Fluid & Electrolyte Q 66

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Which of the following is a gas component of the ABG measurement?

  

    A. Carbon dioxide

    B. Bicarbonate

    C. Hydrogen

    D. pH

    

    

Correct Answer: A. Carbon dioxide

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The gases measured by ABGs are oxygen and carbon dioxide. Bicarbonate and hydrogen are ions; their ratio is measured in the pH. An arterial blood gas (ABG) tests explicitly blood taken from an artery. ABG analysis assesses a patient’s partial pressure of oxygen (PaO2) and carbon dioxide (PaCO2).

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Option B: The measured HCO3 uses a strong alkali that liberates all CO2 in serum, including dissolved CO2, carbamino compounds, and carbonic acid. The calculation only accounts for dissolved CO2; this measurement using a standard chemistry analysis will likely be called a “total CO2”.

Option C: Hydrogen is not present in blood as gas and, therefore, does not exert partial pressure. However, pH, which measures hydrogen ion activity, is a conventional part of every arterial blood gas determination. The normal range for blood pH is 7.35 to 7.45.

Option D: The pH electrode measures the potential difference between a measuring electrode (which contains the sample in contact with a special glass membrane permeable only to H+ ions) and a reference electrode (which has a known, stable pH). From the voltage across these electrodes, the sample pH is calculated.

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