When do you consider your patient hypotensive? Many providers would answer with some sort of systolic blood pressure measurement, like under 100 or 90 mmhg. Others would argue that MAP should be used to indicate if the patient is hypotensive. Does one indicate organ perfusion better than another? A recent graduate of paramedic school told me that his class mostly focused on blood pressure, so he wasn't really sure why titrating MAP would matter. Let's examine what a regular blood pressure tells us vs our mean arterial pressure.
Systolic Blood Pressure
First of all let's talk about the systolic blood pressure (SBP). The SBP represents the pressure in the arteries during contraction of the left ventricle. This number may seem like the most important, possibly because it is higher and we usually think of the systolic blood pressure as our marker for high or low blood pressure in the acute setting. However, SBP only represents a small portion of what our organs are perfused by. Think about this, does your heart spend more time in systole, or in diastole? Obviously the heart spends more time in diastole. By this logic, where do our organs receive most of their perfusion? Obviously diastole. Don't get me wrong, Systolic blood pressure is still very important, as it is what builds up the pressure in our arteries. SBP can be very important to watch in conditions such as ischemic CVA, where a higher pressure may be needed (160-180 mmHg at times). The arteries are a pressurized system that has the aortic valve at one end, and the capillaries at the other. Since blood should not be able to flow retrograde through the aortic valve, the blood is limited by how fast it can leave the arteries by the resistance from the capillaries. This limited speed of flow through capillaries is what allows the arteries to build up pressure. Blood naturally wants to flow through the capillaries because our bodies are always trying to attain equilibrium. The blood on the arterial side of the capillaries is at a much higher pressure than the blood on the venous side. Blood will not stop flowing until these pressures are equalized (which shouldn't happen unless the heart stops generating a systolic blood pressure).
Diastolic Blood Pressure
What about diastolic pressure (DBP)? DBP is very interesting. Not only do our organs get perfused by DBP the majority of the time, but the heart feeds itself during diastole. Check out this picture by my buddy Joel Porter (@j_a_porter).
When the aortic valve is open during systole, the coronary ostia are blocked off, not allowing for blood flow into the coronary arteries. When the aortic valve is closed during diastole, pressure flows backwards into the coronary arteries, feeding the heart. It is generally said that a DBP of 40 mmHg is needed to perfuse the heart. However, this doesn't take into account the pressure in the left ventricle at the end of diastole. There are two opposing pressures at play when it comes to the ventricular wall, and we will consider both of them during the phase of diastole.
The first pressure is the pressure from the coronary arteries. This pressure is fed by the diastolic blood pressure, which we said should probably be at least 40 mmHg to adequately perfuse the heart (see end of article for a quick note on more specific pressures). These coronary arteries come from the outside of the heart and drive inwards, so the most distal parts of the coronary arteries end at the inner ventricular wall. The second pressure is the direct pressure from the blood that is exerted against the inner wall of the ventricle. This is called the left ventricular end diastolic pressure (LVEDP). This pressure is not able to be directly measured, but if you have a pulmonary artery catheter in place, you can measure a wedge pressure which is said to be a direct representation of the LVEDP. Can you picture these two pressures opposing each other? Let's try something real quick... Face the palm of your hand towards your face. Imagine that your palm is the inside of the ventricular wall, and that your radial artery is a coronary artery. Now, take your other hand and press a finger in the center of your palm, blanching the skin. As you note, the blood coming through your radial artery is attempting to perfuse the skin. By pressing your finger against your palm you are mimicking the pressure of the blood on the inside of the ventricle, at the end of diastole. This is the LVEDP.
Now, take your diastolic blood pressure and subtract your LVEDP. This is the pressure your heart's left ventricle is being perfused with. We call this the Coronary Perfusion Pressure (CoPP). Understanding this is important, because this is the reason we give nitro to chest pain patients. Nitro reduces the LVEDP more than it reduces the DBP. This reduction increases the CoPP. Most people think we administer nitro to dilate coronary arteries, however the dilating action of nitro is questionable. The main mechanism of action is reduction of LVEDP.
The brain has something called a CPP as well, but it stands for cerebral perfusion pressure. The CPP for the brain is calculated by subtracting your ICP from your MAP.
Mean Arterial Pressure
What about MAP? We have talked about why the systolic and diastolic pressures are important, but what about the other number on your monitor? MAP! MAP is the mean arterial pressure. You could also call it the average arterial pressure. MAP takes the systolic and diastolic pressure and averages them out to give you a single number. Think about this equation for figuring out MAP....
(SBP + DBP + DBP) / 3 = MAP
One part systolic, two parts diastolic, divided by 3 to find the average. This is a very common method of figuring out MAP, you will find it in most text books and as a correct answer on a lot of exams and tests. But, this equation is missing something rather important. This missing part does not change the number by a drastic amount, but it's important to our understanding of organ perfusion. So, what is it?
Heart Rate! Heart rate represents the amount of time our organs are under an increased pressure from our systolic blood pressure. Consider the graphs below.
Heart Rate
Here is a 6 second representation of pressure. There are 8 beats of the heart in 6 seconds, so we can see that the heart rate is 80 BPM. This means that the organs were under an increased amount of pressure 80 times that minute. And, look at all that blue! DBP is clearly very important. If you use the MAP formula I mentioned above, you would get a MAP of 93 (BP 120/80. 120 + 80 + 80 / 3 = 93). Now let's look at a patient with the same blood pressure but a different heart rate.
Here we have a heart rate of 40 BPM, half of what we saw above. This means that the organs are only being put under an increased pressure half as many times, but the blood pressure and MAP are still the same. What gives? That really doesn't make much sense. Without a doubt, increased inotropic effect from the heart and vasoconstriction play a role in keeping the blood pressure up when we have a decreased heart rate. However, there is a still a way to correct our MAP for heart rate. Let's take a look at how we can do this (because I'm pretty sure your monitor doesn't do this one for you).
MAP = DBP + [0.33 + (HR x 0.0012)] x [PP]
Let's break that down. We will use our examples above. BP 120/80, heart rate of 80 BPM. Remember the order of operations for this one.
Step 1. HR x 0.0012 = 0.096
Step 2. 0.096 + 0.33 = 0.426
Step 3. 0.426 x the pulse pressure (40) = 17.04
Step 4. 17.04 + the diastolic blood pressure = 97.04
So, we have a higher MAP than using the tradition formula. We're onto something! Can you figure out what the MAP would be if we used a heart rate of 20BPM and a BP of 120/80mmHg? The answer is 93mmHg. Not much of a difference, huh? While this formula is more accurate than the traditional, it doesn't yield very surprising results. Since the last step in the formula is the addition of the DBP, you can always count on your MAP being higher than your DBP. The most important thing to realize is that MAP and HR both play a role in organ perfusion.
What do we consider a stable MAP? Most literature suggests that a MAP of 65 mmHg is necessary, but 70 mmHg is better for organ perfusion (and leaves a little room for error). In general, a MAP under 65 mmHg is an emergency and should be treated without delay. When I evaluate retrievals, I always check to see if the patient was delivered to the receiving hospital with a stable MAP, and I generally use 65 mmHg. Anything under that, and you may start to see acute kidney injury, and other organs being hypo-perfused.
So, Our SBP gives us the spike of pressure we need to perfuse or organs and maintain pressure in our arterial system. The DBP is the steady flow of blood that enters the capillaries and perfuses the organs the majority of the time. The MAP corrected with heart rate is a great marker for general organ perfusion.
The Clinical Impression
So, which should you use??? If only it was that simple. What is the point of perfusing the organs in the first place? So they function correctly, Right? What would be the best marker of organ perfusion? The absence of organ dysfunction! Go through the list of organs you could easily check for dysfunction.
Is the patient presenting with acute altered mental status or confusion? Observed organ dysfunction: = Neuro
Does the patient have diaphoretic, pale, cold skin? Observed organ dysfunction = Integumentary
Does the patient have chest pain, pressure, etc? Observed organ dysfunction: = Cardiovascular
Is the patient putting out <1cc/kg/hour of urine? Observed organ dysfunction: = Renal
Good clinical evaluation of the presence or absence of organ dysfunction combined with quantitative measurements like MAP and BP is the best way to assess organ perfusion!
Take Home Points:
1. Your MAP should be above 65 - 70 mmHg to avoid end organ damage.
2. Normal CoPP (heart): 15 mmHg is generally needed for ROSC (or however high you can get it). However, a 'normal' CoPP is said to be 60-80 mmHg. It was expressed to me by a critical care physician and anesthesiologist that CVP and MAP are reasonable surrogates for PAWP - DBP (MAP - CVP), and that this number should generally be 50-80. Just to be clear, I do not have a reference for what I just stated.
3. Normal CPP (brain): Above 50, may need to be higher in certain cases such increased ICP (MAP - ICP = CPP).
4. Urine output is a great indicator of end organ perfusion. Monitor urine output!
5. Pay attention to signs and symptoms of end organ perfusion that are observable and reported by your patient! Don't only pay attention to the quantitative values.
References:
https://www.ncbi.nlm.nih.gov/pubmed/15558774
https://www.ncbi.nlm.nih.gov/pubmed/12210707
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3334768/#!po=2.27273