Preload and Afterload: A Cardiac Output Overview

Preload and afterload are terms you need to know for your next exam — or the next time you’re taking care of a patient. So you’ll likely be asked to calculate a patient’s afterload and preload, along with memorizing other cardiac-related terms.

In this post, we’re going to turn complicated, frustrating lectures on cardiac output into effortless, piece-of-cake study systems.

Jump to Cardiac Subject

1. How Does Blood Flow Through the Heart
2. What is Cardiac Output (CO)?
   • Cardiac Output Equation
   • Decreased Cardiac Output
3. What is Stroke Volume (SV)?
4. Left Ventricular Hypertrophy
   • Brain Natriuretic Peptides (BNP)
5. Preload vs Afterload
   • Preload
   • Afterload
6. The Slingshot Analogy

Before we go into specifics, let’s first have a quick overview of how the heart functions regarding blood flow regulation.

How Does Blood Flow Through the Heart?

An average person has about five liters of blood that needs to be circulated throughout the body.

The heart pumps blood throughout the body. Blood carries oxygen and plasma that helps infiltrate the veins and arteries, sustaining blood pressure and life.

As a pumping organ, what happens if your heart cannot pump the required blood to the rest of the body? The body gets sick, gets impaired, or eventually dies.

Insufficient blood flow = decreased oxygen distribution = tissue death

Think of it as putting a tourniquet around your finger, cutting the blood supply. Immediately, the finger starts to get pale, cold, and cyanotic. Technically, that happens to all body parts if blood flow and oxygen are cut off.

Now that you better understand blood flow regulation by the heart, let’s touch on cardiac output and stroke volume to get a clearer background on how the mechanisms work.

What is Cardiac Output?

Cardiac output (CO) is the amount of blood ejected from the left ventricle, into the aorta of your heart, then out to the rest of the body in one minute. Normally, an average person has about five liters of blood circulating throughout the body’s systems.

You find patients’ CO with a simple equation (well, simple once you memorize it).

Cardiac Output Equation

This CO equation helps you determine a patient’s cardiac output by calculating the amount of blood pumped by the heart in one minute. Here’s the equation (where CO = cardiac output (liters/min), HR = heart rate (beats/min), and SV = stroke volume (milliliters)):

Heart rate (beats/min.) x Stroke volume (mL) = Cardiac output (l/min.)

Decreased Cardiac Output

If the pumping organ is compromised and there is no or insufficient cardiac output, blood responsible for transporting oxygen that sustains life will decrease; thus, resulting in tissue death.

For instance, blood supply is cut off if a tourniquet is placed around the finger. What follows is a series of manifestations of tissue death – turning pale, cold, and cyanotic. Cardiac output is the sum of a person’s heart rate times the stroke volume.

What is Stroke Volume?

Stroke volume is the amount of blood in one clean pump – in other words, what your heart pumps with each beat. It’s a measurement of how much blood goes through your body every minute. Results depend on the size of the heart and how fast it contracts.

Normal stroke volume will depend on each individual patient, but it’s typically between 50 to 100 mL.

Backing up of traffic (too much blood) causes the left ventricle to swell or inflate because it’s trying its hardest to push blood out, going against the resistance of high blood pressure.

Decreased stroke volume = compromised cardiac output = left ventricular hypertrophy

A client with cardiac failure or congestive heart failure has increased pressure being backed up from the rest of the body because of high blood pressure. So the left ventricle struggles to pump out blood to relieve pressure inside the heart.

For this reason, stroke volume is decreased because the left ventricle cannot pump blood efficiently. A typical example of how stroke volume is affected in cases of heart ailments is when left ventricular hypertrophy happens.

Left Ventricular Hypertrophy

If a client has cardiac failure or congestive heart failure, there is increased pressure being backed up from the rest of the body due to high blood pressure. The left ventricle is doing its best to keep up with the significant pressure to pump out.

When this happens, stroke volume is lessened because the left ventricle cannot pump all at once throughout the body. As a result, swelling of the left ventricle occurs due to its effort to push against the resistance.

Thus, the client is diagnosed with left ventricular hypertrophy. Remember, cardiac output is also compromised when stroke volume is compromised.

Brain Natriuretic Peptides (BNP)

Left ventricle hyperinflation is measured with the Brain Natriuretic Peptides (BNP) – the lab test of choice. When the cardiac output no longer sustains oxygen in the peripheral veins, the brain sends signals to the left ventricle.

BNPs are compensatory mechanisms of the brain, communicating to the left ventricle, calling out its hyperinflation, and informing it that there is decreased oxygen level inside the body.

Inside the brainstem, there’s a vasomotor center that regulates and controls blood pressure. It controls blood pressure, the RAAS system of your kidneys, and the BNP. So BNP is basically telling the left ventricle, “Hey, we need you to take the pressure off.”

The normal BNP is around 100; anything that goes beyond this number indicates a heart problem. Clients with congestive heart failure have a BNP of more than 300. Severe congestive heart failure clients can even reach over 25,000 of BNP.

Normal BNP = less than 100

BNP as high as 300 or more is usually a sign of congestive heart failure (CHF).

Now, when the cardiac output is not meeting the required amount of blood in a minute, and the stroke volume is having a hard time getting pressure off from the heart because of too much resistance, that’s where preload and afterload come in.

Preload vs. Afterload

The terms preload and afterload are used to describe the blood volume present in your heart at any given time. The preload refers to the amount of blood already in your ventricles when you’re ready to pump it out, and the afterload refers to the pressure against which your heart has to pump that blood.

So if your preload is high and your afterload is low, your heart will have an easier time pumping blood throughout your body and keeping up with demand.

If you have too much preload and not enough afterload, then your ventricles will fill up faster than they can pump out all that blood — which can cause a stroke or even death.

On the other hand, if you have too much afterload and not enough preload, your ventricles will have trouble emptying themselves properly, leading to high blood pressure.

Preload

Preload is, in simplest terms, the stretching of ventricles. Ventricles tend to stretch (fill with blood) and squeeze (push out blood) to distribute blood adequately.

However, if too much pressure is backed up due to cardiac issues, the ventricles tend to stretch extensively, taking longer to squeeze, resulting in an abnormal contraction.

Too much stretch = unable to squeeze properly

Afterload

Afterload is a fancy word for the pressure required for the left ventricle to force blood out of the body to exert during systole. In other words, it’s the effort of the ventricle squeezing.

In congestive heart failure, you have a back-up of pressure on the left ventricle, causing it to stretch at great lengths, causing a bigger preload and a struggling afterload.

The Slingshot Analogy

The heart is an intricate organ that causes quite a bit of bewilderment, especially with nursing students. And one of the topics that bring about such confusion is concerned with preload and afterload.

Consider this analogy – the heart is like a slingshot; a slingshot that requires pressure when pulling and energy upon release.

The Pulling Effect

The pulling action of the slingshot is called preload. Preload occurs so that there will be the pooling of blood that will either be pushed into the lungs or to the rest of the body. Preload is also referred to as the diastolic pressure inside the blood vessels.

The Release

Afterload, on the other hand, is the action when the slingshot is released. Basically, afterload is the amount of pressure that the heart has to overcome to enter the next phase, whether the blood will go inside the lungs or the peripherals. Afterload is also known as the systolic pressure inside the blood vessels.

So how are increased preload and afterload managed in a hospital setting?

For clients with congestive heart failure or hypertension, the backing up of pressure will cause a wider stretch, increasing preload and afterload.

Clients with acute myocardial infarction (MI) are given nitroglycerin and morphine to bring down preload and afterload.

• Nitroglycerin relaxes the smooth muscle to allow vasodilation.
• Morphine is a central nervous system (CNS) opioid analgesic that relaxes the heart.

Nitroglycerin and Morphine are often given to bring down the preload and afterload; these two drugs, having a direct effect on the preload and afterload, are commonly given to clients with acute myocardial infarction.

Nitroglycerin dilates the pathways of the heart, relaxing the left ventricle. Morphine, a central nervous system opioid analgesic, also widens and relaxes the blood vessels by acting on the BNP of the brainstem.

Easier Organ Memorization Starts Here

Hopefully, we’ve shed some light on cardiac output, stroke volume, preload, and afterload, and how they are intertwined whenever there are heart issues like congestive heart failure.

To retain this knowledge on CO, SV, Left Ventricular, and Preload & Afterload, you’ll need more resources such as video libraries with rationales, NCLEX study guides, quiz banks, and practice questions. SimpleNursing goes beyond nursing school lectures to help you better grasp one of the trickiest subjects of nursing.

So supplement your nursing education with easier resources. Whether you’re becoming a CVRN or any other type of nurse.

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