Types and Properties of Medical Gases
Medical gases in the clean operating department are an indispensable facility for assisting treatment and rescuing patients' lives.
8.1.1 Medical Gases in the Operating Department
There are many types, mainly oxygen, negative pressure suction, compressed air, nitrogen, nitrous oxide, mixed gases, etc., but at least three gases—oxygen, negative pressure suction air, and compressed air—must be installed in the clean operating room.
The backup amount of the medical gas source shall not be less than three days' usage.
8.1.2 Oxygen
(1) Physical properties. Oxygen is a colorless, odorless, non-irritating combustion-supporting gas. Oxygen must comply with the medical oxygen standard.
Molecular weight: 32
Specific gravity: 1.153 at 0℃
Density: gaseous state 1.429 g/L (at 0℃, 760 mmHg); liquid state 1.14 kg/L (at －183℃)
Critical temperature: －118.4℃
Critical pressure: 5.18 MPa
Liquid expansion ratio: at 1 standard atmosphere (760 mmHg), 1 L of liquid oxygen expands into 865 L of gaseous oxygen (i.e., 1：865).
Oxygen is used to supplement oxygen deficiency in the human body and, when mixed with other types of gases, for special treatments. Because oxygen deficiency in the human body will cause various disorders and dangers, it is required to supplement the body with the necessary amount of oxygen; however, inhaling too much high-concentration oxygen can also cause poisoning. Since treatment requires certain special gases, and such gases have special significance for certain functions of the human body, they would be harmful if used alone. Therefore, oxygen is added in a certain safe proportion and mixed to supply patients, so as to achieve the therapeutic effect without causing danger.
(2) Oxygen gas sources. There are mainly the following three forms.
1) Gaseous oxygen.
① The gaseous oxygen station consists of multiple 40 L, 15 MPa steel cylinders grouped together, divided into two groups A and B, and is composed of a manifold, a switchover pressure-reducing device, a safety pressure-relief device, an alarm device, etc.;
② The number of cylinders in the cylinder group is determined according to the hospital's daily usage, generally 10 per group; the two cylinder groups operate with one in use and one on standby, switching between each other; the high-pressure gas in both cylinder groups is supplied to the high-pressure piping of the manifold and sent into the switchover pressure-reducing device, and after pressure reduction respectively, it is delivered to the operating department and other areas.
③ Gaseous oxygen source. The operation of the two cylinder groups is mainly by automatic switchover, supplemented by manual switchover. If group A is working, group B is on standby. When the pressure in group A drops as the working time extends and falls to a level where it can no longer be maintained, the system will automatically switch to group B to work, continuing to maintain the gas volume required at the terminals; at the same time, group A sends out an alarm signal to notify the duty officer to replace the empty cylinders used by group A with full cylinders as the standby gas source for group B; when group B works until it can no longer maintain normal operation, it automatically switches to group A to continue working, and so the cycle continues endlessly.
④ The gaseous oxygen station must have separate storage locations for empty cylinders and full cylinders, each with its own markings. The reserve of the gas source shall not be less than three days' usage. Full cylinders should not be stored in open-air locations; the liquid oxygen tank and the cylinder loading/unloading platform should have sunshade canopies, which shall be made of non-combustible materials. The angle valves of used cylinders must be closed and protective steel caps must be put on. The ambient temperature of the working cylinder room and the switchover pressure-reducing device area should be guaranteed within the range of 10～35℃; if the ambient temperature is too low, the pressure reducer will freeze up and affect the normal operation of the system.
⑤ The gaseous oxygen station must have explosion-proof measures and good ventilation; within 5 m there must be no air openings of communicating trenches. It should be convenient for trucks transporting cylinders to enter and exit, and cylinder loading and unloading should also be convenient; the construction of the oxygen station must comply with relevant national laws and regulations.
⑥ The material for oxygen transmission pipelines is generally selected as stainless steel pipes or deoxidized copper pipes. The gas transmission pipelines must have good static-conducting grounding, with a grounding resistance of less than 10Ω, and lightning protection measures. The gas transmission system is connected separately to the clean operating department and is not shared with other areas.
⑦ Safety venting and alarm devices must be installed on the system in the gaseous oxygen station: when the output pressure is higher than the maximum designed working pressure by 0.015 MPa, the safety pressure-relief valve opens to discharge the excess pressure, and at the same time the alarm device issues an overpressure alarm signal, ensuring the system the safety of the pipeline and its corresponding equipment; when the output pressure after pressure relief falls below the minimum design pressure by 0.025MPa (at which point an underpressure alarm signal is issued), the safety pressure relief valve should close automatically (no shutoff valves are permitted before or after the safety valve) to ensure the system returns to normal.
⑧ The fire hazard of the gaseous oxygen station falls under Class B; the building should have a fire resistance rating of not lower than Grade II. Within its safe fire protection distance, there must be no basements, underground pits, wells, or trenches, nor offices, wards, etc.; these should be located at least 7m away. The gas station must be kept away from flammable and explosive locations such as boilers and incinerators; its safety distance should meet the requirements of national fire protection codes and also comply with the hospital's hygiene management regulations. The area around the gas station must be kept clean and free of stored waste, and away from garbage stations, morgues, dissection rooms, incinerators, etc.
⑨ Safety vent valves should be installed outdoors, and pipes passing through walls should be fitted with sleeves.
⑩ Sufficient maintenance clearance should be left between gas cylinders/equipment and the walls; the distance between gas cylinders and heating units should be no less than 800mm.
⑪ No part of the heater is allowed to come into contact with the oxygen supply equipment.
⑫ The insulation resistance between electrical equipment and oxygen equipment must not be less than 1MΩ.
⑬ The manifold pipes and pressure reducers must be grounded; their grounding devices must be installed independently, with a grounding resistance of no more than 10Ω.
⑭ All valves, pressure reducers, changeover valves, pressure gauges, and high-pressure metal adapter hoses must be periodically calibrated and replaced as required by the product manuals.
⑮ The outer surface of all oxygen conduits must be painted with white marking paint.
[Reference] For a schematic design of the gaseous oxygen station for medical central gas supply, see Figure 8.1.
[Reference] For examples of the layout design and exterior appearance of the gaseous oxygen station, see Figure 8.2.
Figure 8.1 Schematic Diagram of the Gaseous Oxygen Station Design Figure 8.2 Examples of gaseous oxygen station layout design and exterior appearance
2) Liquid oxygen. A liquid oxygen station mainly consists of a liquid oxygen storage tank, a vaporizer, monitoring and regulating instruments, a distributor, and delivery pipelines.
The liquid oxygen storage tank is the core equipment of a liquid oxygen supply station, so the selection and quality of the liquid oxygen storage tank come first. The following aspects should be mainly considered:
① The reputation of the manufacturer's brand in society. The purchase of liquid oxygen storage tanks is very important; guard against counterfeit and shoddy products, products that pass off inferior goods as quality ones, and products refurbished from old (recycled) units (with surfaces repainted) and then resold.
② A liquid oxygen storage tank is an ultra-low-temperature pressure vessel, so the thermal insulation performance of the liquid oxygen tank must be reliable.
③ The volume is determined according to the gas consumption and the scale of the hospital. Usually, one tank of 3.5–5 m³ each, or two storage tanks of the same specification, are selected—one in use and one as backup—switching between each other and providing mutual protection. If the liquid oxygen source is relatively far away, or the number of beds is large, or the hospital is equipped with a hyperbaric oxygen chamber, and there is permitted installation space within the hospital grounds, a storage tank of somewhat larger volume may be appropriately selected to reduce the frequency of transportation.
④ A storage tank with a pressure of 0.8 MPa is generally sufficient. If the pressure is too high, the one-time investment is high; if the pressure is too low, long-distance delivery is difficult, and the terminal pressure and flow rate are also unstable.
⑤ The selection of the vaporizer should be determined according to the gas consumption conditions within the hospital. Generally, a vaporizer with a capacity of 50 m³/h is sufficient; if there is special gas use (such as a hyperbaric oxygen chamber) and a large number of inpatient beds, it should be appropriately increased to 100 m³/h or larger.
⑥ Distributor. A small container made of stainless steel, generally about 0.3 m³ is appropriate, serving to buffer and stabilize. According to the different destinations, separate connections are made individually from the buffer.
【Reference】 For the layout of the liquid oxygen station (with gaseous oxygen as backup), see Figure 8.3.
The installation location of the liquid oxygen station should take into account the safe distance from other objects.
【Reference】 For the safe distances between the liquid oxygen tank and other objects, see Table 8.1. Table 8.1 Minimum Safety Distances between the Liquid Oxygen Tank and Other Objects (m)
| No. | Related objects | Distance/m |
|---|---|---|
| 1 | Hot work and smoking areas | 5 |
| 2 | Medium- and high-voltage substations | 5 |
| 3 | Areas where small quantities of flammable materials are stored | 5 |
| 4 | Areas where large quantities of flammable materials are stored | 15 |
| 5 | Vent pipes containing flammable gases | 5 |
| 6 | Flammable gas cylinder storage area less than 70Nm³ (15℃, 1013mbar) | 5 |
| 7 | Flammable gas cylinder storage area greater than 70Nm³ (15℃, 1013mbar) | 8 |
| 8 | Requirements for a 20t liquefied petroleum gas storage tank: | |
| 0–1.1t (equivalent volume 0–2.15m³) | 6 | |
| 1.1–4t (equivalent volume 2.15–7.84m³) | 7.5 | |
| 4–6t (equivalent volume 7.84–117m³) | 15 | |
| 60–150t (equivalent volume 117–294m³) | 22.5 | |
| 9 | Pipelines carrying flammable gases or liquids | 3 |
| 10 | Chimneys with open flames or open-flame furnaces | 20 |
| 11 | Offices, canteens and areas where people may gather | 5 |
| 12 | Flanges and joints on pipelines carrying flammable gases or liquids: | |
| Pipe inner diameter less than 25mm | 6 | |
| Pipe inner diameter less than 50mm | 10 | |
| Pipe inner diameter less than 70mm | 15 | |
| 13 | Air intake pipes of compressors/ventilation fans | 5 |
| 14 | Sewers or cable trenches | 5 |
| 15 | Building enclosure walls | 5 |
| 16 | Public roads | 5 |
| 17 | Public railways | 10 |
| 18 | Public vehicle parking areas | 5 |
3) Oxygen generator. The oxygen generator is a system facility composed of multiple pieces of equipment, and the oxygen it produces must meet medical oxygen supply standards. To this end, the entire system, as a complete set of equipment for producing medical oxygen, must be a full set of facilities that has been appraised and approved by a legally authoritative organization before it can be put into use. The oxygen generation station consists of an air compression unit, an air storage and treatment unit, an oxygen–nitrogen separation unit, an oxygen filtration, sterilization and storage unit, monitoring equipment, a filling standby unit, an electrical control unit, and so on.
The specifications and models of the oxygen generator equipment should be selected according to the scale of the hospital and the total consumption at all locations, with a certain margin reserved for future development. Two sets of oxygen generation equipment should be installed (one in use and one on standby), and the equipment should operate mainly in automatic mode with manual operation as backup. The machine room must be kept clean and provide space for maintenance; the environment at the compressor air intake must be free of pollution sources and must be kept clean.
(3) Location and requirements of the central oxygen source supply station.
1) The central oxygen source supply station must not be built in a basement; it should be located in a place with good ventilation, explosion-proof measures, and convenient transportation. There must be no boiler rooms, open flames, or power distribution rooms around the oxygen station; the surfaces of trenches within 5m must be sealed, and no air passage between above and below is allowed.
2) The construction site of the central oxygen source supply station must comply with relevant national laws and regulations to ensure the necessary safety distances. Figure 8.3 Example of the Plan Layout of a Liquid Oxygen Station (with a Gaseous Oxygen Station as Backup)
- The central oxygen supply station has a fire hazard classification of Class B and should be a building with a fire resistance rating of not lower than Grade II 2. The liquid oxygen tank and the gas cylinder loading/unloading platform should be provided with sunshades and rain canopies, and the canopy and its supports should be of non-combustible construction 3. The oxygen pipelines should be provided with grounding devices for eliminating static electricity, and the grounding resistance should not be greater than 10Ω; the liquid oxygen tank should be provided with lightning protection grounding devices 4. The room temperature of the duty room with a control console should be greater than 10℃ 5. The gas cylinder bank room should be equipped with explosion-proof exhaust fans, and the liquid oxygen station site should have explosion-proof lighting 6. The duty room should be provided with one duty telephone 7. Within the fire safety separation distance, there must be no basements, underground pits, underground wells, or underground trenches; combustibles and flammables are not allowed, and offices, wards, public places, and busy roads should be located at least 7m away
3）The central oxygen source supply station must have a properly completed grounding grid and lightning protection measures.
4）The gas supplied to the surgical department should be an independent system; the oxygen transmission pipelines can generally be made of stainless steel or copper, and the terminals should use self-sealing quick-connect sockets.
The schematic diagram of the medical central oxygen generation system is shown in Figure 8.4.
8.1.3 Negative Pressure Suction
（1）Applications of negative pressure suction in the clean surgical department.
1）Removing blood around the incision and other excess matter during surgery to ensure the smooth progress of the operation.
2）Removing contaminants from the patient's body and other life-threatening excess matter.
（2）Composition of the negative pressure suction station.
The equipment of the negative pressure suction station mainly includes vacuum pumps, buffer tanks, distributors, gas-liquid separators (cooling equipment), drainage devices, filtration and sterilization devices, silencing facilities, and electrical control equipment.
[Reference] The process layout diagram of the negative pressure suction station (taking a water-ring vacuum pump as an example) is shown in Figure 8.5.
（3）The vacuum pump is the core equipment of the negative pressure suction system; high-quality products featuring high performance, low power consumption, pollution-free operation, and long service life must be selected. Generally, water-ring vacuum pumps are chosen, and their pumping speed is mainly determined based on the total number of operating rooms, hospital ward beds, and ICU beds. (a) System
(b) Process
Figure 8.4 Schematic diagram of the medical central oxygen generation system
- High-efficiency screw air compressors No. 1 and No. 2
- Primary precision filter
- Two-stage refrigerated dryers No. 1 and No. 2
- Intermediate precision filter
- Final precision filter
- Air storage tank
- Oxygen generators No. 1 and No. 2
- Oxygen storage tank
- Sterilizing filters No. 1 and No. 2
- Manifold
- Oxygen purity monitor
- Total flow meter
- Power distribution box and piping system
- Oxygen compressor
- Cylinder filling device
[Reference] For the main units of the negative pressure equipment, see Figure 8.6.
(4) The vacuum pumps of the negative pressure suction station should be provided with standby units, generally consisting of 2–3 pumps. Each vacuum pump operates according to a preset program, and all pumps should participate in automatic cyclic operation; for example, if a negative pressure station is designed with two vacuum pumps A and B, pump A operates while pump B is on standby; if during peak gas consumption pump A's pumping speed cannot meet the demand or it fails, pump B will also be automatically put into operation to ensure normal gas supply. The next time, pump B operates while pump A is on standby, and pump A can also be put into operation when necessary. In this way, the pumps continuously cycle, protecting and substituting for each other, ensuring the normal operation of the negative pressure vacuum system. It is abnormal for a 'standby pump' to remain in the standby position all year round. When a negative pressure system is relatively large and the number of operating rooms is considerable, the pumping speed of the vacuum pumps must be high; otherwise, normal use during surgery cannot be guaranteed; if two pumps are selected (one in use and one on standby), the pump power will be relatively large, and the inrush current at the instant of pump startup is very high. Since vacuum pumps start and stop very frequently during peak surgical periods, the interval from shutdown to the next startup is too short for high-power motors, which, on the one hand, wastes energy and, on the other hand, easily causes burnout. For this reason, it is more economical to design the vacuum pump station with three pumps—one in use, one assisting, and one on standby—using three smaller vacuum pumps A, B, and C. During holidays, afternoons, and nights when surgery is not at its peak, one pump operating is sufficient to maintain general negative pressure suction gas usage. When surgery reaches its peak, the suction gas consumption increases accordingly, and another pump is automatically put into operation to assist Figure 8.5 Process layout diagram of the negative pressure suction station
operation, so as to ensure normal use during surgery. The working procedures of the three pumps $A$, $B$, and $C$ are as follows: first, when the main working pump $A$ cannot maintain normal use, pump $B$ is automatically brought in to assist, and pump $C$ serves as backup. If, during the operation of pumps $A$ and $B$, one of them fails, the faulty pump automatically drops out and pump $C$ is switched in to assist in the work; at the next start-up, pump $B$ becomes the main working pump, pump $C$ assists, and pump $A$ becomes the backup pump; at the third start-up, pump $C$ is the main working pump, pump $A$ is the assist pump, and pump $B$ is the backup pump. In this way the cycle continues endlessly: every pump is a main working pump, and at the same time an assist pump and a backup pump. This design method is both safe and reliable and saves energy and operating costs; therefore, the circuit design of the electrical control cabinet is required to adopt computer program design, so as to ensure that the working procedures of each pump operate normally.
(5) The vacuum pumps of the negative pressure suction station operate mainly by automatic start/stop, supplemented by manual start/stop, and are provided with fault and insufficient-pressure alarm devices.
(6) The negative pressure suction station should be built at a location away from the building of the clean operating department, so as to prevent vibration and noise during equipment operation from being transmitted to the operating rooms; the suction station may be located in the basement or set up independently. Waste matter and harmful excess substances are removed thoroughly by means of the negative pressure system, and this removal process is the suction process. The material drawn in contains large quantities of bacteria, and discharging it along the system piping into the outside atmosphere is not allowed, because these substances contain large numbers of pathogenic bacteria; otherwise the bacterial concentration in the atmosphere would far exceed the hygienic standards, seriously polluting the surrounding environment, which would violate the national environmental protection law. In order for the air at the discharge outlet of the suction system to meet safety requirements (i.e., less than 500cfu/m³ in the air), disinfection and sterilization filtration devices must be added to the suction system to block the large quantities of bacteria within the system and subject them to elimination treatment, so that the air meets the requirements of environmental hygiene standards before being discharged into the outdoor atmosphere.
(7) The terminal connector should be of the self-sealing quick-connect type, and a filter screen of about 10 mesh should be provided on the connector to prevent fibers in the operating room from entering the system with the airflow and clogging the piping. The connector is connected to the suction point via a flexible hose and should be equipped with a pressure regulating device, so that adjustment can be made according to the requirements of suction at different body sites and for patients of different ages, preventing excessive negative pressure from damaging the soft tissues inside the body and causing medical accidents; at the same time, a buffer liquid storage bottle should be inserted in between, and when the liquid is full the air source should be cut off automatically to prevent waste from flowing into the system piping and causing blockage. The buffer bottles generally consist of two 5-liter bottles used in series: the first one fills up while the second serves as backup; at the same time, the second bottle also serves to separate gas from liquid, so that the waste is less likely to be drawn into the piping. The impeller—the only rotating part of the 2BV—forms a liquid ring of the working fluid inside the elliptical pump casing through rotation. The working fluid simultaneously serves three functions: sealing medium, compression medium, and cooling medium, with no wear and no lubrication required.
During the discharge phase, the liquid ring gradually approaches the hub, discharging the pumped medium axially from the discharge port.
Because the impeller is eccentric relative to the rotating liquid ring, the liquid moves back and forth in the spaces between the blades, just like a piston moving in a cylinder, producing suction and compression of the pumped medium.
During the suction phase, the liquid ring gradually moves away from the hub, drawing the pumped medium axially in through the suction port.
Figure 8.6 Reference for Selection of Negative-Pressure Suction Pump
8.1.4 Compressed Air
(1) Compressed air is drawn in from the atmosphere and compressed; therefore, like the atmosphere, it is a mixture of various gases, while air itself is a colorless, odorless, non-flammable gas. Within the surgical department, compressed air mainly serves as the power source for equipment such as pneumatic drills and pneumatic saws; it is also used as the power source for ventilators, as well as mixed with other gases for therapeutic purposes, so it must be oil-free, clean, and odor-free.
(2) The compressed air supply is provided centrally by the central station. The central station should be located away from the surgical department to prevent vibration and noise from equipment operation from disturbing the surgical department. (3) The compressed air station consists of the following main equipment: oil-free air compressors, dryers, filters, disinfection and sterilization devices, gas storage tanks, electrical control units, cooling equipment, etc. The compressor is the core equipment of this system and must be highly efficient, low in power consumption, and highly reliable; the selection of the compressor's specifications and models should be determined according to the number of operating rooms and the total amount of other necessary gas consumption.
[Reference] See Figure 8.7 for the schematic process flow diagram of the compressed air station.
Figure 8.7 Schematic diagram of the process flow of the compressed gas station
T. Thermometer P. Pressure gauge R. Safety valve
(4) The compressed air source station must be equipped with two sets of equipment, one in use and one on standby. The air compressor is the core equipment of this system and must have a simple, reliable, and durable structural configuration, a low failure rate, low power consumption, and high efficiency; the start-stop procedure of the air compressor must be primarily automatic start-stop, with manual start-stop as a supplement.
(5) The environment of the air compressor station must be clean and kept away from the morgue, garbage stations and garbage transport routes, incinerators, boiler rooms, and other such places; the quality of the collected air must be good, and when necessary, an air filter should be added at the air compressor inlet, or a suction pipe should be added to draw high-quality air from overhead.
(6) The compressed air delivery conduits should be made of copper pipes or stainless steel pipes. The terminals should use self-sealing quick-connect couplings.
[Reference] For the process plan layout of the compressed air station, see Figure 8.8.
8.1.5 Nitrous Oxide
(1) Laughing gas (N$_2$O) is a colorless, odorless, non-irritating gas; it is also an inert, non-toxic gas that is easily and safely soluble in water and alcohol, with a slightly aromatic odor; it only releases free oxygen at high temperatures and thus supports combustion. Its physical properties are as follows:
- Molecular weight: 44.027
- Specific gravity: 1.53
- Critical temperature: 26.5℃
- Critical pressure: 7.41MPa
(2) Laughing gas, also known as dinitrogen monoxide, is mainly used for surgical anesthesia. During surgery, if the patient needs to be anesthetized, the anesthesia machine can be used to adjust the ratio of oxygen and laughing gas, i.e., (N$_2$O)+(O$_2$), according to the specific required amount, and supply it to the patient for inhalation into the body, through the Figure 8.8 Example of process plan layout of a compressed air station
...is delivered throughout the body through the action of the internal functional organs and anesthetizes the nerves, so as to achieve the purpose of painlessness. Since nitrous oxide is delivered to the human body together with oxygen, there is sufficient oxygen to maintain the oxygen level required within the patient's body; when the patient's operation is finished, the nitrous oxide is discontinued and oxygen inhalation is given, using oxygen to displace the nitrous oxide remaining in the body, so the patient will regain consciousness very quickly, with no residue left in the body and no after-effects, which is conducive to postoperative recovery. Therefore, the use of nitrous oxide for surgical anesthesia is very safe, and it is also a very ideal anesthetic.
(3) Under a certain pressure, nitrous oxide becomes a liquid and is filled into steel cylinders. The volume of the cylinders is generally 40L. When in use, it becomes gaseous after pressure reduction and is delivered through transmission piping to the corresponding operating rooms of the clean operating department.
(4) The nitrous oxide station is located in a non-clean area adjacent to the operating department, at a position with good ventilation and convenient cylinder transportation. The gas source of the nitrous oxide station is generally arranged in two groups, with two cylinders per group being sufficient. Since the amount of nitrous oxide used in the operating rooms is extremely small, the number of cylinders should not be too large; otherwise, waste will result. The two groups of nitrous oxide cylinders should be designed with an automatic switchover function, and should also have a manual operation function as a backup; the nitrous oxide central supply station must be equipped with an alarm device. The alarm functions cover: insufficient cylinder pressure, and overpressure and/or underpressure of the output pressure after pressure reduction. The alarm must be connected to the duty room or the nurses' station, using audible and visual signals simultaneously.
The output pressure of the nitrous oxide station is 0.4MPa.
The nitrous oxide transmission piping is generally stainless steel or copper tubing, which runs directly into the operating room and terminates at self-sealing quick-connect sockets; the connection to the anesthesia machine is made through an external plug-in device and a transmission hose. The layout of the nitrous oxide room is shown in Figure 8.9.
8.1.6 Nitrogen Gas System
(1) The physical properties of nitrogen are as follows:
- Molecular weight: 28.016
- Specific gravity: 0.967 3 (relative to air at 10°C, 760 mmHg)
- Density: 1.2505 g/L
- Critical density: 0.311 g/L
- Critical pressure: 3.46 MPa
Figure 8.9 Schematic diagram of equipment layout in a separate nitrous oxide room
(2) In the operating room, nitrogen is mainly used as a power source for high-speed pneumatic drills, pneumatic saws, and the like; since nitrogen is a safe gas that poses no life-threatening danger and introduces no cross-infection factors, medical staff often use nitrogen as the power source when cutting bone and making openings in Class I deep, highly difficult surgeries, which is relatively safe; however, when large amounts of nitrogen are used in a sealed operating room and disperse into the space, they will reduce the oxygen content of the space and pose a hazard to the personnel in the room; therefore, attention should be paid to this issue in the design of clean operating rooms.
(3) The nitrogen station should be located in the non-clean area adjacent to the surgery department; the nitrogen station should be well ventilated, and cylinder transportation should be convenient.
(4) The gas source of the nitrogen station consists of cylinder banks of 40 L, 15 MPa cylinders; each bank is generally equipped with 2–4 cylinders, and the two banks switch with each other—one bank in operation, one on standby. When the working bank is nearly depleted, an alarm signal is issued to notify the relevant staff to replace the cylinders; when the pressure drops to 0.8–1 MPa, the system automatically switches to the other bank to continue working, at which point the cylinders of the previously used bank can be replaced and kept on standby, waiting to be switched over for use again next time. This continuous cycle constitutes a continuous gas supply process. If the automatic switching fails, it can be accomplished by manual operation. The gas reserve should be no less than three days' consumption. Copper pipes or stainless steel pipes are generally selected for the nitrogen delivery conduits. The nitrogen in the cylinders is delivered to the operating room through the switching device and then, after pressure reduction, to the operating room.
(5) The terminal pressure for nitrogen use is 0.8–0.9 MPa; the terminal connector adopts a self-sealing quick-connect coupler; in use, the external adapter mates with the quick connector and is connected to the pneumatic equipment through an external hose.
8.1.7 Mixed Gas
(1) Mixed gases are mainly several special-function gases that cannot be used on their own; they must be mixed with oxygen before they can be inhaled into the human body to realize their functions.
(2) Mixed gases come in the following forms. 1) Carbon dioxide gas is mixed with oxygen to form carbonic acid gas, in the proportion of $5%mathrm{CO_2}+95%mathrm{O_2}$. The carbon dioxide is used to constrict blood vessels and reduce bubbles inside the vessels, so that blood flows smoothly through the vessels.
2) Helium is mixed with oxygen at a ratio of 80% helium + 20% oxygen; helium is a very light gas, and the resulting mixture weighs only 1/3 of the weight of the same volume of air. The density of helium is 1/7 that of oxygen, so the helium mixture diffuses easily; based on these properties, it can be used for conditions such as asthma and airway stenosis, where it has a soothing effect.
The anesthesia machine has a gas escape valve, generally installed on the inspiratory side of the anesthesia machine; it is normally kept closed and is only opened temporarily when needed.
8.1.8 Waste Gas Recovery and Discharge Device
(1) The waste gas recovery and discharge device system is a very important facility in a clean operating room; it is a system that helps ensure good air quality inside the operating room. Without the treatment process of this device, the residual gas exhaled by patients, which contains the anesthetic gas mixture, would harm the medical staff.
(2) The waste gas recovery and discharge device is located inside the operating room; an interface is provided on the gas pendant or standby terminal, and the outlet of the waste gas discharge pipe is docked with its quick connector to complete the waste gas discharge process.
(3) The pressure of the waste gas recovery and discharge power source is generally controlled within a vacuum range of 5–10 mmHg. If the negative pressure is too high, the anesthetic gas mixture coming out of the anesthesia machine cannot enter the patient's mouth and is discharged directly outdoors through the discharge duct, and it may also pose a danger to the patient; if the negative pressure is too low, the power is insufficient and the waste gas exhaled by the patient cannot be discharged. Therefore, the waste gas discharge equipment is required to be equipped with pressure regulation and display devices to meet the respiratory needs of different patients. There are generally two types of waste gas discharge power sources. One is to use a low-pressure, high-flow negative pressure pump as the power source; generally, one system serving 3–5 operating rooms is appropriate. However, the negative pressure pump method is not very stable, because it is affected by changes in the number of operating rooms in use and by fluctuations during the start-up and shutdown of the pump. The other is the jet method, which uses the jet principle to generate negative pressure power to discharge the waste gas; once it is adjusted according to the patient's condition, it will not change, because each operating room is independent and is not affected by other operating rooms or the system. This is a relatively stable, relatively ideal, and relatively convenient waste gas recovery and discharge method.
(4) The pipes of the waste gas recovery and discharge system are generally galvanized steel pipes or plastic or nylon pipes. The diameter of the conveying pipes should be as large as possible, because the waste gas is exhaled from the patient's mouth, the pressure is extremely limited, and the negative pressure inside the guide pipe is also limited. In order to ensure that the exhaled gas is discharged smoothly, it is beneficial to make the diameter of the discharge guide pipe as large as possible.
8.1.9 Argon Gas System
(1) In the operating room, argon is mainly used for the protection of the argon knife, reducing the amount of bleeding during surgery.
(2) The argon gas source is supplied in cylinders. The steel cylinder can be moved into the operating room and connected directly to the anesthesia machine through a pressure reducing device, or a supply station can be set up with pipeline delivery, where the gas is connected to the anesthesia machine through the quick plug-in couplings on the gas pendant or wall-type terminal via a connecting device. With the centralized supply method, the operating room is relatively clean and also relatively safe, ensuring that the operation proceeds normally. The conveying pipes are generally deoxidized copper pipes or stainless steel pipes.