ANESTHETIC MACHINES

The primary function of any anesthetic machine is to deliver a precise amount of oxygen and volatile anesthetic under controlled conditions to patients undergoing general anesthesia.

ANESTHETIC MACHINE Oxygen is be delivered at a controlled rate A liquid anesthetic is converted to a gas, then mixed with oxygen, and delivered to the patient 3) Gases exhaled from the patient are disposed of OR re-circulated (after removing the CO2) REMEMBER: You can also use an anesthesia machine to deliver oxygen to a patient in distress. DON’T turn the vaporizer on! You do not want to anesthetize these patients!

COMPONENTS OF THE ANESTHESIA MACHINE Can be broken down into 4 parts: Compressed gas supply Gas tanks, their pressure gauges, and pressure reducing valves (may not be visible) Anesthetic machine Flowmeter, vaporizer Breathing circuit Unidirectional valves, hoses, reservoir bag, pop-off valve, CO2 canister, O2 flush, pressure manometer, negative pressure relief valve Scavenging System Disposes of excess and waste anesthetic gases

PART 1

Following the flow of oxygen GAS CYLINDER Carrier gas Gas that is compressed in a metal cylinder and held under pressure Available in various sizes “E” holds 660 L of oxygen and is attached to the anesthesia machine “H” cylinders hold 6900 L of oxygen and stand separate from the machine Tanks are delivered and picked up by the oxygen supply company

Gas Cylinder Tanks are color coded for safety and recognition Oxygen tanks: GREEN (U.S.), white (Canada) Nitrous oxide tanks: blue (U.S.) Carbon dioxide tanks: gray (U.S.) Oxygen cannot flow out of the tank unless the outlet valve has been turned to the left. (Righty tighty, lefty loosey) Small cylinders attach to the anesthetic machine via a YOKE. Large cylinders are attached via hoses or pipes. Gas lines may also be used to bring in the gas from another room.

GAS CYLINDERS In addition to color-coded tanks, the yokes are gas-specific. A nitrous oxide tank cannot be attached to an oxygen yoke. The pin indexing system that is used to attach it is also specific to its specific gas. A full tank of oxygen has a pressure of 1900-2200 psi. The volume in liters can be determined by multiplying the psi by 0.3 for an E tank and 1.7 for an H tank.   0.3 x psi on gauge   L/min to be delivered This formula will allow you to know how long your tank will last

TANK PRESSURE GAUGE Display of the pressure of oxygen in the tank. Reads zero when it is empty, when tank is turned off, and all gas has been removed from the machine Actual amount in tank is displayed when the tank is turned on Check this gauge BEFORE an anesthetic procedure (especially the lengthy ones).

TANK PRESSURE GAUGE Change when the pressure is no lower than 100 psi (some clinics will require tanks to be changed much earlier). Refill line is at 500 psi.

NITROUS OXIDE A full E cylinder contains 760 psi. Nitrous oxide is present in liquid and gas forms in the tank. When the tank is turned on, liquid evaporates into a gas as other gas leaves the tank. The pressure of the tank doesn’t change because of the constant replacement of the gas until all liquid has been volitalized. The gauge will not drop until almost empty. Anesthetist should change the tank as soon as 500 psi is reached.

PRESSURE REDUCING VALVE Regulates the pressure of the gas leaving the tank and going into the anesthesia machine Allows a constant flow of gas into the machine, despite pressure changes within the tank Reduces the pressure of oxygen that leaves the tank at 2200 psi to a safer 50 psi. In cases of small tanks and gas lines, the line pressure is preset at 50 psi. Only the tank pressure gauge is visible.

PART 2

Now that the gas is in the machine, it’s job is to mix with the anesthetic and be delivered to the patient FLOW METER Allows the flow rate of oxygen traveling through the machine to be adjusted by the anesthetist. Oxygen does not reach the patient unless this is turned on (neither does the anesthetic gas. Calculated in Liters per minute Separate flow meters for each gas These knobs can be distinguished from one another via labeling, touch, or color

FLOWMETER CONT’D The dial of the flowmeter is turned on and either a ball or rotor rises to the selected flow rate. Read at the center of the ball or top of the rotor Gas travels through the flowmeter

VAPORIZER Converts the liquid anesthetic agent (usually isofluorane or sevofluorane) into a vapor. Adds controlled amount of these vapors to the carrier gas Next stop as oxygen travels from the flowmeter The flowmeter must be on to deliver anesthesia to the patient as it needs to be mixed with a carrier gas.

VAPORIZER Similarly, the vaporizer must be on to deliver any anesthesia to the patient. Otherwise, only the carrier gas will be delivered The mixture of the anesthetic gas and carrier gas is known as FRESH GAS. Once mixed, fresh gas cannot return to the vaporizer. Use the correct anesthetic with the correct vaporizer! (Don’t put sevo in the iso machine)

VAPORIZER Amount of anesthetic liquid left in vaporizer is visible in the indicator window. Check BEFORE your surgery to see if it needs refilling! If for some reason the vaporizer is tipped over (usually the whole machine), turn the vaporizer off and run oxygen only through the machine for 15 minutes to flush it out.

PRECISION VS. NONPRECISION VAPORIZERS Precision vaporizers delivers a precise, controlled amount of anesthetic to the patient Expressed as a % which is chosen based on an anesthetic’s MAC and the patient’s requirements $ Commonly used anesthetics can reach concentrations as high as 30% + if they are not controlled

PRECISION VS. NONPRECISION VAPORIZERS Nonprecision vaporizers are simple, cheaper, and are typically used for anesthetics with low vapor pressure such as methoxyfluorane

PART 3

BREATHING CIRCUIT The system that brings the fresh gas from the vaporizer to the patient and takes the expired gases from the patient May contain unidirectional valves, reservoir bag, pop off valve, CO2 canister, O2 flush, negative pressure relief valve, pressure manometer.

UNIDIRECTIONAL VALVES One-way valves that allow the flow of fresh gas to enter the inhalation valve and exit the exhalation valve. Valve is either a rigid disk or a flap that flutters as gas flows past it Inhalation valve opens as patient inhales, anesthetic enters the hose, then the endotracheal tube and the patient CO2 and anesthetic gases are then exhaled, travel down the hose and through the unidirectional exhalation valve. This valve prevents the expired gases from traveling back to the patient before the CO2 is removed

RESERVOIR BAG Also called a rebreathing bag Fills as gases enter the circuit or patient exhales, deflates as patient inhales FUNCTIONS Stores gas Helps in determining correct endotracheal tube placement. Movement of bag with breaths = tube in trachea Allows assessment of respiratory rate and depth

4) Allows “bagging” of the patient - reverse atelectasis if present -removal of CO2 and anesthetic that builds up when respirations have decreased in volume -assist or control ventilation – esp if in respiratory arrest Volume of bag should be minimally 60mL/kg (round up) See pg 119 for reference chart Sizes range from ½ L to 30 L (1,2,3L commonly used on small animals)

SIGNS THAT YOUR RESERVOIR BAG IS NOT THE RIGHT SIZE: Overinflates rapidly if too small. If overinflated, animal will have problems exhaling Bags should consistently be ~ ¾ full Very little movement of bag with breaths if too big

POP-OFF VALVE AKA pressure relief valve Allows excess gas to leave the breathing circuit and be scavenged Prevents the build-up of excess gas or pressure within the circuit. If the pressure were allowed to build up (forgot to open pop off valve), the alveoli in the lungs could rupture KEEP the pop-off valve OPEN unless you are bagging the patient (or if you have a low-flow/closed system)

POP OFF VALVE In some instances, the degree that the pop off valve is opened changes with the flow rate and how full the reservoir bag is

HOW DO I KNOW WHEN THE GRANULES ARE EXHAUSTED? CO2 ABSORBER Gases that don’t exit the machine via the pop off valve go into the CO2 canister and are then returned to the patient CO2 canister usually contains soda lime that removes CO2 from other gases breathed out. “Exhaused” soda lime granules no longer absorb CO2. HOW DO I KNOW WHEN THE GRANULES ARE EXHAUSTED? Color change to violet, off-white or pink depending on the brand. Based on pH. CO2 saturated granules are hard and brittle, new ones can be chipped and crumbled Once color becomes abnormal, it is possible that it changes back to normal within hours

CO2 CANISTER WITH SODA LIME

O2 flush When activated, O2 bypasses the flowmeter and vaporizer and enters the circuit High flow rate (up to 75L/min!) Never use with a non-rebreathing system Can use to fill the bag, help a critical patient, dilute the anesthetic

PRESSURE MANOMETER/GAUGE Measures pressures of gas within the circuit in cm H20 Eyes on this while bagging your patient DO NOT GO ABOVE 20 CM H20!

NEGATIVE PRESSURE RELIEF VALVE When an active scavenging system is utilized, if negative pressure is detected in the circuit, this valve opens and allows room air in. Particularly in instances when there is excessive suction Also in instances where the O2 flow rate is too low or the tank runs out of oxygen Better for the patient to breathe room air than no air