Lecture 4: Mechanics of the ventricular wall Flashcards
(34 cards)
During systole what is notable about the dimension changes of the ventricles?
There is substantial changes the internal dimensions of the left ventricular chamber.
What does left ventricle ejection fraction vary between?
50-60% at rest
85% during exercise.
What does a range of ejection fraction observations from 50-85% conflict with?
These observations conflict with the fact that cardiac muscle cells can develop force over a relatively limited range of lengths
Shorten ~15%
Look at page 31 for the types of strain
now
What is special about the sarcomere-length tension relationship in cardiac muscle?
The length tension relationship does not have a descending limb
This is because of the passive stiffness of sarcomeres generated by the titan.
What can the force-sarcomere length tension relationship be divided into?
Two curves:
Active Force
Passie Force
What does the passive force length tension relation curve explain?
The passive stiffness of sarcomeres so that there is no descending limb of the active force (L-T relation).
Describe the passive force LT-relationship:
- 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
Why can’t cardiac muscle be extended beyond 2.4micrometers?
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.
How is cardiac muscle sensitivity different to skeletal muscle?
Cardiac muscle is more sensitive to length change than skeletal muscle.
At what point in the force-sarcomere length tension relationship is active force start being generated?
At 1.6micrometers active force is generated. (1.3 in skeletal muscle)
What factors contribute to the active force curve on the force-sarcomere length tension relationship?
Length dependant relationship:
- Increased stretch increases troponin c sensitivity to Ca
- Increased stretch increases the number of activated stretch sensitive Ca channels.
How is ventricular wall deformation measured?
In terms of strain
What is Strain?
Strain quantifies changes in dimension with respect to a undeformed reference state.
In a 3d body what is it possible to define?
- 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.
During systole what is a big difference in deformation between the outer and inner wall?
The Inner wall changes shape to a much larger extent than the inner wall.
What allow the inner wall to change to a greater extend?
Myocardial layers and shear.
How much can the inner wall deform compared to the percent change in fibre length?
The inner wall circumference can shorten by 30% while the fibres only shorten by 15%
What allows the the inner wall to change circumference by 30% while the outer wall remains much the same?
Fibre orientation
Describe fibre orientation across the heart wall
Midwall fibres are circumferential
Endocardial fibres are longitudinal
In what direction are the strains on the endocardium?
Large circumferential strains at the endocardium are in the cross fibre direction
What does experimental evidence indicate about the large change in circumference but operating within myocyte contraction length?
Results support the suggestion that TRANSMURAL ROTATION of fibre orientation ensures that myocytes function within the appropriate sarcomere length range throughout the cardiac cycle
What does sarcomeres functioning within the appropriate length range mean?
This normalises myofibril stress, work and oxygen consumption.
Whats the role of ventricular torsion (twisting) during systole?
During systole twisting of the LV wall helps normalise sarcomere length throughout the cardiac cycle and hence contributes to the efficient of ejection
