TONOMETER

Typical Number in Hospital:

Cost Bands:

References: 6,9

The basic principle of a tonometer is that when a pressurized vessel is partly collapsed by an external object the circumferential stresses are removed and the internal and external pressures are equal. This approach has been used to measure intra-ocular pressure, intra-uterine pressure during pregnancy, and to a lesser extent the intraluminal pressure or pulse waveform in some arteries.

The instrument for this is an applanation tononeter which consists of a disc and outer guard-ring which flattens the wall of the sphere or cylinder and records the pressure being applied on the central disc. For the eye a non-contact type exists which identifies the pressure within the eye by flattening a small section of the cornea by a pulse of air whose force increases linearly with time. A collimated beam of light is reflected from this section of the cornea on to a detector, and the quantity of light detected is a maximum when the test section of the cornea is flat. See also Applanation tonometer, Guard ring tocograph and Tocodynamometer.