Lecture 21: Cardiac Output Flashcards
Preload
- Extent of ventricular filling
- Workload imposed on the heart before contraction begins
- Also called end-diastolic volume
Afterload
The resistance that the ventricles must overcome to empty its content
Contractility
- The strength of contraction at any given end-diastolic volume
- Agents that produce an increase in contractility are said to have positive inotropic effects
Positive inotropic agents increase
rate and force of contraction
Examples of positive inotrophic agents
- Norepinephrine
- Epinephrine
Sympathetic stimulation increases stroke volume by
strengthening cardiac contraction and also by enhancing venous return
What shifts the Frank-Starling curve to the left
sympathetic stimulation
Agents that produce a decrease in contractility are said to have what kind of effects
Negative inotropic effect
Negative inotropic agents decrease both
the rate of tension development and the peak tension
Example of negative inotropic agent
Propranol
Two types of control influence stroke volume
Intrinsic control
Extrinsic control
Intrinsic control
The inherent ability of the heart to vary the stroke volume
Intrinsic control depends on
the length-tension relationship of cardiac muscle
Length tension relationship of skeletal muscle
Resting muscle length is approx. at optimal length (I0) at which maximal tension can be developed during a subsequent contraction
When the skeletal muscle is longer or shorter than the optimal length, the subsequent contraction
is weaker
For cardiac muscle, the resting cardiac muscle fiber length is _____ than optimal
less
An increase in cardiac muscle fiber length
increases the contractile force of the heart on the following systole
Main determinant of cardiac muscle fiber length is
the degree of diastolic filling
The greater the extent of the diastolic filling,
the larger the end-diastolic volume and the more the heart is stretched
The more the heart is stretched,
the longer the initial cardiac fiber length before contraction
An increased cardiac muscle fiber length before contraction leads to
a greater force on the subsequent cardiac contraction which leads to a greater stroke volume
The intrinsic relationship between the end-diastolic volume and stroke volume is known as
The Frank-Starling Law of the Heart
Two factors that extrinsically control the stroke volume
- Cardiac sympathetic nerves
- Epinephrine
These factors enhance the contractility of the heart
Increased contractility triggered by norepinephrine and epinephrine
- Increased Ca influx
- Extra Ca allows the myocardial fibers to generate more force through greater cross-bridge cycling than they would without sympathetic influence
