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Crit Care Nurse 2002 Oct; 22(5): 16-18
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Letters to the Editor

Additional step in ABG analysis

The article "Assessing Tissue Oxygenation" (June 2002:22–40) contains a comprehensive overview of arterial blood gas analysis, which will prove to be a valuable resource for nurses and other healthcare professionals in the intensive care environment. The steps outlined are useful in determining an acid-base imbalance involving either the metabolic or respiratory systems and the effectiveness of attempted compensation. However, severely ill patients who develop multiple organ failure frequently present with several acid-base abnormalities occurring simultaneously. Therefore, I routinely add an additional step in the analysis of arterial blood gases to determine if another primary acid-base process is present.

The purpose of the additional step is to determine the expected compensation for the primary disorder. If the actual compensation falls within the calculated range, then a second disorder does not coexist. If the calculated value does not match the measured value, then a mixed disorder is present or compensation has not had time to occur. The expected compensation is calculated by using one of 4 formulas based on the primary process: metabolic acidosis, metabolic alkalosis, respiratory acidosis, or respiratory alkalosis. Metabolic conditions are generally compensated fairly quickly by the respiratory system by eliciting an alteration in the PCO2 level. The Winter’s formula predicts the expected degrees of compensation in a stable, steady-state metabolic disorder:



If the actual PCO2 is higher than calculated with Winter’s formula, then a respiratory acidosis is mostly likely present in addition to the metabolic acid-base disorder. If the PCO2 is greater than 50 to 55 mm Hg, then respiratory acidosis is almost certainly present. On the other hand, if the actual PCO2 is lower than that calculated with Winter’s formula, especially if PCO2 is less than 40 mm Hg, then a respiratory alkalosis is most likely present in addition to primary metabolic condition. The larger the difference between the calculated and actual values, the more likely that the deviation is clinically significant.

In a respiratory condition, the nurse must first determine if the process is acute or chronic. It is extremely important to know the clinical history to properly apply the formulas. An acute state is defined as less than 8 hours, and it takes 24 hours for the kidneys to achieve maximal compensation, which produces a chronic state. For every decrease of 10 mm Hg in PCO2 from normal (40 mm Hg) in respiratory alkalosis, the bicarbonate level should decrease from normal (24 mmol/L) by 2 mmol/L in an acute condition and 5 mmol/L in a chronic condition. Conversely, every increase of 10 mm Hg in PCO2 seen in respiratory acidosis should cause an increase of 1 mmol/L in bicarbonate for acute conditions and an increase of 4 mmol/L in chronic conditions.

Many of the medical calculators such as Med-math and Medcalc have Winter’s formula as a standard calculation. I have found that the more you use the formulas, the easier they are to recall and commit to memory. I believe that this final step will prove to be a useful tool in the diagnosis of complex overlapping conditions that nurses face daily in their critically ill patients.

Selected References

    Longenecker JC. High-Yield: Acid-Base. Philadelphia, Pa: Lippincott-Williams & Wilkins; 1998. Marino PL. The ICU Book. 2nd ed. Philadelphia, Pa: Williams & Wilkins; 1998.
Michelle Kirschner, RN, MSN, APRN, CNP, CCRN
 Cincinnati, Ohio

 

The authors respond

We appreciate the letter written in response to our article, and agree with the use of Winter’s formula as a handy bedside tool in the diagnosis of acid-base disorders. We would also like to emphasize that in the critically ill patient, mixed acid-base disorders can coexist.

Barbara Berry, RN, PhD
 Agnes Eugine Pinard, RN, BSN, MS/HSA
 Miami, Fla





This Article
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Right arrow Articles by Kirschner, M.
Right arrow Articles by Pinard, A. E.

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