Lecture 14: Contractility and Cardiac Output Flashcards
What is cardiac output?
Amount of blood ejected by each ventricle in one minute
How do you calculate cardiac output?
Cardiac Output = Heart Rate x Stroke Volume
What are the four factors that can affect cardiac output?
Heart Rate
Myocardial Contractility
Preload
Afterload
What is preload?
Easy definition: Amount of blood in ventricles before start of contraction
Proper definition: Amount of stretch in muscle fibers of heart at beginning of left ventricular systole
Describe the Frank-Starling relationship.
Volume of blood ejected by ventricle depends on how much blood was in left ventricle at end of diastole
What is afterload?
Resistance ventricles must overcome to eject blood to open aortic/pulmonary valve
If there is a greater amount of afterload, what happens to the contraction?
Contraction is slower
-more force needs to be generated and length tension curve is not ideal
What is contractility?
How hard myocardium contracts for a given preload
What is contractility generally proportional to?
Amount of calcium available to troponin on actin filaments of contractile apparatus
If heart rate is increased, what happens to contractility?
Increase in contractility (inotropic effect)
-more calcium enters cell and taken up by SR
How do cardiac glycosides work?
Goal: Increase sarcoplasmic Ca2+ concentration inside cells to boost muscle contraction and treat heart failure
Cardiotonic drugs inhibit Na+/K+ pump (more Na+ inside cell). This inhibits Na+/Ca2+ exchanger, causing increased sarcoplasmic Ca2+ concentration
How do you calculate stroke volume?
Stroke Volume = End Diastolic Volume - End Systolic Volume
*should be around 70 mL
How do you calculate ejection fraction?
Ejection Fraction % = Stroke Volume/End Diastolic Volume
If preload increases, what happens to cardiac output and contractility?
Both increase
If afterload increases, what happens to cardiac output and contractility?
Cardiac output decreases. Therefore, heart rate and contractility must increase to compensate
Describe what happens with a sympathetic, positive inotropic effect?
Contractility and Cardiac Output increase
- β adrenergic activation
- phosphorylation of SR calcium channels
- phosphorylation of (inhibitory) troponin I
Describe what happens with a parasympathetic, negative inotropic effect?
Atria only affected
-less calcium enters the cell and more potassium leaves the cell
Describe what is going on with the ventricular pressure volume loop.
- Left Ventricle is filling with blood (minimal change in pressure)
- Mitral Valve closes and pressure builds (no chance in volume)
- Aortic valve opens and pressure continues to increase but volume starts to go down because of ejection.
- Aortic valve closes and left ventricle relaxes. Pressure decreases but there is no change in volume.
What is isovolumetric contraction?
When volume stays the same, but pressure increases
What is isovolumetric relaxation?
When volume stays the same, but pressure decreases
What changes in the ventricular pressure volume loop if there is an increased preload?
Left ventricular volume increases but pressure levels stay same
What compensation occurs when there is an increased preload?
Increased stroke volume
Increased force of contraction
What changes in the ventricular pressure volume loop if there is an increased afterload?
Left ventricular pressure increases
Starting volume is increased (not as much blood is ejected)
What compensation occurs when there is an increased afterload?
Reduced stroke volume
Reduced ejection fraction
In what conditions do we see an increased afterload?
Aortic stenosis
Hypertension
What changes in the ventricular pressure volume loop if there is increased contractility?
Left ventricular pressure increases
Starting volume is decreased (more blood is ejected)
What compensation occurs when there is increased contractility?
Increased stroke volume
Increased ejection fraction
