Overview of anesthetic machines and circuits (Proceedings)

Many types of anesthetic machines are now available for veterinary use.  Retired machines from human hospitals are also commonly used by veterinarians.  Regardless of the type selected, one must remember that there is no single circuit that is most appropriate for the variety of patient types and sizes seen by veterinarians.  There are four basic functions of anesthetic breathing circuits:

• Delivery of oxygen to the patient (most veterinary machines utilize 100% oxygen)

• Delivery of the anesthetic gas

• Assistance with ventilation

• Removal of exhaled carbon dioxide from the patient

Rebreathing of carbon dioxide can be prevented in different ways, depending on the type of anesthetic circuit selected.

• Incorporation of one-way valves in the breathing circuit (circle systems)

• Use of a chemical absorber to remove carbon dioxide

• Dilution of the gas in the circuit with a high flow of fresh oxygen

The purpose of today's talk is to focus on some common issues and questions about anesthetic machines and circuits.

Gas source

Anesthesia machines are traditionally separated into two systems: the high pressure (gas source) system and the low pressure system when the gas reaches the flow meter.  For economical reasons, most gases are stored in a compressed state.  A full “E” tank of oxygen, which is the size of tank most commonly fitted for use directly on the anesthesia machine, is pressured to 2200 psi and contains about 700 L of oxygen.  This is too much pressure for the flow meter to handle, so a regulator reduces the pressure to a level that the flowmeter can handle, usually about 50-60 psi.  How do you know how much oxygen is left in the tank?  A tank that is half full will register a pressure of 1100 psi and contain about 350 liters.  At one liter per minute of oxygen flow, this tank would last for 350 minutes.  Always remember that dropping or knocking pressurized gas cylinders can damage them, with the potential result of serious personnel or structural damage.  All medical gas cylinders and equipment utilize an indexing system that prevents a mix-up between gases.

Flow meters

Flow meters are delicate instruments, calibrated for one gas only.  In other words, oxygen flow meters are only used for oxygen, medical air flow meters only for air, etc.  Care should be taken not to overtighten the knob when turning off the flow meter.

Oxygen flush valve

The oxygen flush valve is a safety feature of an anesthetic machine, which allows oxygen to bypass flowmeters and vaporizers and enter the breathing circuit at high flow and pressure (40-70 L/min).  This valve should be used when you want the patient to WAKE UP, not to fill the rebreathing bag to a more comfortable level of gas at the start of the anesthesia process.  It should never be used when a non-rebreathing circuit is connected to a patient.

Pop-off valve

Pop-off valves are really a high pressure relief valve, and as such are a safety feature of an anesthetic machine.  They function to protect the patient from high airway pressures.  The valve can be left open if the patient is breathing spontaneously---if the rebreathing bag was squeezed, then all of the gas would exit via the scavenging system.  The valve can be partially closed or adjusted to release gas from the circuit to the scavenging system at varying peak circuit pressures.  Thus, if you plan to assist ventilation, the valve needs to be partially closed when the rebreathing bag is squeezed to allow positive pressure to be generated in the circuit.  If the valve is fully closed, the patient could be exposed to very high airway pressure, as none of the gas would exit the scavenging system.

Types of breathing circuits and classification

• Rebreathing circuit (circle)

• Closed system

• Semi-closed system

• Nonrebreathing circuits (semi-open)

• Chamber and mask induction (open)

Attributes of circle systems (rebreathing circuits)

• Economy: less oxygen and anesthetic used

• Less loss of body heat and moisture

• Do not know exactly how much anesthetic is in the system without a gas analyzer.  Concentration will depend on oxygen flow rate and time constants of system

• Dependent on proper functioning of one-way valves

• Utilize a chemical absorbent to remove carbon dioxide

Operation of circle systems

• Closed circuit: oxygen is supplied just sufficient enough to meet patient's metabolic oxygen needs and no more (4-7 ml/kg/min)

• Semi-closed circuit: higher oxygen flow rates are used-these rates are somewhat arbitrary and can range from 10-40 ml/kg/min and higher.  At some point, very high flow rates will convert a circle system into a nonrebreathing circuit.  The advantage of operating the circle in a semi-closed fashion is that is allows the operator to make changes in anesthetic concentration within the circle more quickly.  It is less economical to operate however, and more anesthetic will be released to the scavenging system.

• Chemical absorbent (sodalime or baralyme) will become exhausted with use.  There is a color change when the chemical reaction is occurring (white to purple or blue).  The heat produced by the chemical reaction can be felt on the canister while the absorbent is functioning.  Fresh sodalime is friable: Spent granules are hard and not crushable.  The color indicator will revert to white when not in use, so white granules do not necessarily mean the absorbent is fresh.

Attributes of non-rebreathing circuits

• Requires high fresh gas flow rates to remove CO2 (150-500 ml/kg/min) depending on the type of circuit used

• Less work of breathing for the patient

• Recommended for small patients

• Can change the level of anesthetic in the circuit very quickly

• Not economical to use due to the high fresh gas flow rate

• More likely to produce patient body heat loss

• Several types available

How do you calculate cost of operating the circuit?

“MAC” is the minimal alveolar concentration of an anesthetic that produces immobility in 50% of subjects exposed to a supramaximal noxious stimulus.  It is the way we compare the potency of the inhaled anesthetics or the amount of the agent required to produce general anesthesia.  Most surgeries require about 1.5 MAC levels in order to adequately anesthetize the patient.  The MAC level of halothane in dogs is 0.87%, isoflurane is 1.3%, and sevoflurane is 2.3%.  Thus, it requires more sevoflurane to maintain a general anesthesia than halothane or isoflurane and should be taken into account when doing a cost analysis.

Isoflurane costs about $0.10 /ml and sevoflurane is roughly $0.80/ml.  A liquid ml yields 227 mls of halothane vapor, 195 mls of isoflurane vapor and 183 mls of sevoflurane vapor at room temperature.  Depending on the oxygen flow rate and the vaporizer setting, you can calculate the cost of the inhalant for a procedure.  For example:  one hour of anesthesia for a dog under isoflurane at 2 l/min oxygen flow and 2 % vaporizer setting will cost…

• 2 % isoflurane/100 x 2  liters per minute = 0.04 liters per minute

• 0.04 liters per minute x 60 min= 2.4 liters per hour or 2400 ml of isoflurane used/hour.

• 2400 ml divided by 195 ml vapor/ml liquid = 12.3 ml of liquid isoflurane used

• 12.3 ml of liquid x $0.10 /ml = $1.23 per hour.

• If you decrease the oxygen flow rate, you will decrease the amount of agent that is used during the procedure.  In general, 30 ml of oxygen flow/kg of body weight/min is recommended for most circle systems.

If we use sevoflurane as an example…one hour of anesthesia for a dog under sevoflurane at 2 l/min oxygen flow and 2.75 % vaporizer setting will cost…

• 2.75% sevoflurane/100 x 2 liters per minute = 0.055 liters per minute

• 0.055 liters per minute x 60 min=3.3 liters per hour or 3300 ml of sevoflurane used /hour

• 3300 ml divided by 183 ml vapor/ml liquid = 18 ml of liquid agent used

• 18 ml x $0.80 = $14.40 per hour.

The use of premeds and analgesics will reduce the amount of sevoflurane and isoflurane that is necessary to maintain a patient.