Interpreting Central Venous 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. I'm going to talk
now about our central venous pressure trace and the kind of information that we can get
from a central venous pressure trace, and also about how a central venous pressure trace
is generated and what implications that has.
So again, it's important to remember that when we're measuring a CVP, we are in fact
measuring a pressure in the central circulation, so in the superior vena cava or in the right
atrium, depending on where the tip of the catheter is. But in fact, what's happening
is this pressure is a consequence physiologically of volume changes in the superior vena cava
or the right atrium in the setting of the compliance of those two systems.
What we see here when we look at a CVP trace, we see a volume moving in and out of the right
atrium or the superior vena cava being represented as a pressure here. This pressure -- we're
going to talk about this trace as one bead of the central venous pressure. You can see
that there's a couple of waves here. So this first wave on the upstroke here is known as
the A wave.
And you can see that is actually occurring - we can see with the simultaneous A line
trace - as a late event in diastole. So here is end diastole in the arterial blood pressure
traced right before the onset of systole. And if we look up here at the EKG, that same
interval corresponds with the atrial contraction of the EKG. So this is an end diastolic event.
And what this represents-- this is the pressure generated in the right atrium or the superior
vena cava when the atrium contracts in a late diastole. And that is the pressure that's
generated. And that pressure is a consequence of the volume of blood that's moving into
the atrium and the compliance of the atrium.
So you can imagine a circumstance where, if you have a very compliant right atrium, even
if you have a lot of volume moving in with the atrial contraction, the A wave is not
going to be very big. By the same token, if I have a very non-compliant right atrium,
a very young patient, and I have a lot of volume moving - and let's say they've been
given a big volume infusion - and I have a very forceful atrial contraction, I'm going
to see a big A wave here.
The next part of this CVP waveform is a C wave. You can see with a simultaneous A line
trace that the C wave of occurs during the onset of ventricular systole. And if we look
up here at the EKG, that same time interval corresponds to the R wave. What the C wave
represents is bowing of the tricuspid valve back into the right atrium during systole
such that there is a transient decrease in the atrial compliance and an increase in atrial
pressure. During tachycardia, the C wave commonly becomes merged with the A wave. And under
those circumstances, it's almost impossible to differentiate the A and the C waves.
This little down slope is known as the x descent. And the x descent actually represents-- we're
going to be talking about systole now here. See, now we're under the systolic portion
of the arterial line trace and we're into the QRS of the EKG. And it also represents
a combination of factors. It represents the atrium relaxing. And as a consequence of that,
the tricuspid valve apparatus is descending towards the apex of the ventricle. And that,
in turn, results in the formation of this x descent.
Now, the next thing we see here is the V wave. And every human being that has a CVP line
in has a V wave. The V wave is a late systolic event. You can see with the simultaneous A
line trace that the V wave is occurring during late systole. And again, if we look up here
at the EKG, this interval corresponds to the T wave of the EKG.
The V wave is a normal physiologic wave, and it occurs as a consequence of filling of the
right atrium during ventricular systole. So the height of the V wave is going to be totally
dependent on the compliance of the right atrium and the volume of blood that is coming back
into the right atrium during ventricular systole. So a very large V wave can, in fact, be due
to the presence of tricuspid regurgitation, because there's actually regurgitation of
blood into the right atrium during systole, and that's what augments the V wave. But it
also could be due to an instance where there's a lot of volume coming into the right atrium
during ventricular systole in a circumstance where the atrium isn't very compliant.
Another thing to remember about V waves is it's possible to have a big volume of tricuspid
regurgitation without having a big V wave, and the reason that would happen is that if
the right atrium were very, very compliant. So if a patient had tricuspid regurgitation
for a very long period of time and their right atrium had gotten distended and very compliant,
it's conceivable that they could regurgitate a lot of volume back into the right atrium
without generating a high V wave, because the pressure we see on the CVP trace, remember,
is due to the combination of the volume regurgitated and the compliance of the chamber it's regurgitating
into. So a big volume into a very compliant chamber is not going to give us a big V wave.
If a patient acutely developed tricuspid regurgitation and had a very small right atrium and suddenly
have a lot of volume regurgitating back into a small non-compliant right atrium, we would
expect the V wave to be enormous.
OK. So now the next part of the CPV trace is what's known as the Y descent, and that's
this long segment here. And what that represents-- that's an early diastolic event. So here's
the Y descent. And we're starting to be under the diastolic portion of the A line trace.
We're under the diastolic portion of the EKG.
And what the Y descent represents is early diastolic emptying of the right atrium, obviously
during the very earliest parts of diastole. So we have volume leaving the right atrium
into the ventricle. So we see the pressure drop as the volume moves into the ventricle.
And then now we move into the next A wave, which represents contraction of the atrium,
which gives us another A wave here and another pressure increase.
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