APPLANATION TONOMETER

Typical Number in Hospital: 1

Cost Bands: 1,5

References: 6,9

The pressure within an elastic-walled sphere or cylinder such as the eye, an artery, or the pregnant uterus can be measured from outside the walls by the use of an applanation tonometer. This may be a mechanical device consisting of a flat ring in which there is a concentric disc maintained flush with the surface of the outer ring but connected to a force transducer. When a section of the wall is flattened by the device the pressure required on the central disc to keep the wall flat is equal to the pressure within the sphere or cylinder. The pressure is calculated as the force applied divided by the area of the inner disc. Large devices of this type are commonly used to measure the intra-uterine pressure during labour. This device is sometimes called a tocodynamometer or guard ring tocograph, and the measuring instrument called a cardiotocograph, which also records heart rate. Smaller devices have been used for measuring pressures within arteries and also within the eye by direct application to the cornea.

A non-contact applanation tonometer exists for measuring intra-ocular pressures. Flattening of a small section of the cornea is achieved by a blast of air with force increasing linearly with time. Applanation is detected optically by shining a collimated light beam on to the test section on the cornea and detecting the reflection. As the cornea becomes flattened the light intensity received reaches a maximum as the cornea becomes a plane mirror. The whole operation takes a few milliseconds which avoids the need for medication or any effects from blink responses. A computer is normally used to control the air blast and to identify the timing of the peak light intensity. Such devices are used in the ophthalmology department.

A carotid pulse transducer used in timing of the left ventricular ejection time may also be a small applanation tonometer.