Performance of the Lack circuit

It is incumbent upon the anaesthetist to assure himself that his anaesthetic machine is working within safe limits. I should like to describe a simple test to check for the presence of leaks, to assess the magnitude and thereby whether they have significance. Leaks in an anaesthetic machine are of two types. Those in the high pressure section of the machine, between the cylinder and the rotameter bank will present as failure of flow on the rotameter and are readily observed, but those occurring between the rotameter bobbin and the airway are less easy to detect. If the leak is selective of one gas a radical alteration of the composition of the gases presented at the airway will occur. This can be particularly hazardous if the leak should be in the top end of the oxygen rotameter or any point before the nitrous oxide flow joins that of the oxygen. In this instance the oxygen concentration in the inspired gases can fall to critically low levekL4 The flow of gases in the low pressure section of an anaesthetic machine is also the power used to drive many commonly used ventilators. Any leak in this section of the machine will be magnified when it is used in series with a gas driven ventilator. Engineers, after servicing a machine, undertake a pressure leak test to detect any leak;5 this can be readily carried out with equipment available in the anaesthetic room. The principle is to apply a known pressure across the machine outlet and measure the flow of oxygen on the rotameter required to maintain that pressure a t a constant level. The equipment needed is a pressure gauge calibrated up to 130 mmHg and a suitable means of connecting it to the outlet of the anaesthetic machine to be tested. A mercury column or any anaeroid oscillotonometer or simple pressure gauge can be used. To proceed with the test; turn on the gas supply to the machine, turn off the rotameters and turn the selector valve on the machine to the circuit under test. Then connect the pressure gauge to the outlet with an effective seal, turn on the oxygen at the rotameter slowly and stabilise the flow at which the pressure manometer will read a steady 120 mmHg. The volume measured on the rotameter is a measure of the leak. Under these circumstances, an acceptable leak is 200-300 ml oxygen per minute. This is a severe test and will detect troubles from leaks when the machine is used with gas driven ventilators. Machines with low pressure blow off valves are more difficult to test in this way, as the sustained pressure on testing will have to be below the valve pressure, and flows acceptable as leaks will obviously be lower (e.g. if 30 mmHg oxygen/ min. is maintained the acceptable leak flow will be 50-75 ml).