Acid-Base Disorders

A Simple Approach

BP Kavanagh, HSC

1st Step

Check the ABG result or validity:

[H] nM = [PaCO2] 24/[HCO3-]

In range : 7.25 - 7.55,

pH = 7.X

[80-X] = approx. [H] nM

Normal Values• pH 7.35 - 7.45

• PaCO2 35 - 45 mmHg

• [HCO3-] 22 - 26 mmol.L-1

• Anion Gap 10-14 mmol.L -1 [Assumes Protein 40 g/l - should reduce AG by 3, for every

10 g/l decrease in plasma protein]

pH = 7.4

There is no acid-base disorder

OR

There are more than one, and they’re perfectly balanced, not compensated

[Q: How many types can co-exist?]

HypoventilationBrain InjuryCNS Depressants

MyoneuralChest WallLung ParenchymaAirwaysMechanical Ventilation

Brain

Lung

HyperventilationAnxietyDrugsEncephalopathyPregnancy

Mechanical VentilationPulmonary FibrosisPulmonary Edema

Brain

Lung

Ventilation

Alv. vent. = min. vent. - dead space vent

= [f.VT] - [f.VD]

= f.[VT - VD]

= [f / VT].[1 - VD / VT]

• Frequency• Tidal Volume• Physiologic Deadspace

Anion Gap

No Osmole GapKetonesLactateUremiaASA/ParaldehydeRhabdomyolysis

Osmole GapMethanolEthylene GlycolEthanol

Non-Anion Gap

Dilution of HCO3-

Normal SalineTPN

Loss of HCO3-

GI LossRenal Loss

Diarrhea

Ureteric Diversion[Cl- exchanged for HCO3

-]

GI Loss of HCO3-

Renal Loss of HCO3-

1. RTA

2. Acetazolamide

3. Steroid Deficiency

ECF Expansion

HCO3-

Cl- NS

TPN

HCO3-

Cl-

ECF Vol.

Cl- mmol.

HCO3- mmol.

Cl- conc.

HCO3- conc.

Metabolic Alkalosis - CausesECF Contracted [Ur Cl- < 10] Remote Diuretics [ECF low, but no current Cl- spill]

Gastric Losses [Aspiration or Vomiting]

ECF Normal or Expanded [Ur Cl- > 20]

Current Diuretics

Excess Steroid Effect

Excess Renin Effect

Bartters Syndrome

Administration of HCO3-

Post Hypercapnia [Ur Cl- < 10]

ECF Contraction

HCO3-

Cl- Diuresis

HCO3-

Cl-

ECF Vol.

Cl- mmol.

HCO3- mmol.

Cl- conc.

HCO3- conc.

[Secondary Hyperaldo.]

Cl-

2nd Step

• What's the pH, PaCO2, & the HCO3- ?

• If the pH is > 7.45,

there's a primary alkalosis

• If the pH is < 7.35,

there's a primary acidosis

3rd Step

• Calculate the Anion Gap

• If > 20, Calculate Anion Gap ‘Excess’

• Add the ‘Excess’ to the Bicarbonate

The Anion GapIf assess a patient for all known causes of

an anion gap:

AG [mmol/l] % Confirmed

15 30

20 75

25 95

Anion Gap Rules1 If > 20, there’s likely a cause to find

2 AG does not rise to compensate

3 Changes in AG should be matched with changes in HCO3

- [titrated, mole for mole]

If AG is increased > 201. Assume it began normal [12 mmol/l]

2. Therefore an excess has developed

3. Assume the HCO3- began normal

4. If [HCO3- + xs] < normal HCO3

-

additional primary loss of HCO3-

5. If [HCO3- + AGxs] > normal HCO3

-

additional primary source of HCO3

-

Primary AG Metabolic Acidosis Only

Na+

Cl-

AG

HCO3-

Cl-

AG

HCO3-

AG xs

Before After

Primary AG Metabolic Acidosis, WithPrimary Non-Gap Metabolic Acidosis

Na+

Cl-

AG

HCO3-

Cl-

AG

HCO3-

AG xs

Before After

Primary AG Metabolic Acidosis, WithPrimary Metabolic Alkalosis

Na+

Cl-

AG

HCO3-

Cl-

AG

HCO3-

AG xs

Before After

Summary Steps1. Clinical Status

2. Verify Results

3. Determine the Primary Problem[pH, PaCO2, HCO3

-]

4. Calculate AG

5. If AG > 20, calculate AG excess

6. Add to HCO3- [compare to normal range]

Case #1• pH 7.5• PaCO2 29

• HCO3- 24

Case #2• pH 7.2• PaCO2 70

• HCO3- 25

Case #3• pH 7.55• PaCO2 40

• HCO3- 38

Case #4• pH 7.34• PaCO2 60

• HCO3- 31

Case #6

• pH 7.5• PaCO2 20

• HCO3- 15

• Na+ 140• Cl- 103

Case #7

• pH 7.4• PaCO2 40

• HCO3- 24

• Na+ 145• Cl- 100

Case #8

• pH 7.5• PaCO2 20

• HCO3- 15

• Na+ 145• Cl- 100

Case #9

• pH 7.1• PaCO2 50

• HCO3- 15

• Na+ 145• Cl- 100

Case #10

• pH 7.15• PaCO2 15

• HCO3- 5

• Na+ 140• Cl- 110