Lecture 4: Mechanics of the ventricular wall

Question 1

During systole what is notable about the dimension changes of the ventricles?

Answer 1

There is substantial changes the internal dimensions of the left ventricular chamber.

Question 2

What does left ventricle ejection fraction vary between?

Answer 2

50-60% at rest

85% during exercise.

Question 3

What does a range of ejection fraction observations from 50-85% conflict with?

Answer 3

These observations conflict with the fact that cardiac muscle cells can develop force over a relatively limited range of lengths

Shorten ~15%

Question 4

Look at page 31 for the types of strain

Answer 4

now

Question 5

What is special about the sarcomere-length tension relationship in cardiac muscle?

Answer 5

The length tension relationship does not have a descending limb

This is because of the passive stiffness of sarcomeres generated by the titan.

Question 6

What can the force-sarcomere length tension relationship be divided into?

Answer 6

Two curves:

Active Force
Passie Force

Question 7

What does the passive force length tension relation curve explain?

Answer 7

The passive stiffness of sarcomeres so that there is no descending limb of the active force (L-T relation).

Question 8

Describe the passive force LT-relationship:

Answer 8

• Takes no force to extend cardiac muscle passively up to 2micrometers, beyond this the LT-relationship becomes highly non-linear with a limit stop at 2.4micrometers

Abbreviation:

• up to 2micrometers no force
• beyond this highly non-linear with limit stop at 2.4 micrometers

Question 9

Why can’t cardiac muscle be extended beyond 2.4micrometers?

Answer 9

The connective tissue matrix organisation is such that cardiac muscle cannot normally extend beyond 2.4micrometers, Thus ionise not possible to extend sarcomere length so that the descending limb of the LT-relationship is observed.

Question 10

How is cardiac muscle sensitivity different to skeletal muscle?

Answer 10

Cardiac muscle is more sensitive to length change than skeletal muscle.

Question 11

At what point in the force-sarcomere length tension relationship is active force start being generated?

Answer 11

At 1.6micrometers active force is generated. (1.3 in skeletal muscle)

Question 12

What factors contribute to the active force curve on the force-sarcomere length tension relationship?

Answer 12

Length dependant relationship:

• Increased stretch increases troponin c sensitivity to Ca
• Increased stretch increases the number of activated stretch sensitive Ca channels.

Question 13

How is ventricular wall deformation measured?

Answer 13

In terms of strain

Question 14

What is Strain?

Answer 14

Strain quantifies changes in dimension with respect to a undeformed reference state.

Question 15

In a 3d body what is it possible to define?

Answer 15

• Three tensile or compressive strains which represent extension or compression in different co-ordinate directions. (x,y,z)
• Three shear strains which represent changes in shape relative to the reference state.

Question 16

During systole what is a big difference in deformation between the outer and inner wall?

Answer 16

The Inner wall changes shape to a much larger extent than the inner wall.

Question 17

What allow the inner wall to change to a greater extend?

Answer 17

Myocardial layers and shear.

Question 18

How much can the inner wall deform compared to the percent change in fibre length?

Answer 18

The inner wall circumference can shorten by 30% while the fibres only shorten by 15%

Question 19

What allows the the inner wall to change circumference by 30% while the outer wall remains much the same?

Answer 19

Fibre orientation

Question 20

Describe fibre orientation across the heart wall

Answer 20

Midwall fibres are circumferential

Endocardial fibres are longitudinal

Question 21

In what direction are the strains on the endocardium?

Answer 21

Large circumferential strains at the endocardium are in the cross fibre direction

Question 22

What does experimental evidence indicate about the large change in circumference but operating within myocyte contraction length?

Answer 22

Results support the suggestion that TRANSMURAL ROTATION of fibre orientation ensures that myocytes function within the appropriate sarcomere length range throughout the cardiac cycle

Question 23

What does sarcomeres functioning within the appropriate length range mean?

Answer 23

This normalises myofibril stress, work and oxygen consumption.

Question 24

Whats the role of ventricular torsion (twisting) during systole?

Answer 24

During systole twisting of the LV wall helps normalise sarcomere length throughout the cardiac cycle and hence contributes to the efficient of ejection

Question 25

What is the rearrangement of muscle layers during systole known as?

Answer 25

Shear deformation

Question 26

Does transmural rotation of fibre orientation and ventricular torsion explain the dimensional changes during systole?

Answer 26

NO

Question 27

What is the other factor that contributes to dimensional changes during systole?

Answer 27

• Transmural rotation
• Ventricular torsion
• Myocyte Laminar Shearing (rearrangement)

Question 28

Describe why transmural rotation alone doesn’t explain the extent of dimensional change?

Answer 28

• During systole the endocardial region change its transmural dimension (thickness) by around 100%
• However myocyte diameter only increases by around 8% as cells shorten by 15%.

Question 29

What else explains the large deformations during systole?

Answer 29

The large deformations are explained by the rearrangement (shearing) of muscle layers across the ventricular wall during systole.

Question 30

What is maximum shearing associated with?

Answer 30

In the subendocardium maximum shearing deformation is aligned with the myocyte layers.

Question 31

What can computers modelling investigate?

Answer 31

Myocardial function

• Wall stress
• Electrical activation
• Perfusion

Question 32

What do computer models rely on?

Answer 32

Accurate structural and material property data:

• Geometry
• Fibre/sheet architecture
• Mechanical and electrical properties.

Question 33

What is uniform across the heart wall

Answer 33

Sarcomere lengths are relatively uniforms cross the heart wall but orientation changes gradually.

Question 34

Look up page 35

Answer 34

now