1. Blood Gas Analysis – The Basics
Dr Satheesh K Kutty MD. DCH. MRCPCH. Consultant
PICU AL JAHRA

2. OBJECTIVES Upon completion of this session our audience will :
Know the common terminologies used in acid base disturbances
Know the normal ranges of arterial blood gas values
Have a systematic approach towards ABG interpretation
Identify the primary cause and assess the compensatory response
Provide an insight into the oxygenation status of the patient

3. Introduction
A normal pH is a necessity for the optimal functioning of cellular enzymes and the metabolic activities.
It can be potentially life threatening if there is an imbalance in the acid base status due to acidosis or alkalosis.
ABG/BGA provides 4 key aspects of information namely pH, PaCO2, HCO3, and PO2.
Blood gas analysis combined with noninvasive monitoring, aids in the assessment and management of sick patients and provides an insight into their ventilation, oxygenation and metabolic status.

4. Why should we do an ABG? Aids in establishing a diagnosis
Helps guide the treatment plan
Aids in ventilator management
Improvement in acid/base management allows for optimum function of medications
Acid/base status may alter electrolyte levels critical to patient status/care.

5. Components of BGA Blood Gas pH, HCO3 BE Acid Base balance PaCO2
Ventilation
PaO2, SaO2
Oxygenation

6. Components of BGA(contd.)
In modern BGA machines we also get the levels of :
K Na Cl Ca Lactate Hb Hct Glucose MethHb COHb etc.

7. Normal BGA values Parameter Normal values pH 7.35 to 7.45 PaCO2
4.7— 6 KPa (35 to 45 mm Hg)
HCO3
22 to 26 mmol/L
B.E
-2 to +2 mEq /L
PaO2
10—13KPa (80 to 100 mm Hg)
SaO2
95% to 100%

8. Reference ranges for arterial blood gasespH
PaO2
PaCO2
HCO3ˉ
Base excess
7.35 – 7.45
80 – 100* mmHg
35 – 45 mmHg
22 – 26 mmol/L
–2 – +2 mmol/L
10.6 – 13.3 kPa
4.7 – 6.0 kPa
Reference ranges for venous blood gases
PvO2
PvCO2
7.32 – 7.43
25 – 40 mmHg
41 – 50 mmHg
23 – 27 mmol/L
* age and altitude dependent (see text) Kilopascals: to convert pressures to kPa, divide mmHg by 7.5

9. Common Terminologies
Buffer system functions to keep pH in normal range. (H2CO3 buffer)
pH (is the negative of the logarithm to base 10 of) the concentration of hydrogen ions or H+(measured in units of moles per litre).
Hypoxemia : Less PaO2 (< 10 KPa or <80 mm Hg)
Hypercarbia : High PaCO2 (> 6KPa or > 45 mm Hg)
Hypocarbia (Hypocapnia) : Low PaCO2 (<4.6 KPa or <35 mm Hg)

10. Common Terminologies pH Respiratory 6.0 4.5 Respiratory PaCO2
ACIDOSIS NORMAL ALKALOSIS pH
Respiratory Respiratory
PaCO2
Metabolic Metabolic
HCO3

11. Homeostasis
In our body the pH of the blood is maintained by several buffering systems, the main one being the Carbonic acid Bicarbonate system.
H2O + CO2 ↔ H2CO3 ↔H+ + HCO3
Carbonic acid links the respiratory and metabolic systems.
Any change in this equilibrium is rectified following Le Chatelier’s principle.
When any system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to (partially) counteract the effect of the applied change and a new equilibrium is established.

12. Major homeostatic mechanisms
I- BUFFERS
1st line of defense
Includes:
Extracellular buffers-HCO3, Ammonia etc.
Intracellular- protein
Acting within seconds
Action : Remove or release H+
II- Lungs
2nd line of defense
Acting with minutes to hrs
Action: Elimination or retention of CO2
III- Kidney
3rd line of defense
Delayed, within hours to days
Action: Retention of HCO3,Reduction of fixed acids and elimination of H+
 Extracellular buffers include bicarbonate and ammonia, whereas proteins and phosphateact as intracellular buffers;

13. Compensation Acid base Disorder Mechanism of compensation
Metabolic acidosis
Increase minute ventilation
Metabolic alkalosis
Decrease minute ventilation
Respiratory acidosis
Increasing reabsorbtion of bicarbonate and increasing excretion of H+ by the kidneys
Respiratory alkalosis
Decreasing reabsorption of HCO3 and excretion of H+ ions by the kidneys.

14. STEPS OF
ABG INTERPRETATION

15. Normal BGA values Parameter Normal values pH 7.35 to 7.45 pCO2
4.7— 6 KPa (35 to 45 mm Hg)
HCO3
22 to 26 mmol/L
B.E
-2 to +2 mEq /L
PO2
10—13KPa (80 to 100 mm Hg)
SaO2
95% to 100%

16. If Respiratory – ACUTE or CHRONIC?
STEP 1
Is this ABG Authentic?
STEP 2
Look at the pH
STEP 3
Look at the PaCO2
STEP 4
If Respiratory – ACUTE or CHRONIC?
STEP 5
Look at the HCO3
STEP 6
If METABOLIC – ANION GAP?
STEP 7
Is COMPENSATION adequate?
STEP 8
If High gap Metabolic Acidosis – “The Delta Ratio (∆/∆)”
STEP 9
Oxygenation status

17. Before we start… Remember following,
CO2
Is a Respiratory acid
pH and HCO3
Moves in same direction
pH and PCO2
Moves in opposite direction
If HCO3 and PCO2 are moving in same direction
Simple disorder
If HCO3 and PCO2 are moving in opposite direction
Mixed disorder
Remember following,
If pH is altered it is uncompensated disorder.
When pH is normal it is difficult to distinguish primary change from compensatory change

18. Approximate [H+] (mmol/L)pH
Approximate [H+] (mmol/L)
7.00
100
7.05 89
7.10 79
7.15 71
7.20 63
7.25 56
7.30 50
7.35 45
7.40 40
7.45 35
7.50 32
7.55 28
7.60 25
7.65 22
STEP 1
Assess the internal consistency of the values using the Henderseon-Hasselbach equation:
[H+] = 24(PaCO2)            [HCO3-]
If the pH and the [H+] are inconsistent, the ABG is probably not valid.

19. STEP 2: Look at the pH Is there alkalemia or acidemia present?
pH=(  )           Normal or mixed disorder
pH<7.35              Acidosis
pH>7.45            Alkalosis

20. STEP 3: Look at the PaCO2 Keeping step I in mind look at PaCO2
pH<7.35
PaCO2>45mm Hg
Primary Respiratory acidosis
pH>7.45
PaCO2<35mmHg
Primary Respiratory Alkalosis

21. STEP 4A : If Respiratory – ACUTE or CHRONIC?
Acute respiratory acidosis
PaCO2 >45 mm Hg
(i.e.> 6KPa) with an accompanying acidemia (i.e. pH < 7.35).
Chronic respiratory acidosis
PaCO2  >45 mm Hg
(i.e.> 6KPa) with normal or near-normal pH secondary to renal compensation and elevated serum bicarbonate levels (i.e. >30 mEq/L).

22. STEP 4B : Look at direction of change in the pH & PaCO2
In primary respiratory disorders, the pH and PaCO2 change in opposite directions
In metabolic disorders the pH and PaCO2 change in the same direction.

23. Acid base MNEMONIC MD- Metabolic Direct
Metabolic Acidosis
↓P H
↓PaCO2
Metabolic Alkalosis
↑P H
↑PaCO2
Respiratory Acidosis
Respiratory Alkalosis
saME direction= MEtabolic
REverse direction=REspiratory

24. STEP 5: Look at the HCO3 pH<7.35 HCO3<22mEq/L
Primary metabolic acidosis
pH>7.45
HCO3>26mEq/L
Primary metabolic alkalosis

25. STEP 6 : Look at the anion Gap
If metabolic acidosis, then look at the Anion Gap
ANION GAP(AG) is calculated as : S. Na – (Cl + HCO3)
Normal Anion Gap : mEq/L
If elevated (> than 16), then acidosis due to MUDPILES
If anion gap is normal, then acidosis likely due to diarrhea, RTA.

26. STEP 7 : Is the compensatory response adequate or not?

27. Compensation formulas
Disorder
Expected compensation
Correction factor
Metabolic acidosis
PaCO2 = (1.5 x [HCO3-]) +8
± 2
Acute respiratory acidosis
Increase  in  [HCO3-]= ∆ PaCO2/10
± 3
Chronic respiratory acidosis (3-5 days)
Increase  in  [HCO3-]= 3.5(∆ PaCO2/10)
Metabolic alkalosis
Increase in PaCO2 = (∆HCO3-)
Acute respiratory alkalosis
Decrease in  [HCO3-]= 2(∆ PaCO2/10)
Chronic respiratory alkalosis
Decrease in  [HCO3-] = 5(∆ PaCO2/10) to 7(∆ PaCO2/10)
NOTE: If the observed compensation is not the expected compensation, it is likely that more than one acid-base disorder is present.

28. Alternative to Calculating Compensation for Respiratory Disorders
Acute
Chronic
Respiratory acidosis
[HCO3- ] increases 1 mEq/L for each 10 mmHg (1.33 kpa) PaCO2 is above 40 mm Hg (5.33kpa)
[HCO3- ] increases 4 mEq/L for each 10 mmHg(1.33 kpa) PaCO2 is above 40 mm Hg (5.33kpa)
Respiratory alkalosis
[HCO3- ] decreases 2 mEq/L for each 10 mmHg(1.33 kpa) PaCO2 is below 40 mm Hg(5.33kpa)
[HCO3- ] decreases 5 mEq/L for each 10 mmHg(1.33 kpa) PaCO2 is below 40 mm Hg(5.33kpa)

29. Alternative to Calculating Compensation for Metabolic Disorders
In a metabolic disorder with appropriate compensation, the PaCO2 (in mmHg) is approximately the same as the 1st 2 digits of the pH after the decimal point.
For example :
7.27 25 11 pH
PaCO2
HCO3

30. Step 8A: Assess the relationship between increase in anion gap and decrease in [HCO3-].
Assess the ratio of the change in the anion gap (∆AG ) to the change in  [HCO3-] (∆[HCO3-]): ∆AG/∆[HCO3-]
This ratio should be between 1.0 and 2.0 if an uncomplicated anion gap metabolic acidosis is present.
If this ratio falls outside of this range, then another metabolic disorder is present:
If  ∆AG/∆[HCO3-] < 1.0, then a concurrent non-anion gap metabolic acidosis is likely to be present.
If  ∆AG/∆[HCO3-] > 2.0, then a concurrent metabolic alkalosis is likely to be present. 
It is important to remember what the expected “normal” anion gap for your patient should be, by adjusting for hypoalbuminemia

31. STEP 8: CO EXISTANT METABOLIC DISORDER – “The Delta Ratio (∆/∆)”
In case of High AG metabolic acidosis, another disorder?
Ideally, ∆Anion Gap = ∆HCO3
For each 1 meq/L increase in AG, HCO3 will fall by 1 meq/L
∆ Anion Gap = Measured AG – Normal AG (i.e. Measured AG – 12)
∆ HCO3 = Normal HCO3 – Measured HCO3 (i.e. 24 – Measured HCO3)
 Delta ratio = ∆ Anion gap/∆ [HCO3-] or ↑anion gap/ ↓ [HCO3-] 
            = (AG – 12)
( 24 - [HCO3-] )

32. ∆AG/ ∆HCO3- = 1  Pure High AG Metabolic Acidosis
∆ AG/ ∆ HCO3- > 1 Associated Metabolic Alkalosis
∆ AG/ ∆HCO3- < 1  Associated Normal AG Metabolic Acidosis

33. STEP 9 : Oxygenation status
PaO2 in a BGA gives us 2 basic information:
Insight into the cause of hypoxia (if PaO2 < 10).
Assessment of adequacy of gas exchange.

34. Oxygenation status(contd.)
Calculating alveolar-arterial oxygen difference PAO2 – PaO2 can help us differentiating the causes.
PAO2 = (pB - 47 ) FiO2 - PaCO2 / Respiratory quotient
Normal PAO2 – PaO2 = mmHg.
Hypoxia with normal difference is due to hypoventilation.
Hypoxia with elevated difference is due to shunt or to ventilation /perfusion mismatch such as pneumonia

35. Assessing adequacy of gas exchange:
PO2/FiO2 ratio:
> = normal
= Acute lung injury
< = ARDS

36. Tic Tac Toe Method Arterial Blood Gas (ABG) Interpretation
ABG problems can be solved work using the tic-tac-toe method. All you have to do is make a blank chart similar to this:

37. 6. Mark the Chart
Using the lab result values, mark them on your tic-tac-toe. Let’s begin with this sample problem:
Eg. 1: pH: 7.26, paCO2: 32, HCO3: 18
Using the normal values reference chart in the first step, determine where the values should be under in the tic-tac-toe. In the given example :
pH of 7.26 is LOW = ACID so place pH under Acid paCO2 of 32 is LOW = BASE so place paCO2 under Base HCO3 of 18 is LOW = ACID so place HCO3 under Acid
ACID
BASE

38. In this step, determine at which column matches up with the pH.
 Match it up
In this step, determine at which column matches up with the pH.
In the given example, HCO3goes with pH.
HCO3 is considered Metabolic (shown in step 3), and both are under Acid, so this example implies Metabolic Acidosis.
ACID
BASE

39. Determine compensation
 The last step is to determine if the ABG is Compensated, Partially Compensated, or Uncompensated. Here’s the trick:
If pH is NORMAL, PaCO2 and HCO3 are both ABNORMAL = Compensated
If pH is ABNORMAL, PaCO2 and HCO3 are both ABNORMAL = Partially Compensated
If pH is ABNORMAL, PaCO2 or HCO3 is ABNORMAL = Uncompensated
Therefore this ABG is METABOLIC ACIDOSIS, PARTIALLY COMPENSATED .

40. By applying the steps above, interpret the following ABGs:
Eg. 2:
By applying the steps above, interpret the following ABGs:
pH:7.44, PaCO2: 30, HCO3: 21
pH is NORMAL = NORMAL so place pH under Normal PaCO2 is LOW = BASE so place PaCO2 under Base HCO3 is LOW = ACID so place HCO3 under Acid
*Since the acidity of the blood is determined by the value of the pH, determine whether the normal pH is SLIGHTLY ACIDIC or SLIGHTLY BASIC. In this example, pH is NORMAL but SLIGHTLY BASIC therefore it is ALKALOSIS.
In this case PaCO2 goes with pH. PaCO2 is considered Respiratory (shown in step 3), and both are under Basic, so this example implies Respiratory Alkalosis. The HCO3 is also abnormal. When pH is NORMAL and PaCO2 and HCO3 are both ABNORMAL, it indicates FULL COMPENSATION.

41. Therefore this ABG is RESPIRATORY ALKALOSIS, FULLY COMPENSATED.

42. Eg.3 Ph PaCO2 10 KPa HCO3 40
Acid
Normal
Base
CO2 pH
HCO3
Fully compensated respiratory acidosis

43. Eg.4 Ph PaCO KPa HCO3 13
Acid
Normal
Base
HCO3 pH
CO2
Fully compensated respiratory alkalosis

44. Eg.5 Ph PaCO2 6.5KPa HCO3 31
Acid
Normal
Base
CO2 pH
HCO3
Fully compensated metabolic alkalosis

45. Eg.6 Ph 7.36 PaCO2 4 KPa HCO3 15 Fully compensated metabolic acidosis
Normal
Base
HCO3 pH
CO2
Fully compensated metabolic acidosis

46. Eg.7 Ph PaCO KPa HCO3 21
Acid
Normal
Base
HCO3 pH
CO2
Partially compensated Respiratory alkalosis

47. Eg.8 Ph PaCO KPa HCO3 21
Acid
Normal
Base pH
HCO3
CO2
Partially compensated metabolic acidosis

48. Eg.9 Ph 7.54 PaCO2 3.2 HCO3 25 Uncompensated Respiratory alkalosis
Acid
Normal
Base
HCO3 pH
CO2
Uncompensated Respiratory alkalosis

49. Thank You