Anesthesia machine and scavenging (Proceedings)

Understanding the operation and maintenance of the anesthesia deliver system (ADS) is essential for operator and patient safety. One should spend time understanding the below information regarding compressed gas & ADS safety as well as the function and its various breathing systems.

Compressed Gases

There are two types of gases you may see used in veterinary anesthesia. Oxygen (green or white colored tank) and Nitrous Oxide (blue tank). The diameter index and safety system is a coding system used with compressed gas cylinders. The cylinders are attached to the machine by a length of high-pressure hose connected to a diameter index safety system (DISS) fitting on the machine. An 'E' cylinder has a similar safety system call the pin index safety system (PISS). E cylinders are held in a yoke. These safety connections prevent a person from accidentally connecting the wrong gas to the regulator or hose. For example, attaching a nitrous oxide (N2O) cylinder where an oxygen cylinder needs to go.

Compressed Gas Storage & Handling

• Never leave a cylinder sitting upright. Store E cylinders in a rack and secure H cylinders to a wall

• Separate full from empty cylinders. Always keep compressed gases away from heat and electrical wiring.

• It is ideal to have a minimum of 20 feet distance between cylinders and combustibles. Open cylinder valves slowly and completely.

Cylinder Capacity

Cylinder types

Oxygen (O2)

Room air contains 21% oxygen. Patients under general anesthesia will have much higher inspired oxygen. Most veterinary anesthesia machines will deliver oxygen only at 100%. This will enter the breathing system and be delivered to the patient with or without inhalation anesthetic gases. An oxygen concentration of 21% is adequate for an awake patient. Anesthetized patients encounter respiratory depression from anesthesia; therefore without supplemental oxygen hypoxemia is real possibility.

Nitrous Oxide (N2O)

This is a gas that may be used in addition to oxygen. It's MAC in small animals range from 180- 200%. Using N2O as a second gas in veterinary patients will provide additional analgesia as well as less cardiovascular compromise. It's storage and delivery is similar to oxygen. N2O exists as a liquid of room temperature. The N2O pressure gauge reads only pressure of gas within the cylinder. As gas leaves the tank, more liquid evaporates and enters gas state. The result is that the cylinder pressure gauge will not change until all of the liquid has evaporated. You know when to change the tank because a full E cylinder will weigh about 8kg but 6kg when empty. Once liquid nitrous oxide is exhausted, the gauge will begin to change; leaving you with about 5-10 minutes remaining.

Flowmeter

The flowmeter allows the anesthetist to set the gas flow rate(s) that are delivered to patient. It contains a ball or bobbin that rises within a glass tube to a height proportional to the flow of gas through tube. The gas flow rate is read at widest diameter of ball or bobbin. When using the N2O, the N2O:O2 ratio should never exceed 2:1 as the patient may develop hypoxemia. Avoid excessive torque when closing the flowmeter knob. This may break or strip the pin valve. The gas passing through the flowmeter is reduced from 50 psi (pounds per square inch) to 15psi.

Oxygen flush valve (quick flush)

The oxygen quick flush bypasses the vaporizer and delivers O2 directly to the patient's breathing system (PBS). It will deliver between 35-75L/min. and is used to fill the PBS with 100% pure oxygen. This oxygen flush valve can be used to dilute anesthetic in the PBS or rapidly fill the reservoir bag. DO NOTUSE THE QUICK FLUSH VALVE ON A PATIENT ATTACHED TO A NONREBREATHING SYSTEM, because pressure is pushed directly into the patient instead of the patient hoses.

Precision and Non-Precision Vaporizers

Precision Vaporizer- Out of Circuit (VOC)

These are precision vaporizers that are located outside of the PBS. (PBS includes unidirectional valves, hoses, CO2absorber, pop-off valve, and reservoir bag.) Most precision vaporizers have temperature compensating bypass mechanisms; therefore temperature does not affect the concentration delivered. The vaporizer converts volatile anesthetic liquid to anesthetic vapor and adds it to the carrier gas (O2 and N2O). The carrier gas passes through the vaporizer and a portion of vaporized anesthetic is picked up. The vaporizer is labeled, and color-coded a particular anesthetic. If a vaporizer is filled with the wrong anesthetic, drain and flush with O2 at 1L/min until dry (minimum of 8 hrs).

Non-Precision Vaporizer- In Circuit (VIC)

This vaporizer is a glass jar containing a wick. The wick absorbs anesthetic liquid, allowing a greater surface area for vaporization. The concentration of anesthetic delivered is unknown (unless measured with gas measurement tools). The VIC can vary the output concentration by opening or closing the control valve.

All Vaporizers

DO NOT TIP OVER OR SHAKE VAPORIZERS. If liquid anesthetic enters bypass channels of the vaporizer, a potentially lethal dose of anesthetic may be delivered to the patient. If this occurs, run O2 at 1L/ min through the machine with vaporizer off for 15min prior to attaching to a patient. When transporting vaporizers make sure they are empty and dry.

Adjustable Pressure Limiting (APL) Valve or Pop-Off Valve

The APL valve limits the amount of pressure buildup that can occur during manual or spontaneous ventilation. When the user adjusts the APL valve to trap more gas inside the PBS, a spring inside the APL valve is compressed according to how much the user turns the APL valve. Closing the valve is usually required in order to administer intermittent positive pressure ventilation (IPPV) to your patient. Try to keep this valve open at all times unless delivering manual, controlled, or assisted (IPPV). If left closed, pressure can build up in the PBS and result in cardiopulmonary injury. Always check pressure of breathing circuit. Make sure the PBS pressure gauge stays at zero centimeters water pressure unless positive pressure is being delivered.

Carbon Dioxide Absorber

Common types of CO2 absorbents are Soda Lime and Bara Lime. The active ingredients react with exhaled CO2 and water. This is an effective method for removing carbon dioxide from a rebreathing circle system. Handle absorber with care or use gloves. Do not inhale dust of granules. Breathing protection may be necessary when pouring new granules into the canister. Monitor time used as these granules will eventually cease to absorb CO2. Some granules turn purple when chemical reaction is taking place, others do not. When replacing the absorber, do not tightly pack granules. At least 1/2 inch of air space should be left between the granules and the top of the canister to allow unimpeded airflow through the canister. Gently shake canister during and after filling to prevent channels from forming within the granules. Also check regularly for worn gaskets and improper seals, which could cause leaks in your PBS.

Patient Breathing Systems (PBS)

The function of any PBS is to deliver oxygen with anesthetic gases, and eliminate carbon dioxide.

Circle Rebreathing System

In the circle system, gases flow in a circular pattern through an inspiratory and expiratory (one-way) direction. Carbon dioxide (CO2)rebreathing is prevented through soda lyme absorption. Other exhaled gases are allowed to be rebreathed by the patient. There are more components and more airway resistance compared to that of a non-rebreathing system. The equipment is relatively expensive.

Non-rebreathing System

With a non-rebreathing system, there is no device for absorbing CO2. Fresh gas MUST wash out CO2, thus the reason for a high flow rate. There is less resistance and very few components keeping this system more economical. There is more gas waste and atmosphere pollution due to high flow rates.

Diagram

Fresh Gas Flow (FGF)

Rebreathing system: Non-rebreathing breathing system:

Semi-closed flow 20-50 mL/kg/min Open flow 200-300 mL/kg/min

Low flow 10-20 mL/kg/min

Pollution & Waste Scavenging

In the past, anesthetic gases were discharged into room air. In recent times, questions have been raised as to possible hazards from trace amounts of anesthetic gases. There is research implicating that anesthetic vapors may induce spontaneous abortion in humans. March 1977, the National Institute for Occupational Safety and Health (NIOSH) published "Criteria for a Recommended Standard...Occupational Exposure to Waste Anesthetic Gases and Vapors" (NIOSH Publication No. 77-140).This document contained a recommendation that occupational exposure to halogenated anesthetic agents, such as halothane, and Isoflurane be kept below 2 parts per million (ppm). The solution to this is to SCAVENGE EVERYTHING! Adhere to a no leak technique. (<300ml). Use cuffed endotracheal tubes, tight fittings, low oxygen flows, good ventilation and routine leak tests.

Scavenging Methods

Two common methods by which to scavenge are passive and active. There are three passive methods. One being the floor drop. This should only be used when outdoors with adequate ventilation. Charcoal absorption is another passive method. Be sure to monitor the weight of the charcoal canister as they will exhaust. Try to keep your fresh gas flow on the flowmeter below two liters per minute as you may get pollution coming from the bottom of the canister with higher flow rates. A simple hose out the window or outer wall is a third example of passive scavenging. For active scavenging, you need a central vacuum and an interface on or near your anesthesia machine for proper scavenging. Don't forget to scavenge gas sampling monitors, capnographs, induction chambers, recovery rooms and to maintain proper ventilation when filling vaporizers. Try to fill vaporizers at the end of the day or when very little personnel are in the anesthesia environment. Use detection badges twice a year to monitor anesthetic gas exposure. Keep gas exposure below 2 ppm.

References

Davey A, Diba A: Ward's Anaesthetic Equipment 5th ed. UK, Elselvier Saunders, 2005

Dorsch J, Dorsch S: Understanding Anesthesia Equipment 4th ed. Baltimore, Williams & Wilkins, 1999

Greene, Stephen A: Veterinary Anesthesia and Pain Management Secrets, Philadelphia, Hanley & Belfus, Inc., 2002

Tranquilli W, Thurmon J, Grimm K: Lumb, Jones', Veterinary Anesthesia and Analgesia 4th ed. Iowa, Blackwell Publish, 2007