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I'm precepting in CVICU which so far has been great. I'm curious about how an arterial line works. I understand what it's for, but don't understand how it works. Is pressure going through the line to keep the vessel open?
Gosh, there really is nothing better for teaching about pressure monitoring than an Arterial Line. The catheter is essentially the same as an IV cathelon that we nurses are well acquainted with. In fact, many A-lines are started by Resp Therapists and I have known plenty of them who prefer to start A-Lines with #22 cathelons. So there is no mystery about what is in the Pt.
Then there is a tubing set that goes from the Pt to the Transducer. Which is the little device that has the wire going to the monitor joined to the tubing coming down from the pressure bag. It has the little 'pig-tail' that flushes the A-Line. So what are each of these three things?
First the wire that goes from the 'transducer' to the monitor...well, that is how the wave-form and the numbers show up on the screen. A "transducer" takes the pressure from the 'wave' of pressure in the artery that radiates up the tubing and translates it into electicity. This electricity then goes up the wire to the monitor and gives you Blood Pressures and Art Wave Forms. (And by the way---very important---if the wave form looks bad you SHOULD NOT TRUST the BP the wave form produces on the monitor!!!!)
2nd, the pressure bag is just a way of keeping the right amount of fluid flowing thru the A-line. It pushes HARD against the transducer and the transducer allows 3cc/hr of NSS thru the transducer into the A-Line. Think of it as a TKO IVFluid for the A-Line. And that the pressure pushing against the transducer keeps the wire getting the right signal.
So what about #3, the pig-tail? Well, when you pull the pig-tail, it opens the transducer to the full force of the NSS in the pressure bag. This does two things. The first is obvious--it 'flushes' the line which clears the accumulated blood, fibrinogen and similar 'stuff' that would eventually clot it off. Second, it produces a 'square' wave on the monitor. (Try it with your next A-Line--quickly pull the pig-tail then release it. Watch the monitor. You'll see a 'square wave' on the screen.)
Why is a 'square wave' significant to old RNs like yer ol' Papaw John? Because once upon a time--when dinosaurs roamed the earth--we had to assemble A-Line and other transducers from kits that were autoclaved and wrapped in sterile packages. We had to fill the transducer with sterile saline and thump and shake the assembled kit to get all the bubbles out. If a bubble or other obstruction to perfect passage of pressure in the artery to the the screen was present, the pig-tail would not produce a 'square-wave'. It would produce a 'leaning wave' or a 'curved wave' or whatever. But not square. So the 'square-wave' produced by the quick jerk on the pig-tail shows that the pressure is perfectly transmitted through the system to the monitor.
So the Art-line has a way of keeping itself open and of showing a wave and pressure on the monitor. And all the other 'pressure monitoring' lines we use in critical care (PA Pressures, ICP Pressures) are roughly the same.
Next--you need to find what "ZEROing" the pressure lines does. But I've gone on far too long now to deal with that.
I couldn't help but smile :spin: at your comment about the accuracy of the arterial waveform. One night, a million years ago, I had a patient that went south on me. He had an arterial line that had been working just fine all night. When I called the Code, the art line was reading a systolic of about 40. The doctor said that it wasn't accurate tho we had no pulse, no respirations and we could see compressions on the arterial waveform. Of course it was accurate--we had a dead patient! What a boob! Makes you wonder sometimes how they can be trusted to take care of people
Well, as I said above--there is no better way to teach about pressure lines than an arterial line. So what are we doing when we 'zero' a pressure sensing monitor? We are compensating for the weight of the air.
Does that seem weird? That air has weight? Well, it's true. We walk around on the face of the planet at the bottom of a 'sea' of air. If you dive to the bottom of the pool to get something that fell in and sank--you expect to feel the pressure of 10 or 12 feet of water. Well, air is of course much lighter that water but there is roughly a MILE OF AIR over your head right now.
So imagine I have a Pt here in Fla with an art line. When I assess this Pt, I 'zero' the line by reciting the little jargon that I first learned 25yrs ago: OFF to the patient. (I turn the stopcock towards the Pt). OPEN to air. (I take the cap off the stopcock.) ZERO the line. (I push the zero button on the monitor.) SQUARE the wave. (I pull the ponytail and make sure there is a vertical up and a vertical down on the waveform.)
Now what have I done? I've put the local Florida barametric pressure into the monitor as the zero point. The arterial pressure on the screen (say--120/70) means: 120 mm mercury ABOVE THE ZERO POINT and 70 mm mercury ABOVE THE ZERO POINT.
Now lets magically transport me and my patient to Denver Colorado which is 1700 meters above sea level. The 'mile high city'. Now there is a LOT less pressure on the whole system and the same heart beat and the same real physiological pressure inside the patient appears on the monitor like a WHOLE LOT MORE. I don't know exactly how much more but we could figure it out with conversions of mm of mercury and meters of air--but I don't want to do that and you don't want to either, right? So what do we do instead of trying to figure out the difference between the way the monitor records the pressure in Fla and in Denver? We RE-ZERO the art line to the new barometric pressure!! (We're pretty clever, us nurses!!)
And that is what we're doing when we 'zero' a pressure line. We're setting the 'zero-point'.
Now--let's face it--an art line deals with a STRONG pressure wave. When I forget to take my Vasotec, my BP would be like 170/100. So precisely 'zeroing' my art line would not change the reading very much. But with other pressure lines (Pulmonary Arteries, Intra-Cranial pressures) you're talking about patients who really need precision!!! There is a big difference between a PA pressure of 20 and 28 for example--or an intra-cranial difference of 6 or 14--THAT'S a BIG DIFFERENCE.
So you should learn from the git-go to ZERO the pressure lines. It's not an optional, maybe-you-should-but-what-the-heck nursing procedure. It's an absolute necessity to GET IT RIGHT.
Hi Papaw John,
I enjoyed your explanation on A-lines so much that I am asking if you will please give us your breakdown of a Swan-Ganz or other PAP lines.
I appreciate your simplicity yet through descriptions as well as your " little jargon" hopefully you have one of those for these lines too.
Just to drive Papaw's point about the importance of zeroing your art lines home...
I work with kids, everybody knows that. Our kids could be only hours old, or they could be the size of a Sherman tank. The itty-bitties are the ones who demand the utmost accuracy in monitoring, because they don't have much room for change. One night a few weeks back, we had several itty-bitties in the unit, two post-op cardiacs and a liver transplant, at opposite ends of the unit. Now these kiddies are prone to pressure issues and often need several times their circulating volume in fluid boluses in the first days post-op and may be on one or more inotropes to help 'em out. On this particular night, around 2330, all of the itty-bitties started having pressure issues. The wave-forms on their monitors had not changed, they all were text-book perfect. But the readings we were getting were dramatically lower. Cuff pressures were all very close to the previous art line pressures, and much higher than the art line was currently giving us. The kids were well-perfused and their heart rates had not risen. Our resident on call that night isn't particularly bright and has had it drilled into her that we believe the art line over the cuff at all times. So I'm helping out at the first bedside to raise the alarm (baby 48 hours out from the CV OR)... we gave fluid, a lot of fluid, with no change in pressure on the art line but a great response on the cuff. So the resident decides to start norepinephrine... Once it was running, the response was underwhelming so she had the nurse crank up the rate. Well, now we've got a great art line pressure, complete with whip, the cuff pressure is sky-high and the baby's heart rate is about 90. So now the norepi is off... That was when I made a tour around the unit to see if anyone else was needing my help. Oh, little What's-his-name in Bed 13 has the identical problem? And little Sweetie in Bed 8 too? What's going on, I'm wondering. So I took a poll. (I'm famous for taking polls.) Three for three, none of the nurses had included rezeroing their lines in their troubleshooting bag of tricks. I was stunned. It's one of the first things I do when there's a sudden and unexplained change in pressures. None of these nurses were green either. Imagine their surprise when rezeroing the lines magically fixed their kids! Later we found out that a storm front had blown in at around 2330 and dumped six inches of snow on us. Moral of the story? You might need to rezero your lines a couple of times a shift depending on the weather!