Underdamping/Overdamping of Arterial Pressure Waveforms

[drlee]

I ran into some confusion on what causes underdamping/overdamping of arterial pressure waveforms. According to Big Blue, increasing the LENGTH of the tubing causes underdamping, whereas in the Hall question book (q. 85), increasing the LENGTH of the tubing causes overdamping. Can anyone clear this confusion?

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[Idiopathic]

We went over this recently and it was actually wrong in one of the practice questions that we were studying.

Increasing the length of the tubing definitely overdampens the waveform, as does overly compliant tubing. Hall et al. are correct here

 

[pgg]

This was indirectly tested in the 1994 ITE Book A #46:

During direct intra-arterial measurement of blood pressure
(A) the mean arterial pressure will be unaffected by small air bubbles in the system
(B) an underdamped system will overestimate the mean arterial pressure
(C) an underdamped system will overestimate the diastolic pressure
(D) long tubing from the catheter to the transducer will tend to decrease resonance in the system
(E) calibration of the transducer by opening a stopcock to air at the sternal angle will overestimate the mean arterial pressure

The key says the answer is A. Bubbles cause overdamping, which doesn't affect MAP, so this is correct.

Underdamped systems underestimate (or accurately measure) diastolic pressure but provide accurate MAPs, so B & C are wrong.

Miller (6th ed p 1279-1280) recommends placing the transducer "5 cm below the sternal border in the fourth intercostal space" so E is wrong because zeroing at the sternal angle would underestimate the MAP.

If D is wrong (as the board claims), the implication is that long tubing either increases or doesn't change resonance in the system. Increased resonance = underdamping so what I'm taking away from this is that the board thinks (thought) that longer tubing causes underdamping. This is at odds to Hall and what intuitively and conceptually makes sense, but agrees with Big Blue.

I think that this old board question is wrong and may have been thrown out. Since much of Big Blue is based on old/remembered questions, I suspect this may be where Jensen got the bad info. I sent this off to Jensen using his errata form a few months ago but never got a response.

Idiopathic is right; long tubing contributes to dampening. Systolic pressure is underestimated; diastolic pressure is overestimated (or accurate); MAP is accurate.

 

[kim5142]

I ran into some confusion on what causes underdamping/overdamping of arterial pressure waveforms. According to Big Blue, increasing the LENGTH of the tubing causes underdamping, whereas in the Hall question book (q. 85), increasing the LENGTH of the tubing causes overdamping. Can anyone clear this confusion?

I was reading the SOCCA resident's guide and had the same question. Not sure if anyone still cares, or if the question was settled long ago.

According to the SOCCA 2014 resident's guide section 8 on pulse wave monitoring: their reasoning for the "underdamping" effect as tube length increases is explained "as the pulse travels from the aorta to the periphery, the systolic pressure is amplified by reflected waves from the periphery. This pulse amplification results in distal measurements (e.g. radial artery) having a greater systolic pressure and slightly lower diastolic pressure compared to more proximal measurements (e.g. femoral artery).

I would still think the reflected waves would be traveling the opposite direction and result in overdamping. I'd be interested to hear any additional insight on the topic. Thanks.

 

[caligas]

Anybody used something called a "r.o.s.e" filter to smooth out underdamped systems?


Used it years ago but can't find them now.

 

[Ronin786]

Anybody used something called a "r.o.s.e" filter to smooth out underdamped systems?


Used it years ago but can't find them now.

We have ROSE adapters at my shop. What I do though is attach a 3cc syringe to the stopcock and open to all directions. It achieves the same thing.

 

[Planktonmd]

Anybody used something called a "r.o.s.e" filter to smooth out underdamped systems?


Used it years ago but can't find them now.

Just inject a small air bubble in the line it will fix the problem and it's free!

 

[Planktonmd]

This was indirectly tested in the 1994 ITE Book A #46:



The key says the answer is A. Bubbles cause overdamping, which doesn't affect MAP, so this is correct.

Underdamped systems underestimate (or accurately measure) diastolic pressure but provide accurate MAPs, so B & C are wrong.

Miller (6th ed p 1279-1280) recommends placing the transducer "5 cm below the sternal border in the fourth intercostal space" so E is wrong because zeroing at the sternal angle would underestimate the MAP.

If D is wrong (as the board claims), the implication is that long tubing either increases or doesn't change resonance in the system. Increased resonance = underdamping so what I'm taking away from this is that the board thinks (thought) that longer tubing causes underdamping. This is at odds to Hall and what intuitively and conceptually makes sense, but agrees with Big Blue.

I think that this old board question is wrong and may have been thrown out. Since much of Big Blue is based on old/remembered questions, I suspect this may be where Jensen got the bad info. I sent this off to Jensen using his errata form a few months ago but never got a response.

Idiopathic is right; long tubing contributes to dampening. Systolic pressure is underestimated; diastolic pressure is overestimated (or accurate); MAP is accurate.

Resonance happens in a one sided open tube filled with fluid when the length of the tube equals 1/4 wave length of the mechanical wave.
So if we assume the mechanical wave travels at the speed of sound in water: 1482 m/s, and the frequency is 70 BPM(pulse frequency) = 1 HZ , the wave length would be 1482/1 = 1482 meters, and 1/4 wave length (resonance length) will be 370 meters.
So... the closer your tube to 370 meters, the closer it is to the resonance frequency, which means in our example that the longer tube causes more resonance and as a result more amplification of the wave.
So... longer tube = less damping of systolic wave.
Easy isn't it?

 

[caligas]

Just inject a small air bubble in the line it will fix the problem and it's free!

yes, works well, just don't flush the bubble!

 

[IkeBoy18]

seeing that the MAP in under/overdamped systems is accurate, in practice, is it worth it to fix it?

 

[norwood]

just don't flush the bubble!

... Which is why I'd never intentionally put air in one of those systems. Even if everyone else involved with the case knows not to flush it (CRNAs, residents, breaks, etc) the system also slowly (few cc's per hour, IIRC) auto-flushes. It's designed that way, so that it won't clot.

You can see this effect if your techs make a-line setups ahead of time -- they'll develop a little puddle under the end of the tubing.

 

[caligas]

seeing that the MAP in under/overdamped systems is accurate, in practice, is it worth it to fix it?

I find sometimes in cardiac that surgeons will focus on the systolic and will ask you to bring the pressure down if there is a lot of "whip" causing a falsely elevated systolic. I'd rather damp the system than have to explain it to them.