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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

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simple rules for the interpretation of arterial blood gases
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
“Mantra”

1 for 1

10 for 7

1

4

2

5

mantra3
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
Examples

pH = 7.33, PCO2=30, HCO3=15

diagnosis

examples5
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
Suggestion:

Delete the word compensation from your acid base vocabulary.

mantra7
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
Definition

Acidemia

Acidosis

definition9
Definition

Acidemia – an abnormally low pH (high proton concentration)

definition10
Definition

Acidemia – an abnormally low pH (high proton concentration)

Acidosis – a process which produces an excess of protons

example
Example
  • pH=7.22, PCO2=38, HCO3=15
  • diagnosis
example12
Example
  • pH=7.22, PCO2=38, HCO3=15
  • diagnosis:

Met and Resp acidosis

or

Undercompensated Met Acidosis

example13
Example
  • pH=7.38, PCO2=28, HCO3=16
  • diagnosis
example14
Example
  • pH=7.38, PCO2=28, HCO3=16
  • diagnosis: Met Acid with Resp Alk or Overcompensated Met Acid
example15
Example
  • pH=7.47, PCO2=47, HCO3=34
  • diagnosis
example16
Example
  • pH=7.47, PCO2=47, HCO3=34
  • diagnosis metabolic alkalosis
mantra17
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

example18
Example
  • pH=7.51, PCO2=42, HCO3=34
  • diagnosis
example19
Example
  • pH=7.51, PCO2=42, HCO3=34
  • diagnosis Met and Resp alkalosis
example20
Example
  • pH=7.42, PCO2=52, HCO3=34
  • diagnosis
example21
Example
  • pH=7.42, PCO2=52, HCO3=34
  • diagnosis metabolic alkalosis and respiratory acidosis
example22
Example

pH=7.35, PCO2=60, HCO3=32

diagnosis

example23
Example

pH=7.35, PCO2=60, HCO3=32

diagnosis chronic respiratory acidosis

example24
Example

pH=7.25, PCO2=60, HCO3=26

diagnosis

example25
Example

pH=7.25, PCO2=60, HCO3=26

diagnosis acute respiratory acidosis

example26
Example

pH=7.30, PCO2=60, HCO3=29

diagnosis

example27
Example

pH=7.30, PCO2=60, HCO3=29

diagnosis acute on chronic resp acid

example28
Example

pH=7.45, PCO2=25, HCO3=17

diagnosis

example29
Example

pH=7.45, PCO2=25, HCO3=17

diagnosis chronic respiratory alkalosis

mantra30
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

example31
Example

pH=7.33, PCO2=30, HCO3=15

Na= 147, Cl=110

diagnosis

what is an anion gap
What is an Anion Gap?
  • Qualitative
  • Quantitative
  • Why not include H+ ? K+ ?
what is an anion gap33
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
DDx of AG Met Acid
  • Ketones
  • Uremia
  • Salicylates
  • Methanol
  • Alcohols
  • Lactate
  • Ethylene Glycol
  • What are the unmeasured anions?
additional rule
Additional Rule

The HCO3 normally falls 1 for every 1 increase in anion gap, with the exception of sepsis.

the anions 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
What Lowers Anion Gap?
  • Hypoalbuminemia
  • Lithium
  • Myeloma
example38
Example
  • pH = 7.20, HCO3=10, PCO2=25
  • Na=140, Cl=110
  • diagnosis
example39
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
DDx of NonAG Met AcidUSED CARS
  • Ureterosigmoidostomy / Fistulae
  • Saline
  • Early Renal Failure
  • Diarrhea
  • CAI
  • AAs
  • RTA
  • Supplements
urine anion gap
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.
example42
Example
  • pH = 7.35, HCO3=19, PCO2=35
  • Na=140, Cl=100
example43
Example
  • pH = 7.35, HCO3=19, PCO2=35
  • Na=140, Cl=100
  • diagnosis AG metabolic acidosis and metabolic alkalosis
example44
Example
  • pH = 7.31, HCO3=15, PCO2=30
  • Na=138, Cl=112
  • diagnosis
example45
Example
  • pH = 7.31, HCO3=15, PCO2=30
  • Na=138, Cl=112
  • diagnosis non AG metabolic acidosis
example46
Example
  • pH = 7.12, HCO3=10, PCO2=33
  • Na=138, Cl=95
  • diagnosis
example47
Example
  • pH = 7.12, HCO3=10, PCO2=33
  • Na=138, Cl=95
  • diagnosis AG metabolic acidosis and metabolic alkalosis and respiratory acidosis
example48
Example
  • pH = 7.38, HCO3=14, PCO2=23
  • Na=138, Cl=95
example49
Example
  • pH = 7.38, HCO3=14, PCO2=23
  • Na=138, Cl=95
  • diagnosis AG metabolic acidosis and metabolic alkalosis and respiratory alkalosis
example50
Example
  • pH = 7.40, HCO3=24, PCO2=40
  • Na=140, Cl=96
  • diagnosis
example51
Example
  • pH = 7.40, HCO3=24, PCO2=40
  • Na=140, Cl=96
  • diagnosis AG metabolic acidosis and Metabolic alkalosis
example52
Example
  • pH = 7.40, HCO3=24, PCO2=40
  • Na=140, Cl=104
example53
Example
  • pH = 7.40, HCO3=24, PCO2=40
  • Na=140, Cl=104, albumin=1.1
last rule
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
example55
Example
  • pH = 7.40, HCO3=24, PCO2=40
  • Na=140, Cl=104, albumin=1.1
example56
Example
  • pH = 7.40, HCO3=24, PCO2=40
  • Na=140, Cl=104, albumin=1.1
  • Diagnosis: Anion Gap Metabolic Acidosis Metabolic Alkalosis
dogmatic statement
Dogmatic Statement
  • You cannot interpret arterial blood gases without looking at the electrolytes.
dogmatic statement58
Dogmatic Statement
  • You cannot interpret arterial blood gases without looking at the electrolytes.
  • Be sure to check the albumin as well.
summary
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

summary60
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
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!

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