A must-see for beginners | Understanding the anesthesia machine from the working principle of the bellows

Modern anesthesia machines are of different shapes and colors, and many different functions have been developed, but their most basic and most important function is to assist or control breathing.

Regarding the working principle of the anesthesia machine, it is very complicated to elaborate.
The core air circuit is the essence of the anesthesia machine.
This article briefly explains the entire breathing process through the working principle of the anesthesia machine bellows.

Clinically, the classic control method of anesthesia machine is pneumatically controlled electric.
The most obvious sign is the ventilator bellows.
The bellows in the control of breathing is equivalent to a balloon.

When the central oxygen supply enters the anesthesia machine, it will be divided into three tributaries: fresh airflow, rapid oxygenation, and ventilator airflow; these three are independent of each other, but the airflow that the anesthesiologist can adjust is only fresh airflow, and the ventilator airflow is self-regulated and does not participate in the gas.
Exchange, the figure below is a schematic diagram of a simple air circuit of an anesthesia machine.

Drawing: Qiu Feilong's manual control mode is obviously only suitable for assisted breathing or spontaneous breathing.
The balloon and APL valve are only effective at this time; the gas exhaled by the patient enters the storage balloon through the exhalation valve without carbon dioxide absorption When the pressure in the airway exceeds the setting pressure of the APL valve, the gas will overflow.
The inhalation is mainly from fresh gas and storage airbags.
When the flow of fresh gas exceeds the patient’s minute ventilation, there will be no reabsorption in the manual mode .

After the machine control mode is converted to the machine control mode, each breath will have the ventilator airflow and the patient's breathing airflow working at the same time, which is called a dual-circuit system ventilator, which is pneumatic and electronic control; the two airflows are not mixed and exist in the wind box Inside and outside.

When exhaling, the exhaled air is mixed with fresh air to fill the wind box, and the excess air will be discharged through the POP-OFF valve on one side (the self-starting pressure is generally 1-2cmH2O by default).

When inhaling, the airflow of the ventilator enters the enclosed space between the bellows and the shell, and the air in the bellows is driven into the breathing circuit by pressure compression, while a small part of the gas controls the POP-OFF valve to be closed (as shown in the figure below).

The picture is from "Morgan Anesthesiology" to understand the working principle, how does the anesthesia machine accurately control every breath? In fact, it is extremely inaccurate to judge the tidal volume of the patient based on the readings of the falling bellows.
The difference in tidal volume in each controlled breath is affected by the following four factors: Ventilator-fresh airflow linkage We know the POP of the ventilator during the entire inhalation phase The -OFF valve is closed, so as to ensure that all the preset gas enters the breathing circuit, but there is a big problem.
At this stage, the fresh airflow is also being fed! For example, the fresh air flow rate is 4L/min, the inhalation-expiration ratio is 1:2, and the frequency is 12 beats/min; each tidal volume will increase the extra amount from the fresh air flow=4000×0.
33/12=110ml, this increase is called respiration Machine-fresh airflow chain.

Circuit compliance standard adult circuit compliance is 5ml/cH2O.
If the peak pressure of the circuit is 20cH2O, 100ml can be lost during the inhalation phase; for this reason, the pediatric circuit is designed to be more rigid, and the compliance drops to 1.
5-2.
5ml/ cH2O.

Gas compression driving the gas in the loop will also cause gas compression loss, which is about 3% of the tidal volume under normal circumstances.
If the tidal volume is 500ml, gas compression can cause a loss of about 15ml.

Ventilator or patient’s airway leak.
This part also includes the gas sampled during carbon dioxide gas monitoring, and its manifestation is gas leak.

Modern advanced anesthesia machines have many ways to reduce the difference in tidal volume.

The self-test of the anesthesia machine is mainly to check the system compliance and circuit leakage.
Based on these results, the bellows is automatically adjusted, but note that the circuit leakage is not compensated; after some anesthesia machines continuously monitor the flow of fresh air, the ventilator automatically subtracts the ventilator-fresh The interlocking amount of airflow is called pre-adjustment; in some cases, the actual tidal volume is continuously monitored at the inhalation valve, and adjustment and compensation are performed after comparing the set tidal volume, which is called feedback adjustment.

In addition, the more advanced anesthesia machine can realize electric control, which can deliver fresh air flow only in the expiratory phase, thereby canceling the ventilator-fresh air flow linkage, achieving more accurate tidal volume, and more suitable for small-weight pediatric patients; of course; , As the most classic air-controlled electric anesthesia machine, it is still the mainstream.

This article is based on the observation that the bellows does not drop at the end of the high-flow ether discharge at the end of the pediatric surgery.
After being puzzled, consult the gas circuit engineer of the Koman anesthesia machine, and refer to the "Morgan Anesthesiology" in detail.
I hope it will be rewarding for you! Recommendation: What is the appropriate concentration of inhaled oxygen during surgery?