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.
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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