Simple Rules for the
Interpretation of Arterial
Blood Gases
Nicholas Sadovnikoff, MD, FCCM
Assistant Professor, Harvard Medical School
Co-Director, Surgical Intensive Care Units
Brigham and Women’s Hospital
Boston, MA
Kuwait City, Kuwait
November 24, 2011
“Mantra”
1 for 1
10 for 7
1
4
2
5
Mantra
Metabolic Acidosis: 1HCO3 => 1PCO2
Metabolic Alkalosis: 10  HCO3 => 7 PCO2
Acute Resp Acid: 10  PCO2 = 1  HCO3
Chronic Resp Acid: 10  PCO2 = 4  HCO3
Acute Resp Alk: 10  PCO2 => 2  HCO3
Chronic Resp Alk: 10  PCO2 => 5  HCO3
Examples
pH = 7.33, PCO2=30, HCO3=15
diagnosis
Examples
pH = 7.33, PCO2=30, HCO3=15
Diagnosis:
metabolic acidosis or
Metabolic acidosis with respiratory compensation
or
metabolic acidosis with compensatory respiratory
alkalosis
Suggestion:
Delete the word
compensation from
your acid base
vocabulary.
Mantra
Metabolic Acidosis: 1HCO3 => 1PCO2
Metabolic Alkalosis: 10  HCO3 => 7 PCO2
Acute Resp Acid: 10  PCO2 = 1  HCO3
Chronic Resp Acid: 10  PCO2 = 4  HCO3
Acute Resp Alk: 10  PCO2 => 2  HCO3
Chronic Resp Alk: 10  PCO2 => 5  HCO3
Definition
Acidemia
Acidosis
Definition
Acidemia – an abnormally low pH
(high proton concentration)
Definition
Acidemia – an abnormally low pH
(high proton concentration)
Acidosis – a process which
produces an excess of protons
Example
• pH=7.22, PCO2=38, HCO3=15
• diagnosis
Example
• pH=7.22, PCO2=38, HCO3=15
• diagnosis:
Met and Resp acidosis
or
Undercompensated Met Acidosis
Example
• pH=7.38, PCO2=28, HCO3=16
• diagnosis
Example
• pH=7.38, PCO2=28, HCO3=16
• diagnosis:
Met Acid with Resp Alk
or
Overcompensated Met Acid
Example
• pH=7.47, PCO2=47, HCO3=34
• diagnosis
Example
• pH=7.47, PCO2=47, HCO3=34
• diagnosis
metabolic alkalosis
Mantra
Metabolic Acidosis: 1HCO3 => 1PCO2
Metabolic Alkalosis: 10  HCO3 => 7 PCO2
Acute Resp Acid: 10  PCO2 = 1  HCO3
Chronic Resp Acid: 10  PCO2 = 4  HCO3
Acute Resp Alk: 10  PCO2 => 2  HCO3
Chronic Resp Alk: 10  PCO2 => 5  HCO3
Example
• pH=7.51, PCO2=42, HCO3=34
• diagnosis
Example
• pH=7.51, PCO2=42, HCO3=34
• diagnosis
Met and Resp alkalosis
Example
• pH=7.42, PCO2=52, HCO3=34
• diagnosis
Example
• pH=7.42, PCO2=52, HCO3=34
• diagnosis
metabolic alkalosis and
respiratory acidosis
Example
pH=7.35, PCO2=60, HCO3=32
diagnosis
Example
pH=7.35, PCO2=60, HCO3=32
diagnosis
chronic respiratory acidosis
Example
pH=7.25, PCO2=60, HCO3=26
diagnosis
Example
pH=7.25, PCO2=60, HCO3=26
diagnosis
acute respiratory acidosis
Example
pH=7.30, PCO2=60, HCO3=29
diagnosis
Example
pH=7.30, PCO2=60, HCO3=29
diagnosis
acute on chronic resp acid
Example
pH=7.45, PCO2=25, HCO3=17
diagnosis
Example
pH=7.45, PCO2=25, HCO3=17
diagnosis
chronic respiratory alkalosis
Mantra
Metabolic Acidosis: 1HCO3 => 1PCO2
Metabolic Alkalosis: 10  HCO3 => 7 PCO2
Acute Resp Acid: 10  PCO2 = 1  HCO3
Chronic Resp Acid: 10  PCO2 = 4  HCO3
Acute Resp Alk: 10  PCO2 => 2  HCO3
Chronic Resp Alk: 10  PCO2 => 5  HCO3
Example
pH=7.33, PCO2=30, HCO3=15
Na= 147, Cl=110
diagnosis
What is an Anion Gap?
• Qualitative
• Quantitative
• Why not include H+ ?
K+ ?
What is an Anion Gap?
• Qualitative – unmeasured anions
• Quantitative - Na+ - Cl– - HCO3–
• Why not include H+ - too small
K+ - always ~ 4
DDx of AG Met Acid
•
•
•
•
•
•
•
•
Ketones
Uremia
Salicylates
Methanol
Alcohols
Lactate
Ethylene Glycol
What are the unmeasured anions?
Additional Rule
The HCO3 normally
falls 1 for every 1
increase in anion gap,
with the exception of
sepsis.
The Anions of Sepsis
• HPLC studies have failed to
identify the anions responsible
for the  AG in sepsis.
• Lactate accounts for a portion.
• H+ is likely from ATP hydrolysis
which drives the fall in
bicarbonate and is not coupled
to lactate production.
What Lowers Anion Gap?
• Hypoalbuminemia
• Lithium
• Myeloma
Example
• pH = 7.20, HCO3=10, PCO2=25
• Na=140, Cl=110
• diagnosis
Example
• pH = 7.20, HCO3=10, PCO2=25
• Na=140, Cl=110
• diagnosis
AG Met acid and
Non-AG met acid
DDx of NonAG Met Acid
USED CARS
•
•
•
•
•
•
•
•
Ureterosigmoidostomy / Fistulae
Saline
Early Renal Failure
Diarrhea
CAI
AAs
RTA
Supplements
Urine Anion Gap
• UAG = Na+ + K+ - Cl• largely reflects NH4+ and therefore is
usually negative due to renal
ammoniagenesis
• In non-AG metabolic acidosis, a
negative UAG implies extra-renal
cause of the disorder.
• If positive, renal ammoniagenesis is
likely impaired.
Example
• pH = 7.35, HCO3=19, PCO2=35
• Na=140, Cl=100
Example
• pH = 7.35, HCO3=19, PCO2=35
• Na=140, Cl=100
• diagnosis
AG metabolic acidosis and
metabolic alkalosis
Example
• pH = 7.31, HCO3=15, PCO2=30
• Na=138, Cl=112
• diagnosis
Example
• pH = 7.31, HCO3=15, PCO2=30
• Na=138, Cl=112
• diagnosis
non AG metabolic acidosis
Example
• pH = 7.12, HCO3=10, PCO2=33
• Na=138, Cl=95
• diagnosis
Example
• pH = 7.12, HCO3=10, PCO2=33
• Na=138, Cl=95
• diagnosis
AG metabolic acidosis and
metabolic alkalosis and
respiratory acidosis
Example
• pH = 7.38, HCO3=14, PCO2=23
• Na=138, Cl=95
Example
• pH = 7.38, HCO3=14, PCO2=23
• Na=138, Cl=95
• diagnosis
AG metabolic acidosis and
metabolic alkalosis and
respiratory alkalosis
Example
• pH = 7.40, HCO3=24, PCO2=40
• Na=140, Cl=96
• diagnosis
Example
• pH = 7.40, HCO3=24, PCO2=40
• Na=140, Cl=96
• diagnosis
AG metabolic acidosis and
Metabolic alkalosis
Example
• pH = 7.40, HCO3=24, PCO2=40
• Na=140, Cl=104
Example
• pH = 7.40, HCO3=24, PCO2=40
• Na=140, Cl=104, albumin=1.1
Last rule!
• For every 1.0 g/dl that the serum
albumin is below 4.0, the upper
limit of the anion gap is
decreased by 2.5
• Normal upper limit = 12
• Alb 3.0 upper limit = 9.5
• Alb 2.0 upper limit = 7.0
• Alb 1.9 upper limit = 4.5
Example
• pH = 7.40, HCO3=24, PCO2=40
• Na=140, Cl=104, albumin=1.1
Example
• pH = 7.40, HCO3=24, PCO2=40
• Na=140, Cl=104, albumin=1.1
• Diagnosis:
Anion Gap Metabolic Acidosis
Metabolic Alkalosis
Dogmatic Statement
• You cannot interpret
arterial blood gases without
looking at the electrolytes.
Dogmatic Statement
• You cannot interpret
arterial blood gases without
looking at the electrolytes.
• Be sure to check the
albumin as well.
Summary
Metabolic Acidosis: 1HCO3 => 1PCO2
Metabolic Alkalosis: 10  HCO3 => 7 PCO2
Acute Resp Acid: 10  PCO2 = 1  HCO3
Chronic Resp Acid: 10  PCO2 = 4  HCO3
Acute Resp Alk: 10  PCO2 => 2  HCO3
Chronic Resp Alk: 10  PCO2 => 5  HCO3
Summary
Metabolic Acidosis: 1HCO3 => 1PCO2
Metabolic Alkalosis: 10  HCO3 => 7 PCO2
Acute Resp Acid: 10  PCO2 = 1  HCO3
Chronic Resp Acid: 10  PCO2 = 4  HCO3
Acute Resp Alk: 10  PCO2 => 2  HCO3
Chronic Resp Alk: 10  PCO2 => 5  HCO3
But don’t forget to check the anion gap and the
serum albumin!
Thank you for your attention
Metabolic Acidosis: 1HCO3 => 1PCO2
Metaboli Alkalosis: 10  HCO3 => 7 PCO2
Acute Resp Acid: 10  PCO2 = 1  HCO3
Chronic Resp Acid: 10  PCO2 = 4  HCO3
Acute Resp Alk: 10  PCO2 => 2  HCO3
Chronic Resp Alk: 10  PCO2 => 5  HCO3
But don’t forget to check the anion gap and the
serum albumin!
Thank you for your attention