ABG Jujitsu: Mastering Arterial Blood Gas Interpretation

Pulmcast's ABG Interpretation Guide breaks down arterial blood gas analysis into a clear framework: assess pH to determine acidosis or alkalosis, compare PaCO₂ and HCO₃⁻ to identify respiratory vs metabolic causes, evaluate compensation, and check oxygenation. This step-wise approach enhances diagnostic accuracy in acute care.

ABG Interpretation Calculator

pH: pCO₂ (mmHg): HCO₃⁻ (mEq/L):

Na⁺ (mEq/L): Cl⁻ (mEq/L):

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WHY ABG Interpretation?

The body needs to keep blood pH in a very tight range; typically between 7.35 and 7.45. Outside of this range, bad things can happen; Abnormalities in:

• Electrolyte concentrations - esp calcium and potassium

• Protein folding

• Membrane potentials

• Cellular function

May seem like a narrow range - remember that pH is a logarithmic scale. Small changes in pH mean much larger changes in serum proton (H+) concentration

• We are constantly exposed to substances or changes in metabolism that, if left uncompensated, would result in large changes in pH. Our body is able to keep pH in such a narrow range through 3 main mechanisms:

  • Bicarbonate buffer equation - Resist changes in pH through LeChatlier’s principle

  • The lungs - Hyper or hypo-ventilate

  • The kidneys

    • Secrete or retain acid

    • Secrete or retain bicarbonate

    • Produce de novo bicarbonate

  • One honorable mention - hemoglobin, acts as a potent buffer by binding carbon dioxide to form carbaminohemoglobin

History of ABG interpretation

• Van Slyke - popularized a method using Bronsted-Lowry definition of Acids/bases and the Henderson-Hassalbach equation

  • Bronsted-Lowry define acids as something that donates proton in solution and bases as something that accepts proton in solution

  • Henderson hassalbach equation relates pH, pCO2, Bicarbonate.

• Donald Van Slyke was a Dutch American Biochemist who popularized the method of acid-base interpretation that most of us use to this day all the way back in the 1950s

  • His methods at the time were only able to measure blood pCO2 and blood pH. Serum bicarbonate was calculated from this values

  • Most of the van slyke method involved interpreting patterns that arose from abnormalities in pH, bicarbonate, and pCO2 alone

There is a different, more quantitative approach to acid-base that takes a lot more things into consideration, like the effects of chloride, lactate, and the strong-ion difference on acid-base status

• This called the Stewart approach, popularized by Peter Stewart back in the 1980s. Stewart used about 6 equations inspired by many different fields of analytical chemistry to interpret physiologic acid-base status.

• More resources:

  • Emcrit Acid-Base series

  • Explore more episodes on critical care topics

• pH <7.35 - it’s an Acid

• pH >7.45 - it’s a Base

• What makes sense?

• Carbon dioxide = Acid

  • High acid (CO2) should give you respiratory acidosis, low acid should give you respiratory alkalosis

• Bicarb = base

  • High base (bicarb) should give you metabolic alkalosis, low base should give you metabolic acidosis

Further investigation -

• If metabolic disorder present, is the respiratory compensation adequate

  • Metabolic Acidosis: Winters formula - 1.5 x [HCO3-] + 8 (+/-2)

  • Metabolic alkalosis: 0.7 x [HCO3-] + 20 (+/- 5)

If respiratory disorder is present, is it acute or chronic? Is there metabolic compensation?

• For every 10 mmHg change up or down from “40”, pH will change:

  • Resp acidosis: decr by 0.08

  • Resp alkalosis: incr by 0.08

  • Resp acidosis: decr by 0.03

  • Resp alkalosis: incr by 0.03

Equation: [Na+] - ( [Cl-] + [HCO3-] )

May need to correct for hypoalbuminemia, hyperglycemia

Only for elevated AG. Equation: (AG - 12) + [HCO3-]

If >26, concomitant metabolic alkalosis

If >22, concomitant non-AG metbaolic acidosis