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 HCO 3 - Na + K+K+K+K+ Cl - HCO 3 -

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 (HCO 3 - ) helps “buffer” changes in pH  Total CO2 (TCO 2 ): 97% HCO 3 -, 3% dissolved gases; reflects HCO 3 - 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 (O 2 and CO 2 ) that circulate around in our blood  We measure oxygen (pO 2 ) and carbon dioxide (pCO 2 )  The “p” stands for partial pressure, and we measure it in mmHg

7. Blood Gases  Oxygen (O 2 ) –Oxygen is what our cells use to live off of (no oxygen = cell death) –Measuring p0 2 tells us if there is enough oxygen circulating around for cells to survive

8. Blood Gases  Carbon dioxide (CO 2 ) –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 CO 2 O2O2

9. Blood Gas Parameters  Oxygen (O 2 ) –normal p a O 2 = >85 mmHg –if p a O 2 < 80 mmHg, provide O 2 support –if p a O 2 < 60 mmHg while on O 2 support, consider ventilator therapy –To be accurately assessed, pO 2 must be measured from an arterial sample

10. Blood Gas Parameters  Carbon dioxide (CO 2 ) –normal pCO 2 = 35-45 mmHG –if pCO 2 < 35 mmHg then is hyperventilation –if pCO 2 > 45 mmHg then is hypoventilation –if pCO 2 > 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 HCO 3 - and CO 2 –maintenance of electroneutrality (same number of positive and negative charged particles)

13. Henderson-Hasselbalch Equation pH = pK a + log [salt] / [acid] pH = 6.1 + log [HCO3-] / 0.3pCO 2 pH is a function of the ratio of the HCO 3 - and the pCO 2

14. Henderson-Hasselbalch Equation  Derived: CO 2 + H 2 0 H 2 CO 3 H + + HCO 3 -

15. Carbonic Acid Buffer System  Derived: CO 2 + H 2 0 H 2 CO 3 H + + HCO 3 - respiratory metabolic respiratory metabolic control control 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 [HCO 3 - ] or –an abnormality in the pCO 2

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 HCO 3 - to become too low low HCO 3 - = metabolic acidosis

21. Metabolic alkalosis Some acidic substance has been lost from the body, causing the HCO 3 - to become too high high HCO 3 - = metabolic alkalosis

22. Respiratory Acidosis Abnormal breathing has caused CO 2 to build up in the body high CO 2 = respiratory acidosis

23. Respiratory Alkalosis Abnormal breathing (hyperventilation) has caused too much CO 2 to be lost from the body Low CO 2 = 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: CO 2 + H 2 0 H 2 CO 3 H + + HCO 3 -  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 -, HCO 3 -, Ph -, proteins -

31. Anion Gap all cations = all anions Measured cations + unmeasured cations = measured anions + unmeasured anions (Na + + K + ) + unmeasured cations = (Cl - + HCO 3 - ) + unmeasured anions

32. Anion Gap (Na + + K + ) + unmeasured cations = (Cl - + HCO 3 - ) + unmeasured anions (Na + + K + ) - (Cl - + HCO 3 - ) = unmeasured anions- unmeasured cations (Na + + K + ) - (Cl - + HCO 3 - ) = 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