Blood Pressure – Vital Knowledge for EMS

The blood pressure is one of the most ubiquitous diagnostic tools used in medicine and has a sacred role in EMS. Every EMT and Paramedic needs to be able to get an accurate blood pressure from every patient, every time. It is so widely regarded throughout medicine as a useful diagnostic tool that it’s considered to be one of the “Vital Signs” and pretty much everyone reading this has either taken someone’s blood pressure, and/or has had theirs taken many times.

Of course we know that the blood pressure is the measure of the heart’s ability to pump blood throughout the body. It’s simple, right: Cardiac Output – Vascular resistance = BP. The blood pressure is represented as a number *slash* number, or “Something *over* something” measured in “mmHg” (millimeters of mercury). These numbers represent the “Systolic” and the “Diastolic” pressures, with the Systolic blood pressure meaning the peak fluid pressure of blood flowing through the arteries at “systole”, or the heart’s peak contractile force; and the Diastolic blood pressure measuring the pressure of blood in the arteries when the heart is at “diastole”, or at rest. EMS people use the blood pressure to see how well the patient is “Perfusing” or circulating blood and the oxygen and nutrients it carries to the end tissues it supplies. “Hypotension” is too low of a blood pressure and can result in tissue damage, tissue death, and/or Shock; and “Hypertension” is too high of a blood pressure and can result in all kinds of short and long-term damage to the body, including heart disease, kidney disease, stroke, and many other chronic conditions. In EMS, we use the blood pressure as an important diagnostic tool in such things as trauma to measure blood loss, and also in medical care to determine shock or cardiac compromise.

But we all know the basics, right? Good, if you’re an EMT, you probably should know all that. However, you may not have heard these terms:

  • Pulse Pressure: The difference between the Systolic Blood Pressure and the Diastolic blood pressure. For example, a patient with a BP of 120/80 has a Pulse Pressure of 40mmhg.
  • Stroke Volume: A measure of the volume of blood ejected with each beat. (Stroke volume + Pulse rate = Cardiac Output)
  • Preload: A measurement of the pressure left in the vascular system during Diastole (Or “Left Ventricular End Diastolic Pressure” I’m just going to call it preload)
  • Afterload: The pressure that chambers of the heart must generate in order to pump blood. In the case of the Left Ventricle, it’s the pressure it must create through contraction in order to pump blood into the aorta.

(For everything else you’ve ever wanted to know about blood pressure, read this: “Overview of Blood Pressure” by John Ross)

What if there were more things that taking a patient’s blood pressure could tell you about them?

There are, of course. The blood pressure is way more useful as a diagnostic tool than most EMTs and Paramedics realize. Here are some of the things that the simple blood pressure can help you learn about your patients and the care they need:


It can diagnose Orthostatic Hypotension

Have you ever seen a medical provider take “Orthostatic Blood Pressures?” These are taken as three consecutive blood pressure measurements taken with the patient in the Supine (laying down), Sitting upright, and Standing position. To properly perform this, have the patient lay supine for five minutes and take a baseline blood pressure measurement. Then have the patient sit upright, wait two minutes then take their blood pressure. Repeat with the patient in a standing position. If the patient gets dizzy for more than a minute with positional changes, that’s a positive sign for orthostatic hypotension, as is a drop in systolic blood pressure by 20mmhg between readings.

What does this mean?

Well, it can mean that the patient is dehydrated, is experiencing hypovolemic shock, has some type of cardiac compromise or an arrythmia, is anemic, has a problem regulating their blood pressure, has an electrolyte imbalance, and a few other conditions. It can also be caused by medications such as Beta Blockers or even Viagra. Orthostatic Hypotension is also a common cause of Syncope, or fainting. It’s an important assessment finding to record in your patient care report and to pass on to the receiving facility.

(Read More?


It can help diagnose a Thoracic Aneurism

The arms are the most common places where the blood pressure is measured. The blood pressure cuff aka a “Sphygmomanometer” is wrapped around the arm at the bicep and applies pressure to occlude the brachial artery. The brachial artery is supplied by the subclavian artery, of which there are the Right and the Left subclavian arteries respectively. It has been shown that there may be a normal 10 to 20mmHg difference in blood pressure between the arms in a small minority of patients. Therefore it is important to take blood pressure readings from both arms when diagnosing hypertension. It is also useful to note when there is a difference in readings above 20mmHg from one arm to another. This can be a sign of Increased intra-thoracic pressure, a Thoracic Aneurism, or something called “Subclavian Steal Syndrome”.

In a thoracic aneurism, a condition with a mortality rate reaching up to 80%, the aortic arch in the chest is compromised. This results in severe pain (usually described as “ripping” or “tearing”), hypotension, and usually death if it ruptures. As the aneurism tears, it compromises the entrance to the right subclavian artery before the left, causing the blood pressure in the right arm to drop. This is an important diagnostic tool to use in diagnosing chest pain and should be documented.


 It can help detect increased intrathoacic pressure and other conditions

The thoracic cavity is the area commonly called the chest and is the area above the diaphragm protected by and enclosed in the rib cage. As we know, there are a lot of important things in there that humans need functioning properly in order to, you know, live. Pulsus Paradoxus is a condition where the heart’s pumping capacity is compromised by the thoracic pressure and the blood pressure rises and falls with inspiration and exhalation. The blood pressure drops (and sometimes even the radial pulse disappears) with inspiration and rises again with exhalation based upon the volume/pressure of air in the chest. The “paradox” results from the fact that you can hear cardiac beats on auscultation of (listening to) the chest, but cannot detect them with the blood pressure and/or pulse.

What does this mean?

Lots of conditions can cause Pulsus Paradoxus and roughly they can be broken down into three groups: Cardiac causes, Pulmonary Causes, and Other causes.

First, let’s give a nod to the other causes, the non-cardiac and non-pulmonary causes, which are Anaphylaptic Shock and an obstruction of the superior vena cava.

The cardiac causes can be:   (and THANK YOU Wikipedia for being smarter than me and very accessible)

  • cardiac tamponade – A “bruise” of the heart resulting in the pericardial sac filling with blood that cannot escape and compromises cardiac function. (Treated with a pericardiocentesis, which some EMS providers can do in the field. I can).
  • constrictive pericarditis – Inflammation or purulent (puss-filled) infection of the heart which compromises pumping ability.
  • pericardial effusion – Fluid around the heart
  • pulmonary embolism – A blockage in the pulmonary artery or vein
  • cardiogenic shock – Impaired pumping ability of the heart due to cardiac damage or other compromise. Commonly seen in severe myocardial infarctions. (Heart attacks)

It can also be caused by pulmonary (lung) conditions, such as a tension pnuemothorax, COPD, and sometimes in severe and acute asthma, where the patient traps so much inhaled air in the lungs that they cannot exhale the excess pressure due to the inflammation of the air passages.

When you see these signs, make sure to take multiple blood pressure measurements to trend the patient’s progression. Calculate their Pulse Pressures, as cardiac tamponade, tension pneumothorax,  and other conditions are characterized by narrowing of pulse pressure and compromised cardiac output also resulting in hypotension.


 It can help detect a closed head injury, stroke, or Intracranial Hemorrhage (<– that’s an excellent link)

Cushing’s Triad, aka Cushing’s reflex, is a group of symptoms that has been shown to reveal increased intracranial pressure (ICP), the pressure within the cranial vault around the brain. This reflex shows three distinct signs which are predictive of Stroke (both ischemic and hemorrhagic), intracranial bleeding, head trauma, and some other conditions that raise ICP. These signs are:

  • Slowed pulse rate
  • Markedly increased systolic pressure (high BP) with widened pulse pressure, as the diastolic pressure usually stays normal, and:
  • Irregular breathing (Cheyne-Stokes pattern respirations)

Any time you suspect an injury or condition that may raise ICP, check the blood pressure and look for Cushing’s Reflex. It can help you zero in on the patient’s condition.


Here are some tips for making sure your blood pressures count:

  • Automatic BP cuffs do an ok job of measuring the blood pressure in a routine setting, but they have weaknesses. They cannot detect pulsus paradoxus, they give wildly inaccurate readings in bradycardia (slow heart rate), and they’re very much affected by the bumps in the road felt in the back of an ambulance. TAKE AT LEAST ONE OR TWO MANUAL BLOOD PRESSURES.


  • Can’t hear the systolic pressure? Take a palpated blood pressure by feeling the radial pulse while you deflate the cuff. The first pulse you feel = a reasonably accurate systolic pressure.


  • As with a lot of diagnostic tools, the first blood pressure measurement is a spot-check. The second reading creates a trend and reveals a lot more information. Take them every 5-10 minutes on critical patients, and every 10-15 on stable ones, keep mindful of the pattern.

This is by no means an exhaustive list, but it should give you some more respect for the humble blood pressure. As always, follow your local protocols and medical orders and this article isn’t meant as medical advice. Keep learning out there.

Also, feel free to add things in the comments section. I’d love to see what I missed.


Want to learn more stuff about stuff? Check out:



  • Cardiac output = stroke volume * pulse rate.

    You wrote addition instead of multiply.

    • Ron Stewart

      I think he changed it to subtraction. Still not the multiplication that it needs to be.

  • Cardiac output = stroke volume * pulse rate.

    You wrote addition instead of multiply.

  • Anonymous

    Typo in the first equation, too: Blood pressure = CO multiplied by SVR.

    You put a minus sign.

  • Anonymous

    Typo in the first equation, too: Blood pressure = CO multiplied by SVR.

    You put a minus sign.

  • MAP or CPP is another function of the BP I teach students. The mean arterial pressure is the diastolic pressure doubled and added to the systolic, then divided by three. Most automatic BP machines do this calculation for you and display (under the BP). What does it mean? You want your MAP to be greater than 60 to get oxygenated blood to the brain. IF it is less 60, you better be treating for shock. 

    • Medic768

      CPP or coronary perfusion pressure is important in teaching proper CPR but not something we measure in the pre hospital setting. I don’t allow my interns to use a automatic BP cuff either, it is important to get a real feel for your patients pulse and pressure which a machine can’t do as good as you. MAPs are important for continuous care, but when treating for emergency medicine, simple systolic and diastolic readings are the most useful tools in the field. We only have a few minutes with these patients and until paramedics are allowed to provide care in home, we should focus our teachings on emergency care.

  • medic/RN 4240

    You also forgot to add what the normal pulse pressure should be (30-50)  less than 30 can mean shock and widening PP can point to a neuro problem.

  • Dlarratt

    Great article! Thanks. Even the type-o’s gave me a chance to research and learn more! It really conntected the dots!

  • Too Old To Work

    I speak of postural blood pressures only to condemn the practice. It’s rarely of any diagnostic value in the field and I’ve seen all too many EMTs who will find a hypotensive patient and sit him up to “see if he’s postural”. Believe me, if the guy has a supine BP of 80 and a pulse of 120, he’s effing postural!

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  • Tj Edwards

    There’s some ozzie research that suggests ‘in trauma’ a differential of 15 mmhg over 15 minutes is significant either way. Too often & you can skew the results if using the same arm constantly.

    Respect for the post!


  • i’d never heard of preload or afterload before… thanks for the info, dude!

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