Surgical Critical Care

Arterial Blood Gas Analysis allows you to assess oxygen and carbon dioxide levels in the blood, as well as the patient's plasma acid / base status. Things you want to ask yourself when examining an ABG:

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• Is the patient hypoxemic?

  • If PaO2 is < 80 mmHg, a patient is hypoxemic​

  • If it is between 60 - 79 mmHg, they are considered mildly hypoxemic. 

  • If the measurement is between 40 and 59 mmHg, they are considered moderately hypoxemic

  • If it is less than 40 mmHg, your patient is severely hypoxemic

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• Is the patient hypercarbic and if so, does it matter?

  • PaCO2 > 45 = hypercarbia​

  • When evaluating a person's CO2, you must also look at the patient's pH! An elevated PaCO2 in the setting of an normal pH suggests a compensatory process to maintain a pH within normal range

  • If the pH reflects acidosis... then the hypercarbia is clinically significant. It's causes need to be diagnosed and treated.

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• Does the patient have acidemia, and if so, what kind? What are the clinical implications? (i.e. Does it need to be fixed?)

  • When the pH drops below 7.35 it is a signal that the body is not maintaining homeostasis as it normally does and there is likely a pathologic cause that needs support / correction. ​

  • While lower pH (more acidic plasma) causes more O2 to be released at the tissue level, it also globally vasodilates the peripheral vasculature diverting perfusion away from vital organs, (i.e. overriding the patient's natural ability to divert blood to where it is most needed...). So the 'win' of better O2 delivery becomes severly diminished by the 'loss' of directed blood flow. 

  • Acidemia will be from one of three general categories:

    • Purely Respiratory​

    • Purely Metabolic

    • Mixed / Compensatory Respiratory + Metabolic

  • Since CO2 levels directly affect cerebral vascular dilation AND can affect Cerebral perfusion pressures in patients with severe traumatic brain injuries, management of tight CO2 ranges are sometimes mandated despite pH measurements. 

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• Does the patient have alkalemia and if so, what kind? What are the clinical implications? (i.e. Does it need to be fixed?)

  • Alkalemia affects respiratory drive. The brain's response to a high pH is to slow breathing so the patient will retain CO2 (i.e. lower the pH to a more normal range). The amount of pH rise to create clinically significant respiratory suppression is much higher than the amounts typically seen in the ICU setting. ​

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One fairly, easy to understand reference on ABG analysis exits on the "Life in The Fast Lane" website. This site is from an EM / ICU perspective and covers many great critical care topics. This  Acid / Base Chart by Jeremy Rogers, shows how to think through evaluating causes of pH aberrancies in ABGs. 

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