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.
Assess the ventilatory setting, oxygenation and acid base status.
Assess the response to an intervention.
Regulate electrolyte therapy.
Establish preoperative baseline parameters.
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
Radial artery ( most common )
Radial is the most preferable site used
i. It is easy to access
ii. It is not a deep artery which facilitate
stabilization and puncturing
iii. The artery has a collateral blood
Blood gas kit OR
23-26 gauge needle
Stopper or cap
Plastic bag & crushed
Vial of heparin
Bar code or label
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
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
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.
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
Acts within few seconds
Acts within few minutes
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−),
21. RENAL REGULATION
Occurs via 3 mechanism:
i. reabsorption of the filtered HCO3
ii. excretion of titratable acids,
iii. production of ammonia
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 )
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
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.
= 24 x
𝐻+ = ± 2 of 𝐻𝐶𝑂3
blood ; ifnot then the ABG is invalid and not
H+ ion pH
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.
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
RULE 6: For a primary respiratory disorder where the 𝐻𝐶𝑂3
is abnormal , determine the expected 𝐻𝐶𝑂3
chronic respiratory disorder
6a : For a chronic respiratory acidosis, if
is lower than expected incomplete renal response
is higher than expected secondary metabolic alkalosis
6b : For a chronic respiratory alkalosis, if
is higher than expected incomplete renal response
is lower than expected secondary metabolic alkalosis
36. ANION GAP
Normally, measured cation(MC) + unmeasured cation(UC) = measured anion(MA) + unmeasured
MC – MA = UA-UC
Measured cation = 𝑁𝑎+
and Measured Anion = 𝐶𝑙−
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 =
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
ii. GI loss of bicarbonate
History to be noted
If no history + , check for Urine Anion Gap
Urine Anion Gap( UAG) = Urine 𝑁𝑎+
In RTA UAG more positive
In GI Loss of 𝐻𝐶𝑂3