ECG

The spread of excitation through the atria, ventricles and conducting system is brought about by what

local electrical currents

What is an action potential

the abrupt reversal of the membrane potential to a positive value

In the active depolarised region, the interior of the membrane is _______ charged while in the resting zone ahead is ________ charged

positively

negatively

Conduction velocity in atria

0.5 m/s

Conduction velocity in AV node

0.05 m/s

Conduction velocity in Bundle of His, bundle branches

1.0 m/s

Conduction velocity in Purkinje fibres

5.0 m/s

Conduction velocity in Ventricles

0.5 m/s

Depolarisation of atria & ventricles goes in what direction

Top to bottom

Does repolarisation occur spontaneously

yes

In what direction does repolarisation occur in atria & ventricles

Bottom to top

Resting membrane potential cells are _______ charged inside and __________ charged outside

Negatively charged inside

Positively charged outside

During atrial depolarisation cells are _______ charged inside and __________ charged outside

Positively charged inside

Negatively charged outside

During ventricular repolarisation cells are _______ charged inside and __________ charged outside

Negatively charged inside

Positively charged outside

What is an ECG a recording of

a recording of potential changes at the skin surface, resulting from depolarisation and repolarisation of heart muscle

How does cardiac excitation have any effect on currents around the body

The spread of cardiac excitation creates currents in the extracellular fluid.

The currents generate small potential differences across the body surface of around 1mV

What can ECGs be recorded by and on (& at what speed)

Recorded by a sensitive voltmeter connected to metal electrodes on the skin surface

It is recorded on a strip of moving paper or computer screen to produce the ECG trace

The paper speed is set at 25mm/s (1 large division every 0.2 sec)

The magnitude of the skin potential difference depends on what

the mass of the myocardium that is activated

Does the surface ECG detect activity of the SA and AV nodes

No - just activity of atria and ventricles

The electrical current set up by atrial and ventricular depolarisation is a _____

vector (has both magnitude & direction)

Positive deflection by depolarisation towards = R

Positive deflection by depolarisation away = T

What part of the cardiac cycle do we not see on an ECG? Why?

atrial repolarisation

It is hidden by the QRS complex

The overall direction of depolarisation changes between going towards / away from the detecting electrode which is placed on the leg. Depolarisation in different parts of the heart goes in different directions. In what direction does it go from:

The SA node:

Ventral septum:

Ventricle:

Ventricle last portion:

Repolarisation of heart:

The SA node: Towards AV node - towards electrode - positive deflection (p wave)

Ventral septum: Away from electrode - small negative deflection (q wave)

Ventricle: Towards electrode - large positive deflection (r wave)

Ventricle last portion: Away from electrode - small negative deflection (s wave)

Repolarisation of heart: Away from electrode - positive deflection (t wave)

Why is atrial repolarisation not seen

It occurs at the same time as the QRS

It’s too small to have an effect

What did Einthoven invent

The original electrocardiographic lead system

How did the original electrocardiographic lead system work

Recording electrodes are placed on:
• left arm (LA)
• right arm (RA)
• left leg (LL)
• a fourth electrode on the right leg acts as an electrical earth

A bipolar lead was used in the original electrocardiographic lead system. What does this mean

Recordings are made between 2 electrodes

There are 3 limb leads used in the original electrocardiographic lead system. What was the function of each

Lead I : records potential difference between LA and RA (LA is positive, RA is negative)

Lead II : records potential difference between LL and RA (LL is positive, RA is negative)

Lead III : records potential difference between LL and LA (LL is positive, LA is negative)

Mean QRS axis

The average direction in which the dipole causing the QRS complex is oriented

Usually lies within the 0 º to 90 º quadrant down and to the left.

It corresponds to the anatomical pathway the wave of depolarisation takes

What is that called

Einthoven triangle
oriented in the frontal plane of the body (2 dimensional)

What does the Einthoven triangle represent

2 negative electrodes (RA & LA) and 1 positive electrode are placed on the person’s 2 arms and left leg. By swapping the position of the positive electrode, the direction of the ECG vector changes to go between:

+ 90º with + on leg (aVF - augmented voltage left foot)

- 30º with + on left arm (aVL - augmented voltage left arm)

- 150º with + on right arm (aVR - augmented voltage right arm)

The 3 augmented limb leads aVR, aVL, aVF are unipolar/bipolar

unipolar

What is the hexaxial reference system

The unipolar (aV) and bipolar limb leads (standard) record electrical activity of the heart in the frontal plane of the body

The main deflection is positive for all leads except the aVR

Place the right leads on the right lines using the hexaxial system

- lead I : 0º
- lead II : +60º
- aVF: +90º
- lead III: +120º
- aVR: -150º
- aVL: -30º

When an ECG is taken using electrodes on the chest, how many leads are there and what are they called

6

Called V₁, V₂, V₃, V₄, V₅, V₆,

There are still electrodes on the La, Ra & LL & right foot is earthed

The chest leads examine the heart in what plane

horizontal

Do chest leads give large/small ECG deflections

large

V₁ versus V₆

V1: negative wave - S wave

V6: positive wave - R wave

The depolarisation of the left ventricle is far greater than in the right ventricle

Being a vector, the LV depolarisation cancels out the RV depolarisation

Both V1 and V6 only see a wave of depolarisation heading towards V6 (V1 is somewhat cancelled out)

V1 lead: RS

V6: QR

ECG is recorded at what speed

25mm/s (5 large boxes, 25 small squares)

Spaces between waves are called?

segments

Most important segment and why?

ST segment

Used to test for ischemic activity in the heart

What does the PR interval represent

time from initial depolarisation of the atria to initial depolarisation of the ventricle

Which interval correlates with conduction time through the AV node

PR interval

PR interval length

0.12 - 0.2 s

Sinus arrhythmia

the physiological slowing of firing by the SA node during expiration and acceleration during inspiration

A phasic rise in vagal activity occurs during expiration which is initiated by lung stretch receptors which slows the heart rate

During inspiration vagal activity is reduced and heart rate increases

(this is normal)

first degree AV-nodal block effect on PR interval

the conduction velocity through the AV node is slowed (due to injury), the PR interval is lengthened (>0.20 s)

second degree AV-nodal block * frequently asked

the AV node conducts only some of the supraventricular depolarisations:
- some P waves are followed by QRS complexes
- other P waves are not followed by a QRS complex

In this example there are 3 P waves for every 2 QRS complexes (3:2 ratio)
- other ratios are possible (e.g, 2:1)

Caused by increased injury to the AV node

third degree AV-nodal block

There is complete electrical block between the atria and the ventricles

The AV node fails to conduct any signals

P waves occur regularly

QRS complexes occur regularly but with low frequency (secondary pacemaker)

PVC

Premature ventricular contraction

The heartbeat doesn’t originate within the atria (SA node) like normal. It originates within the ventricle

These ventricular beats disturb the normal rhythm of the heart (arrhythmias)

PVC on an ECG

The PVC is not preceded by a P wave

The next QRS would have been expected at the arrow

The shape, amplitude and duration of the PVC are abnormal

The wave of depolarisation is not conducted by the normal conducting pathways

The diastolic interval after the PVC is long this compensatory pause allows for increased ventricular filling before the next beat

What’s an ectopic beat

A PVC

How does injured heart tissue affect depolarisation

In a normal heart at point d the 2 currents cancel each other out. In an injured heart, the conduction is interrupted.

Depolarisation passes back through injured area and area G is induced to depolarisation again but too early!

An ectopic beat can occur

Ventricular fibrillation

Re-entry can occur causing a series of rapid uncoordinated excitations

These rapid un-coordinated excitations are collectively referred to as ventricular fibrillation (VF)

There is no cardiac output during VF

Death follows within minutes

(Normally, an electrical impulse travels through the heart tissue once, then stops (because the cells behind it are refractory). In re-entry, the impulse doesn't stop — it circles back into areas that are no longer refractory and keeps re-exciting the tissue.)

What can cause Ventricular fibrillation

Ventricular fibrillation is often a fatal consequence to myocardial ischaemia (loss of blood flow to a potion of the muscle in the heart), anaesthetic overdose or electrocution

During ischaemia, ventricular ectopics can occur

How is ventricular fibrillation treated

An electrical shock is delivered through pads or paddles placed on the chest