The Clinical Approach To Acid-base Disorders

By Author: Ibrahim Machiwala
Total Articles: 463
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Methods to Interconvert pH and [H+]:Drop the 7 and and Decimal Point Rule
pH Drop 7 and Decimal Point Difference from 40 [H+] nmol/L
7.40 40 0 40
7.38 38 2 42
7.42 42 2 38

Methods to Interconvert pH and [H+]:The 0.1 pH Change Rule:
PH Conversion Factor [H+] nmol/L
6.90 100 / 0.8 125
7.00 100 100
7.10 100 X 0.8 80
7.20 100 X 0.8 X 0.8 64

Henderson Equation:

24 X Pco2
[H+] = --------------
[HCO3-]

A patient has diabetic ketoacidosis and the following laboratory data: pH=7.10, Paco2= 30 mm Hg, [Hco3-] = 13 mmol/L, AG= 25 mEq/L, what do you conclude?

pH= 7.10, thus [H+] is 70-80 nmol/L
AG is 25, thus added anion concentration is 25-12= 13
24 X Pco2
[H+] = --------------
[HCO3-]

80?24 X 30/ 13
80?56

Tests Used in Acid-Base Diagnoses:

The anion gap in plasma
The osmolal gap in plasma
The urine net charge
The osmolal gap in urine
The urine pH
The urine Pco2

Anion Gap in Plasma:
...
... The difference between measured major positive and negative charges
[Na+] -[Cl-] - [HCO3-]
The normal value is 12 ? 2 mEq/L
Expect close to 1:1 reciprocal change in anion gap and [HCO3-]
The anion gap changes with blood pH, but this change is small: 0.5 mEq/L for each 0.1 unit change in pH
An increased anion gap may be the only clue that metabolic acidosis is present in a mixed acid-base disorder

Charge Balance:
Cations (mEq/L) Anions
(mEq/L)
Na+ 140 CL- 103
K+ 4 HCO3- 25
Ca2+ 5 Proteins: albumin 16
Mg2+ 2 Organic 4
H+ 0.00004 (40 nmol/L) Other inorganics: HPO4-SO4 3

Example: Lactic acid?H+ and Lactate:
Plasma
(mEq/L) [Na+] [Cl-] [HCO3-] Anion Gap
Normal 140 103 25 12
+10 mmol/L 140 103 15=25-10 22=12+10
lactic acid

If the concentration of albumin in plasma is half of normal, what adjustments should be made when interpreting the plasma anion gap?
Include all the major positive charges in solution: K+: 4 mmol/L
The normal value for AG would be 16 instead of 12
albumin has a valence of 16 (16 mEq/L) and concentration of 4 g/dL and 0.5 mmol/L
The expected value for plasma AG should be reduced when albumin falls to 2 mg/dL

Patients with MM may have a protein in plasma that bears a net positive charge. What the impact of this protein on the value of the plasma anion gap?
IgG myeloma adds a lysine-rich or arginin-rich protein in plasma
This paraproteins carry a net positive charge
These unmeasured positive charges are associated with measured Cl-
The plasma AG falls, or may become negative if the concentration of paraproteins is high enough

The Osmolal Gap in plasma:
The osmolal gap is useful means of detecting the presence of uncharged molecules in plasma
Calculated osmolality 2[Na+] + [Glucose] (mmol/L) + [Urea] (mmol/L)
Osmolal gap is the difference between measured and calculated osmolality of plasma
Increased osmolal gap indicates the presence of an unmeasured compound that is not charged: most likely alcohol

Conversion Between mg/dL and mmol/L
Constituent Molecular Weight mg/dL mmol/L
Glucose 180 90 5
Urea 60 30 5
BUN 28 14 5

The Urine Net Charge:
Measured NH4+ excreted with Cl-
Most hospital biochemistry laboratory do not routinely measure NH4+ in the urine
In normal urine the major cations are Na, K and NH4+ and the major anions are Cl- and HCO3-
NH4+ is usually excreted along with Cl-
Thus if are plentiful in urine, there will be a much greater quantity of Cl- than the measured cations Na+ plus K+
If the sum of Na+ and K+ is greater than Cl-, there will be no electrical room for NH4+ unless there are large amount of unmeasured anions in the urine

The Urine Net Charge:
[Cl-] > [Na+] + [K+] = high [NH4+ ]

[Cl-] < [Na+] + [K+] = either a low [NH4+ ] or excretion of
NH4+ with an anion other than Cl-

The Osmolal Gap in Urine:
Is used to detect NH4+
In chronic metabolic acidosis, it is expected to find more than 200 mmol of NH4+ excreted each day
This is excreted usually with Cl- and the urine net charge is very negative
If the urine net charge is not negative and the patient is acidotic, consider the osmolal gap

Osmolal Gap:
Measured osmolality minus calculated
Calculated osmolality= 2 ([Na+] + [K+]) + [Glucose] + [Urea] all in mmol/L
When osmolal gap is high, thus unmeasured anions exist in large amount
Those usually be ketoacids anions, drug metabolites or hippurate (toluene intoxication)

Example:
[Na+] 50
[K+] 50
[Cl-] 25
[NH4+] 200
Glucose 0
[Urea] 250
Osmo urine, measured 850
Osmo urine, calculated 450
Osmo gap 400

Expected Responses to Primary Acid-Base Disorders:
Disorder Response
Metabolic acidosis Drop the 7 and the decimal point from the pH
For every mmol/L fall in HCO3 from 25, Paco2 drops 1 mm Hg
Metabolic alkalosis Drop the 7 and the decimal point from the pH
For every mmol/L rise in HCO3 from 25, Paco2 should rise 0.7 mm Hg
Acute respiratory acidosis For every twofold increase in Paco2, the plasma HCO3 should increase by 2.5 mmol/L from 25
Chronic respiratory acidosis For every mm Hg increase in Paco2, the plasma HCO3 should increase by 0.3 mmol/L from 25
Acute respiratory alkalosis For every mm Hg fall in Paco2, the plasma H+ should fall by 0.8 mmol/L from 40
Chronic respiratory alkalosis For every mm Hg in Paco2, the plasma HCO3 should fall by 0.5 mmol/L from 25

Guidelines for the Diagnosis of Mixed Disorder:
Calculate the plasma AG, if it is very low or negative, there is probably an error in one of the electrolytes
Henderson equation, detect errors
If AG is elevated more than 5 mEq/L, the patient probably has metabolic acidosis
Compare the magnitude of the fall in plasma HCO3 with the increase in plasma anion gap: they should be similar
A rise in plasma AG that is less than the fall in plasma HCO3 suggest that a component of metabolic acidosis involves loss of NaHCO3 or renal tubular acidosis is present
A rise in plasma AG that is much greater than the fall in HCO3 suggest that there is a coexistent metabolic alkalosis (additional source of HCO3)
In metabolic acidosis or alkalosis, look for the expected change in PCO2

A 23-year-old woman with rheumatoid arthritis increased her dose of salicylates because of a flare-up. She then developed epigastric pain and vomited frequently for 2 days. She went to the local hospital, where the following blood results were obtained:
H+ 20 nmol/L
pH 7.70
Paco2 25
Anion gap 17

Causes:
Metabolic alkalosis: vomiting
Respiratory alkalosis: salicylate intoxication
Metabolic acidosis with high AG: salicylic acid

A 50-year-old woman underwent intestinal bypass for morbid obesity. Because she was having 10-15 watery stools per day, she was treated with tincture of opium and found somnolent and somewhat hypotensive the next morning. Plasma values were:
Na+ 130 H+ 96 nmol/L
K+ 3.2 pH 7.02
Cl- 102 Paco2 40
Albumin 4 HCO3 10

Causes:
Metabolic acidosis: diarrhea
Lactic acidosis: hypotension
Ketoacidosis: starvation
Respiratory acidosis: suppression of ventilation

Plasma Normal
PH 7.40
HCO3 25
AG 12
PCO2 40

Plasma Normal
PH 7.40 7.30
HCO3 25 15
AG 12 12
PCO2 40 30

Plasma Normal Loss of NaHCO3
(10 mmol/L)
PH 7.40 7.30
HCO3 25 15
AG 12 12
PCO2 40 30

Plasma Normal Loss of NaHCO3
(10 mmol/L)
PH 7.40 7.30 7.13
HCO3 25 15 5
AG 12 12 22
PCO2 40 30 15