ABG Jujitsu

Physiology

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:

  • pH <7.35 - Acid

  • pH >7.45 - 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

 

Attributions

coming soon

ARDS/Hypoxemia, Critical Care, MedEdJohn HeislerComment