Preload vs. Afterload

  1. Confused here! With an occluded left coronary artery (ischemia), would it more likely to see a decreased preload or decreased afterload? Thanks so much!
  2. 18 Comments

  3. by   ckh23
    Look at the anatomy of the coronary vessels. Also find out when the heart is perfused, during systole or diastole?
  4. by   StudentOfHealing
    What does the left coronary artery supply? (hint: The strongest (in my opinion most important chamber)).

    Compare & Contrast: Preload and Afterload.

    How does afterload relate to the left ventricle?

    Think Think Think.
  5. by   Esme12
    Welcome to AN! The largest online nursing community!

    We are happy to help with homework....but you need to tell me what you think first and I will help you find the answer yourself! You have been given a good head tell us what you think and we will help you from there!
  6. by   LoriBSN2b
    We just did this today and I'm confused too. In through right atrium yet preload affects left ventricle. I think.
  7. by   ♪♫ in my ♥
    You need to start with a clear understanding of what 'preload' and 'afterload' mean.

    Preload refers to the amount of blood that is in the heart prior to contraction... venous return to the atria... think 'CVP'

    Afterload refers to the flow resistance against which the heart must pump... think 'SVR'
  8. by   StudentOfHealing
    When trying to understand SVR, I like to imagine a water hose with a pump at the end. (I saw this somewhere on AN - a few months back when I first learned this)

    The water hose = Systemic Vasculature.

    The pump = Left Ventricle.

    How HARD you must PUMP to propel the water FORWARD is the afterload.

    To take things even further, we then can say that certain things like (arteriosclerosis) means you have a higher SVR - meaning you must pump harder.

    In regard to your original question: Use all the information we've given you.

    There are three main things you must know - knowing these things WILL give you the answer

    Define: Preload & Afterload.
    What does the left coronary artery supply?
  9. by   Esme12
    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.
    ECG Learning Center - An introduction to clinical electrocardiography

    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 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 Pressure/BP010.htm
  10. by   Esme12

    The autonomic nervous system receptors act as on/off buttons that control the various sympathetic and parasympathetic effects in the body. When these buttons are turned on or off, things happen in your body. If you learn about these receptors and their actions described below, you will be able to understand what a beta-blocker does or what to expect from an alpha agonist medication or how cocaine can be bad for you.

    The sympathetic receptors
    The types of sympathetic or adrenergic receptors are alpha, beta 1, and beta 2. Alpha-receptors are located on the arteries. When the alpha receptor is stimulated by epinephrine or norepinephrine, the arteries constrict. This increases the blood pressure and the blood flow returning to the heart. The blood vessels in skeletal muscles lack alpha-receptors because they need to stay open to utilize the increased blood pumped by the heart.

    Fight or flight
    Remember the fight or flight response? It would not make sense to take blood from other parts of the body and pump it to the muscles so we can run away or defend ourselves if the blood vessels in the skeletal muscles are also constricted and cannot benefit from the increased blood circulation providing extra oxygen and nutrients.

    So what happens if the alpha-receptors are blocked? The arteries dilate. Thus an alpha-blocker medication causes vasodilation and can be used to treat hypertension.

    Next are the Beta receptors. Beta 1 receptors are located in the heart. When Beta 1 receptors are stimulated they increase the heart rate and increase the heart's strength of contraction or contractility.

    The Beta 2 receptors are located in the bronchioles of the lungs and the arteries of the skeletal muscles. When these receptors are stimulated, they increase the diameter of the bronchioles to let more air in and out during breathing and they dilate the vessels of the skeletal muscles so they can receive the increased blood flow produced by stimulating the alpha and beta 1 receptors.

    So......if norepinephrine or epinephrine is the neurotransmitter of the sympathetic nervous system and it interacts with all the receptors we just described, then we know that norepinephrine or epinephrine stimulates the alpha, beta 1 and beta 2 receptors and thus it is an alpha agonist, a beta 1 agonist and a beta 2 agonist.

    When epinephrine or adrenaline are administered to a patient, alpha, beta 1 and beta 2 agonist are effected; the will be an expected increase in blood pressure, increased heart rate, increased cardiac contractility, dilation of the bronchioles in the lungs and dilation of the vessels in the skeletal muscles. You can also stimulate a single receptor site such as a Beta 2 agonist medication like an albuterol inhaler that stimulates Beta 2 receptors in the lungs then we can dilate the bronchioles in the patient with bronchospasm without causing excessive stimulation of the heart.

    When you use Beta 1 antagonist medication more commonly called a Beta blocker such as metoprolol (or other drugs ending in ‘olol') which blocks Beta 1 receptors thus decreasing heart rate and contractility which decreases blood pressure for the hypertensive patient and decreases the chance of a dysrhythmia after a heart attack by controlling the heart rate.

    The sympathetic receptors can be over-stimulated by the non-therapeutic use of substances like cocaine and methamphetamines. Or the excessive use or overdose of sympathomimetic medication like pseudoephedrine or those used to treat attention deficit disorders. Severe alcohol withdrawal may also induce sympathetic overdrive.

    Excessive stimulation of the sympathetic receptors can result in dangerously high blood pressure, tachycardia, dysrhythmias and hyperthermia, any one of which may cause organ damage with the real potential for organism death.

    Parasympathetic receptors
    Ok....let's switch to the parasympathetic or cholinergic receptors. These are easier since there are only two types, muscarinic receptors and nicotinic receptors. To make this even easier get rid of the nicotinic receptors they are involved in muscle contraction and are affected by substances such as curare (used on those poison tipped arrows) that cause muscle paralysis by blocking these nicotinic receptors. Medications such as succinlycholine are available to block the nicotinic receptors and induce paralysis necessary for certain medical procedures.

    There is one parasympathetic receptor you must learn, the muscarinic receptor. When this receptor is stimulated, it causes a decrease in the heart rate, a decrease in heart contractility and a decrease in the size of the bronchioles. When we are at rest, we can slow down and conserve energy. The parasympathetic nervous system helps us do this. What would happen if the muscarinic receptors are blocked? ......... the heart rate and contractility will increase, there will be dilation of the bronchioles and less production of secretions in the body.

    This is the exact effect of atropine, a drug used to counteract too much parasympathetic activity such as from over-stimulation of the vagus nerve or the effects of certain chemical warfare nerve agents and organophosphate poisoning. Atropine is a parasympatholytic, we can also call it a parasympathetic antagonist or parasympathetic blocker or an anticholinergic medication.
    All these terms mean the same; it means they block the action of acetylcholine at the parasympathetic receptors. The effect of blocking any receptor causes the opposite effect that would be expected from stimulating the receptor.

    Ipratroprium is another example of a parasympathetic blocker medication but this one is inhaled so most of the effect occurs in the lungs, and when it blocks parasympathetic receptors in the lungs it causes the bronchioles to dilate and decrease production of secretions like mucus. That makes ipratroprium useful in the patient with COPD who produces excessive pulmonary mucous and in combination with albuterol for any wheezing patient.

    It is important to remember that it is the balance between the sympathetic and parasympathetic nervous system that keeps our automated body functions in balance and working properly. Outside forces, including drugs, medications or poisons can change the functioning of the autonomic nervous system. And it is wise to keep in mind that all medications are potential toxins that have some beneficial side effects.

    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
  11. by   ♪♫ in my ♥
    Holy cow, Esme...

    Quite the dissertation there.

    Wish I'd had it when I was a nursing student...
  12. by   Esme12
    All you have to do is ask....I love cardiac...LOL
  13. by   ♪♫ in my ♥
    Quote from Jg2626
    Confused here! With an occluded left coronary artery (ischemia), would it more likely to see a decreased preload or decreased afterload? Thanks so much!
    Think about the definitions...

    Preload describes the amount of blood filling the heart... would that be affected by damaged cardiac tissue?

    Afterload describes the squeeze against which the heart is pumping... would that be affected by damaged cardiac tissue?
  14. by   olanikeu
    Esme12, I learn a lot fom your post about preload and afterload.Thanks