“abg” is the abbreviation for arterial blood gas (analysis)
Often when I enter a patient’s room, after my introduction, I ask the patient if the doctor (or nurse) informed them that an ‘abg’ was ordered and the answer is almost inevitably “no”. So I explain that the reason I am there is to draw some blood for this test and, again, almost inevitably the patient states, they already did that or they already took blood, can’t you use some of that? or, indicating the IV, can’t use take it from this? The answer is always ‘no’ and then I explain.
It used to irritate me that the physician – or “provider” is the new term because sometimes it’s a nurse practitioner or physician’s assistant who is the ‘provider’ – didn’t consult the patient, ask permission from the patient, or otherwise communicate with the patient about this, or any other procedure, but I’m over it now; I have my ‘routine’ with patients who’ve been ordered an abg. Many times the topic of an abg is avoided because, as is popularly assumed, ‘it hurts worse than any other blood draw’…but, in my experience as a healthcare worker and as a patient, this isn’t true.
An arterial blood gas is the analysis of the gases from the blood drawn from an artery, not a vein, which is the reason that every other blood draw for lab work cannot be used in place of the arterial blood. The ‘gas’ is both the oxygen and carbon dioxide that bound to the hemoglobin of the red blood cells and the ratio of both is indicative, mostly, of how well the lungs are working: transferring oxygen from the air within the lungs TO the blood and removing the carbon dioxide FROM the blood to be exhaled via the lungs. The arterial blood, usually drawn from the radial artery at the location of either wrist, is blood that has most recently left the lungs and is circulating throughout the body distributing that oxygen to all organs and tissues. Veinous blood is the blood returning from all organs and tissue back to the lungs to ‘drop off’ the carbon dioxide. The most accurate reflection of how well the lungs are working is the blood that has just left them; the arterial blood.
The radial artery is the most frequently used site to draw blood from for several reasons: it’s the most accessible (at most a watch may need to be removed), it’s more ‘superficial’ (that means smaller, shorter needle is used because the artery is so close to the surface of the tissue), it’s the largest of the two arteries in the lower arm (the other artery is the ulnar artery) and lastly, if for some reason the radial artery is damaged (it can occur but it’s rare), the ulnar artery will also supply the hand with arterial blood. Other arteries that can be used but are less frequently used are: the branchial artery located at the inside of the elbow and the femoral artery located in the groin.
In assessing where to draw arterial blood, we palpate (feel) for the pulse at the wrist. We try to avoid using the artery of the dominant hand but we use the artery of either arm with the best pulse and the logic of this choice is because we ‘feel’ for where the pulse is and aim the needle to this area. Unlike vein draws where a tourniquet is used to make the vein bulge and, therefore, visible, the artery is not blocked in this way because we need the blood flow from the lungs/heart to draw the sample.
A very common fear about an abg draw is pain. Personally, any needle entering any part of my body is painful; I’ve had a radial artery draw that I didn’t feel was as painful as the typical antecubital (inside of the elbow) veinous draw. One of my pet-peeves is when another staff member has ‘warned’ the patient that what I’m about to do “really hurts“. This is a disservice to the patient as every patient’s pain threshold is unique to them and every technician’s technique is different as is every situation that requires the lab work in the first place. Communication and creating as a relaxed environment as possible is what I’ve found works the best. Relaxed patient = relaxed muscles = easier needle insertion and draw. I’ve rarely missed and frequently have patients state either that wasn’t that bad or it didn’t hurt at all. It’s (almost) all in the set up and technique.
Besides pain threshold, the radial nerve runs adjacent to the radial artery. Sometimes, inadvertently, this nerve is struck and when that happens, much like breaking a tooth and exposing the nerve, it’s an obvious and undeniable pain causing tingling up the arm and sometimes causing the hand to contract. Quickly partially withdrawing and repositioning the needle allows the nerve to restabilize and the pain to dissipate, however, the abg process must continue and it’s best to not withdraw the needle completely.
Once the artery is punctured very little blood needs to be obtained for the analysis so most of the time, depending on variables such as heart rate, dehydration, patient cooperation, the actual time the needle is in the arm is short. Pressure is applied to an arterial puncture site longer than venous – especially if one is on a ‘blood thinner’ – because arteries have more smooth muscle and connective tissue, thus, blood is under more pressure within the artery and there is a more likelihood of a hematoma (blood clot under the skin) forming if insufficient pressure is applied post draw.
Lastly, the ‘numbers’ or results of an abg analysis is to determine if the oxygen and carbon dioxide is within normal limits and different institutions’ labs may vary slightly in what is within the normal window: carbon dioxide or pCO2 should be between 35-45 mmHg (millimeters of mercury) and oxygen or pO2 should be above 80mmHg but most definitely above 60mmHg depending on patient/circumstances/therapies. One caveat is that some people (usually smokers) may have results that deviate from ‘normal’ – they may have a high carbon dioxide level that is normal for them and this range is sometimes upwards of 60mmHg but is typically in the 50s. And always we strive for a normal pH 7.35-7.45mmHg. Many factors contribute to an abnormal pH including a high or low pCO2 and this is about as complicated in blood chemistry as I want to get here.
For reasons having to do with state licensing, in NH it is within the legal scope of practice for a respiratory therapist to draw an abg sample and because (we) have the most experience, we are usually the ‘go to’ person but that isn’t a hard and fast rule; doctors, nurse practitioners, physicians’ assistants and, I believe, paramedics who work in the hospital setting can as well. In an emergency, whomever these aforementioned staff has access, ability and confidence will draw for expediency.
I’ll leave you with one last thought: if it’s ever recommended that an abg be obtained, always ask the provider why – be informed. Lab technicians and respiratory therapists are usually paged and, without any other information besides patient name and room number, arrive to do what is asked/ordered. If a patient hasn’t been provided information about what to expect and why and is inquiring, I try to find the ordering provider before I perform the procedure but that can delay results and treatment. Remember, an ABG is an analysis of how well your lungs are transferring gases, that’s the only reason for this lab work. pH can be obtained from a veinous lab sample. Once you’ve agreed to have the lab draw done, just relax. The more relaxed you are, the more relaxed the drawer of your blood is, thus, the easier and less painful the whole procedure will be for both parties.
*Linkz to this site for “linky party”