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Transducers in Invasive Pressure Monitoring, by Dr. James DiNardo.

Hi, my name is Jim DiNardo. I'm a Professor of Anesthesia at Harvard Medical School, and

one of the cardiac ICU attendings here at Children's Hospital Boston. Today I'm going

to be talking about invasive monitoring, specifically arterial pressure monitoring and central venous

pressure monitoring. We're going to spend a little bit of time talking about transducers

and how they work.


Transducers are a system that converts a mechanical signal, which in this case-- in the case of

pressure monitoring-- both for arterial lines and central venous pressure lines, is a pulsatile

signal, and it's converted through the transducer, and then through this cable converted to a

digital signal pressure waveform, which is what you see on the monitor. And we're not

going to spend a lot of time talking about electronically how that works, but suffice

it to say that in order for a transducer to work, it has to be connected by a continuous

column of fluid to the fluid in the patient's body in the system that your monitoring, ideally

with no bubbles in it.

Because as we'll talk about the presence of bubbles in the transducer system degrades

the conversion of the pressure signal to the electronic signal that we see on the monitor

by damping out the pulses in the system. So, we have this continuous volume of fluid and

we have the transducer generally hooked to a flush system. In this case, this is normal

saline with a little bit of Heparin added running in about three mLs an hour, which

is pretty typical, And that may vary from institution to institution. And that's just

the volume of fluid necessary to keep the system free of clot and to prevent any thrombus

forming on the ends of the catheters, which will also degrade the quality of the system

and obviously creates a potential risk to the patient.

Zeroing the Transducer.

When you zero a transducer, basically what you're doing is telling the transducer system

and subsequently the monitor that the pressure being sent to the transducer is basically

atmospheric pressure.

So the way that's typically done is that we take the transducer and we turn the stopcock

off towards the patient so that the only pressure the transducer is seeing now is atmospheric

pressure. And that's the circumstance under which we zero a transducer.

That tells the transducer to discount any other pressure except the atmospheric pressure

in the room. And ideally, the transducer would be here at the level of the patient's heart

or near the level of the patient's heart.

So once the transducer is zeroed, we now have this continuous column of fluid from the patient

vessel, in this case the artery, to the transducer. And as a consequence of that, we would get

a reliable trace here of the arterial blood pressure.

Importance of Placement.

The other important thing to keep in mind is that once the transducer is appropriately

zeroed to atmosphere, where it lies relative to the patient will influence the pressure

that we see on the screen. So for example, we know exactly how much pressure a column

of saline exerts in terms of millimeters of mercury.

So one centimeter of water is equivalent to about 0.8 millimeters of mercury. And just

keep that number in mind because what that means is that if I had zeroed this transducer

and I take it and I drop it down below the patient, the patient's arterial blood pressure

is going to rise in millimeters of mercury-- and that is both the systolic and the diastolic

and the mean blood pressure-- by an amount equal to the weight, if you will, of this

column of fluid.

So if I take the transducer and I drop it five inches below the patient, that's equal

to about nine millimeters of mercury of pressure. So this transducer now which was zeroed to

atmosphere is now seeing the patient's arterial pressure plus the weight of this column of

fluid five inches or so below the patient. And obviously, if I lower it even further,

I'm going to increase the weight of this column of fluid. And the pressure is going to be

even higher.

By the same token, if I take the transducer and put it up here, I'm now going to lower

the patient's arterial pressure by a quantity equal to this column of fluid again, but in

the other direction. So the point being that the patient's arterial pressure now, to reach

the transducer, has to overcome the pressure loss generated by raising the height of the

transducer. And in this case, if I raise it up five or so inches, I'm going to lower the

patient's arterial blood pressure by about nine or so millimeters of mercury.

Now, with an A line, that might not be so important. I mean, if your arterial blood

pressure is 100 and I drop the transducer on the floor, I may raise your arterial blood

pressure by, say, 15 or 20 millimeters of mercury, which is substantial but not a huge

amount. Where it becomes more important is the central venous pressure transducer.

If I'm dealing with a pressure of nine millimeters of mercury and the transducer is down below

the patient that same distance and I add nine millimeters of mercury now to the number I'm

seeing here, in essence I've increased the patient's CVP erroneously by 100%. The point

being that the position of a transducer is important in both instances.

But when you're measuring pressure in low pressure systems such as the CVP, it really

makes an enormous difference. And if you're using the CVP to make clinical decisions,

there's a big difference between a CVP of nine and a CVP of 18. So it's important to

remember that if I had zeroed this transducer and I take it and I drop it down below the

patient or if I raise it above the patient, it would give you an inaccurate pressure measurement.

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