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Ten things everyone needs to know about blood gases

Ten things everyone needs to know about blood gases. Doug Pursley , M.Ed., RRT Program Director Ozarks Technical Community College Springfield, MO. 1. There is a difference between classification and interpretation of blood gases.

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Ten things everyone needs to know about blood gases

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  1. Ten things everyone needs to know about blood gases Doug Pursley, M.Ed., RRT Program Director Ozarks Technical Community College Springfield, MO

  2. 1. There is a difference between classification and interpretation of blood gases

  3. Classification vs. Interpretation • Classification – systematic arrangement according to established criteria

  4. Classification: The High/Lo or Arrow System • 54 year old narcotic overdose: • pH 7.22 (7.40) • PaCO2 70 (40) • HCO3 27 (24) • Partially compensated respiratory acidosis according to the arrow system

  5. Classification vs. Interpretation • Interpretation – the act of applying an explanation to the results

  6. Interpretation • Incorporates calculations, baseline values, electrolytes (Na+/Cl-), and clinical picture into the equation to come up with a more precise explanation of the patient’s acid-base status

  7. 2. Hydrolysis happens

  8. Hydrolysis = = + +

  9. CO2 + H2O = H2CO3 = H+ + HCO3 • Starting at a PaCO2 of 40 mmHg, HCO3 will increase from normal by 1 for every 10 acute increase in PaCO2 • Starting at a PaCO2 of 40 mmHg, HCO3 will decrease from normal by 2 for every 10 acute decrease in PaCO2 • Simple physiochemical event and occurs almost immediately

  10. Correct interpretation when one knows about hydrolysis…

  11. Starting at a PaCO2 of 40 mmHg, HCO3 will increase from normal by 1 for every 10 increase in PaCO2 • 54 year old narcotic overdose: • pH 7.22 • PaCO2 70 (three tens above 40) • HCO3 27 (HCO3 expected to increase by 3) • Expected HCO3 same as Actual HCO3 so… • Acute respiratory acidosis • Base excess is zero in this case

  12. 3. Always evaluate the base excess/deficit

  13. Normal BE/BD is -2 to +2 • With the exception of hydrolysis, tends to follow HCO3 • Values above +2 indicate compensation for respiratory acidosis or metabolic alkalosis • Values below -2 indicate compensation for respiratory alkalosis or metabolic acidosis • Normal value for base does not exclude metabolic acidosis IF there is also a co-existing and competing metabolic alkalosis and visa versa

  14. General rule of thumb • If the pH and PaCO2 are both out of whack in opposite directions, and the base is normal, start your interpretation out with ACUTE RESPIRATORY… • Possible exceptions: • Co-existing Metabolic Acidosis AND Metabolic Alkalosis • Triple acid-base disturbances

  15. 4. Indirect metabolic assessment can be performed by comparing the actual pH with predicted respiratory pH

  16. Starting at pH of 7.4 and PaCO2 of 40… • pH will increase by 0.1 for each 10 acute decrease in PaCO2 • pH will decrease by 0.06 for each 10 acute increase in PaCO2 • If PaCO2 suddenly changes from 40 to 30, pH will change from 7.40 to 7.50 • If PaCO2 suddenly changes from 40 to 60, pH will change from 7.40 to 7.28

  17. If actual pH is greater than the predicted respiratory pH by more than 0.03, the patient will have a base excess • If actual pH is less than the predicted respiratory pH by more than 0.03, there will be a base deficit • If both are within 0.03 of each other, then the base will be normal most likely reflecting an acute respiratory disturbance

  18. 5. The anion and bicarbonate gaps are important parts of evaluating acid-base disorders

  19. Two types of metabolic acidosis • High AG metabolic acidosis • Accumulation of acid in the plasma • Lactic acidosis, DKA, azotemic renal failure, toxins • Normal AG metabolic acidosis • Loss of base from the plasma • Diarrhea • Intestinal drainage tubes • Renal tubular acidosis (RTA)

  20. Anion Gap • Represents the concentration of unmeasured anions in the plasma • Unmeasured anions also referred to as “acid anions” such as: • Lactate (lactic acidosis) • Acetoacetate (ketoacidosis) • Sulphate, phosphate (azotemic renal failure)

  21. Anion Gap • Derived from subtracting measured cations from measured anions • Clinical equation is: • Na+ - Cl- - HCO3- = Anion Gap

  22. Anion Gap • Normal value is 12 • If value is increased, there is an accumulation of acid anions in the plasma • If AG is 20-29, there is 67% chance of high AG metabolic acidosis • If AG is >30, there is 100% chance of high AG metabolic acidosis

  23. Anion Gap • If High AG acidosis is the only acid-base disorder, then there should be a 1:1 correlation between rise in the gap and fall in HCO3 • Example: If AG goes from 12 to 24, HCO3 should go from 24 to 12 • A high AG with a normal or high HCO3 indicates extra HCO3 on board from an additional metabolic alkalosis or compensation for respiratory acidosis

  24. Normal AG Metabolic Acidosis • Due to loss of base rather than accumulation of acids • Anion Gap will be normal • Common causes are: • Renal tubular acidosis • Intestinal drainage tubes • Diarrhea

  25. Bicarbonate Gap • Represents the change in AG from normal minus the change in HCO3 from normal • Normal value is zero, plus or minus 6 • Values greater than +6 indicate: • Metabolic alkalosis or compensation for respiratory acidosis • Values less than -6 indicate: • Normal AG gap metabolic acidosis or compensation for respiratory alkalosis

  26. Hard way to calculate BG • ∆ AG - ∆ HCO3 • = [AG - 12] – [24 – HCO3] • = [(Na – Cl – HCO3) – 12] – [24 – HCO3]

  27. Easy way to calculate BG • BG = Na – Cl - 36 • Comes from cancelling out the terms in the equation below: • BG = [(Na – Cl – HCO3) – 12] – [24 – HCO3]

  28. 6. Know the rules for compensation

  29. Compensation for respiratory acidosis • HCO3 will increase by 4 for every 10 chronic increase in PaCO2 above 40 • Kidneys respond to chronic respiratory acidosis by retaining HCO3 • Takes 3-4 days to reach a maximum value

  30. Compensation for respiratory alkalosis • HCO3 will decrease by 5 for every 10 chronic decrease in PaCO2 below 40 • Kidneys respond to chronic respiratory alkalosis by excreting HCO3 • Takes 3-4 days to reach a maximum value

  31. Compensation for metabolic acidosis • Expected PaCO2 in metabolic acidosis will follow “One Point Five Plus Eight Rule” or Winter’s Formula: • PaCO2 expected = (1.5 x HCO3) + 8 • Compensation starts immediately and becomes complete in several hours • Limit of compensation is PaCO2 of about 8-10

  32. Compensation for metabolic alkalosis • Expected PaCO2 in metabolic alkalosis will follow the “Point Seven Plus 20 Rule” • PaCO2 expected = (0.7 x HCO3) + 20 • Compensation starts immediately and becomes complete in several hours • Limit of compensation traditional thought to be PaCO2 of 60 but more recent studies indicate CO2 may be linear with the HCO3

  33. 7. All acid-base values should calculate out according to the Henderson-Hasselbalch equation

  34. H-H Equation

  35. 30 second accuracy check with a $15 calculator • Multiply PaCO2 by 0.03 (converts to H2CO3) • Divide HCO3 by H2CO3 • Log the result • Add 6.1 • Result should be within 0.03 units of the measured pH • If not, acid base is inconsistent (could be HCO3 calculation error, sensor error, or transcription error)

  36. 8. Total oxygen content (Cao2) is the best index of oxygenation

  37. CaO2 • CaO2 = (1.34 x Hb x SO2) + (PO2 x 0.003) • Best index of blood oxygen because it is a measure of total oxygen in blood

  38. Other Oxygen indices

  39. PaO2 • Measure of oxygen dissolved in plasma • Indication of the amount of oxygen available to combine with Hb • Can be normal or high in anemia, COHb, and MetHb • PaO2 of 60 is not bad if Hb and pH are normal • Direct correlation with SaO2 • Absent dyshemoglobinemia, if a patient’s PaO2 is 60, his SaO2 will be 90% assuming a normally shaped O2-Hb curve

  40. SaO2calc • An estimation of the oxygen saturation from the O2Hb curve • Plots pH with PO2 to determine saturation • Not accurate in COHb and MetHb

  41. SaO2CO-ox • The actual, fractionalized value of oxygen saturation measure by a CO-oximeter • Most accurate SaO2

  42. SpO2 • Uses red and infrared wavelengths of light to determine saturation • Measures “functional” saturation i.e. assumes only oxygen is attached to Hb • Overestimates saturation in COHb • Migrates towards 85% in MetHb

  43. All saturations • In severe anemia i.e. Hb 4 g/dl, SaO2calc , SaO2CO-ox , and SpO2 can all be high and the patient could still be have tissue hypoxia

  44. a/A ratio • Index of shunting • PaO2 ÷ PAO2 • Normal value is 0.9 (i.e. PaO2 90 ÷ PAO2 100) • Lower limit of normal is 0.75 • The lower the number, the worse the shunt

  45. 9. The P/F ratio is a good index of shunting, but it does have a limitation

  46. P/F ratio • Index of shunting • Simplified version of a/A ratio • PaO2 ÷ FIO2 • Normal value is 400-500 • PaO2 90 ÷ FIO2 0.21 = 429 • The lower the number, the worse the shunt • Not accurate in severe hypercarbia

  47. 10. The alveolar air equation has actual clinical application

  48. Alveolar Air Equation • PAO2 = (PB – 47 x FIO2) – (PaCO2 x 1.2) • Needed to calculate PaO2/PAO2 ratio and A-a gradient • A modification of the formula can be used to estimate device FIO2 in normal subjects

  49. Case 1 • A adult male patient recently brought to the ER has the following ABG on a nasal cannula at 2 l/m: • 3:49 am • pH 6.83 • PaCO2 139 • PaO2 138 • HCO3 23 • Base -13

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