Alright #MedTwitter & #MedStudentTwitter, I’m going to interrupt your regularly scheduled #COVID19 programming to talk about Blood Pressure, Shock & Using the Force (#MayTheFourthBeWithYou) to assess it! So,
Where would a measured blood pressure be highest?
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Where would a measured blood pressure be highest?
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Blood Pressure (BP) in the body behaves as a hydraulic system and undergoes a process known as “systolic amplification.”
When two ocean waves run into each other, the resulting wave is a smaller but taller spike!
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When two ocean waves run into each other, the resulting wave is a smaller but taller spike!
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This is exactly what happens in the body thanks to vascular bifurcations and narrowing! As the initial hydraulic wave runs into bifurcations and bounces backward, it gets added to the next wave.
Amplification of systolic pressure is the result of reflected waves!
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Amplification of systolic pressure is the result of reflected waves!
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This means that the measured BP at the dorsalis pedis artery is highest! But, see the waveform below? It’s the highest but for the shortest period of time (the wave is tall and narrow instead of short and wide).
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The above tweet also reveals that no matter the site of BP measurement, because of the fact that the higher pressures are for shorter periods of time, the AVERAGE pressure is the same!
We call this average the MAP or the Mean Arterial Pressure.
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We call this average the MAP or the Mean Arterial Pressure.
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This is why it’s more important to focus on MAP than a single site pressure reading that is low.
What if, however, the MAP is low? Well, let’s derive the determinants of MAP!
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What if, however, the MAP is low? Well, let’s derive the determinants of MAP!
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We know that in a hydraulic circuit, flow(Q) is directly related to the driving pressure (force to move stuff) and indirectly related to resistance. So,
Q = (Driving Pressure) / Resistance
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Q = (Driving Pressure) / Resistance
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In the body, the flow is cardiac output (CO), the driving pressure is the forward pressure (MAP) minus the “push back” pressure (the RAP or right atrial pressure) and the resistance is SVR or the systemic vascular resistance. We then get:
CO = (MAP - RAP) / SVR
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CO = (MAP - RAP) / SVR
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Rearrange w/ algebra and we get:
MAP = CO * SVR + RAP.
In normal people, the RAP = 0 so we can more or less say that MAP = CO * SVR.
So, back to our original question, what if the MAP is low? Let’s walk through an example of using this formula to organize our thinking!
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MAP = CO * SVR + RAP.
In normal people, the RAP = 0 so we can more or less say that MAP = CO * SVR.
So, back to our original question, what if the MAP is low? Let’s walk through an example of using this formula to organize our thinking!
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We see a low MAP.
Let’s see if the extremities are warm or cool, because this tells us about SVR. Cool extremities imply a high SVR and vice versa.
Our patient has cool extremities.
So, MAP = CO * SVR and SVR is high, so we must have a CO issue.
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Let’s see if the extremities are warm or cool, because this tells us about SVR. Cool extremities imply a high SVR and vice versa.
Our patient has cool extremities.
So, MAP = CO * SVR and SVR is high, so we must have a CO issue.
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CO or Cardiac Output = Heart Rate (HR) * Stroke Volume (SV). We see the patient is tachycardic, so the issue is in the SV. Either the heart isn’t moving blood forward OR something is preventing the blood from filling the heart!
You see a hugely elevated JVP on the patient.
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You see a hugely elevated JVP on the patient.
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BOOM. You are done. You know you have an SV problem and you see the elevated JVP - this means something is preventing the blood from being pumped. Your differential is tamponade, RV failure, & tension pneumothroax. Anything preventing the filling or destroying the pump.
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You look with an ultrasound and see a huge tamponade. After a pericardiocentesis, the blood can fill the heart again, the JVP goes down, the SV increases, your CO normalizes and MAP = CO * SVR is all back to normal!
Congratulations, you just saved a life!
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Congratulations, you just saved a life!
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To recap:
- blood pressure acts as a hydraulic system and undergoes systolic amplification
- this amplification makes point measures less valuable than MAP
- deriving the MAP = CO * SVR makes sense when you understand the system
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- blood pressure acts as a hydraulic system and undergoes systolic amplification
- this amplification makes point measures less valuable than MAP
- deriving the MAP = CO * SVR makes sense when you understand the system
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- when you have a low MAP, thinking through the formula MAP = CO * SVR is an easy way to make your thinking efficient, clear, and high yield! Go and save lives!
Fin.
Fin.
