Lab
ABG Interpretation
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ABG Interpretation
, Arterial Blood Gas Interpretation
See Also
Arterial Blood Gas
Venous Blood Gas
Technique
Step 1 - Determine primary abnormality
Determine Acidosis versus alkalosis
pH <7.35: Acidosis
pH >7.45: Alkalosis
Determine Metabolic versus Respiratory
Primary Metabolic Disorder
pH changes in same direction as bicarbonate, pCO2
Metabolic Acidosis
Serum ph decreased
Serum bicarbonate and paCO2 decreased
Metabolic Alkalosis
Serum ph increased
Serum bicarbonate and paCO2 increased
Primary Respiratory Disorder
pH changes in opposite direction bicarbonate, pCO2
Respiratory Acidosis
Serum ph decreased
Serum bicarbonate and paCO2 increased
Respiratory Alkalosis
Serum ph increased
Serum bicarbonate and paCO2 decreased
Technique
Step 2 - Sharpen the diagnosis
Calculate the
Anion Gap
Helpful in
Metabolic Acidosis
Helpful in mixed acid-base disorders
Calculate
Osmolar Gap
Helpful in
Metabolic Acidosis
Calculate
Urinary Anion Gap
Helpful in
Non-Anion Gap Metabolic Acidosis
Distinguishes renal from extra-renal cause
Technique
Step 3 - Determine Compensation
Metabolic Acidosis
PaCO2
decreases 1.2 mmHg per 1 meq/L bicarbonate fall
Also calculate the
Anion Gap
PaCO2
= (1.5 * Bicarb) + 8
Metabolic Alkalosis
PaCO2
increases 6 mmHg per 10 meq/L bicarbonate rise
Acute
Respiratory Acidosis
pH decreases 0.08 for each 10 mmHg
PaCO2
increase from 40 mmHg
Bicarbonate increases 1 meq/L per 10 mmHg
PaCO2
rise from 40 mmHg
Chronic
Respiratory Acidosis
pH decreases 0.03 for each 10 mmHg
PaCO2
increase from 40 mmHg
Bicarbonate increases 3.5 to 4 meq/L per 10 mmHg
PaCO2
rise from 40 mmHg
Acute
Respiratory Alkalosis
pH increases 0.08 for each 10 mmHg
PaCO2
decrease from 40 mmHg
Bicarbonate decreases 2 meq/L per 10 mmHg
PaCO2
decrease from 40 mmHg
Chronic
Respiratory Alkalosis
pH increases 0.03 for each 10 mmHg
PaCO2
decrease from 40 mmHg
Bicarbonate decreases 5 meq/L per 10 mmHg
PaCO2
decrease from 40 mmHg
Minimum bicarbonate in respiratory compensation is typically 12-15 meq/L
Technique
Step 4 - Define Associated Abnormalities
Calculated PaCO2
Useful in High
Anion Gap Metabolic Acidosis
Defines concurrent respiratory abnormalities
Excess Anion Gap
EAG > 30 mEq/L:
Metabolic Alkalosis
present
EAG < 23 mEq/L:
Metabolic Acidosis
present
References
Ghosh (2000) Fed Pract p. 23-33
Killu and Sarani (2016) Fundamental
Critical Care
Support, p. 93-114
Rutecki (Dec 1997) Consultant, p. 3067-74
Rutecki (Jan 1998) Consultant, p. 131-42
Arieff (1993) J Crit Illn 8(2): 224-46 [PubMed]
Narins (1982) Am J Med 72:496 [PubMed]
Narins (1980) Medicine 59:161-95 [PubMed]
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