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dr. Husnil Kadri, M.Kes Biochemistry Departement Medical Faculty Of Andalas University Padang Arterial Blood Gases • • • • Aids in establishing a diagnosis Helps guide treatment plan Aids in ventilator management Improvement in acid/base management allows for optimal function of medications • Acid/base status may alter electrolyte levels critical to patient status/care Logistics • When to order an arterial line -– Need for continuous BP monitoring – Need for multiple ABGs • Where to place – (with antikoagulant) – A. Radial – A. Femoral – A. Brachial – A. Dorsalis Pedis – A. Axillary The Components Desired Ranges: – pH ; 7.35 - 7.45 – PaCO2 ; 35-45 mmHg – PaO2 ; 80-100 mmHg – HCO3 ; 21-27 – O2sat ; 95-100% – Base Excess ; +/-2 mEq/L Arterial Blood Gases • Reflect oxygenation, gas exchange, and acidbase balance • PaO2 is the partial pressure of oxygen dissolved in arterial blood • SaO2 is the amount of oxygen bound to hemoglobin Base Excess Definition: The amount of a strong acid (like HCl) needed to bring blood to 7.40. • Assumes 100% oxygenation, 37oC, and pCO2 of 40. Normal = 0 Used to calculate the metabolic component of an acid-base disturbance. Base Excess calculations Calculated the same way, in practice, as SID: Buffer Base (SID) = HCO3- + AHCO3 calculated by pH & pCO2 (blood gas machine) A- calculated using pH & hemoglobin (whole blood) OR A- calculated using albumin & phos (plasma) BE = Buffer Base – “expected buffer base” (expected if pH = 7.4 and pCO2 = 40) Indicators of hypoxaemia and hypoxia Arterial blood gases PO2 SO2 pH PCO2 Lab Findings 80-100 mm Hg (normal) 60-80 mm Hg (mild hypoxemia) 40-60 mm Hg (moderate hypoxemia) <40 mm Hg (severe hypoxemia) 95%-97% (normal) <90% (may indicate hypoxemia) 7.35-7.45 (normal) <7.35 (acidemia) >7.45 (alkalemia) 35-45 mm Hg (normal) >45 mm Hg (hypoventilation) <35 mm Hg (hyperventilation) 8 Is it Respiratory or Metabolic? 1. Respiratory Acidosis • Increased pCO2 >50 2. Respiratory Alkalosis • Decreased pCO2<30 3. Metabolic Acidosis • 4. Metabolic Alkalosis • Decreased HCO3 <18 Increased HCO3 >30 Compensated or Uncompensated—what does this mean? 1. Evaluate pH—is it normal? Yes 2. Next evaluate pCO2 & HCO3 • pH normal + increased pCO2 + increased HCO3 = compensated respiratory acidosis • pH normal + decreased HCO3 + decreased pCO2 = compensated metabolic acidosis Compensated vs. Uncompensated 1. Is pH normal? No 2. Acidotic vs. Alkalotic 3. Respiratory vs. Metabolic • pH<7.30 + pCO2>50 + normal HCO3 = uncompensated respiratory acidosis • pH<7.30 + HCO3<18 + normal pCO2 = uncompensated metabolic acidosis • pH>7.50 + pCO2<30 + normal HCO3 = uncompensated respiratory alkalosis • pH>7.50 + HCO3>30 + normal pCO2 = uncompensated metabolic alkalosis Causes of Acidosis • Respiratory – Hypoventilation – Impaired gas exchange • Metabolic – Ketoacidosis • Diabetes – Renal Tubular Acidosis • Renal Failure – Lactic Acidosis • Decreased perfusion • Severe hypoxemia Causes of Alkalosis • Respiratory – Hyperventilation due to: • Hypoxemia • Metabolic acidosis • Neurologic –Lesions –Trauma –Infection • Metabolic – Hypokalemia – Gastric suction or vomiting – Hypochloremia Mixed Metabolic Acidosis and Chronic Respiratory Alkalosis Examples: • Sepsis • Addition of respiratory alkalosis to metabolic acidosis further decreases HCO3- but pH may remain normal • Lactic acidosis plus respiratory alkalosis due to severe liver disease, pulmonary emboli, or sepsis 14 Mixed Metabolic Alkalosis and Chronic Respiratory Acidosis Examples: • Patient with COPD receiving glucocorticoids or diuretics • pCO2 and HCO3- are increased by both conditions, but pH is neutralized 15 Mixed Alkalosis, Severe Example: • Postoperative patient with severe hemorrhage stimulating hyperventilation [respiratory alkalosis] plus massive transfusion and nasogastric drainage [metabolic alkalosis] 16 Mixed Chronic Respiratory Acidosis and Acute Metabolic Acidosis Examples: • COPD [chronic respiratory acidosis] with severe diarrhoea [metabolic acidosis]. pH is too low for pCO2 of 55 mmHg in chronic respiratory acidosis, indicating low pH due to mixed acidosis, but HCO3- effect is offset 17 Mixed Metabolic Acidosis and Metabolic Alkalosis Examples: • Gastroenteritis with vomiting [metabolic alkalosis] and diarrhoea [metabolic acidosis due to loss of HCO3-]; surprisingly normal findings with marked volume depletion 18 Serum Values in Acid-Base Disturbances Condition Na+ Cl- HCO3- pCO2 mmol/L mmol/L mmol/L mmHg Normal 140 105 25 40 7.40 Metabolic acidosis 140 115 15 31 7.30 Chronic respiratory alkalosis 136 102 25 40 7.44 Mixed metabolic acidosis and 136 chronic respiratory alkalosis 140 Metabolic alkalosis 108 14 24 7.39 92 36 48 7.49 100-102 28 50 7.37 90 67 7.40 Chronic respiratory acidosis 140 Mixed metabolic alkalosis and 140 chronic respiratory acidosis 40 pH 19 Serum Values in Acid-Base Disturbances Condition Na+ Cl- HCO3- pCO2 pH mmol/L mmol/L mmol/L mmHg Normal 136-145 100-106 24-26 35-45 7.35-7.45 Metabolic alkalosis 139 89 35 47 7.49 Respiratory alkalosis 136 102 20 30 7.44 Mixed alkalosis, mild 139 92 32 39 7.53 Mixed alkalosis, severe 139 92 32 30 7.63 Mixed chronic respiratory acidosis and acute metabolic acidosis Mixed metabolic acidosis and metabolic alkalosis 136 102 22 55 7.22 140 103 25 40 7.40 20 Summary of Pure and Mixed Acid-Base Disorders Decreased pH Normal pH Increased pH pCO2 Respiratory acidosis with or without incompletely compensated metabolic alkalosis or coexisting metabolic acidosis Respiratory acidosis and compensated metabolic alkalosis Metabolic alkalosis with incompletely compensated respiratory acidosis or coexisting respiratory acidosis Normal pCO2 Metabolic acidosis Normal Metabolic alkalosis Source: Adapted from Friedman HH. Problem-oriented medical diagnosis, 3rd ed. Boston: Little, Brown. 1983 21 References • Anisman, S. Base Excess & Strong Ion Theories. ppt. 2003. • Klee, V. Arterial Blood Gas Analysis.ppt. 2012. • Perkins, J. ABG Interpretation. ppt. 2012. • Rashid, FA. Respiratory Mechanisms in Acid-Base Homeostasis.ppt. 2005.