depolarisation
  • It is measured from the start of the P wave to 1st deflection of the QRS complex
    < irrespective of whether QRS complex begins with Q wave or R wave
25
Q

What is the normal duration of the PR interval

A

The normal duration is 120-200ms

3-5 small squares

26
Q

Describe what may cause PR interval prolongation

A

It could be 1st-degree heart block

27
Q

Describe the QRS complex in a typical ECG

A
  • Ventricular depolarisation
  • Large muscle mass of the Left ventricle means QRS predominantly shows the LV signal

Q wave: any initial negative deflection
R wave: any positive deflection
S wave: any negative deflection after R wave

28
Q

Give the normals waves for the QRS complex

QRS duration, R wave height, S wave depth

A

Normals values:

  • QRS duration: < 120ms
  • R wave: can be variable (not always)
  • S wave depth: < 30mm
29
Q

Describe what could be the cause of wide QRS complexes

A

Frequent, wide QRS complexes:

- Ventricular ectopic beats

30
Q

Describe what large R waves could signify, and potentially what may cause them

A

Large R waves - Left ventricular hypertrophy

Causes:

  • Aortic stenosis
  • Hypertension (compensatory hypertension)
  • Athletes (due to large ventricular muscle mass)
31
Q

Describe Q waves in a typical ECG

A
  • Normal Q waves can be found in leads facing the left ventricles (leads I, II, aVL, V5, V6)
  • They occasionally occur in lead III
32
Q

Give the normal ranges for Q waves

A

Normal:

  • < 2 mm in depth (2 small squares)
  • < 40ms in duration (1 small square)
33
Q

Describe the causes of an abnormally long Q wave

A

Past infarction of heart muscle

34
Q

Describe the ST segment in a typical ECG

A

ST-segment

  • QRS complex ends at the J point
  • So, ST-segment: J point to start of T wave

It signifies:
- End of ventricular depolarization to the beginning of repolarisation
(muscle is depolarised and is contracting; isoelectric does not mean inactive

It is usually level +/- 1mm from baseline - may slope upwards slightly

35
Q

Describe what an ST-segment elevation is,

and potential causes:

A

ST-segment elevation:
- This is when the ST segment is seen to be positioned higher (elevated)

Causes:

  • STEMI: ST-elevation myocardial infarction
  • Pericarditis: ST elevation would be seen over most of the leads
36
Q

Describe the QT interval in a typical ECG

A

It is the total time for depolarisation and repolarisation to occur

37
Q

Describe what a prolonged QT interval can cause:

A

A prolonged QT interval or ‘Long QT syndrome can cause:

- Serious arrhythmias when the person is exercising or is stressed

38
Q

Describe the T wave in a typical ECG

A
  • Represents ventricular repolarisation
  • It is asymmetrical - steeper on the ascent
  • Rarely exceeds 10mm (height)
39
Q

Describe the U wave in a typical ECG

A
  • Small deflection after T wave

- Many ECG’s have no discernible U wave

40
Q

Name and describe the different available anti-arrhythmic drugs

A

4 classes of drugs:

Class I: Sodium channel blockers (lidocaine)
- Reduce the maximum rate of repolarisation - used to treat ventricular dysrhythmias

Class II: b-adrenoceptor antagonists (atenolol)
- Used to treat tachyarrhythmias. They decrease mortality in post-myocardial infarction

Class III: potassium channel blockers (amiodarone)
- These slow repolarisation and prolong cardiac action potential, thereby increasing the refractory period

Class IV: calcium channel antagonists (verapamil)
- They block L-type calcium channels, slow conduction in the SA node and AV node. They are used to treat supraventricular tachycardias (SVTs)

Others:

  • Adenosine (slows AV conduction) - used in SVTs
  • Digoxin (increases vagal tone - via CNS) - slows AV conduction, used in SVTs

Medicine 2 - 04 Atrial Fibrillation (6 decks)

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