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

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VetStat

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

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Electrolytes

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

Na
Cl-
HCO3-
K
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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

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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

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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

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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

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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

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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

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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

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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

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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)

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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

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Henderson-Hasselbalch Equation

• Derived
• CO2 H20 H2CO3 H HCO3-

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Carbonic Acid Buffer System

• Derived
• CO2 H20 H2CO3 H HCO3-
• respiratory metabolic
• control control

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Acid-base Balance

• To maintain the blood pH
• Kidneys will alter HCO3-
• Lungs will alter pCO2

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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

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Primary Acid-Base Abnormalities

• Normal pH 7.34 7.44
• pH lt 7.34 acidemia (emiaon the blood)
• pH gt 7.44 alkalemia

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Primary Acid-Base Abnormalities

• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis

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Metabolic Acidosis

• Some acidic substance has built up in the body,
  causing the HCO3- to become too low
• low HCO3- metabolic acidosis

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Metabolic alkalosis

• Some acidic substance has been lost from the
  body, causing the HCO3- to become too high
• high HCO3- metabolic alkalosis

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Respiratory Acidosis

• Abnormal breathing has caused CO2 to build up in
  the body
• high CO2 respiratory acidosis

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Respiratory Alkalosis

• Abnormal breathing (hyperventilation) has caused
  too much CO2 to be lost from the body
• Low CO2 respiratory alkalosis

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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

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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

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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

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Compensation

• Respiratory compensation happens very quickly
  pant, pant, pant
• Metabolic compensation takes a few days

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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

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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

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Anion Gap

• Cations positively charged particles (positive
  ions)
• Na, K, Ca, Mg
• Anions negatively charged particles (negative
  ions)
• Cl-, HCO3-, Ph-, proteins-

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Anion Gap

• all cations all anions
• Measured cations unmeasured cations measured
  anions unmeasured anions
• (Na K) unmeasured cations
• (Cl- HCO3-) unmeasured anions

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Anion Gap

• (Na K) unmeasured cations
• (Cl- HCO3-) unmeasured anions
• (Na K) - (Cl- HCO3-) unmeasured anions-
  unmeasured cations
• (Na K) - (Cl- HCO3-) anion gap

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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)

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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

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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

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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

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Blood Gas Interpretation

• Looking at anion gap and chloride concentration
  provide a means of identifying a couple of
  specific causes of metabolic acidosis

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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

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Ionized Calcium (Ca)

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