What are we trying to figure out:
We want to know if it is possible to determine the pressure of the inlet for the catheter that returns deoxygenated blood to the ECMO system to help prevent collapsing of tissue outside the inlet which in turn would cause tissue damage to the vessels involved. A blockage due to tissue collapse also could potentially interrupt blood flow to the patient creating a serious event. Below is a basic description of how Extra Corporeal Membrane Oxygenation works.
A Basic ECMO Circuit:
The basis of ECMO is to provide oxygenation and blood flow for the body when the lungs or heart are not able to. The circulating pump controls the flow of blood through the body and the ECMO Circuit. Cannulas are inserted into the body to the heart to provide blood flow. The blood is pumped through an oxygenation membrane then through a heat exchanger and on into the body. At the same time deoxygenated blood is also removed from the body by the same circulating pump to be reoxygenated and the cycle continues.
Pressure transducers are used throughout the system to ensure blood is flowing properly.
- P1: Pre-Oxygenation Membrane Pressure (Standard Nation Wide)
- Determines if there is problem with the Oxygenator
- P2: Post-Oxygenation Membrane Pressure (Standard Nation Wide)
- Determines if there is a problem with flow into the patient
- P3: Return Pressure before Pump (Standard Nation Wide)
- Determines if there is a problem with blood return from the patient Located on a much lower plane than the patient
ECMO Cannula Design
When the cannulas are inserted into the body they are placed in the circulatory system like below. The cannula tip is designed to pull deoxygenated blood through several holes at the SVC and IVC while simultaneously pushing oxygenated blood into the right atrium of the heart.
Is to basically guess at the pressure at the catheter based on the P3 pressure which is located below the patient by a vertical difference of 1-4 feet. Clinicians adjust the height of the patient to change the pressure of P3 or adjust the flow rate of blood if oxygenation and blood pressures allow.
What have we done so far?
A new pressure transducer P4 was introduced into the system at the same height as the patient to possibly induce the pressure we are looking for.
I was brought in to map out the pressure differences between P3 and P4 at different heights on a chart. But I felt this was not helpful as it does not take into consideration the catheter and flow of blood. It also only demonstrates the effect of gravity on pressures.
And then I remembered from school that there was an equation that could probably solve the question they are wanting.
I think Bernoulli’s equation applies to this situation directly but I am not sure and I have no idea how to apply it to this complex system. And since I am an avid user of Super User and Stack Overflow I thought Physics would be able to help out in this matter!
Things I think we need to know to formulate a way to predict the pressure:
- Density of blood: 1060 kg/M3
- The unit of measurement for Flow: cc/min
- The unit of measurement for the pressure transducers: mm/Hg
- Diameter and length of the ECMO catheter
- Diameter and Length of tubing from catheter inlet to P3
- Diameter and Length of tubing from catheter inlet to P4
- Vertical difference between catheter inlet and P3
Let me know what else would be needed and how we can work out a solution for this or if you have any ideas on how to simplify this problem please feel free to comment.
Ideally the solution would explain how get the pressure as well as possibly provide a formula we could us to by inputting the required variables to return an estimated possible pressure value.
Unfortunately are just clinicians and have almost no knowledge of fluid dynamics so your help is greatly appreciated!
I got a little bit more information that will help with the problem it turns out that the manufacturers have a pressure drop table for each cannula type that tells the pressure drop of the cannula at different flow rates that looks similar to this:
We are going to contact the manufactures and find out where they measure the pressure at for this table and if it is where our P4 is then this should greatly reduce the work needed for this problem completely negating the need for the P3 sensor in the problem.
If am right we should be able to just walk backwards from the P4 and add in the pressure drop to get the the pressure inside the cannula then it would be just a matter of figuring out the RA pressure from that.