3. Determinants fo Cardiac Output Flashcards
List the determinants of cardiac output.
- VENOUS RETURN
- Preload
- Afterload
- Contractility
- HR
Energetically, are HR and SV equivalent to increase cardiac output? Explain.
No. SV primarily drives cardiac output b/t it’s linked to venous return
Equation for work done by heart
Work = force x distance
- force = directly proportional to pressurization of blood
- distance = ejection distance (how far fluid is squeezed)
Define external work.
Movement of blood from the ventricles to the great vessels (stroke work)
Equation for stroke work
Stroke work = arterial pressure x SV
Define internal work.
- Work done against structural elements in the heart
- Mostly dissipated as heat
What happens to internal and external work when you increase SV? Explain.
Increase SV –> filling more or empying more completely –> maximizes external work (based on stroke work equation) and minimizes internal work
For a given SV, which is more energetically costly: an increase in volume or pressure? Why?
PRESSURE.
High BP –> heart has to keep squeezing during isovolumetric contraction –> additional work done on blood that pressurizes it to vessel-damaging pressures –> heart can’t contract as far –> lost stroke volume
What happens to the external and internal work when you increase volume load?
External work increases more than internal work
Define preload.
- Ventricular filling
- Central venous pressure
Define afterload.
Increased aortic pressure
What is the effect of preload on SV?
Increased SV
What is the effect of afterload on SV?
Decreased SV
List the 2 venous compartments.
- Central venous compartment
- Peripheral venous compartment
Central vs. Peripheral venous compartment
CENTRAL:
- provides preload for ventricular filling
- always less than 1L
PERIPHERAL:
- 3.5-4L sitting in peripheral veins
- 4x as large as central venous compartment at rest
What is the significance of veins’ high compliance?
Can have large changes in volume w/ little change in pressure
Where is the majority of the blood volume located? What % of blood volume is this?
60% of blood volume is peripheralized in venous compartment
Equation for flow of venous return.
What pressure gradient b/t the venous compartments favors venous return?
- High peripheral venous pressure
- Low central venous pressure
How does muscular contraction enhance venous return?
Contract a muscle —> presses on veins —> no backflow distally b/c of closed valves —> only direction of flow is back to the heart
How does the venous muscle pump affect the volume and pressure of blood it pumps?
- Amount of blood that gets pumped is proportional to how much muscle you activated
- Pressure at which it’s pumped out is proportional to how hard you contract that muscle
What is the relation b/t ventricular volume and tension?
Pre-loaded resting cardiac muscle length affects the magnitude of developed tension
At steady state, venous return equals __?
Cardiac output
How does central venous pressure affect venous return? Cardiac output?
- Decrease venous return
- Increase cardiac output
What does the equilibrium values of central venous pressure tell us?
- CVP sufficient to stretch the heart for cardiac output
- CVP low enough against driving pressure of venules to get same amount of drainage from venules
How does afterload affect blood flow?
Increases amount of time isovolumetric contraction lasts –> impedes forward flow –> decreases SV
How does preload affect blood flow?
Define contractility.
How much force of contraction generated for a given unit of preload
How does increased contractility affect the heart and blood flow?
For same length and ventricular filling:
- greater peak tension
- greater shortening
- greater SV
What are the short and long term ways to increase contractility?
- Short term: catecholamine (norepinephrine) stimulation
- Long term: cardiac hypertrophy (increase myofibrils per myocyte)
What changes w/ contractile state?
End Systolic Pressure Volume Relation (ESPVR)
