arterial blood gases , a guide to pg students of anesthesiology

Arterial blood gases

1. Arterial blood gas analysis Presented by: dr Tetikcha Gurung Moderated by : dr ram rai

2. INTRODUCTION ABG provides rapid information on three physiological processes: Ventilation( reflected by Oxygenation status( assessed primarily by Acid Base Balance ABG analysis  essential for diagnosing and managing the patient’s oxygenation status , ventilation failure and acid base balance.  

3. INDICATION Assess the ventilatory setting, oxygenation and acid base status. Assess the response to an intervention. Regulate electrolyte therapy. Establish preoperative baseline parameters.

4. contraindication An abnormal modified Allen’s test. Local infection or distorted anatomy at puncture site. Severe peripheral vascular disease of the artery. Active Raynaud’s Syndrome

5. site Radial artery ( most common ) Brachial artery Femoral artery Radial is the most preferable site used because: It is easy to access It is not a deep artery which facilitate palpation, stabilization and puncturing The artery has a collateral blood circulation

6. equipments Blood gas kit OR 1ml syringe 23-26 gauge needle Stopper or cap Alcohol swab Disposable gloves Plastic bag & crushed ice Lidocaine (optional) Vial of heparin (1:1000) Bar code or label

7. Methodology PREPARATORY PHASE: Record patient inspired oxygen concentration. Explain the procedure to the patient. Heparinize the needle. Donot leave excess heparin in the syringe ↑↑ heparin  ↑↑ dilutional effect  ↓↓ and ↓↓ Wait at least 20 minutes before drawing blood for ABG after changing settings of mechanical ventilation, after suctioning the patient or after extubation .  

8. After preparing the site, the artery is palpated for maximum pulsation In case of radial artery , Modified Allen test is done. Skin and subcutaneous tissue may be infiltrated with local anesthetic agent if needed The needle is inserted at 45 in radial, 60 in brachial and 90 in femoral. Ensure no air bubble Air Bubble has 0 mmHg Air Bubble + Blood = ↑↑ ↓↓ Place the capped syringe in the container of ice immediately Maintain firm pressure on the puncture site for 5 minutes.  

9. Modified allen’s test Test to determine collateral circulation is present from the ulnar artery in case thrombosis occur in the radial artery.

10. ABG syringe must be transported at the earliest to the laboratory for early analysis via cold chain ABG sample should always be sent with relevant information regarding Temperature.  

11. Most ABG analyzer report data at normal body temperature  patient body temperature affect the value of 0.1mL of consumed / dL of blood in 10 min in patients with normal TLC level  leucocytosis causes ↓↓pH and ↑↑ level  immediate chilling / analysis ++  

12. COMPLICATION Arteriospasm Infection Hematoma Hemorrhage Distal Ischemia Gangrene AV Fistula

13. INTERPRETATION OF abG OXYGENATION ALVEOLAR VENTILATION ACID BASE BALANCE

14. ACID BASE BALANCE Acid base balance is defined by the concentration of hydrogen ion The hydrogen ion concentration in aqueous solution is expressed by pH which is defined as negative logarithm( base 10 ) of [ pH = log(1/ [ -log [  

15. The acid base equilibrium is described using Henderson Hasselbach Equation:

16. BICARBONATE BUFFER SYSTEM Acts within few seconds RESPIRATORY REGULATION Acts within few minutes RENAL REGULATION Acts in hours to days

17. Chemical buffer Buffer = base molecule and its weak conjugate acid. pKa = dissociation ionization constant  pH at which acid is 50 % dissociated and 50% undissociated. pKA indicates strength of the acid There are 2 buffer system: Extracellular buffer system Intracellular buffer system

18. Extracellular buffer system It includes : Bicarbonate buffer system( pKa =6.1) and Phosphate buffer system( pKa =6.8). Bicarbonate Buffer System ( ) The base = bicarbonate and its weak acid conjugate= carbonic acid CO 2 + H 2 O carbonic anhydrase H 2 CO 3 H + + HCO 3 -  

19. Intracellular buffer It includes : Hemoglobin buffer ( HbH /Hb) Other protein buffer ( PrH / Pr −) Phosphate buffer (H2PO4 −/HPO4 2−),

20. Hemoglobin buffer system

21. Respiratory regulation

22. Renal regulation Occurs via 3 mechanism: reabsorption of the filtered excretion of titratable acids, production of ammonia  

23. ANALYTE Normal Value Units pH 7.35 - 7.45 PCO2 35 - 45 mm Hg PO2 72 – 104 mm Hg` [HCO3] 22 – 30 meq/L SaO2 95-100 % Anion Gap 9 + 3 meq/L B.E +2 to -2 meq /L

24. DEFINITIONS ACID: molecule that can act as a proton (H+) donor BASE: molecule that can act as a proton acceptor. ACIDEMIA:A blood pH less than 7.35 ALKALKEMIA : a blood pH greater than 7.45 ACIDOSIS – presence of a process which tends to  pH by virtue of gain of H +  or loss of HCO 3 - ALKALOSIS – presence of a process which tends to  pH by virtue of loss of H + or gain of HCO 3 -

25. Simple Acid Base Disorder/ Primary Acid Base disorder – a single primary process  of acidosis or alkalosis due to an initial change in PCO 2 and HCO 3 . Compensation - The normal response of the  respiratory system or kidneys to change in pH induced by a primary acid-base disorder The Compensatory responses to a primary Acid Base disturbance are never enough to correct the change in pH they only act to reduce the severity. Mixed Acid Base Disorder – Presence of more than one acid base disorder simultaneously .

26. Buffer Base: It is total quantity of buffers in blood including both volatile(Hco 3 ) and nonvolatile (as Hgb,albumin,Po 4 ) 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

27. The H + In extracellular fluid is determined by balance between the pCO 2 and HCO 3 - in the fluid. This relationship is expressed as H + = 24 x (pCO 2 / HCO 3 )

28. STEPWISE APPROACH TO ACID BASE ANALYSIS

29. STEP 1: Check for authenticity STEP 2: : Identify the primary Acid Base disorder STEP 3: Evaluate the Secondary Response STEP 4: Calculate Anion Gap

30. STEP 1 : CHECH FOR AUTHENTICITY [H+] neq /l = 24 X (PCO 2 / HCO 3 ) Calculate it from the ABG report and if this value is equal to H+ in the report,the ABG report is authentic. Alternatively subtract the last two digits of the pH( e.g 20 in Ph 7.20) from 80, this value is approximately equal to the H+ concentration in the ABG report. = 24 x = ± 2 of of venous blood ; ifnot then the ABG is invalid and not compatible   H + ion pH 100 7.00 79 7.10 63 7.20 50 7.30 45 7.35 40 7.40 35 7.45 32 7.50 25 7.60

31. STEP 2 : IDENTIFY THE PRIMARY ACID BASE DISORDER. RULE 1 : If the and /or pH is outside the normal range  acid base disorder RULE 2: if the and pH are both abnormal, compare the directional change 2a: if and pH or pH  primary metabolic acid base disorder 2b : if and ↓ pH  primary respiratory acid base disorder  

32. RULE 3: if the pH is abnormal, the condition is a mixed metabolic and respiratory disorder. 3a: if is abnormal, directional change in  type of respiratory disorder 3b: if pH is abnormal , the directional change in pH metabolic disorder.  

33. STEP 3 : EVALUATE THE SECONDARY RESPONSE

34. RULE 4: For a primary metabolic acidosis , if measured is higher than expected  secondary respiratory acidosis and measured is less than expected  secondary respiratory alkalosis.  

35. 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

36. 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]

37. RULE 5: For a primary respiratory disorder, a normal or near normal  acute RULE 6: For a primary respiratory disorder where the is abnormal , determine the expected for a chronic respiratory disorder 6a : For a chronic respiratory acidosis, if is lower than expected  incomplete renal response is higher than expected  secondary metabolic alkalosis 6b : For a chronic respiratory alkalosis, if is higher than expected  incomplete renal response is lower than expected  secondary metabolic alkalosis  

38. 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

39. ANION GAP Normally, measured cation(MC) + unmeasured cation(UC) = measured anion(MA) + unmeasured anion(UA). MC – MA = UA-UC Measured cation = and - ( = UA –UC UA - UC = Anion Gap(AG)= 8-12mEq/L Corrected Anion Gap= Anion Gap + 2.5(4.5- albumin of patient) 1g/dL of Albumin contribute to 3 mEq /L of Anion Gap  

40. HAGMA( High Anion Gap Metabolic Acidosis) Anion gap is high because fall in is not compensated by Causes K- Ketoacidosis due to endogenous causes/ acid  Diabetes, Alcohol , Starvation U-U remic Acidosis S-S alicylate/Paraldehyde M-M ethanol E-E thylene Glycol L-L actic Acidosi  

41. NAGMA(Normal Anion Gap Metabolic Acidosis) A.k.a Hyperchloremic Metabolic Acidosis Fall in is compensated by Causes:   Causes of nongap metabolic acidosis - DURHAM Diarrhea, ileostomy, colostomy, enteric fistulas Ureteral diversions or pancreatic fistulas RTA type I or IV, early renal failure Hyperailmentation , hydrochloric acid administration Acetazolamide, Addison’s Miscellaneous – post-hypocapnia, toulene , sevelamer, cholestyramine ingestion

42. CALCULATE ANION gaP - ( = UA –UC; = 140 and = 106 UA - UC = Anion Gap(AG)= 8-12mEq/L In case of HAGMA, calculate Delta Ratio= 1-2 Delta Ratio = = When Delta Ratio < 1 ; ∆ increased disproportionately  HAGMA+NAGMA When Delta Ratio >2 ; ∆ decreased disproportionately  HAGMA+ metabolic alkalosis When Delta Ratio 1-2  HAGMA  

43. If a patient has normal anion gap , cause may be RTA GI loss of bicarbonate History to be noted If no history + , check for Urine Anion Gap Urine Anion Gap( UAG) = Urine +(Urine In RTA  UAG more positive In GI Loss of  UAG negative.  

44. 33636222222222222222222879566