Acid-Base Analysis examples.presentation

Evaluation and Analysis of
Acid-Base Disorders
Taylor Sawyer DO
Resident Pediatrics
TAMC

Why Acid-Base ?
 Complicated
 Confusing
 Time consuming

Reference:
Western Journal of Medicine. Aug 1991; 155: 146-151

Objectives:
 Introduction to equipment used and variables
involved in acid-based problems
 Simplified discussion on acid-base disorders
 Systematic approach to acid-base
interpretation
 3 Quick and easy rules on acid-base disorders

Acid-Base Analysis, What do You
Need?
 Blood gas (pH, CO2)
 Serum chemistry (Na, Cl, HCO3)
 Calculator
 30 seconds

The Tools: Blood Gas
 Few drops of blood (< 100 μl)
into cartridge
 Cartridge placed into analyzer
 Foil pouch in the cartridge
containing a calibrated buffered
solution with analytes in know
concentration is punctured and
flows over sensors (calibration)
 Blood sample then pushed onto
sensors
 Measurements performed

i-STAT 7
 I stat 7:
1. Sodium
2. Potassium
3. Ionized calcium
4. pH
5. PCO2
6. PO2
7. Hematocrit
– Bicarbonate*
– Total carbon dioxide*
– Base excess*
– O2 saturation*
– Hemoglobin*
(* denotes calculated result)

The Tools: Serum Chemistry
 COBAS INTERGA 800
– Uses four separate methods
of analysis on each sample
– Can perform up to 72
different tests
– Can handle 185 samples
tubes
– Can do up to 850 serum
chemistries per hour
– Direct measurements of
electrolytes and HCO3

ABG: 7.40 / 40 / 80 / 24 / 0
– pH
– PaCO2
– PaO2
– HCO3
– BE

Acid-Base Normals:
pH= 7.40 (7.35 - 7.45)
PCO2 = 40 (35 - 45)
HCO3 = 24 (22 - 26)

Acidemic vs. Alkalemic
pH < 7.35 = Acidemic
pH > 7.45 = Alkalemic
Separate term for pH to allow description of the
net effect of multiple respiratory and metabolic
abnormalities

Rule 1
 Look at the pH. Whichever side of 7.40 the pH
is on, the process (CO2, HCO3) that caused it
to shift that way is the primary abnormality.
Principle: The body does not fully compensate
for a primary acid-base disorder

Keep It Simple:
 CO2 = Acid
– CO2 =  pH (acidemia)
–  CO2 =  pH (alkalemia)
 HCO3 = Base
–  HCO3 =  pH (alkalemia)
–  HCO3 =  pH (acidemia)

Four Primary Disorders:
 PCO2 < 35 = respiratory alkalosis
 PCO2 > 45 = respiratory acidosis
 HCO3 < 22 = metabolic acidosis
 HCO3 > 26 = metabolic alkalosis
– Can have mixed pictures with compensation
– Can have up to 3 abnormality
simultaneously (1 respiratory + 2 metabolic)
– The direction of the pH will tell you which is
primary!

Example # 1:
Blood gas: 7.50 / 29 / 22
 Alkalemic
 Low PCO2 is the primary (respiratory alkalosis)
 No metabolic compensation = acute process
Acute Respiratory Alkalosis

Example # 2:
Blood gas: 7.25 / 60 / 26
 Acidemic
 Elevated CO2 is primary (respiratory acidosis)
 No metabolic compensation= acute process
Acute Respiratory Acidosis

 Acidemic
 Elevated CO2 is primary (respiratory acidosis)
 Metabolic compensation has occurred = chronic
process
Chronic Respiratory Acidosis with
Metabolic Compensation*
*true metabolic compensation takes 3 days (72hrs)
Example # 3:
Blood gas: 7.34 / 60 / 31

Chronic Respiratory Acidosis with
Metabolic Compensation

Example # 4:
Blood gas: 7.50 / 48 / 36
 Alkalemic
 Elevated HCO3 is primary (metabolic alkalosis)
 Respiratory compensation has occurred =
acute /chronic ?
Metabolic Alkalosis with Respiratory
Compensation*
*Respiratory compensation takes only minutes

Metabolic Alkalosis with
Respiratory Compensation

Example # 5:
Blood gas: 7.20 / 21 / 8
 Acidemic
 Low HCO3 Is primary (metabolic acidosis)
 Respiratory compensation is present
Metabolic Acidosis with Respiratory
Compensation

Acid-Base Analysis examples.presentation

Anion Gap (AG):
 The calculated difference between the
positively charged (cations) and negatively
charged (anions) electrolytes in the body:
AG= Na+
- (Cl-
+ HCO3
-
)
 Normal AG = 12 ± 2 (10 – 14)

Anion Gap
 Also can be though of as the concentration of
the excess unmeasured anion in the serum
 Total body cations = total body anions (net 0)
Normal Measured:
Na - (Cl + HCO3) = + 12
Normal Unmeasured:
anions - Cations = - 12
--------
net = 0

Rule 2
 Calculate the anion gap. If the anion gap is 
20, there is a primary metabolic acidosis
regardless of pH or serum bicarbonate
concentration
Principle: The body does not generate a large
anion gap to compensate for a primary
disorder (anion gap must be primary)

Why is this true?
1. AG > 20 is more than 4 standard deviations from the
mean and therefore unlikely due to chance.
2. Although a modest increase in anion gap does occur
in patients with metabolic or respiratory alkalosis
(increase negatively charged serum proteins), even in
severe alkalosis this increase is almost never > 20
3. A specific cause for an anion gap can be found in less
than 30% of cases with a anion gap less than 20, as
compared to 77% of those with AG > 20, and 100%
with AG > 30*
* Gabow et al. Diagnostic Importance of an increased serum anion gap. N Engl J
med. 1980; 303:854-858

So,
 The presence of an anion gap  20 is highly
predictive of the presence of an underlying
identifiable primary metabolic acidosis

Rule 3
 Calculate the excess anion gap (total anion gap –
normal anion gap) and add this value to the
measured bicarbonate concentration:
– if the sum is > than normal bicarbonate (> 30) there is an
underlying metabolic alkalosis
– if the sum is less than normal bicarbonate (< 23) there is an
underlying nonanion gap metabolic acidosis
1. Excess AG = Total AG – Normal AG (12)
2. Excess AG + measured HCO3 = > 30 or < 23?
 Principle: 1 mmol of unmeasured acid titrates 1 mmol
of bicarbonate (  anion gap =  [ HCO3])

Why is this true?
 For each 1 mmol acid titrated by the carbonic
acid buffer system, 1 mmol of HCO3 is lost via
conversion to CO2 and H2O and 1 mmol of the
sodium salt of the unmeasured acid is formed.
1 mmol  in HCO3 = 1mmol in AG
 Therefore, the sum of the new (excess) anion
gap and the remaining (measured)
bicarbonate values should equal the normal
bicarbonate concentration

HCO3 Added
 If :
Excess AG + Measured HCO3 = > normal HCO3 (30)
 Then:
Some additional disorder has added HCO3 to the
extracellular space (metabolic alkalosis)

HCO3 Removed
 If :
Excess AG + Measured HCO3 = < normal HCO3 (23)
 Then:
Some additional disorder has removed HCO3 from
the extracellular space (nonanion gap metabolic
acidosis), e.g. renal or GI loses

Is This Really True?
 Published reports do indicate that a reciprocal
relationship between increased anion gap
and decreased HCO3 does exist in
uncomplicated organic acidosis*
 Due to multiple buffering systems in the body it
may not always be a one-to-one relationship
 Bicarbonate is the major extracellular buffer
* Naris et al. Anion gap and Serum Bicarbonate. N Engl J Med 1980;
303: 161

Remember the Rules
1. Look at the pH: (< or > 7.40?) whichever caused the
shift (CO2 or HCO3) is the primary disorder
2. Calculate the anion gap: if AG  20 there is a
primary metabolic acidosis (regardless of pH or HCO3)
3. Calculate the excess anion gap, add it to HCO3:
Excess AG = Total AG – Normal AG (12)
Excess AG + HCO3 = ?
If sum > 30 there is an underlying metabolic alkalosis
If sum < 23 there is an underlying nonanion gap metabolic
acidosis

Example # 1
Blood gas: 7.50 / 20 / 15
Na= 140, Cl = 103
 Alkalemic
 Low CO2 is primary (respiratory alkalosis)
 Partial metabolic compensation for chronic condition?
 AG = 22 (primary metabolic acidosis)
 Excess AG (AG – 12) + HCO3 = 25 (no other primary
abnormalities)
 Respiratory Alkalosis and Metabolic
Acidosis
The patient ingested a large quantity of ASA and had
both centrally mediated resp. alkalosis and anion gap
met. Acidosis associated with salicylate overdose

Example # 2
Blood gas: 7.40 / 40 / 24
Na= 145, Cl= 100
 pH normal
 Excess AG (AG – 12) + HCO3 = 33 ( underlying
metabolic alkalosis)
 Metabolic Acidosis and Metabolic Alkalosis
This patient had chronic renal failure (met. acidosis)
and began vomiting (met. alkalosis) as his uremia
worsened. The acute alkalosis of vomiting offset the
chronic acidosis of renal failure = normal pH

Example # 3
Blood gas 7.50 / 20 / 15
Na= 145, Cl = 100
 Alkalemic
 Low CO2 is primary (respiratory alkalosis)
 Excess AG (AG – 12) + HCO3 = 33 (underlying
metabolic alkalosis)
 Respiratory alkalosis, Metabolic Acidosis
and Metabolic Alkalosis
This patient had a history of vomiting (met. alkalosis),
poor oral intake (met. acidosis) and tachypnea
secondary to bacterial pneumonia (resp. alkalosis)

How Many Primary Abnormalities
Can Exist in One Patient?
 Three primary abnormalities is the max
because a person cannot simultaneously
hyper and hypoventilate
 One patient can have both a metabolic
acidosis and a metabolic alkalosis – usually
one chronic and one acute

Example # 4
Blood gas: 7.10 / 50 / 15
Na= 145, Cl= 100
 Acidemic
 High CO2 and low HCO3
-
both primary (respiratory
acidosis and metabolic acidosis)
 AG = 30 (metabolic acidosis is anion gap type)
 Excess AG + HCO3 = 33 (underlying metabolic
alkalosis)
 Respiratory Acidosis, Metabolic Acidosis
and Metabolic Alkalosis
This is an obtunded patient (resp. acidosis) with a
history of emesis (metabolic alkalosis) and lab findings
c/w diabetic ketoacidosis (metabolic acidosis w/ gap)

Example # 5
Blood gas: 7.15 / 15 / 5
Na= 140, Cl= 110
 Acidemic
 Low HCO3
-
primary (metabolic acidosis)
 AG= 25 (metabolic acidosis is anion gap type)
 Excess AG + HCO3 = 18 (underlying nonanion gap
metabolic acidosis)
 Anion Gap and Nonanion gap Metabolic
Acidosis
Diabetic ketoacidosis was present (anion gap met.
acidosis). Patient also had a hyperchloremic nonanion
gap met. acidosis secondary to failure to regenerate
bicarbonate from ketoacids lost in the urine.

Conclusions:
 To do accurate acid-base evaluations you need
both blood gas and serum chemistry
 Use a systematic approach
 Remember the 3 rules
 “normal” blood gases may not be normal
 It is important to identify all the underlying acid-
base in order to appropriately treat the patient

Acid-Base Analysis examples.presentation

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