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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? |
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There is no answer at this time. |
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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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