Sun21November0437PM 13

Which one of the following parameters is a derived measurement from laboratory blood gas analysis?

(Please select 1 option)

Temperature

PaO2

pH Incorrect answer selected

PaCO2

Standard bicarbonate This is the correct answer

Explanation

pH, PaO2 and PaCO2 are directly measured.

Standard bicarbonate, base XS and oxygen saturation are indirectly measured. Standard bicarbonate is derived from the use of nomograms.

pH: pH electrode. Two half-cells, which develop an electrical potential when connected together. Reference electrode (mercury/mercury chloride) [calomel] maintains a constant potential and is bathed in KCl. Measuring electrode (silver/silver chloride) is exposed to H+ ions that pass though sensitive glass An electrical potential develops proportional to [H+].

PaO2: Oxygen electrode (Clark/Polarographic). Measures oxygen tensions by oxidation/reduction reactions, which generates an electrical current. A potential difference of 700 mV is applied across the electrodes. Platinum cathode and silver anode immersed in an electrolyte solution. Current is a measure of O2 diffused across a polypropylene membrane.

At cathode O2 + 4 e- + 2 H2O 4 OH-

At anode Ag + Cl- = AgCl + e-.

PaCO2: Severinghaus electrode (modified pH electrode). Allows CO2 to undergo a chemical reaction that produces hydrogen ions

[H+] a PCO2 in contact with the membrane of the electrode.

Operates on the principle of electric potential between electrodes

CO2 + H2O H2CO3 H+ + HCO3-

Oxygen content is a derived measurement.

This is the sum of oxygen dissolved in plasma and chemically bound to haemoglobin as determined by the calculation:

Oxygen content (CaO2) = (PaO2 × 0.003) + (SaO2 × 1.34 × Hb) where haemoglobin concentration is expressed in g/dL.

The temperature of blood is a direct measurement.

Answer Statistics

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4%

2

2%

3

17%

4

8%

5

70%

Times answered: 247