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I remember sitting in the little pizza place across from campus going over flashcards before our test with two of my classmates. It was drilled into us to learn the normal lab values and vital signs and I learned them and passed the tests. Great! However, I am the kind of learner who can learn everything you need to know for a test, pass the test with flying colors and then completely miss the application of it to the real world of patient care. Not good for my patients. Luckily there are people much smarter than me that I can go ask and learn from. We will discuss what the four main blood pressure figures mean. Please know that I feel very nervous about getting this info wrong! If something is incorrect or needs further explanation, please let me know!
Systolic Blood Pressure (SBP): This tells us the pressure that an amount of blood is exerting on the vessel as it leaves the heart.
Importance: It is an indicator of whether or not the heart (left ventricle) is pushing with enough force to push enough blood (stroke volume) out of the heart to the organs and tissues. Normal is <120mmHg but >95mmHg
Diastolic Blood Pressure (DBP): This tells us the arterial tone or resting pressure of the vessels through which the blood is returning to the heart.
Importance: As the blood is pushed through the body, it returns to the heart at a certain pressure. A low pressure may indicate that not enough blood is returning to the heart which then causes a lower SBP. Normal is <80mmHg but >65mmHg
Pulse Pressure (PP): This is simply the difference between the systolic and diastolic blood pressure.
Importance: Normally the pulse pressure is from 30-40. This number can subtly indicate a soon to be significant change. For example, if the pulse pressure starts to decrease it could indicate hypovolemia.
Mean Arterial Pressure (MAP): There is a formula for this one that you probably don’t need to memorize, but understanding MAP is very important. MAP is the pressure of the blood against the target organs (kidneys, brain…you know the ones you want to keep working)
Importance: Generally speaking, most people need a MAP of 60mmHg to adequately perfuse their organs. In critical care we will sometimes see orders written for MAP greater than 50 or 55 and you’ll definitely be assessing for altered mental status and kidney function.
Interpreting your patient’s blood pressures and watching trends are going to vastly increase your understanding of patients in critical care. Having a little more insight into the numbers that make up the blood pressure and their meaning to the life and health of your patient may actually scare you a bit. Or make you a jumpy nurse…not to say that has ever happened to me. By grasping the significance of the blood pressure (not just seeing a number and saying “oh this person has hypertension” or “maybe I should hold this metoprolol for a blood pressure that low”) I have been able to monitor closely and start interventions quickly to help prevent life-threatening complications. Hopefully, you too will feel a little smarter about interpreting blood pressures!