Ensuring proper and safe use of the cryotherapy machine

Cryotherapy machines, also known as cryosurgery machines, continue to be widely used for surgical procedures of the eye such as retinal detachment repair, cataract extraction, glaucoma and so on. Cryotherapy machines (Figure 1) control the release of a compressed gas, stored under high pressure in a cylinder. When the foot pedal is depressed, the gas is regulated to a lower working pressure and pumped to the metal tip of the cryoprobe, where it expands rapidly. This reduction in pressure cools the gas and the tip freezes very quickly. Release of the foot pedal causes warm low-pressure gas to fl ow through the probe for defrosting. The main compressed gases used for cryotherapy are carbon dioxide (CO2) and nitrous oxide (N2O). Nitrous oxide is more effective for cryotherapy, but in many low-resource settings carbon dioxide is often much less expensive and more readily available. The pressure gauge on the back of the machine (in the case of the model in Figure 1) indicates how much gas is inside the cylinder. The pressure gauge in the front indicates the regulated or ‘working’ pressure that is applied to the probe. This pressure can be adjusted using the knob on the front, according to the strength of cooling that is desired and the type of probe used, as per the manufacturer’s recommendations. The higher the working pressure the greater the cooling. Many probes have a thermocouple that measures the temperature at the tip and this measurement is displayed on the temperature gauge on the front of the machine. Most standard cryotherapy machines do not have to be connected to an electrical outlet but may require a small battery to power the temperature gauge.