EMT Exam: Understanding Blood Pressure

You will need to know all about blood pressure for the EMT exam. You will encounter situations with blood pressure frequently as an EMT. One way of talking about blood pressure is with this formula:

Blood pressure (BP) = Cardiac output (CO) × systemic vascular resistance (SVR) (the size of the vasculature)

Before you panic, rest assured — you won’t be calculating BP this way! However, the formula quickly illustrates what really counts in maintaining blood pressure.

Cardiac output is the amount of blood that is sent out of the left ventricle in 1 minute. It, too, has a formula:

Cardiac output (CO) = Heart rate (HR) × stroke volume (SV) (the amount of blood squeezed out per contraction)

Again, don’t panic — no math is needed here either. But put both formulas together and this is what you get:

BP = (HR × SV) × SVR

What does all this mean? The human body controls its blood pressure through essentially one of three ways: by adjusting heart rate, stroke volume, or systemic vascular resistance. In fact, blood pressure is usually controlled via a combination of the three.

A patient is having an AMI that’s targeting the heart’s electrical system. The result is that the heart rate slows down. If everything stayed the same, blood pressure would drop. But it doesn’t: The body constricts its arterial beds in the skin and other parts of the body so that SVR increases. The result is that the patient turns pale, the skin cools, and blood pressure remains near normal.

So, if the primary function of the cardiovascular system is to maintain perfusion throughout the body, it makes sense that keeping a handle on moment-to-moment changes in blood pressure controls the system.

Baroreceptors in the carotid arteries do exactly that. As the receptors sense a drop in BP, they send signals to the brain that, in turn, sends signals to the body to do such things as increase heart rate, increase the strength of ventricular contractions, constrict appropriate arteries, and even decrease the amount of water being filtered by the kidneys so that more fluid stays within the bloodstream.

The process is really more complicated, but in a nutshell, that’s how the body keeps its perfusion within a very narrow band of pressure.

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