Determinants of Cardiac Output
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What is cardiac output?
it is the liters of blood pumped by each of the ventricles per minute.
What is the normal value of cardiac output?
WHat is the formula for cardiac output?
CO = HR x SV
What can cardiac output do?
It can continuously adjust to meet the body's demand. If the body is exercising, the cardiac output may increase from 5 L/min. to 15 L/min. or more to meet the body's metabolic needs.
What one of two things must change to change the cardiac output?
Heart rate or Stroke volume
What usually happens with a well trained athletes stroke volume and heart rate?
- increased stroke volume
- decreased heart rate
What is the most common way for an increase in heart rate?
an increase in sympathetic activity
What is the most common way for an decrease in heart rate?
an decrease in parasympathetic activity
What is stroke volume?
the average amount of blood pumped out of each ventricle in 1 minute.
What is the formula for stroke volume?
EDV - ESV = SV
End Diastolic Volume - End Systolic Volume = Stroke Volume
What are the three primary mechanisms that regulate EDV and ESV?
- 1.) Preload
- 2.) Afterload
- 3.) Inotropy
A change in what primarily alters EDV?
A change in what primarily alters ESV?
Afterload & Inotropy
What is preload?
It is the initial stretching of the cardiac myocytes prior to contraction: therefore it is related to the sarcomere length at the end diastole.
Since we can't measure sarcomere length, we must measure by diastolic volume or pressure.
So Preload is the "amount of blood in the ventricle at end-diastole"
What does preload describe...
Preload describes the volume that fills the ventricle at the end of passive filling and the atrial contraction. The factors that influence preload.
What are the factors that determine ventricular preload?
- 1.) Increase in outflow resistance & Afterload (AV stenosis)
- 2.) Increase in venous blood pressure (CVP)
- 3.) Ventricular compliance
- 4.) Increase in Heart rate
- 5.) Increase in Atrial contraction
- 6.) Increase in inflow resistance (MV stenosis)
- 7.) Ventricular systolic failure
Compliance and the ventricle can be defined as...
the ratio of a change in volume divided by a change in pressure.
The pressure generated at a given volume is determined by what?
the compliance of the ventricles or the ease by which the ventricle fills.
What is the Frank-Starlings Law of the heart?
Amount of stretch is equal to the amount of contraction.
Increasing venous return and ventricular preload leads to an increase in stroke volume.
What happens if there is changes in the left atrial pressure?
The SV of the left ventricle must adjust to match the right ventricle.
What is afterload?
it is the load against which the heart must contract to eject blood. The greater the pressure in the aorta the greater the afterload.
What is the major component of afterload?
the aortic pressure, because it is the pressure the ventricle must overcome to eject blood. The greater the pressure in the aorta the greater the afterload.
What two things cause an increase in afterload?
- 1.) Hypertension
- 2.) aortic valve stenosis
Because it limits the ability of the stretch against the increased afterload.
Effects of Increased afterload
An increased in afterload will have a negative effect on cardiac muscle shortening and stroke volume.
What is Inotropy?
the force of contraction or the contractile of the myocyte.
What are changes in inotropy caused by?
Changes in inotropy are caused by intrinsic cellular mechanisms that regulate the interaction between actin and myosin independent of changes in sarcomere length.
What is the energy at which the myocytes contract?
What is the arterial pressure against which they have to expel the blood?
What produces a larger stroke volume?
What reduces the stroke volume?
A rise in TPR will do what to SV?
cause a decrease in SV.
TPR is directly related to pressure.
What is another law that influences the stroke volume?
Law of Laplace
What is the Law of Laplace?
It is the relationship that exists between forces within the walls of any curved fluid container (ventricles) and the pressure of it's contents.
What is wall tension?
it is the tension or isometic contraction generated by individual myocytes.
What is significant about the pressure around the ballon?
the pressure is the same all the way around the ballon.
What is significant about the wall tension around the ballon?
the wall tension is greatest where the radius is the greatest.
What is the formula for the maximum wall tension?
- T = PR
- tension = radius x pressure
What is the formula for the lesser wall tension?
- T = PR /2
- tension = pressure x radius / 2
What happens as ventricular volume increases?
- the radius increases
- so than a larger force is reguired from each individual muscle cell to produce any given intraventricular pressure.
What is the relationship between Frank-Starling mechanism and LaPlace Law?
What happens when there is an increase in chamber size?
- increase in contractile force (Frank-Starling)
- increase in afterload (LaPlace's)
- decrease in mechanical efficiency of the ventricle
What pathological situation would make the Law of Laplace worse?
What happens to the stroke volume when there is an increase in afterload?
increase in stroke volume
What is two instances where the stroke volume is low, but the contractility is high?
- 1.) massive hemorragh- large amount of blood loss
- 2.) dehydration
What is the myocardial oxygen consumption?
the amount of oxygen used by the heart (Muscle).
How much of the myocardial ATP is used in cellular processes or basel metabolism?
25 % of the myocardial ATP
How much of the myocardial ATP is used in producing muscle contraction?
75% goes to producing muscle contraction
What will reductions in cardiac preload tend to cause?
a reduction in the energy required for isovolumetric contraction.
What will cause a reduction in the work load of the heart?
an reduction in preload and afterload.
What is one way to decrease preload?
a decrease in volume through urination with the help of directics drugs.
What is one way to decrease afterload?
decrease the blood pressure with the help of Beta Blocks drugs.
What do you need to have the most efficient heart (Cardiac output)?
low heart rate and a high stroke volume.
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