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The Basics of Blood Gas and Acid-base

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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 ... – PowerPoint PPT presentation

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Title: The Basics of Blood Gas and Acid-base


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
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 gt85 mmHg
  • if paO2 lt 80 mmHg, provide O2 support
  • if paO2 lt 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 lt 35 mmHg then is hyperventilation
  • if pCO2 gt 45 mmHg then is hypoventilation
  • if pCO2 gt 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 lt 7.34 acidemia (emiaon the blood)
  • pH gt 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- gt 36, then is a strong ion
    alkalosis, usually hypochloremic alkalosis
  • If Na - Cl- lt36, 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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