Anesthesia apparatus is a device that is an integral part of modern surgery, resuscitation and anesthesiology. This is a universal device that provides the opportunity during surgical interventions and various medical procedures to realize the delivery of special, precisely dosed mixtures of gas and volatile narcotic substances to the patient. Anesthesia apparatus not only provides adequate anesthesia, but can also be used to organize artificial and auxiliary ventilation. This device allows doctors to constantly monitor the main vital parameters of the patient and makes it possible to successfully carry out complex medical interventions that require the use of anesthesia.
The optimal conditions for gas exchange in the patient's lungs are the main task of the anesthesia apparatus. This means that during anesthesia it is this device that will ensure the supply of sufficient oxygen to the patient’s lungs, eliminate (eliminate) the carbon dioxide that the patient exhales, and maintain optimal humidity and temperature of the inhaled gas mixture.
All anesthesia devices include the following elements: a reservoir of gases; gas dosimeter and vaporizer for volatile narcotic substances; respiratory circuit. Each of these elements has its own specific function and its own technical characteristics.
The gas reservoir is the very cylinders that contain the gases used in anesthesia. Most often it is oxygen and nitrous oxide. In this case, oxygen cylinders are painted in blue, and with nitrous oxide - in gray. Due to the different color of the cylinders, it is easier for doctors to navigate in the configuration of the anesthesia apparatus. 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.
Gearboxes are divided into several types: with adjustable pressure, with unregulated pressure, as well as equipped with a device that prevents freezing of gases in the channel of the gearbox.
Gas dosimeter and vaporizer
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 in a unit equipped with a chamber in which gas mixing occurs. When several gases simultaneously enter the chamber (oxygen and nitrous oxide), they are mixed and are transferred to the patient as a mixture. What exactly will be the concentration in this case depends on the magnitude of the flow, which is installed on each of the flowmeters. So, for example, under the conditions of supplying 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, vaporizes them. It is in this form that the patient inhales them. To date, vaporizers are used that allow accurate dosing of narcotic substances. The technical indicators of each of them are customizable.
The respiratory circuit is the "heart" of the anesthesia apparatus and the most complex of its elements. It is the respiratory circuit that provides the direct supply of oxygen and anesthetics to the patient's airways from dosimeters and vaporizers, and is also responsible for the removal of exhaled gases from his lungs. To date, there are two types of respiratory circuit: with and without reversion. Reversion in this case is the complete or partial inhalation by the patient of the gas-narcotic mixture that he had exhaled before.
Irreversible breathing circuit
The patient inhales oxygen and anesthetics either from the air sac or from the atmosphere. The air he exhales enters the atmosphere. Based on what is used as a reservoir of gases (atmosphere or breathing bag), a non-reversible breathing circuit can be open or half-open.
Open irreversible breathing circuit
The simplest example of such a breathing circuit is the use of a conventional facial mask. It was in this way that the first anesthesia in history was given. A liquid narcotic drug, for example, 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 used for this type of anesthesia are Esmarch and Vancouver masks. They are a metal frame, 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. Moreover, the method has more than serious drawbacks. First of all, it excludes the possibility of dosing the substance used for anesthesia as accurately as possible. Impossible also artificial ventilation of the lungs. The operating room air is contaminated with fumes of a narcotic substance, 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 his stomach. Therefore, anesthesia with the help of a mask plays a rather auxiliary role and is used in small operations that are carried out for young children.
Another example of an open irreversible circuit is anesthesia using anesthesia machines. During the inhalation of the patient, air passes through the evaporator, where it is saturated with a certain amount of narcotic substance, after which it enters the breathing unit through the hose. With its help, the gas moves either to the mask or to the endotracheal tube, in other words, it enters the patient’s respiratory tract. The exhaled gas is then released into the atmosphere through the exhalation valve. Due to the fact that the breathing unit has special furs, it is possible to carry out artificial ventilation of the lungs. As a result, anesthesia in such an open way with the help of the apparatus combines all the advantages of anesthesia with a mask, but it is devoid of most of the disadvantages inherent in this method. Nevertheless, it should be mentioned that large moisture losses persist, as well as the fact of contamination of the operating room with fumes of narcotic substances.
It should be noted that anesthesia in an open way is especially important when surgical interventions are necessary in the so-called "field" conditions, when it is not possible to deliver oxygen cylinders. At the same time, some modern anesthesia devices of the open type have dosimeters for oxygen and nitrous oxide, as a result of which they can be turned into devices with a half-open circuit.
Half open non-reversible breathing circuit
The semi-open non-reversible respiratory circuit has a number of advantages. Its difference from the open one is that during its use, the carrier gas does not come from the atmosphere, but from the apparatus. The gas that the patient exhales while completely escaping into the atmosphere.
The undeniable advantage of this method is the ability to give the patient 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 shortcomings of the semi-open non-reversible respiratory circuit, it should be noted a very significant consumption of oxygen and substances used for anesthesia, significant atmospheric pollution in the operating room with vapors of drugs, as well as significant loss 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 an 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 narcotic drug 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 there is a need for artificial ventilation of the lungs, the free end of the exhalation tube in the phase of inhalation is clamped with a finger. At the same time, the lungs are filled directly as a result of the flow of fresh gas, and exhalation occurs when the nozzle opening is unclenched, removing the finger.
Another, more convenient modification of this system is the Rice 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-shaped tube. The tailpipe of this bag is in communication with the atmosphere. The advantage of such a system is that artificial ventilation can be carried out with one hand, compressing the bag and at the same time covering its tail opening, which opens in the exhalation phase. In addition, this bag makes it possible to track 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, drugs, moisture and heat. It was from these considerations that scientists thought about developing a respiratory 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) again 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 ensured.
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. In order for this purification to be as complete as possible, an active fresh absorber should be used. One serving of this substance is designed for approximately two hours of operation. Before each anesthesia, a fresh absorber is recommended to be placed in the adsorber. The absorbent is stored in hermetically sealed containers.
As for the dosage of narcotic substances when using closed and half-closed systems, special analyzers are used to determine it.
However, in any case, the use of closed and semi-closed systems is allowed only by qualified specialists.
Other characteristics of anesthesia machines
In addition to the characteristics of the respiratory 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. By the type of anesthetic fan, the devices are divided into devices with a pneumatic fan and with a fan on an electric drive. In addition, there are anesthesia machines completely devoid of ventilator.
Some anesthesia machines use a mechanical rotameter, while others use an electronic rotameter.
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 optimal temperature and humidity of the respiratory mixture, which makes it possible to avoid malfunctions in the patient's respiratory system.
Equally important is the battery life. It is imperative that the machine continues to function even if the power is suddenly cut off. Only under this condition, doctors can finish the surgery without problems. In different devices, battery life varies from 45 minutes to two hours.
In terms of dimensions, anesthesia devices are divided into wall, portable and floor models. In ambulances, the most compact devices are most often used.
Finally, some manufacturers of equipment intended for anesthesia produce both universal devices and devices designed for patients of a certain age - adults or children.