Contents
The anesthesia machine is a device that is an integral part of modern surgery, resuscitation and anesthesiology. This is a universal device that provides an opportunity during surgical interventions and various medical procedures to implement the supply of special, precisely dosed mixtures of gaseous and volatile drugs to the patient. The anesthesia machine not only provides adequate anesthesia, but can also be used to organize artificial and assisted ventilation. This device allows physicians to constantly monitor the patient’s vital parameters and makes it possible to successfully perform complex medical interventions that require the use of anesthesia.
Optimal conditions for gas exchange in the patient’s lungs are the main task of the anesthesia machine. This means that during anesthesia, it is this device that will ensure that a sufficient amount of oxygen enters the patient’s lungs, eliminate (eliminate) carbon dioxide that the patient exhales, and also maintain optimal humidity and temperature of the inhaled gas mixture.
Anesthesia device
All anesthesia machines include the following elements: gas reservoir; gas dosimeter and evaporator for volatile narcotic substances; breathing circuit. Each of these elements has its own specific function and its own technical characteristics.
Gas tank
The gas reservoir is the same cylinders that contain the gases used in anesthesia. Most often it is oxygen and nitrous oxide. At the same time, cylinders with oxygen are painted blue, and with nitrous oxide – gray. Due to the different colors of the cylinders, it is easier for physicians to navigate the configuration of the anesthesia machine. With a relatively small volume, a rather large volume of gas is stored in cylinders due to compression. In order to reduce the pressure at the outlet of the cylinders and maintain it at a constant, not too high level, special reducers are used.
Reducers are divided into several types: with adjustable pressure, with unregulated pressure, and also equipped with a device that prevents freezing of gases in the reducer channel.
Gas dosimeter and evaporator
The key to successful management of the depth of anesthesia and the guarantee that overdose can be avoided is the presence of a dosimeter in the anesthesia machine. This is a special technical element that makes it possible to supply the patient with the exact proportions of gases – oxygen and nitrous oxide. Most modern anesthesia machines are equipped with so-called rotametric dosimeters, or rotameters. As a rule, two rotameters are provided for oxygen: for flow up to 2 l/min and for flow up to 10 l/min.
Rotameters designed for various gases are combined into a unit equipped with a chamber in which the gases are mixed. When several gases (oxygen and nitrous oxide) enter the chamber at the same time, they mix and, as a mixture, enter the patient. What exactly the concentration will be in this case depends on the flow rate that is set on each of the rotameters. For example, under conditions of supply of 2 liters of oxygen per minute and 6 liters of nitrous oxide per minute from the mixing chamber, the patient will receive a mixture that contains 25 percent oxygen and 75 percent nitrous oxide.
The dosed supply of liquid narcotic substances is carried out using a special device that turns these drugs into steam, in other words, evaporates them. It is in this form that the patient inhales them. To date, evaporators are used that allow accurate dosing of narcotic substances. The technical indicators of each of them are customizable.
breathing circuit
The breathing circuit is the “heart” of the anesthesia machine and the most complex of its elements. It is the breathing circuit that directly supplies oxygen and anesthetics to the patient’s airways from dosimeters and evaporators, and is also responsible for removing exhaled gases from his lungs. Today, there are two types of breathing circuit: with reversion and without it. Reversion in this case is the complete or partial inhalation by the patient of the gas-narcotic mixture that was previously exhaled by him.
Non-reversible breathing circuit
The patient inhales oxygen and anesthetics either from the counterlung or from the atmosphere. The air he exhales is released into the atmosphere. Depending on whether the gas reservoir is the atmosphere or the counterlung, the non-reversible breathing circuit is either open or semi-open.
Open non-reversible breathing circuit
The simplest example of such a breathing circuit is the use of a conventional face mask. It was in this way that the first anesthesia in history was administered. A liquid narcotic, such as ether, chloroform, etc., was dripped onto the face mask. The patient inhaled its vapors mixed with atmospheric air, and exhaled into the surrounding atmosphere.
The simplest devices that have been used for anesthesia of this type are Esmarch and Vancouver masks. They are a frame made of metal, which is covered with several layers of gauze. The advantages of such devices are their extreme simplicity, as well as the absence of the so-called “dead space effect” and breathing resistance. At the same time, the method also has more than serious drawbacks. First of all, it eliminates the possibility of dosing the substance used for anesthesia as accurately as possible. It is also impossible to artificially ventilate the lungs. The air in the operating room is polluted with narcotic vapors, which can affect the doctor. A very significant loss of moisture and heat is also recorded. A similar method cannot be used for operations on the face and neck, on the open chest. It is also excluded during surgical interventions, during which the patient is located on the stomach. Therefore, anesthesia with a mask plays a rather auxiliary role and is used for minor operations that are performed on young children.
Another example of an open non-reversible circuit is anesthesia using anesthesia machines. During the patient’s inhalation, the air passes through the evaporator, where it is saturated with a certain amount of narcotic substance, after which it enters the breathing attachment through the hose. With its help, the gas moves either to the mask or to the endotracheal tube, in other words, enters the patient’s airways. The exhaled gas is released into the atmosphere through the exhalation valve. Due to the fact that the breathing attachment has special furs, it is possible to carry out artificial ventilation of the lungs. As a result, conducting anesthesia in a similar open way using the apparatus combines all the advantages of anesthesia with the help of a mask, but at the same time it is devoid of most of the disadvantages inherent in this method. However, it should be mentioned that large moisture losses persist, as does the fact that the operating room is contaminated with narcotic vapors.
It should be noted that open anesthesia is especially important when surgical interventions are required in the so-called “field” conditions, when it is not possible to deliver cylinders with oxygen. At the same time, some modern open-type anesthesia machines have dosimeters for oxygen and nitrous oxide, as a result of which they can be converted into machines with a semi-open circuit.
Semi-open non-reversible breathing circuit
The semi-open non-reversible breathing circuit has a number of advantages. Its difference from the open one is that during its use, the flow of carrier gas does not come from the atmosphere, but from the apparatus. The gas that the patient exhales is completely released into the atmosphere.
The indisputable advantage of this method is the ability to supply the patient with mixtures that contain a high percentage of oxygen, as well as the chance of using gaseous narcotic substances. In addition, both semi-open and open hardware methods make it possible to control the inhaled concentration of the narcotic substance.
If we talk about the disadvantages of a semi-open non-reversible breathing circuit, then we should note a very significant consumption of oxygen and substances used for anesthesia, significant air pollution in the operating room with vapors of medicines, as well as significant losses of heat and moisture by the body.
Special valveless, semi-open, non-reversible systems are used for anesthesia in young children. This, for example, can be the Air system, which consists of a T-shaped tube with a cross section of about 15 mm. A hose is put on one of the pipes, into which the gas-narcotic mixture is supplied, the second pipe is connected to the endotracheal tube or mask, and through the third, which remains open, the exhaled gas is released into the atmospheric air. If it becomes necessary to artificially ventilate the lungs, the free end of the expiratory tube is clamped with a finger during the inhalation phase. At the same time, the lungs are filled directly as a result of the flow of fresh gas, and exhalation occurs when the opening of the pipe is opened by removing the finger.
Another, more convenient modification of this system is the Ries system. A rubber tube with a rubber bag, the volume of which is about 500 ml, is put on the free branch pipe of the T-tube. The tail pipe of this bag is open to the atmosphere. The advantage of such a system is that artificial ventilation can be carried out with one hand, squeezing the bag and at the same time blocking its tail opening, which opens during the exhalation phase. In addition, this bag makes it possible to monitor respiratory movements and gas pressure.
Reversible breathing circuit
The need to create reversible breathing circuits was dictated by the desire to minimize the loss of gases, narcotic substances, moisture and heat. It is from these considerations that scientists thought about developing a breathing circuit in which the gas-narcotic mixture exhaled by the patient is either completely (in a closed system) or partially (in a semi-closed system) re-inhaled by the patient.
The use of systems of this type is fraught with two difficulties. Firstly, there is a need to eliminate high concentrations of carbon dioxide, and secondly, a given concentration of narcotic substances should be provided.
The removal of carbon dioxide is carried out using adsorbers that contain a granular chemical carbon dioxide absorber. After the gas-narcotic mixture exhaled by the patient is in the adsorber, it is purified from an excess of carbon dioxide. To make this cleansing as complete as possible, an active fresh scavenger should be used. One serving of this substance is designed for approximately two hours of work. Before each anesthesia, it is recommended to place a fresh absorber in the adsorber. The absorbent is stored in hermetically sealed vessels.
As for the dosage of narcotic substances when using closed and semi-closed systems, special analyzers are used to determine it.
However, in any case, the use of closed and semi-closed systems is only permitted for qualified personnel.
Other characteristics of anesthesia machines
In addition to the features of the breathing circuit, there are other important characteristics of anesthesia machines.
So, these devices are distinguished by the type and amount of gases used. There can be either two or three. According to the type of anesthetic ventilator, devices are divided into devices with a pneumatic ventilator and with an electric ventilator. In addition, there are anesthesia machines that are completely devoid of a ventilator.
Some anesthesia machines use a mechanical flowmeter, while others use an electronic flowmeter.
In addition, anesthesia machines are classified according to the possibility or impossibility of using low-flow anesthesia. Devices that allow the use of low-flow anesthesia make it possible to significantly reduce the consumption of often very expensive anesthetics. In such devices, reverse circuits are used. They also make it possible to maintain the temperature and humidity of the respiratory mixture at an optimal level, so that it is possible to avoid malfunctions in the patient’s respiratory system.
Battery life is also important. It is extremely important that the machine continues to function even if the power goes out unexpectedly. Only under this condition, doctors will be able to complete the surgical intervention without any problems. In different devices, battery life varies from 45 minutes to two hours.
According to the overall dimensions, anesthesia machines are divided into wall-mounted, portable and floor models. In ambulances, the most compact devices are most often used.
Finally, some manufacturers of anesthesia equipment produce both universal devices and devices designed for patients of a certain age – adults or children.