Acid-Base Disorders 1 (Quick Guide)

Question 1

Discuss Winter’s formula

Answer 1

Used to predict appropriate respiratory compensation in metabolic acidosis

PCO2 = 1.5 x HCO3 + 8 +/- 2

Measured = Predicted –> pure met acid
Measured > Predicted –> met acid + respi acid
implies inadequate ventilation, don’t give sodium bicarbonate
Measured < Predicted –> met acid respi alk

Question 2

Discuss delta ratio

Answer 2

∆AG / ∆HCO3
<0.4 –> Pure NAGMA
0.4 -0.8 –> NAGMA + HAGMA
0.8 - 2.0 –> Pure HAGMA
>2.0 –> HAGMA + met alk (or preexisting compensated resp acid)

normal AG is 12. normal HCO3 is 24

Question 3

another take on expected compensation in metabolic acidosis

Answer 3

for every dec in HCO3 by 1 mEq/L, there is dec in PCO2 by 1 mm Hg

In a maximally-compensated metabolic acidosis (which takes about 12-24 hours), Winter’s formula applies:
Expected PaCO2 = [(1.5 x HCO3) +8] +/- 2

Question 4

Potential indications for bicarbonate therapy in metabolic acidosis

Answer 4

1. Severe hypobicarbonatemia (<4 mEq/L)
   insufficient buffer concentrations may lead to extreme increases in acidemia with small increaeses in acidosis
2. Severe acidemia (pH <7.00 to 7.15) in cases of HAGMA, with signs of shock or myocardial iritability that has not responded to supportive measures including
   - adequate ventilation
   - fluid resuscitation
3. Severe hyperchloremic acidemia
   Lost bicarbonate must be regenerated by kidneys and liver, which may require days

Question 5

Compensation in respiratory acidosis

Answer 5

acute:
inc in HCO3 by 1 mEq/L for every inc in PCO2 by 10 mmHg

chronic:
inc in HCO3 by 3.5 mEq/L for every inc in PCO2 by 10 mmHg
with near-normal pH

Question 6

How to determine if respiratory acidosis is acute or chronic?

Answer 6

Ratio of change in pH and change in PCO2
(delta pH / Δ PCO2)
>0.8: concomitant metabolic acidosis
=0.8:acute
0.33-0.8: acute on chronic
=0.33: chronic
<0.33: concomitant metabolic alkalosis

Question 7

Discuss respiratory alkalosis

Answer 7

predicted relationship:
dec in [H+] by 1-mmol results from dec in PCO2 by 1 mmHg

“Chornic respiratory alkalosis is unique among the acid base disorders in that its compensation may be complete*
- bicarbonaturia and dec in acid excretion requires 6 to 72 hours to develop fully
- at at least 1 week to normalize pH

Question 8

Albumin and metabolic acidosis

Answer 8

Hypoalbuminemia may mask the presence of a high AG metabolic acidosis (HAGMA).
For every drop in the albumin level by 1 g/dL, the normal AG range should be lowered by approximately 2.5

Question 9

Compensation in metabolic alkalosis

Answer 9

As a guideline, PCO2 in patients with significant metabolic alkalosis should rise by 0.7 mm Hg for each 1 mEq increase in HCO3

The PCO2 also rarely rises above 55 mm Hg in compensation for metabolic alkalosis