Cardiac Muscle Mechanics Flashcards
Which is most important in increasing cardiac output: Heart rate, or stoke volume? Explain.
Heart rate because it can increase by up to 200% while stroke volume can only increase up to 50%
Does a 100% increase of heart rate result in a corresponding 100% increase in cardiac output?
No, it results in a greater than 100% increase in cardiac output because stroke volume increases also!
What three factors regulate stroke volume?
Preload, afterload, and inotropy (contractility)
What is preload?
The initial stretching of the cardiac myocyte prior to contraction.
What is the relationship between sarcomere length and force of contraction?
Increased stretch of sarcomeres results in stronger contraction
What were Ernest Starling’s major experimental findings relating to filling pressure and ventricular contractile force?
When central venous pressure (venous return to the RV) increased, the RV end-diastolic volume and pressure also increased. This led to increased RV contractile force and a peak RV pressure, which transferred to the pulmonary circulation and into the left heart. After a few heart beats, LV filling and contractility also increased, increasing stroke volume.
What is the Frank-Starling mechanism aka Starling’s law of the heart?
Increased venous return –> increased ventricular filling –> increased preload –> increased contractile force –> increased stroke volume
Which two physiologic processes explain increased myocyte contractility with increased sarcomere stretch. Explain each.
- Overlap: At a sarcomere length of less than 2 micrometers the opposing actin filaments overlap or buckle, interfering with cross-bridge formation. From 2.0 to 2.4 micrometers thick and thin filaments are more optimally aligned for cross-bridge formation and force development.
- Length-dependent activation: Contractile proteins (esp. TnC) increase in sensitivity to [Ca2+] when they are stretched.
How is compliance related to the Frank-Starling mechanism?
Increased compliance increases sarcomere stretch and end-diastolic volume, and decreased compliance decreases sarcomere stretch and end-diastolic volume.
What is the difference between heterometric and homeometric regulation?
Heterometric regulation is the regulation of stroke volume by altering preload via venous return/filling pressure and is sarcomere length-dependent.
Homeometric regulation is done through the alteration of afterload and is not dependent on sarcomere length.
What is afterload?
The load against which the heart must contract to eject blood.
What contributes to afterload for the left ventricle? What about the right ventricle?
Aortic pressure for the left ventricle. Pulmonary pressure for the right ventricle.
How does wall stress relate to intraventricular pressure, ventricular radius, and ventricular wall thickness?
wall stress = P x r / h
where P is the intraventricular pressure, r is the radius of the ventricle, and h is the wall thickness.
Afterload ________ (increases or decreases) in ventricular dilation.
Afterload ________ (increases or decreases) in ventricular hypertrophy.
In dilation, afterload increases and in hypertrophy afterload decreases.
How does afterload affect myocyte shortening velocity?
Increased afterload decreases shortening velocity.
Explain how preload increases as a result of increased afterload.
An increase in afterload will result in decreased stroke volume due to a smaller ejection fraction, leaving more blood in the chamber after systole (ESV). In subsequent heart beats, venous return will add more blood to the already partially filled ventricle, leading to increased stretch and an increase in stroke volume.
Can changes in afterload affect Vmax (myocyte shortening velocity) on a Frank-Starling curve?
No, Vmax remains the same.
What is inotropy/contractility?
An intrinsic measure of cardiac function that is not affected by extrinsic factors like preload and afterload.
Can changes in inotropy/contractility change Vmax on a Frank-Starling curve?
Yeah
What effect on the end-systolic pressure-volume relationship (ESPVR) line does increasing inotropy have?
An increase in inotropy would increase the slope of the ESPVR line, resulting in increased stroke volume at any preload or afterload.
