The Basic Working Principle Of Anesthesia Machine

The  anesthesia machine  can be divided into the following main sub-systems from the working principle: the gas supply system, the control circuit system, the breathing and ventilation circuit system, and the clearing system, as well as a set of system function and breathing circuit monitors. Some anesthesia machines also have monitors and alarms to easily display the values and changes of certain physiological variables and parameters related to cardiopulmonary function or the concentration of anesthetics and other gases in the breathing mixture.

The following mainly describes the composition of the anesthesia machine and the functions of each part from the working principle.

1. Gas supply and control loop system

The anesthesia machine needs a lot of oxygen when it works, which is usually obtained from the central air supply system of the hospital or from the oxygen cylinder. Reduce the pressure to the suitable working pressure of the anesthesia machine, the central gas supply system does not need a regulator, because the gas has dropped to about 4 kg, and the suitable working pressure of the anesthesia machine is 3-6 kg. Most anesthesia machines have an alarm system for oxygen source failure. If the oxygen pressure is below 2.8 kg, the machine will reduce or cut off the flow of other gases and activate the alarm.

The flow of each gas in the continuous flow device is controlled and displayed by a flow meter. The flow meter can be mechanical or an electronic sensor with LCD. After the gas passes through the control valve and the flow meter , into the low-pressure circuit, and if necessary, through the vaporizer, and then supply it to the patient. In a good anesthesia machine, the flow control mechanism of nitrous oxide and oxygen should be linked, so that the ratio of oxygen to nitrous oxide will never drop to the minimum value (0.25L/min).

2. Breathing and ventilation circuit system

Most anesthesia machines provide a continuous flow of oxygen and anesthetic gas, called a circulatory system. In this type of anesthesia machine, there are two main breathing circuits, tight and semi-closed.

In the closed breathing circuit, the gas exhaled by the patient is completely returned to the circulatory system after removing carbon dioxide; in the semi-closed breathing circuit, part of the gas exhaled by the patient enters the circulatory system, and part of it is discharged from the circulatory system. In the circulatory system, the supply flow of fresh gas below 1L/min is called low flow anesthesia, and the fresh gas flow below 0.5L/min is called minimum flow anesthesia.

Manual ventilation requires the operator to continuously squeeze the storage bag to make the patient breathe. During a long operation, the operator is not only very tired, but also affects other work. Therefore, an automatic ventilator is often used to mechanically allow the patient to breathe. The ventilator forces anesthesia gas mixture into the patient circuit and breathing system, receiving both the patient’s exhaled gas and fresh gas. The anesthesiologist can adjust parameters such as tidal volume, respiratory rate, inspiratory ratio and minute ventilation according to the patient’s condition, and adjust the ventilation mode to meet the various needs of the patient.

3. Clear the system

The removal system of the anesthesia machine, also known as the carbon dioxide absorption system, consists of 1-2 carbon dioxide absorption tanks, which are filled with soda lime or barium lime. The main function is to remove carbon dioxide from the patient’s exhaled breath.

4. Monitoring and alarm system

The anesthesia machine has a set of monitoring-related devices according to different configurations, such as monitoring of airway, physiological aspects, anesthetic gas concentration, and monitoring that can indirectly reflect the depth of anesthesia and the degree of muscle relaxation in patients.

Most of the monitoring systems of anesthesia machines are only equipped with a basic monitoring device as the platform of the system. The monitoring contents include: airway pressure, inhaled tidal volume, minute ventilation, respiratory rate and related alarm systems. Other monitoring equipment can be purchased separately and added to the anesthesia machine system.

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