Cardiac Output: Stroke Volume, Preload, & Afterload Pt 5

SimpleNursing.com, with Mike Linares, is here once again to turn complicated, frustrating lectures into effortless, piece-of-cake study systems.

Right now, we will be discussing the following:

  1. Cardiac output (CO)
  2. Stroke volume (SV)
  3. Preload
  4. Afterload

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

Heart Regulation of Blood Flow

An average person has about five liters of blood that needs to be circulated throughout the body; therefore, as a pumping organ, if your heart cannot pump the required blood to the rest of the body, what happens? The body gets sick, gets impaired, or eventually dies.

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

  • 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’s what happens to all parts of the body if blood flow and oxygen are cut off.

Now that you have a better understanding of blood flow regulation by the heart, we go to our main topic.

Cardiac Output

Cardiac output is the amount of blood that’s ejected from the left ventricle, into the aorta of your heart, then out to the rest of the body in one minute.

                        Heart rate x Stroke volume = Cardiac output (one minute)

Stroke Volume

Stroke volume is the amount of blood in one clean pump. How is this seen or applied in the clinical setting?

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 left ventricle struggles to pump out blood to relieve pressure inside the heart. For this reason, stroke volume is decreased because the left ventricle is unable to pump blood efficiently.

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

How Left Ventricle Hyperinflation is Measured

To determine left ventricular hyperinflation, the lab test of choice is the Brain Natriuretic Peptides (BNP).

When the cardiac output no longer sustains oxygen in the peripheral veins, the brain sends signals to the left ventricle.  Brain natriuretic peptides 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.

There’s a vasomotor center in the brainstem that 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.”

  • 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

Preload is, in simplest terms, the stretching of ventricles. So ventricles tend to stretch (fill with blood) and squeeze (push out blood). If there is too much pressure filling the ventricles, they tend to extend to the point of not having a proper contraction.

  • Too much stretch = unable to squeeze properly

Afterload

Afterload is the degree of pressure inside the aorta to push or eject blood. Afterload is just a fancy word for the pressure required for the left ventricle to force blood out of the body. So, afterload is just the effort of the ventricle to squeeze. In cases of congestive heart failure (CHF) or hypertension, 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.

How are increased preload and afterload managed in a hospital setting?

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

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

Hopefully, this was able to help you have a better grasp at one of the trickiest subjects of nursing.

For more useful tips and information, visit SimpleNursing.com. On this site, you can check out our Patho Bible – The Top 70 Diagnoses that are commonly seen in a clinical setting.

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