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Interpretation of
Mixed Acid Base Disorders-Mixed Acid Base Disorders-
No Da Vinci Code
by
Surg Capt Aamir Ijaz,
Consultant Pathologist / Assoc Prof of Pathology,
PNS SHIFA
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ARTERIAL BLOOD GASES
• Measured on a
Blood Gas Analyser.
• Sample of whole • Sample of whole
blood is directly
injected into the
analyser without
delay
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Site for Sample Collection• Best site ?
• Why ?
• No vein adjacent to Radial Artery
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arterial blood venous blood
pH 7.35-7.45 7.31-7.40
COPD
7.27
Arterial or Venous Blood ??
PCO2 35-45mmHg 41-51mmHg
(4.7-6.0 kPa) (5.5-6.8kPa)
HCO3- 22-28mmol/l 25-29mmol/l
PO2 > 80mmHg 38-42 mmHg
(>10.6kPa) (5.1-5.6kPa)
54mmHg
31mmol/l
40mmHg
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Solution ?
Mention site on Lab Form
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Measured and Calculated Tests
Measured parameters:
–pH
– Partial Pressure of Carbon
dioxide (PCO2)dioxide (PCO2)
–Partial Pressure of oxygen (PO2)
• Calculated parameters:
– HCO3 using Henderson Hasselbalch equation
–Base Excess (BE)
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ABG measurement –
Some unique things
• Samples to reach lab within 15 minutes
• No incubation or reagent mixing
• Instant results• Instant results
• Invalid if result delayed
• “bedside use” with some precautions
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ABG measurement –
An expensive test
• Quite expensive investigation
• Under-utilized
• Cost of even one test per day is also • Cost of even one test per day is also
very high
• Proper interpretation is required for
optimum use
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ABG measurement –
High diagnostic yield
95 %
60
80
100
Percentage ABGs
25%
15%
10%
0
20
40
60
Percentage
ABGs CBC
Electrolytes Urinalysis
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Types of Acid Base Disorders
• Single (simple) disorders
• Double (mixed) disorders
• Triple (mixed) disorders• Triple (mixed) disorders
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Simple (Single) Disorders
• Metabolic Acidosis
• Metabolic Alkalosis
• Respiratory Acidosis• Respiratory Acidosis
• Respiratory Alkalosis
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Double Disorders
• Metabolic and Respiratory Acidosis
• Metabolic and Respiratory Alkalosis
• Metabolic Alkalosis and Respiratory
Acidosis
• Metabolic Acidosis and Metabolic • Metabolic Acidosis and Metabolic
Alkalosis
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Triple Disorders
Metabolic Acidosis, Metabolic Alkalosis
and Respiratory Alkalosis:
beyond “salicylate poisoning”beyond “salicylate poisoning”
Any high anion gap acidosis
complicated by vomiting and
hyperventilation can cause this
disorder
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Pattern of ABG results from
CCU,ICU and NICU
in 45 days (n=114)
46%
6%
46%
48%
Simple Dsiorders Mixed Disorders Normal Results
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Distribution of Mixed Acid Base Disorders
in a Critical Care Units (PNS SHIFA)
in 45 days (n=67)
34%56%
10%
Double Acidosis Double Alkalosis Opposing disorders
66%
34%
Metabolic Acidosis+Respiratory Alaklosis
Metabolic Alkalosis +Respiratory Acidosis
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Analysing ABG Reports
• Clinical features
• Results of other investigations e.g.
electrolytes etcelectrolytes etc
• ABG result necessary for confirmation
of diagnosis
• Biochemical opinion may be given even
with meagre information
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Treatment PrinciplesofAcid-BaseDisturbances
Correct analysis(alwaysbe alert to the presence of mixeddisturbances)
The 3-Cs
Correct disturbance(buy time, don’t over do)
Correct disease(a MUST if possible)
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Decoding an ABG Report
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Look at pH
• Low pH-----Acidosis
• High pH----Alkalosis
The Decoder
• High pH----Alkalosis
• Normal pH--- A normal pH does not rule out existence of an acid base disorder (see below)
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If pH is abnormal Examine
PCO2 and HCO3 relationship
• If PCO2 and HCO3 change in the same direction, it is a single disorder.
Example:.
The Decoder
Example:.
• in Metabolic Acidosis (i.e. decreased HCO3), PCO2 should decrease. If it is decreased, consider Metabolic Acidosis with Respiratory Compensation (Single Disorder)
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If pH is abnormal Examine PCO2
and HCO3 relationship (Cont)
• If PCO2 and HCO3 change in the opposite
direction, it is a double disorder
The Decoder
Example:
• in Metabolic Acidosis (i.e. decreased HCO3),
PCO2 should decrease. If it is increased,
consider Metabolic Acidosis with Respiratory
Acidosis (Double Disorder)
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Examine pH and HCO3 relationship
(For single disorders)
If pH and HCO3change in the same
direction primary abnormality is metabolic
– Examples:
The Decoder
• in metabolic acidosis both pH and HCO3
decrease
• in metabolic alkalosis both pH and HCO3
increase
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Examine pH and HCO3 relationship
(For single disorders) (Cont)
If pH and HCO3 change in the opposite
direction primary abnormality is
respiratory
The Decoder
respiratory
• Examples: – in respiratory acidosis pH decreases and HCO3
increases
– in respiratory alkalosis pH increases and HCO3
decreases
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If pH is normal Examine PCO2 and
HCO3 relationship again
If PCO2 and HCO3 grossly change in the same direction, it is also a double disorder
Examples:
• in Metabolic Acidosis (i.e. decreased HCO3),
The Decoder
• in Metabolic Acidosis (i.e. decreased HCO3), PCO2 should decrease to a certain extent. If it is decreased too much, consider Metabolic Acidosis with Respiratory Alkalosis (Double Disorder)
• in Respiratory Acidosis (i.e. increased PCO2), HCO3 should increase to a certain extent. If it is increased too much, consider Respiratory Acidosis with Metabolic Alkalosis (Double Disorder)
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If pH, PCO2 and HCO3 all are normal
Normal Acid Base Status !!(----but please be careful of values on the extreme of (----but please be careful of values on the extreme of
reference ranges, which may be due to a fully
compensated disorder like Chronic Respiratory
Alkalosis or mild double disorders)
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ANION GAP
• It is a measure of anions other than
HCO3 and Chloride
• Biochemical Basis:• Biochemical Basis:
Always:
CATIONS = ANIONS
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ANION GAP
(Biochemical Basis)
• Major Cat-ions:
– Na = 140 mmol/L
– K = 4 mmol/L – K = 4 mmol/L
– Ca = 4.5 mmol/L
– Mg = 1.5 mmol/L
TOTAL CATIONS = 150 mmol/L
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ANION GAP
(Biochemical Basis)• Major Anoins:
– Cl = 100 mmol/L
– HCO3 = 27 mmol/L
– proteins = 15 mmol/L
– PO3 = 2 mmol/L
– SO3 = 1 mmol/L
– Organic Acids = 5 mmol/L
TOTAL ANIONS = 150 mmol/L
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Anion Gap:
• Calculated as following:
(Na + K) - (Cl + HCO) = 18 mmol/L
Range : 7 – 18 mmol/LRange : 7 – 18 mmol/L
• It is only a lab derived index
• There can Never be an anion gap in any
condition because electroneutrality is
always maintained in the plasma
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High Anion Gap (Hyperkalaemic Metabolic Acidosis)
• Increased unmeasured Anions• Ketoacidosis
• Lactic Acidosis
• Renal Failure
• Poisoning • Poisoning
– Methanol
– Ethanol
– Salicylates
– others
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Normal Anion Gap (Hypokalaemic Metabolic Acidosis)
• Diarrhoea:
– Loss of HCO3
– Loss of K
• Renal Tubular Acidosis• Renal Tubular Acidosis
• Acetazolamide therapy
• Uretro-illeal shunt
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DELTA RATIO
The delta ratio is used for the determination of
a mixed acid base disorder in an elevated
anion gap metabolic acidosis
Measured anion gap – Normal anion gap
Normal [HCO3-] – Measured [HCO3-]
or
(anion gap – 12)
(24 - [HCO3-])
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Delta Ratio Assessment
Guideline
< 0.4 Hyperchloraemic normal anion gap acidosis
0.4 - 0.8 • Renal failure
• Combined high AG & normal AG acidosis
1 to 2 •Uncomplicated high-AG acidosis
•Lactic acidosis: 1.6 (average value)
>2 A pre-existing elevated HCO3 level due to:
–a concurrent metabolic alkalosis, or
–a pre-existing compensated respiratory acidosis
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Predicted HCO3
Predicted [HCO3] = 24 -[Anion Gap-12]
• If patient's [HCO3] > Predicted [HCO3]
– Metabolic acidosis + Metabolic Alkalosis
• If patient's [HCO3] < Predicted [HCO3]
– Compensatory response to resp alkalosis
– Metabolic acidosis + Metabolic Alkalosis
– Compensatory response to resp acidosis
If patient's [HCO3] =Predicted [HCO3]
– Simple Metabolic acidosis
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Other Interpretation Assistants
• Flenley’s Graph
• Electronic Calculators (Skyscape for
desktop, ipads etc).desktop, ipads etc).
• Equations (or Formulas) – usually
required for interpreting more subtle
changes in ABGs.
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Flenley’s graphBy plotting the result of the pH, HCO3 and Paco2 on the graph, a mixed ABG
disturbance is present if the point falls outside the arms of the CROSS.
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Learning Points
• ABG samples should be sent to the lab immediately and ask for results instantly
• Mixed disturbances are very • Mixed disturbances are very common in critically ill patients because of multiple pathologies
•Always be on the lookout for Mixed Disturbance
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Acknowledgement
Dr Humaira Ashraf
FCPS Part II Trainee
Chemical Pathology
PNS SHIFA