Cvs 8: The Electrocardiogram

Front 1

How is an ECG generated?
- Voltage
- Amplitude

Back 1

VOLTAGE:
- Potential differences between resting and active parts of the myocardium lead to the flow of current in the extracellular fluid
Currents in ecf generate the ECG and are opposite in charge to the myocardial cells
- The ECG records the spread of depolarisation and repolarisation = intracellular recording is negative at rest and positive during systole so the charge of the ecf will be opposite to this
- The currents that generate the ECG flow both ahead of the propagating action potential and behind it
- The action potential is long in the heart - very long compared with the time it takes to propagate over the myocardium = the active zone will occupy the entire ventricular myocardium for 200-300ms before returning to rest
- Depolarisation spreading towards an electrode gives a positive voltage
- Depolarisation spreading away from an electrode gives a negative voltage (with respect to earth)
- Repolarisation spreading towards the electrode gives a negative voltage
- Repolarisation spreading away from the electrode gives a positive voltage

AMPLITUDE:
- The amplitude of the signal depends on:
1. The mass of myocardium from which it is recorded
• In the heart – small structures (SA node, AV node, bundles of His, Purkinje fibres) will be too small to give a measurable signal.
• In the heart – the left ventricle will dominate over the right in normal physiological conditions. This will affect the electrical axis of the heart → will appear to spread towards LH as larger mass.
• The signal from the atria will be smaller than from the ventricles.
• Signals will become larger in conditions of hypertrophy (e.g. in RH hypertrophy axis will appear to spread towards the RH rather than the LH)
2. The rate of depolarisation or repolarisation
• With repolarisation, the rate of change of voltage with time is less than with depolarisation
• Signals associated with repolarisation are smaller than those associated with depolarisation. Repolarisation of the atria will not be measurable in the ECG.
3. The orientation of propagation, relative to the electrode (see diagram)
4. The proximity of the electrode to the myocardium from which it is recording
• Different electrode positions will be ‘looking at’ the nearest part of the heart. There is a relationship between the aspect of the heart that may be affected by disease and the electrode or lead from which the record comes.
- The size of the signal will be dependent on the amount of insulation between the electrode, which is placed on the skin, and the heart, which generates the electrical signal → thinner people have larger ECGs.
- Recordings may be subject to interference from signals from skeletal muscle (so must lie patient down and relax them).

Front 2

What do the different waves signify on an ECG?

Back 2

- The T wave is repolarising so has an opposite sign to the other signals, but is also travelling in the opposite direction and therefore is usually the same as the QRS wave
- R is always positive, so if there are 2 peaks, it will be R and R'

Front 3

How can the spread of the heartbeat be assessed using an ECG?

Back 3

- The heart has a complex geometry.
- To assess the spread of the heart beat through this complex structure, the ECG looks at the spread of the heart beat in both:
• the coronal (using limb leads)
• the transverse plane (using chest leads)

Front 4

Describe the limb leads and what they show

Back 4

- Use limb leads to record the spread of the heart beat in the coronal plane (front to back)
- Connections to left arm, right arm, and left leg. Right leg is connected to earth (ground)
- Bipolar limb leads = record the voltage difference between 2 electrodes:
• Lead I: left arm – right arm
• Lead II: left leg – right arm
• Lead III: left leg – left arm
- Augmented limb leads = compare electrode relative to the ground and amplify x2 to scale:
• aVL: left arm
• aVR: right arm
• aVF: left leg

Front 5

Describe where the chest leads are positioned and what they indicate

Back 5

- Use chest leads to record the spread of the heart beat in the transverse plane
- Connections to positions on the chest in front of the heart; record between these leads and earth
• V1: right edge of the sternum 4th intercostal space
• V2
• V3
• V4: fifth intercostal space, midclavicular line
• V5
• V6: mid-axillary line, fifth intercostal space
- Progression of R wave = R wave gets larger as move around the chest as the electrical axis is towards left and this is where repolarisation begins

Front 6

How can you determined heart rate from an ECG?

Back 6

Heart rate = 300 / number of large squares

Front 7

Describe the action potential in ischaemia e.g. in MI

Back 7

- The cardiac action potential is shortened in ischaemia: owing to activity of potassium channels that open in a compromised metabolic state.
- As a result there will be current flow between the ischaemic and the normal myocardium during systole giving rise to a signal during the interval between QRS and T.

Front 8

Describe angina on an ECG

Back 8

- The cardiac action potential is shortened in ischaemia: the ischaemic region has returned to rest during the ST interval
- But the non-ischaemic region is still active
- Contraction = squashes artery so 1st effects seen on the inner surface, which never gets away from its resting potential
- Myocardium anterior to this is healthy, so can see a current passing backwards from healthy to unhealthy parts
- Recorded with an ST depression as flowing backwards

Front 9

Describe the ECG recording in MI

Back 9

- Acute MI has severe transmural ischaemia
- Current flows away from ischaemic region to healthy region
- The cardiac action potential is shortened in ischaemia: the ischaemic region has returned to rest during the ST interval
- Unaffected heart is still depolarised
- Records an ST elevation (hence STEMI) = can indicate the degree of ischaemia

Front 10

When considering an ECG how should it be assessed?

Back 10

1. Is there any electrical activity?
2. What is the ventricular rate? = 300/R-R interval. Usually 60-120bpm
3. Is the QRS rhythm: regular or irregular (or regularly irregular)?
4. Is the QRS complex normal width? (normally <3 small squares)
5. Is there atrial activity present?
6. Is atrial activity related to ventricular and how = PR interval? Normally 3-5 small squares

Front 11

How would you determine a first degree heart block on ECG?

Back 11

- Constantly prolonged PR interval
- Indicates that it is taking longer than usually to leave the AV node

Front 12

Describe how complete heart block appears on ECG

Back 12

- 3rd degree heart block
- High number of P waves per minute and each arises completely independent of the QRS complex

Front 13

Describe the pattern of atrial fibrillation on the ECG

Back 13

- No clear P waves
- Baseline is wiggly
- QRS normal but irregular
- V1 shows flutter waves

Front 14

Describe the pattern of ventricular fibrillation on the ECG

Back 14

- No QRS complexes and baseline wanders
- No organised contraction

Front 15

How can you tell a left and right bundle branch block on an ECG

Back 15

On left bundle branch block:
- V1 looks like a W
- V6 looks like a M (with an eye of faith)
- Remember WiLLiaM
For right branch block the other way around = remember MaRRoW

Front 16

How can you tell on an ECG if there is an axis deviation?

Back 16

- Normal axis range = -30 to +90
- Remember UDDL and DUUR
- Left deviation
• Lead I = up
• Lead II = down
• Lead III = down
- Right deviation:
• Lead I = Down
• Lead II = up
• Lead III = up