Brief introduction to anesthesia modes
Anesthesia is an indispensable part of medical procedures, and an effective anesthesia modality ensures patient safety and comfort during surgery. Closed, semi-closed, and semi-open inhalation anesthesia modalities are common, each with different characteristics and functions. Closed anesthesia is suitable for surgeries requiring a stable level of anesthesia, semi-closed anesthesia is suitable for milder needs, and semi-open inhalation anesthesia is suitable for surgeries requiring more anesthetic. When choosing an anesthesia modality, physicians need to comprehensively consider the specific circumstances of the patient and the surgery, ensuring a safe and effective anesthesia process.
1. Closed-loop anesthesia mode
Closed-loop ventilation systems are equipped with a reservoir bag to allow for complete re-inhalation of exhaled gas. They feature a one-way valve and neutralize carbon dioxide. These systems include recirculation loop systems with a fully closed APL valve. The fresh gas intake is exactly equal to the patient’s intake; carbon dioxide is absorbed and all gas is re-inhaled, with the APL valve completely closed.
Closed-loop anesthesia is a mode of anesthesia with complete airway control. Both inhaled and exhaled gas are controlled by the anesthesia machine, and carbon dioxide in exhaled gas is absorbed by soda lime. In closed-loop mode, the anesthesia system accurately controls the patient’s anesthetic state by adjusting drug dosage and respiratory parameters. Its advantages include a stable depth of general anesthesia over a long period, facilitating artificial ventilation and respiratory management, but resulting in greater respiratory resistance and instrument dead space. This mode is suitable for longer surgeries and can maintain a stable level of anesthesia. Closed-loop methods include cyclic and reciprocating types, with the cyclic closed-loop method being more commonly used.
2. Semi-closed anesthesia mode
Semi-closed breathing systems are equipped with a reservoir bag to allow partial re-inhalation of exhaled gas, a one-way valve to neutralize carbon dioxide, and a low fresh airflow. These systems include recirculation loops with the APL valve at least partially open. Partial re-inhalation exists, and both the fresh gas flow rate and APL valve are at intermediate values.
Semi-closed anesthesia is a mode between closed and open inhalation anesthesia. In semi-closed mode, the anesthetic system controls the patient’s airway pressure to some extent but allows the patient to breathe spontaneously. This mode is suitable for surgeries requiring less anesthetic, providing relatively free breathing and a lighter level of anesthesia. Its characteristic is that it falls between open and closed methods, with some degree of re-inhalation of exhaled gas, controlled according to the valve and gas flow rate used. The prescribed carbon dioxide re-inhalation volume is less than 1% in semi-open systems and more than 1% in semi-closed systems.
3. Semi-open anesthesia mode
Semi-open systems are equipped with a reservoir bag, preventing re-inhalation. Examples include the Mapleson circuit or a recirculation circuit operating at a high fresh gas flow rate (FGF > minute ventilation VE). There is no re-inhalation; the fresh gas flow rate is higher than the minute ventilation.
Semi-open inhalation anesthesia is a relatively traditional mode of anesthesia. In this mode, the anesthetic is delivered to the patient’s respiratory system via inhalation. This mode is suitable for surgeries requiring large amounts of anesthetic and can provide a deeper level of anesthesia. It refers to the complete expulsion of inhaled gas (anesthetic mixture) and exhaled gas into the atmosphere, preventing re-inhalation. Its advantages include simplicity, small respiratory resistance and dead space, but rapid escape of anesthetic gas makes it difficult to maintain a constant depth of anesthesia.
4. Open anesthesia mode
Open-type ventilation systems (including inhalation and open-drip anesthesia) lack a reservoir, functional re-inhalation of exhaled gas, tubing, and valves. In these systems, patients can directly inhale the atmosphere.
Open-type drip anesthesia ventilation systems are no longer used in modern medicine, but they have historical significance and are still occasionally used in some developing countries. The Schimmelbusch wire mask is one of the simplest anesthetic ventilation devices. This device attaches gauze to a wire mask, and liquid volatile anesthetics (ether, chloroform, or halothane) are dripped onto the gauze. When the patient inhales through the mask, air passes through the gauze, causing the anesthetic liquid to evaporate, thus delivering a high concentration of anesthetic to the patient. In this system, the patient’s exhaled gas is not re-inhaled. Its advantages lie in its simplicity and minimal equipment requirements.
Inhalation ventilation systems deliver anesthetic gas through a mask to the patient’s face, without the mask directly contacting the patient’s face. This system is most commonly used for pediatric patients because, in these cases, directly attaching the mask to a child’s face can be difficult or result in resistance. High-flow-rate inhalation is also used in ophthalmic surgery. In this system, especially when the gas flow rate is sufficiently high, there is virtually no re-inhalation of exhaled air.