Arterial Blood Gas Interpretation Dr. Shinjan Patra

OBJECTIVES  ABG Sampling  Interpretation of ABG  Step wise approach to decode an ABG  Acid Base status  Electrolyte status

ABG – Procedure and Precautions Site-Ideally Radial Artery Brachial Artery Femoral Artery.  Ideally - Pre-heparinised ABG syringes - Syringe should be FLUSHED with 0.5ml of 1:1000 Heparin solution and emptied.

Precautions DO NOT LEAVE EXCESSIVE HEPARIN IN THE SYRINGE HEPARIN DILUTIONAL HCO 3 EFFECT PCO 2 Only small 0.5ml Heparin for flushing and discard it Syringes must have > 50% blood. Use only 2ml or less syringe.

Precautions Ensure No Air Bubbles Contact with air bubbles- PO2 Pco2 ABG Syringe must be transported at the earliest to the laboratory for EARLY analysis via COLD CHAIN

Other Considerations – Any change in body temp at the time of sampling leads to alteration in values detected by the electrodes – ABG Analyser is controlled for Normal Body temperatures – ABG Sample should always be sent with relevant information regarding O 2, FiO 2 status and Temp.

Step Wise approach to Interpret the ABG At first we will go through the normal values-

ANALYTENormal ValueUnits pH PCO mm Hg PO272 – 104mm Hg` [HCO3]22 – 30meq/L SaO % Anion Gap12 + 4meq/L ∆HCO3+2 to -2meq/L

Steps Step 1- Is the ABG authentic? Step 2- Acidosis Or Alkalosis Step 3- Respiratory or Metabolic Step 4- If respiratory is it acute or chronic? Step 5- Is there any compensation or not? Step 6- Is the compensation adequate? Step 7- Anion-Gap calculations & implications

Is the ABG authentic? [ H+] neq/l = 24 X (PCO 2 / HCO 3 ) If this equation fulfills then it is authentic This equation derives from- Henderson-Hasselbalch equation pH = log HCO x PCO 2

Acidosis Or Alkalosis  Look at pH < acidemia >7.45 – alkalemia

Respiratory or Metabolic IS PRIMARY DISTURBANCE RESPIRATORY OR METABOLIC?  pH PCO 2 or pH PCO 2 -METABOLIC  pH PCO 2 or pH PCO 2 - RESPIRATORY  There may be mixed acidosis or alkalosis also.

If respiratory is it acute or chronic? We will determine it by evaluating the value of x from this equation- ppH= x x ∆Pco 2 /10 If this value of x is close to 0.08 then it is acute If this value of x is close to 0.03 then it is chronic

Is there any compensation or not? If respiratory acidosis or alkalosis-then [HCO3] increased or decreased respectively. If metabolic acidosis or alkalosis- then Pco2 decreased or increased respectively.

Is the compensation adequate? Respiratory compensation(in underlying metabolic disorder) Pco2 will decrease by 1.25 mm Hg per mmol/l decrease in [HCO3](metabolic acidosis) Pco2 will increase by 0.75 mm Hg per mmol/l increase in [HCO3](metabolic alkalosis)

Metabolic Compensation(in underlying Respiratory disorder) -For every 10mm Hg change in Pco2 [Hco3] will increase 1meq/l(acute resp. acidosis) [Hco3] will increase 4meq/l(chr. resp. acidosis) [Hco3] will decrease 2meq/l(acute resp. alkalosis [Hco3] will decrease 5meq/l(chr. resp. alkalosis)

Anion-Gap Calculations AG= [Na+] – {[Cl-]+[HCO3-]} Normal Value= meq/l It mostly includes the unmeasured anions such as- Albumin, phosphates, sulfate & organic anions.

AG variation in Acidosis High anion gap acidosis- Lactic acidosis, Ketoacidosis, Toxins ingestion, Renal failure(acute & chronic) Normal anion gap acidosis- GI bicarbonate loss(diarrhea, external pancreatic drainage), Renal acidosis(RTA)

Relationship Between ∆AG & ∆[Hco3] If there is increased AG present then we should see the ∆AG/ ∆[Hco3] value to assess the cause. If it is <1= Concurrent non-anion gap metabolic acidosis also present. If it is 1-2= Uncomplicated anion-gap metabolic acidosis present If it is >2= Associated Metabolic Alkalosis also present

Case scenarios & interpretation