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Acid Base Balance and ABG Interpretation

Acid Base Balance and ABG Interpretation. Dr Kim Khaw Anaesthesia and Intensive Care. Objectives. How to perform ABG sampling How to Interpret ABG How to Make Use of ABG Clinical Diagnosis Clinical Implications of Disturbance Management Monitoring. Where is Arterial Blood Sampled from?.

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Acid Base Balance and ABG Interpretation

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  1. Acid Base Balance and ABG Interpretation Dr Kim Khaw Anaesthesia and Intensive Care

  2. Objectives • How to perform ABG sampling • How to Interpret ABG • How to Make Use of ABG • Clinical Diagnosis • Clinical Implications of Disturbance • Management • Monitoring

  3. Where is Arterial Blood Sampled from? • Any artery in the body…… ………..EXCEPT Pulmonary Artery • Commonly from: • Radial artery- Non dominant arm • Femoral artery • Brachial artery

  4. Radial Artery • Easily accessible • Collateral circulation • Confirm prior to procedure • Allen’s Test

  5. Allen’s Test

  6. Allen’s Test

  7. Arterial Sampling • Single arterial stab • Insertion of Arterial Catheter

  8. How to do it

  9. Heparinised Syringe

  10. How to do it

  11. Arterial Sampling • Single arterial stab • Insertion of Arterial Catheter

  12. Arterial Catheter Monitoring: • Arterial Blood Pressure • Arterial Blood Gas

  13. Handling Arterial Blood • Kept in ice at 4 ºC • Analyzed ASAP • Red and white cells continue to metabolize oxygen and produce CO2

  14. Arterial Blood Gas Analysis • Blood gas analyser PICTURE

  15. Compact Analysers

  16. Complications ofArterial Sampling • Haematoma • Thrombosis • Ischaemia • Infection

  17. Basic Medical Science – Data Interpretation • Foundation for Data Interpretation • Basic Diagnosis of Acid base Status: • pH • PaCO2 • HCO3 •  Abnormality

  18. ABG Parameters • pH • PCO2 • PaO2 • HCO3 –a –- actual bicarbonate • HCO3 –s –- standardised bicarbonate • BE - Base Excess

  19. pH • A way of expressing ACIDITY • Better application for chemistry than Physiology • Negative logarithm of hydrogen ions pH = - log10 [H+]

  20. pH Biological range is: 20 - 160 nmoles/l Normal range is 35 - 45 nmoles/l (pH 7.35 - 7.45) pH 6.8 7.1 7.4 7.7 [H+] 160 80 40 20 (nmol/l) DOUBLING of [H+] changes pH by 0.3

  21. pH • Physiologically is influenced by MANY substances • BUT we interpret the ABG by looking at only a FEW measurable parameters :- => PCO2 => HCO3/BE (Buffers)

  22. PaCO2 • Body production is constant 200-250ml/min • Kept in narrow range by respiratory centre:-

  23. PaCO2 & pH Biological range: 2.7 - 10.6 kPa Normal range: 4.6 - 6.0 kPa pH 7.2 7.3 7.4 7.5 7.6 PaCO2 (kPa) 10.6 8 5.3 4 2.7 pH change by 0.1 for every  PaCO2 2.6kPa  pH by 0.1 (Resp. Acidosis)  PaCO2 1.3kPa  pH by 0.1 (Resp. Alkalosis)

  24. Actual Bicarbonate - HCO3-a • HCO3-a (Actual value measured) • Influenced by: Metabolic acids (or base) Acid + HCO3-  Salt + Water Carbon dioxide CO2 + H2O  H2CO3  H+ + HCO3-

  25. New Parameters • Standardised Bicarbonate • Base Excess • PaO2

  26. Standardised Bicarbonate • Derived from the actual bicarbonate • Imaginary value assuming a standard PaCO2 of 5.3 kPa • Thus the influence from CO2 is standardised or eliminated • The value reflects purely metabolic influence

  27. Base Excess Definition: • Amount of titratable base in the blood to neutralize to a pH of 7.4 (at PaCO2 of 5.3kPa and temp of 37º C) Implications: • If it is +ve → Alkalosis • If it is –ve → Acidosis • Normal range  2 • Clinically significant if  7 mmol/l

  28. Base Excess & Standardised HCO3- • Standardised HCO3- & Base Excess reflects specifically: Metabolic Acidosis or Alkalosis

  29. PaO2 • Partial Pressure of Oxygen in blood • Relative to the inspired Oxygen %: • “Index of oxygenation of the blood” • PaO2 = PiO2 - PaCO2/RQ + k (Alveolar Gas Equation)

  30. Alveolar Gas Equation - PaO2 • For Practical Purposes: PaO2 (kPa) = O2 (%) – 10

  31. Alveolar Gas Equation PaO2 = PiO2 - PaCO2/RQ + k • PaO2 = FiO2 - 10 kPa = % Inspired O2 - 10 Breathing Air: • PaO2 =21 - 10 kPa • PaO2 =11 kPa Breathing 40% oxygen: • PaO2 =40 - 10 kPa • PaO2 =30 kPa

  32. Anion Gap The blood plasma is electrically neutral. Therefore the sums of the positive and negative charges are equal:

  33. Anion Gap • [Na + K] - [HCO + Cl] • If >16  Fixed Acidic Material in Body • Na - [HCO + Cl] • If >12  Fixed Acidic Material in Body

  34. Summary Standardised Bicarbonate • Metabolic status without respiratory influence Base Excess Metabolic status: • Alkalosis (+ve) • Acidosis (-ve) Partial Pressure of Oxygen • Should be Inspired Oxygen % - 10

  35. Clinical Diagnosis Acid Base Status: • What is the pH abnormality? • Is it Respiratory or Metabolic • What is the Magnitude? • Is there any compensatory changes? • If not, WHY NOT? • Are there any tricks?

  36. Clinical Diagnosis Gas Exchange Status: • Oxygenation • Is there any abnormality? • PaO2 = Inspired O2% -10 • Ventilation • Is there any abnormality? •  PaCO2= Hypo ventilation • PaCO2= Hyper ventilation

  37. Case 1 In a class demonstration, medical students have to take arterial blood sample from an unfortunate colleague who ‘volunteered’. He is perfectly healthy otherwise.

  38. Case 2 A 74-year-old lady complained of pain one hour after a total hip replacement surgery. She was given an injection of morphine for analgesia. Shortly afterwards she was noted to be drowsy and the house officer took an arterial blood for investigation.

  39. Actual Bicarbonate - HCO3-a Carbon dioxide CO2 + H2O  H2CO3  H+ + HCO3- • Alveolar Gas Equation • PaO2 = PiO2 - PaCO2/RQ + k

  40. Case 3 A patient who suffered from gastroenteritis had been vomiting for several days.

  41. Case 4 A pedestrian who was involved in a road traffic accident was brought to the emergency department bleeding heavily from a compound fracture of the femur. She was hypotensive, cold and clammy on arrival, and was immediately taken to the operating theatre for surgery.

  42. Case 5 An elderly patient with chronic obstructive airways disease presented to the emergency department with increasing dypsnoea.

  43. Case 6 A 16-year-old girl with insulin-dependent diabetes presents with fever, lethargy and shortness of breath. She has not taken insulin for the past two days. Her arterial blood shows:

  44. Case 7 A young man was hit by a truck and brought to the emergency department in a comatosed state. An arterial blood shows:

  45. Situations where Arterial Blood Gas would be useful

  46. StrokePathophysiology • Subarachniod Haemorrhage •  Intracranial pressure •  Cerebral Blood Flow

  47. Management Principles: • Cerebral Blood flow  Perfusion Pressure • Perfusion Pressure  MAP - ICP Management: • Evacuation of blood clots • Reduce the ICP – hyperventilation, drugs, drain CSF Role of ABG: • PaCO2 as guide to ventilation • Arterial catheter measuring MAP

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