2. Pakistan Society Of Chemical Pathologists Distance Learning Programme In Chemical Pathology Lesson No 9 Interpretation of Electrolytes and Arterial Blood Gasses Reports By
Surg Commodore Aamir Ijaz, FCPS, FRCP (Edin)
Prof of Pathology / Consultant Chemical Pathologist
BUM&DC and PNS SHIFA Karachi
Dr Muhammad Anwar Magsi, MPhil
Assoc Prof of Pathology, Bolan Med College, Quetta
(Trainee FCPS-Part II PNS SHIFA Karachi
Dr Humaira Asif, FCPS
Assist Prof of Pathology, BUM&DC Karachi
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3. Basic Approaches to Acid base Disorders
The traditional approach to acid-base analysis, which relies upon the definition of the hydrogen ion concentration in blood as a function of the ratio between the PCO2 and the serum bicarbonate concentration.
Strong Ion Difference : The "strong ion difference" or "SID", the difference between the completely dissociated cations and anions in plasma. Because not all strong ions can be measured, the "apparent SID" or "SIDapp" is calculated from the net charge of the major strong cations: Na+, K+, Ca++, Mg++, and the major strong anions: Cl- and lactate. The independent variables responsible for changes in acid-base balance are the strong ion difference (SID), plasma concentration of nonvolatile weak acids or buffers (ATot), and arterial carbon dioxide tension (PaCO2).

4. pH Negative log of hydrogen ion concentration.
It is actually an indicator of the concentration of hydrogen ions {H+}
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5. Body Buffers Intracellular Buffers Extracellular Buffers Proteins
Haemoglobin
Extracellular Buffers
Bicarbonate
Phosphate
Ammonium
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6. Bicarbonate Buffer System
It is a mixture of carbonic acid (H2CO3) and bicarbonate (HCO3)
It is the most abundant extracellular buffer system.
Most rapid buffer system but not the most efficient system.
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7. Handerson-Hasselbach Equation
pH = pK + Log {HCO3} / {H2CO3} Where K is the overall equilibrium constant For bicarbonate system pK is 6.1 pH of Plasma = Log of 20/1 7.4 = ( Log of 20 is 1.3)
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8. pH of the Bicarbonate Buffer System in Plasma
To keep pH at 7.4 {HCO3} should be 20 times of {H2CO3}. An important implication is that H2CO3 remains in such low concentration because it constantly dissociates into CO2 and water i.e. H2CO3 H20 + CO2 So concentration of H2CO3 is directly proportional to Partial Pressure of CO2 (PCO2) Therefore we can say: pH = HCO3 PCO2
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9. Changes in Bicarbonate Buffer System
Respiratory Component:
PCO2is the respiratory component. High blood PCO2is respiratory acidosis and decreased PCO2is respiratory alkalosis.
Metabolic Component: Describes HCO3
High blood HCO3 is metabolic alkalosis and decreased HCO3 is metabolic acidosis.
Compensation: The buffer system compensates in response to abnormality in the other system to bring the pH normal or near normal
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10. Laboratory Investigations for Acid Base Disorders
For Biochemical Diagnosis:
Plasma Bicarbonate.
Blood Gases: pH
PCO2
PO2
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11. Laboratory Investigations For Acid Base Disorders (Cont)
For Aetiological diagnosis:
Electrolytes (Na+ and K+)
Renal Function Tests
Plasma Glucose
Plasma Lactate
Urine and Plasma Ketones
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12. ABG Analysis
Arterial Blood Gasses analysis is an extremely helpful test in critically ill patients. .

13. Types of Acid Base Disorders
Single
Simple Disorders
Double
Mixed Disorders
Triple
Mixed Disorders

14. Simple (Single) Disorders
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Single or simple or primary acid base disorder evoke a compensatory response that produce secondary acid base disturbance and reversion of pH toward(rarely)to normal
Respiratory Alkalosis

15. Double Disorders Metabolic Acidosis Metabolic Alkalosis
Respiratory Alkalosis
Metabolic Alkalosis
Respiratory Acidosis
Metabolic+
Respiratory
Alkalosis
Metabolic+ Respiratory Acidosis
When two primary acid base disturbance arises simultaneously in the same patient the complex is called mixed acid base disorder or double disorder.

16. Triple Disorders Triple disorder Metabolic Alkalosis Respiratory
If three primary disturbance occurs together in the patient is described as having triple acid base disorder.
Metabolic
Acidosis
Triple disorder

17. Interpretation of ABG Report
Vital for Emergency Management
Instant Report and Instant Interpretation
Immediate Correction

18. How to Interrupt Quickly
Good Understanding of Pathophysiology
Softwares
Mobile Apps

19. One Minute Decoder
Can works in tandem with Mobile Apps (or Equations) For :
Identification of Primary Disorder
Evaluation of Compensatory Process

20. Salient Features of ‘One Minute Decoder’
Question 1: Acidosis or Alkalosis ?
Question 2: Primary disorder is Metabolic or Respiratory ??
Question 3: Single or Double disorder ???

21. MIXED DISORDERS
Both metabolic and respiratory abnormalities may be present in the same patient.
Clinically more common than single disorders
Examples:
Renal Failure with pneumonia i.e. metabolic acidosis and respiratory alkalosis.
Patient with hypoxia with vomiting from upper GI i.e. respiratory acidosis and metabolic alkalosis.
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22. Interpreting ABG Report
Pathophysiological principles
By “One Minute Decoder”
By Equations
Flenley’s Graph
Electronic Calculators (Skyscape for desktop, Smart phones etc).
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23. Decoding an ABG Report
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24. Question 1: Acidosis or Alkalosis ? Look at pH
The Decoder
Question 1: Acidosis or Alkalosis ? Look at pH
Low pH-----Acidosis
High pH----Alkalosis
Normal pH--- A normal pH does not rule out existence of an acid base disorder (see below)
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25. The Decoder
Question 2: Primary disorder is Metabolic or Respiratory ?? Examine pH and HCO3 relationship
If pH and HCO3 change in the same direction primary abnormality is metabolic
Examples:
in metabolic acidosis both pH and HCO3 decrease
in metabolic alkalosis both pH and HCO3 increase
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26. Examine pH and HCO3 relationship (Cont)
The Decoder
Examine pH and HCO3 relationship (Cont)
If pH and HCO3 change in the opposite direction primary abnormality is respiratory
Examples:
in respiratory acidosis pH decreases and HCO3 increases
in respiratory alkalosis pH increases and HCO3 decreases
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27. The Decoder
Question 3: Single or Mixed disorder When pH is Abnormal Examine PCO2 and HCO3 relationship
If PCO2 and HCO3 change in the same direction it is a simple disorder
Examples:
in simple metabolic acidosis both HCO3 and PCO2 decrease
in simple metabolic alkalosis both HCO3 and PCO2 increase
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28. Examine PCO2 and HCO3 relationship (Cont)
The Decoder
Examine PCO2 and HCO3 relationship (Cont)
If PCO2 and HCO3 change in the opposite direction it is a double disorder
Examples:
in double acidosis both HCO3 decrease and PCO2 increases
in double alkalosis both HCO3 increases and PCO2 decreases
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29. Normal pH Double opposing disorder
Fully compensated disorder (rare in chronic respiratory alkalosis)
Normal Acid Base Status
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30. Double opposing disorders
Normal pH (usually)
Grossly abnormal HCO3 and PCO2
Examples :
Mixed Respiratory Acidosis and Metabolic Alkalosis
Mixed Respiratory Alkalosis and Metabolic Acidosis
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31. If pH, PCO2 and HCO3 all are normal
--- Normal Acid Base Status !!
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32. Flenley’s graph
By 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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33. Interpretation of ABG Reports
Look at pH
Step-1
Examine pH and HCO3
Step 2
PCO2 and HCO3
Step 3
Mobile App e.g. Skyscape (or Equations)
Step 4
1. Acidosis, alkalosis, or normal?
primary disturbance respiratory or metabolic?
3. primary respiratory disturbance, is it acute or chronic?
4.If metabolic disturbance, is the respiratory system compensating OK
5. If metabolic acidosis, is there an increased anion gap?
6. an increased anion gap metabolic acidosis, are there other derangements?
Delta Ratio for Triple Disorders
Step 5

34. Complete or Partial If pH < 7.35 or > 7.45
Partial or incomplete compensation
If pH = 7.35 – 7.45
Complete Compensation (only in Chronic Respiratory Alkalosis)

35. Anion Gap (AG) It is a measure of anions other than HCO3 and Chloride
Biochemical Basis:
Always:
CATIONS = ANIONS
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36. AG (Biochemical Basis)
Major Cat-ions:
Na = mmol/L
K = mmol/L
Ca = mmol/L
Mg = mmol/L
TOTAL CATIONS = mmol/L
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37. AG (Biochemical Basis)
Major Anions:
Cl = mmol/L
HCO = mmol/L
proteins = mmol/L
PO = mmol/L
SO = mmol/L
Organic Acids = mmol/L
TOTAL ANIONS = mmol/L
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38. AG =(Na+ K+) - (Cl- + HCO3-) or
=Na+ - (Cl- + HCO3-)
Range : 7 – 18 mmol/l
Cations
Anion Gap = -
It is a measure of anions other than HCO3 and Chloride
Biochemical Basis:
Always:
CATIONS = ANIONS
Anions

39. Anion Gap: It is only a lab derived index
There can Never be an anion gap in any condition because electro-neutrality is always maintained in the plasma
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40. AG corrected for low albumin (AG corr)
Albumin accounts for about 90% of the mean value AG and albumin is reversely correlated with HCO3
When the level of serum albumin decreases, AG will decrease equally
AG is concealed by hypoalbuminemia
HCO3 will increase accordingly (low Alb alkalosis)

41. AG corr Albumin: 40 (35-50) g/L×0.25* = 10 (9-13) mmmol/L
* coefficient for converting Alb g/L into Alb mmol/L.

42. Steps in Diagnosis of a Triple Disorder
Diagnose Double Disorder
Examine AG
Examine [HCO3]
Example:
Double Disorder : Low PCO2+ High [HCO3]
(i.e. Resp Alk + Met Alk)
Triple Disorder : Low PCO2+ High [HCO3] + AG > 16
(i.e. Resp Alk + Met Alk+ High AG Met Acidosis)

43. Diagnosis Triple Disorder
Metabolic Alkalosis
Respiratory Alkalosis
High AG
Metabolic Acidosis

44. Steps in Diagnosis of a Triple Disorder
Diagnose Double Disorder
Examine AG
Examine [HCO3]
Example:
Double Disorder: High PCO2+ [HCO3] higher than the upper limit of compensation
(i.e. Resp Acidosis + Met Alk)
Triple Disorder: High PCO2+ [HCO3] higher than the upper limit of compensation
+ AG > 16
(i.e. Resp Acidosis + Met Alk + High AG Met Acidosis)

45. Diagnosis Triple Disorder
Metabolic Alkalosis
High AG
Metabolic Acidosis
Respiratory Acidosis

46. Other situations of Triple Disorders
Combined High AG and Normal AG Met Acidosis in a patient with Resp Alkalosis
Low Albumin Met Alkalosis with Met Acidosis and Respiratory Alkalosis

47. Important Fact
Respiratory Acidosis and Respiratory Alkalosis never occur together in the same patient !!!

48. Delta Ratio Measured anion gap – Normal anion gap (anion gap – 12)
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-])

49. Delta Ratio Assessment Guideline
< 0.4
Hyperchloraemic normal anion gap acidosis
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

50. Predicted HCO3 If patient's [HCO3] > Predicted [HCO3]
Predicted [HCO3] = 24 -[Anion Gap-12]
If patient's [HCO3] > Predicted [HCO3]
Metabolic acidosis + Metabolic Alkalosis
Compensatory response to resp acidosis

51. If patient's [HCO3] =Predicted [HCO3]
Simple Metabolic acidosis
If patient's [HCO3] < Predicted [HCO3]
Compensatory response to resp alkalosis

52. Example Blood Gases Electrolytes pH 7.58 PCO2 2.1mmHg HCO3 19mmol/L
Na 137 mmol/L
K 5.2 mmol/L
Cl 89 mmol/L
Anion Gap 33 mmol/L
Blood Gases
pH 7.58
PCO mmHg
HCO3 19mmol/L

53. Interpretation of ABG Reports
Look at pH
Step-1
Examine pH and HCO3
Step 2
PCO2 and HCO3
Step 3
Mobile App e.g. Skyscape (or Equations)
Step 4
1. Acidosis, alkalosis, or normal?
primary disturbance respiratory or metabolic?
3. primary respiratory disturbance, is it acute or chronic?
4.If metabolic disturbance, is the respiratory system compensating OK
5. If metabolic acidosis, is there an increased anion gap?
6. an increased anion gap metabolic acidosis, are there other derangements?
Delta Ratio for Triple Disorders
Step 5

54. Thank you