The body needs to keep blood pH in a very tight range, 7.35 - 7.45.
Outside of this range you have abnormalities in electrolyte concentrations (esp K and Ca), membrane potentials, protein folding and cellular function
May seem narrow - its because small changes in pH mean larger changes in H+ concentration (logarithmic scale)
We are constantly exposed to substances/metabolic changes that would result in large changes in pH. Our body works against this by:
Bicarbonate buffer equation: LeChatlier’s principle
The lungs: hypo or hyperventilation
The kidneys: secrete or retain acid, secrete or retain bicarb, produce de novo bicarb
Honorable mention: Hgb acts as potent buffer by binding CO2 to form carbaminohemoglobin
Hx of ABG Interpretation
Donald Van Slyke was a dutch American chemist who popularized the Bronsted Lowry definition of acids/bases and the Hendersen Hassalbach equation back in the 1950s
Acids donate protons to solution, bases accepts protons in solution
Henderson Hassalbach - related pH, pcO2, bicarb
We still use this definition today. There is a different, more quantitative approach to acid base that takes a lot more into account - the effects of chloride, lactate and the strong ion difference.
This is called the Stewart approach and was popularized by Peter Stewart back in the 1980s; its complicated, and a useful clinical skill, but beyond scope of this episode. Read more:
5 Steps To ABG Interpretation
-emia: in the blood vs -osis: the condition of having
Can use rule of 4’s to remember normals: 7.40 (pH)/40 (pCO2)/ 24(bicarb)
What makes sense?
Respiratory -> pCO2
Replace “pCO2” with the word “acid”
If your pCO2 (acid) is ↑, what will happen to your pH?
↓ More acidotic - respiratory acidosis
If your pCO2 (acid) is ↓, what will happen to your pH?
↑ More alkalotic - respiratory alkalosis
Metabolic -> bicarbonate
Replace “bicarbonate” with the word “base”
If your bicarb (base) is ↑, what will happen to your pH?
↑ More alkalotic - metabolic alkalosis
If your bicarb (base) is ↓, what will happen to your pH?
↓ More acidotic - metabolic acidosis
A) Respiratory compensation for metabolic acidosis
1.5 x bicarbonate + 8 + 2 = New normal pCO2
If the pCO2 on the gas is equal to the new normal pCO2 per winter’s formula, you are COMPENSATED (not necessarily if pH is normal)
B) Chronic respiratory diseases (i.e. COPD) will have higher than normal bicarbonate levels and will much better tolerate high pCO2 levels (little change in pH with pCO2 changes)
10 mmHg change in pCO2 ->
pH change by 0.08 for acute process
pH change by 0.03 or less for chronic process
[Na+] - ([Cl−] + [HCO3])
All your plus’ should equal all of your minuses. If it doesn’t (there’s extra minuses in there), you have an anion gap
Causes of HAGMA: Dissociates into acid into the blood (acid producing process)
Causes of NAGMA: bicarb loss
Bicarb in NAGMA? Yes!
Bicarb loss —> replacing bicarb makes sense
Bicarb in HAGMA? Quite a debate
Bicarb can ruin intracellular pH, cause more dissociation of whatever is elevating your anion gap (bicarb pushes are hypertonic - incr plasma tonicity)
Will temp fix pH but pH can worsen very quickly later
Whether or not you push bicarb, will you have an affect on mortality? Probably not
Good to push bicarb if: pH severely low OR you have an out (i.e. you are getting ready to start CRRT, go to OR for source control)
Answers the question: does patient have NAGMA on top of HAGMA, should I give a little bicarb?
Delta gap is often used interchangeably with delta ratio and corrected bicarbonate
You only calculate this if anion gap is high
Corrected bicarb (surrogate for delta gap) -
Anion gap - 12 + bicarbonate
Corrected Bicarb <22 - you also have a NAGMA
Corrected Bicarb 22-26: Just HAGMA
Correct Bicarb >26 - you also have metabolic alkalosis