Welcome back to the tasty morsels of critical care podcast.
Today we are going to do our best to charm the yellow snake of the intensive care unit and cover the pulmonary artery floatation catheter. Like a lot, indeed practically all of these topics, I do not in any way consider myself to have great expertise in the topic but I have had to upskill as much as I possibly can in lieu of the typical mis spent youth doing cardiac anaesthesia that most of my colleagues have had.
As such the source list for this post is quite varied in terms of its references.
The focus here will be on the basis, the nuts and bolts of how to put in and what type of numbers you might obtain from a PAC.
The insertion carries a lot of similar complications to any typical central vascular access procedure. But the big ones come from the fact that you’re trying to place the catheter through the heart rather than in close proximity to it. Perforation is of course a real possibility but perhaps more likely are nasty arrhythmias precipitated by the catheter irritating the myocardium. Expect to see this more in the cold, shocked post bypass patient or in someone who’s already having a lot of arrhythmias.
The PAC is also famous for the knots it can manage to tie itself into that can make extraction more than a little challenging.
There are lots of good materials online on insertion so I’ll only mention a few basics in passing. The tiny little balloon at the tip catches the flow of the venous return and pulls the catheter along with the flow.
In the absence of flurosocopy it can be tricky to know quite where the tip of the catheter is at any given time so we use the changes in waveforms to tell us what chamber or vessel the tip is at any given time. The pattern we expect to see should be CVP waveform, RV waveform then PA waveform and finally a wedged waveform. If all plays ball the you should those patterns at roughly 20cm, 30cm, 40cm and 50 cm respectively. The challenge is usually transitioning from the RV to the PA and the key change in waveform to look for is the “step up” in the diastolic pressure from the RV waveform which has a diastolic in the low single digits to a PA diastolic which is in the low double digits.
Once the procedure bit is done we typically take a CXR looking for the tip. Typically the natural curve of the catheter leads it to ending up in the right PA most commonly though this is by no means guaranteed. It can be tricky to tell from a simple CXR but ideally we want the tip in a West zone 3 part of the lung, typically in the inferior portions. West zones may be a distant memory from medical school but for our purposes the estimate of the left atrial pressure produced by our pulmonary capillary wedge pressure is only valid when the alveolar pressure is less than the pulmonary venous pressure, a situation that exists only in West zone 3. If you’re in zone 3 you should be able to see a and v waves (analagous to the a and v waves of the CVP waveform)
In some of the linked papers at the end there are some excellent images of troubleshooting various waveforms. One of the more useful ones was dealing with the failing RV (the very scenario where a PAC is likely to be needed) In this scenario, the RV diastolic pressures can approach the PA diastolic pressures with a loss of the “step up” as you move into the PA. The key difference to note in this scenario is that when the PAC is in the RV the diastolic run off (the period before the next ejection) is upsloping and the disatolic run off is downsloping when the PAC is in the PA.
There are lots of measurements we can take from the PAC. Directly measured PA pressures are of course useful but the typical catheters used these days also have a thermodilution filament built in so that we can measure continuous cardiac output (on the principle that the RV cardiac output is equivalent to the LV cardiac output). The contemporary catheters use semi random pulses of heat (up to 44 degrees) in order to calculate a thermodilution cardiac output. In general it needs at least a 15% difference in CO to be detectable and it averages things over 5-10 minutes rather than from beat to beat. There is often a “stat CO” measure that averages it over a more like 60 seconds.
In another success of marketing over function there is typically a continuous oxygenation sensor at the tip of the catheter. This gives a continuous reading of the true mixed venous oxygenation but is probably worth calibrating with an actual co-oximetry reading from a blood gas taken from the tip of the catheter.
With a PAC in place we have the potential for measuring the pulmonary capillary wedge pressure which given a long number of assumptions can allow us to infer things like a left atrial pressure or left ventricular end diastolic pressure, key variables for assessing the filling status of the left heart. The principle involves the tip being in a west zone 3 branch vessel, the balloon is then blown up creating a theoretical continuous column of blood between the tip of the catheter and the left atrium. Once wedged the displayed number will typically be a mean, however the PAOP should be obtained at end expiration and in end diastole which often means reviewing a screenshot with your monitor and using a cursor to identify the pressure, timed at the onset of the QRS. There of course are lots of subtleties and caveats to the number obtained and even more about how to respond to it.
Finally if you want to be really hard core there is a way of compensating for the effect of high levels of PEEP (>10) on the PAOP. The transmission index (TI) gives you a “corrected” PAOP taking this into account. The TI is calculated by looking at the PAOP in inspiration and expiration. The difference between these two numbers is then divided by the driving pressure on the ventilator, this is your TI. The corrected PAOP is then the measured PAOP minus the total PEEP multiplied by the TI. This type of maths does not translate well to audio format and indeed there are actually several of these calculations available just to make it even more confusing.
There is a substantial literature behind the utility, or lack thereof of the PAC that has led to a massive decline in its use preceding the mid noughties when i started practicing. However they remain a key tool in the intensivists arsenal and if you deal with sick hearts on a regular basis it’s vital you have a decent grasp on charming the yellow snake.
Irwin & Rippe Chapter 19 (an excellent source of a textbook if you want detail on any topic not particularly well served by Oh)
Deranged physiology has as expected an even higher level of excruciating details for those interests, presented of course in an excellent fashion.