The sensor consists of a lithium-selective electrode in a flow-through cell. It is disposable, sterilized by gamma irradiation and foil packed. The sensor is connected to a three-way tap on the arterial line and a small peristaltic pump restricts the flow through it to 4.5 ml/min. The flow-through cell is made of polycarbonate and designed with an eccentric inlet so that the blood swirls past the tip of the electrode.
The lithium-selective electrode is made of polyurethane with a central lumen. A wick, which is soaked in heparinised saline when the cell is first primed, makes the electrical connection between the blood in the cell in the vicinity of the tip of the electrode and the remote reference. This arrangement ensures adequate constancy of the voltage of the reference which is far enough from the blood to avoid a significant temperature effect. The electrode is filled with a reference material which provides a constant ionic environment and supports the membrane which is dip cast. The membrane is made of polyvinyl chloride and contains a lithium ionophore to make it selectively permeable to lithium ions.