Kingdom of Bahrain Arabian Gulf University College of
Kingdom of Bahrain Arabian Gulf University College of Medicine and Medical Sciences Unit I – Problem 3 – Clinical Acid-Base Disturbances Prepared by: Ali Jassim Alhashli Based on: Kaplan Step 2 CK Internal Medicine
Acid/Base Disturbances (ABG Interpretation) • • First of all, you must know the normal values of ABG: – p. H = 7. 35 – 7. 45… (7. 4). – PCO 2 = 35 – 45 mm. Hg… (40 mm. Hg). – HCO 3 = 22 – 26 m. Eq/L… (24 m. Eq/L). What are the steps which you must follow when interpreting an ABG result? – First, look to the p. H and comment if the patient has acidosis or alkalosis? • Acidosis: p. H < 7. 35 • Alkalosis: p. H > 7. 45 – Then, comment whether this acid-base disturbance is respiratory or metabolic? • Respiratory acidosis (↑PCO 2): look for any causes of hypoventilation which leads to accumulation of CO 2 that is considered to be an acid: – COPD. – Obesity. – Sleep apnea. – Depression of respiratory drive by opiates. – Effusions. – Suffocation. • Respiratory alkalosis (↓PCO 2): look for any causes of hyperventilation which leads to washout of CO 2 from the lungs: – Pain. – Anxiety. – Severe anemia. – Pulmonary embolism.
Acid/Base Disturbances (ABG Interpretation) • What are the steps which you must follow when interpreting an ABG result? – First, look to the p. H and comment if the patient has acidosis or alkalosis? • Acidosis: p. H < 7. 35 • Alkalosis: p. H > 7. 45 – Then, comment whether this acid-base disturbance is respiratory or metabolic? (continued) • Metabolic acidosis (↓HCO 3): causes are classified according to the anion gap (anion gap = (Na) – (HCO 3 + Cl)… normal value = 8 -12): – Low anion gap: » Multiple myeloma. » Low albumin level. » Lithium. – Normal anion gap: » Diarrhea. » Renal tubular Acidosis (RTA). » Ureterosigmoidostomy (it is a surgical procedure in which ureter are connected to sigmoid colon and considered as a part of treatment of bladder cancer in which urinary bladder has to be removed). – High anion gap (mnemonic: LA MUD PIE): » L: Lactate. » A: Aspirin. » M: Methanol. » U: Uremia. » D: DKA. » P: Propylene glycol. » I: Isoniazid and Isopropyl alcohol. » E: Ethyelen glycol. • Metabolic alkalosis (↑HCO 3): can be caused by the following: – Loss of H: vomiting, Conn syndrome or diuretics. – HCO 3 retention: bicarboate administration. – Movement of H into cells: hypokalemia.
Acid/Base Disturbances (ABG Interpretation) • What are the steps which you must follow when interpreting an ABG result? – First, look to the p. H and comment if the patient has acidosis or alkalosis? • Acidosis: p. H < 7. 35 • Alkalosis: p. H > 7. 45 – Then, comment whether this acid-base disturbance is respiratory or metabolic? – After that, look if there is compensation by body systems trying to bring the p. H back to normal. • There are 2 classifications: – Full compensation: in which p. H completely returns back to normal range. – Partial compensation: in which p. H doesn’t return back to normal range. Notice that with presence of compensation, both CO 2 and HCO 3 should move in the same direction (for example, if there is respiratory acidosis (↑CO 2) → there will be retention of HCO 3). If they do not move in the same direction → this indicated the presence of a mixed disorder. • Compensation of respiratory acidosis: – Acute respiratory acidosis: HCO 3 increases 1 m. Eq/L for each 10 mm. Hg increase in PCO 2. – Chronic respiratory acidosis: HCO 3 increases 4 m. Eq/L for each 10 mm. Hg increase in PCO 2. • Compensation of respiratory alkalosis: – Acute respiratory alkalosis: HCO 3 decreases 2 m. Eq/L for each 10 mm. Hg decrease in PCO 2. – Chronic respiratory alkalosis: HCO 3 decreases 5 m. Eq/L for each 10 mm. Hg decrease in PCO 2. • Compensation of metabolic acidosis: – Expected PCO 2 = 1. 5 x (HCO 3) + 8 (± 2) • Compensation for metabolic alkalosis: – Expected PCO 2 = 0. 7 x (HCO 3) + 20 (± 5)
Acid/Base Disturbances (ABG Interpretation)
Acid/Base Disturbances (ABG Interpretation) • • Case (1): – An elderly female from a nursing home was transferred to hospital because of profound weakness and areflexia. Her oral intake had been poor for a few days. Current medication was a diuretic tablet. – Arterial Blood Gases: • p. H = 7. 58 • p. CO 2 = 50 mm. Hg • p. O 2 = not provided. • HCO 3 = 44. 4 mmol/l – Patient has metabolic alkalosis because HCO 3 is high and explains p. H value. – Compensation for her metabolic alkalosis: • Expected p. CO 2 = 0. 7 (HCO 3) + 20 mm. Hg (range: ± 5) 0. 7 (44) + 20 = 55 (50 -55) – Therefore, p. CO 2 is within the expected range. – Final comment: pure partially compensated metabolic alkalosis. Case (2): – A known case of COPD with the following ABG results: • p. H = 7. 34 • p. CO 2 = 44 mm. Hg • HCO 3 = 28 mmol/l – Patient has chronic respiratory acidosis because p. CO 2 is high and explains p. H value. – Compensation for chronic respiratory acidosis: • HCO 3 increases 4 m. Eq/l for each 10 mm. Hg rise in p. CO 2 • In this patient, p. CO 2 is 5 mm. Hg above the baseline. Therefore, HCO 3 should increase by 2 m. Eq/l (24 + 2 = 26 mmol/l). You can notice that patients HCO 3 is more than 26 mmol/l (indicating the presence of metabolic alkalosis) – Final comment: chronic respiratory acidosis with metabolic alkalosis.
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