Interpreting Arterial Pressure Waveforms, 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. And I'm going
to talk a little bit now about what information we can gather from looking at an arterial
line trace.
Arterial System.
To start with, it's important to remember that the shape of the arterial trace seen
on a monitor is really due to the interaction of essentially two variables, which is the
patient's stroke volume-- that is the volume of blood that's ejected into the arterial
system with each beat-- and the compliance of the arterial system into which the blood
is being ejected. So when we're looking at A line traces, it's important to remember
that although the pulse pressure can be an index of stroke volume, it depends entirely
on the compliance of the patient's arterial system.
And the reason this is important is that, let's assume for a moment that we have two
patients, one who is young and has a very compliant arterial system, and one who is
older or, in fact, very elderly and has a very non-compliant arterial system. And let's
assume that those patients have the same stroke volume.
The patient with the compliant arterial system is going to have a much narrower pulse pressure--
that is the difference between the systolic and the diastolic pressure-- than the elderly
patient with the non-compliant aorta, who's going to have a very wide pulse pressure because
the same stroke volume is injected, essentially, in the circumstance of the older patient,
something resembling more of a lead pipe, where the entire stroke volume will be displayed
as a pressure and then a deterioration of that pressure.
And in the example of a younger person with a very compliant arterial system, the stroke
volume in essence will almost be completely damped out by an infinitely compliant system.
Therefore the volume ejected will appear more like a straight line in an infinitely compliant
arterial system. And it will appear as a square wave in a non-compliant system.
Dicrotic Notch.
Now, the other thing that people talk about a lot when they look at A line traces is the
dicrotic notch, which is a notch on the descending limb of the A line trace. And oftentimes,
people will say that this corresponds to aortic valve closure. But dicrotic notch really is
not actually aortic valve closure. What it is, is it's a reflection of the reflected
wave from the periphery being seen on the arterial trace.
The less compliant your arterial system becomes, that is, in general the older you become,
the more rapidly the pulse wave is transmitted down your arterial system. And when it's propagated
down your arterial system, it's also reflected back. So you can imagine that the faster the
wave goes down the arterial system, and the faster it's reflected back, the earlier in
the arterial line trace the dicrotic notch will appear.
So in young children with compliant arterial systems, you often see a very clear dicrotic
notch. In elderly patients, the dicrotic notch actually is essentially lost, because it comes
back so early that it's summated it on top of the arterial line tracing. You often see
in elderly patients that they have a whip, or underdamping in their arterial line trace,
which actually in many circumstances is just early reflection of a pulse from the periphery,
summated on their A line trace.
Now having said that, if you have a transducer system that is not very well damped because
of the kind of tubing you're using, you may actually see an A line trace that has a big
whip on it, which is due to underdamping, which means that the system is just too responsive
to reflected waves, and you get a trace which has a big spike on it. By the same token,
with bubbles in the system what you get is an overdamped system, which in the extreme
will just give you a flatline trace. There will be absolutely no pulsation whatsoever.
Invasive Versus Noninvasive Blood Pressure Monitoring.
Now one of the questions that comes up when you're talking about A line traces is oftentimes
you'll see a discrepancy between an automated blood pressure trace and an arterial line
trace. And it creates a fair amount of concern in many circumstances. The important thing
to remember is that that difference is almost entirely due to the fact that invasive A line
in an automated blood pressure cuff measures the same physiologic parameter, which is blood
pressure, but they do it in two entirely different ways.
So the A line, actually, is transduction of a pressure waveform. Automated blood pressure
cuffs work by actually measuring flow through an artery. So they're two different ways of
measuring the same phenomena. And one of the important points about that is the mean blood
pressure determined by a noninvasive blood pressure cuff and an arterial line in a given
patient will almost always be the same.
When an automated blood pressure cuff determines systolic blood pressure, it uses oscillometry
to determine when pulsations, or flow, recommence as the blood pressure cuff pressure is lowered
from a high pressure. So when an automated blood pressure cuff blows up, it inflates
to a pressure that exceeds arterial blood pressure. And then it begins to deflate.
And at the point where it starts to detect pulsations again, which is the recommencement
of flow, it identifies that as the systolic blood pressure. As it continues to deflate,
it's capable of detecting the amplitude of the pulsations, and when it detects the point
of maximal amplitude of pulsations, it labels that as the mean blood pressure.
And then as it continues to deflate, at the point where it loses any detection of pulsation,
it identifies that as diastolic blood pressure-- that is, complete commencement of arterial
flow. So the pressure is low enough now that it's not occluding the artery at all.
So in order of accuracy, as compared to invasive blood pressure monitoring with a non invasive
pressure, the mean blood pressure is the most accurate, the systolic is the second most
accurate, and the diastolic blood pressure-- as determined by a non invasive blood pressure
cuff-- is the least accurate.
So one of the phenomena that you see when you use automated blood pressure cuffs, if
you have an arterial line trace that has a big spike on it, a lot of whip on it, either
because your arterial system is underdamped or because you're dealing with an elderly
patient that has prominent reflected wave phenomena, one of the things that you'll notice
is that the systolic blood pressure, as determined by the noninvasive blood pressure cuff, will
be substantially lower than the systolic blood pressure determined by the arterial line.
And the reason for that is that as the cuff starts to deflate, the area under the spike
on the invasive blood pressure is very narrow, and the noninvasive blood pressure cuff is
incapable of detecting the small amount of flow that occurs during that very short time
period as it deflates. And it really won't detect the arterial systolic blood pressure
until there's a more substantial volume of flow beneath it.
So again, when we're comparing the two, the mean is most likely to be similar in the two
modalities. The systolic is generally likely to be accurate-- although, again, there's
likely to be big discrepancies between the systolic obtained by a noninvasive cuff and
the systolic obtained during invasive blood pressure monitoring when there's a lot of
whip or a big spike on the A line trace.
And the diastolic, as obtained by noninvasive blood pressure monitoring, tends to be, in
many circumstances, substantially different than that obtained by invasive blood pressure
monitoring. And that's due entirely to the method by which noninvasive blood pressure
is determined.
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