BLOOD GAS ANALYSER

Typical Number in Hospital: 6

Cost Bands: 4,5

References: 2,6,9

The partial pressures of oxygen and carbon dioxide in the blood are important indicators of the acid-base balance of the patient. Samples of arterial blood may be taken to the laboratory, but it is also common to have a blood gas analyser on hand in the intensive care unit, in the special care baby unit, and the labour ward. Modern analysers only require a very small sample of blood, but are relatively expensive and internally complex.

The partial pressure of oxygen (pO2) measurement usually employs the Clark-type polarographic electrode in which oxygen diffuses through a membrane and is detected in a potassium chloride solution as the quantity of electric current generated between a glass-coated platinum electrode and a silver/silver chloride reference electrode. The necessary reactions occur under a polarizing voltage of 600-800 mV and the total current is limited by the very small exposed area on the platinum cathode (e.g. 20 [u]m diameter). The membrane is usually polypropylene although it can be Teflon.

Reactions in the Clark electrode are very sensitive to temperature and so exact control is necessary. The system is calibrated using two gases of known oxygen concentration. Measurement of carbon dioxide concentration is usually achieved in an assembly including two chambers, one for the specimen and a second containing a pH electrode bathed in a buffer solution of bicarbonate and sodium chloride. The two chambers are separated by a semipermeable membrane usually made of Teflon. This allows dissolved carbon dioxide to pass through but blocks the passage of charged particles. Carbon dioxide diffuses across the membrane to produce the same concentration in both chambers and the pH in the inner chamber increases or decreases respectively. The pH of the sample is measured by an inbuilt pH meter.

Blood gas estimations can also be performed non-invasively. Methods include infrared oximeters which typically fit on to the earlobe, or gas-sensitive membrane electrodes which may be fitted onto the skin (transcutaneous pO2 and pCO2 meters) or made as part of an intra-arterial probe. These latter types, though important, are still troublesome in use.