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

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, PCO 2 =30, HCO 3 =15 diagnosis

Examples pH = 7.33, PCO 2 =30, HCO 3 =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, PCO 2 =38, HCO 3 =15 diagnosis

Example pH=7.22, PCO 2 =38, HCO 3 =15 diagnosis: Met and Resp acidosis or Undercompensated Met Acidosis

Example pH=7.38, PCO 2 =28, HCO 3 =16 diagnosis

Example pH=7.38, PCO 2 =28, HCO 3 =16 diagnosis: Met Acid with Resp Alk or Overcompensated Met Acid

Example pH=7.47, PCO 2 =47, HCO 3 =34 diagnosis

Example pH=7.47, PCO 2 =47, HCO 3 =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, PCO 2 =42, HCO 3 =34 diagnosis

Example pH=7.51, PCO 2 =42, HCO 3 =34 diagnosis Met and Resp alkalosis

Example pH=7.42, PCO 2 =52, HCO 3 =34 diagnosis

Example pH=7.42, PCO 2 =52, HCO 3 =34 diagnosis metabolic alkalosis and respiratory acidosis

Example pH=7.35, PCO 2 =60, HCO 3 =32 diagnosis

Example pH=7.35, PCO 2 =60, HCO 3 =32 diagnosis chronic respiratory acidosis

Example pH=7.25, PCO 2 =60, HCO 3 =26 diagnosis

Example pH=7.25, PCO 2 =60, HCO 3 =26 diagnosis acute respiratory acidosis

Example pH=7.30, PCO 2 =60, HCO 3 =29 diagnosis

Example pH=7.30, PCO 2 =60, HCO 3 =29 diagnosis acute on chronic resp acid

Example pH=7.45, PCO 2 =25, HCO 3 =17 diagnosis

Example pH=7.45, PCO 2 =25, HCO 3 =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, PCO 2 =30, HCO 3 =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 – - HCO 3 – 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 HCO 3 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, HCO 3 =10, PCO 2 =25 Na=140, Cl=110 diagnosis

Example pH = 7.20, HCO 3 =10, PCO 2 =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 NH 4 + 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, HCO 3 =19, PCO 2 =35 Na=140, Cl=100

Example pH = 7.35, HCO 3 =19, PCO 2 =35 Na=140, Cl=100 diagnosis AG metabolic acidosis and metabolic alkalosis

Example pH = 7.31, HCO 3 =15, PCO 2 =30 Na=138, Cl=112 diagnosis

Example pH = 7.31, HCO 3 =15, PCO 2 =30 Na=138, Cl=112 diagnosis non AG metabolic acidosis

Example pH = 7.12, HCO 3 =10, PCO 2 =33 Na=138, Cl=95 diagnosis

Example pH = 7.12, HCO 3 =10, PCO 2 =33 Na=138, Cl=95 diagnosis AG metabolic acidosis and metabolic alkalosis and respiratory acidosis

Example pH = 7.38, HCO 3 =14, PCO 2 =23 Na=138, Cl=95

Example pH = 7.38, HCO 3 =14, PCO 2 =23 Na=138, Cl=95 diagnosis AG metabolic acidosis and metabolic alkalosis and respiratory alkalosis

Example pH = 7.40, HCO 3 =24, PCO 2 =40 Na=140, Cl=96 diagnosis

Example pH = 7.40, HCO 3 =24, PCO 2 =40 Na=140, Cl=96 diagnosis AG metabolic acidosis and Metabolic alkalosis

Example pH = 7.40, HCO 3 =24, PCO 2 =40 Na=140, Cl=104

Example pH = 7.40, HCO 3 =24, PCO 2 =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, HCO 3 =24, PCO 2 =40 Na=140, Cl=104, albumin=1.1

Example pH = 7.40, HCO 3 =24, PCO 2 =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