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The Future of the anaesthetic machine
By Wilfried
Buschke
Anaesthesia Ventilation is
accompanied by a reversal of the physiologic intra pulmonary pressure
conditions and administration of potential toxic substances. Other additional
effects are related to pollution and a negative effect on airway conditioning.
To overcome
these adverse effects, future anaesthetic machines should incorporate:
-
a breathing system for closed system anaesthesia,
-
a ventilator with advanced ventilation modes and optimized spontaneous
breathing capabilities,
-
integrated control of intravenous drugs.
A breathing system for closed system
anaesthesia
Anaesthesia
research has focused on the development of ventilators with rebreathing systems
and closed systems for more than 80 years. The benefits of a closed system were
already described by Waters in 1926:
-
Reduction of pollution,
-
Improved breathing gas conditioning,
-
Consumption of volatile agents will be reduced so that the choice of the
agent becomes independent of price.
By supplementing
this concept with modern measurement and feedback control systems, it is now
possible to add quantitative anaesthesia to the scenario: i.e. monitoring and
documentation of the uptake and consumption of O2, N2O
and volatile agents.
The ZEUS
anaesthesia machine incorporates a new breathing system which combines a
classic circle system with the closed system. An electronic vaporizer delivers
the volatile agent directly into the breathing system.
With the
help of state-of-the-art sensors and computer technology it is possible to
control the target of anaesthesia (delivery of oxygen, adequate anaesthesia)
using feedback control systems. Controlling fresh-gas flows and fresh-gas
settings for the agent concentration will become superfluous as the control mechanism
has been optimized for quick wash-ins and wash-outs as well as low gas
consumption in the steady state.
A ventilator that is optimized for spontaneous
breathing
Compared to
intensive care respirators, ventilation quality in anaesthesia could not be
improved substantially, up till now. This was due to the complex requirements
of the rebreathing circuit and because the right technology was not available.
The new
ZEUS ventilator "Turbo Vent" uses a totally new ventilation drive: a
blower located in the inspiratory limb. The blower’s rotation is controlled
according to the required PIP (positive inspiratory pressure) and PEEP. The
blower drive is a so-called pressure source and has several benefits compared
to ventilators with a flow source (bellow-type ventilator):
-
the patient can breathe spontaneously at any time,
-
the PEEP pressure does not collapse (real CPAP) if the patient inhales,
-
inspiratory flow goes up to 180 L/min.
With these
preconditions, the ZEUS ventilator features –amongst others– the following new
ventilation modes:
-
volume-controlled ventilation with decelerating inspiratory flow
(AutoFlow),
-
synchronized pressure-controlled ventilation (BIPAP),
-
CPAP and Pressure Support Ventilation.
Although
functionality is comparable to an ICU ventilator, the new ventilator
incorporates all anesthesia requirements, for instance high manual ventilation
quality as well as inspiratory and expiratory valves to ensure backup
ventilation.
Integrated control of intravenous drugs
During the
last decade new intravenous drugs have been developed, and today Intravenous
Anaesthesia (TIVA, IVA) has its place beside volatile anaesthesia in many
developed countries. As a result, in addition to volatile anaesthesia, a modern
anaesthesia machine should be able to control the administration of IV-drugs.
The ZEUS
touch screen display can be used as an IV drug controller. All required
settings are stored in a drug library. The means of administration can be
selected by the operator for e.g. an absolute rate or body mass related
settings. With the help of a 3-compartment model plasma and effect site
concentrations are calculated.
The user
can store his preferences in a setup file. Because of this, the machine can be
configured easily for all types of anaesthesia techniques (volatile
anaesthesia, balanced anaesthesia, TIVA).
Summary
A new
anaesthesia machine with a blower ventilator and electronic vaporizer enables
closed system anaesthesia and new capabilities in anaesthesia ventilation. The
built-in control system allows for efficient administration of volatile agents.
The central system display allows all kinds of anaesthesia techniques to be
controlled, including intravenous anaesthesia.
References:
1. Waters RM (1926) Advantages and
technique of carbon dioxid filtration
with inhalation anesthesia. Anesth.
Analg 5:160-65
3. Jantzen J-P (1998) Minimal flow,
closed circuit, quantitative anaesthesia:
Technical realisation. Acta Anaesthesiol
Scand 42 (Suppl.112):53-56
4. Jantzen J-P, Kleemann PP (1992)
Closed circuit anaesthesia: implications
for airway climate and malignant
hyperthermia - experimental studies
in pigs. Min Anest 58 (Suppl.1):77-78
5. J-P, Jantzen (2002)
Quantitative Anästhesie-Back to the Future
Draeger Journal June
2002 – 49. DAK
Clinical Performance of Closed System Anaesthesia
with Conventional Anaesthetic Machines
Jan A. Baum
Closed system anaesthesia
is the most efficient method to apply inhalation anaesthetics. It is only
realized if, at any moment during the course of anaesthesia, the fresh gas
volume equals that amount of gas taken up by the patient. The total gas uptake,
on its part, is the sum of the gas volumes of each single gaseous component of
the anaesthetic gas taken up by the patient. It becomes the more difficult for
the anaesthetist to estimate the total gas uptake of the individual patient the
more complex is the composition of the anaesthetic gas. If, for instance, the
carrier gas consists of a mixture of nitrous oxide and oxygen, the nitrous
oxide uptake follows a power function whereas the oxygen uptake remains nearly
constant during the course of anaesthesia. Thus, to exactly follow the total
gas uptake would require continuous adaptation of the fresh gas volume by
frequent alterations at the gas flow controls. This is impossible when
conventional anaesthetic machines are used, but requires technically
sophisticated devices electronically controlling fresh gas supply by closed
loop feedback. Thus, the main obstacle for any realisation of closed system
anaesthesia with conventional anaesthetic machines is the power function
characteristic of the uptake of any of the anaesthetic gas components, as it
holds, for instance, for nitrous oxide.
Consistent omission of the
use of nitrous oxide as a carrier gas component results in a total gas uptake
being mainly determined only by the oxygen consumption of the patient, which
remains nearly constant during the course of anaesthesia. This holds likewise
for pure oxygen or a carrier gas mixture
consisting of air and oxygen, as, due to its extremely low solubility in blood
and tissues, nitrogen uptake itself is negligible. Compared with the oxygen
consumption, the small amount of anaesthetic vapour, taken up by the patient,
quantitatively also does not play a significant part in total uptake. In
clinical practice the fresh gas flow can be reduced to just that amount of
oxygen being taken up by the patient. It can be calculated by applying Brody´s
formula and is about 250 mL/min in an average body weight adult patient. An initial
high flow phase, however, has to preced flow reduction to establish a
sufficient concentration of the volatile anaesthetic. This is indispensable as
the delivery of volatile anaesthetics in conventional anaesthetic machines
still is linked to the fresh gas flow. After ten minutes, however, generally a
sufficient concentration of the volatile anaesthetic is already established.
During that time the volatile´s uptake has become as low that even with fresh
gas flows as low as 0.25 mL/min - or even lower - a sufficient amount of
anaesthetic vapour can be delivered into the system to maintain the aspired
concentration within the circuit.
Due to the omission of
nitrous oxide, the routine supplemental injection of 0.1-0.2 mg fentanyl or
0.5-1.0 mg alfentanil during induction is recommended to replace the missing
analgesic effect. The missing hypnotic effect can be replaced by increasing the
anaesthetic´s expiratory concentration by only 0.2-0.25 times the MAC of the
chosen anaesthetic. According to clinical experience 1.0-1.2 vol% isoflurane,
2.0-2.2 vol% sevoflurane, and 4.0-5.0 vol% desflurane (expiratory
concentrations), in general, will guarantee sufficient anaesthetic depth. Thus,
applying following scheme enables the anaesthetist to perform closed system
anaesthesia with conventional anaesthetic machines in routine clinical
practice. Initial high-flow phase: 3.0 L/min air and 1.0 L/min oxygen
(alternatively 4.0 L/min oxygen), vaporizer setting for isoflurane 2.5 vol%,
sevoflurane 3.5 vol%, and desflurane 6.0 vol%. After ten minutes fresh gas flow
reduction to 0.25 L/min oxygen, vaporizer setting for isoflurane 5.0 vol%,
sevoflurane 8.0 vol%, and desflurane 10.0 vol%. Sevoflurane and desflurane are
especially suitable to be applied with closed system anaesthesia. When using
isoflurane, however, at such low flows hardly a concentration of 1.2 vol% can
be maintained, but also the concentration of only 0.9-0.8 vol% guarantees
sufficient anaesthetic depth in most of
all surgical cases. The use of conventional anaesthetic machines featuring
fresh gas flow compensation of the ventilator and a gas reservoir facilitates
the performance of closed system anaesthesia.
The key to realize closed
system anaesthesia with conventional anaesthetic machines in clinical practice
is the use of simply composed carrier gases.
