2The 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.
3ANESTHETIC MACHINEOxygen is be delivered at a controlled rateA liquid anesthetic is converted to a gas, then mixed with oxygen, and delivered to the patient3) 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!
4COMPONENTS OF THE ANESTHESIA MACHINE Can be broken down into 4 parts:Compressed gas supplyGas tanks, their pressure gauges, and pressure reducing valves (may not be visible)Anesthetic machineFlowmeter, vaporizerBreathing circuitUnidirectional valves, hoses, reservoir bag, pop-off valve, CO2 canister, O2 flush, pressure manometer, negative pressure relief valveScavenging SystemDisposes of excess and waste anesthetic gases
6Following the flow of oxygen GAS CYLINDERCarrier gasGas that is compressed in a metal cylinder and held under pressureAvailable in various sizes“E” holds 660 L of oxygen and is attached to the anesthesia machine“H” cylinders hold L of oxygen and stand separate from the machineTanks are delivered and picked up by the oxygen supply company
7Gas 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.
8GAS CYLINDERSIn 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 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 deliveredThis formula will allow you to know how long your tank will last
9TANK 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 machineActual amount in tank is displayed when the tank is turned onCheck this gauge BEFORE an anesthetic procedure (especially the lengthy ones).
10TANK PRESSURE GAUGEChange 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.
11NITROUS OXIDEA 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.
12PRESSURE REDUCING VALVE Regulates the pressure of the gas leaving the tank and going into the anesthesia machineAllows a constant flow of gas into the machine, despite pressure changes within the tankReduces 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.
14Now that the gas is in the machine, it’s job is to mix with the anesthetic and be delivered to the patientFLOW METERAllows 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 minuteSeparate flow meters for each gasThese knobs can be distinguished from one another via labeling, touch, or color
15FLOWMETER CONT’DThe 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 rotorGas travels through the flowmeter
16VAPORIZERConverts the liquid anesthetic agent (usually isofluorane or sevofluorane) into a vapor.Adds controlled amount of these vapors to the carrier gasNext stop as oxygen travels from the flowmeterThe flowmeter must be on to deliver anesthesia to the patient as it needs to be mixed with a carrier gas.
17VAPORIZERSimilarly, the vaporizer must be on to deliver any anesthesia to the patient. Otherwise, only the carrier gas will be deliveredThe 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)
18VAPORIZERAmount 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 vaporizeris tipped over (usually thewhole machine), turn thevaporizer off and run oxygenonly through the machine for15 minutes to flush it out.
19PRECISION VS. NONPRECISION VAPORIZERS Precision vaporizers delivers a precise, controlled amount of anesthetic to the patientExpressed 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
20PRECISION VS. NONPRECISION VAPORIZERS Nonprecision vaporizers are simple, cheaper, and are typically used for anesthetics with low vapor pressure such as methoxyfluorane
22BREATHING CIRCUITThe system that brings the fresh gas from the vaporizer to the patient and takes the expired gases from the patientMay contain unidirectional valves, reservoir bag, pop off valve, CO2 canister, O2 flush, negative pressure relief valve, pressure manometer.
23UNIDIRECTIONAL 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 itInhalation valve opens as patient inhales, anesthetic enters the hose, then the endotracheal tube and the patientCO2 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
24RESERVOIR BAG Also called a rebreathing bag Fills as gases enter the circuit or patient exhales, deflates as patient inhalesFUNCTIONSStores gasHelps in determining correct endotracheal tube placement. Movement of bag with breaths = tube in tracheaAllows assessment of respiratory rate and depth
254) 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 arrestVolume of bag should be minimally 60mL/kg (round up) See pg 119 for reference chartSizes range from ½ L to 30 L (1,2,3L commonly used on small animals)
26SIGNS THAT YOUR RESERVOIR BAG IS NOT THE RIGHT SIZE: Overinflates rapidly if too small. If overinflated, animal will have problems exhalingBags should consistently be ~ ¾ fullVery little movement of bag with breaths if too big
27POP-OFF VALVE AKA pressure relief valve Allows excess gas to leave the breathing circuit and be scavengedPrevents 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 ruptureKEEP the pop-off valve OPEN unless you are bagging the patient (or if you have a low-flow/closed system)
28POP OFF VALVEIn some instances, the degree that the pop off valve is opened changes with the flow rate and how full the reservoir bag is
29HOW DO I KNOW WHEN THE GRANULES ARE EXHAUSTED? CO2 ABSORBERGases that don’t exit the machine via the pop off valve go into the CO2 canister and are then returned to the patientCO2 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 crumbledOnce color becomes abnormal, it is possible that it changes back to normal within hours
31O2 flushWhen activated, O2 bypasses the flowmeter and vaporizer and enters the circuitHigh flow rate (up to 75L/min!)Never use with a non-rebreathing systemCan use to fill the bag, help a critical patient, dilute the anesthetic
32PRESSURE MANOMETER/GAUGE Measures pressures of gas within the circuit in cm H20Eyes on this while bagging your patientDO NOT GO ABOVE 20 CM H20!
33NEGATIVE 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 suctionAlso in instances where the O2 flow rate is too low or the tank runs out of oxygenBetter for the patient to breathe room air than no air