How are preload and afterload measured?

  1. Can someone explain to me how preload and afterload are measure? My book says that preload is measure by ventricular end diastolic measures but does not go into detail, and it does not say anything about how afterload is measure. If anyone can explain it to me, I would be very appreciative. Thank you.
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  2. 5 Comments

  3. by   TheCommuter
    Welcome! Your request for help with understanding preload and afterload has been moved to the Nursing Student Assistance forum: http://allnurses.com/nursing-student-assistance/
  4. by   Esme12
    Welcome To AN! The largest online nursing community!

    Preload
    is about volume. If the water pressure is low, the water output will be a trickle, not enough to water and sustain your pretty garden. If the volume is too much, it will back up your plumbing system (Right-sided heart failure engorged liver, systemic edema, etc, or left-sided failure pulmonary edema.)

    Afterload is about pressure or resistance. If there is a link or narrowing in your garden hose, the volume will back up AND the output will drop. Think of it like a garden hose.........

    A good analogy is that your hose has gone form a garden hose to a fire hose or developed huge leaks and it will either get more blood to the right places easier (decreasing afterload so the heart doesn't have to work as hard and will therefore work better) ...or you will not get the blood to where it need to go until you "patch it up" (constrict the blood vessels to a more normal size like the vasodilation that occurs with sepsis)or stop the bleeding by sealing the leak(the source of hemorrhage)....OR....

    Gross analogy, but it works...Think about flushing a toilet......you flush, and then flush again right away......nothing happens right? This is because the tank doesn't have time to fill....PRELOAD is decreased in the tank.

    What if the toilet is plugged up? When you flush, it backs up....this is too much afterload.

    Preload = Volume
    Afterload = Pressure/Resistance
  5. by   Esme12
    Cardiac output is the volume of blood pumped by the heart per minute (mL blood/min). Cardiac output is a function of heart rate and stroke volume. The heart rate is simply the number of heart beats per minute.

    The stroke volume is the volume of blood, in milliliters (mL), pumped out of the heart with each beat. Increasing either heart rate or stroke volume increases cardiac output.

    Cardiac Output in mL/min = heart rate (beats/min) X stroke volume (mL/beat)

    An average person has a resting heart rate of 70 beats/minute and a resting stroke volume of 70 mL/beat. The cardiac output for this person at rest is:
    Cardiac Output = 70 (beats/min) X 70 (mL/beat) = 4900 mL/minute.

    The total volume of blood in the circulatory system of an average person is about 5 liters (5000 mL). According to our calculations, the entire volume of blood within the circulatory system is pumped by the heart each minute (at rest). During vigorous exercise, the cardiac output can increase up to 7 fold (35 liters/minute)

    Cardiac Output
  6. by   Esme12
    A post of mine from a long time ago....

    Preload is stretch. The amount of volume being returned to the right side of the heart from systemic circulation.
    Afterload is squeeze. The amount of resistance the left side of the heart has to overcome in order to eject blood.

    The right side of the heart, composed of the right atrium and ventricle, collects and pumps blood to the lungs through the pulmonary arteries. The lungs refresh the blood with a new supply of oxygen, making it turn red.

    Oxygen-rich blood, "red blood," then enters the left side of the heart, composed of the left atrium and ventricle, and is pumped through the aorta to the body to supply tissues with oxygen.

    Four valves within your heart keep your blood moving the right way. The tricuspid, mitral, pulmonary and aortic valves work like gates on a fence. They open only one way and only when pushed on. Each valve opens and closes once per heartbeat — or about once every second.

    A beating heart contracts and relaxes. Contraction is called systole, and relaxing is called diastole.

    During systole, the ventricles contract, forcing blood into the vessels going to your lungs and body — much like ketchup being forced out of a squeeze bottle. The right ventricle contracts a little bit before the left ventricle does. The ventricles then relax during diastole and are filled with blood coming from the upper chambers, the left atria (contains oxygenated blood from the lungs/luminary vein)and right atria (from the Superior/inferior vena cava). Then the cycle starts over again.

    The heart is nourished by blood vessels called coronary arteries extend over the surface of your heart and branch into smaller capillaries. The heart also has electrical wiring, which keeps it beating. Electrical impulses begin high in the right atrium and travel through specialized pathways to the ventricles, delivering the signal to pump. (PQRST) The conduction system keeps the heart beating in a coordinated and normal rhythm, which in turn keeps blood circulating. The continuous exchange of oxygen-rich blood with oxygen-poor blood is what keeps us alive.

    http://www.learntheheart.com/EKGreview.html
    ECG Learning Center - An introduction to clinical electrocardiography
    ECGpedia





    Preload is about volume. If the water pressure is low, the water output will be a trickle, not enough to water and sustain your pretty garden. If the volume is too much, it will back up your plumbing system (Right-sided heart failure engorged liver, systemic edema, etc, or left-sided failure pulmonary edema.)

    Afterload is about pressure or resistance. If there is a link or narrowing in your garden hose, the volume will back up AND the output will drop. Think of it like a garden hose.........

    A good analogy is that your hose has gone form a garden hose to a fire hose or developed huge leaks and it will either get more blood to the right places easier (decreasing afterload so the heart doesn't have to work as hard and will therefore work better) ...or you will not get the blood to where it need to go until you "patch it up" (constrict the blood vessels to a more normal size like the vasodilation that occurs with sepsis)or stop the bleeding by sealing the leak(the source of hemorrhage)....OR....

    Gross analogy, but it works...Think about flushing a toilet......you flush, and then flush again right away......nothing happens right? This is because the tank doesn't have time to fill....PRELOAD is decreased in the tank.

    What if the toilet is plugged up? When you flush, it backs up....this is too much afterload.

    Preload = Volume
    Afterload = Pressure/Resistance

    CV Physiology | Pressure/BP010.htm
  7. by   Mavrick
    I suppose the simplest answer to your question is in "mm of Hg", which is a unit of measurement of pressure.

    Ideally you would stick a probe into the ventricle and just before a contraction would read a number on a pressure measuring device, that is pre-load. To measure after-load, you would need to measure the pressure in the aorta just before that valve opens and blood is pushed out.

    It is much more complicated. If want to be technically correct it's about the stretch of the sarcomere and its ability to snap back into a forced contraction.

    A good visual of pre-load is a balloon filled with air. When it is filled with a little bit of air and you release it, you get a little poof. When you put a lot of air in the balloon (more volume, more stretch, more pressure) and then release it you get a big rush of air.


    A good visual for after-load is opening a door on a windy day. On a breezy day the door is relatively easy to open, on a really blustery day there is much more air pressure to have to push against to get that door open.

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