OXIMETER

Typical Number in Hospital: 3

Cost Bands: 3

References: 2,9

This is usually a device which can distinguish between oxyhaemoglobin and reduced haemoglobin by measuring and comparing the absorption of red and infrared light. Bench models exist which take blood in a cuvette, but the technique can also be applied in vivo. In vivo models usually clip on to the ear and shine light through the ear-lobe (the heat from the lamp also acts to increase the blood circulation in the ear). On the other side of the ear-lobe are filters and photocells.

The in vivo devices have not found wide acceptance because there are errors due to the quality of the blood being sampled (i.e. it is not arterial blood), small falls in the oxygen content of the blood cannot be detected, and biological variations in the total haemoglobin level render a single measurement inaccurate.

The technique is much more accurate if blood samples can be taken, especially if blood can be haemolysed (red cells ruptured). This can be done completely outside the body but in the cardiac catheterization laboratory blood samples may be drawn down the catheter for on-line analysis. This is particularly useful in identifying leaks between the chambers of the heart that may occur in some septal defects. The oxyhaemoglobin content of the right heart would be raised if blood from the left heart were able to leak through. In some models the light absorption/reflection characteristics of the blood are detected remotely using fibre-optic coupling from the external measuring device to the blood inside the body.

Use of a fibre-optic oximeter can improve speed of response of cardiac output estimation and provide intensive care monitoring of blood oxygen saturation.