The Anesthesia Machine and Breathing System

Educational Objectives

The goal of this program is to improve the awareness of common features of the anesthesia workstation, sources of anesthesia machine failure, and common problems encountered with the use of the anesthesia machine. After hearing and assimilating this program, the clinician will be better able to:

1. Describe common features of the anesthesia workstation.
2. Discuss sources of anesthesia machine failure.
3. Identify common problems encountered with the use of the anesthesia machine.

Summary

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Anesthesia workstation: preferred term; includes anesthesia machine, breathing circuit, monitoring system; newer workstations more diverse, variable; essential components hidden from view, complex, integrated computer controls; must be fully knowledgeable about workstation; read manuals; use checklist for specific workstation; know components of self-tests for machines; 3 sections of workstation — anesthesia machine (includes gas supplies, pressure regulators, flowmeters, vaporizers, fresh gas outlet, monitors, alarms, protective devices); breathing circuit (includes circuits, CO₂ absorber, ventilators, tubing, scavenging); monitors (respiratory, cardiovascular, other physiologic variables), alarms

Common features: inlet for hospital pipeline and compressed gases, inlet for compressed gas cylinders; pressure regulators to reduce pipeline, tank pressures to safe levels; fail-safe devices; flowmeters to control amount of delivered gas; vaporizers for adding volatile anesthetic agents to carrier gas; common gas line through which compressed gases mixed with volatile agent enter breathing limb; fresh gas valve with O₂ supply; breathing limb (circle system, O₂ analyzer, gas-sampling lines, barometer to measure inspiratory rate and volume, airway-pressure monitor, mechanical ventilator scavenger system, often humidifiers or heaters, filters)

Additional monitors: required by ASTM for modern anesthesia machines; continuous breathing-system pressure, exhaled tidal volumes, ventilatory CO₂ concentration, anesthetic vapor concentration, inspired O₂ concentration, O₂ supply pressure, prioritized alarm system

Resistance to flow of gases: pressure drop to overcome resistance as gases move through circuit or airway, influenced by flow rate, type of flow; measured in mm Hg per flow rate or cm Hg/L/sec

Types of flow: laminar flow — orderly, smooth, particles move in line parallel to wall; flow fastest in center, slower toward walls, pressure drop described by Hagen–Poiseuille law (proportional to length, viscosity, flow rate, inverse to 4th power of radius of tube); turbulent flow — disorderly, flow lines not parallel; particles move all directions including sideways and opposite of flow; flow rate equal across diameter of tube, Hagen–Poiseuille law does not apply (pressure drop influenced by gravity, friction, density, length, flow rate, inverse to 5th power of radius); because impact of density, helium has usual velocity but low density, useful in relieving resistance of turbulent flow; turbulent flow generalized (when flow exceeds critical flow rate as defined by high Reynolds number) or localized (at constrictions, curves, bends, valves, irregularities)

Resistance: reflected by flow-volume loops, discrepancy between peak and plateau pressures when using volume-controlled mode of ventilation with 20% inspiratory hold; peak pressure reflects compliance, resistance to flow; plateau pressure reflects compliance only

Sources of resistance: difference between peak and plateau ~2 cm H₂O to 5 cm H₂O; if higher, look for causes of increased resistance in airway circuit or patient; circuit sources of resistance and turbulent flow — circle system, breathing circuit’s corrugated tubing, usually trivial; heat and humidifier exchange devices, filters, generally minimal but problematic if condensation or if secretions accumulate, should be avoided with spontaneous breathing, especially in children; endotracheal (ET) tube most important site of resistance; increased resistance and turbulent flow, increased work of breathing if patient doing their breathing during spontaneous or assisted ventilation

Level of resistance: no agreement on what level excessive; be aware of how much resistance caused by each component; use components with least resistance

Minimizing circuit resistance: use gas-conducting pathways of minimal length, maximal diameter, without sharp curves or sudden changes; beware of use of filters and heat and moisture exchangers (HMEs)

Gas-flow controlling valves (flowmeters): control gas flow; vertical tubes or electronic devices; glass or plastic tubes mounted vertically, tapered, become wider moving upward; inside tubes, floats (bobbin or ball); specific for each gas, subject to damage, causing leaks, low pressure; employ principle of constant pressure; pressure gradient of gas flow around float balances weight of float; as flow of gas increases, velocity of flow increases, gradient increases, float rises; tube lumen increases velocity of flow, so gradient falls until again it equals weight of float, thus height of bobbin reflects flow rate; if leaks in O₂ flowmeter tube, hypoxic mixture could be delivered to patient, so O₂ flowmeter mounted last

Anesthesia-delivery system: high-pressure section — gas cylinders, pipelines to high pressure reduction valves (50-2200 psi); intermediate section — between low-pressure reduction valve to flowmeter (~12 psi); low-pressure section — beyond flowmeters, in circle system breathing bag (<1 psi, or <50 mm Hg)

Safety systems: alarms for apnea, disconnect, inspired O₂ saturation; activate alarms, set at sensitive threshold, near current settings; sound light, annunciators, messages and sound message, sound alarm; alarms monitor for low pipeline pressure, low tank pressure, low cylinder pressure, low airway pressure while ventilator in use

Systems protecting against delivery of hypoxic mixtures: O₂ fail-safe, nitrogen-O₂ ratio limiting system, PISS, DISS, inspired O₂ saturation monitor; O₂ fail-safe — monitors pressure coming into machine from pipelines or cylinders, turns off all gas flow if pressure <~30 psi; did not prevent dialing in hypoxic mixture by excess N₂O if O₂ available; several O₂-N₂O proportioning systems developed; original Ohmeda system (Link 25) had ratcheted chain-linked interconnection, prevented O₂ delivery <30%, or N₂O delivery >70%; North American Dräger system monitors pressure in 2 gas-flow systems, keeps O₂ ratio ≥30%; Diameter Index Safety System (DISS), special connectors on lines from wall gas outlets to workstation prevent misconnection; Pin Index Safety System (PISS), pins and holes on gas cylinders, yokes on workstation to ensure proper tanks connected to proper locations; machine must have dedicated monitor for inspired O₂ saturation (adequate functioning part of daily preuse check)

O₂ failure: if O₂ cylinders off, O₂ pressure machine lines falls when wall O₂ fails; if O₂ cylinders on, automatically feed O₂ at ~40 psi;, alarms on older machines may not sound until O₂ cylinders empty; after machine checkout, turn off O₂ cylinders; newer workstation should alarm when line pressure falls; line-pressure gauge should inform loss of line pressure; room alarms in operating room (OR) wall should sound when O₂ supply lost; if pipeline O₂ lost, open up O₂ cylinder on workstation, check pressure, calculate O₂ amount left; reduce O₂ use by hand-ventilating with breathing bag, avoids O₂ use to drive bellow-driven ventilators; reduce fresh gas flow, turn off auxiliary O₂-supply flowmeters; call for extra O₂ cylinders; notify hospital authorities

Ventilator disconnect: when ventilation controlled by mechanical ventilator, continuous use of device capable of detecting disconnection, must give audible signal when alarm threshold exceeded; how alarm triggered varies (eg, tidal volume, minute ventilation, peak airway pressure, capnography); default limits often set too low

Gases: include O₂, air, N₂O, helium, heliox, CO₂

Gas pressures: source from pipeline or wall (~50 psi) or from cylinders (~700 psi to 2200 psi, depending on gas); pressure in tanks decreased to ~45 psi by reducing valves before joining pipeline gas; if pipeline pressure >45 psi, flow will not come out of cylinders; when pipeline pressure <45 psi, gas flows out of cylinders; pressure then reduced to ~12 psi before going to flowmeters

E-cylinders: color coded; in US, O₂ green, N₂O blue, air yellow; may differ in other countries; pressures, volumes vary with gas; O₂ and air, gases in cylinders, full tanks at ~2200 psi contain ~600 L, volume directly proportional to pressure; N₂O and CO₂, liquids in cylinders, full tanks at ~750 psi contain ~1600 L; when gas in liquid form, pressure does not show how much gas left in cylinder until three-quarters empty

O₂ flush valve: fills ventilation system, receives O₂ from pipeline or cylinders after initial pressure reduction valve, pressure at ~50 psi (~2500 mm Hg); flushed O₂ bypasses vaporizer, potentially exposing to high pressures (modern circle systems limit to ~50 cm H₂O to 70 cm H₂O); pressing flush valve dilutes concentration of anesthetic vapor; if flush valve sticks in open position, can cause barotrauma; if partially open, can dilute anesthetic agent, contribute to awareness

Leaks: occur in high-pressure circuit, low-pressure circuit, circle, connection to patient, ET tube, lungs; detected by capnograph, failure of bellows or breathing bag to refill, need to increasingly tighten adjustable pressure threshold device, loss of exhaled volume, odor of anesthetic gases in room

Circuits: various types with multiple classification systems, conflicting definitions (eg, open, semiopen, semiclosed, closed); include circle systems, Mapleson circuits, self-inflating nonrebreathing bags (eg, bag valve mask [Ambu device])

Circle system: most commonly used system; 3 valves (inspiratory, expiratory, pop-off valve), CO₂ absorber, reservoir bag; can provide none or total rebreathing of expired gas, excluding CO₂; operated in semiopen, semiclosed, or closed nonbreathing mode; arrangement of components important for optimal functioning

Flow: fresh gas inflow occurs just before inhalation 1-way check valve, to inspiratory corrugated limb tubing, to Y-piece, to patient; expiratory corrugated limb tubing through exhalation 1-way valve, through tubing to CO₂-absorbing canister, back to start

Ventilator switch: teed into tubing between exhalation check valve and CO₂-absorbing canister, bag-ventilator selector switch, connects flow to ventilator or breathing reservoir bag and accompanying adjustable pressure-release, or APL, valve

Dead space: despite size and length, due to 1-way check valves, only dead space in circle system, Y-piece and extensions to patient

Rebreathing: CO₂ rebreathing prevented by CO₂ canister; on other hand, rebreathing of other expired gases depend on fresh gas flow; when fresh gas flow=minute ventilation, almost no rebreathing of expired gas, so CO₂ absorber may be unnecessary; as fresh gas flow drops, amount of rebreathing rises; minimal fresh gas flow applies only enough O₂ and agent to match O₂ consumption and agent uptake; requires ~250 mL O₂/min in 70-kg adult

Low-flow or closed systems: low flow=fresh gas flow 0.5 L/min to 2 L/min; closed system=fresh gas flow only balances uptake (~300 mL/min); advantages — conserves heat, H₂O, agent, decreases pollution, economic; disadvantages — slow concentration changes of agents and gases to patient, changes in fresh gas flow may change delivered tidal volume, may be discrepancy between concentration dialed on vaporizer and inspired concentration of agents

Changes in fresh gas flow: affect time it takes for changes in dialed O₂ concentration by flowmeters and of agents by vaporizer; time estimated from time constant (time to achieve 63% equilibrium); takes ~3 time constants to achieve near equilibrium; time constant=volume÷fresh gas flow rate; typical volume of circle system ~5 L, if fresh gas flow=5 L/min, then time constant=5÷5, or ~1 min, so it takes ~3 mins to 4 mins for equilibrium; if fresh gas flow=0.5 L/min, then time constant=5÷0.5, or 10 min, so it takes >30 mins to reach equilibrium; to change concentration, need to temporarily raise fresh gas flows until equilibrium reached, then go back to low flows

Vaporizer-to-patient concentration: low gas flow increases discrepancy between concentration of agent out of vaporizer and inspired concentration to patient; when fresh gas flow ≥ minute ventilation, some inspired gas comes from exhaled gas from patient, has less agent because of removal by lungs; if minute ventilation=5 L/min and fresh gas flow=0.5 L/min, then >90% inspired gas from exhaled gas, only 10% from fresh gas flow, so discrepancy between that dialed and what patient receives; when fresh gas flow=minute ventilation, almost no discrepancy between inspired concentration to patient and that dialed on vaporizer

Noncircle systems: Mapleson described and classified noncircle systems as Class A through F, depending on components and arrangements; some had reservoir bags and/or valves; some expired gas goes out to atmosphere, some goes back into inspiratory limb; amount of rebreathing dependent partly on fresh gas flow and other characteristics; require relatively high fresh gas flows to prevent CO₂ rebreathing; most systems replaced by circle system, but Bain circuit and Jackson-Rees circuit (both modifications of Mapleson D system) sometimes used

Rebreathing and dead space: inhalation of previously expired gas; may include CO₂; influenced by circuit type, component function, fresh gas flow, dead space (mechanical in breathing circuit, physiologic in patient)

Rebreathing: causes retention of heat, moisture, but decrease in O₂, anesthetic agent, possible increase in CO₂; important to prevent CO₂ rebreathing; in circle system, accomplished by CO₂ absorber, proper functioning of 1-way check valves; in other circuits (eg, Mapleson), highly dependent on fresh gas flow

Causes of rebreathing in circle system: incompetent inspiratory or expiratory valves and failed CO₂ absorber; evaluated or recognized mainly by capnograph with elevation of baseline CO₂ (normally <1-2 cm H₂O, or mm Hg), sloping of inspiratory curve

CO₂ absorbents: original and traditional agents, eg, soda lime, SodaSorb; barium hydroxide lime (Baralyme) largely removed from US market; newer agents have less or no strong bases (eg, calcium hydroxide lime [Amsorb, Amsorb Plus], lithium hydroxide [Litholyme]); consist of granules of absorbing material; granule size compromise between absorbency and resistance; smaller size increases both absorbency and resistance; usual mesh size ~4 to 8 spaces per linear inch; exhaustion of CO₂absorbers indicated by pH-sensitive dyes; soda lime uses ethylene blue, changes from white to purple, but after time color may revert to white, capacity lost; others turn purple, remain permanent; best precaution, change canisters regularly, monitor inspired CO₂ levels

Absorbent problems: CO₂ absorbents can cause toxicity of volatile agents (eg, compound A with sevoflurane, production of CO with desflurane); heat, fire can occur when sevoflurane becomes dry with use of Baralyme, produces inflammable products (eg, formaldehyde), these have been largely eliminated by avoiding drying, eliminating use of Baralyme; sevoflurane can result in production of compound A, may be renal toxin; production of compound A worse with fresh absorbent, dry absorbent, soda lime, high sevoflurane concentrations, low fresh gas flows, higher absorbent temperature; US Food and Drug Administration (FDA) limits fresh gas flow with sevoflurane to >1 L/min during first 2 MAC-hrs, >2 L/min thereafter; absorbents can also produce CO, worse with desflurane, modest with isoflurane, very little with halothane or sevoflurane; aggravated by dry absorbent, increased absorbent temperature, high desflurane concentrations, low fresh gas flows, small patients

Anesthesia Patient Safety Foundation: recommends absorbents that do not degrade significantly when exposed to volatile agents; avoid desiccation by regularly changing absorbent and avoiding O₂ flow through absorbent at end of day

Modern absorbents: newer agents with less or without strong base widely adopted and recommended because do not produce strong alkaline; advantages — do not produce CO or compound A, fires less likely; disadvantages — 50% less absorbent, more expensive

Vaporizers: volatile anesthetic agents liquid at room temperature; at equilibrium, some vapor in atmosphere above liquid (amount dependent on vapor pressure, in turn dependent on agent, temperature); concentration expressed as percent or mL/dL, calculated as vapor pressure÷total barometric pressure; vapor pressure of isoflurane at 20°C ~240 mm Hg, thus concentration above liquid isoflurane at sea level ~31%; sevoflurane vapor pressure ~160 mm Hg, concentration 21%; desflurane vapor pressure ~660 mm Hg, concentration 87%

Types of vaporizers: conventional variable bypass vaporizer — isoflurane, sevoflurane; dual-gas blender — desflurane; Datex-Ohmeda Aladin cassette — all 3 agents; Maquet series injector — not commonly used in United States; traditional Tec vaporizers — sevoflurane and isoflurane; variable-bypass, flow-over-wick temperature-compensated, agent-specific, out-of-system vaporizers; based on temperature, desired concentration; fresh gas flow entering these vaporizers divided, some gas goes through vaporizing chamber where exposed to vapor above liquid, producing concentration described; concentrated vapor then passed back to join fresh gas flow to produce desired concentration; desflurane has very high vapor pressure and relatively low potency, cannot be used with contemporary variable-bypass vaporizers; led to specialized vaporizer that contains separate circuits for fresh gas flow and desflurane vapor

Desflurane vaporizer: pure desflurane generated in separate vaporizing chamber or sump, then heated to constant 39°C to produce vapor of pure desflurane at vapor pressure ~1300 mm Hg; based on amount of fresh gas flow through unit and desflurane concentration dial setting; appropriate amount of pure desflurane vapor added to fresh gas flow to produce desired concentration; desflurane vaporizers have special alarms for low volume, tilt, power failure, abnormal output; automatic shutoff if low desflurane level, vaporizer tilts, power failure, or if failure of pressure controlling devices inside vaporizer; alarm should sound when turned off

Datex-Ohmeda Aladin cassette vaporizer: plug-in cassette; contains vaporized liquid; agent-specific vapor changers for halothane, enflurane, isoflurane, sevoflurane, desflurane; variable control knob on outlet from vapor chamber into bypass chamber, controlled by computer; flowmeter measures fresh gas flow and flow out of vapor chamber, adjusts flows to produce desired vapor concentration; computer system receives information from anesthesia machine regarding flow of gases, amount of fresh gas flow, type of agent in vapor chamber, temperature and pressure in vapor chamber, desired concentration of anesthetic on dial; based on data, computer adjusts valves in chamber to assure proper mixture goes to patient

Maquet system: injector available for halothane, isoflurane, enflurane, but uncommon in US

Hazards with vaporizers: misfilling with wrong agent, tipping, overfilling, underfilling, leaks

Factors influencing vaporizer output: temperature, but modern vaporizers temperature compensated; extreme fresh gas flows, output linear, range of 0.25 L/min to 10 L/min; presence of N₂O in carrier, at low flow, desflurane output from vaporizer reduced by ~20% in presence of N₂O; tilting, tipping, filling with wrong agent; putting agent with higher vapor pressure in vaporizer designed for agent with lower vapor pressure gives false-high concentration (eg, if isoflurane [vapor pressure 250 mm Hg] into sevoflurane vaporizer designed for vapor pressure 75 mm Hg, produces excessive concentration of agent); altitude can affect function of desflurane vaporizers; output constant fraction or percent for given dial setting; if dial set at 6%, at high altitude it puts out same concentration (6%), however, because partial pressure lower, MAC equivalent also lower; in contrast, other Tec vaporizers put out constant partial pressure for given dial setting, therefore at altitude, these put out same partial pressure and same MAC equivalent, but fraction, concentration elevated at higher altitudes since partial pressure relative to barometric pressure higher (eg, at 1.2 setting, it produces 9 mm Hg vapor pressure, but now 1.5%)

Factors influencing inspired volume: discrepancies occur between inspired volume delivered to patient vs that delivered by ventilator, breathing bag, that reported by volume monitor; differences in inspired O₂ concentration or anesthetic agent vs that delivered from common gas outlet

Factors influencing tidal volume delivered to patient: in older ventilators, influenced by changing fresh gas flows; modern ventilators designed to compensate for change in fresh gas flow; however, volume delivered can be affected by wasted ventilation due to gas compression, compression or distension of breathing system, or leaks; generally, compression effects small, but leaks may be significant

Tidal volume discrepancies: detected by volume-measuring device inserted between T-piece and ET tube; also assessed by volume-measuring devices on expiratory limb; reflect changes due to fresh gas flow changes from leaks, but do not detect defects caused by, gas compression; can also observe chest wall movement, breath sounds and monitor blood gases, end-tidal CO₂

Gas-concentration discrepancies: caused by rebreathing, air dilution, leaks, uptake of agent by breathing-system components; already discussed discrepancies in time for dialed concentrations to equilibrate with circuit; fresh gas flow also affects gas concentration delivered to patient; actual concentration delivered to patient influenced by fresh gas flow; when fresh gas flow greater than minute ventilation, concentration delivered to patient same as concentration delivered to circuit once equilibrium achieved; however, as fresh gas flow decreased to ≥ minute ventilation, some inspired gas to patient will be expired gas; amount=difference between fresh gas flow and minute ventilation; if minute ventilation=5 L/min and fresh gas flow=1 L/min, 80% of gas delivered, expired gas; because of uptake of volatile agent and O₂, volatile agent and O₂ going to patient ≥ that delivered from fresh gas flow going to circuit

Humidity: amount of H₂O vapor in gas; expressed as absolute humidity (ie, mass of H₂O per volume, expressed in mg/L) or relative humidity (ie, amount vs capacity when fully saturated at given temperature, expressed as %); saturated=maximum H₂O gas can hold; varies with temperature; at 20°C, gas can hold ~19 mg/L (20 mm Hg H₂O) and at 37° C, ~44 mg/L (47 mm Hg)

Humidifying: gases supplied dry at room temperature; by time delivered to alveoli, fully saturated with H₂O and warmed to body temperature with H₂O and heat delivered by respiratory tract; nose principal HME in body; when air gets to trachea, ~31°C and ~89% saturated; ET tube or supraglottic airway bypasses nasal humidifier

Impact of dry gas during anesthesia: uncertain, probably greater in pediatrics, long procedures, patients at increased risk of pulmonary complications; potential adverse effects — damage to respiratory tract, heat loss (minor), fluid loss (minor), and ET tube absorption; effects of dry inhaled gases may include damage to respiratory tract with thickened secretions, decreased ciliary function, impaired surfactant activity, increased mucosal susceptibility to injury, bronchoconstriction, atelectasis, coughing; recommended minimum humidity 2 mg/L to 44 mg/L

How to increase humidity: CO₂ absorbent, breathing of exhaled gases, low fresh gas flow, moisturizing tubes and reservoir bags, use of coaxial circuits (eg, Bain circuit), HMEs (“artificial noses,” passive or regenerative humidifiers), humidifiers

Humidifiers: cheap, easy, simple, effective; also effective bacterial and viral filters, and avoid overheating or excessive humidification; heated humidifiers no longer commonly used during anesthesia; partly because of associated complications (infection, disconnection, leaks in breathing circuit, overheating, overhumidification, increased volume and compliance in circuit)

Circle system: amount of humidity gradually increases with time, then stabilizes; sources of humidity from exhaled gases if fresh gas flow reduced, absorbent, H₂O during CO₂ neutralization; after ~6 mins at fresh gas flow 0.5 L/min to 2 L/min, humidity ~20 mg% to 25 mg%; higher with decreased fresh gas flow, increased ventilation, wet tubing

Scavenging: advocated because of unproven concerns that chronic exposure to low concentrations of inhaled anesthetic agents hazardous to OR personnel; National Institute for Occupational Safety and Health (NIOSH) recommendations for minimal environmental concentrations of volatile agents — 2 ppm for isolated halogenated agents, 25 ppm for N₂O, 0.5 ppm for halogenated agents when used with N₂O, and 50 ppm for N₂O when used alone in dental facilities; American Society of Anesthesiologist (ASA) recommendations — waste anesthetic gases should be scavenged; use appropriate work practices to minimize exposure; educate personnel working in areas where waste anesthetic gases may be present regarding possible hazards and management; insufficient evidence to recommend routine monitoring or for routine medical surveillance

Sources of gases that need scavenging: anesthetic machine on circuit, pressure-release valve, ventilator, gas-sampling device, ventilator drive, leaks, and if fresh gas flow and agents not turned off at end of case; other sites include masks, noncuffed ET tubes, and while filling vaporizer; several types of scavenging systems — active or passive, closed or open; all should have positive and negative pop-off valves; scavenging facilitated by use of low fresh gas flow and changing air in room ≥15×/hr

Hazards of scavenging: positive or negative pressure inside scavenger system may interfere with scavenging, delivery of ventilation to patient, failure to connect between machine and disposal system

Workstation preuse checks: O₂ analyzer calibration, low-pressure circuit check leak test, circle system test; testing for leaks in low-pressure system depends on whether check valve proximal to common gas outlet; checklist on many machines; use negative-pressure test with suction bulb; if no checklist, use positive-pressure leak test

Universal negative-pressure leak test: first verify machine master switch and flow valves off; attach suction bulb to common fresh gas outlet, squeeze bulb repeatedly until fully collapsed, and then verify bulb stays collapsed for ≥10 secs; then open 1 vaporizer at time and repeat steps, finally remove suction bulb and reconnect fresh gas hose to fresh gas outlet; be aware when leak test automatically performed by machine; necessary to repeat leak test self-test with each vaporizer dial turned to “on” position to detect internal vaporizer leaks; if flowmeter included to O₂ supply, must be turned off or will appear as leak

Circle system test: includes positive-pressure test and flow test; positive-pressure test — set all gas flows to 0 or minimum, close pop-off valve, occlude Y-piece, then pressurize system to ~30 cm H₂O; ensure pressure remains fixed for ≥10 secs, then open pop-off valve and ensure pressure decreases to 0; flow test — inhale and exhale through Y-piece or mask and observe that valves open and close proximally; then selectively close end of expiratory and inspiratory limbs separately to ensure not able inhale or exhale, respectively, to ensure competency of valves; can also test with ventilator in circuit connected to breathing bag

Other checks: ensure adequate O₂ supply in E-cylinders and functioning emergency backup (eg, Ambu bag, jet ventilator, extra O₂ supply, tubing)

Checklist remarks: checklists often do not emphasize tank pressure must read 0 before turning on to check pressure and do not recommend checking inspiratory and expiratory valves separately; 1993 FDA anesthesia apparatus checklist recommendations outdated; ASA Committee on Equipment and Facilities issued preanesthesia checklist procedure

Example Scenarios

Case 1: 1.5 hrs into case with patient on ventilator, receiving tidal volume 600 mL at 10 L/min, fresh gas flow 2 L/min; low O₂ pressure alarm sounds; pipeline pressure 0; O₂ flowmeter indicates no flow; you immediately turn on O₂ cylinder, pressure reads 2000 psi, O₂ flow resumes with 95% inspired O₂

Question 1: How long can you run anesthetic in this manner? Answer: ~80 min; fresh gas flow 2 L/min; O₂ flow (driving ventilator) using ≥6 L/min to maintain minute ventilation, thus utilizing ~8 L/min, full E-cylinder contains ~660 L, then should last ~84 mins

Question 2: What should you do to extend this time? Answer: First, turn off ventilator if using O₂ to drive it (true on some machines; others piston driven, run off electricity); second, reduce fresh gas flow

Case 2: After your case has settled down, you reduce N₂O and O₂ flows from 2 L/min to 0.5 L/min; inspired O₂ concentration gradually falls to <50%

Question 1: What might explain this fall? Answer: Uptake of N₂O has gradually decreased while O₂ constant; at these relatively low flow rates, concentration of N₂O in circuit will gradually rise and O₂ will gradually fall; other causes of low FiO₂ — loss of O₂ and pressure or flow from pipeline due to central line failure, local obstruction or disconnect, wrong gas going into O₂ pipeline, misconnection of pipelines to anesthesia machine, wrong gas in O₂ cylinder or wrong attachment; failure of O₂-N₂O proportional system, adding second non-O₂ gas (eg, air, helium, CO₂), break in flowmeters, air entrapment from negative pressure, leak in bellows, error in monitoring of inspired O₂ concentration; if raise O₂ flow back to 10 L/min, and yet inspired concentration continues to fall

Question 2: When concentration falls below 90%, what action to take? Answer: If time permits, go through differentials and correct identified causes; if all else fails or out of time, ventilate with self-inflating valve with air, or attach to separate O₂ source other than another O₂ tank from anesthesia machine, if suspect wrong gas attached to anesthesia machine or wrong gas coming out of pipelines

Case 3: During case, circulating nurse in room who happens to be pregnant complains that she smells anesthetic agent around machine

Question 1: How to troubleshoot? Answer: See above sources vaporizer leak, spilled anesthetic liquid or open canister or open vaporizer filter cap, leak in vaporizer, failure with scavenger system or leak elsewhere in circuit or end-tidal ET tube

Case 4: During laparoscopy, end-tidal CO₂ progressively rises >50 mm Hg

Question 1: What is differential diagnosis? Answer: Could be from increased CO₂ production, hyperthermia, hypermetabolism, or absorption of unusual amounts of CO₂ from abdomen; rebreathing due to malfunction of CO₂ absorber or bypass around CO₂absorber; incompetent inspiratory or expiratory valves; leak in inspiratory limb or addition of CO₂ into fresh gas flow; lower-than-needed alveolar ventilation because of increased dead space or low minute ventilation

Case 5: During case, low minute-ventilation volume alarm sounds

Question 1: What activates alarm? Answer: Already reviewed these issues; limits need to be adjusted for each patient and each case

Question 2: What is included in differential diagnosis and how to troubleshoot problem? Answer: Look at capnograph; if patient ventilated, check if ventilator bellows emptying and filling properly; check ability to drive gas into patient; check if ventilator on or ventilator failure; outflow resistance from high airway pressure; inadequate volume in circuit from loss of gas supply into anesthesia machine or circuit; getting gas back from patient; disconnect; leaking cuff; air leak from lung; nasogastric tube inserted in trachea; suction applied to bronchoscope; leak in system; problem not found, ventilate with self-inflating breathing bag

Case 6: During different case, high airway-pressure alarm sounds

Question 1: What are implications? Answer: High airway pressure may indicate high pressure in lungs, resulting in barotrauma and/or reduced cardiac output; may also indicate obstruction of flow from ventilator to patient, could lead to hypoventilation

Question 2: How to troubleshoot? Answer: Differential diagnoses include obstruction of gas flow to patient, reduced lung compliance, increased resistance to gas flow in circuit, reversed PEEP valve, kinked corrugated tubing, cuff overinflating end of ET tune, kinked or plugged ET tube, endobronchial intubation or foreign body or secretions in airway, gas building up in circuits, failure of ventilator-release valve, excessive pressure or vacuum in scavenger, adjustable relief valve will not open or fails, gas building up in lung, air trapping from ball-valve phenomenon, PEEP or inadequate expiratory time

Question 3: What are some causes of low end-tidal CO₂? Answer: Excessive ventilation, error in ventilator settings, hypothermia, deep anesthesia, low cardiac output, pulmonary embolism, error in monitoring system (eg, miscalibration, loose connection)

Case 7: Question 1: What happens if electrical supply into machine is lost? Answer: Modern anesthesia machines should have battery output (~30 min); battery backup powers ventilator controls, alarms, and integral monitors of machine; battery does not power external appliances connected to auxiliary power outlet on anesthesia machine (eg, major monitor, airway multigas monitor, cardiac output monitor)

Question 2: What to do about total electrical power failure to OR? Answer: Loss of all monitors, loss of all lighting, loss of all electrical monitoring equipment besides anesthesia machine, includes some suction, cell savers, electrocautery, defibrillator, microscope and if on cardiopulmonary bypass, heart-lung machine; think about and be prepared for managing total loss of electrical power

Case 8: After weekend, start case in same-day surgery center using desflurane; after intubation, notice pulse-oximeter saturation gradually falls from 100% to 94%; peripheral venous blood gas, O₂ 190 mm Hg

Question 1: What might be going on? Answer: Causes of low pulse-oximeter reading include low FiO₂, hypoventilation, pulmonary dysfunction, monitor error, carboxyhemoglobinemia, methemoglobin, blue dye in bloodstream; peripheral venous gas showing PO₂ of 190 mm Hg excludes low PO₂ as cause; likely possibility carboxyhemoglobin; read as ~90% oxyhemoglobin and 10% deoxyhemoglobin; if high enough, causes modest reduction in pulse-oximeter reading, should be considered in differential diagnosis of low pulse-oximeter reading; diagnosis confirmed through multi-wavelength O₂ saturation analysis in laboratory; ask for carboxyhemoglobin saturation measurement; carboxyhemoglobinemia not detected by conventional blood-gas analyzers, which estimate saturation from PaO₂ (normal); CO generated by CO₂ absorber when exposed to inhalation agents, especially desflurane through dry CO₂ absorber, if fresh gas flow left on for long periods of time (eg, over weekend), especially if rebreathing bag absent or pop-off valve left open (flow of greatest resistance from fresh gas flow backwards, through CO₂ absorber, into environment); CO₂ absorber becomes dry and at risk of developing hyper-CO next day (Monday morning syndrome)

Readings

Dorsch JA, Dorsch SE: Understanding Anesthesia Equipment. 5th ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2008; Ehrenwerth J et al, eds: Anesthesia Equipment: Principles and Applications. 2nd ed. Philadelphia, PA: Elsevier Saunders; 2013; 7th ed. Philadelphia, PA: Churchill Livingstone/Elsevier; 2010; Rutort KT, Eisenkraft JB: The anesthesia workstation and delivery systems for inhaled anesthetics. In: Barash PG, eds. Clinical Anesthesia. 8th ed. Appendix-Chapter 25; Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2017:644-705; Rose G, Mclarney JT: Anesthesia Equipment Simplified. 1st ed. Edition; New York, NY: McGraw-Hill; 2014; Sandberg W et al: MGH Textbook of Anesthesia Equipment. 1st ed. Philadelphia, PA: Elsevier Saunders; 2010.

Disclosures

For this activity, the faculty and planning committee reported nothing to disclose.

Acknowledgements

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 0 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 0 CE contact hours.

Lecture ID:

ANBR190109

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.

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