Fachinformation
|
|
|
|
|
|
||
|
Fachinformation
|
||
   
|
High Flow-Closed Circuit: Feed Back Controlled Inhalation AnaesthesiaThe Road to "Physiological" Artificial Ventilation. Jan-Peter A.H. Jantzen et al. Abstract: Objectives of anesthesia ventilation include maintenance of normoxaemia, normocapnia, airway climatisation and MAC-based dosage of volatile anesthetic agents. If these goals are to be achieved automatically, e.g. by means of feed back control, regulated and controlled variables need to be defined. Clinically relevant controlled variables are oxygen saturation (sO2), end-tidal carbon dioxide concentration (etCO2), water content of respiratory gases and end-tidal concentration of volatile anesthetic agents (etVA). Corresponding regulated variables include inspiratory oxygen concentration (FIO2), water content of inspiratory gas (FIH2O), inspiratory concentration of volatile anesthetic agents and - for all variables - ventilatory minute volume. In particular if gases other than oxygen are used (e.g. N2O or N2), system volume also needs to be controlled. Obviously, fresh gas flow rate (FGF) is neither a controlled nor a regulated variable for any of the qualities pertinent to anesthesia ventilation; rather to the contrary, FGF adds noise to the system and any change of FGF adversely affects equilibria of minute ventilation, FIO2 and volatile agent concentration. However, most conventional anesthesia workstations (per ISO 5358 or EN 740) allow user adjustable fresh gas flow (FGF), adding a source of error to all control systems. We have designed an anesthetic workstation, complying with EN 740, which has eliminated FGF as an independent variable and which allows closed loop feed back control of relevant variables. It is a closed system without valves, offering further advantages with respect to work of breathing, airway climatisation, cost, environmental protection and quantitative anesthesia. The PhysioFlex is a computer controlled high flow-closed circuit workstation, relying on three hierarchically arranged levels of feed back control: oxygen, volume and agent. Controlled variables are FIO2, system volume and etVA, corresponding regulated variables are the volumes of oxygen, N2O/N2 and liquid VA, digitally fed into the system. All variables are adjusted automatically (with reference to the computer’s physiology and pharmacokinetics databank) to the respective setpoint selected by the user. Integration of an activated-charcoal filter allows reduction of VA concentration without opening the system. High flow within the system provides lagfree equilibrium of all concentrations, allowing feed back control without oscillation. A back up safety system protects against wrong dosage resulting from erroneous measurements (single fault condition). The digital administration of oxygen allows online monitoring of oxygen uptake, reflecting the patients VO2. Everyone has learnt to trust feed back control systems in daily life and sometimes considers them superior to manual control, e.g. the autopilot in aviation. Applying the same philosophy to anesthesia ventilation relieves the clinician from some needless and distracting routines, giving him more time to take care of his patient.
Key Words: Anaesthesia;
inhalation, technique, quantitative Introduction With the introduction of curare into clinical practice, tracheal intubation and controlled ventilation became the preferred techniques for prolonged surgical procedures. Accordingly, the anaesthetic apparatus became more complex; while Morton’s Ether Inhaler was designed only for the evaporation of sulphur ether, the scope of contemporary anaesthetic workstations is broader: "System
for the administration of inhalation anaesthesia which includes one or
more actuator modules, their particular monitoring and alarm modules and
essential hazard protection modules." In consideration of potential hazards resulting from application of sophisticated technology, standards have been drawn up to define essential safety features. As a result of globalization, national standards (e.g. ASA, ASTM, BSI, DIN) are progressively being replaced by international standards (CEN, ISO). We have designed an integrated anaesthesia workstation per EN 740, based on physiological considerations and relying on computer controlled closed loop feed back systems.
The
History During the sixties of this century, the anaesthesia machine had to be adapted to the properties of halogenated agents, which successively replaced ether, chloroform and cyclopropane. Halothane, due to its high potency and narrow therapeutic margin of safety, required a sidestream vaporiser, outside of the breathing circuit. This brought an end closed circuit anaesthesia, which had bee popular since 1924 (3). The high fresh gas flows now becoming the standard were associated with increased leakage and pollution of the theatre, prompting the installation of expensive scavenging systems. Dosage of carrier gases relied upon calibrated flowmeters; these „Rotameters" were patented in 1908 by Küppers for industrial use and manufactured in the Rota-Werke at Aachen, Germany. Their use in anaesthesia machines was pioneered by M. Neu in 1910. Volumetry and analytical monitoring (oximetry [EN 12598], capnometry [EN 864] and anaestheticometry [EN ISO 11196]) were not available then. In order to be „on the safe side", anaesthesiologists tended to give „a little more of everything"; in particular fresh gas including oxygen, and minute volume. This benevolent habit had undesirable consequences: oxygen toxic lung damage (4) and retinopathy of prematurity, hypocapnic cerebral ischemia (5) and pollution of the operating room (6). With the advent of oximetry (both in-line and pulse oximetry [EN 865]) and capnometry, FIO2 and minute ventilation could be reduced without putting the patient at risk of hypoxaemia or hypercapnia. The use of inappropriately high fresh gas flow rates, however, survived the introduction of anaestheticometry (7). Traditionally anaesthesiologists had used the scale of the vaporiser as an indicator of the amount of volatile anaesthetic (VA), administered to their patient; in rebreathing systems with side stream vaporisers, this required fresh gas flow rates close to the minute volume (8). From the engineering aspect, the flow dependent inaccuracy of rotameters and vaporisers had to be overcome. Anaestheticometry actually should have eliminated the dosage problems that result from the dissociation of FIVA from FFGVA with low fresh gas flow rates, and thus allowed the renaissance of low-flow techniques. Such a development was, however, not appreciated by clinicians, nor readily supported by the major manufacturers of anaesthesia machines. Only recently in the United States, anaesthesiologists, in an effort to contain costs, began advocating the wide spread use of low flows. In this effort they found support from an unlikely source, the manufacturers of recently introduced expensive volatile agents. Until now, this had to be accomplished with outdated technology, because so far the major equipment manufacturers have failed to do away with copper kettle vaporisers and rotameters. The design elements "common gas outlet" and user adjustable fresh gas flows (FGF) were sacrosanct, despite their poorly defined relevance for the safe conduct of anaesthesia. Analysing the goals of anaesthesia ventilation in terms of cybernetics, it is obvious, that FGF is neither a controlled, nor a regulated variable for any of the qualities of anaesthesia (Table 1). Quite to the contrary, user adjustable FGF must be seen as noise, interfering with important variables like airway pressure (9), tidal volume (10, 11), I:E ratio, FIO2 (12) or FIVA (8). Furthermore, use of rebreathing systems precluded the practice of quantitative anaesthesia, an old dream of physiologically oriented anaesthesiologist.
The
Idea A much criticised
feature of all currently available anaesthesia machines is that they lack
ergonomy. Jet aeroplane cockpits and automobile dash boards are designed
with user ease in mind. Such thought guided the design of the PhysioFlex
workstation. Obviously,
functional design is not prohibitive to aesthetics: in 1992, the PhysioFlex
was awarded a design prize at the Industrie Forum Design Hannover.
The
Solution Quantitative, closed circuit anaesthesia, relying on three levels of computerised closed loop feed back control (oxygen, system volume and volatile agent; Table 3). Tidal volume is generated by (one to four) membrane chambers, allowing for exact volumetry, minimal compressible volume and unresisted spontaneous breathing (Figure 2). System volume is measured by a carrier gas (N2O or air) bolus injection/dilution technique and used to correct dynamic volume measurements for compliance. A high, unidirectional flow of 60 L*min-1, generated within the system by a blower, providing a low time constant despite a very low fresh gas flow into the system and allowing unidirectional flow without valves (Figure 2). An activated charcoal-filter, allowing rapid elimination of volatile anaesthetics without opening the system (13). A computer screen as machine-user-interface, displaying three windows with analogue information, taken into account human physiological perception capabilities. Digital information is limited to what is required by law (EN 740); the most important variable - etCO2 - is highlighted (Figure 3). A separate watchdog system monitors oxygen, volume and volatile agents. This redundant monitoring guards against errors in the feed back control system resulting from false readings in the primary sensors. For reasons of ergonomy, the top of the workstation, including the user interface, can be turned around 360°, such as to face the user whatever the patients position during surgery. For simplicity, the main controls are limited to left-right cursors to select, and up-down cursors to modify any variable; changes take effect after hitting the ¿ -key (return/Ok). All other control buttons are designed as soft keys and appear only when needed. Ventilator settings, though also user adjustable, are automatically adapted to the relevant physiological variables gender, age and bodyweight. With this information and reference to the databanks on the solid state hard disk (e.g. the Radford normogram and the prime dose algorithm for volatile agents) the computer calculates ventilatory and anaesthetic requirements and offers a setting which warrants normoventilation and normoxia in most cases. In order to keep the system volume as small as possible, only the minimal number of volume chambers (see above) required for the individual patient is activated. Data from physiological databases are obtained under STPD conditions (standard temperature and pressure, dry), which must be taken into account. Gas uptake information is based on STPD, ventilatory data calculations rely on the actual BTPS conditions (body temperature and pressure, saturated). The PhysioFlex is equipped with an accumulator, providing mains-independent power supply for 45 minutes. A RS 232 interface allows downloading of data for offline analysis.
Technical
details The gas concentrations in the system can be adjusted or changed by the user at any time. PhysioFlex also has a compliance-correction function and an automatic leakage compensation function, which have no effect on respiration. Circulation
pump (Blower) Gas
dosage valves. Anaesthetic
agent delivery Membrane
chambers Temperature
sensor Oxygen
sensor CO2,
N2O, anaestheticometer (agent sensor) CO2
absorber Anaesthetic
agent filter
The PhysioFlex allows quantitative anaesthesia, cost savings for gases and vapours in the range of 70%, as compared to non-rebreathing systems, and prevents pollution of the theatre. Integrity of the tracheo-bronchial epithelia is maintained, which is not the case with non-rebreathing systems (Figure 4) (14). On-line monitoring of oxygen uptake (VO2) proved useful to assess depth of anaesthesia, quantify metabolic effects of hypothermia (15) and provides an early warning system for malignant hyperthermia (14). Uptake information is also helpful to assess reperfusion after release of a Tourniquet, or to detect gas leakage from the lung following pulmonary surgery. Due to software control of the system, further developments can easily be incorporated; additional features currently under evaluation include etpCO2-based feed back control of minute volume (16), "intelligent" weaning modes and non-invasive measurement of cardiac output (partial rebreathing technique).
Everyone has learnt to trust feed back control systems in daily life and sometimes considers them superior to manual control, e.g. the autopilot in aviation. Applying the same philosophy to anaesthesia ventilation relieves the clinician from needless and distracting routines, giving him more time to take care of his patient. The use of unnecessarily high fresh gas flow rates counteracts cost containment efforts and violates pollution regulations and needs justification.
References
|
zurück
