Arterial Line - transducer and zeroing

[ICUatUCI]

I'm getting confused about zeroing and transducer placement for arterial line. From my understanding, at least where I'm at, they zero using stopcock close to where the transducer is. Once it is zeroed, we don't need to re-zero but just move the transducer according to pt. Is this true?

The following is the question from Hall's book.

20-gauge arterial line connected to a transducer that is located 20 cm below the level of the heart. The system is zeroed at the stopcock located at the wrist while the patient’s arm is stretched out on an arm board. How will the arterial line pressure compare with the true blood pressure (BP)?

A. It will be 20 mm Hg higher

B. It will be 15 mm Hg higher

C. It will be the same

D. It will be 15 mm Hg lower

I thought it would be B but correct answer is C. Is the BP the same because it was zeroed and transducer/pt's position never changed thus BP reading is accurate.

If so, if I were to move the pt's bed higher, in order to get accurate reading, I either have to rezero and not move the transducer OR move up the transducer the same distance as pt's bed?

Thank you

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[IlDestriero]

You can put the transducer at the level of the heart and zero it there or leave it wherever and zero it at the patient’s wrist, which is essentially heart level. Doesn’t matter, though I zero it at the transducer at the heart or COW depending on what I want to track. The advantage of putting the transducer at the level of the heart is you can easily go up or down if the bed moves instead of guessing it was here and I need to go up or down x inches. Just keep it at the heart or brain level. The easiest is to hook it to a stationary support on the bed. That way as the bed goes up or down, the transducer does as well. That won’t work with tburg/reverse t though.
Some GE company people said its most accurate to zero at transducer then zero again at the patient. He Claims it’s more accurate that way. As I cannot explain what that would be, I don’t do if. Our Cardiac team does. They also believe it matters. Whatever.
If anyone can explain why, I’d love to hear it. Perhaps many don’t accurately place the transducer at the heart to begin with. Then it might matter, though you could just do it distally and call it a day.
I’m a “do it correctly the first time and move on” kind of guy and I found more zero errors when I tried distally than at the transducer, so that’s why I do it there.
Has anyone else heard of this double zero nonsense. It makes me wonder if they don’t trust their software to do it and the 2 attempts are somehow processed by the filter/software? It’s all a black box to me, and I’m not sure they share much of their proprietary stuff anywayx


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Il Destriero

 

[pgg]

I thought it would be B but correct answer is C. Is the BP the same because it was zeroed and transducer/pt's position never changed thus BP reading is accurate.

Think about what
1) what it means to zero the transducer
2) what pressure the transducer sees
and you can work through any variation of these questions.

Zeroing means that the transducer ignores whatever pressure it sees when it is zeroed. By opening the stopcock at the wrist, when the transducer is zeroed, you’re teaching it to subtract out the pressure of a 20 cm of H2O column between the wrist and the transducer.

When the stopcock is then opened to the patient again, the transducer “sees” the patient’s blood pressure plus that 20 cm column of water in the tubing - but you’ve told it (zeroed it) to subtract that 20 cm H2O, so the displayed reading is accurate.

Raise the bed 20 cm without moving he transducer, and now there’s a 40 cm column of water added to the patient’s BP. You’ve zeroed out 20, so the reading would be 20 cm H2O higher (= 15 mmHg, don’t forget the cmH2O to mmHg conversion).

Common variant of this question: what if you raise the wrist 20 cm? Now there’s a 40 cm column of water between the wrist and the transducer, but also a -20 cm column between the body and the wrist, so the reading is still accurate.

f you zero the a-line at the wrist, you will get an accurate BP so long as you maintain the same vertical distance between the patient and the transducer. The sensible thing to do is to zero the line with the transducer level with the patient, and keep it level with the patient, and none of those fluid column stuff is relevant.

If so, if I were to move the pt's bed higher, in order to get accurate reading, I either have to rezero and not move the transducer OR move up the transducer the same distance as pt's bed?

Yes.

 

[dhb]

pgg you're a smart dude so i'm going on a limb contradicting you although i'm not sure i am since your post was kind of obscur to me.

But here it goes:
Your question 1) what it means to zero the transducer?

Zeroing the transducer means that when you open the stopcock the transducer is seeing the atmospheric pressure and defines it at 0. So that when the transducer is back inline it gives you the pressure in the arterial system.

If you raise the transducer at the level of the head the pressure will be lower than at the level of the heart by as many cm H2O that it was lifted and if it drops to the floor well the opposite.

Zeroing the trasducer at different heights makes no sense since the atmospheric pressure doesn't change.

 

[BLADEMDA]

Zeroing and leveling the art line
Zeroing and leveling are occasionally used interchangeably, but they are not the same thing. They tend to occur together in the clinical setting, but the terms describe different processes. Zeroing exposes the transducer to atmospheric pressure via an open air-fluid interface, and leveling assigns this zero reference point to a specific position on the symbolic fluid-filled column that is the patient‘s body.

"Zeroing"can be defined as "the use of atmospheric pressure as a reference standard against which all other pressures are measured". The canonical college definition is "a process which confirms that atmospheric pressure results in a zero reading by the measurement system". The device is zeroed when the air-fluid interface is opened to atmospheric pressure (otherwise it would read diastolic blood pressures of ~ 760mmHg). Atmospheric pressure varies little between the intensivists' eye level and the supine patients' aortic root level, and so strictly speaking the zeroing of an arterial line can take place with the transducer lying anywhere. Re-zeroing must occasionally take place as both the transducer and the atmospheric pressure will gradually drift away from the calibration point.

"Leveling" can be defined as "the selection of a position of interest at which the reference standard (zero ) is set". The canonical college definition is "a process which determines the position on the patient you wish to be considered to be your zero." For reasons of convenience this tends to happen at the same time as zeroing the system to atmospheric pressure (which also sets the reference "0 mmHg" standard), but theoretically one could zero the transducer to atmosphere and then swing it wildly all around the room before levelling it against a reference point on the frightened patient.

The system is conventionally "leveled" at the phlebostatic axis, which is a reference level we have used since probably 1945. The phlebostatic axis corresponds roughly with the position of the right atrium, and his level has generally been accepted as the ideal reference level for measure the pressure of the blood returning to the heart. It was therefore adopted as the reference level for CVP measurement. For arterial pressure measurements, at least since 2001 or so we have been also leveling the arterial lines at the phlebostatic axis. Prior to that, some units leveled their arterial lines at the level of the catheter insertion site. The specific reference point for the arterial transducer is actually the aortic root, but because it is very close to the right atrium the two reference levels are essentially the same.

For every 10cm below the phlebostatic axis, the art line will add 7.4mmHg of pressure.

One may sometimes be interested in leveling the arterial line at another point. Essentially, the level at which you zero the arterial line will measure the arterial pressure at that level. Which means that if your patient is in some sort of unconventional position (eg. sitting bolt upright) you may wish to measure at the level of the tragus instead. An art line leveled at the level of the external auditory meatus will measure the arterial pressure in the Circle of Willis, which is a representation of cerebral perfusion pressure. Various eminent society guidelines recommend that for the use of cerebral perfusion pressure as a therapeutic target, the reference level should be somewhere around the middle cranial fossa. Whether this matters or not is a subject of some debate.


The arterial line pressure transducer setup

 

[ICUatUCI]

Think about what
1) what it means to zero the transducer
2) what pressure the transducer sees
and you can work through any variation of these questions.

Zeroing means that the transducer ignores whatever pressure it sees when it is zeroed. By opening the stopcock at the wrist, when the transducer is zeroed, you’re teaching it to subtract out the pressure of a 20 cm of H2O column between the wrist and the transducer.

When the stopcock is then opened to the patient again, the transducer “sees” the patient’s blood pressure plus that 20 cm column of water in the tubing - but you’ve told it (zeroed it) to subtract that 20 cm H2O, so the displayed reading is accurate.

Raise the bed 20 cm without moving he transducer, and now there’s a 40 cm column of water added to the patient’s BP. You’ve zeroed out 20, so the reading would be 20 cm H2O higher (= 15 mmHg, don’t forget the cmH2O to mmHg conversion).

Common variant of this question: what if you raise the wrist 20 cm? Now there’s a 40 cm column of water between the wrist and the transducer, but also a -20 cm column between the body and the wrist, so the reading is still accurate.

f you zero the a-line at the wrist, you will get an accurate BP so long as you maintain the same vertical distance between the patient and the transducer. The sensible thing to do is to zero the line with the transducer level with the patient, and keep it level with the patient, and none of those fluid column stuff is relevant.




Yes.

Thanks for the explanation. It is more clearly to me. I like your common variant question. I initially thought there would be increased in BP reading but since it is just the wrist and not the body, BP reading is still accurate. Thanks again

 

[btbam]

In the heart rooms, we zero with stopcock open at the chest -phlebostatic axis- and have the transducer set up attached to the bed on a pole so no need to worry about re-zeroing when bed goes up and down.

 

[Ronin786]

This is a stupid and confusing question like all board questions seem to be.

The trick here is that they say it's zeroed at the stopcock near the patient's wrist. Which idiot zeroes an arterial line that way? The correct way is to zero at the stopcock located near the transducer because there is no column of fluid in between. Then you can throw your transducer across the room and it won't matter as long as you throw the patient with it.

 

[sevoflurane]

In the heart rooms, we zero with stopcock open at the chest -phlebostatic axis- and have the transducer set up attached to the bed on a pole so no need to worry about re-zeroing when bed goes up and down.

Shouldn't be different in any room.

 

[fakin' the funk]

Zeroing and leveling are different.

You zero to atmospheric pressure.

You level at the horizontal level of the organ whose pressure you're interested in.

 

[Aether2000]

Blade, you rock! Love your explanation.

 

[deleted697535]

"Off to the patient open to air"
"Press Zero."

Doesn't matter where the flip you zero it. I usually zero mine about 20 mins before the patient even arrives in the OR and hang it on the pole ready to go...

Zeroing has to do with atmospheric pressure not the patient

To level it I tape it to the pillow for supine cases most often etc etc

 

[BLADEMDA]

Since I keep things KISS because I work with CRNAs I ZERO and LEVEL at the same time. This keeps things nice and simple. It's what most of you do on a daily basis.

That said, Zeroing and LEVELING are not the same thing (more of a BOARD question than real world).

 

[Iso4ane]

Zeroing and leveling are occasionally used interchangeably, but they are not the same thing.

The zeroing of an arterial line can take place with the transducer lying anywhere.

I was literally thinking about this and testing this the other day. The difference between zeroing and leveling is a big deal and very real. If you saw me in the OR that day, you would've of thought I was a idiot/crazy. Patient was stable, raised transducer, lowered transducer, open to atm (as if we were zeroing), up, down, back up, (re-zero at higher level) turn transducer back to patient, lowered transducer to below patient (re-zeroed) the transducer. Repeat. Figured out where you zero doesn't matter, made sense on paper, but testing it out made something click.

 

[lunaire]

To add to above, the only relevant time to re-zero is if you're doing flight medicine, hyperbaric medicine, or if your wire got disconnected and you get a question mark on your monitor. Unless you feel your ears pop, the atmospheric pressure you zeroed to didn't change much, and you don't need to re-zero.

 

[dukethin]

I still don’t get this question. If the transducer is below the level of the phlebostatic axis (in this case, it’s 20cm below), why is the weight of the column not added to “actual” blood pressure resulting in an overestimation of the blood pressure? Why is the answer not B?

 

[deleted697535]

Because it's not levelled at the transducer in this particular case. It's levelled at the stop cock at the wrist.

It's a bs trick question.
I've never seen anyone in real life zero an art line that way and if I ever do I'll punch them