Physics Exam 3 SG P2.pdf

PHYSICS: EXAM 3 STUDY GUIDE PART 2 Work done to compress a gas requires the expenditure of energy (manifested as heat)...

PHYSICS: EXAM 3 STUDY GUIDE PART 2 Work done to compress a gas requires the expenditure of energy (manifested as heat) Adiabatic: no exchange of heat with the surroundings, yet the temperature of the gas rises o Gas is compressed to store, then when you hook it up to the anesthesia machine, the pressure inside that tube was atmosphere (14.7 psi) and now it is very quickly pressurizing to tank pressure (2,000 psi) ▪ Gay-Lussac’s gas law – as pressure increases, temperature increases with a constant volume o Tubing will become extremely hot o If any particles or dirt is anywhere in tubing or connecter, they will combust ▪ “crack” cylinder before use to push out any particles that may be in connector Joule-Thompson Effec (Joule-Kelvin Effect): expansion of gas causes cooling o When do compressed gases expand in anesthesia? o Nitrous Oxide tanks have gas and liquid; as gas is pulled off the top, the liquid continuously expands to gas. Continuous expansion will cause low temperature, and tank will frost Critical temperature: the temperature above which a particular gas is unable to be liquified through the application of pressure (maximum temperature that it can be liquified) o Critical temperature of Nitrous – 36.5C o Critical temperature of Oxygen – -118C Critical pressure: the pressure where a gas is able to be liquified at its critical temperature Vapor pressure: movement of molecules between liquid and vapor states in equilibrium Compressed gas – any material or mixture in the container having either: o A vapor pressure > 40 psia at 70F o A vapor pressure 104 psia at 130F o Any liquid flammable material with a vapor pressure > 40 psia at 100F Regulation of compressed medical gasses o Pharmacopeia of US or National Formulary: preparation and purity specifications o State Department of Transportation (DOT): design, construction, testing, marketing, labelling, filling, storage, maintenance, and transport Gap Piping Systems o Central supplies—O2, N2O, Air, N2 o Problems: ▪ excess pressure ▪ inadequate pressure ▪ alarm dysfunction ▪ crossover of gases ▪ gas contamination (pipes mismatched) ▪ fires ▪ leaks ▪ depletion of reserve Oxygen Cylinder o Oxygen remains gas under high pressure o tank pressure falls linearly as the gas flows from the tank o Gauge pressure accurately reflects amount of gas remaining in cylinder Nitrous Oxide Cylinder  o Nitrous is liquid at high pressure, at ambient temp (20 C) o Gas above the liquid remains at constant pressure independent of how much liquid remains in cylinder o Gauge pressure will only start to fall once all the liquid is evaporated and you only have 1 tank of volume of gas left. Will fall rapidly as residual gas flows from cylinder o Will read 745 psig until no more liquid Nitrous o Have to find volume based on weight of tank, then use molecular weight of substance, vs weight of volume in tank to find volume, then use (1 mole = 22.4L gas) to calculate L flow remaining Medical compressed gases o oxygen, nitrous oxide, medical air, Entonox (50/50 nitrous and oxygen), Heliox, Xenon Gas Formul US Color Int. Color PSI @ 21C State L Capacity a Oxygen O2 Green White 2200 (max) Gas 660 Nitrous N2O Blue Blue 745 Gas + liquid 1600 Air Yellow B&W 1800 Gas 600 Carbon dioxide CO2 Gray Gray 838 Gas + liquid 1590 Helium He Brown Brown 2000 Gas 500 Nitrogen N2 Black Black 2200 Gas 660 MUST check labels to verify what gas is inside a tank – colors differ and tanks are reuseable Cylinder will also have material of tank, maximum filling pressure (psig), serial number and inspection date marked on it o Checked for leaks and structural integrity with overpressure test q 10 years E cylinder o nominal value of 4.8L at 14.7psi o Max fill 1900psig holds 660L o small cylinder packed valve—female type port not unique to gas type  H cylinder o nominal value of 43.6L at 14.7psi o Max fill 2200psig holds 6900L o large cylinder packed valve—male type of outlet port has unique diameter and threads Pin Index Safety System: pins are placed complementary to specific holes in the tank; 2 pins in a unique configuration are used to identify each gas o Prevents mounting of wrong cylinder o Pins complementary in the tank yoke Quick Couplers o Each has specific pin configuration for individual gas. This and attached hose should be color coded for specific gas Diameter Index Safety System (DISS): threaded connections in which the diameters of the threads are specific for each of the gases o Required on all anesthesia machine gas inlets o Prevents cross connection of gasses o Standardized o False assurance—can have wrong outlet on it, connected to wrong tank, etc. Gas cylinders can never be left standing upright and unsecured o Will break at neck if they fall over and release that intense pressure – ‘unguided missile’ with enough force to penetrate a brick wall  Liquid oxygen o Stored in liquid form at large facilities – ‘unlimited’ volume, but requires specific technology and equipment o Liquid O2 stored at -160C, pressure inside storage vessel ~85 psig o Insulated with vacuum to maintain temperature o Oxygen gas pulled from top of container, heated, then passed through pressure regulator to bring to pipeline pressure (50 psi) o In rapid use, temperature and pressure may fall, so control valve will cause liquid O2 to pass through vaporizer to add heat and maintain pressure in tank o Safety valve vent will let pressure out if O2 starts to heat and vaporize in temperature control failure Medical vacuum—used for Bovie smoke exhaled air and volatiles. o Not CO2, goes into absorber Nitrogen in OR used for air tools MATH PROBLEMS 4 types of breathing circuits o Variations include the 1) type of CO2 absorber 2) unidirectional valves used o Open ▪ No reservoir, no rebreathing ▪ Requires high-flow rate (FGF > MV) ▪ Does not allow for controlled ventilation, not delivery of precise inspired gas concentrations ▪ Anesthesia machine cannot be made into an open circuit, this would be HFNC, t-piece o Semi-open ▪ Reservoir, no rebreathing ▪ Requires high flow rate (FGF > MV) ▪ Anesthesia machine should be semi-open on induction and emergence, non-rebreather mask o Semi-closed ▪ Reservoir, partial rebreathing ▪ Uses low flow rate (FGF < MV) ▪ Most of the time, the anesthesia machine is semi-closed during maintenance o Closed ▪ Reservoir, complete rebreathing ▪ Uses low flow rate (FGF < MV) ▪ Only adding in exact amount of gas that is consumed by the body Rebreathing o from mechanical deadspace o prevented by inspiratory and expiratory valves in circuit How is CO2 removed from a breathing circuit? o High flow – CO2 is washed out with fresh gas o Low flow – CO2 is removed via a chemical reaction with a CO2 absorbent Mechanical Dead space: o Y-piece, elbow, ETT o Y-piece is only part of circle system that is dead space, elbow and ETT are not technically part of circle system but are mechanical dead space o Dead space is where inhaled and exhaled gasses comingle ▪ Circle system has one-way gas flow, except for in Y-piece o Increasing tubing length does not increase dead space, only resistance ▪ Increases WOB for the patient on emergence to overcome resistance to maintain gas flow volume  Factors that influence rebreathing o Fresh gas flows (FGF) o Mechanical dead space o Design of breathing system Effects of a rebreathing system o Heat and humidity retention o Decreased waste of gasses and anesthetic o Decreased resistance o Altered inspired gas tensions (O2, CO2, anesthetic) Resistance in breathing system o Compliance of tubing – distensibility (ml/cm H20) ▪ When patient breathes in, the force will first cause flexion of the tubing, then when tubing has reached its maximum flexion the flow will move through to the patient ▪ “loss to compliance” o Laminar vs turbulent flow ▪ potentially increase patient work requirements Potential cause of decreased inspired volume than the prescribed amount o Loss to compliance o Wasted ventilation o Breathing system leaks Potential cause of increased inspired volume than the prescribed amount o In a closed system (vent), FGF > absorption of gas by patient or rate or loss via leaks ▪ uncommon in modern vents o Failure of APL/pressure relief valve Circle system components: o absorber (cannister, housing, baffles, side/center tube, bypass) o absorbents o unidirectional valves o I/E ports o Y-connector o FG inlet o APL valve o Pressure gauge o breathing tubes o reservoir (rebreathing bag) o vent o bag/ventilator selector switch o respiratory gas monitor sensor o airway pressure monitor o respirometer o PEEP valve (optional) o filters (optional) o heated humidifier (optional)  Flow of gas through a circle system o Exhale through Y-piece (dead space) o Expiratory limb o Exhalation check valve (unidirectional flow) o Bag/vent selector switch ▪ Reservoir bag and APL valve (spontaneous or you ventilating) ▪ Vent and pressure relief valve (mechanical ventilation) o CO2 absorber (removes CO2) o Fresh gas inlet (replaces O2 and anesthetic) o Inhalation check valve (unidirectional flow) o Inspiratory limb o Inhaled through Y-piece (dead space) Only cleans the gas that the patient is breathing in Rotation of bag/vent selector switch permits substitution of anesthesia machine ventilator for reservoir bag Volume of reservoir bag is determined by fresh gas inflow and APL In the circle system what comprises dead space? o Y-piece Resistance to breathing in circle system—minimal Relationship between inspired and delivered concentration o difference varies inversely with breathing systems internal volume o nitrogen, carbon dioxide, oxygen, anesthetic agents Breathing circuit vigilance aids o oxygen analyzer o pressure alarms ▪ threshold pressure alarm alerts to partial disconnect o capnometry o agent monitoring o flow meter/volume meter o mass spectroscopy o stethoscope  APL: adjustable pressure limiting valve; controls how much pressure you can push into a patient when bag ventilating o Opens and closes to scavenger o O (open) to 70 (closed) o Max when ventilating should be 20mmHg, above that you risk opening the esophageal sphincter Check valves: prevent backflow and only allow unidirectional flow, which keeps the inhale and exhale limbs from becoming dead space o Valve failure – stuck closed: airway obstruction ▪ Take pt off vent and bag ventilate them o Valve failure – stuck open: dead space and rebreathing CO2 ▪ Convert to semi-open system and increase FGF > MV to wash out CO2 As fresh gas is added through the gas inlet, that amount has to be released somewhere so the volume is not continuously increasing o If bagging, out APL valve to scavenger o If ventilating, out pressure relief valve to scavenger Mapleson breathing systems: o Typically used for pediatrics and transport o Semi-open circuits o Inhaled and exhaled gasses travel through same tubing – requires HIGH FGF to washout CO2, no scrubber, risk for rebreathing and hypercarbia if gas flows are too low o Requires flows if 20-25Lpm or 2.5x MV ▪ Wastes gas, wastes anesthetic, no heat and humidification ▪ Circle system only takes 1-2Lpm o Mapleson A—Best for spontaneous ventilation o Mapleson B o Mapleson C o Mapleson D—Best for controlled ventilation ▪ Mapleson D Bain Modification FGF enters prior to the corrugated tubing Pro: gas is warmed Con: kinks easily o Mapleson E o Mapleson F  Methods of CO2 removal from a system o #1—Chemical absorption o Washout by fresh gas o Separation of exhaled gas from fresh gas o Use of T-piece without reservoir Soda lime is most common CO2 absorbent o 80% calcium hydroxide, 15% water, 4% sodium hydroxide, 1% potassium hydroxide (activator), silica o Irregular in shape to increase surface area o Size is 4-8 mesh: optimizes surface area without increasing resistance o Smaller creates resistance, larger not enough surface area o Shake before use to prevent air from channeling, and turn off gas flows when not in use to not dry out the water = desiccation o Desiccated soda lime doesn’t work and creates CO Ethyl violet o white when fresh, turns purple on exhaustion Degradation of anesthetics by CO2 absorber o Sevoflurane breaks down into Compound A ▪ Nephrotoxic in rates, no studies in humans ▪ In the presence of water, Sevo produces Methanol, which promotes breakdown of Compound A into Compound B and other low toxicity products C, D, and E Desiccated absorber = no breakdown of Compound A ▪ Manufacturer recommendation to run at 2L o Desflurane flow through a desiccated CO2 absorber will produce CO o Either agent running through a desiccated CO2 absorber will produce heat and become very hot Inhaling dry gasses without heat and humidity increases mucus viscosity (mucus plugging), promotes ciliary dysfunction, and causes hypothermia HMEF: Heat and Moisture Exchange Filter o Hydrophobic filter that precipitates water vapor on the filter, and on next inhalation the patient gets that water back o May also offer some bacterial and viral filtration, but not adequate for infectious pts  Benefits of Low Flow o save volatile o retain heat and moisture Scavenger system o Removes waste gasses from system via APL valve or vent pressure relief valve and a vacuum pulls gas out (active removal) and releases outside of OR (roof, into atmosphere) o Has adjustment knob to control vacuum pressure o Points of exit for waste gas in circle system (dump to scavenger): ▪ APL valve ▪ reservoir bag ▪ ventilator relief valve o Transfers gasses from APL valve or vent pressure relief valve via 19mm or 30mm tubing to scavenger interface o If suction is too high – pulling air from patient breath, low TV and hypoxia ▪ Negative pressure relief valve – opens at -0.25 cmH2O and sucks air from atmosphere into system to prevent loss of volume ▪ Collapsed reservoir ▪ if suction is too high, creates negative pressure on patient o If suction is too low – gasses back up and have increased pressure to patient, barotrauma ▪ Positive pressure relief valve – opens at 4 or 5 cmH2O and releases to atmosphere to prevent backup of pressure if adjustment knob is too low ▪ Overdistention of reservoir o Purple tubing indicates scavenger system Sidestream sampler: takes gas from inhaled limb of system, analyzes, dumps tested gas to scavenger o Last chance to prevent administration of hypoxic gas mixture