Step by step to cracking that cryptic sheet ! All credits to Dr Tiny Nair MD,DM, Consultant cardiologist, PRS Hospital, Thiruvananthapuram.

1. A PG’S GUIDE TO ABG

2. CAN YOU TRUST THE REPORT?
 Henderson-Hasselbalch shortcut:-
 80 – Last two digits of pH = (24 X pCO2)/HCO3
 Balanced equation = Correct ABG.

3. pA(lveolar)O2 SHENANIGANS
 pAO2 = 150 – (pCO2 X 1.25) [@ room air]
 If pa(rterial)O2 is low, calculate:-
 pAO2 – paO2 = dO2 = >15 or <15.

4. dO2 REVELATIONS I
 dO2 > 15
› COPD
› ILD
› Pneumonia
› Pulmonary edema
› Right to left shunt

5. dO2 REVELATIONS II
 dO2 < 15
› Low FiO2
› CNS diseases
› High altitude
› Respiratory muscle disease
› Upper airway obstruction

6. PRIMARY DISORDER
 Resp and Metab are on opposite sides of
acidosis and alkalosis.
 Easy :- Primary disorder tallies with pH.

7.  (Resp + Metab) acidosis/alkalosis.
 Calculate (Δ Value/Normal value) for
pCO2 and HCO3.
 Larger value is the primary.

8. COMPENSATION
Resp Metabolic
Acidosis Alkalosis
HCO3 pCO2 X 0.7
X 0.1
X 0.4 I
N
C
a/c R
c/c E
pCO2 A HCO3
S
HCO3 I
X 0.2 N pCO2 X 1.2
X 0.5 G
Alkalosis Acidosis

9. JUMPING THE ANION GAP
 Always calculated for metabolic
acidosis.
 AG = Na – (Cl + HCO3) = <10 normally.
 High AG ≥ 10
 ΔAG = Excess anions = AG – 10 (Anions
neutralized by HCO3)
 Original HCO3 = ΔAG + HCO3

10. ΔAG REVELATIONS
 ΔAG = ΔHCO3; All is well.
 ΔAG > ΔHCO3 = HAG Met Acid + Meta
Alk.
 ΔAG < ΔHCO3 = HAG Met Acid + NAG
Met Acid.

11. NAGMA – SO WHAT?
 Calculate Urine AG = (Na + K) – Cl
 Normal UAG = 0 ≈ GI loss (diarrhoea)
 UAG > 0 ≈ Urinary loss (RTA)

12. APPENDIX
 High RBS needs corrected Na.
 Corrected Na = Na + [(RBS-100)/100] X 1.4.

13. HCO3 THERAPY
 pH < 7.2 and HCO3 < 5 – 10 mEq/l
 Inadequate ventilatory compensation
 Elderly on β-blockers in severe acidosis
with compromised cardiac function
 Concurrent severe AG and NAGMA
 Severe acidosis with renal
failure/intoxication

14. HOW MUCH HCO3
 HCO3 deficit = (ideal – actual) X 0.5 X
lean body wt.

15. NOW GO AND BREAK THAT
CRYPTIC ABG!