Cardiac cycle

Question 1

1. Define cardiac cycle and go through the stages.
2. What proportion of cardiac cycle is systole and diastole?
3. What is SVR and PVR?
4. What are the peak pressures for the LV and RV?

Answer 1

1. The time takne to complete one systole and one diastole. See diagram below.
2. 1/3 Systole and 2/3 diastole. The faster the HR this approaches 50:50. THis maximeses SV ejection but compromises diastolic filling and coronary perfusion.
3. SVR= resistance the heart pumps against and is approx 5x higher than PVR
4. Normal peak LV pressure is 120mmHg and RV is 25mmHg

Question 2

Draw a diagram showing the following:

Ventricular contractions

Arterial Pressure

Left atrial pressure

Discuss the stages of each part including values of pressures where possible.

What pathology can be seen in the atrial pressure waves?

Answer 2

Peak LV pressure is approx 120 mmHg. Total volume ejected into aorta is the stroke volume (SV), typically 70 mls.

A trace for the RV would have the same morphology but occur at much lower pressures.

Left atria:

a-wave: Atrial contraction delivering approx 30% of volume to LV

c-wave: Isovolumetric contraction, bulging back of MV into LA so small ↑ pressure

x-descent: As the ventricle contracts this pulls the fibrous atrio-ventricular rings towards the apex of the heart. This comparatively lengthens the atria and causes atrial pressure to fall

v-wave: LA pressure rises due to venous return accumulating in the atria throughout systole whilst the MV is closed

y-descent: MV opens, blood flows into ventricle hence LA pressure falls

Pathology:

AF = absent ‘a’ waves

Tricuspid Regurgitation = Prominent ‘v’, loss ‘c wave ’ & ‘x descent’

AV Junction block = regular cannon ‘a’

Complete Heart Block = irregular cannon ‘a’

Question 3

Pressure volume loops

Draw the LV pressure loop and describe each stage

Answer 3

A pressure-volume loop shows the volume of blood ‘moved’ by the LV and how much pressure is generated to achieve this. The loop represents one cardiac cycle and has volume (mls) on the x axis and pressure (mmHg) on the y axis.

Valve opening, closure, stroke volume and work done can all be shown from the loop.

Valves:

A - MV opens

B - MV closes

C - AV opens

D - AV closes

B→C = Isovolumetric Contraction

D→A = Isovolumetric Relaxation

Stroke volume: SV = LVEDV - LVESV

Work done: Pressure x volume (area inside of loop)

Question 4

Preload

What is pre-load? Draw a diagram to show the affects of increase pre-load on the pressure volume loop

Answer 4

Preload is defined as the end diastolic stretch or tension of the ventricular wall, best shown on a pressure-volume loop by the left ventricular end diastolic volume (LVEDV) on the x-axis. At increased LV volumes the shape of the LV filling curve changes from linear to a gradually increasing gradient.

This can be explained as the heart is relatively easy to fill at normal physiological volumes but as filling continues it becomes progressively more difficult. Aims to prevent over-filling.

The slope of curve = ΔP/ΔV. This is called elastance (and is the reciprocal of compliance).

Increasing preload typically increases SV until overdistension occurs (the Frank Starling Relationship)

Question 5

Contractility

Draw a diagram to show contractility and the effects of inotropes on it.

Answer 5

Contractility is the intrinsic ability of the heart to do mechanical work for a given pre- and after load. It is shown by the slope of end-systolic pressure line, i.e the angle of the end-systolic pressure point with the x-axis.

This contractility line is known as Ees.

If contractility increases then Ees has an increased slope and is therefore rotated up and to the left eg; if catecholamines administered

Question 6

Afterload

What is it and draw a diagram to show what happens if there is an increase in afterload

Answer 6

Afterload is the ventricular wall tension required to eject the stroke volume. It is indicated by slope of straight line joining LVEDV form x-axis to the end-systolic point of the loop (Ea).

If after load increases in isolation then the gradient moves up and to the right.

Question 7

Coronary blood flow

Describes what happens to coronary artery blood flow during the cardiac cycle

Answer 7

Normal conronary blood flow is 200-250ml/min (adult) which is equal to 5% of cardiac output.

O2 extraction is high (55-60%) compared to the rest of the body as a whole (25%).

Coronary perfusion pressure (CorPP) is driving pressure for the coronary circulation. Generated by the difference between aortic pressure and intracardiac pressures and hence varies throughout the cardiac cycle.

The LCA is exposed to considerable pressure from LV during systole. This leads to compression and flow virtually ceases during systole. However, flow through the coronaries to the RV and atria will occur lower pressures.

The immediate endocardial layer directly absorbs O2 from the blood within the cavity. The rest of the heart muscle relies on coronary perfusion.