5 - Measurement of Cardiac Output Flashcards
What is cardiac output?
The cardiac output (in volume blood per unit time, L/min) is the blood flow required to match the metabolic demands of the tissue, especially for oxygen delivery and is equal to the stroke volume multiplied by the heart rate
How can you determine cardiac output?
- the sum total of the blood flows to individual tissues OR…
- venous return
What is the equation for cardiac output?
CO = SV x HR
CO = cardiac output SV = stroke volume HR = heart rate
What factors affect cardiac output?
Only factors which influence stroke volume or heart rate
What factors influence heart rate?
- Parasympathetic and sympathetic stimulation
- Alterations in rate of diastolic depolarization
- Alterations in the slope of phase 4 in the SA nodal cell action potential
What does sympathetic stimulation do to the heart rate?
Increase conduction velocity through the AV ode by increasing the slope of phase 4
Faster heart rate
What does parasympathetic stimulation do to the heart rate?
Decrease conduction velocity through the AV by decreasing the slope of phase 4
Slower heart rate
What factors regulate stroke volume?
- Inotropic state of myocytes
- Contraction and relaxation of myocytes
- Dimensions of the ventricles
- Structure of the ventricles
How do we determine stroke volume?
We look at preload and afterload
What is preload?
Diastolic load, that is, the load on the heart at the end of filling
The filling of the ventricle causes stretch, increasing the length of the sarcomere (contractile unit)
What is afterload?
Systolic load
The amount of work it takes to move the contents of the ventricles into the aorta and beyond
The afterload is related to the contractile EFFICIENCY… That is the force generated and the rate at which it is generated to deal with the preload
How would you theoretically determine the preload?
If we measure a single muscle fiber and add a weight, the muscle fiber will stretch corresponding to a particular tension… This is our preload
What does the Frank-Starling Law of the Heart describe?
The Frank-Starling Law of the Heart denotes that the work done by the heart is a function of the length of the muscle fiber.
The amount of work done by the heart is directly proportional to end diastolic volume
Force of ventricular contraction is a function of ventricular end-diastolic muscle length - Related to Ca++ sensitivity, within limits
Why would you measure the end diastolic volume?
End diastolic volume = the amount of blood back in the heart after filling
The more volume that returns to the heart, the more that can be expelled
If there is more end diastolic volume, the heart must generate a higher stroke volume to accommodate this larger volume
Describe the relationship between end diastolic volume and stroke volume
Fairly linear… If the end diastolic volume goes up, the stroke volume goes up
This is the case until the end diastolic volume gets very large and the heart cannot keep up
How does this linear relationship work? How does the heart know to increase stroke volume with increased end diastolic volume?
Changes in the end diastolic volume generate changes in the diastolic pressure because of the increase in tension
The increased tension is felt by the elastic components of the contractile apparatus, especially titin
What is meant by venous return?
The RATE of volume returning to the heart
What is the speed at which the blood returns to the heart dependent upon?
The pressure within the peripheral venous compartment and the pressure within the central venous compartement (superior and inferior vena cava and right atrium)
The difference in these pressures creates a “driving force” for the return
What is the effect of increased pressure within the central venous compartment?
If central venous pressure (or right atrial pressure) increases only a few mmHg, venous return will greatly diminish (the driving force will decrease)