“Interpreting Arterial Pressure Waveforms” by Jim DiNardo, MD, FAAP for OPENPediatrics

“Interpreting Arterial Pressure Waveforms” by Jim DiNardo, MD, FAAP for OPENPediatrics


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. Please help us improve the content by providing
us with some feedback.