Why Do Doctors Order Blood Gases?

Why do doctors order blood gases (ABG)?  Gases can be a doctors best friend.  They can help you define mixed acid base disorders that may not be intuitively obvious.   If you want to be a great internist, you have to be great at blood gas interpretation and you have to be able to do it quickly and efficiently. You have to understand what all the numbers mean and you have to get a good clinical sense of how to interpret them and how to change management based on their result.  You also have to be able to do it without pulling out your formula books. In six years as a hospitalist. I have never calculated what the compensatory responses should be. I just know. 

Sometimes blood gases change your management or your medical opinion on what's happening. Take for example my patient with advanced multiple sclerosis. She presented through the emergency department with "oropharyngeal bleeding of unclear etiology". Here was her original basic metabolic profile?
Na 137 K 4.0 CL 99 HCO3 36 BUN 35 CR 1.0
Her medications included Baclofen, Lasix and and aspirin and that's about it. When I first saw the data, my first inclination was to attribute the elevated bicarb to chronic hypercapnic respiratory failure with a compensatory metabolic alkalosis. It would make sense. Multiple sclerosis can imply a failing respiratory drive.  But I had the respiratory folks draw a blood gas anyway. To confirm my suspicions and to clarify the degree of hypercapnea.

Here is what I got
pH 7.47 pCO2 46 pO2 58
And this is why obtaining a blood gas can be important in deciding exactly what is going on. In this case, I was wrong. The patient had hypercapnea but it wasn't the primary process. As you can see, having an alkalotic pH by definition excludes a primary respiratory acidosis. What we have here is a primary metabolic alkalosis with incomplete compensatory respiratory acidosis.

A close look at the labs show a significant BUN/Cr ratio. While I originally thought this to be secondary to blood passing through the GI track, in retrospect, it is likely an indication of a significant state of volume depletion secondary to chronic Lasix therapy.  I originally planned on providing no saline support. But my mind changed when I saw the alkalotic nature of the blood gas. I initiated IV fluids and discontinued the diuretic therapy and gave it some time.

With the diagnosis made, repeating the gase would be an exercise in academia, something us community guys generally avoid.  The first two things I do when I see a patient is
  1. Review the vital signs
  2. Calculate the anion gap and review the bicarb status to determine if  blood gases are warranted.
I have saved many a lives and had significant changes in management due to surprise findings ABGs. You can't really know what's going on unless you have confidence in your patient's acid-base status.  That requires internists to  become highly competent in the evaluation of blood gas physiology across any number of clinical situations, some of which can be acute and life threatening in nature. If you're going to save lives, you  must have a strong grasp of the fundamentals. You must see thousands of them to make it automatic.

The evaluation of acid base situations is a core learning principle for all medical students and something that internists must have a strong foundation in understanding. This process is automatic for me. Some clinical scenarios are more complicated than others, but knowing your way through acid base will save lives your entire career.  That is a great feeling to have.  If you aren't highly competent in acid-base, you are not capable of practicing a full scope of primary care medicine.

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