Simple Rules for the Interpretation of Arterial

Blood Gases

Nicholas Sadovnikoff, MD, FCCM

Assistant Professor, Harvard Medical SchoolCo-Director, Surgical Intensive Care Units

Brigham and Women’s HospitalBoston, MA

Kuwait City, KuwaitNovember 24, 2011

“Mantra”

1 for 110 for 7

1425

Mantra

Metabolic Acidosis: 1HCO3 => 1PCO2Metabolic Alkalosis: 10 HCO3 => 7 PCO2Acute Resp Acid: 10 PCO2 = 1 HCO3Chronic Resp Acid: 10 PCO2 = 4 HCO3Acute 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 orMetabolic acidosis with respiratory compensation ormetabolic acidosis with compensatory respiratory alkalosis

Suggestion:

Delete the word compensation from your acid base vocabulary.

Mantra

Metabolic Acidosis: 1HCO3 => 1PCO2Metabolic Alkalosis: 10 HCO3 => 7 PCO2Acute Resp Acid: 10 PCO2 = 1 HCO3Chronic Resp Acid: 10 PCO2 = 4 HCO3Acute 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 acidosisorUndercompensated 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

• diagnosismetabolic alkalosis

Mantra

Metabolic Acidosis: 1HCO3 => 1PCO2Metabolic Alkalosis: 10 HCO3 => 7 PCO2Acute Resp Acid: 10 PCO2 = 1 HCO3Chronic Resp Acid: 10 PCO2 = 4 HCO3Acute 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

• diagnosisMet and Resp alkalosis

Example

• pH=7.42, PCO2=52, HCO3=34

• diagnosis

Example

• pH=7.42, PCO2=52, HCO3=34

• diagnosismetabolic alkalosis and

respiratory acidosis

Example

pH=7.35, PCO2=60, HCO3=32

diagnosis

Example

pH=7.35, PCO2=60, HCO3=32

diagnosischronic respiratory acidosis

Example

pH=7.25, PCO2=60, HCO3=26

diagnosis

Example

pH=7.25, PCO2=60, HCO3=26

diagnosisacute respiratory acidosis

Example

pH=7.30, PCO2=60, HCO3=29

diagnosis

Example

pH=7.30, PCO2=60, HCO3=29

diagnosisacute on chronic resp acid

Example

pH=7.45, PCO2=25, HCO3=17

diagnosis

Example

pH=7.45, PCO2=25, HCO3=17

diagnosischronic respiratory alkalosis

Mantra

Metabolic Acidosis: 1HCO3 => 1PCO2Metabolic Alkalosis: 10 HCO3 => 7 PCO2Acute Resp Acid: 10 PCO2 = 1 HCO3Chronic Resp Acid: 10 PCO2 = 4 HCO3Acute 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?

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

Additional Rule

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

• diagnosisAG Met acid andNon-AG met acid

DDx of NonAG Met AcidUSED 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

• diagnosisAG metabolic acidosis andmetabolic 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

• diagnosisnon 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

• diagnosisAG metabolic acidosis andmetabolic 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

• diagnosisAG metabolic acidosis andmetabolic 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

• diagnosisAG 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: 1HCO3 => 1PCO2Metabolic Alkalosis: 10 HCO3 => 7 PCO2Acute Resp Acid: 10 PCO2 = 1 HCO3Chronic Resp Acid: 10 PCO2 = 4 HCO3Acute Resp Alk: 10 PCO2 => 2 HCO3 Chronic Resp Alk: 10 PCO2 => 5 HCO3

Summary

Metabolic Acidosis: 1HCO3 => 1PCO2Metabolic Alkalosis: 10 HCO3 => 7 PCO2Acute Resp Acid: 10 PCO2 = 1 HCO3Chronic Resp Acid: 10 PCO2 = 4 HCO3Acute 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: 1HCO3 => 1PCO2Metaboli Alkalosis: 10 HCO3 => 7 PCO2Acute Resp Acid: 10 PCO2 = 1 HCO3Chronic Resp Acid: 10 PCO2 = 4 HCO3Acute 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