CHAPTER 18 The Anesthesia Machine and Vaporizers
A more modern and correct name for an anesthesia machine is an anesthesia delivery system. The job of the first anesthesia machines was to supply a mixture of anesthetizing and life-sustaining gases to the patient. A modern anesthesia delivery system performs these functions and also ventilates the patient and provides a number of monitoring functions. The most important purpose is to help the anesthesiologist keep the patient alive, safe, and adequately anesthetized. Anesthesia machines have become rather standardized. Currently there are two major manufacturers in the United States: Drager and Datex-Ohmeda.
2 Describe the plumbing of an anesthesia machine to create an overview of its essential interconnections
Oxygen (O2) and nitrous oxide (N2O) are available on almost every anesthesia machine. Most commonly the third gas is air, but it can also be helium (He), heliox (a mixture of He and O2), carbon dioxide (CO2), or nitrogen (N2). If the third gas does not contain O2 (as do air and heliox), it is possible to deliver a (dangerous) hypoxic mixture to the patient.
Usually the gas source for anesthesia machines in hospitals is from a centralized wall or pipeline supply. An emergency backup supply of gases is stored in tanks called E-cylinders attached to the rear of the anesthesia machine. These tanks should be checked daily to ensure that they contain an adequate backup supply in case of pipeline failure.
4 Since the flow rates of N2O and O2 are controlled independently, can the machine be set to deliver a hypoxic mixture to the patient?
Different machine manufacturers have different ways of protecting the patient from hypoxic mixtures. The Drager’s ORMC (oxygen ratio monitor and controller) senses the oxygen flow rate and controls N2O flow pneumatically. Datex-Ohmeda’s Link-25 system mechanically links the O2 and N2O flow knobs to ensure that the proportion of N2O to O2 remains in a safe range as the N2O flow is increased.
Oxygen and air in the E-cylinders is pressurized up to approximately 2200 psig, but the anesthesia machine needs to work with gas at an initial pressure of about 50 psig, slightly less than the pressure of the gas from the wall supply. A regulator performs this function. Each gas is regulated separately. Gas from both the cylinder and the wall supply encounters a check valve that selects the gas source with the highest pressure for use by the machine. Under normal circumstances the wall supply is used preferentially, and the tank supply is spared for use if the wall supply fails.
For practical purposes, wall gases are continuous in volume availability, as long as the central oxygen supply is refilled. Wall gas pressures are typically about 55 psig. Tank pressure is generally regulated by the first-stage regulator to 45 psig. The anesthesia machine preferentially chooses the source with the highest pressure. As long as everything is working correctly, the wall supply is used rather than the tank supply. Use of the wall supply oxygen is preferable because it is available in greater volume, is cheaper, and preserves the tank supply for use only in emergency situations.
7 The hospital supply of oxygen is lost. The gauge on the O2 tank reads 1000 psi. How long will you be able to deliver oxygen before the tanks are empty?
Contemporary anesthesia machines have two sources of gases: the wall outlet and E-cylinders attached to the machine itself. The cylinders are color coded and usually left shut off, being saved for use in an emergency, but a normally functioning anesthesia machine and normal wall outlet oxygen pressures will use the wall outlet preferentially.
A full green E-cylinder of O2 has a pressure of 2000 psi and contains about 625 L of O2. Since the O2 is a compressed gas, the volume in the tank correlates linearly with the pressure on the gauge. Therefore a pressure of 1000 psi means that the O2 tank has about 312 L of gas left.
Besides delivering oxygen flow directly to the patient, oxygen also powers the ventilator bellows, and the patient’s minute ventilation approximates the driving flow of the bellows. Thus, if a patient is receiving an oxygen flow of 2 L/min and a minute ventilation of 8 L/min, 10 L of oxygen will be drained from the oxygen tank every minute. A tank with 312 L remaining will last for 31 minutes at this rate. To reduce consumption of the tank oxygen, decrease the oxygen flow rate and hand-ventilate; also request that additional oxygen tanks be brought to the room.
8 A new tank of N2O is installed, and the pressure gauge reads only about 750 psig. Why is the pressure in the N2O tank different from the pressures of other gases?
Air and O2 are compressed gases and cannot be compressed to liquids in a room temperature environment because the critical temperature (the temperature at which a gas can be compressed into a liquid) is exceeded. However, at room temperature N2O condenses into a liquid at 747 psi. E-cylinders of N2O contain the equivalent of about 1600 L of gas when full, whereas E-cylinders of O2 and air hold only about 600 L. The pressure in the N2O tank remains constant until the N2O has been vaporized. About only a quarter of the initial volume of N2O remains when the cylinder pressure drops, although accurate estimation would require weighing the cylinder, knowing the empty (tare) weight of the cylinder, and determining the moles of N2O remaining.
In contrast, being a compressed gas, the volume of gas remaining in an oxygen or air cylinder is directly proportional to the pressure. The pressure of a filled cylinder is 2200 psig. A pressure reading of 1100 would suggest that the tank is half full or has about 300 L remaining.