Cheater's shortcuts to Blood Gas Interpretation

  1. Blood Gas interpretation is and remains one of the major stumbling blocks for a lot of nurses. The shortcuts I am about to outline will not tell you EVERYTHING you need to know off of a blood gas report but they will tell you the essentials. I am deliberately going to write these as simply as possible so anyone (hopefully) can understand them.

    The first method is simply to us a "Nomogram" there are several out there from the original by Sigaard-Anderson to variations on the theme. Some are easier to use than others. They are ideal if you cannot be bothered or simply do not need to use the information often enough to remember the steps needed to analyse gases.

    Here is an example of a nomogram


    To use it - simplest way - take a square piece of paper and place the left hand edge against the patient's PCO2 on the bottom axis and align the top edge of your paper with the Ph on the right hand axis - where the corner of your square falls should be where your patien't balance is. i.e. if the corner of the paper is in the shaded area for acute respiratory acidosis then that is the analysis. Sometimes the corner falls outside the shaded areas and that is often because the patient has a "mixed" result i.e. the patient has both metabolic and respiratory acidosis (and in that case a big dose of big trouble)

    The second method I use is a "rule of thumb". The rule of thumb Acute disorders = for every 1 mmhg rise or fall in the PCO2 the PH should change by .01 in the opposite direction.

    The range for normal PH is 7.35 - 7.45

    The range for normal PCO2 is 35 - 45

    If we take 7.40 as the ideal PH and 40 as the ideal PCO2 then our rule of thumb works this way.

    I look at my PH and it is 7.2 - so .2 less than the ideal - by my rule of thumb I should expect my PCO2 to be 60 (approx - remember this is a rule of thumb) if it is then all the PH shift can be accouted for as respiratory acidosis therefor the patient has a primary uncompensated respiratory acidosis.
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    About gwenith

    Joined: Jul '02; Posts: 10,261; Likes: 233
    icu nurse


  3. by   versatile_kat
    Thanks a lot for the nomogram gwenith. I've printed it off and will bring it to work for the nurses to make copies of. You're absolutely right, ABG's tend to be one of the harder concepts for nurses to grasp (new grad and experienced alike). Thanks for the info!