ABG Analysis

ABG AnalysisABG AnalysisTheoretical conceptsFew practical pointsPractice CasesConcepts6 step approach to ABG analysis

http://www.thoracic.org/clinical/critical-care/clinical-education/abgs.php

Step 1Is it valid?Henderson-Hesslebach equation[H+] = 24 x (PCO2 / [HCO3 -] )[H+] = 80 (Last two digits after decibel in pH )Check validity for 7.40/40/100/24

pHApproximate [H+](mmol/L)7.001007.05897.10797.15717.20637.25567.30507.35457.40407.45357.50327.55287.60257.6522Step 2Acidemia?- pH < 7.35 ORAlkalemia?- pH > 7.45

Usually represents primary disorderAcidosis or alkalosis can still be present, even with normal pH (7.35-7.45)Step 3Is it primary acidosis or alkalosis?AcidosisRespiratorypH PaCO2 AcidosisMetabolic pH HCO3

AlkalosisRespiratorypH PaCO2 AlkalosisMetabolicpH HCO3

Step 4Is there appropriate compensation?

Usually compensation is NOT complete.Compensation 1 Metabolic disordersCalculate Expected PCO2Met. Acidosis: (1.5 x HCO3)+ (8 2)Met. Alkalosis: 40 + (0.6 x HCO3) 2Calculate any superimposed respiratory disorder by comparing measured & expected PCO2.Compensation 1 Respiratory disorders

Calculate Expected pH AcuteResp. acidosis: Expected pH= 7.4 - (0.008 x CO2)Resp. alkalosis: Expected pH= 7.4 + (0.008 x CO2) ChronicResp. acidosis: Expected pH= 7.4 - (0.003 x CO2)Resp. alkalosis: Expected pH= 7.4 + (0.003 x CO2)

CompensationDisorderChange in primary electrolyteCompensationTime frameMetabolic Acidosis 10 less HCO310 less PCO2ImmediateMetabolic Alkalosis10 more HCO36 more PCO2ImmediateRespiratory Acidosis(chronic)10 more PCO24 more HCO3Hours-daysRespiratory Alkalosis(chronic)10 less PCO25 less HCO3Hours-daysAcute Respiratory disorder10 change in PCO21-2 change in HCO3HoursStep 5Is there Anion Gap?

Normal AG up to 12 meq/L (10 if you are nephrologist)Correct for hypoalbuminemia (Add 2.5 to AG for each 1gm of albumin drop below 4)If elevated AG, consider calculating the osmolal gap

Step 6If elevated AG, Calculate delta-delta gapTo identify superimposed non-AG metabolic disorder

Calculation: HCO3 + Anion Gap (AG - 12)If 22-30: Isolated AG metabolic acidosisIf >30: Superimposed metabolic alkalosisIf 10: Renal etiology (RTA)If normal or < -10: Non-Renal (Diarrhea)Few practical pointsWhats worse- Acidosis or Alkalosis?

Does hypercarbia also lead to hypoxemia and vice versa?

Bicarbonate therapyFor severe metabolic acidosis (pH below 7.1-7.0)

Acidemia leads to:Decreased myocardial contractility Fall in cardiac output/HypotensionPulmonary venoconstrictionDecreased binding of norepinephrine to its receptors Adversely affects cell functions such as enzymatic reactions, ATP generation etc.

Bicarbonate therapyIs it harmless?

Inappropriate bicarbonate therapy can lead to:Shifts in ionized calciumEventually increased CO2, work of breathingIncreased serum lactate and ketosisIn vitro studies show that intracellular alkalinization hastens cell death following anoxiaFall in blood pressure and cardiac output Few studies show increase mortality

Bicarbonate therapyNot recommended routinely in DKACurrently not part of ACLS protocolNot recommended for LA induced Metabolic acidosis, unless severe (atleast below 7.00) or severe bicarbonate deficitDoes it decrease the risk of contrast induced AKI?PaO2/PCO2 issues62 y/o CM with severe COPD intubated for acute hypercapneic respiratory failure.Day 2 ABG: 7.55/36/65/32/91% sat on PRVC @ 18/500/5/40%.Vitals: 110/60, RR-30, HR-90, Sat-92%, Afebrile

What do you think?Osmolar GapSave yourself from M&Ms!!Always check if AG metabolic acidosis present!!Measured calculated osmolarityOSM gap = Measured OSM (2[Na+] - glucose/18 BUN/2.8)Normal below 10.Raised in other alcohol ingestion: Ethylene Glycol, Methanol, Isopropyl alcohol, Acetone

20A-a GradientHelps determine source of hypoxemiaNormal around 5-10Increased if:Diffusion problemV-Q mismatchRight to left shunt

Calculation: PAO2 PaO2PAO2 = 150 (if RA) (1.25 X PCO2) Thank You