Cardiac Cycle

Question 1

Draw and label a pressure-volume loop

Answer 1

Question 2

Describe the stages in ventricular systole, diastole and atrial systole

Answer 2

Ventricular systole

1. Low LVP at start
2. Isovolumetric contraction - contraction of V causes a rapid increase in VP. This momentarily causes a back flow into the atrium causing mitral valve to close
3. Ejection of blood into aorta only occurs once LVP > aortic P
4. At LVP peak, blood ejection stops, backflow into ventricle closes aortic valve

Ventricular diastole:

1. V muscles relax, P decreases but LVP > atrial P
2. Once LVP < atrial P, mitral valve opens and ventricle fills
3. Phase of rapid filling followed by phase of slow filling until atrial systole

Atrial systole:

• no marked effect, 90% blood already in V

Question 3

What are the 4 heart sounds?

Answer 3

1. Lub - turbulent blood against mitral valve
2. Dub - turbulent blood against semi-lunar valve
3. Whoosh - rapid relaxation of ventricles
4. Atrial contraction

Question 4

Define ECG

Answer 4

A record of the average electrical potential generated in the heart muscle and graphed in terms of voltage against time during the different phases of the cardiac cycle.

Question 5

Draw and label an ECG trace. Including intervals and segments.

Answer 5

Question 6

How is the P wave generated?

Answer 6

• SAN cells in RA depolarise, causes depolarisation of adjacent RA cells
• Wave of depolarisation moves from R to L
• Creates a dipole, with L being more +ve than R
• +ve closer to +ve electrode, causes ECG trace to deflect up
• Straight line after = delay as depolarisation passes through AVN

Question 7

How is the Q wave formed?

Answer 7

• Wave of depolarisation travels down interventricular septum (bundle of HIS)
• Depolarisation spreads from L to R
• -ve end of dipole closer to +ve electrode, causes ECG trace to deflect down

Question 8

How is the R wave formed?

Answer 8

• Endocardium depolarises before the epicardium
• Creates large dipole (lots of cardiomyocytes in L ventricle)
• +ve end of dipole closer to +ve electrode, ECG trace deflects up

Question 9

How is the S wave formed?

Answer 9

• Depolarisation finishes spreading through both ventricles
• ECG returns to baseline
• -ve end of dipole closer to +ve electrode, ECG deflects down

Question 10

How is the T wave formed?

Answer 10

• Epicardium repolarises first before endocardium
• epicardial cells become +ve and create a dipole with -ve endocardial cells (which are still depolarised)
• +ve end of dipole closer to +ve electrode, trace deflects up

Question 11

What is the significance of the intervals on the ECG trace?

Answer 11

• PR - time between atrial and ventricular depolarisation
• QT - length of time ventricles remain depolarised
• PP - time between atrial depolarisations
• RR - time between ventricular depolarisations

Question 12

What is the clinical significance of ECG’s?

Answer 12

• RR interval can be used to work out heart rate
• Arrythmias can be identified
• Gives info on electrolyte balance
• Can be used to determine chamber enlargement