Measured PaOQ more than 4 mm Hg from the calculated value Measured I ICO more than rnFql from the calculated value

APPLICATION OF BASIC PATHOPHYSIOLOGY

When given an ABG for interpretation, it is essential to use an approach that is focused yet comprehensive.8 An algorithm illustrating this concept is shown in Figure 28-1. Using this algorithm, Step 1 is to identify all abnormalities in the pH, PaC02, and/

or 1 3 and then decide which abnormal values are primary and which are compensatory. This is best done by initially looking at the pH. Whichever side of 7.40 the pH is on, the process that caused it to shift to that side is the primary abnormality. If the arterial pH is lower than 7.40 (acidemia), an elevated PaC02 (greater than 45 mm i irn~

Hg or 6 kPa, respiratory acidosis) or a lowered 1 (less than 22 mEq/L or mmol/L, metabolic acidosis) would be the primary abnormality. If the arterial pH is higher than 7.40 (alkalemia), a decreased PaCC>2 (less than 35 or 4.7 kPa, respiratory

IICO-

alkalosis) or an increased 1 3 (greater than 25 mEq/L or mmol/L, metabolic alkalosis) would be the primary abnormality. Once the primary disorder is established, Step 2 is to apply the formulas from Table 28—1 to assess whether compensation is appropriate and to look for concurrent processes.

Step 1: Establish the Primary Disorder pH less than 7.35 (acidemia)

PaCOi greater than 45 mm Hg

Respiratory acidosis

HCO^ less than 22 mmot/L

Metabolic acidosis

pH greater than 7.45 (alkalemia)

PaC02 less than 35 mm Hg

Respiratory alkalosis

HCO3 greater than 26 mmot/L

Metabolic alkalosis

Stop 2: Assess Adequacy of Compensation

Respiratory acidosis

Acue

Metabolic acidosis

Chronic

Expected AHC03 = (0.35 x APaC02)

Respiratory alkalosis

Acute

Expected AHCO5 = (0,2 x APaC02)

Metabolic alkalosis

Chronic

Stop 3: Calculato the Anion Gap

1

Anion gap = Na* - (CI" 4 HCO^)

Anion gap less than 12: No anion gap metabolic acidosis

Anion gap 12 or more: Anion gap metabolic acidosis

Step 4: Calculate the Excess Gap

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