Inhaled anesthetic Delivery Systems Sahmeddini MD Department of Anesthesia Shiraz medical university

Inhaled anesthetic Delivery Systems

Safety Standards ANSI - (American National Standards Institute) 1979 -- Standards set for all machines sold in the U.S. ASTM -- (American Society for Testing and Materials 1988 1994: ASTM F1161-94 2000: ASTM F1850-00

To comply with the 2000 ASTM F1850-00 standard Continuous breathing system pressure Exhaled tidal volume Ventilatory carbon dioxide concentration Anesthetic vapor concentration Inspired oxygen concentration Oxygen supply pressure Arterial oxygen saturation of hemoglobin Arterial blood pressure Continuous electrocardiogram.

Functions of anesthesia machine Convert supply gases from high pressure to low pressure Convert liquid agent to gas Deliver in a controlled manner Provide positive pressure for ventilation Alert the provider to malfunction Prevent delivery of a hypoxic mixture

Testing Specific Components of the Anesthesia Delivery System 1)Calibration of the oxygen analyzer 2) The low-pressure circuit leak test 3) The circle system tests.

High Pressure System Receives gasses from the high pressure E cylinders attached to the back of the anesthesia machine. (2200 psig for O2, 745psig for N2O)

Usually not used, unless pipeline gas supply is off High Pressure System Receives gasses from the high pressure E cylinders attached to the back of the anesthesia machine 2200 psig for O2 745 psig for N2O Usually not used, unless pipeline gas supply is off

High Pressure System

Hanger Yoke Hanger Yoke: orients and supports the cylinder Providing a gas-tight seal Ensuring a unidirectional gas flow into the machine

Pin Index Safety System(PISS) Prevents tank swaps Pin positions Air 1-5 Oxygen 2-5 Nitrous oxide 3-5

Pin Index Safety System(PISS)

Two sources of gas: Pipeline 50 psig Tanks »Oxygen: 2200 psig »Nitrous oxide: 745 psig »Both reduced to 45 psig upon entering the machine

Tank H Tank E Tank

E Size Compressed Gas Cylinders Cylinder Characteristics Oxygen Nitrous Oxide Air Colour White Blue Black State Gas Liquid and gas Contents (L) 625 1590 Empty Weight (kg) 5.90 Full Weight (kg) 6.76 8.80 6.50 Pressure Full (psig) 2000 750 1800

Approximate remaining time# Oxygen cylinder pressure(psig) 200 .oxygen flow rate(L/min)

Intermediate Pressure System

Intermediate Pressure System Receives gasses from the regulator or the hospital pipeline at pressures of 40-55 psig

Pipeline Inlet Connections Mandatory N2O and O2,usually have air and suction too Inlets are non interchangeable due to specific threading as per the Diameter Index Safety System (DISS)

Diameter Index Safety System (DISS)

Oxygen Pressure Failure Devices Machine standard requires that an anesthesia machine be designed so that whenever the oxygen supply pressure is reduced below normal, the oxygen concentration at the common gas outlet does not fall below 19%

Oxygen Pressure Failure Devices A Fail-Safe valve is present in the gas line supplying each of the flow meters except O2. This valve is controlled by the O2 supply pressure and shuts off or proportionately decreases the supply pressure of all other gasses as the O2 supply pressure decreases

Oxygen Pressure Failure Devices Historically there are 2 kinds of fail-safe valves Pressure sensor shut-off valve (Ohmeda) Oxygen failure protection device (Drager)

Pressure Sensor Shut-Off Valve Oxygen supply pressure opens the valve as long as it is above a pre-set minimum value (e.g.20 psig). If the oxygen supply pressure falls below the threshold value the valve closes and the gas in that limb (e.g..N2O), does not advance to its flow control

Pressure sensor shut-off valve

Oxygen Failure Protection Device (OFPD) Based on a proportioning principle rather than a shut-off principle. The pressure of all gases controlled by the OFPD will decrease proportionately with the

Oxygen failure protection device

Oxygen Supply Failure Alarm The machine standard specifies that whenever the oxygen supply pressure falls below a manufacturer specified threshold (usually 30 psig) alarm shall blow within 5 seconds.

Limitations of Fail-Safe Devices/Alarms Fail-safe valves do not prevent administration of a hypoxic mixture because they depend on pressure and not flow. Do not prevent hypoxia from accidents such as pipeline crossovers or a cylinder containing the wrong gas.

OXYGEN FLUSH VALVE By passes vaporizer Delivers large volumes of oxygen to breathing circuit Is under high(er) pressure caution!!!

OXYGEN FLUSH VALVE Receives O2 from pipeline inlet or cylinder reducing device and directs high, unmetered flow directly to the common gas outlet (downstream of the vaporizer) Machine standard requires that the flow be between 35 and 75 L/min The ability to provide jet ventilation Hazards: May cause barotraumas Dilution of inhaled anesthetic

Second-Stage Reducing Device Located just upstream of the flow control valves Receives gas from the pipeline inlet or the cylinder reducing device and reduces it further to 26 psig for N2O and 14 psig for O2 Purpose is to eliminate fluctuations in pressure supplied to the flow indicators caused by fluctuations in pipeline pressure

Low Pressure System Extends from the flow control valves to the common gas outlet Consists of: Flow meters Vaporizer mounting device Check valve Common gas outlet

Flow Meter Assembly

Flow Meter When the flow control valve is opened the gas enters at the bottom and flows up the tube elevating the indicator The indicator floats freely at a point where the downward force on it (gravity) equals the upward force caused by gas molecules hitting

Flow Meter Standards Oxygen flow control knob Physically different Larger and projects further Different shape All knobs are colour coded Knobs are protected

Electronic flow sensors Some newer anaesthesia workstations have now replaced the conventional glass flow tubes with electronic flow sensors that measure the flow of the individual gases. These flow rate data are then presented to the anaesthesia care provider in either numerical format, graphic format, or a combination of the two.

Cracked tubes In the presence of a flow meter leak (either at the “O” ring or the glass of the flow tube) a hypoxic mixture is less likely to occur if the O2 flow meter is downstream of all other flow meters

Proportioning Systems Mechanical integration of the N2O and O2 flow control valves Maintain a minimum 25% concentration of oxygen with a maximum N2O:O2 ratio of 3:1

Proportioning Systems

Proportioning Systems

Proportioning Systems

Vaporizers A vaporizer is an instrument designed to change a liquid anesthetic agent into its vapor and add a controlled amount of this vapor to the fresh gas flow

Vaporizers

Classification of Vaporizers Methods of regulating output concentration Concentration calibrated Method of vaporization Flow-over Bubble through Injection Temperature compensation: Thermocompensation Supplied heat

Applied Physics Vapor pressure Based on characteristics of agent Varies with temperature Boiling point: Vapor pressure equals atmospheric pressure Latent heat of vaporization: Heat required to change liquid into a vapor Comes from liquid environment

Ohmeda and Drager Characteristics Variable bypass Flow over Temperature compensated Agent specific Out of circuit

Basic Design Gas enters vaporizer Flow is split Majority is by passed Some enters vaporizing chamber Saturated gas leaves chamber Diluted by bypass gas Delivered to patient

Generic Bypass Vaporizer Flow from the flow meters enters the inlet of the vaporizer The function of the concentration control valve is to regulate the amount of flow through the bypass and vaporizing chambers Splitting Ratio = flow though vaporizing chamber/flow through bypass chamber

Factors that Effect Output Flow rate Accurate at most flows Lower than dial setting at both extremes of flow Temperature Vapor pressure varies with temp Accurate at 20 - 35 C

Factors that Effect Output Intermittent back pressure Retrograde flow Higher than dial setting especially at low flows and high ventilator pressures Carrier gas composition N2O causes transient drop

Carbon Dioxide Absorbents Two formulations of carbon dioxide absorbents are commonly available today: soda lime calcium hydroxide lime Baralyme, or barium hydroxide lime

CO2 Absorption (con’t) Soda lime –94% calcium hydroxide –5% sodium hydroxide –1% potassium hydroxide –silica to harden granules –ethyl violet as an indicator

CO2 Absorption (con’t) Ethyl violet is the pH indicator added to both soda lime Ethyl violet changes from colorless to violet when the pH of the absorbent decreases as a result of carbon dioxide absorption. Unfortunately, in some circumstances ethyl violet may not always be a reliable indicator of the functional status of absorbent

Anesthesia Ventilators The ventilator on the modern anesthesia workstation serves as a mechanized substitute for the hand of the anesthesia care provider in manipulating the reservoir bag of the circle system, or another breathing system.

Ventilators Classified by: Power source pneumatic electric both Drive mechanism double circuit driven by oxygen

Ventilator Problems (con’t) Leak in bellows assembly Mechanical problems Electrical problems

Setting the Ventilator Based on the principle that PaCO2 is directly proportional to alveolar ventilation

AV X CO2 = AV X CO2 (what you have) (what you want) AV = alveolar ventilation CO2 = carbon dioxide If you know 3, you can solve for the 4th

The Circuit: Circle System Arrangement is variable, but to prevent re-breathing of CO2, the following rules must be followed: Unidirectional valves between the patient and the reservoir bag Fresh-gas-flow cannot enter the circuit between the expiratory valve and the patient Adjustable pressure-limiting valve (APL) cannot be located between the patient and the inspiratory valve

Circle System Advantages: Relative stability of inspired concentration Conservation of respiratory moisture and heat Prevention of operating room pollution PaCO2 depends only on ventilation, not fresh gas flow Low fresh gas flows can be used Disadvantages: Complex design = potential for malfunction High resistance (multiple one-way valves) = higher work of breathing The final three points of the advantages section are meant to contrast with the Bain circuit

The Adjustable Pressure Limiting (APL) Valve User adjustable valve that releases gases to the scavenging system and is intended to provide control of the pressure in the breathing system Bag-mask Ventilation: Valve is usually left partially open. During inspiration the bag is squeezed pushing gas into the inspiratory limb until the pressure relief is reached, opening the APL valve. Mechanical Ventilation: The APL valve is excluded from the circuit when the selector switch is changed from manual to automatic ventilation

The Adjustable Pressure Limiting (APL) Valve

Scavenging Systems Scavenging is the collection and removal of vented anesthetic gases from the OR Since the amount of anesthetic gas supplied usually far exceeds the amount necessary for the patient, OR pollution is decreased by scavenging

Scavenging Systems Workers should not be exposed to an eight hour time-weighted average of > 2 ppm halogenated agents (not > 0.5 ppm if nitrous oxide is in use) or > 25 ppm nitrous oxide.

Evidence of harm to anesthesia personnel from waste gases is suggestive but unproved (strongest relationship is N2O and reproductive difficulties).

Type of Scavenging Systems Scavenging may be active (suction applied) passive (waste gases proceed passively down corrugated tubing through the room ventilation exhaust grill of the OR).

Hazards of scavenging Obstruction distal to interface Occupational exposure Barotrauma or inability to ventilate

High-Pressure System Checkout    Check Oxygen Cylinder Supply.       Open the O2 cylinder and verify that it is at least half full (≈1000 psi)       Close the cylinder. Check Central Pipeline Supplies.       Check that hoses are connected and pipeline gauges read about 50 psi.

Low-Pressure System Check Initial Status of Low-Pressure System. a.    Close the flow control valves and turn the vaporizers off.    b.    Check the fill level and tighten the vaporizers’ filler caps.

Perform Leak Check of Machine Low-Pressure System. Verify that the machine master switch and flow control valves are OFF   Attach a “suction bulb” to the common (fresh) gas outlet    Squeeze the bulb repeatedly until it is fully collapsed.      Verify that the bulb stays fully collapsed for at least 10 seconds.        Open one vaporizer at a time and repeat steps c and d as above.        Remove the suction bulb and reconnect the fresh gas hose.

Turn on Machine Master Switch and All Other Necessary Electrical Equipment. Test Flow Meters.   a.    Adjust the flow of all gases through their full range while checking for smooth operation of the floats and undamaged flow tubes.    b.    Attempt to create a hypoxic O2/N2O mixture and verify correct changes in flow and/or alarm.

Calibrate O2 Monitor a. Ensure that the monitor reads 21% in room air. b.    Verify that the low-O2 alarm is enabled and functioning.    c.    Reinstall the sensor in the circuit and flush the breathing system with O2.    d.    Verify that the monitor now reads greater than 90%.

Check Initial Status of Breathing System. a.    Set the selector switch to “Bag” mode.    b.    Check that the breathing circuit is complete, undamaged, and unobstructed.    c.    Verify that CO2 absorbent is adequate.    d.    Install the breathing circuit accessory equipment (e.g., humidifier, positive end-expiratory pressure [PEEP] valve) to be used during the procedure.

Perform Leak Check of Breathing System. Set all gas flows to zero (or minimum).      Close the APL (pop-off) valve and occlude the Y-piece.      Pressurize the breathing system to about 30 cm H2O with an O2 flush.      Ensure that the pressure remains fixed for at least 10 seconds.        Open the APL (pop-off) valve and ensure that the pressure decreases