

ABG InterpretationArterial Blood Gas Extracellular fluid pH is determined by the concentration of hydrogen ions This relationship is expressed as: Normal Values Normal RegulationNormally there is very tight control and a change in either PCO2 or HCO3 due to a pathological process results in a change in the other variable as a compensatory process. Estimating Hydrogen Ion Concentration from pHThe change in hydrogen ion concentration with pH is curvilinear and therefore to mentally estimate accurately. Roughly you can get away with assuming a linear relationship. Start with the fact that at pH 7.4 the proton concentration is 40. Then for each change in pH by 0.01 the hydrogen ion concentration will change by 1 nEq/L. Examples: PH 7.32 = 40 (base value for 7.4) + 8 (1 neQ/L for each 0.01 change between 7.4 – 7.32) = 48 PH 7.52 = 40  12 = 28 nEq/L Intuitively, you add to the hydrogen ion concentration as the pH drops. A second way that is slightly more complex is also slightly more accurate because it more closely follows the true curvilinear relationship. When the blood is acidotic each 0.01 change in pH from 7.4 results in an increase in proton concentration by 1.25. When the blood is alkalotic, each 0.01 change in pH from 7.4 results in a decrease in proton concentration by 0.8. Examples PH 7.32 = 40 (base value for 7.4) + (8 x 1.25) (1.25 neQ/L for each 0.01 change between 7.4 – 7.32) = 50 PH 7.52 = 40  12 = 30.4 nEq/L As you can see there is little difference in the values obtained by the two equations until the extremes of pH are reached. Use the first method for pH values that are relatively close to 7.4 and use the second equation when the pH value is farther away. 7 Questions for ABG Interpretation
Step 1: Is the ABG internally consistent?Checking internal consistency is a fancy way to say, “do these numbers make sense?” Use the equations above to convert pH to proton concentration. Then plug the numbers into the equation: [H+] (nEq/L) = 24 x (PCO2/HCO3) If both sides are equal the ABG is internally consistent. I’m not sure how much variation is expected and acceptable, but question an ABG that doesn’t match up. Step 2: Is there acidemia or alkalemia?
If the pH value is less than 7.36 there is acidemia (acidic serum). If the pH is greater than 7.44 there is alkalemia (the serum is alkalotic). This helps defines the basic problem without delving into the cause. If the pH is normal there may still be a mixed acidbase disorder. Step 3: What is the Primary Base Disorder
The HCO3 and CO2 should move in the same direction for a simple acidbase disorder. One is due to the primary pathophysiological problem and the other is due to compensation. Compensation will not entirely correct a disorder. If they are moving in opposite directions there are two acidbase disorders present.
Step 4: What is the anion gap?The anion gap is calculated only when there is a metabolic acidosis. The anion gap calculates unmeasured anions. If there is a large amount of unmeasured anions then this is the cause of the metabolic acidosis. If the number of unmeasured anions is within normal limits then we can assume that the metabolic acidosis is due to a loss of bicarb. Equations below are taken from Marino’s “The ICU Book”: Positive ions = Negative ions Rearrange to get: The normal range for the AG is 3 to 11 mEq/L. Albumin usually makes up a large amount of the unmeasured anions. Therefore, in patients who are hypoalbuminemic the anion gap may be falsely normal. The AG should be adjusted for the albumin level. True AG = Calculated AG + 2.5(4.5 – measured albumin (g/dL)) When all is said and done, if a patient has a metabolic acidosis with an elevated anion gap (> 11 mEq/L) then the patient is said to have an Anion Gap Metabolic Acidosis. Step 5: What is the delta gap?The delta gap is only calculated when there is an anion gap metabolic acidosis. The goal of the delta gap is to detect other concomitant metabolic disturbances. When there is a solitary anion gap metabolic acidosis there is a 1 mmol/L fall in bicarbonate for every 1 mmol/L rise in the anion gap. Δ Anion gap = Δ [Bicarbonate] If the change in bicarbonate (right side of the equation) is higher than expected the patient may simultaneously have a normal anion gap metabolic acidosis. If the change in the anion gap (left side of the equation) is higher than expected the patient may simultaneously have a metabolic alkalosis. Step 6: Is there Compensation?Changes in respiratory drive can occur very rapidly to compensate for metabolic disorders. Changes in HCO3 take place in the kidneys to compensate for respiratory disorders and take longer to establish. HCO3 is reabsorbed in the proximal tubules and this can be adjusted to increase or decrease the total amount of bicarbonate in the body. For the first 6 hours of a respiratory acidbase disorder there will be no compensation. At 612 hours acute compensation will develop. Chronic compensation develops over days. Compensation will never completely correct the primary acidbase disorder. Metabolic acidbase disorders
Respiratory acidbase disorders
What does an “uncompensated” acidbase disorder imply? Mixed… Step 7: What is the cause?Interpreting a VBG


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