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Blood Pressure Equation

Apr 04, 2016

When the doctor takes your blood pressure, they are measuring how hard your heart is pumping to circulate your blood. They use an inflatable cuff on your arm that fills with air to squeeze the blood vessel walls. There is a pressure gauge attached and the doctor or a nurse listens to the heartbeats. Understanding blood pressure equation and common cardiac equations accurately goes a long way in avoiding medical mistakes. It is important for medical staff to put the cuff on properly for these numbers to be correct.

What Do Blood Pressure Readings Include?

The blood pressure equation is determined by two different factors: Systolic blood pressure, and Diastolic blood pressure. These two numbers can speak volumes about how the heart is functioning and the entire circulatory system. In more critical cases, doctors will also look at pulse pressure and vascular resistance. Here is what they do:

Systolic Blood Pressure

This number tells you about the left ventricle of the heart and the effort it is putting in to push blood throughout the body. When you see a blood pressure reading of 110/70, the 110 is the systolic pressure. 110 is a pretty good number and tells us the heart doesn’t have to work too hard to get the blood out to the body. If you see a number higher than 120, the heart is having to work a little harder.

Diastolic Blood Pressure

This is the number that lets you know what the pressure is when your heart relaxes and refills with blood. On a blood pressure reading of 110/70, this is the bottom number. 70 tells you that your heart relaxes and refills just fine. If the number is 80 and up then your heart is working a little harder to relax and refill.

Pulse Pressure

The pulse pressure is the difference between the systolic and diastolic blood pressure numbers. If the blood pressure is, 120/80 then the pulse pressure is 40. If the pulse pressure is “wide”, meaning there is a very large difference between the top and bottom numbers, it indicates something is going on in the body such as hyperthyroidism, shock or trauma, or any condition that relaxes the blood vessels too much. If the pulse pressure is a small amount of numbers between, this indicates things like blood loss, rapid heart rate, or congestive heart failure. This number is extremely important in the critical care setting, especially when monitoring fluid or blood loss in trauma patients.

Vascular Resistance

This used to be called peripheral resistance and is used to indicate the amount of resistance the heart must overcome in order to get the blood out to the body. A few of the things that may increase the resistance in the blood vessels and make it harder to pump blood are: thicker blood, constricted blood vessels, and constriction of the tiny capillaries in the lungs. When the blood is more dilute and blood vessels are relaxed, blood can flow more freely through the body.

Blood Pressure Equation and Cardiac Equations: Types and Formula

The blood pressure equation actually includes a few different calculations: the stroke volume equation, the cardiac output equation, and the mean arterial pressure. Let’s take a look at each of these:

Stroke Volume Equation

This measures the actual volume of blood that your heart pumps out of your ventricles with each beat. The doctor will order a test called an echocardiogram.They will look at how full the ventricle is when full during the relaxing and filling phase, then they look again to see how much blood is left in the ventricle at the end of the heartbeat. Whatever is left they subtract that amount from the beginning. The equation looks like this:

              SV (Stroke Volume) = EDV (End Diastolic Volume) – ESV (End Systolic Volume)

For example; a healthy male that weighs 150 pounds has a EDV around 120ml. It is around 50 ml for the ESV, so the SV is 70ml.

Cardiac Output Equation

The cardiac output equation tells us the amount of blood the heart is able to pump out to the body to give it nutrients, oxygen, and needed chemicals to live. This is determined by taking the Stroke Volume and multiplying it by the Heart Rate. The equation looks like this:

       CO (Cardiac Output) = SV (Stroke Volume) x HR (Heart Rate)

The determined stroke volume above was 70ml. The patient’s heart rate is 70 beats per minute. We multiply the SV of 70ml with the HR of 70 and we get 4900 which converts into 4.0 L per minute for the cardiac output.

Mean Arterial Pressure

When you have your blood pressure taken in the hospital or doctor’s office on a machine, you will notice towards the bottom there is another number under the diastolic and systolic. You may see the MAP, which is mean arterial pressure. It describes average blood pressure in an individual. The MAP should be between 70 and 110. It is dangerous for this number to be less than 60 otherwise your important organs will not receive enough oxygen. The equation for mean arterial pressure looks like this:


In someone who has a blood pressure of 120/70 you would do this: 70 x 2 = 140 + 120 = 260/3 = 86.7 MAP

A mean arterial pressure of 103 is considered within normal range and the body and organs are getting enough blood, oxygen and nutrients. You usually only see this number looked at when patients are in the intensive care unit.

What Is The Blood Pressure Equation Used For?

The blood pressure equation, including the stroke volume equation and the cardiac output equation, tells the doctor how stable or critical patients are. Your heart, kidneys, brain, nervous system, and hormones all work in unison to keep your blood pressure in a good range. Normal blood pressure is anything 120 or less systolic and 80 or less diastolic. If the blood pressure is too high, this can cause damage to the kidneys or the heart can fail. Low blood pressure can also be serious and any severe fluctuations needs to be addressed and treated as soon as possible.