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The Basics of Blood Gas and Acid-base

The Basics of Blood Gas and Acid-base. Kristen Hibbetts, DVM, DACVIM, DACVECC. VetStat. Measures 3 categories of results (Chemistry) Electrolytes Blood gases Acid-base status. Electrolytes. Electrolytes keep the cells functioning We pay the most attention to

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The Basics of Blood Gas and Acid-base

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  1. The Basics of Blood Gas and Acid-base Kristen Hibbetts, DVM, DACVIM, DACVECC

  2. VetStat • Measures 3 categories of results • (Chemistry) Electrolytes • Blood gases • Acid-base status

  3. Electrolytes • Electrolytes keep the cells functioning • We pay the most attention to • Na+, K+, Cl- and HCO3- Na+ Cl- HCO3- K+

  4. Electrolytes • Sodium (Na+): maintains plasma volume (osmolality) and blood pressure • Potassium (K+): important for cell membrane excitability • Chloride (Cl-): moves with sodium to maintain plasma volume, and important in acid-base regulation • Bicarbonate (HCO3-) helps “buffer” changes in pH • Total CO2 (TCO2): 97% HCO3-, 3% dissolved gases; reflects HCO3- when respiratory function is normal

  5. Electrolytes • We must maintain normal levels of electrolytes in our blood to maintain normal cell function • Clinicians can alter their fluid administration to either add or dilute certain electrolytes

  6. Blood Gases • Blood gases are literally gases (O2 and CO2) that circulate around in our blood • We measure oxygen (pO2) and carbon dioxide (pCO2) • The “p” stands for partial pressure, and we measure it in mmHg

  7. Blood Gases • Oxygen (O2) • Oxygen is what our cells use to live off of (no oxygen = cell death) • Measuring p02 tells us if there is enough oxygen circulating around for cells to survive

  8. O2 CO2 Blood Gases • Carbon dioxide (CO2) • Carbon dioxide is what is left over when the cell uses the oxygen • The job of the lungs is to breath in the oxygen and breath out the carbon dioxide

  9. Blood Gas Parameters • Oxygen (O2) • normal paO2 = >85 mmHg • if paO2 < 80 mmHg, provide O2 support • if paO2 < 60 mmHg while on O2 support, consider ventilator therapy • To be accurately assessed, pO2 must be measured from an arterial sample

  10. Blood Gas Parameters • Carbon dioxide (CO2) • normal pCO2 = 35-45 mmHG • if pCO2 < 35 mmHg then is hyperventilation • if pCO2 > 45 mmHg then is hypoventilation • if pCO2 > 60 mmHg, consider ventilator therapy • Can be appropriately measured on venous or arterial sample

  11. Acid-base Balance • The acidity of the blood is measured as pH • The blood has a very specific pH range where everything works adequately pH = 7.34 – 7.44

  12. Acid-base Balance • pH is maintained by multiple methods: • use of a buffer system consisting of HCO3- and CO2 • maintenance of electroneutrality (same number of positive and negative charged particles)

  13. Henderson-Hasselbalch Equation pH = pKa + log [salt]/[acid] pH = 6.1 + log [HCO3-]/0.3pCO2 pH is a function of the ratio of the HCO3- and the pCO2

  14. Henderson-Hasselbalch Equation • Derived: CO2 + H20 H2CO3 H+ + HCO3-

  15. Carbonic Acid Buffer System • Derived: CO2 + H20 H2CO3 H+ + HCO3- respiratory metabolic control control

  16. Acid-base Balance • To maintain the blood pH: • Kidneys will alter [HCO3-] • Lungs will alter pCO2

  17. Acid-base Balance • When there is an abnormality in the blood pH, we can often blame it on either: • an abnormality in the [HCO3-] or • an abnormality in the pCO2

  18. Primary Acid-Base Abnormalities • Normal pH = 7.34 – 7.44 • pH < 7.34 = acidemia (“emia”=on the blood) • pH > 7.44 = alkalemia

  19. Primary Acid-Base Abnormalities • Metabolic acidosis • Metabolic alkalosis • Respiratory acidosis • Respiratory alkalosis

  20. Metabolic Acidosis Some acidic substance has built up in the body, causing the HCO3- to become too low low HCO3- = metabolic acidosis

  21. Metabolic alkalosis Some acidic substance has been lost from the body, causing the HCO3- to become too high high HCO3- = metabolic alkalosis

  22. Respiratory Acidosis Abnormal breathing has caused CO2 to build up in the body high CO2 = respiratory acidosis

  23. Respiratory Alkalosis Abnormal breathing (hyperventilation) has caused too much CO2 to be lost from the body Low CO2 = respiratory alkalosis

  24. Acid-base Interpretation • When you see an abnormal pH on a blood gas, you can then determine whether it is abnormal due to metabolic processes or respiratory processes • This is essential to figure out the best way to treat the patient

  25. Compensation • Remember that the body will try to fix the abnormal pH itself with the following equation: CO2 + H20 H2CO3 H+ + HCO3- • HOWEVER, compensation rarely returns the pH completely back to normal

  26. Compensation • A metabolic acidosis, will always have a mild respiratory alkalosis to go with it • A respiratory acidosis will always have a mild metabolic alkalosis to go with it • etc

  27. Compensation • Respiratory compensation happens very quickly … pant, pant, pant • Metabolic compensation takes a few days

  28. Mixed Acid-base Process • When two separate processes are happening at the same time • Is very different from normal compensation • i.e. mixed metabolic acidosis and respiratory acidosis

  29. Anion Gap Based on rule of electroneutrality The sum of all cations in the body is the same as the sum of all anions in the body cations = anions

  30. Anion Gap • Cations = positively charged particles (positive ions) • Na+, K+, Ca++, Mg++ • Anions = negatively charged particles (negative ions) • Cl-, HCO3-, Ph-, proteins-

  31. Anion Gap all cations = all anions Measured cations + unmeasured cations = measured anions + unmeasured anions (Na+ + K+) + unmeasured cations = (Cl- + HCO3-) + unmeasured anions

  32. Anion Gap (Na+ + K+) + unmeasured cations = (Cl- + HCO3-) + unmeasured anions (Na+ + K+) - (Cl- + HCO3-) = unmeasured anions- unmeasured cations (Na+ + K+) - (Cl- + HCO3-) = anion gap

  33. Anion Gap • Normal anion gap is around 20 • A high anion gap means there are a lot of extra unmeasured anions present • These are usually: lactic acid, ketoacids, uremic acids (BUN, creatinine), ethylene glycol (antifreeze)

  34. Anion Gap • Normal anion gap is around 20 • A low anion gap usually means there are a lot fewer unmeasured anions present • This is usually low protein

  35. Strong Ion Difference (SID) Based on rule of electroneutrality Simplified: The difference between strong cations and strong anions in plasma is constant Very, very, very simplified: Na+ - Cl- = 36

  36. Strong Ion Difference (SID) Very, very, very simplified: Na+ - Cl- = 36 If Na+ - Cl- > 36, then is a strong ion alkalosis, usually hypochloremic alkalosis If Na+ - Cl- <36, then is a strong ion acidosis, usually hyperchloremic acidosis

  37. Blood Gas Interpretation • Looking at anion gap and chloride concentration provide a means of identifying a couple of specific causes of metabolic acidosis

  38. Ionized Calcium (Ca++) • Calcium is important for proper muscle and nerve cell function • Of the total body Ca++, approximately: • 40% is bound to albumin • 10% is associated with other substances • 50% is ionized • Only ionized Ca++ is biologically active and therefore immediately available to the body

  39. Ionized Calcium (Ca++) • Hypercalcemia (increased Ca++) causes muscle weakness • Hypocalcemia (low Ca++) causes muscle spasm and rigidity, sometimes to the point of seizure

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