What Is an Anesthesia Machine?

The anesthesia machine sends the anesthetic to the patient's alveoli through a mechanical circuit to form the partial pressure of the anesthetic gas, which diffuses into the blood and directly inhibits the central nervous system, thereby producing the effect of general anesthesia. The anesthesia machine is a semi-open anesthesia device. It is mainly composed of anesthesia evaporation tank, flow meter, folding bellows ventilator, breathing circuit (including inhalation and exhalation one-way valve and manual airbag), and corrugated tubing.

The most common breathing circuit for anesthesia is the "circulatory system". Two one-way flaps allow gas to flow into a circulation circuit that chemically absorbs carbon dioxide. In this system, fresh gas from the anesthesia machine enters the breathing circuit downstream of the carbon dioxide absorption tank and upstream of the inhalation one-way valve. The incoming fresh gas is mixed with the original gas in the loop system, flows through the suction one-way flap, and flows through the reusable or disposable corrugated pipe to the Y-shaped pipe. The patient's exhaled gas flows through the other branch of the circulatory system (exhalation) and enters the storage balloon through the exhaled one-way valve. By squeezing, a positive pressure is generated in the airbag, forcing the collected gas to pass through the carbon dioxide absorption device. Since the fresh air flowing into the circuit system consumes much more gas than the patient and the absorbent, it is necessary to install such a safety valve between the expiratory one-way valve and the carbon dioxide absorption tank. When the pressure exceeds a specified threshold, excess gas can escape. The absorption tank contains sodium carbonate lime (a mixture of sodium, potassium, and calcium hydroxide) or barium hydroxide lime (a mixture of barium hydroxide, octahydrohydrate, and calcium hydroxide). These substances absorb carbon dioxide through chemical reactions, and release heat and water at the same time (the water released can moisturize the air in the circulation system). When the absorption capacity is exhausted, the indicator changes color. The absorbent tank must be designed to allow easy replacement of the absorbent. The APL valve for venting too much gas is usually a spring-loaded valve. The spring tension controls the circuit pressure. If the patient breathes spontaneously, the safety valve is in the open position, and the breath inhales and exhales air with minimal resistance. If the patient is deeply anesthetized and deeply paralyzed, the anesthesiologist can partially or fully close the safety valve to squeeze the airbag to fill the lungs with gas to help and control the patient's breathing. The exhaust gas discharged from the safety valve should be guided to the operating room through the exhaust pipe to avoid the danger of trace anesthesia gas to the health of the operating room staff.

After the patient has completed the induction of anesthesia, the air anesthesia machine is connected to a closed mask or tracheal tube. During inhalation, the anesthetic mixed gas enters the patient through the opened inhalation flap; during exhalation, the inhalation flap is opened and at the same time the inhalation flap is closed, exhaling the exhaled gas. Foldable bellows can be used when assisted or controlled breathing is used. Depress during inhalation and pull up during exhalation to ensure that the patient has sufficient ventilation. At the same time, according to the actual needs, adjust the ether switch to maintain a stable level of anesthesia.

The disadvantage of this device is that the concentration of ether is low and it can only be used for the maintenance of anesthesia. Moreover, the consumption of ether is large and it is easy to cause environmental pollution.

DC Anesthesia Machine

The DC anesthesia machine is composed of high-pressure oxygen, a pressure reducer, a flow meter and an anesthetic liquid evaporator. As shown in Figure 2-1-2. The device can only provide oxygen and adjust the concentration of anesthetics for inhaled gas. Other devices must be connected in series with the output site to perform anesthesia.

Tightly closed anesthesia machine

The device supplies the patient with a low flow of anesthesia mixed gas through the unidirectional flow of the escape gas valve (gate). The exhaled gas enters the CO2 absorber through the expiratory valve and is reused. Its structure is mainly composed of oxygen and nitrous oxide devices, gas flow meters, evaporators, and CO2 absorbers; one-way valves, breathing tubes, gas escape valves, and airbags, as shown in Figure 2-1-3. Show.

Modern anesthesia machines are also equipped with ventilator internal airway pressure, expiratory flow, end-expiratory CO2 concentration, inhalation anesthetic concentration, oxygen concentration monitor, hypoxic alarm and hypoxic-nitrous oxide automatic protection devices. Figure 2-1-4 is an actual anesthetic gas path diagram. This is a closed loop anesthesia circuit. Before anesthesia, the patient must first be given a certain amount of oxygen (usually 3 to 5 minutes), and then anesthesia is performed.

Composition and function of anesthesia machine

The anesthesia machine is composed of the following parts from the structure: frame, external circuit, anesthesia ventilator, anesthetic vaporizer, flow meter, and monitoring system. The anesthesia machine consists of four main sub-systems in principle: the gas supply and control circuit system, the breathing and ventilation circuit system, the purge system, and a set of system functions and a breathing circuit monitor. Some anesthesia machines also have monitors and alarms to indicate the values and changes of certain physiological variables and parameters related to cardiopulmonary function or the concentration of gases and anesthetics in breathing gas mixtures. Generally, manufacturers only provide fewer monitoring and alarm combinations for standard products.

The following explains the composition and function of the anesthesia machine mainly from the working principle:

Anesthesia circuit system

Because the anesthesia machine requires a large amount of oxygen to work, it is usually obtained from the hospital's central gas supply system or an oxygen cylinder. Each gas entering the circuit from the cylinder passes through a filter, a one-way vent valve, and a regulator, which reduces the pressure to the appropriate working pressure of the anesthesia machine. The central gas supply system does not require a regulator because the gas has dropped to about 0.4MPa. The appropriate working pressure of the anesthesia machine is 0.3 ~ 0.6MPa. Most anesthesia machines have an oxygen source fault alarm system. If the oxygen pressure is below 0.28MPa, the machine will reduce or cut off the flow of other gases and activate the alarm.

The flow rate of each gas in the continuous flow device is controlled by the flow meter and displayed by the flow meter. The flowmeter can be mechanical or an electronic sensor with LCD. After passing through the control valve and flow meter, the gas enters the low-pressure circuit, if necessary, it also passes through the evaporation tank, and then is supplied to the patient. With a good anesthesia machine, the flow control mechanism of laugh gas and oxygen should be linked. Only in this way will the ratio of oxygen to laugh gas never drop to the minimum value (0.25L / min).

Anesthesia machine system

Most anesthesia machines provide a continuous flow of oxygen and anesthetic gas, called the circulatory system. In this type of anesthesia machine, there are two main breathing circuits, tightly closed and semi-closed. In the tightly closed breathing circuit, the gas exhaled by the patient is returned to the circulatory system after removing CO2. In the semi-closed type, part of the gas exhaled by the patient enters the circulatory system and part of it is discharged out of the circulatory system. In the circulation system, the supply flow of fresh gas below 1L / min is called low flow anesthesia, and the flow of fresh gas below 0.5L / min is called minimum flow anesthesia.

Manual ventilation requires the operator to continuously squeeze the airbag manually to allow the patient to breathe. When the operation is performed for a long time, the operator is not only very tired, but also affects other work. Therefore, an automatic ventilator is often used to mechanically enable the patient to breathe. The ventilator forces the mixed gas of anesthesia into the patient circuit and respiratory system, receiving the exhaled and fresh gas from the patient. Anesthesiologists can adjust parameters such as tidal volume, breathing rate, breathing ratio, and minute ventilation according to the patient's situation. Adjust the ventilation mode to meet the various needs of the patient.

3.Clear the system

Also known as carbon dioxide absorption system, it consists of 1-2 CO2 absorption tanks (sodium lime tanks), which contain sodium lime or barium lime.

CO2 absorption tank It is used to remove CO2 from the patient's exhaled breath.

Anesthesia machine monitoring and alarm system

The anesthesia machine has a set of monitoring-related devices according to different configurations, such as monitoring for monitoring the airway, physiological aspects, the concentration of anesthesia gas, and indirectly reflecting the depth of anesthesia and muscle relaxation of the patient.

The monitoring system of most anesthesia machines is only equipped with a basic monitoring device as a platform for the system. The monitoring content includes: airway pressure, inhaled tidal volume, minute ventilation, respiratory frequency and related alarm systems. Additional monitoring required can be purchased separately and added to the system.

In addition, the anesthesia workstation also needs to be equipped with an anesthesia information management system. This system can receive, analyze, and store information related to clinical and administrative management of anesthesia, automatically collect information from the monitor and automatically generate anesthesia record sheets.