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Chemistry
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Acid-Base Balance and Blood Gases
What term describes an arterial pH below 7.35?
Acidemia.
Acidemia describes the blood pH. Acidosis is the process that pushes pH down, and it can be present while pH is still in range.
What term describes an arterial pH above 7.45?
Alkalemia.
Alkalemia describes the blood pH. Alkalosis is the process that pushes pH up, and it can be present while pH is still in range.
Which blood-gas variable represents the respiratory component of acid-base balance?
pCO₂.
Ventilation controls how much carbon dioxide the body retains.
Which variable represents the metabolic component of acid-base balance?
Bicarbonate (HCO₃⁻).
A primary fall in bicarbonate marks metabolic acidosis, and a primary rise marks metabolic alkalosis.
What primary change produces respiratory acidosis?
What primary change produces respiratory alkalosis?
A decrease in pCO₂.
Increased CO₂ elimination raises pH.
What is the primary bicarbonate change in metabolic acidosis?
Bicarbonate decreases.
Acid gain or bicarbonate loss lowers pH.
What is the primary bicarbonate change in metabolic alkalosis?
Bicarbonate increases.
Acid loss or base gain raises pH.
Which variable changes to compensate for a primary metabolic acid-base disorder?
pCO₂, through a change in ventilation.
Compare the measured pCO₂ with the value an expected-compensation rule predicts.
Which variable changes with renal compensation for a primary respiratory disorder?
Bicarbonate.
A chronic respiratory disorder shows more renal compensation than an acute one, because the kidneys need time to adapt.
State Winters formula for expected pCO₂ in metabolic acidosis, with its units.
Expected pCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 mm Hg.
Bicarbonate is entered in mmol/L, and the result is in mm Hg.
In metabolic acidosis, what does a pCO₂ above the range Winters formula predicts suggest?
Additional respiratory acidosis.
In metabolic acidosis, what does a pCO₂ below the range Winters formula predicts suggest?
Additional respiratory alkalosis.
Why can a pH within the reference interval still accompany a mixed disorder?
Opposing primary processes can offset their effects on pH.
Check pCO₂ and bicarbonate against the expected compensation.
Which three variables does a conventional blood-gas analyzer measure directly?
pH, pCO₂, and pO₂.
The analyzer calculates the other reported values, such as bicarbonate, from these three.
How does a conventional blood-gas analyzer obtain the bicarbonate it reports?
It is calculated from the measured pH and pCO₂.
A chemistry panel measures total CO₂ separately.
How does exposure to room air usually change blood-gas pCO₂ and pH?
pCO₂ falls and pH rises.
CO₂ escapes into room air, where its partial pressure is much lower.
In which direction does trapped room air drive specimen pO₂?
Toward the oxygen partial pressure of room air.
The error can raise or lower pO₂, depending on the specimen's starting pO₂.
Why can excess liquid heparin distort a blood-gas specimen?
It dilutes the blood and alters dissolved gas values.
Underfilling a syringe that contains liquid heparin raises the heparin-to-blood ratio.
Which two pieces of collection information does blood-gas interpretation require?
Whether the specimen is arterial or venous, and the inspired oxygen at collection.
Venous pCO₂ runs several mm Hg higher and pH several hundredths lower than arterial, and expected pO₂ depends on the inspired oxygen.
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