Acid-Base Disorders A Simple Approach BP Kavanagh, HSC

1st Step Check the ABG result or validity: [H] nM = [PaCO 2 ]  24/[HCO 3 - ] In range : , pH = 7.X [80-X] = approx. [H] nM

Normal Values pH PaCO mmHg [HCO3 - ] mmol.L -1 Anion Gap10-14 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? ]

Hypoventilation Brain Injury CNS Depressants Myoneural Chest Wall Lung Parenchyma Airways Mechanical Ventilation Brain Lung

Hyperventilation Anxiety Drugs Encephalopathy Pregnancy Mechanical Ventilation Pulmonary Fibrosis Pulmonary Edema Brain Lung

Ventilation Alv. vent. = min. vent. - dead space vent = [f.V T ] - [f.V D ] = f.[V T - V D ] = [f / V T ].[1 - V D / V T ] Frequency Tidal Volume Physiologic Deadspace

Anion Gap No Osmole Gap Ketones Lactate Uremia ASA/Paraldehyde Rhabdomyolysis Osmole Gap Methanol Ethylene Glycol Ethanol

Non-Anion Gap Dilution of HCO 3 - Normal Saline TPN Loss of HCO 3 - GI Loss Renal Loss

Diarrhea Ureteric Diversion [ Cl - exchanged for HCO 3 - ] GI Loss of HCO 3 -

Renal Loss of HCO RTA 2. Acetazolamide 3. Steroid Deficiency

ECF Expansion HCO 3 - Cl - NS TPN HCO 3 - Cl - ECF Vol. Cl - mmol. HCO 3 - mmol. Cl - conc. HCO 3 - conc.

Metabolic Alkalosis - Causes ECF 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 HCO 3 - Post Hypercapnia [Ur Cl - < 10]

ECF Contraction HCO 3 - Cl - Diuresis HCO 3 - Cl - ECF Vol. Cl - mmol. HCO 3 - mmol. Cl - conc. HCO 3 - conc. [Secondary Hyperaldo.] Cl -

2nd Step What's the pH, PaCO 2, & the HCO 3 - ? 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 Gap If assess a patient for all known causes of an anion gap: AG [mmol/l]% Confirmed

Anion Gap Rules 1 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 HCO 3 - [ titrated, mole for mole ]

If AG is increased > Assume it began normal [12 mmol/l] 2. Therefore an excess has developed 3. Assume the HCO 3 - began normal 4. If [HCO xs] < normal HCO 3 -  additional primary loss of HCO If [HCO AGxs] > normal HCO 3 -  additional primary source of HCO 3 -

Primary AG Metabolic Acidosis Only Na + Cl - AG HCO 3 - Cl - AG HCO 3 - AG xs Before After

Primary AG Metabolic Acidosis, With Primary Non-Gap Metabolic Acidosis Na + Cl - AG HCO 3 - Cl - AG HCO 3 - AG xs Before After

Primary AG Metabolic Acidosis, With Primary Metabolic Alkalosis Na + Cl - AG HCO 3 - Cl - AG HCO 3 - AG xs Before After

Summary Steps 1. Clinical Status 2. Verify Results 3. Determine the Primary Problem [pH, PaCO 2, HCO 3 - ] 4. Calculate AG 5. If AG > 20, calculate AG excess 6. Add to HCO 3 - [ compare to normal range ]

Case #1 pH 7.5 PaCO 2 29 HCO Case #2 pH 7.2 PaCO 2 70 HCO

Case #3 pH 7.55 PaCO 2 40 HCO Case #4 pH 7.34 PaCO 2 60 HCO

Case #6 pH 7.5 PaCO 2 20 HCO Na Cl - 103

Case #7 pH 7.4 PaCO 2 40 HCO Na Cl - 100

Case #8 pH 7.5 PaCO 2 20 HCO Na Cl - 100

Case #9 pH 7.1 PaCO 2 50 HCO Na Cl - 100

Case #10 pH 7.15 PaCO 2 15 HCO Na Cl - 110