What is stroke volume
How to calculate stroke volume is one of the most encountered questions due to the importance of the concept of stroke volume in cardiac physiology. As a result, understanding how to find stroke volume is essential because it would help you understand cardiac physiology and appreciate its applications in real life. Stroke volume is the amount of blood ejected from your ventricles after one ventricular contraction. Typically, this number should be 70 ml of blood.
This number sometimes varies since stroke volume factors include preload, afterload, contractility, and heart rate. Moreover, these factors can be affected by other diseases. For example, patients with hypertension will have increased afterload and patients with heart failure will have increased preload.
Don’t be afraid of these terms; all of them will be simplified in this article. In this article, we will go through what is stroke volume, how to calculate stroke volume, and what are the different diseases and factors that affect stroke volume. And once you deeply understand it, you will love it!
Stroke volume equation,
Many formulas can be used to calculate the stroke volume. The most common stroke volume equation is:
Stroke volume = end-diastolic volume – End-systolic volume. Where:
Stroke volume = Volume ejected on one beat (mL)
End-diastolic volume = volume in the ventricle before ejection (mL)
End-systolic volume = volume in the ventricle after ejection (mL)
Calculation Step by Step
- Determine the end-diastolic volume by knowing how much blood is present in the ventricle before it contracts (can be done by another equation: Ejection fraction=SV/EDV)
- Determine the end-systolic volume by knowing how much blood is present in the ventricle after it contracted.
Here is how to calculate stroke volume. Example: An echocardiography of A 44-year-old woman shows that the amount of blood in her ventricles before it contracts is 140ml. The amount of blood in the ventricle after it contracted is 70 mL. How to calculate stroke volume?
Solution:
- The amount of blood in her ventricles before it contracts = EDV = 140ml
- The amount of blood in the ventricle after it contracted = ESV = 70 mL
- SV = EDV-ESV = 140-70 = 70mL
Stroke volume definition,
Stroke volume definition is simple. It is a measure of how effective the heart is in pumping blood each beat. Typically, the cardiac cycle is composed of ventricular contraction followed by ventricular relaxation.
When you try to palpate your radial artery, the pulsations you feel are caused by your heart successfully pumping enough blood every beat. Similarly, suppose you exercise for few minutes. In that case, you will feel stronger pulsations, meaning that your blood is pumping more blood (more stroke volume) to satisfy the needs of your muscles.
This means that any disease that causes weakness in your heart muscles will affect your stroke volume. To be more specific, some diseases affect preload, afterload, or contractility and, thus, affect your stroke volume since these are the factors that affect stroke volume.
How to find stroke volume
Although the stroke volume definition is constant, the different ways it can be calculated can make it somewhat complex. The equation written above is not the only way you can calculate the stroke volume. In real life, people describe how efficient the heart is in pumping blood in terms of cardiac output, not stroke volume.
But wait. What is cardiac output? It refers to how much blood your heart pumps per minute, not beat. Here is how to calculate stroke volume from the cardiac output:
Cardiac output (CO) = SV x Heart rate (HR) Where:
CO = the amount of blood pumped by the ventricle / minute
HR = how many times you heart beats / minute
SV = the amount of blood pumped / beat.
Here is an excellent analogy to simplify it for you:
Suppose you drink one bottle of water each day. How many bottles of water do you drink per year?
We will need to multiply the amount of water you drink each day (equivalent to how much blood is pumped per beat – SV) by how many days present in the year (equivalent to how many beats are in one minute – HR)
This will yield how many bottles you drink in a year (how much blood is ejected in one minute – CO)
As you can see, stroke volume is proportional to the cardiac output. But scientists prefer to use CO since it also considers a crucial variable, which is HR. For instance, imagine your heart is pumping enough blood per beat (average stroke volume). Still, the actual number of beats per minute is lower than expected. Does that mean that your heart is working well? Of course not. Even though the stroke volume is okay, the number of beats is so low that it can’t pump enough blood to other organs. This is why scientists prefer to use cardiac output.
Factors that affect stroke volume
Factors that affect stoke volume can be frustrating to learn. Let’s go through them one by one.
Preload
Preload is the initial stretching of muscle cells before contraction. More blood means more stretching of the ventricles. Think of it as a balloon. More water means more stretch.
The implication is that more stretching of the ventricles means more muscular contractions. It is similar to a rubber band. The stronger you stretch it, the stronger it rebounds. Stronger ventricular contractions mean larger stroke volume and, hence, cardiac output.
Preload can be high in some normal conditions like during exercise. This is because during exercise, blood vessels contract making it easier for blood to be squeezed from the veins to the heart. This will increase your stroke volume during exercise. On the other hand, in heart failure, preload is high because the heart itself can’t pump blood, causing accumulation of blood in the ventricles. In this case, even though the preload is high, the stroke volume is low. This is because the cardiac muscles are sick and can’t pump blood.
Afterload
Afterload is defined as the amount of pressure that the heart has to work against in order to eject blood. Think of it as pushing against a wall made of stone vs. a wall made of plastic. Of course, the plastic one will be pushed harder by you.
This is because the amount of pressure you had to work against was considerably low compared to the wall made of stone.
Similarly, the heart has to work against pressure. This pressure is caused by your arteries. If your blood vessels are contracted so hard, they will generate high pressure that your heart will have to work against. So, the heart won’t be able to pump enough blood. Conversely, if your blood vessels dilate, your heart will find significantly lesser pressure to push against. As a result, more blood is pumped, or you can say that stroke volume increases (and cardiac output as well).
Patients with hypertension have elevated afterload, which causes thickening of the heart muscles due to the pressure it chronically pushes against.
Contractility
This one is easy. Contractility refers to how strong each contraction of your heart. Stronger contractions tend to occur during sympathetic activation, as in exercise.
Stronger contractions mean more blood is pumped (increased stroke volume). Many diseases and drugs can affect the contractility of the heart either by increasing or decreasing it.
Heart rate
This one is a little bit tricky. What is the relation between how fast the heart contracts and the stroke volume?
Example #1
If you increase the heart rate without increasing the contractility, you will decrease the stroke volume. This is because when the heart contracts faster, it doesn’t have time to fill with blood. So, less blood filling means less blood pumped.
Example #2
Nevertheless, in real life, that never happens. This is because every time you increase the heart rate, you will increase the contractility as well. The reason is that the sympathetic nervous system innervates both the nodal cells that determine the heart rate and the muscles that determine contractility. The increase in contractility will compensate for the decreased filling time and result in an overall increase in stroke volume.
So, we can say that Example #1 is not real and happens only in labs.
Conclusion
Stroke volume is defined as the amount of blood pumped by the heart per beat. The stroke volume formula is as the following: SV=EDV-ESV. Other equations like CO=HR x SV can calculate it.
Stroke volume is affected by many factors like preload, afterload, contractility, and heart rate. Also, increasing the heart rate without increasing the contractility will decrease SV because of reduced filling time. However, suppose you increase the contractility and heart rate together. In that case, this will increase stroke volume because the increased contractility will compensate for the decreased filling time.
