Breathin Basics—ABG’s

“ABG’s” is another one of those medical acronyms that you hear all the time when you have asthma or lung disease. And if you’ve ever been unfortunate enough to be hospitalized for your asthma, no doubt you’ve had one of these tests done on you.

ABG stands for “Arterial blood gases”. An ABG is a blood test that measures the PO2 (oxygen) and PCO2 (carbon dioxide) in arterial blood ( blood that comes from an artery vs a vein). The Ph (the acidity/alkalinity of the blood ), HCO3 (Bicarbonate buffers) and SAO2 (O2 saturation) are also determined.

The test is performed by collecting a small sample of blood from a peripheral artery (not a vein) usually in your wrist where you feel your pulse. Because you can’t see arteries, the person drawing the sample has to go entirely by feel, which means it can often take more than one poke to hit the blood vessel ( and yes…it can hurt like crazy too).

ABG’s are usually only obtained on asthmatics who are experiencing severe respiratory distress and are not responding well to treatment. Because ABGs can only indicate a persons breathing status at that particular moment, they are often repeated several times during the course of a hospitalization. Patients who are in critical condition and who require frequent ABGs will usually have a special catheter inserted into the artery (called an Arterial Line). Whenever an arterial blood sample is needed, it can be drawn directly from a special port on the catheter instead of having to poke the patient with a needle every time.

Normal ABG values would look something like this:

Ph 7.35-7.40 PCO2 35-40 PO2 80-100 HCO3 24 O2 SAT 97-100%

All 5 of these parameters are used in evaluating the respiratory status of a patient, but for the sake of this discussion the value we’re most interested in is the PCO2 (carbon dioxide). CO2 is a waste product of cellular metabolism and because we get rid of it by exhaling it out of our lungs, measuring how much CO2 is in our blood gives us a good indication of how well our lungs are doing their job. The faster and deeper we breath, the more CO2 we expel (we call this “Hyperventilation”). The slower we breath, the more CO2 we retain (we call this “Hypoventilation”). During normal breathing, the body maintains just the right level of CO2 (35-40)

Here’s a scenario of what can happen to your CO2 levels during a severe flare;
During a severe asthma attack it becomes very difficult to breath. As a result, you’re forced to use more breathing muscles than you normally would (what we call accessory muscle use) in order to get the air in and out of your lungs. This extra muscle use causes more CO2 (waste) to be produced. Ever notice that you breath faster during an asthma attack? It’s not only from air hunger. The body’s first line of defense against rising CO2, is to breath faster in order to blow it off and keep the levels within a safe range. However, if the work of breathing gets too severe, the lungs are unable to expel the CO2 fast enough and blood levels continue to rise …we call this “Respiratory failure”. The work of breathing can become so overwhelming, that the person begins to tire out and could eventually stop breathing all together… we call this “Respiratory Arrest”. Hopefully this will never happen to you.

Too much CO2 can make the blood very acidic (decreased Ph). If the CO2 blood levels get too high or the Ph too low, it can cause damage to the vital organs such as the brain and heart. As a second line of defense the kidneys will hold on to more sodium bicarbonate which helps buffer the extra acidity.Bladder activity is also increased to help get rid of the acidic compounds. The problem with this 2nd line of defense, is that it takes much longer to kick in.

One way we can quickly the lower CO2 in someone who’s in respiratory failure, is to blow the CO2 off by mechanical means…either with BIPAP or a ventilator. By placing someone on a ventilator, we can control how much air moves in and out of a persons lungs thereby regulating how much CO2 moves out.

The example above, is of course an over simplification of what can occur during a severe attack. There are many other factors involved, but the basic goal in treating a critically ill asthmatic is to open up their airways and normalize their blood gas values.

Here you can see an arterial line in my left wrist and some bruising from previous arterial line attempts in the right arm.

As a Respiratory Therapist Ive done thousands of ABG’s on patients, and as a severe asthmatic myself, Ive had hundreds of them done on me. Here are some of my actually blood gas results between 2005 and 2008 while a patient in the UCSF hospital intensive care unit.

**Following a more recent hospitalization(4/2013) I wrote a post regarding Bipap and how it effects ABGs**.