A basic understanding of the anesthesia delivery system and its components is important to the provision of safe patient care.
The current voluntary consensus standard describing the features of a contemporary anesthesia workstation is the American Society for Testing and Materials (ASTM) F1850–00, published in 2000 and reapproved in 2005.
The ASTM F1850–00 standard calls for an integrated and prioritized alarm system, breathing pressure monitoring, and exhaled volume or ventilatory CO2 monitoring.
Pin-index and DISS ensure that the correct medical gas enters the correct part of the anesthesia machine. The "fail-safe" valve (pressure sensor shut-off valve, O2 failure protection device) prevents the flow of nitrous oxide (N2O) or other gases if the O2 supply pressure is not adequate, but it does not ensure O2 flow. An O2 analyzer in the patient circuit is essential to detect a hypoxic mixture. It should be automatically enabled and the low O2 alarm set whenever the machine is capable of delivering an anesthetic gas mixture.
The O2 and N2O flow controls are interlinked so that a gas mixture containing 25% or greater of O2 is created at the flowmeters when N2O and O2 are in use. Use of a third or fourth gas (eg, helium) may "defeat" this feature.
A variable bypass anesthesia vaporizer creates a saturated vapor concentration of the anesthetic and then dilutes it to clinically desirable concentrations. Contemporary vaporizers for halothane, isoflurane, enflurane, and sevoflurane are variable-bypass, concentration-calibrated, and temperature-compensated types.
Vaporizers are agent specific. Erroneous filling must be avoided; agent-specific filling devices should be used.
The anesthesia workstation should be checked each day before anesthetizing the first patient and whenever any change has been made to the system. A shortened checkout should precede each administration of anesthesia. The checkout procedure should follow the directions given in the machine's operation and maintenance manual. Because of the diversity of the newer workstations, in 2007, the American Society of Anesthesiologists published guidelines to act as a template for developing preanesthesia checkout procedures.
Use of free-standing vaporizers downstream from the common gas outlet can be hazardous and should be avoided. Such vaporizers are often used on pump oxygenators for cardiopulmonary bypass procedures.
Anesthesia ventilators are traditionally pneumatic and of "bag-in-a-bottle" or "double-circuit" design. In traditional ventilators, a standing bellows design, in which the bellows descend on inspiration and ascend on expiration, is preferred because it makes a leak in the breathing system more obvious (ie, the bellows do not refill). In Dräger workstations, an electronically driven piston in a cylinder replaces the traditional bellows.
In a traditional ventilators, the delivered tidal volume (VT) may differ from the VT setting because delivered VT is influenced also by the fresh gas flow, the inspiratory-to-expiratory ratio, the breathing circuit compliance, and the peak inspiratory pressure. Newer designs (eg, GE-Datex Anesthesia Delivery Unit [ADU], Aisys, Smart Vent, Dräger Apollo, Fabius GS, Narkomed 6400) use various approaches to ensure that VT ...