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Blood gas and acid base evaluation

Blood gas and acid base evaluation. Chapter 20. Step one. Normal Ph 7.35 – 7.45 PCO2 35 – 45 mmHg ( 31 – 39 ) PO2 60 – 100 mmHg BE -2 - +2 HCO3 24 AG 8 – 16 Na , K , Cl Ph = pKa + log X HCO3/PCO2 x Solubility A-a gradient N < 20 mmHg O2 – Hb dissociation scale. Step 1.

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Blood gas and acid base evaluation

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  1. Blood gas and acid base evaluation Chapter 20

  2. Step one • Normal • Ph 7.35 – 7.45 • PCO2 35 – 45 mmHg ( 31 – 39 ) • PO2 60 – 100 mmHg • BE -2 - +2 • HCO3 24 • AG 8 – 16 • Na , K , Cl • Ph = pKa + log X HCO3/PCO2 x Solubility • A-a gradient N < 20 mmHg • O2 – Hb dissociation scale

  3. Step 1 • Ph = pKa + log X HCO3/PCO2 x Solubility • CO2 + H2O – H2CO3 – H + HCO3 • A-a gradient N < 20 mmHg • O2 – Hb dissociation scale

  4. Step two • Is there an OXYGENATION problem • PO2 • Saturation • Must always be interpreted with FiO2 • PF Ratio = PO2/FiO2 • Normal > 400 • ALI < 300 • ARDS < 200

  5. Step 2 • Reason for oxygenation problem • Work out A-a gradient • Decide if it is an intrinsic lung problem

  6. Step 3 • Is there a VENTILATION problem • Look at PaCO2 • <35 hyperventilation • >45 hyperventilation • Remember that ventilation always interpreted with pH in mind

  7. Step 4 • What is the pH • Normal • Normal • Compensated • Mixed • Alkalosis • Acidosis

  8. Step 5 • Does the PaCO2 FIT IN with the pH • Alkalosis + PaCO2 < 40 mmHg = Respiratory Alkalosis • Acidosis + PaCO2 > 40 mmHg = Respiratory acidosis • If the PaCO2 does not fit in with the ph , move to next step • If the PaCO2 fit in with Ph = PRIMARY ABNORMALITY

  9. Step 6 • Look at the HCO3 • Does the SBC fit in with the pH • Alkalosis + SBC > 24 = Metabolic alkalosis • Acidosis + SBC < 24 = Metabolic acidosis • This will be the primary abnormality if the PaCO2 did not fit in with pH

  10. But wait , there is more • Remember that basic 4 abnormalities are • Resp acidosis • Metab acidosis • Resp alkalosis • Metab alkalosis • Uncompensated disorders • Compensated disorders • Mixed disorders • Chronic versus acute

  11. Step 7 • Determine the degree of COMPENSATION • Rules apply ( tables ) • Thus the body always tries to compensate for the primary abnormality • If compensation as expected = normal compensation • If compensation not as expected = mixed disorder

  12. Step 8 • Calculate the ANION GAP if metabolic acidosis • AG = Cations – Anions = 8-16 mEq/L = unmeasured anions • AG = (Sodium + Potasium) – (Choride + Bicarbonate) • Acidosis + AG > 16 Mm = High anion gap metabolic acidosis • Acidosis + AG normal = Low anion gap metabolic acidosis

  13. Step 9 • Look at the Haemotological variables • Haematocrit • Haemoglobin • Abnormal haemoglobins

  14. Step 10 • Look at the metabolites and electrolytes • Lactate • Glucose • Calcium • Sodium • Potassium • Magnesium

  15. Intracellular pH • 6.8 • Neutral at body temp of 37 degrees celcius • Hydrogen = Hydroxyl • Dissociation of water and other electrlytes

  16. Administration of 8.4% NaHCO3 • Rather treat the cause • Ph < 7.1 metabolic acidosis • Acidific toxins etc to aid renal excretion • Hypernatraemia , hyperosmolality , fluid overload

  17. Chloride/Sodium Ratio • 0.74-0.78 • > in hyperchloraemic metabolic acidosis • 0.9% Saline , Voluven , Venofundin

  18. Sodium • Hyponatraemia • Hypervolaemichyponatraemia • Normovolaemichyponatraemia • Hypovolaemichyponatraemia • < 130 Mm – No elective surgery • < 120 Mm – Neurologic derangements • Central pontinedemyelinization if corrected too fast

  19. Sodium • Hypernatraemia • Hypervolaemic • Normovolaemic • Hypovolaemic • > 150 no elective surgery • Brain oedema if corrected too fast

  20. Potassium • Hypokalaemia • < 3,5 • Causes • Redistribution, Decreased intake, Increased losses • Symptoms and signs • Cardiovascular , musculoskeletal

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