This document provides an overview of arterial blood gas analysis. It discusses how ABG provides information about ventilation, oxygenation, and acid-base balance. The key points covered include indications for ABG, contraindications, sample collection sites and techniques, complications, acid-base physiology including buffer systems and compensation mechanisms, and a stepwise approach to ABG interpretation.

1. ARTERIAL BLOOD GAS ANALYSIS
PRESENTED BY: DR TETIKCHA GURUNG
MODERATED BY : DR RAM RAI

2. INTRODUCTION
 ABG provides rapid information on three physiological processes:
i. Ventilation( reflected by PaCO2)
ii. Oxygenation status( assessed primarily by PaO2 andSaO2
iii. Acid Base Balance
 ABG analysis  essential for diagnosing and managing the patient’s oxygenation status , ventilation
failure and acid base balance.

3. INDICATION
 Assess the ventilatory setting, oxygenation and acid base status.
 Assess the response to an intervention.
 Regulate electrolyte therapy.
 Establish preoperative baseline parameters.

4. CONTRAINDICATION
 An abnormal modified Allen’s test.
 Local infection or distorted anatomy at puncture site.
 Severe peripheral vascular disease of the artery.
 Active Raynaud’s Syndrome

5. SITE
 Radial artery ( most common )
 Brachial artery
 Femoral artery
 Radial is the most preferable site used
because:
i. It is easy to access
ii. It is not a deep artery which facilitate
palpation,
stabilization and puncturing
iii. The artery has a collateral blood
circulation

6. EQUIPMENTS
Blood gas kit OR
 1ml syringe
 23-26 gauge needle
 Stopper or cap
 Alcohol swab
 Disposable gloves
 Plastic bag & crushed
ice
 Lidocaine (optional)
 Vial of heparin
(1:1000)
 Bar code or label

7. METHODOLOGY
PREPARATORY PHASE:
 Record patient inspired oxygen concentration.
 Explain the procedure to the patient.
 Heparinize the needle.
• Donot leave excess heparin in the syringe
• ↑↑ heparin  ↑↑ dilutional effect  ↓↓ HCO3
−
and ↓↓pCO2
 Wait at least 20 minutes before drawing blood for ABG after changing settings of mechanical
ventilation, after suctioning the patient or after extubation.

8.  After preparing the site, the artery is palpated for maximum pulsation
 In case of radial artery , Modified Allen test is done.
 Skin and subcutaneous tissue may be infiltrated with local anesthetic agent if needed
 The needle is inserted at 45 in radial, 60 in brachial and 90 in femoral.
• Ensure no air bubble
Air Bubble has pO2 − 150 mm Hg and pCO2 −0 mmHg
Air Bubble + Blood = ↑↑ pO2 and ↓↓ pCO2
 Place the capped syringe in the container of ice immediately
 Maintain firm pressure on the puncture site for 5 minutes.

9. MODIFIED ALLEN’S TEST
 Test to determine collateral circulation is present from the ulnar artery in case thrombosis occur in the
radial artery.

10.  ABG syringe must be transported at the earliest to the laboratory for early analysis via cold chain
 ABG sample should always be sent with relevant information regarding O2, FiO2 status and Temperature.

11. COMPLICATION
 Arteriospasm
 Infection
 Hematoma
 Hemorrhage
 Distal Ischemia
 Gangrene

12. ACID BASE BALANCE
 Acid base balance is defined by the concentration of hydrogen ion
 The hydrogen ion concentration in aqueous solution is expressed by pH which is defined as negative
logarithm( base 10 ) of [H+
].
pH = log(1/ [H+
]) = -log [H+
].

13.  The acid base equilibrium is described using Henderson hasselbach Equation:

14.  BICARBONATE BUFFER
SYSTEM
 Acts within few seconds
 RESPIRATORY
REGULATION
 Acts within few minutes
 RENAL REGULATION
 Acts in hours to days

15. CHEMICAL BUFFER
 Buffer = base molecule and its weak conjugate acid.
 pKa= dissociation ionization constant  pH at which acid is 50 % dissociated and 50% undissociated.
 pKA indicates strength of the acid
 There are 2 buffer system:
i. Extracellular buffer system
ii. Intracellular buffer system

16. EXTRACELLULAR BUFFER SYSTEM
 It includes : Bicarbonate buffer system(pKa=6.1) and Phosphate buffer system(pKa=6.8).
1. Bicarbonate Buffer System(H2CO3/HCO3
−
)
The base = bicarbonate and its weak acid conjugate= carbonic acid
CO2 + H2O carbonic anhydrase H2CO3 H+ + HCO3
-

17. INTRACELLULAR BUFFER
 It includes :
i. Hemoglobin buffer(HbH/Hb)
ii. Other protein buffer(PrH/Pr−)
iii. Phosphate buffer(H2PO4 −/HPO4 2−),

18. HEMOGLOBIN BUFFER SYSTEM

19. RESPIRATORY REGULATION

21. RENAL REGULATION
 Occurs via 3 mechanism:
i. reabsorption of the filtered HCO3
−
ii. excretion of titratable acids,
iii. production of ammonia

23. DEFINITIONS
 ACID: molecule that can act as a proton (H+) donor
 BASE: molecule that can act as a proton acceptor.
 ACIDEMIA:A blood pH less than 7.35
 ALKALKEMIA : a blood pH greater than 7.45
 ACIDOSIS – presence of a process which tends to  pH by virtue of gain of H + or loss of HCO3
-
 ALKALOSIS – presence of a process which tends to  pH by virtue of loss of H+ or gain of HCO3
-

24.  Simple Acid Base Disorder/ Primary Acid Base disorder – a single primary process of acidosis or
alkalosis due to an initial change in PCO2 and HCO3.
 Compensation - The normal response of the respiratory system or kidneys to change in pH induced by
a primary acid-base disorder
The Compensatory responses to a primary Acid Base disturbance are never enough to correct the
change in pH they only act to reduce the severity.
 Mixed Acid Base Disorder – Presence of more than one acid base disorder simultaneously .

25.  The H+ In extracellular fluid is determined by balance between the pCO2 and HCO3
- in the fluid.
 This relationship is expressed as
H+ = 24 x (pCO2/ HCO3 )

26. STEPWISE APPROACH TO ACID BASE ANALYSIS

27. ANALYTE Normal Value Units
pH 7.35 - 7.45
PCO2 35 - 45 mm Hg
PO2 72 – 104 mm Hg`
[HCO3] 22 – 30 meq/L
SaO2 95-100 %
Anion Gap 9 + 3 meq/L
B.E +2 to -2 meq/L

28.  STEP 1: Check for authenticity
 STEP 2: : Identify the primary Acid Base disorder
 STEP 3: Evaluate the Secondary Response
 STEP 4: Calculate Anion Gap

29. STEP 1 : CHECH FOR AUTHENTICITY
 [H+] neq/l = 24 X (PCO2 / HCO3)
Calculate it from the ABG report and if this value
is equal to H+ in the report,the ABG report is
authentic.
 Alternatively subtract the last two digits of the
pH(e.g 20 in Ph 7.20) from 80, this value is
approximately equal to the H+ concentration in
the ABG report.
 𝐻𝐶𝑂3
−
= 24 x
𝑝𝐶𝑂2
𝐻+ = ± 2 of 𝐻𝐶𝑂3
−
of venous
blood ; ifnot then the ABG is invalid and not
compatible
H+ ion pH
100 7.00
79 7.10
63 7.20
50 7.30
45 7.35
40 7.40
35 7.45
32 7.50
25 7.60

30. STEP 2 : IDENTIFY THE PRIMARY ACID BASE DISORDER.
 RULE 1 : If the 𝑃𝑎𝐶𝑂2 and /or pH is outside the normal range acid base disorder
 RULE 2: if the 𝑃𝑎𝐶𝑂2 and pH are both abnormal, compare the directional change
 2a: if 𝑡ℎ𝑒 ↑ 𝑃𝑎𝐶𝑂2 and ↑pH or ↓ 𝑃𝑎𝐶𝑂2 and ↓pH  primary metabolic acid base disorder
 2b : if 𝑡ℎ𝑒 ↑ 𝑃𝑎𝐶𝑂2 and↓pH or ↑ pH and ↓ 𝑃𝑎𝐶𝑂2 primary respiratory acid base disorder

31.  RULE 3: if the 𝑃𝑎𝐶𝑂2 or pH is abnormal, the condition is a mixed metabolic and respiratory disorder.
 3a: if 𝑃𝑎𝐶𝑂2 is abnormal, directional change in 𝑃𝑎𝐶𝑂2  type of respiratory disorder
 3b: if pH is abnormal , the directional change in pH metabolic disorder.

32. STEP 3 : EVALUATE THE SECONDARY RESPONSE

33.  RULE 4: For a primary metabolic acidosis , if
measured 𝑃𝑎𝐶𝑂2 is higher than expected secondary respiratory acidosis and
measured 𝑃𝑎𝐶𝑂2 is less than expected  secondary respiratory alkalosis.

34.  RULE 5: For a primary respiratory disorder,
a normal or near normal 𝐻𝐶𝑂3
−
 acute
 RULE 6: For a primary respiratory disorder where the 𝐻𝐶𝑂3
−
is abnormal , determine the expected 𝐻𝐶𝑂3
−
for a
chronic respiratory disorder
 6a : For a chronic respiratory acidosis, if
𝐻𝐶𝑂3
−
is lower than expected  incomplete renal response
𝐻𝐶𝑂3
−
is higher than expected secondary metabolic alkalosis
 6b : For a chronic respiratory alkalosis, if
𝐻𝐶𝑂3
−
is higher than expected incomplete renal response
𝐻𝐶𝑂3
−
is lower than expected  secondary metabolic alkalosis

36. ANION GAP
 Normally, measured cation(MC) + unmeasured cation(UC) = measured anion(MA) + unmeasured
anion(UA).
 MC – MA = UA-UC
 Measured cation = 𝑁𝑎+
and Measured Anion = 𝐶𝑙−
and 𝐻𝐶𝑂3
−
 𝑁𝑎+ - (𝐶𝑙− + 𝐻𝐶𝑂3
−
) = UA –UC
 UA - UC = Anion Gap(AG)= 8-12mEq/L
 Corrected Anion Gap= Anion Gap + 2.5(4.5- albumin of patient)
 1g/dL of Albumin contribute to 3 mEq/L of Anion Gap

37. CALCULATE ANION GAP
 𝑁𝑎+ - (𝐶𝑙− + 𝐻𝐶𝑂3
−
) = UA –UC; 𝑁𝑎+ = 140 and 𝐶𝑙− = 106
 UA - UC = Anion Gap(AG)= 8-12mEq/L
 In case of HAGMA, calculate Delta Ratio= 1-2
Delta Ratio =
∆𝐴𝐺
∆𝐻𝐶𝑂3
−
=
𝐴𝐺−12
24−𝐻𝐶𝑂3
−
 When Delta Ratio < 1 ; ∆ 𝐻𝐶𝑂3
−
increased disproportionately  HAGMA+NAGMA
 When Delta Ratio >2 ; ∆ 𝐻𝐶𝑂3
−
decreased disproportionately  HAGMA+ metabolic alkalosis
 When Delta Ratio 1-2  HAGMA

38.  If a patient has normal anion gap , cause may be
i. RTA
ii. GI loss of bicarbonate
 History to be noted
 If no history + , check for Urine Anion Gap
Urine Anion Gap( UAG) = Urine 𝑁𝑎+
+(Urine𝐾+
− 𝐶𝑙−
)
 In RTA UAG more positive
 In GI Loss of 𝐻𝐶𝑂3
−
 UAG negative.