Anesthetic system
An anesthetic system is a technical device that technically enables regulated ventilation as well as the supply and disposal of breathing air, oxygen and anesthetic gases within the framework of anesthesia (general anesthesia) .
The term anesthesia machine describes an anesthesia workstation as a whole. The anesthesia system is part of the anesthesia machine.
Systematics
One distinguishes
- Non-rebreathing systems from
- Rebreathing systems .
The essence of the distinction lies in the question of where the inhaled air comes from and where the used air is exhaled. The used air is reprocessed in rebreathing systems (elimination of carbon dioxide).
| system | inspiration | Expiration | |
|---|---|---|---|
| open | Surroundings | Surroundings | |
| half open | system | Surroundings | |
| half closed | system | system | Release of an excess into the environment |
| closed | system | system | no surplus tax in the area |
Non-rebreathing systems
Open system
The best known open system is the Schimmelbusch mask ( Curt Schimmelbusch , German surgeon).
It was a wire frame with which gauze compresses were stretched over the mouth and nose so that inhalation anesthetics could be administered. For this purpose the ether was dripped onto the gauze and evaporated there. The problems were that you had to keep the unwieldy mask tight and prevent the gauze from icing (evaporation is an endothermic process).
It is historically one of the oldest systems and has long been the standard instrument for anesthesia in the past. From today's point of view, however, the imperfect controllability of the anesthesia due to uncontrollable anesthetic gas concentrations and the extreme environmental (operating room air) pollution caused by the anesthetic gas are obstacles to further use.
Semi-open system
In the semi-open system, inhalation occurs from an anesthetic system and exhalation into the environment.
These systems have the advantage of low system resistance. A disadvantage, however, is the high gas consumption (oxygen, anesthetic gas). They are therefore relatively uneconomical in terms of resource consumption.
The release of the expired breathing air into the environment pollutes the air in the operating room. However, there are options to discharge the gases into the open, so that workplace pollution from anesthetic gases can be excluded.
The figure shows the functional diagram of a valve from the Ambu series in the inspiration phase .
The yellow rubber valves ensure the directed air flow. They are locked in the rest position.
In the figure above, the inspiration valve is open (inspiration state). In this way, fresh air can get from the anesthesia machine (can be connected on the right) to the patient (below). The exhalation valve (left) is closed, so rebreathing is impossible.
In the expiration phase, the inhalation valve (right) is closed and the expiration valve (left) is open. The exhaled air escapes into the environment.
The Ambu Valve allows for effective combination with an anesthesia machine to restrict the air consumption to the so-called minute volume. There are other semi-open systems that are used with excess (e.g. bain ).
Both ventilation and spontaneous breathing can be achieved with an Ambu valve. This is not possible with various other semi-open systems.
In contrast to semi-closed systems, CO 2 absorbers can be dispensed with in semi-open systems . The Ambu valve variants are therefore used independently of anesthesia in transport respirators in intensive care and rescue medicine .
Rebreathing systems
Rebreathing systems are always circular systems. The patient receives some of the air he exhaled. The exhaled air must be freed from CO 2 . This is done using carbon dioxide absorbers (3), which are mainly filled with calcium hydroxide , which reacts with CO 2 to form carbonate and water. This reaction is exothermic. These properties are used in modern circular systems to warm and humidify the air we breathe.
In terms of resource consumption, circular systems are more economical than semi-open systems. Usually less than three liters of fresh gas are used per minute.
Semi-closed system
The semi-closed system, as it was introduced in principle in 1923 by Ralph Milton Waters (* 1883), is most widely used today. It forms the best compromise between resource consumption, measurement effort and acquisition costs. The illustration of an anesthesia machine with a semi-closed system can be found in the article Anesthesia .
In the present functional diagram , the patient is connected to the Y-piece (5) of the semi-closed circle system. The dome valves (4 and 6) with ceramic plates ensure the direction of the air flow. These small plates close the valves at rest by gravity. Ventilation or spontaneous breathing leads to pressure increases or decreases in the system and hose, which lead to the opening of the respective valves. Overpressure can be generated by compressing the resuscitator bag (1) or by passive exhalation by the patient, and negative pressure by taking a breath of the patient.
Fresh air is supplied to the system via a line (8).
When inhaling, the exothermic absorber reaction warmed and humidified air is supplied to the patient via the open inspiratory valve (4), the silicone tube (green) and the Y-piece (5). The expiration valve (6) is closed due to pressure and gravity. This process can be initiated by self-breathing or ventilation.
During expiration, the exhaled air reaches the anesthesia arm (2) via the Y-piece, the silicone tube and the expiration valve (6). There, excess gas can be released into the environment via an overpressure valve (7) or made available for a new ventilation process in the other direction.
The above Ambu valve and the dome valves are not evidence of the existence of a semi-open or semi-closed system per se. With the removal of the dome valve plates and the installation of an Ambu valve instead of the Y-piece, a semi-closed system can be created, since this would also be a rebreathing system. In practice, however, unnecessarily high system resistances and a certain uncertainty with regard to the air flow direction would be accepted.
Closed system
Closed systems are essentially the same as semi-closed systems, but avoid releasing an excess into the environment. With them, new anesthetic strategies are possible (keyword: quantitative anesthesia).
They have the lowest resource consumption because in the equilibrium ( steady state ) only the oxygen used (about 3–6 ml / kg · min) has to be renewed. However, they have to be sealed in a complex manner. In addition, constant monitoring of the system parameters is necessary for their exact function. This increases the technical effort to such an extent that the investments required are extremely high. For these reasons, closed systems have not yet become widespread.
In connection with the future use of xenon for anesthetic purposes , however, a greater spread of these systems is to be expected.
Individual evidence
- ↑ Christoph Weißer: Anesthesia. In: Werner E. Gerabek, Bernhard D. Haage, Gundolf Keil, Wolfgang Wegner (eds.): Enzyklopädie Medizingeschichte. De Gruyter, Berlin 2005, ISBN 3-11-015714-4 , p. 54 f .; here: p. 54.