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Step by step to cracking that cryptic sheet !All credits to Dr Tiny Nair MD,DM, Consultant cardiologist, PRS Hospital, Thiruvananthapuram.
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A PG’S GUIDE TO ABG
CAN YOU TRUST THE REPORT? Henderson-Hasselbalch shortcut:-
80 – Last two digits of pH = (24 X pCO2)/HCO3
Balanced equation = Correct ABG.
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.
dO2 REVELATIONS I
dO2 > 15
› COPD
› ILD
› Pneumonia
› Pulmonary edema
› Right to left shunt
dO2 REVELATIONS II
dO2 < 15
› Low FiO2
› CNS diseases
› High altitude
› Respiratory muscle disease
› Upper airway obstruction
PRIMARY DISORDER
Resp and Metab are on opposite sides of acidosis and alkalosis.
Easy :- Primary disorder tallies with pH.
(Resp + Metab) acidosis/alkalosis.
Calculate (Δ Value/Normal value) for pCO2 and HCO3.
Larger value is the primary.
COMPENSATION
pCO2 HCO3
RespAcidosis
Alkalosis
MetabolicAlkalosis
Acidosis
HCO3 X 0.1 X 0.4
HCO3
X 0.2X 0.5
pCO2 X 0.7
pCO2 X 1.2
INCREASING
a/cc/c
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
ΔAG REVELATIONS
ΔAG = ΔHCO3; All is well.
ΔAG > ΔHCO3 = HAG Met Acid + Meta Alk.
ΔAG < ΔHCO3 = HAG Met Acid + NAG Met Acid.
NAGMA – SO WHAT?
Calculate Urine AG = (Na + K) – Cl
Normal UAG = 0 ≈ GI loss (diarrhoea)
UAG > 0 ≈ Urinary loss (RTA)
APPENDIX
High RBS needs corrected Na.
Corrected Na = Na + [(RBS-100)/100] X 1.4.
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
HOW MUCH HCO3
HCO3 deficit = (ideal – actual) X 0.5 X lean body wt.
NOW GO AND BREAK THAT CRYPTIC ABG!
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