Approach to Acid Base Problems: Revisited
Problem sets at end of review!
1. pH: Check arterial pH
Principle: The net deviation in pH will indicate whether an acidosis or an alkalosis is
present (but will not indicate mixed disorders)
Guidelines:



IF an acidemia is present THEN an acidosis must be present
IF an alkalemia is present THEN an alkalosis must be present
IF pH is normal THEN Either (no acid-base disorder is present) or (Compensating
disorders are present ie a mixed disorder with an acidosis and an alkalosis)
2. PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
Principle: Each of the simple disorders produces predictable changes in [HCO3] & pCO2.
Guidelines:

IF Both [HCO3] & pCO2 are low THEN Suggests presence of either a Metabolic Acidosis
or a Respiratory Alkalosis (but a mixed disorder cannot be excluded)

IF Both [HCO3] & pCO2 are high THEN Suggests presence of either a Metabolic Alkalosis
or a Respiratory Acidosis (but a mixed disorder cannot be excluded)

IF [HCO3] & pCO2 move in opposite directions THEN a mixed disorder MUST be present
Which disorder is present is dependent on which change is primary and which is
compensatory, and this requires an assessment based on the history, examination & other
results.
3. CLUES: Check for clues in the other biochemistry results
Principle: Certain disorders are associated with predictable changes in other biochemistry
results
Step 3 involves reviewing other results looking for specific evidence of particular disorders.
Some of these 'clues' are outlined in the table below. In most circumstances, these clues are
confirmatory of the expected diagnosis but on occasion can alert to the presence of an
unanticipated second disorder. An elevated anion gap can be particularly useful. Most of these
'clues' are obtained from the biochemistry profile. An alert clinician can often correctly pick the
diagnosis before the gas results are back.
Some Aids to Interpretation of Acid-Base Disorders
"Clue"
Significance
High anion gap
Always strongly suggests a metabolic acidosis.
Hyperglycemia
If ketones also present in urine -> diabetic ketoacidosis
Hypokalemia and/or
hypochloremia
Suggests metabolic alkalosis
Hyperchloremia
Common with normal anion gap acidosis
Elevated creatinine
and urea
Suggests uremic acidosis or hypovolemia (prerenal
renal failure)
Elevated creatinine
Consider ketoacidosis: ketones interfere in the
laboratory method (Jaffe reaction) used for creatinine
measurement & give a falsely elevated result; typically
urea will be normal.
Elevated glucose
Consider ketoacidosis or hyperosmolar non-ketotic
syndrome
Urine dipstick tests
for glucose and
ketones
Glucose detected if hyperglycemia; ketones detected if
ketoacidosis
4. COMPENSATION: Assess the Compensatory Response
Principle: The 6 Rules are used to assess the appropriateness of the compensatory
response.
Guidelines:


If the expected & actual values match => no evidence of mixed disorder
If the expected & actual values differ => a mixed disorder is present
Rule 1 : The 1 for 10 Rule for
Acute Respiratory Acidosis
The [HCO3] will increase by 1 mmol/l for every 10 mmHg elevation in pCO 2
above 40 mmHg.
Expected [HCO3] = 25 + { (Actual pCO2 - 40) / 10 }
Comment: The increase in CO2 shifts the equilibrium between CO2 and HCO3 to result in
an acute increase in HCO3. This is a simple physicochemical event and occurs almost
immediately.
Example: A patient with an acute respiratory acidosis (pCO2 60mmHg) has an actual
[HCO3] of 31mmol/l. The expected [HCO3] for this acute elevation of pCO2 is 25 + 2 =
27mmol/l. The actual measured value is higher than this indicating that a metabolic
alkalosis must also be present.
Rule 2 : The 4 for 10 Rule for
Chronic Respiratory Acidosis
The [HCO3] will increase by 4 mmol/l for every 10 mmHg elevation in
pCO2 above 40mmHg. (Dr. Bolander used 3.5)
Expected [HCO3] = 25 + 4 { (Actual pCO2 - 40) / 10}
Comment: With chronic acidosis, the kidneys respond by retaining HCO3, that is,
renal compensation occurs. This takes a few days to reach its maximal value.
Example: A patient with a chronic respiratory acidosis (pCO2 60mmHg) has an actual
[HCO3] of 31mmol/l. The expected [HCO3] for this chronic elevation of pCO2 is 25 +
8 = 33mmol/l. The actual measured value is extremely close to this so renal
compensation is maximal and there is no evidence indicating a second acid-base
disorder.
Rule 3 : The 2 for 10 Rule for
Acute Respiratory Alkalosis
The [HCO3] will decrease by 2 mmol/l for every 10 mmHg decrease in
pCO2 below 40 mmHg.
Expected [HCO3] = 25 - 2 { ( 40 - Actual pCO2) / 10 }
Comment: In practice, this acute physicochemical change rarely results in a [HCO3]
of less than about 18 mmol/l. (After all there is a limit to how low pCO2 can fall as
negative values are not possible!) So a [HCO3] of less than 18 mmol/l indicates a
coexisting metabolic acidosis.
Rule 4 : The 5 for 10 Rule for a
Chronic Respiratory Alkalosis
The [HCO3] will decrease by 5 mmol/l for every 10 mmHg decrease in
pCO2 below 40 mmHg.
Expected [HCO3] = 25 - 5 { ( 40 - Actual pCO2 ) / 10 }
(range: +/- 2)
Comments:


It takes 2 to 3 days to reach maximal renal compensation
The limit of compensation is a [HCO3] of about 12 to 15 mmol/l
Rule 5 : The One & a Half plus 8 Rule - for a
Metabolic Acidosis
The expected pCO2 (in mmHg) is calculated from the following formula:
Expected pCO2 = 1.5 x [HCO3] + 8
(range: +/- 2)
Comments:



Maximal compensation may take 12-24 hours to reach
The limit of compensation is a pCO2 of about 10 mmHg
Hypoxia can increase the amount of peripheral chemoreceptor stimulation
Example: A patient with a metabolic acidosis ([HCO3] 14mmol/l) has an actual pCO2
of 30mmHg. The expected pCO2 is (1.5 x 14 + 8) which is 29mmHg. This basically
matches the actual value of 30 so compensation is maximal and there is no evidence
of a respiratory acid-base disorder (provided that sufficient time has passed for the
compensation to have reached this maximal value). If the actual pCO2 was 45mmHg
and the expected was 29mmHg, then this difference (45-29) would indicate the
presence of a respiratory acidosis and indicate its magnitude.
Rule 6 : The Point Five plus Twenty Rule - for a
Metabolic Alkalosis
The expected pCO2(in mmHg) is calculated from the following formula:
Expected pCO2 = 0.7 [HCO3] + 20 (range: +/- 5)
Comment: The variation in pCO2 predicted by this equation is relatively large.
The combination of a low [HCO3] and a low pCO2 occurs in metabolic acidosis and in respiratory
alkalosis. If only one disorder is present it is usually a simple matter to sort out which is present.
The factors to consider are:


The history usually strongly suggests the disorder which is present
The net pH change indicates the disorder if only a single primary disorder is present (eg
acidemia => acidosis)

An elevated anion gap or elevated chloride define the 2 major groups of causes of metabolic
acidosis
Check Anion Gap
Anion Gap = [Na] – ([Cl] + [HCO3]) = 12 +/- 4  = 8-16
An elevated Anion Gap always strongly suggests a Metabolic Acidosis.


If AG is 20-30 then high chance (67%) of metabolic acidosis
If AG is > 30 then a metabolic acidosis is definitely present
Practice Problem #1
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.2—(less than normal 7.4) Acidemic
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is low (this does not correlate with arterial pH—not a primary disturbance);
Bicarb is low (this does correlate with the arterial pH—primary!)
Metabolic (because Bicarb involved) Acidosis
Gap or no gap?
Anion Gap = [Na] – ([Cl] + [HCO3]) = 12 +/- 4  = 8-16
= 141 – (111 + 8) =
12 +/- 4
= 22 ≠ 12 +/- 4  elevated anion gap
If Metabolic Acidosis use the formula (Winter’s Formula)
Expected pCO2 = 1.5 x [HCO3] + 8
(range: +/- 2)
= 1.5 x 8 + 8
= 20 is the expected CO2  Actual CO2 was 21 so within +/- 2  appropriate respiratory
compensation
Metabolic Acidosis with elevated anion gap and appropriate respiratory compensation 
Differential Diagnosis is MULEPAK; as well as increase inorganic acids, lab errors, decreased
unmeasured cations, PCN, carbicillin, increased albumin. Use patient’s history to help
differentiate the diagnosis
Practice Problem #2
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.26—(less than normal 7.4) Slightly Acidemic
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is low (this does not correlate with arterial pH—not a primary disturbance);
Bicarb is low (this does correlate with the arterial pH—primary problem!)
Metabolic—because Bicarb involved Acidosis
Gap or no gap?
Anion Gap = [Na] – ([Cl] + [HCO3]) = 12 +/- 4  = 8-16
= 156 – (113 + 10) =
12 +/- 4
= 33 ≠ 12 +/- 4  elevated anion gap
If Metabolic Acidosis use the formula
Expected pCO2 = 1.5 x [HCO3] + 8
(range: +/- 2)
= 1.5 x 10 + 8
= 23 is the expected CO2  Actual CO2 was 23 so within +/- 2  appropriate respiratory
compensation
Metabolic Acidosis with elevated anion gap and appropriate respiratory compensation 
Differential Diagnosis is MULEPAK, increase inorganic acids, lab errors, decreased
unmeasured cations, PCN, carbicillin, increased albumin
Practice Problem #3
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.10—(less than normal 7.4) Acidemic
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is normal (this does not correlate with arterial pH—not the primary
disturbance);
Bicarb is low (this does correlate with the arterial pH—primary problem!)
Metabolic (because Bicarb involved) Acidosis
Gap or no gap?
Anion Gap = [Na] – ([Cl] + [HCO3]) = 12 +/- 4  = 8-16
= 138 – (114 + 12) =
12 +/- 4
= 12 = 12 +/- 4  non anion gap
If Metabolic Acidosis use the formula
Expected pCO2 = 1.5 x [HCO3] + 8
(range: +/- 2)
= 1.5 x 12 + 8
= 26 is the expected CO2  Actual CO2 was 40 so consider a mixed disorder present. Since
the actual pCO2 was 40mmHg and the expected was 26mmHg based on the equations, this
difference (40-26) would indicate the presence of a respiratory acidosis. Why? There is far
more CO2 present than should be with a single disorder. Look at your patient! Does the
patient have respiratory depression or decreased breathing rate? Do the ventilatory settings
need to be increased, if on a ventilator?
Metabolic Acidosis with non elevated anion gap and inappropriate respiratory compensation
(retaining CO2!) Mixed disorder with Respiratory Acidosis.
Differential Diagnosis GI losses or Renal Losses  Must evaluate Urine Net Charge. Review
any history from patient/caregiver that may be helpful?
Practice Problem #4
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.37—(less than normal 7.4) Slightly Acidemic
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is low (this does not correlate with the arterial pH—)
Bicarb is low (this does correlate with the arterial pH—)
IF Both [HCO3] & pCO2 are low THEN Suggests presence of either a Metabolic Acidosis or a
Respiratory Alkalosis (but a mixed disorder cannot be excluded)
The combination of a low [HCO3] and a low pCO2 occurs in metabolic acidosis and in
respiratory alkalosis. The factors to consider are:


The history usually strongly suggests the disorder which is present
An elevated anion gap or elevated chloride define the 2 major groups of causes of
metabolic acidosis
If Metabolic Acidosis use the formula
Expected pCO2 = 1.5 x [HCO3] + 8
(range: +/- 2)
= 1.5 x [HCO3] + 8
1.5 x [18 ] + 8
27 + 8 Expected CO2 = 35 ≠ actual 24 (less CO2  alkalosis)
Gap or no gap?
Anion Gap = [Na] – ([Cl] + [HCO3]) = 12 +/- 4  = 8-16
= 142 – (111 + 18) =
12 +/- 4
= 13 = 12 +/- 4  non anion gap
Metabolic acidosis and respiratory alkalosis must evaluate history
Practice Problem #5
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.51—(more than normal 7.4) Alkalemic
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is elevated (this does not correlate with arterial pH—not primary disturbance);
Bicarb is high (this does correlate with the arterial pH—primary problem!)
Metabolic (because Bicarb involved) Alkalosis
Metabolic Alkalosis use
Δ PCO2 = 0.5 x Δ HCO3 (range: +/- 5)
Δ PCO2 = 0.5 (38-25) (range: +/- 5)
49 - 40= 0.5 (13) (range: +/- 5)
9 = 6.5(range: +/- 5)
= Expected pCO2 is accounted for so appropriate respiratory compensation.
Differential Diagnosis
Exogenous load?; GI or renal loss of excess acid—vomiting or NG loss: volume depletion;
hypokalemia; excess steroid
Differentiate using volume status and urine Cl
Practice Problem #6
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.40—Normal—must confirm that CO2 and Bicarb are normal too!
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is elevated (this does not correlate with arterial pH—indicates pH should be
acidic)
Bicarb is high (this does not correlate with the arterial pH—indicates pH should be
basic!)
IF pH is normal THEN Either (no acid-base disorder is present) or (Compensating disorders
are present i.e. a mixed disorder with an acidosis and an alkalosis) This is a mixed
disorder!!!!
Mixed Disorder Respiratory acidosis and Metabolic alkalosis present
Practice Problem #7
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.38—(less than normal 7.4) slightly acidemic
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is elevated (this does correlate with arterial pH—the primary disturbance);
Bicarb is high (this does not correlate with the arterial pH—not the primary
problem!)
Respiratory (because CO2 involved) Acidosis
Respiratory Acidosis use
Expected [HCO3] = 25 + { (Actual pCO2 - 40) / 10 }
25 + [(70-40)/10]
=28
28 expected HCO3 does not equal actual HCO3 of 40 therefore
compensation likely with Metabolic alkalosis
A patient with an acute respiratory acidosis (pCO2 70mmHg) has an actual [HCO3] of 40
mmol/l. The expected [HCO3] for this acute elevation of pCO2 is 28mmol/l. The actual
measured value is higher than this indicating that a metabolic alkalosis must also be present.
Practice Problem #8
Is the patient acidemic or alkalemic? Look at the pH.
pH is 7.68—(more than normal 7.4)Alkalosis
PATTERN: Look for suggestive pattern in pCO2 & [HCO3]
CO2 is low (this does correlate with arterial pH—primary problem);
Bicarb is high (this does correlate with the arterial pH—primary problem!)
Respiratory Alkalosis
Expected [HCO3] = 25 - 2 { ( 40 - Actual pCO2) / 10 }
= 25 – 2 {( 40 – 35) / 10}
Expected [HCO3] = 24
Does not equal actual bicarbonate (40); actual value is higher than expected
indicates a metabolic alkalosis as well.
Metabolic and respiratory alkalosis
Mixed Disorder review patient history
Review volume status and urine Cl for metabolic alkalosis for metabolic acidosis; evaluate
respiratory status—blowing off too much CO2!