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

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Outline 1. Common Errors During ABG Sampling 2. Components of ABG 3. Discuss simple steps in analyzing ABGs 4. Calculate the anion gap 5. Calculate the delta gap 6. Differentials for specific acid-base disorders

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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 minutes PaCO2 3-10 mmHg/hour PaO2 PaO2 pH d/t lactic acidosis generated by glycolysis in R.B.C.

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Parameter37 C (Change every 10 min) 4 C (Change every 10 min) pH PCO2 1 mm Hg0.1 mm Hg PO2 0.1 vol %0.01 vol % Temp Effect On Change of ABG Values

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FEVERFEVER OR HYPOTHERMIAHYPOTHERMIA 1. Most ABG analyzers report data at N body temp 2. If severe hyper/hypothermia, values of pH & PCO2 at 37 C can be significantly diff from pt’s actual values 3. Changes in PO2 values with temp also predictable Hansen JE, Clinics in Chest Med 10(2), If Pt.’s temp < 37C Substract 5 mmHg Po2, 2 mmHg Pco2 and Add pH per 1C decrease of temperature

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AIR BUBBLES : 1.PO2 150 mmHg & PCO2 0 mm Hg in air bubble(R.A.) 2.Mixing with sample, lead to PaO2 & PaCO2PaO2 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

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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 HEPARINHEPARIN Dilutional effect on results HCO 3 - & 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

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TYPE OF SYRINGE 1. pH & PCO2 values unaffected 2. 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: 1. size of syringe/needle 2. vol of sample HYPERVENTILATION OR BREATH HOLDING HYPERVENTILATION May lead to erroneous lab results

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COMPONENTS OF THE ABG pH : Measurement of acidity or alkalinity, based on the hydrogen (H+) – 7.45 Pao 2 : The partial pressure oxygen that is dissolved in arterial blood mm Hg. PCO 2 : The amount of carbon dioxide dissolved in arterial blood. 35– 45 mmHg HCO 3 : The calculated value of the amount of bicarbonate in the blood. 22 – 26 mmol/L SaO 2: The arterial oxygen saturation. >95% pH,PaO 2,PaCO 2, Lactate and electrolytes are measured Variables HCO 3 (Measured or calculated)

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Contd… 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

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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)

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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

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Normal Values Variable Normal Normal Range(2SD) pH pCO Bicarbonate Anion gap Albumin4 4

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Steps for ABG analysis 1. What is the pH? Acidemia or Alkalemia? 2. What is the primary disorder present? 3. Is there appropriate compensation? 4. Is the compensation acute or chronic? 5. Is there an anion gap? 6. If there is a AG check the delta gap? 7. What is the differential for the clinical processes?

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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 ????

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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

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Step 2: What is the primary disorder? What disorder is present? pHpCO2HCO3 Respiratory Acidosis pH lowhigh Metabolic AcidosispH low low Respiratory Alkalosis pH high low Metabolic AlkalosispH high high

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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

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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

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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 = × (PCO2 - 40) Chronic respiratory acidosis delta pH = × (PCO2 - 40) Acute respiratory alkalosis delta pH = × (40 - PCO2) Chronic respiratory alkalosis delta pH = × (40 - PCO2) From: THE ICU BOOK - 2nd Ed. (1998) [Corrected]

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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

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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

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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

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Step 5: Calculate the anion gap AG used to assess acid-base status esp in D/D of met acidosis AG & HCO 3 - 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

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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

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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] = [4 – 2] = = 24

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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 additional non-gap metabolic acidosis If delta gap 18 – 30 -> no additional metabolic disorders

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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 additional non-gap metabolic acidosis So Metabolic acidosis anion and non anion gap

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Metobolic acidosis: Anion gap acidosis

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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

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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. U AG = U NA + U K – U CL Kaehny WD. Manual of Nephrology 2000; 48-62

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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 – = 12 Most likely due to the diarrhea

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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

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Metabolic alkalosis Calculate the urinary chloride to differentiate saline responsive vs saline resistant Must be off diuretics in order to interpret urine chloride Saline responsive U CL <25Saline-resistant U CL >25 VomitingIf hypertensive: Cushings, Conn’s, RAS, renal failure with alkali administartion NG suctionIf not hypertensive: severe hypokalemia, hypomagnesemia, Bartter’s, Gittelman’s, licorice ingestion Over-diuresisExogenous corticosteroid administration Post-hypercapnia

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Respiratory Alkalosis Causes of Respiratory Alkalosis Anxiety, pain, fever Hypoxia, CHF Lung disease with or without hypoxia – pulmonary embolus, reactive airway, pneumonia CNS diseases Drug use – salicylates, catecholamines, progesterone Pregnancy Sepsis, hypotension Hepatic encephalopathy, liver failure Mechanical ventilation Hypothyroidism High altitude

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Case1. 7.27/58/60 on 5L, HCO , anion gap is 12, albumin is 4.0 1. pH= Acidemia (pH < 7.4) 2. CO 2 = Acid (CO2>40) Opposite direction so Primary disturbance = Respiratory Acidosis 3 &4: Compensation : Acute or chronic? ACUTE CO 2 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.

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Contd. 5: Anion gap –N/A 6: There is an acute respiratory acidosis, is there a metabolic problem too? ΔHCO 3 - = 1 mEq/L↑/10mmHg↑pCO 2 The pCO 2 is up by 18 so it is expected that the HCO 3 - will go up by 1.8. Expected HCO 3 - is 25.8, compared to the actual HCO 3 - of 26, so there is no additional metabolic disturbance. Dx-ACUTE RESPIRATORY ACIDOSIS

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Case.2 7.54/24/99 on room air, HCO , anion gap is 12, albumin is 4.0. 1: pH= Alkalemia (pH > 7.4) 2. CO 2 = Base (CO2<40) pH & pCO2 change in opposite Direction So Primary disturbance = Respiratory Alkalosis 3 &4: Compensation ? acute or chronic? ACUTE ΔCO 2 =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

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Contd… 5:Anion gap – N/A 6: There is an acute respiratory alkalosis, is there a metabolic problem too? ΔHCO 3 - = 2 mEq/L↓/10mmHg↓pCO 2 The pCO 2 is down by 16 so it is expected that the HCO 3 - will go down by 3.2. Expected HCO 3 - is 20.8, compared to the actual HCO 3 - of 20, so there is no additional metabolic disturbance. Dx-ACUTE RESPIRATORY ALKALOSIS

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

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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

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

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Contd…. 5: Anion gap – N/A 6: There is a chronic respiratory alkalosis, is there a metabolic problem also? Chronic: ΔHCO 3 - = 4 mEq/L↓/10mmHg↓pCO 2 The pCO 2 is down by 20 so it is expected that the HCO 3 - will go down by 8. Expected HCO 3 - is 16, therefore there is no additional metabolic disorder. Dx-CHRONIC RESPIRATORY ALKALOSIS

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

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

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

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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

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Case-7 7.6/30/83 on room air, HCO , anion gap = 12, albumin = 4.0 1: pH = Alkalemia (pH > 7.4) 2: CO 2 = Base (CO2<40) SoPrimary Disturbance: Metabolic Alkalosis 3&4: Compensation ? ∆ pCO 2 =0.7 x ∆ HCO 3 - CO 2 will increase by 0.7 (∆HCO 3 - ) 0.7 (28-24) 2.8 = 42.8 Actual CO 2 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

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Case-8 A 50 yo male present with sudden onset of SOB with following ABG 7.25/46/78 on 2L, HCO , anion gap = 10, albumin = 4.0 1: pH = Acidemia (pH < 7.4) 2:CO 2 = Acid (CO2>40) So Primary disturbance: Respiratory Acidosis 3 &4: If respiratory disturbance is it acute or chronic? ACUTE ∆ CO 2 = 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.

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Contd… Anion Gap = 10 (alb normal so no correction necessary) 6: There is an acute respiratory acidosis, is there a metabolic problem too? ∆ HCO 3 - = 1 mEq/L↑/10mmHg↑pCO 2 The HCO 3 - will go up 1mEq/L for every 10mmHg the pCO 2 goes up above 40 The pCO 2 is up by 6 so it is expected that the HCO 3 - will go up by 0.6. Expected HCO 3 - is 24.6, compared to the actual HCO 3 - of 20. Since the HCO 3 - 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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Case-9 7.15/22/75 on room air, HCO 3 - 9, anion gap = 10, albumin = 2.0 1: pH = Acidemia (pH < 7.4) 2:CO 2 = Base (CO2<40) So Primary disturbance: Metabolic Acidosis 3&4: ∆ Compensation ? pCO 2 =1.2 x ∆ HCO 3 - Expected pCO 2 = 1.2 x ∆ HCO 3 - 1.2 (24 -9) 1.2 (15) 18. The expected pCO 2 is 22 mmHg. The actual pCO 2 is 22, which is expected, so there is no concomitant disorder.

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Contd…. 5: Anion Gap = 10 AGc = (4-2) = 15 Anion Gap Metabolic Acidosis 6: Delta Gap: Delta gap = (actual AG – 12) + HCO3 = (15-12) + 9 = 3+ 9 = 12 which is<18 Non-AG Met Acidosis Dx-ANION GAP METABOLIC ACIDOSIS with NON-ANION GAP METABOLIC ACIDOSIS

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