1 The mechanical events of the cardiac cycle

Question 1

Describe the different phases in the cardiac cycle

Answer 1

• Diastole: ventricular diastole (diastolic murmur)
• Ventricular systole: heart murmur

Ventricles contract and relax, allowing them to receive blood from the atria (in tandem)
- At rest, diastole lasts twice as long as systole

Question 2

How many subphases are there in the cardiac cycle?

Answer 2

3 in systole

4 in diastole

Question 3

List all the phases/subphases in the cardiac cycle

Answer 3

Atrial systole

Ventricular systole

• Isovolumetric contraction
• Rapid ejection
• Reduced ejection

Ventricular Diastole

• Isovolumetric relaxation
• Rapid ventricular filling
• Diastasis (reduced ventricular filling)

Question 4

Describe Atrial systole

Answer 4

• Depolarisation of the atria (P wave) leads to atrial contraction
  > see A wave on the atrial pressure curve
  > as Atrial muscle contracts, blood is compressed a little + pressure increases
• A tiny amount of ‘topping off’ completely fills the ventricle
  > Rarely, a 4th heart sound may be heard - S4

Question 5

Describe isovolumetric contraction

Answer 5

1st subphase in ventricular systole
- As ventricles start to contract
- A-V valves close when ventricular pressure exceeds atrial pressure (red pressure curve exceeds yellow)
> Mitral and tricuspid
- Isovolumetric contraction begins at the peak of the R-wave of the ECG
- No real change in the volume of the ventricles during this phase (isovolumetric)
- Pressure in the ventricle is not high enough to exceed the pressure in the aorta

First heart sound (LUB) - S1
- caused by the closing of the A-V valves and associated blood turbulence

Question 6

Describe rapid ejection

Answer 6

It is the next (2nd) subphase of ventricular systole
- When ventricular pressure (red) exceeds that in the aorta or the pulmonary artery
- Semilunar valves open + rapid ejection of blood from the ventricles starts (2/3 of stroke volume leaves in this time)
- C wave in the atrial pressure curve is caused by the slight distension of the A-V valves into the atria (normally not measurable)
> Papillary muscles contract to prevent A-V valves from being pushed into the atria (MITRAL + TRICUSPID)

Question 7

Describe Reduced ejection

Answer 7

Coincides with the T wave of the ECG (ventricular repolarisation)

When ventricular myocytes are starting to repolarise (but some are still contracting)
- Blood flow out of the ventricles continues, but happens more slowly (reduced ejection - 1/3 of stroke volume)

Eventually, (as the ventricle starts to relax) pressure in the ventricle falls below that in the arteries

• This allows some arterial blood to flow towards the heart
• The Semilunar valves close (between L ventricle and aorta is aortic valve)

Question 8

Describe isovolumetric relaxation

Answer 8

End of ventricular systole - start of ventricular diastole

During this and the previous 2 phases, the atria have been filling with blood (atop the closed A-V valves)
- and Atrial pressure has been rising gradually

Blood flow out of the ventricles stops (hopefully they are sufficiently empty - isovolumetric)
> end-systolic volume in ventricles is 50ml normally

The 2nd heart sound (DUP) - S2
- Occurs when the semilunar valves close
(aortic and pulmonary valves)
> This sound may ‘split’ on deep inspiration (caused by differences in pressure in the pulmonary artery when a person breathes deeply
> so pulmonary valve closes later than in a person how doesn’t breathe deeply
> instead of LUB-DUP its LUB-DI-DUP

ON ECG, this is the end of T wave - P wave of next cardiac cycle

Question 9

Rapid ventricular filling

Answer 9

When ventricular pressure falls below atrial pressure, the A-V valves open
- This allows blood to flow from the atria into the ventricles
> Rapid rise in the volume of blood (2/3 of volume)

A 3rd heart sound (S3) may be heard in children, who have thinner chest walls (easier to hear)
- BUT in adults, this is usually a clear sign of cardiac problems
> (CHF) - lots of blood coming back to the heart
> congested - fill with blood (turbulent filling)

Question 10

Describe diastasis (reduced ventricular filling)

Answer 10

Filling of the ventricles continues more slowly, as atrial and ventricular pressure rise
- This continues until the ventricles are almost full (around 120ml) when the whole thing starts again
> (60-100 times a min)

Question 11

Describe the jugular venous pulse

Answer 11

Fluctuations in the atrial pressure curve can be transmitted into the venous side of the circulation, including the internal jugular vein (valveless)

Although the pressures involved are low (4-6mmHg in RA),
- if a normal person lies back at around 45˚, the jugular venous pulse (JVP) may JUST be seen, just above the clavicle, between 2 heads of the SCM

The a, c, and v waves of the JVP match those on the arterial pressure profile
- The JVP can be more pronounced in patients with heart failure of fluid overload

The JVP can be distinguished from the carotid pulse by its biphasic nature and the fact it disappears on palpation

Question 12

Describe when systole occurs (in terms of heart sound)

Answer 12

Systole =
- beginning of 1st heart sound to the beginning of 2nd heart sound

LUB to DUB

Question 13

Describe when systole occurs (in terms of heart sound)

Answer 13

Diastole =
- beginning of 2nd heart sound to beginning of 1st heart sound of next heartbeat

DUB to LUB (DUB)