PaO2, PaCO2 and pH electrodes

Blood gas samples may be analysed by a variety of devices, ranging from large stationary machines, which are often found on wards or in laboratories to smaller handheld devices that can be taken into patient’s homes. Some may have LCD displays or a keypad to enter information and a slot to insert a cartridge, syringe or sample capillary tube. Different devices will suit different blood gas testing services.

The PO2 electrode is known as the Clark electrode, and consists of a silver anode and platinum cathode. The anode and the cathode are suspended in an electrolyte, and there is a semipermeable membrane between this system and the blood sample. Oxygen molecules cross this semipermeable membrane and dissolve in the electrolyte. The oxygen interacts with the silver anode, releasing electrons which generate a flow around a circuit. The current produced is proportional to the PO2 in the blood sample, and the signal is converted to kPa or mmHg.

The PCO2 electrode is known as the ‘Severinghaus’ electrode and includes a bicarbonate solution separated from the blood sample by a CO2 permeable membrane. The carbon dioxide moves through the membrane, reacts with water to create carbonic acid, and the electrode measures the resulting pH of the bicarbonate solution. This signal is converted into a PaCO2.

The pH electrode is a glass electrode that is sensitive to hydrogen ions. It contains a solution of known pH on one side of a glass membrane, and the sample of blood on the other side. The difference in pH between the solution and the blood creates a voltage difference, which can be used to generate a pH measurement.

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