P01

Question 1

What is an ECG?

Answer 1

An ECG provides a visual representation of the spread of electrical events through the heart.
This electrical activity is detected on the surface of the body using recording electrodes.
Leads consisting of a pair of electrodes (one positive and one negative) are used to record the electrical activity

Question 2

What are the electrical events of the heart in the ECG?

Answer 2

See picture.

Question 3

What are the 12 ECG leads?

Answer 3

The 3 standard limb leads I, II, and III.
The 3 augmented limb leads aVR, aVL, and aVF.
The 6 unipolar chest leads V1 to V6.

Question 4

What is the arrangment of the 3 standard leads (limb leads)?

Answer 4

Lead I - RA –> LA (right arm to left arm).
Big upwards deflection, as it goes from negative to positive.
Lead II - RA—>LL (left leg).
Big upward deflection, as goes from negative to positive in the same direction.
Leads I and II look at the heart from a left view.
Lead III - LA–>LL.
Looks at heart from a right view.
See picture.

Question 5

What does a depolarising vector look like?

Answer 5

A depolarising vector that moves toward a positive electrode (i.e. away from a negative electrode) produces a positive deflection on the ECG trace.
A depolarising vector that moves away from a positive electrode (i.e. toward a negative electrode) produces a negative deflection on the trace.
The magnitude of these deflections is greatest when the vector is moving directly toward the electrode (i.e. following the axis of the lead).
A vector that moves at right angles to the axis of a lead produces no deflection on the trace.

Question 6

What does a repolarising vector look like?

Answer 6

The opposite directions to depolarising.
A repolarising vector that moves toward a positive electrode (i.e. away from a negative electrode) produces a negative deflection on the ECG trace.
A repolarising vector that moves away from a positive electrode (i.e. toward a negative electrode) produces a positive deflection on the trace.

Question 7

When a wave of repolarization travels toward a negative electrode, how is this represented on an ECG tracing?

Answer 7

Upward from the baseline.

Question 8

What planes do the ECG leads view the heart in?

Answer 8

The limb leads view the heart in the coronal plane, while the chest leads view the heart in the transverse plane

Question 9

What is the summation of the two arrows?
See picture.

Answer 9

See picture.
Add the arrows tip to tail, then complete the triangle they form.

Question 10

What are the augmented leads?

Answer 10

Augmented leads are unipolar, so sum from the two leads.
aVR is composed of RA, average of LA–LL.
aVL is composed of LA, average of RA–LL.
aVF is composed of LL, average of RA–LA.
See pictures.

Question 11

How does electrical activity arise in the heart, and what pathway does it follow?

Answer 11

Autorhythmic cells of the SAN in the right atrium generate action potentials which travels along this conduction pathway:
AV node
Bundle of His
Left and right bundle branches.
Purkinje Fibres

Question 12

What does the Wiggers diagram show?

Answer 12

see picture

Question 13

What relationship would you expect between the QRS complex of an ECG, and the start of left ventricular contraction (ventricular systole)?

Answer 13

The QRS complex would precede the start of left ventricular contraction.
The QRS complex therefore precedes the increase in left ventricular pressure (which results from ventricular contraction).

Question 14

What is the temporal relationship between the pressure in the left ventricle and heart sounds?

Answer 14

See picture.
The first heart sound is a result of the AV valve closing, which is because the pressure in the ventricle exceeds the atrial pressure.
There is isovolumetric contraction because the AV valve is closed so no change in volume.
The second heart sound is the aortic valve closing, which is because the pressure in the aortic is greater than in the ventricle.
There is isovolumetric relaxation because the aortic valve and AV valve is closed so no change in volume.

Question 15

What are the heart sounds?

Answer 15

Closure of the mitral and tricuspid valves produces the first “lub” sound, while closure of the aortic and pulmonary valves produces the second “dub” sound.

Question 16

What is the correlation of valves opening/closing and pressure changes?

Answer 16

Mitral valve opens - atrial pressure is greater than ventricular pressure.
Mitral valve closes - ventricular pressure is greater than atrial pressure.
Mitral and aortic valve closes - ventricular pressure is greater than atrial pressure but less than aortic pressure.
Aortic valve opens - ventricular pressure is greater than aortic pressure.
Aortic valve closes - aortic pressure is greater than ventricular pressure.

Question 17

Describe the physiological events occurring in the heart muscle represented by each ECG component (the P wave, QRS complex, and T wave). Why does the QRS complex have the largest amplitude?

Answer 17

P-wave - atrial depolarisation, Na+ enters through VGNaC.
QRS complex - ventricular depolarisation, Ca2+ enters through L-type Ca2+ channels.
T wave - ventricular repolarisation, K+ leaves cell through delayed rectifier channels.
QRS has the largest amplitude because there are more cardiac ventricular myocytes than atrial myocytes.

Question 18

Why does the first heart sound occur after the QRS complex?

Answer 18

Because the QRS complex causes depolarisation of the ventricles, which then leads to contraction of the ventricles.
Contraction then increases the pressure in the ventricles, and causes it to increase above the atrial pressure, so mitral and tricuspid valves close - the first heart sound.

Question 19

Why does the second heart sound occur after the T wave?

Answer 19

The t-wave represents ventricular repolarisation - and causes ventricular relaxation and the pressure in the ventricles to decrease.
Second heart sound occurs when the aortic and pulmonary valves close, due to the pressure of the ventricles being less than the aorta and pulmonary artery.

Question 20

Describe the relationships between the first and second heart sounds, and the finger pulse.

Answer 20

First heart sound caused by closure of mitral and tricuspid valves due to ventricular contraction, which leads to ejection of blood from the heart.
Finger pulse is slightly after the first heart sound and the aortic valve opens
There is a slight delay in the pressure wave going from the arteries to the finger.
Second heart sound is closure of the aortic and pulmonary valves, caused by ventricular relaxation. no more blood is ejected.

Question 21

Would the relationship be the same if you had recorded the carotid pulse?

Answer 21

The upstroke of the carotid pulse nearly coincides with S1 or follows it by a very short delay.
Finger: The pulse wave arrives later due to the longer distance and slower transmission through smaller arteries.
Both carotid and finger pulses end before or around S2, as this marks the closure of the semilunar valves and the end of systole.

Question 22

How is HR calculated from an ECG?

Answer 22

Count the number of QRS complexes in 10s then multiply by 6.
For average heart rate, measure the R-R intervals of 4 complexes. Then calculate 60/R-R interval (or 60000), for each, then add and divide the heart rates to find the average.
The average is used because individual R-R intervals vary slightly.

Question 23

What is the R-R interval?

Answer 23

This represents the time from one ventricular depolarisation to the next, and shows heart rate.

Question 24

Why is there normal variation in R-R intervals?

Answer 24

The R–R interval can vary with breathing.
Inspiration → heart rate increases → shorter R–R.
Expiration → heart rate decreases → longer R–R.
The sympathetic branch speeds up the heart rate.
The parasympathetic branch (via the vagus nerve) slows it down.
The balance between these systems causes beat-to-beat changes in R-R intervals.

Question 25

Why do different people have different R-R intervals?

Answer 25

Cardiovascular health Stress BMI Age Medications Fitness levels Genetics Diet

Question 26

Which ECG lead has the biggest amplitude?

Answer 26

Lead II, because it is parallel to the depolarisation from the heart. This is because lead II measures from the right atrium to the left leg. It is a positive deflection because it is a depolarisation from a negative electrode to a positve electrode.

Question 27

What does the P-R interval on the ECG represent?

Answer 27

Time from start of atrial depolarisation to the end of depolarisation - the atria are contracting and ejecting blood. It marks the start of QRS complex - ventricular depolarisation. It shows the delay of the AV node.

Question 28

Can you think of an example when the R-R interval and the P-R interval might change?

Answer 28

During exercise the P-R and R-R interval are shortened. Heart rate is increased, so reduced R-R interval. Ventricular delay is reduced, as ventricular conduction is increased, so P-R interval is reduced. This occurs because of sympathetic stimulation of the SA node.

Question 29

Can you think of an example when the R-R interval and the P-R interval might change due to diseases?

Answer 29

Shortened p-r interval - Wolff-parkinson white syndrome. Pheochromocytoma causes excessive adrenaline release. Adrenaline acts via beta1 adrenoceptors on the SAN to increase the steepness of the pacemaker current and elevate heart rate. So giving a beta-adrenoceptor blocker will reduce the steepness of the pacemaker current to act to lower heart rate.

Question 30

What is responsible for inducing changes in the P-R interval in exercise?

Answer 30

Increased sympathetic tone: Exercise stimulates the sympathetic nervous system, which enhances AV nodal conduction. Reduced vagal tone: Parasympathetic (vagal) influence decreases, further speeding up conduction. Higher heart rate: Faster heart rates naturally compress the cardiac cycle, including the P-R interval.

Question 31

Why is it called a 6 lead ECG when there are only 5 electrodes?

Answer 31

Because there are 6 views of the heart.

Question 32

In the 6 lead ECG, why do different leads have different amplitudes?

Answer 32

The QRS complex reflects ventricular depolarization, which has a dominant vector (usually leftward and downward). Leads aligned with this vector (like Lead II) show larger amplitudes. Leads perpendicular to the vector (like aVR) show smaller or biphasic QRS complexes.

Question 33

How does lead amplitude vary across individuals?

Answer 33

Factors like body fat, muscle mass, or lung inflation can affect how electrical signals travel e.g. obesity or emphysema can reduce QRS amplitude due to increased distance or poor conductivity. Myocardial infarction, bundle branch blocks, or cardiomyopathies can alter the shape and amplitude of the QRS complex in specific leads A larger heart, especially with left ventricular hypertrophy, generates stronger electrical signals, resulting in higher QRS amplitudes The orientation of the heart in the chest affects how well the electrical vector aligns with the ECG leads.