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!