Sasi ARTERIAL BLOOD GAS ANALYSIS

ARTERIAL BLOOD GAS ANALYSIS

1. ARTERIAL BLOOD GAS ANALYSIS Sasidharan , junior resident Department of Pediatrics Moderator : Dr. Ramesh Kumar

2. INTRODUCTION ABG is a routine investigation to monitor acid base balance in patients It provides information about - oxygenation - acid base balance - adequacy of ventilation It helps in making a diagnosis ,assessing severity and titrating the treatment

3. INDICATIONS OF ABG clinical features of : Respiratory failure ( acute or chronic ) Suspected metabolic acidosis 1. Renal failure 2. Cardiac failure 3. Hyperglycemic states associated with diabetes To assess ventilation and acid base abnormalities in a ventilated patient

4. TECHNIQUE Preferred artery - RADIAL Arterial catheter can be left in place if repeated samples are required Needle size - 20 -26 G Angle - < 30 degrees 0.05 ml of heparin for 1 ml of blood - careful with excess heparin ( its weak acid and can alter ABG values )

5. PRECAUTIONS Pull the syringe slowly to avoid air bubbles Air bubbles can alter paO2 by 30 mmHg Cap the needle immediately after sampling Store the sample if delay in processing is anticipated

6. Normal ABG values pH 7.35 – 7.45 PCO 2 35 – 45 mmHg PO 2 80 – 100 mmHg HCO 3 22 – 26 mmol /L BE - 2 - +2 SaO 2 >95%

7. STEPWISE APPROACH OF ABG ANALYSIS STEP 1 Is the pH outside the normal range ? Acidemic – pH<7.35 Alkalemic – pH > 7.45 STEP 2 Is the respiratory (PaCO2) or the metabolic (HCO3 -) component abnormal? Respiratory component alters the arterial paCO2 ( normal range 35-45) Metabolic component alters the serum HCO3 (normal range 20-26)

8. STEP 3 If its respiratory problem, is it acute or chronic. This depends on pH Acute respiratory acidosis – pH decrease = 0.08 * (paCO2 – 40)/10 Chronic respiratory acidosis – pH decrease = 0.03 * ( paCO2 – 40 )/10 Acute respiratory alkalosis – pH increase = 0.08 * ( 40 – paCO2)/10 Chronic respiratory alkalosis – pH increase = 0.03 * ( 40- paCO2) / 10

9. STEP 4 For respiratory component is metabolic component adequate? Respiratory acidosis : Acute : for every 10 increase in CO2 , HCO3 will increase by 1 Chronic : for every 10 increase in CO2 , HCO3 will increase by 3.5 Respiratory alkalosis : Acute : for every 10 decrease in CO2, HCO3 will decrease by 2 Chronic : for every 10 decrease in CO2, HCO3 will decrease by 4

10. STEP 5 If it’s a metabolic disorder, assess compensation by respiratory system. Metabolic acidosis : use winter’s formula : [(1.5 * HCO3 ) + 8 ] ± 2 CO2 concentration should be within this limits. Metabolic alkalosis : For every 10 increase in HCO3 , CO2 increases by 7 If pH is < 7.35 and CO2 more than 55, then it means respiratory acidosis is co-existing

11. STEP 6 Anion gap calculation: If its metabolic acidosis , calculate anion gap AG = Na – ( HCO3 + Cl ) Normal AG : 12 ± 4 If > 12 = high anion gap acidosis If < 12 = non anion gap acidosis

12. DELTA RATIO <0.4 – pure Normal anion gap metabolic acidosis 0.4 to 0.8 – mixed normal anion gap with high anion gap metabolic acidosis 0.8 to 2.0 – pure high anion gap metabolic acidosis > 2.0 – mixed high anion gap metabolic acidosis with metabolic alkalosis ( or pre existing compensated respiratory alkalosis)

13. Case scenario A 6 year old male child k/ c/o ckd presented with complaints of Loose stools 15- 20 episodes x 1 day no blood in stool. Stool watery in consistency. Past history of k/c/o ckd on CAPD.

14. Clinical examination O/E Child was irritable,lethargic Vitals PR-142/min CP/PP – ++/+- cold peripheries + RR-22/min SpO2 – 99 % BP – 69/ 37 mm Hg CFT – 4 sec Signs of dehydration : Sunken eyes +, dry tongue+, skin turgor release prolonged CVS – S1S2 normal.No murmur RS- B/L air entry equal.No added sounds P/A – Soft, no organomegaly CNS- E4V4M6 RBS- 116 mg/dl

15. pH- 7.29 pCO2-25.8 paO2- 47.2 HCO3- 12.3 BE- - 12.3 Na- 135 K-3.8 Cl - 97 AG- 25.5 Step 1 acidosis or alkalosis? pH- 7.29---- ACIDOSIS . Step 2 Metabolic or respiratory? pCO2-25.8 and HCO3- 12.3.0 – METABOLIC Step 3 Compensation? paCO2 has decreased i.e. same direction change. So compensation present. Is the blood gas report internally consistent? { [24*paCO2/HCO3]- 40 } * 0.01 here, { 24(25.8/12.3) -40 } * 0.01= 0.11 Subtracting this value from 7.4, pH expected= 7.29

16. step 4 Is compensation adequate? WINTER’S FORMULA (paCO2= 1.5*HCO3 + 8 +/- 2) Here, 1.5x12.3 + 8 +/- 2= 26.25 +/ - 2 = 24.25- 28.25 Here Pco2 – 25.8 Adequate compensation present. Step 5 Calculate anion gap AG= 129-(97+12.3)= 19.7 HIGH ANION GAP METABOLIC ACIDOSIS PRESENT Step 7 Calculate delta AG and delta HCO3 Delta AG= 7.7. delta HCO3= 11.7 Delta AG/ delta HCO3= 7.7/11.7 = 0.65 Mixed HAGMA, NAGMA.

17. interpretation There is metabolic acidosis Adequate compensation- acute- by respiratory alkalosis Values are internally consistent. There is HAGMA with NAGMA Probable cause is chronic renal failure causing high anion gap metabolic acidosis with dehydration secondary to diarrhoea causing normal anion gap metabolic acidosis.

18. Blood gas analysis after treatment VBG DAY 1 DAY 2 DAY 3 pH 7.29 7.168 7.25 PCO2 25.8 31.8 23.5 PO2 47.2 49.5 45.1 HCO3 12.3 11.3 10.2 BE -12.3 - 16.0 -15.4 Na 129 128 131.2 K 3.5 3.8 3.66 Cl 97 96 99 AG 19.7 20.7 21.8

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