ABG Interpretation
DR ANKIT GAJJAR
MBBS, MD, IDCCM, IFCCM, EDIC
CONSLUTANT INTENSIVIST
Collecting ABG
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Radial is ideal
Preheparinised syringe
Flush syringe with 0.5 ml heparin and empty
Heparin not emptied adequately
- Low HCO3
- Low PCO2
• 50% of the syringe should be filled by blood
• Avoid contact with air
– Increase PO2 if PO2 <150
– Decrease in PCO2
• Marked elevation in WBC
– Decrease in PO2
• Maintain cold chain
Know your machine
• Measured variables
– pH
– PCO2
– PO2
• Calculated variables
– HCO3
- Lactate
- SpO2
- PaO2
- Electrolytes
What does ABG tells us
• Acid Base disorder
– Metabolic acidosis
– Metabolic alkalosis
• Blood gas abnormalities
– Respiratory acidosis
– Respiratory alkalosis
– Hypoxia
– Carbon monoxide poisoning
– Methemoglobinaemia
In the end we will be experts
The Blood Gas Report:
The essentials
• pH
7.4 (7.35-7.45)
• PCO2
40 (35-45)
• PO2
80 (80-100)
• HCO3
24 (24-28)
STEP 1 - Is your ABG valid?
• Henderson-Hasselbalch equation
• [H+]= 24 X pCO2/HCO3
• 80 – (H+) = Decimals after 7
Ph
7.40
PCO2
40
PO2
88
HCO3
24
Na
137
Cl
102
ACID BASE DISORDER
• Step 1 - pH
• Step 2 – Metabolic / Respiratory
• Step 3 – Respiratory acute v/s Chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 Respiratory compensation
– Rule of 1,2,4,5
• Step 5 Metabolic compensation
– Expected pCO2 = (1.5 x HCO3)+8
– Expected pCO2 = (0.7x HCO3)+21
• Step 6 Anion gap
• Step 7 Delta ratio
STEP – 1 Acidemic or Alkalemic
• pH < 7.4 - Acidosis
• pH > 7.4 - Alkalosis
STEP 2 – METABOLIC / RESPIRATORY
Metabolic Acidosis
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Increase acid production
Decrease acid excretion
Increase alkali excretion
In ABG
– pH < 7.40
– HCO3 < 24
Anion gap
• Serum AG = Measured cations - measured
anions = Unmeasured anions - unmeasured
cations
• Serum AG = Na - (Cl + HCO3)
• Reference range is 8 to 16 mmol/l (an average
of 12 mmol/l)
• AG falls by 2.5 meq/L for every 1 g/dL
reduction in the serum albumin concentration
pH
7.05
PCO2
31
PO2
88
HCO3
10
Na
137
Cl
102
Increase anion gap
• Increase unmeasured anions (MUD-PILES)
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Methanol
Uremia
DKA
Paraldehyde
Isoniazid
Lactic Acidosis (shock, sepsis, metformin, linezolid, drugs)
Ethylene Glycol
Salicylates
• Decrease unmeasured cations
– Ca, Mg, K
Normal anion gap acidosis
• Loss of bicarb (HARD UP)
– Hyperchloremic acidosis (Excessive normal saline)
– Acetazolamide
– RTA
– Diarrhea
– Ureterosigmoidostomy / colonic fistula
– Post hypocapnia
Delta gap
• AG – Normal AG = Δ Gap
• Δ gap SHOULD BE = Δ Bicarbonate
• Δ gap/ Δ Bicarbonate = Delta Ratio
• Δ ratio < 1 – Another metabolic acidemia
• Δ ratio > 1 – Another metabolic alkalemia
Look for compensation
• Expected pCO2 = (1.5 x HCO3) + 8 ± 2
• If CO2 is higher then expected concomitant
respiratory acidosis
• If CO2 is lower then expected concomitant
respiratory alkalosis
Ph
7.05 7.43
PCO2
32
16
PO2
88
88
HCO3 10
10
Na
137 137
Cl
102 102
Management of metabolic acidosis
• Identify underlying etiology
• Treat according to underlying etiology
• HCO3
– HCO3 losing pathology
– severe acidosis
• If inadequate respiratory compensation
INTUBATION AND HYPERVENTILATION
Metabolic alkalosis
• Ph > 7.40
• HCO3 > 26
Look for compensation
• Expected pCO2 = (0.7 x HCO3) + 21 ± 2
• PCO2 higher then expected associated
respiratory acidosis
• PCO2 lower then expected respiratory alkalosis
Ph
7.58
PCO2
50
PO2
88
HCO3
44
Na
145
Cl
88
Causes
• Chloride responsive (U Chloride < 20)
– Vomiting / Gastric suction
– Prior use of diuretics
– Posthypercapnia
• Chloride resistant (U Chloride > 20)
– Severe hypokalemia
– Hyperaldosteronism
– Active use of diuretics
– Milk alkali syndrome
Treatment of metabolic alkalosis
• Normal saline
• Potassium replacement
• Acetazolamide
Respiratory acidosis
• pH < 7.35
• PCO2 > 45
Acute v/s Chronic
• Expect change in pH = 0.008 x PCO2
• If pH is higher then expected pH it is chronic
Ph
7.31
PCO2
76
PO2
68
HCO3
38
Na
137
Cl
94
Compensation (Rule of 1,4,2,5)
• Increase in HCO3
• <24 hrs: Expected [HCO3] = 24+[1/10x (PCO2-40)]
10 mmhg increase in PaCO2 increase in HCO3
by 1 mmol/L
• >24 hrs: Expected [HCO3] = 24+[4/10x (PCO2-40)]
10 mmhg increase in PaCO2 increase in HCO3
by 4 mmol/L
Ph
7.31
PCO2
76
PO2
68
HCO3
38
Na
137
Cl
94
Respiratory alkalosis
• Ph > 7.45
• PCO2 > 45
Acute v/s Chronic
• Expect change in pH = 0.008 x PCO2
• If pH is lower then expected pH it is chronic
Compensation (Rule of 1,4,2,5)
• Decrease in HCO3
• <24 hrs: Expected [HCO3] = 24 – [2/10x (40-PCO2)]
10 mmhg decrease in PaCO2 decrease in HCO3 by
2 mmol/L
• >24 hours:Expected [HCO3] = 24 –[5/10 x (40-PCO2)]
10 mmhg decrease in PaCO2 decrease in HCO3 by
5 mmol/L
Ph
7.52
PCO2
30
PO2
66
HCO3
22
Na
130
Cl
100
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
ARE WE READY FOR THE CHALLENGE
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.39
PCO2 93.8
PO2
69.6
HCO3 55.7
Na
145
Cl
86
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.18
PCO2 16
PO2
114
HCO3 6
Na
135
Cl
95
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.25
PCO2 80
PO2
46
HCO3 20
Na
134
Cl
104
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.23
PCO2 23
PO2
64
HCO3 14
Na
130
Cl
94
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.14
PCO2 59.1
PO2
66.3
HCO3 18
Na
138
Cl
102
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.23
PCO2 23
PO2
110
HCO3 14
Na
130
Cl
100
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.26
PCO2 18
PO2
128
HCO3 7
Na
136
Cl
113
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.31
PCO2 33
PO2
88
HCO3 16
Na
134
Cl
113
• Step 1 - pH
• Step 2 – metabolic/respiratory
• Step 3 – resp acute v/s chronic
– change in pH = 0.08 x PCO2 / 10
• Step 4 resp compensation
– Rule of 1,2,4,5
• Step 5 metabolic compensation
– Expected pCO2 = (1.5 x HCO3) + 8
– Expected pCO2 = (0.7x HCO3) + 21
• Step 6 Anion gap
• Step 7 delta ratio
Ph
7.64
PCO2 32
PO2
75
HCO3 33
Na
-
Cl
-
One more gap – Saturation gap
• ABG machiene gives SaO2 from PaO2 – SaO2
chart
• In case of Methemoglobinemia ABG will give
SaO2 higher then SpO2
• ABG machine with co oxymetry will give
measured
– SaO2
– Meth HB
– Carboxy Hb
THANK YOU