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Blood Gas Analysis and it’s Clinical Interpretation

Learn about ABG sampling errors, analyzing steps, anion gap calculations, and more. Understand temperature effects, air bubbles, steady states, and interpretation of ABG components.

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Blood Gas Analysis and it’s Clinical Interpretation

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  1. Blood Gas Analysis and it’s Clinical Interpretation Dr R.S.Gangwar MD, PDCC, FIPM Assistant Professor Geriatric ICU,DGMH

  2. Outline • Common Errors During ABG Sampling • Components of ABG • Discuss simple steps in analyzing ABGs • Calculate the anion gap • Calculate the delta gap • Differentials for specific acid-base disorders

  3. Delayed Analysis • Consumptiom of O2 & Production of CO2 continues after blood drawn • Iced Sample maintains values for 1-2 hours • Uniced sample quickly becomes invalid within 15-20 minutes • PaCO2 3-10 mmHg/hour • PaO2 • pH  d/t lactic acidosis generated by glycolysis in R.B.C.

  4. Temp Effect On Change of ABG Values

  5. FEVER OR HYPOTHERMIA • Most ABG analyzers report data at N body temp • If severe hyper/hypothermia, values of pH & PCO2 at 37 C can be significantly diff from pt’s actual values • Changes in PO2 values with temp also predictable • If Pt.’s temp < 37C Substract 5 mmHg Po2, 2 mmHg Pco2 and Add 0.012 pH per 1C decrease of temperature Hansen JE, Clinics in Chest Med 10(2), 1989 227-237

  6. AIR BUBBLES : PO2 150 mmHg & PCO2 0 mm Hg in air bubble(R.A.) Mixing with sample, lead to  PaO2 &  PaCO2 • To avoid air bubble, sample drawn very slowly and preferabily in glass syringe Steady State: • Sampling should done during steady state after change in oxygen therepy or ventilator parameter • Steady state is achieved usually within 3-10 minutes

  7. Leucocytosis : •  pH and Po2 ; and  Pco2 • 0.1 ml of O2 consumed/dL of blood in 10 min in pts with N TLC • Marked increase in pts with very high TLC/plt counts – hence imm chilling/analysis essential • EXCESSIVE HEPARIN • Dilutional effect on results  HCO3- & PaCO2 • Only .05 ml heperin required for 1 ml blood. • So syringe be emptied of heparin after flushing or only dead space volume is sufficient or dry heperin should be used

  8. TYPE OF SYRINGE • pH & PCO2 values unaffected • PO2 values drop more rapidly in plastic syringes (ONLY if PO2 > 400 mm Hg) • Differences usually not of clinical significance so plastic syringes can be and continue to be used • Risk of alteration of results  with: •  size of syringe/needle •  vol of sample • HYPERVENTILATION OR BREATH HOLDING May lead to erroneous lab results

  9. COMPONENTS OF THE ABG • pH: Measurement of acidity or alkalinity, based on the hydrogen (H+). 7.35 – 7.45 • Pao2 :The partial pressure oxygen that is dissolved in arterial blood. 80-100 mm Hg. • PCO2: The amount of carbon dioxide dissolved in arterial blood. 35– 45 mmHg • HCO3: The calculated value of the amount of bicarbonate in the blood. 22 – 26 mmol/L • SaO2:The arterial oxygen saturation. >95% • pH,PaO2 ,PaCO2 , Lactate and electrolytes are measured Variables • HCO3 (Measured or calculated)

  10. Contd… • Buffer Base: • It is total quantity of buffers in blood including both volatile(Hco3) and nonvolatile (as Hgb,albumin,Po4) • Base Excess/Base Deficit: • Amount of strong acid or base needed to restore plasma pH to 7.40 at a PaCO2 of 40 mm Hg,at 37*C. • Calculated from pH, PaCO2 and HCT • Negative BE also referred to as Base Deficit • True reflection of non respiratory (metabolic) acid base status • Normal value: -2 to +2mEq/L

  11. CENTRAL EQUATION OF ACID-BASE PHYSIOLOGY • Henderson Hasselbach Equation: • where [ H+] is related to pH by • To maintain a constant pH, PCO2/HCO3- ratio should be constant • When one component of the PCO2/[HCO3- ]ratio is altered, the compensatory response alters the other component in the same direction to keep the PCO2/[HCO3- ] ratio constant • [H+] in nEq/L = 24 x (PCO2 / [HCO3 -] ) • [ H+] in nEq/L = 10 (9-pH)

  12. Compensatory response or regulation of pH By 3 mechanisms: • Chemical buffers: • React instantly to compensate for the addition or subtraction of H+ ions • CO2 elimination: • Controlled by the respiratory system • Change in pH result in change in PCO2 within minutes • HCO3- elimination: • Controlled by the kidneys • Change in pH result in change in HCO3- • It takes hours to days and full compensation occurs in 2-5 days

  13. Normal Values

  14. Steps for ABG analysis • What is the pH? Acidemia or Alkalemia? • What is the primary disorder present? • Is there appropriate compensation? • Is the compensation acute or chronic? • Is there an anion gap? • If there is a AG check the delta gap? • What is the differential for the clinical processes?

  15. Step 1: • Look at the pH: is the blood acidemic or alkalemic? • EXAMPLE : • 65yo M with CKD presenting with nausea, diarrhea and acute respiratory distress • ABG :ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1 • ACIDMEIA OR ALKALEMIA ????

  16. EXAMPLE ONE • ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1 • Answer PH = 7.23 , HCO3 7 • Acidemia

  17. Step 2: What is the primary disorder?

  18. Contd…. • Metabolic Conditions are suggested if • pH changes in the same direction as pCO2 or pH is abnormal but pCO2 remains unchanged • Respiratory Conditions are suggested if: • pH changes in the opp direction as pCO2 or pH is abnormal but HCO3- remains unchanged

  19. EXAMPLE • ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5. • PH is low , CO2 is Low • PH and PCO2 are going in same directions then its most likely primary metabolic

  20. EXPECTED CHANGES IN ACID-BASE DISORDERS • Primary Disorder Expected Changes • Metabolic acidosis PCO2 = 1.5 × HCO3 + (8 ± 2) • Metabolic alkalosis PCO2 = 0.7 × HCO3 + (21 ± 2) • Acute respiratory acidosis delta pH = 0.008 × (PCO2 - 40) • Chronic respiratory acidosis delta pH = 0.003 × (PCO2 - 40) • Acute respiratory alkalosis delta pH = 0.008 × (40 - PCO2) • Chronic respiratory alkalosis delta pH = 0.003 × (40 - PCO2) • From: THE ICU BOOK - 2nd Ed. (1998) [Corrected]

  21. Step 3-4: Is there appropriate compensation? Is it chronic or acute? • Respiratory Acidosis • Acute (Uncompensated): for every 10 increase in pCO2 -> HCO3 increases by 1 and there is a decrease of 0.08 in pH • Chronic (Compensated): for every 10 increase in pCO2 -> HCO3 increases by 4 and there is a decrease of 0.03 in pH • Respiratory Alkalosis • Acute (Uncompensated): for every 10 decrease in pCO2 -> HCO3 decreases by 2 and there is a increase of 0.08 in PH • Chronic (Compensated): for every 10 decrease in pCO2 -> HCO3 decreases by 5 and there is a increase of 0.03 in PH • Partial Compensated: Change in pH will be between 0.03 to 0.08 for every 10 mmHg change in PCO2

  22. Step 3-4: Is there appropriate compensation? • Metabolic Acidosis • Winter’s formula: Expected pCO2 = 1.5[HCO3] + 8 ± 2 OR  pCO2 = 1.2 ( HCO3) • If serum pCO2 > expected pCO2 -> additional respiratory acidosis and vice versa • Metabolic Alkalosis • Expected PCO2 = 0.7 × HCO3 + (21 ± 2) OR  pCO2 = 0.7 ( HCO3) • If serum pCO2 < expected pCO2 - additional respiratory alkalosis and vice versa

  23. EXAMPLE • ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5. • Winter’s formula : 17= 1.5 (7) +8 ±2 = 18.5(16.5 – 20.5) • So correct compensation so there is only one disorder Primary metabolic

  24. Step 5: Calculate the anion gap • AG used to assess acid-base status esp in D/D of met acidosis •  AG &  HCO3-used to assess mixed acid-base disorders • AG based on principle of electroneutrality: • Total Serum Cations = Total Serum Anions • Na + (K + Ca + Mg) = HCO3 + Cl + (PO4 + SO4 + Protein + Organic Acids) • Na + UC = HCO3 + Cl + UA • Na – (HCO3 + Cl) = UA – UC • Na – (HCO3 + Cl) = AG • Normal =12 ± 2

  25. Contd… • AG corrected = AG + 2.5[4 – albumin] • If there is an anion Gap then calculate the Delta/delta gap (step 6) to determine additional hidden nongap metabolic acidosis or metabolic alkalosis • If there is no anion gap then start analyzing for non-anion gap acidosis

  26. EXAMPLE • Calculate Anion gap • ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5/ Albumin 2. • AG = Na – Cl – HCO3 (normal 12 ± 2) 123 – 97 – 7 = 19 • AG corrected = AG + 2.5[4 – albumin] = 19 + 2.5 [4 – 2] = 19 + 5 = 24

  27. Step 6: Calculate Delta Gap • Delta gap = (actual AG – 12) + HCO3 • Adjusted HCO3 should be 24 (+_ 6) {18-30} • If delta gap > 30 -> additional metabolic alkalosis • If delta gap < 18 -> additional non-gap metabolic acidosis • If delta gap 18 – 30 -> no additional metabolic disorders

  28. EXAMPLE : Delta Gap • ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5/ Albumin 4. • Delta gap = (actual AG – 12) + HCO3 • (19-12) +7 = 14 • Delta gap < 18 -> additional non-gap metabolic acidosis • So Metabolic acidosis anion and non anion gap

  29. Metobolic acidosis: Anion gap acidosis

  30. EXAMPLE: WHY ANION GAP? • 65yo M with CKD presenting with nausea, diarrhea and acute respiratory distress • ABG :ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1 • So for our patient for anion gap portion its due to BUN of 119 UREMIA • But would still check lactic acid

  31. Nongap metabolic acidosis • For non-gap metabolic acidosis, calculate the urine anion gap • URINARY AG Total Urine Cations = Total Urine Anions Na + K + (NH4 and other UC) = Cl+ UA (Na + K) + UC = Cl + UA (Na + K) – Cl = UA – UC (Na + K) – Cl = AG • Distinguish GI from renal causes of loss of HCO3 by estimating Urinary NH4+ . • Hence a -ve UAG (av -20 meq/L) seen in GI, while +ve value (av +23 meq/L) seen in renal problem. • UAG = UNA + UK – UCL Kaehny WD. Manual of Nephrology 2000; 48-62

  32. EXAMPLE : NON ANION GAP ACIDOSIS • 65yo M with CKD presenting with nausea, diarrhea and acute respiratory distress • ABG :ABG 7.23/17/235 on 50% VM • BMP Na 123/ Cl 97/ HCO3 14 • AG = 123 – 97-14 = 12 • Most likely due to the diarrhea

  33. Metabolic alkalosis • Calculate the urinary chloride to differentiate saline responsive vs saline resistant • Must be off diuretics in order to interpret urine chloride

  34. Respiratory Alkalosis

  35. Case1. • 7.27/58/60 on 5L, HCO3- 26, anion gap is 12, albumin is 4.0 • 1. pH= Acidemia (pH < 7.4) • 2.CO2= Acid (CO2>40) • Opposite direction so Primary disturbance = Respiratory Acidosis • 3 &4: Compensation : Acute or chronic? ACUTE • CO2 has increased by (58-40)=18 • If chronic the pH will decrease 0.05 (0.003 x 18 = 0.054)  pH would be 7.35 • If acute the pH will decrease 0.14 (0.008 x 18 = 0.144) pH would be 7.26.

  36. Contd. • 5: Anion gap –N/A • 6: There is an acute respiratory acidosis, is there a metabolic problem too? • ΔHCO3- = 1 mEq/L↑/10mmHg↑pCO2 • The pCO2 is up by 18  so it is expected that the HCO3- will go up by 1.8. Expected HCO3- is 25.8, compared to the actual HCO3- of 26, so there is no additional metabolic disturbance. • Dx-ACUTE RESPIRATORY ACIDOSIS

  37. Case.2 • 7.54/24/99 on room air, HCO3- 20, anion gap is 12, albumin is 4.0. • 1: pH= Alkalemia (pH > 7.4) • 2.CO2= Base (CO2<40) • pH & pCO2 change in opposite Direction So Primary disturbance = Respiratory Alkalosis • 3 &4: Compensation ? acute or chronic? ACUTE • ΔCO2 =40-24=16 • If chronic the pH will increase 0.05 (0.003 x 16 = 0.048)  pH would be 7.45 • If acute the pH will increase 0.13(0.008 x 16 = 0.128) pH would be 7.53

  38. Contd… • 5:Anion gap – N/A • 6: There is an acute respiratory alkalosis, is there a metabolic problem too? • ΔHCO3- = 2 mEq/L↓/10mmHg↓pCO2 • The pCO2 is down by 16 so it is expected that the HCO3- will go down by 3.2. Expected HCO3- is 20.8, compared to the actual HCO3- of 20, so there is no additional metabolic disturbance. • Dx-ACUTE RESPIRATORY ALKALOSIS

  39. Case-3 • 7.58/55/80 on room air, HCO3- 46, anion gap is 12, albumin is 4.0. Ucl -20 • 1: pH= Alkalemia(pH > 7.4) • 2:CO2= Acid (CO2>40) • Same direction so Primary disturbance = Metabolic Alkalosis • 3&4: Compensation: • ∆ pCO2=0.7 x ∆ HCO3- • The HCO3- is up by 22.CO2 will increase by 0.7x22 = 15.4. Expected CO2 is 55.4, compared to the actual CO2 of 55, therefore there is no additional respiratory disturbance.

  40. contd • 5: No anion gap is present; and no adjustment needs to be made for albumin. Metabolic Alkalosis • Urinary chloride is 20 meq/l (< 25 meq/l)so chloride responsive, have to treat with Normal saline. Dx-METABOLIC ALKALOSIS

  41. Case-4 • 7.46/20/80 on room air, HCO3- 16, anion gap = 12, albumin = 4.0 • 1: pH = Alkalemia (pH > 7.4) • 2:CO2 = Base (CO2<40) • So Primary disturbance = Respiratory Alkalosis • 3 &4: Compensation? acute or chronic? Chronic • ΔCO2 =40-20= 20. • If chronic the pH will increase 0.06 (0.003 x 20 = 0.06)  pH would be 7.46. • If acute the pH will increase 0.16 (0.008 x 20 = 0.16) pH would be 7.56.

  42. Contd…. • 5: Anion gap – N/A • 6: There is a chronic respiratory alkalosis, is there a metabolic problem also? • Chronic: ΔHCO3- = 4 mEq/L↓/10mmHg↓pCO2 • The pCO2 is down by 20  so it is expected that the HCO3- will go down by 8. Expected HCO3- is 16, therefore there is no additional metabolic disorder. • Dx-CHRONIC RESPIRATORY ALKALOSIS

  43. Case-5 • 7.19/35/60 on 7L, HCO3- 9, anion gap = 18, albumin = 4.0 • 1: pH = Acidemia (pH < 7.4) • 2:CO2= Base (CO2<40) • So Primary disturbance: Metabolic Acidosis • 3&4: Compensation ? ∆ pCO2=1.2 x ∆ HCO3- • CO2 will decrease by 1.2 (∆HCO3-)  1.2 (24-9) 18. 40 – 18= 22 Actual CO2 is higher than expected Respiratory Acidosis • 5: Anion Gap = 18 (alb normal so no correction necessary)

  44. Contd….. 6: Delta Gap: • Delta gap = (actual AG – 12) + HCO3 = (18-12) + 9 = 6 + 9 = 15 which is<18 Non-AG Met Acidosis • Dx-ANION GAP METABOLIC ACIDOSIS with NON-ANION GAP METABOLIC ACIDOSIS with RESPIRATORY ACIDOSIS

  45. Case-6 • 7.54/80/65 on 2L, HCO3- 54, anion gap 12,albumin = 4.0 , Ucl 40 meq/l • 1: pH = Alkalemia (pH > 7.4) • 2:CO2= Acid (CO2>40) • So Primary disturbance: Metabolic Alkalosis • 3&4: Compensation? ∆ pCO2=0.7 x ∆ HCO3- • CO2 will increase by 0.7 (∆HCO3-)  0.7 (54-24) 2140 + 21 = 61 Actual CO2 is higher than expected Respiratory Acidosis

  46. Contd…. • 5: Anion Gap = 12 (alb normal so no correction necessary) • Urinary chloride is 40 meq/l (> 25 meq/l)so chloride resistant. So treatment would be disease specific and repletion of potassium • Dx-METABOLIC ALKALOSIS with RESPIRATORY ACIDOSIS

  47. Case-7 • 7.6/30/83 on room air, HCO3- 28, anion gap = 12, albumin = 4.0 • 1: pH = Alkalemia (pH > 7.4) • 2:CO2= Base (CO2<40) • SoPrimary Disturbance: Metabolic Alkalosis • 3&4: Compensation ? ∆ pCO2=0.7 x ∆ HCO3- • CO2 will increase by 0.7 (∆HCO3-)  0.7 (28-24) 2.8 40 + 2.8 = 42.8 Actual CO2 is lower than expected Respiratory Alkalosis • Anion Gap = 12 (alb normal so no correction necessary) • See urinary chloride for further Dx. • Dx-METABOLIC ALKALOSIS with RESPIRATORY ALKALOSIS

  48. Case-8 • A 50 yo male present with sudden onset of SOB with following ABG 7.25/46/78 on 2L, HCO3- 20, anion gap = 10, albumin = 4.0 • 1: pH = Acidemia (pH < 7.4) • 2:CO2= Acid (CO2>40) • So Primary disturbance: Respiratory Acidosis • 3 &4: If respiratory disturbance is it acute or chronic? ACUTE • ∆ CO2 = 46-40= 6 • If chronic the pH will decrease 0.02 (0.003 x 6 = 0.018)  pH would be 7.38 • If acute the pH will decrease 0.05 (0.008 x 6 = 0.048) pH would be 7.35.

  49. Contd… • Anion Gap = 10 (alb normal so no correction necessary) • 6: There is an acute respiratory acidosis, is there a metabolic problem too? • ∆ HCO3- = 1 mEq/L↑/10mmHg↑pCO2 • The HCO3- will go up 1mEq/L for every 10mmHg the pCO2goes up above 40 • The pCO2 is up by 6  so it is expected that the HCO3- will go up by 0.6. Expected HCO3- is 24.6, compared to the actual HCO3- of 20. Since the HCO3- is lower than expected Non-Anion Gap Metabolic Acidosis (which we suspected). • Dx-RESPIRATORY ACIDOSIS with NON-ANION GAP METABOLIC ACIDOSIS

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