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Q: aRTERIAL BLOOD GASES ( No Answer,   9 Comments )
Question  
Subject: aRTERIAL BLOOD GASES
Category: Health > Medicine
Asked by: bloodboss-ga
List Price: $10.00
Posted: 03 Apr 2004 08:49 PST
Expires: 03 May 2004 09:49 PDT
Question ID: 324536
HOW LONG AFTER AN ABG IS DRAWN AND PUT ON ICE CAN IT BE DELAYED IN
TESTING WITH ACCURATE RESULT AND IF YOU DRAW VENOUS BLOOD INSTEAD OF
ARTERIAL, CAN YOU BE A PHLEBOTOMIST AND NOT AN RESPIRATORY TECH OR
NURSE?
Answer  
There is no answer at this time.

Comments  
Subject: Re: aRTERIAL BLOOD GASES
From: alkali-ga on 03 Apr 2004 10:36 PST
 
Bloodboss,

Your ABG sample cannot be delayed at all. It must be put on ice and
delivered immediately to the lab for analysis. Blood contains cells
that continue to metabolize the oxygen in the sample and produce
carbon dioxide. There will be a continuous degradation of the test
results over time, and even an immediate analysis is only an
approximation of what the values were in the patient at the time the
sample was drawn.

You cannot draw venous blood for an arterial blood gas test. The
values will be sufficiently wrong that inappropriate treatment might
be given if the physician were unaware of the conditions under which
the sample was acquired.

If no one authorized to take an arterial sample (which requires
special training, informed consent of the patient, and very particular
care of the site where the sample was taken to prevent serious
complications) is available and yet one is required, it is possible to
obtain an arterialized capillary sample. This is not ideal, and should
be clearly marked as such on the sample container to prevent
misinterpretation of the results.

Best of luck!

Alan Kali
Subject: Re: aRTERIAL BLOOD GASES
From: rapscallion-ga on 04 Apr 2004 14:37 PDT
 
The ABG sample should be cooled on ice and delivered ASAP for
analysis, as already mentioned. Moreover the sample should be sealed
from air to prevent loss of CO2.

From "Nunn's Applied Respiratory Physiology, 4th Ed." by John F. Nunn
(Butterworth, 1993), Appendix E, Fig E.1 (which is a time-correction
nomogram assuming storage at room temperature):

PCO2 rises about 0.07 mmHg per minute;
pH falls about 0.0006 per minute;
PO2 falls about 1-3 mmHg per minute.

Implication: The sample should be analyzed within about 10 minutes to
avoid clinically significant errors.
Subject: Re: aRTERIAL BLOOD GASES
From: rapscallion-ga on 04 Apr 2004 14:48 PDT
 
From "Clinical Application of Blood Gases, 2nd Ed." by Shapiro BA,
Harrison RA, Walton JR (Year Book Medical Publishers, 1977), Table
14-2, p.157:

At 4 Deg C:
pH changes 0.001 per 10 minutes
PCO2 changes 0.1/10 mmHg per 10 min.
PO2 changes 0.01 vol% per 10 min.

They state on p.158: "As a general rule, arterial blood samples should
be cooled immediately after being drawn; if this is done, a delay of
up to 1 hour will have little effect on the results."
Subject: Re: aRTERIAL BLOOD GASES
From: rapscallion-ga on 04 Apr 2004 14:50 PDT
 
Correction to my second post: "PCO2 changes 0.1 mmHg per 10 min."
Subject: Re: aRTERIAL BLOOD GASES
From: alkali-ga on 05 Apr 2004 11:08 PDT
 
Rapscallion,

Thank-you for the excellent textbook answers. Having run several ICUs,
CCUs and a number of busy emergency departments, however, I can tell
you that they are, practically speaking, entirely wrong.

In the real world, when people are given latitude with time-critical
samples such as described in the textbooks you cite, errors are
multiplied. A sample that should be cooled to 4 degrees celsius is
often cooled inadequately due to various human failings and the
inevitable time pressure of the critical care environment. This is
complicated by the fact that there is no practical way of measuring
the temperature of the sample inside a closed container. Worse, still,
the sample may be overcooled leading to cell lysis and further
derangement of the results.

The only way to ensure accurate arterial blood gas results is by
stipulating immediate cooling and delivery to the lab along with
flagging the specimen as urgent so that the lab processes it as close
to immediately as practically possible. In my units, I always
stipulated that ABG samples should be conveyed to the lab directly by
a person detailed to do so and trained sufficiently to understand not
to dawdle, chat or visit the cafeteria on the way. A volunteer,
"candystriper" or nurse's assistant is acceptable given these
provisos.

Any protocol that allows for delay in delivering ABG samples to the
lab inevitably suffers from the "oops" factor. Busy personnel put the
specimen aside, intending to deliver it within the specified time
frame, then some evolving crisis distracts them and the specimen sits
there. This happens often in critical care situations. Having obtained
hundreds (at least) of ABG samples, I can assure you that the process
is painful, unpleasant and risky. If I were a critical care patient, I
should be very unhappy at having to undergo a repeat because a sample
went astray.

The final reason that no ABG protocol should allow for delay is that
arterial blood gases are normally only drawn in critical care
situations, and the results are needed immediately. ABGs are the gold
standard for assessing oxygenation, and ensuring adequate oxygenation
is the sine qua non of critical care. That is why we teach
"airway-breathing-circulation" as the first steps in resuscitation. If
you do not require the results of an arterial blood gas determination
urgently, you should think twice about doing the test. There are many
other less invasive ways of assessing the same factors that do not
require an aterial stab. Conversely, having done the test, it behoves
one to obtain the results as close to immediately as practically
possible, since abnormalities in the test may easily result in death
during any delay.

While the textbooks are certainly correct from a scientific
perspective, I know of no critical care setting that has incorporated
the concept of intentional delay into their arterial blood gas
protocols.

As my old flight instructor used to say, "Don't try it if you only
think you can do it; even when you're sure, you are still going to be
wrong some of the time."

Alan Kali
Subject: Re: aRTERIAL BLOOD GASES
From: alkali-ga on 05 Apr 2004 11:49 PDT
 
On second thought, rapscallion, your answers are obviously not wrong.
They probably represent impeccable science, and to the extent that the
bloodboss wanted the most accurate scientific determination of the
rate of deviation from initial conditions of an arterial blood gas
sample, you gave the correct answer.

I assumed from the second part of bloodboss' question that this was a
practical rather than scientific query, but my answer is only part of
the story.

Sometimes I am a bit of a pompous old blowhard.

Alan Kali
Subject: Re: aRTERIAL BLOOD GASES
From: rapscallion-ga on 05 Apr 2004 18:00 PDT
 
alkali:
"pompous old blowhard" -- Been there, done that... :)

You are right to stress the no-delay rule, for all the real-world
reasons you give. However it's not hard to get the sample put on ice
immediately, even if transport is a bit delayed. Ideally the ICU or OR
has a blood-gas analyzer close at hand. I used to run my own samples,
so I had no one else to blame.
Subject: Re: aRTERIAL BLOOD GASES
From: alkali-ga on 05 Apr 2004 18:26 PDT
 
Rapscallion,

"Ideally the ICU or OR has a blood-gas analyzer close at hand."

Ooh! I never had one of those. What a good idea!

Now I feel like a pompous old blowhard with the emphasis on "old".

;-)

Alan Kali
Subject: Re: aRTERIAL BLOOD GASES
From: bloodboss-ga on 06 Apr 2004 05:51 PDT
 
thank you all for the input.  as a nurse, I identify, but more
importantly, my empathy is truly focused on the facts described of the
patient.

and age, by the way, is just a number!!!

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