NEWER ANESTHESIA WORKSTATIONS
With the introduction of new technology often comes the need for
adaptation of current technology to successfully allow its incorporation into existing
systems. Otherwise, a more comprehensive redesign of an entire anesthesia system
may be necessary. One such example of adaptation in the anesthesia workstation can
be seen with two new design variations of the circle breathing system. The first
of these is found on the Datex-Ohmeda S/5 ADU (see Fig.
9-1
), and the second is incorporated into the Dräger Narkomed 6000
and Fabius GS workstations. Because use of the circle system is fundamental to the
day-to-day practice for most anesthesiologists, a comprehensive understanding of
these new systems is crucial for their safe use.
Datex-Ohmeda S/5 ADU
The Datex-Ohmeda S/5 ADU debuted as the AS/3 ADU in 1998. Along
with its more comprehensive safety features and integrated design that eliminated
glass flow tubes
and conventional anesthesia vaporizers in exchange for a computer screen with digital
fresh gas flow scales and the built-in Aladin Cassette Vaporizer system, the machine
had a radically different appearance. It is not until closer inspection that the
other unique properties of the ADU begin to stand out. The principal difference
in the ADU's circle system lies in the incorporation of the specialized "D-lite"
flow and pressure transducer fitting into the circle at the level of the Y-connector.
On most traditional circle systems, exhaled tidal volume is measured by a spirometry
sensor located in proximity to the expiratory valve. The placement of the D-lite
fitting at the Y-connector provides a better location to perform exhaled volume measurement,
allows airway gas composition and pressure monitoring to be done with a single fitting
(instead of multiple fittings) added to the breathing circuit, and allows assessment
of inspiratory and expiratory gas flow and therefore generation of complete flow-volume
spirometry. The relocation of the spirometer sensor to the Y-connector also makes
it possible to move the location of the fresh gas inlet to the "patient's side" of
the inspiratory valve without adversely affecting accuracy of exhaled tidal volume
measurements.
This atypical arrangement of the circle system, with the fresh
gas entering on the patient's side of the inspiratory valve, is advantageous for
several reasons. It is likely to be more efficient in delivering fresh gas to the
patient while preferentially eliminating exhaled gases. It is less likely to cause
desiccation of the carbon dioxide absorbent (see "Interactions of Inhaled Anesthetics
with Absorbents"). Other notable changes on the S/5 ADU circle system include a
compact proprietary carbon dioxide absorbent canister design that can be changed
during ventilation without loss of the circle system's integrity and the relocation
of the inspiratory and expiratory unidirectional valves from a horizontal position
to a vertical position on the compact block assembly just below the absorbent canister.
[35]
[36]
[52]
Reorientation of the unidirectional valves reduces the breathing circuit's resistance
encountered by a spontaneously ventilated patient. The vertically oriented, unidirectional
valves only have to be tipped away from the vertical position to be opened, unlike
conventional horizontal valve disks, which have to be physically lifted off the valve
seat against gravity to be opened.
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