Arterial Line Question - page 3

by wonderbee 66,352 Views | 35 Comments

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?... Read More


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    Great info here but not one mention of where the transducer needs to be. Curious. Can't tell you how many times I've been called in to a room by a new nurse in a panic over a pressure reading just to find the transducer hanging off the bed.

    The answer, of course, is the phlebostatic axis-mid axillary line, 4th intercostal space (ie, in line with the heart). Many folks use armbands to hold the transducer and that's fine but keep in mind that if you turn the patient the reading is false- if the transducer is lower than the heart the reading is falsely high, higher than the heart reads falsely low.
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    Central venous pressure is zeroed to the phlebostatic axis.
    Art lines are zeroed to the level of the aortic root- best done a bit more anterior.
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    Quote from TakeBack
    Central venous pressure is zeroed to the phlebostatic axis.
    Art lines are zeroed to the level of the aortic root- best done a bit more anterior.
    Interesting. In almost 20 years I've never heard of leveling to the aortic root.
    I did a google search of it and after looking at the first 20-30 hits, every single one of them said to level the a-line to the phlebostatic axis. It's all I've ever done and I wonder how much of an actual difference it would make?
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    Effect of Variable Transducer Level, Catheter Access, and Patient PositionCHEST October 2001 vol. 120 no. 4 1322-1326 Measurements and main results: For each transducer level, five systolic and diastolic pressures were measured and used to calculate five corresponding mean arterial pressures (MAPs) at each access site. When transducers were at the aortic root, MAP corresponding to aortic root pressure was obtained in all positions regardless of catheter access site. When transducers were moved to the level of catheter access, as current recommendations suggest, significant errors in aortic MAP occurred in the reverse Trendelenburg position. The same trend for error was noted in the Trendelenburg position but did not reach statistical significance. Conclusions: (1) Current recommendations that suggest placing the transducer at the level of catheter access regardless of patient position are invalid. Significant errors occur when subjects are in nonsupine positions. (2) Valid determination of direct arterial BP is dependent only on transducer placement at the level of the aortic root, and independent of catheter access site and patient position.
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    I hadn't seen that either, learn something new every day. A couple observations though.
    1. the efficacy of reverse trendelenberg has been pretty much dismissed in all the literature I've seen and in my clinical practice so I really don't use it.
    2. the conclusions state that there was a difference but not statistically significant in the trendelenberg position but then goes on to say "significant errors occur when subjects are in nonsupine positions." So... which is it?
    3. Is the slight difference between the position of the aortic root and the phlebo axis going to be clinically significant?
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    Quote from txdude35
    I hadn't seen that either, learn something new every day. A couple observations though.
    1. the efficacy of reverse trendelenberg has been pretty much dismissed in all the literature I've seen and in my clinical practice so I really don't use it.
    2. the conclusions state that there was a difference but not statistically significant in the trendelenberg position but then goes on to say "significant errors occur when subjects are in nonsupine positions." So... which is it?
    3. Is the slight difference between the position of the aortic root and the phlebo axis going to be clinically significant?
    If you are referring to the use of reverse-T for BP management, I routinely see that it works for short term management- an acutely hypertensive pt will get some BP benefit from the venous pooling.That study shows more sig differences in reverse T than others.Re: the signfiicance of the difference:Circulation. 1995 Oct 1;92(7):1994-2000.Anatomically and physiologically based reference level for measurement of intracardiac pressures."CONCLUSIONS: External fluid-filled transducers should be used with the goal of removing hydrostatic pressure and other influences so that the presence of subatmospheric pressure during diastole in any of the cardiac chambers is accurately measured. To achieve this goal, intracardiac pressure should be referenced to an external fluid-filled transducer aligned with the uppermost blood level in the chamber in which pressure is to be measured. The current practice of referencing the zero level of LV diastolic pressure to an external fluid-filled transducer positioned at the midchest level results in systematic overestimation due to hydrostatic effects and produces physiologically significant error in the measurement of diastolic intracardiac pressure."
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    Cool, thanks for the info.
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    I have a couple thoughts from the last few posts-
    1- I always zero to phlebostatic... the aortic root propostion is new to me... thanks for the exposure. We do typically keep our alines and CVPs on the same holder though so it's not very feasible.
    2- in post 23 "conclusions" mentions "Current recommendations that suggest placing the transducer at the level of catheter access regardless of patient position are invalid. Significant errors occur when subjects are in nonsupine positions." I've never heard of such a policy? Who would even imagine this is practical?
    3- I'm confused by some discussion here about REVERSE T-burg... what about it is "dismissed"? We actually use it more for practical reasons than for interventional reasons- ie to keep HOB close to 30* for a vented pt who also has a fem line for IABP or CRRT. I believe neuro ICU uses it for increased ICP pts also but I try to shy away from those rooms when I float.
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    I have a question about getting all the air out of the bag before you prime the line and then > to the 300mm of pressure. For years I have always taken great pains to remove the air before I prime the pressure bag. I read ? an article some whre that stated it was a old sacred cow but I can't remember where I read it or maybe it was a seminar. Anyway it kinda made sense in that it was most important to have the line free of air and that the small amt of air in the bag would be pushed to the top when the bag was>to 300mm and would not enter the system. It was stressed the importance of never lying the bag on the bed during transfer or transport in that the the air in the chamber could enter the tubing. The question was raised about if the fluid got low in your system wouldn't air get pushed in the tubing and the answer was that the pressure in the body would be greater if the fluid ran out and you would see blood in the tubing then. So I have looked on several sites regrding this and it's about 60-40 in favor of getting the air out before you prime the the tubing. We don't have a policy that states this and I am currently a preceptor. Any hard facts and or opinions? Any thing evidence based?
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    I know zeroing the art line and flushing it are done together but I am unsure which is done first. Or, does it matter? Thanks in advance..


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