The basic principle of anesthesia machine
1 Working principle
The function of the anesthesia machine is mainly to output anesthetic gas, so that the patient undergoes surgery under anesthesia. Therefore, it is necessary to have a gas supply device first, and the gas supplied is O2, air or N2O. In the past, compressed O2 or air and liquid N2O stored in an air cylinder were used for supply. Nowadays, most major urban hospitals have built a central gas supply system to provide the above three gases. All applications in clinical anesthesia require blood pressure reduction to ensure constant low pressure and safety. Normally, the pressure is reduced to 3kg/cm2. When entering the anesthesia machine to the breathing loop, the air flow must be reduced to milliliters per minute through a flow meter before it can be used for the patient. Because there is a one-way valve in the loop, the inhaled or exhaled gas runs in a certain direction, and there is a soda lime tank between the breathing loops. Therefore, normal breathing can be performed in the loop of the anesthesia machine, oxygen or anesthetic gas can be inhaled, and CO2 in the exhaled gas is absorbed when it flows through the soda lime tank.
2. Supply of anesthetic gas
Except that N2O is directly input to the loop through the flowmeter and mixed with O2 for patient inhalation, the rest are all volatilized by the anesthetic liquid contained in the evaporator and output the anesthetic vapor. And supply the patient to inhale according to a certain concentration, so the evaporator can be described as the core component of the anesthesia machine, which is related to the depth of anesthesia and the safety of the patient.
The simplest anesthetic evaporator is to pass a certain amount of O2, air or N2O+O2 mixture (called diluent gas) in the space above the container containing inhaled anesthetics, and a small part of the gas flows into the evaporation chamber through the regulating valve. Saturated anesthetic vapor (also called carrier gas) is taken away, and the diluent airflow and the carrier airflow are mixed at the junction of the output port to become an airflow containing a certain percentage of anesthetic vapor, which enters the breathing loop for the patient to inhale.
The methods used by the gas to bring out the anesthetic vapor through the evaporation chamber are as follows: (1) The flow-over type, the carrier gas blows from the anesthetic liquid surface to take away the anesthetic vapor molecules. Most of the evaporators used in anesthesia machines are of this type (called inflatable plenum), and the airflow enters the evaporation chamber actively, and the room has a positive pressure. ⑵ Draw-over type, the difference is that the anesthetic vapor is brought out by the force of the patient's inhalation, so the evaporation chamber is negative pressure. The resistance to the air flow must be very low (such as an air anesthesia machine). ⑶ Bubble through type, the carrier gas penetrates the anesthetic liquid to form numerous small bubbles, thereby increasing the volatilization area. ⑷Dropper is a controlled drop of anesthetic liquid (or micro-pump syringe) into the breathing loop to evaporate for the patient to inhale. ⑸Combined type: The airflow can not only brush over the liquid surface, but also have the function of penetrating the liquid medicine to form bubbles. The evaporator of the DMN-86 multifunctional anesthesia machine designed by our hospital has three functions: brushing, sucking and penetrating bubbling.
In order to output a constant and correct concentration of anesthetic, modern anesthetic vaporizers have temperature and pressure compensation devices, such as Drager19-I vaporizers. The principle of the Tec 6 evaporator for desflurane is more complicated.
