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Arterial Blood Gases

Richard Stretton Respiratory Registrar. Arterial Blood Gases. Arterial Blood Gases. Seen as complicated Misunderstood Important An easy way and a hard way. Objectives. Develop an organised system for looking at blood gases Be able to comment on the arterial pO 2 in relation to the FiO 2

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Arterial Blood Gases

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  1. Richard Stretton Respiratory Registrar Arterial Blood Gases

  2. Arterial Blood Gases • Seen as complicated • Misunderstood • Important • An easy way and a hard way

  3. Objectives • Develop an organised system for looking at blood gases • Be able to comment on the arterial pO2 in relation to the FiO2 • Interpret acid base disturbance and it’s significance in the acutely unwell

  4. What Are We Measuring? • pH • pO2 • pCO2 • HCO3 • Base Excess

  5. Acid Base Balance • pH is carefully controlled • Enzymatic Function relies on pH control • Buffers • Haemoglobin • BICARBONATE • Ammonium • Phosphate

  6. Striking the Balance H+ + HCO3- H2CO3  CO2 + H2O • When you’ve got too much H+, lungs blow off CO2 • When you can’t blow off CO2, kidneys try to get rid of H+

  7. 5-step approach • Assess Oxygenation • Determine Acid-Base Deficit • Determine the respiratory component • Determine the metabolic component • Which is primary and which is secondary

  8. 5-step approach • Assess Oxygenation • Determine Acid-Base Deficit • Determine the respiratory component • Determine the metabolic component • Which is primary and which is secondary

  9. 5-step approach • Assess Oxygenation pO2 = 10 -13 kPa on air • Is the patient hypoxic? • Is there a significant A-a Gradient? A-a Gradient is the difference in concentration of oxygen between the Alveolus (A) and the artery (a) Normal <3 A-a Gradient = PAO2 – (PaO2 + PaCO2/0.8)

  10. I shouldn’t say this but… v.v.v.v. rough guide Inspired O2 - (pO2 + pCO2) Add together pO2 and pCO2 from your blood gas Take this away from the concentration of Oxygen the patient is breathing With an upper limit of normal of about 5

  11. 5-step approach • Assess Oxygenation • Determine Acid-Base Deficit • Determine the respiratory component • Determine the metabolic component • Which is primary and which is secondary

  12. 5-step approach • Determine Acid-Base Deficit • pH>7.45 alkalaemia • pH<7.35 acidaemia • Acidosis - a process causing excess acid to be present in the blood. Acidosis does not necessarily produce acidaemia • Alkalosis - a process causing excess base to be present in the blood. Alkalosis does not necessarily produce alkalaemia.

  13. 5-step approach • Assess Oxygenation • Determine Acid-Base Deficit • Determine the respiratory component • Determine the metabolic component • Which is primary and which is secondary

  14. 5-step approach • Determine the respiratory component Does this explain the acid-base deficit? • PaCO2: >6.0 kPa - respiratory acidosis • <4.7kPa - respiratory alkalosis

  15. 5-step approach • Assess Oxygenation • Determine Acid-Base Deficit • Determine the respiratory component • Determine the metabolic component • Which is primary and which is secondary

  16. 5-step approach • Determine the metabolic component. Does this explain the acid-base deficit? • HCO3 <22 mmols/l - metabolic acidosis >26 mmols/l - metabolic alkalosis

  17. Remember…… H+ + HCO3- H2CO3  CO2 + H2O • When you’ve got too much H+, lungs blow off CO2 • When you can’t blow off CO2, kidneys try to get rid of H+

  18. 5-step approach • Assess Oxygenation • Determine Acid-Base Deficit • Determine the respiratory component • Determine the metabolic component • Which is primary and which is secondary

  19. 5-step approach • Which is primary and which is secondary? Remember • Compensation doesn’t always completely restore pH to the normal range • A mixed picture may be present

  20. 5-step approach • Assess Oxygenation • Determine Acid-Base Deficit • Determine the respiratory component • Determine the metabolic component • Which is primary and which is secondary

  21. Assumptions • CO2 changes are related to respiratory changes • HCO3 changes relate to metabolic changes • Overcompensation does not occur • Respiratory compensation is rapid • Metabolic compensation is slow

  22. Respiratory Acidosis Any cause of hypoventilation • CNS depression • Neuromuscular disease • Acute or chronic lung disease • Cardiac arrest • Ventilator malfunction

  23. Respiratory Alkalosis Any cause of hyperventilation • Hypoxia • Acute lung conditions • Anxiety • Fever • Pregnancy • Hepatic failure • Some central CNS lesions

  24. Metabolic Acidosis

  25. Metabolic Alkalosis Loss of acid or gaining alkali • Vomiting • Diarrhoea • Diuretics (and hypokalaemia generally) • Ingestion of alkali

  26. Reminder of normal values • pH 7.35 – 7.45 (H+ = 35 -45) • pO2 10 - 13 kPa on air • pCO2 4.6 - 6.0 kPa • HCO3 25 - 35 mmols/l • Base excess ± 2.0

  27. Lets get going…….. • Working out acidosis/alkalosis and compensation is usually the bit people struggle with • So…..

  28. Outcome codes

  29. Translate Uncompensated Metabolic Acidosis

  30. Translate Uncompensated Respiratory Acidosis

  31. Translate Uncompensated Respiratory Alkalosis

  32. Translate Compensated Metabolic Acidosis or Compensated Respiratory Alkalosis

  33. Translate Compensated Respiratory Acidosis or Compensated Metabolic Alkalosis

  34. Translate Decompensated Respiratory Acidosis

  35. What Now? • Now you can determine any acid base pattern • Convert the numbers into high/low/normal • Convert that into acid/alkali • What is primary, what is compensation? • Distinguish between uncompensated, compensated, and decompensated

  36. Nomenclature • Uncompensated Respiratory Acidosis • Acute Type 2 Respiratory Failure • Compensated Respiratory Acidosis • Chronic Type 2 Respiratory Failure • Decompensated Respiratory Acidosis • Acute on Chronic Type 2 Respiratory Failure

  37. Case 1 • Young female admitted with overdose of unknown tablets and smelling of alcohol pO2 12 kPa on air pH 7.24 PaCO2 2.5 HCO3 8 • Metabolic Acidosis with respiratory compensation

  38. Case 2 • Elderly male admitted from nursing home with one week history of fever and vomiting pO2 12 kPa on 4l by mask pH 7.49 PaCO2 6.3 HCO3 35 • Metabolic alkalosis with respiratory compensation

  39. Case 3a • Middle aged man admitted with cough sputum and haemoptysis. Life-long smoker pO2 4 on air pH 7.19 PaCO2 9.7 HCO3 28 • Acute respiratory acidosis with no time for metabolic compensation

  40. Case 3b • Middle aged man admitted with cough sputum and haemoptysis. Life-long smoker pO2 6 on air SpO2 92% pH 7.32 PaCO2 10.0 HCO3 39 • Acute respiratory acidosis with no time for metabolic compensation

  41. Case 4 • Middle aged man post cardiac arrest. Breathing spontaneously on endotracheal tube pO2 35 on 15l via reservoir mask pH 6.9 PaCO2 8.9 HCO3 13 • Mixed metabolic and respiratory acidosis

  42. Case 5 • Elderly lady with congestive cardiac failure pO2 9 on 40% oxygen pH 7.64 PaCO2 3.5 HCO3 29 • Respiratory alkalosis secondary to pulmonary oedema. • Acute as no metabolic compensation

  43. Case 6 • Young diabetic male admitted with chest infection, vomiting and drowsiness pO2 12 on air pH 7.31 PaCO2 1.6 HCO3 6.0 • Acute metabolic acidosis with respiratory compensation

  44. Case 7 • 54 yr-old lady post MI. Acutely unwell, cold, clammy, hypotensive and oliguric pO2 10 on 60% oxygen pH 6.99 PaCO2 7.8 HCO3 14 • Mixed pattern of respiratory and metabolic acidosis

  45. Case 8 • 50 yr-old man admitted with exacerbation of long-standing bronchial asthma. Respiratory rate of 18 pO2 5.1 on 60% oxygen pH 7.39 PaCO2 5.8 HCO3 26 • Severe type I respiratory failure

  46. Questions ?

  47. The 6th step… • If an acidosis is present work out the anion gap to help determine cause. • Anion Gap is the difference between the measured positive and negatively charged ions. • It gives an estimate of the unmeasured ions in the serum • Unmeasured – proteins, sulphates

  48. Anion Gap • Anion Gap = [Na+K] –[CL+HCO3] • Normal anion gap 10-18

  49. Metabolic Acidosis • Increased anion gap (added acid) • Renal failure • Ketoacidosis • Lactic acidosis • Salicylate/Tricyclic overdose

  50. Metabolic Acidosis • Decreased anion gap (loss of bicarbonate) • Renal tubular acidosis • Diarrhoea • Carbonic anhydrase inhibitors • Ureteral diversion • Chloride administration

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