Welcome to ICU Primary PrepCast. Hi, my name is Swapnil and I'm joined by Dr. Mike Clifford from Melbourne. Welcome, Mike. Thank you, Swapnil.
I'm just noticing that you've got your Navy haircut going there. It's very short. That's right. It's Army haircut. now so i need to change my haircut all right you're going through all of the services that's right anyways let's dive into today's snippet this is about arterial line and physics behind the pressure transducer so my the question has been asked around the explain the resonance and its significance and effects of damping on invasive arterial blood pressure measurement and again this was a bit of confusing for candidates how much to write and what what is expected it did.
Yeah. Okay. So there's what you need to know, and then there's what you can get down on paper. And they're probably reasonably similar, but not completely similar. This is one of my favorite topics.
And I think if there's one, because what defines an ICU patient, one is an endotracheal tube and the other is an art line, because both of those things mean you need one to one nursing. Because if your art line falls out, you'll bleed to death. If your ET tube falls out and you're paralyzed, you'll die. So an ICU patient is someone with an art line. give or take.
So being able to look at an arterial trace and see what it looks like and what that means and what the problems are when you're looking at that trace are very important to intend. Also, historically, they were a bit of a fiddle. You had to adjust them. You had to adjust their damping. You had to optimize the waveform, often with little screws and screwdrivers or injecting air into the system.
They were a pain, right? And you really had to know what you were doing. These days it's all taken care of. They get mass produced in a factory.
They've got little tiny silicone wafers that are balanced to provide multiple wheat stone bridges within the system. So they're highly sophisticated pieces of kit and they're disposable and the cost is about $20, right? But there's been an enormous evolution in the sophistication of those systems.
But having said that, the problems with them are the same, right, and the nature of their design is still the same. So you have to understand why. There's all this physics involved in this, what would seem to be a relatively simple thing. So what's resonance, right?
The natural frequency is the frequency of any object that it will adopt if it's perturbed from rest. So if you strike a guitar string, it will resonate, right? It'll produce a sound. And if it's in tune, it'll produce that note.
And it doesn't matter how many times a day you come and hit it, it'll make the same sound. So every... Every piece of oscillating system will have its own natural resonant frequency.
Now, resonance will occur when you subject that string to an oscillating force close to its natural frequency. So if I come along to a guitar string and I tap it, if I continually tap it at the frequency at which it's ringing, it will get louder and louder and louder and louder, okay? And eventually the string will probably disrupt. And many people have seen those YouTube videos of bridges in the United States swinging in the breeze and then falling apart. And what's happening is that the gusts of wind are striking that bridge while it's oscillating close to its natural resonant frequency.
You may also find that if you're walking across a footbridge, sometimes there's no shaking, and then all of a sudden someone will walk past the bridge and it'll start to shake. And it's not because they're heavier than anybody else. It's just that their footsteps happen to be landing at the resonant frequency.
So every monitoring system will also have a natural frequency, okay? And if the monitoring frequency is too low... The frequencies in the monitored pressure waveform will overlap with that natural frequency of the measurement system.
So as a result, the system will resonate and any pressure waveforms on the monitor will be exaggerated. Okay, so that's an underdamped trace. It'll wobble all over the place. So when you design a natural frequency of the arterial line system, you want it to be at least 10 times the fundamental frequency of the system. So in most clinical systems, the natural frequency is at least 10 to 15 hertz, which is much higher than the fundamental frequency of an arterial waveform, which is, you know.
60 to 120 beats a minute, so one to two hertz. So the faster the heart rate and the steeper the systolic pressure upstroke, the greater the dynamic response demands of that system will be. It has to get better and better, okay? So the resonant frequency of that system can be increased by, you guessed it, a short, wide, stiff catheter, because that makes the resonant frequency higher.
So that's why we use short, wide, stiff catheters. That's why we use stiffer tubing on our arterial system, okay? So damping is what you get when you decrease the amplitude.
of the oscillation, okay? And it's a result of energy loss from the system. And we incorporate the degree of damping.
because it's desirable and necessary for any accurate blood pressure measurement. There is always some damping inherent in any system because your blood pressure is striking your saline column and moving the saline column along the pipe, and the pipe will expand ever so slightly. So there's always a degree of damping occurring. But you can see more damping if you look at some patients because there might be an air bubble in the system, or there might be blood clot, or the catheter might be partially kinked.
And these all reduce the deflection of the little transducer diaphragm or the wafer crystals inside the system. So it will reduce the size of the waveform. The damping coefficient is the description of the tendency of any apparatus to extinguish those oscillations.
And we like to have a damping coefficient of about 0.6 to 0.64. And that would be called optimal damping. And that gives us the best balance of the speed of response and the accuracy of the system.
And a good arterial line trace would have a dichronic notch, for example, and often two oscillations after what was called the flush test. So in the old days when you used to flush a transducer system with a little, you'd lift up a little diaphragm and that would allow a flush through and then you'd let it go quickly and you get bounce in the arterial line. And some systems out there today still have that.
And what you want is one to two bounces in that system. So in terms of problems with the system, underdamping, you'll get resonance will occur, causing the signal to oscillate too much and overshoot. And the overshoot.
will result in a higher systolic blood pressure and an overestimation of the systolic blood pressure and an underestimation of the diastolic blood pressure. Your mean arterial pressure will tend to be unaffected. It will be very rapid in response. And if you do a little square wave or flush test, you'll get multiple oscillations that will keep going. Overdamping, the high frequency components of the arterial waveform will be lost and you will get loss of the dichroic notch and you might lose some of the other details in the waveform.
And again, it will underestimate your systolic blood pressure. And it may overestimate your diastolic publish. So the two pressures close together.
And again, your mean may well be accurate, but the system will be very slow to respond to changes. And if you did a square wave test, you might only get one oscillation. So that would be over damped. And that's really resonance and damping.
That's as much as you need to know. And we will always ask in those questions, what are the effects of over damping? What are the effects of under damping? And invariably you're losing information. Your mean usually is accurate.
Thank you. and you just need to have in your head a couple of reasons why it may occur. So the next time you're on the unit and you're doing a ward round, have a look at the arterial pressure trace and see whether you think it's underdamped or overdamped, but then have a look at where the catheter is placed. Is it in the foot of a little old lady or is it in the femoral artery?
And if it is damped, see if you can troubleshoot why. And occasionally you'll find a bubble floating near the transducer system and if you flush it out, you'll make everything look better and everyone will think you're a hero and you'll get a job next year. Thanks, Mike.
I thought the flush test is to get rid of people, not to employ people. Well, yes, if you flush it the wrong way and you get blood all over your consultant, you won't be coming back next year. That's right.
Because nursing staff will tell you if you mess up with their sheets. Correct. So that's the end of our today's snippet. We'll be back with another snippet in a week's time.
Till then, goodbye and have a nice time. See you later, everybody. Have fun with your outline.