Transcript Blood Gases and Electrolyte Balance for Dummies
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
-
Na + , K + , Cl and HCO 3 -
Na + Cl HCO 3 K +
Electrolytes
Sodium (Na + ): maintains plasma volume (osmolality) and blood pressure Potassium (K + ): important for cell membrane excitability Chloride (Cl regulation ): moves with sodium to maintain plasma volume, and important in acid-base 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
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
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
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
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 O 2 CO 2
Blood Gas Parameters
Oxygen (O 2 )
–
normal p a O 2
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if p a O 2 = >85 mmHg < 80 mmHg, provide O 2
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if p a O 2 < 60 mmHg while on O 2 consider ventilator therapy support support,
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To be accurately assessed, pO 2 must be measured from an arterial sample
Blood Gas Parameters
Carbon dioxide (CO 2 )
–
normal pCO 2 = 35-45 mmHG
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if pCO 2 < 35 mmHg then is hyperventilation
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if pCO 2 > 45 mmHg then is hypoventilation
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if pCO 2 therapy > 60 mmHg, consider ventilator
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Can be appropriately measured on venous or arterial sample
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
Acid-base Balance
pH is maintained by multiple methods:
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use of a buffer system consisting of HCO 3 and CO 2
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maintenance of electroneutrality (same number of positive and negative charged particles)
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
Henderson-Hasselbalch Equation
Derived: CO 2 + H 2 0 H 2 CO 3 H + + HCO 3 -
Carbonic Acid Buffer System
Derived: CO 2 + H 2 0
respiratory control
H 2 CO 3 H + + HCO 3 -
metabolic control
Acid-base Balance
To maintain the blood pH:
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Kidneys will alter [HCO3-]
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Lungs will alter pCO2
Acid-base Balance
When there is an abnormality in the blood pH, we can often blame it on either:
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an abnormality in the [HCO 3 ] or
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an abnormality in the pCO 2
Primary Acid-Base Abnormalities
Normal pH = 7.34 – 7.44
pH < 7.34 = acidemia (“emia”=on the blood)
pH > 7.44 = alkalemia
Primary Acid-Base Abnormalities
Metabolic acidosis Metabolic alkalosis Respiratory acidosis Respiratory alkalosis
Metabolic Acidosis
Some acidic substance has built up in the body, causing the HCO 3 low to become too low HCO 3 = metabolic acidosis
Metabolic alkalosis
Some acidic substance has been lost from the body, causing the HCO 3 too high to become high HCO 3 = metabolic alkalosis
Respiratory Acidosis
Abnormal breathing has caused CO 2 build up in the body to high CO 2 = respiratory acidosis
Respiratory Alkalosis
Abnormal breathing (hyperventilation) has caused too much CO 2 body to be lost from the Low CO 2 = respiratory alkalosis
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
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
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
Compensation
Respiratory compensation happens very quickly … pant, pant, pant
Metabolic compensation takes a few days
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
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
Anion Gap
Cations = positively charged particles (positive ions)
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Na + , K + , Ca ++ , Mg ++
Anions = negatively charged particles (negative ions)
–
Cl , HCO 3 , Ph , proteins -
Anion Gap
all cations = all anions Measured cations + unmeasured cations = measured anions + unmeasured anions (Na + + K + ) + unmeasured cations = (Cl + HCO 3 ) + unmeasured anions
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
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)
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
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
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
Blood Gas Interpretation
Looking at anion gap and chloride concentration provide a means of identifying a couple of specific causes of metabolic acidosis
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
Ionized Calcium (Ca
++
)
Hypercalcemia (increased Ca ++ ) causes muscle weakness
Hypocalcemia (low Ca ++ ) causes muscle spasm and rigidity, sometimes to the point of seizure