Morgan and M. chapters 2-4

what is ALARP and what are its principles?

A basic principle of radiation safety is to keep exposure "as low as reasonably practicable" (ALARP). The principles of ALARP are protection from radiation exposure by the use of time, distance, and shielding. Exposure can be reduced to the provider by increasing the distance between the beam and the provider. Physical shields are usually incorporated into radiological suites and can be as simple as a wall to stand behind or a rolling leaded shield to place between the beam and the provider ecause protective eyewear has not been consistently used to the same degree as other types of personal protection, radiation-induced cataracts are increasing among employees working in interventional radiology suites.

compare mapleson A, B, C and D

The APL valve in Mapleson A, B, and C circuits is located near the face mask, and the reservoir bag is located at the opposite end of the circuit.

what are the components of the high pressure circuit?

The High-pressure circuit consists of those parts which receive gas at cylinder pressure hanger yoke (including filter and unidirectional valve) yoke block cylinder pressure gauge cylinder pressure regulators

what are the components of the intermediate pressure circuit?

The Intermediate pressure circuit receives gases at low, relatively constant pressures (37-55 psi, which is pipeline pressure, or the pressure downstream of a cylinder regulator) pipeline inlets and pressure gauges ventilator power inlet Oxygen pressure-failure device (fail-safe) and alarm flowmeter valves oxygen and nitrous oxide second-stage regulators oxygen flush valve

what are the components of the low pressure circuit?

The Low-pressure circuit includes components distal to the flowmeter needle valves flowmeter tubes vaporizers check valves (if present) common gas outlet

which Mapleson circuit is the best for controlled ventilation? why?

The Mapleson D circuit is efficient during controlled ventilation, because fresh gas flow forces alveolar air away from the patient and toward the APL valve. Interchanging the position of the APL valve and the fresh gas inlet transforms a Mapleson A into a Mapleson D circuit Thus, simply moving components completely alters the fresh gas requirements of the Mapleson circuits. The Bain circuit is a coaxial version of the Mapleson D system that incorporates the fresh gas inlet tubing inside the breathing tube This modification decreases the circuit's bulk and retains heat and humidity better than a conventional Mapleson D circuit as a result of partial warming of the inspiratory gas by countercurrent exchange with the warmer expired gases. A disadvantage of this coaxial circuit is the possibility of kinking or disconnection of the fresh gas inlet tubing. Periodic inspection of the inner tubing is mandatory to prevent this complication; if unrecognized, either of these mishaps could result in significant rebreathing of exhaled gas.

why does the APL valve need to be open during spontaneous breathing?

The adjustable pressure-limiting (APL) valve should be fully open during spontaneous ventilation so that circuit pressure remains negligible throughout inspiration and expiration . As anesthetic gases enter the breathing circuit, pressure will rise if the gas inflow is greater than the combined uptake of the patient and the circuit. Gases may exit the circuit through an APL valve, controlling this pressure buildup. Exiting gases enter the operating room atmosphere or, preferably, a waste-gas scavenging system. All APL valves allow a variable pressure threshold for venting. The APL valve should be fully open during spontaneous ventilation so that circuit pressure remains negligible throughout inspiration and expiration. Assisted and controlled ventilation require positive pressure during inspiration to expand the lungs. Partial closure of the APL valve limits gas exit, permitting positive circuit pressures during reservoir bag compressions.

briefly describe the anestehsia machine

The anesthesia machine receives medical gases from a gas supply, controls the flow and reduces the pressure of desired gases to a safe level, vaporizes volatile anesthetics into the final gas mixture, and delivers the gases to a breathing circuit that is connected to the patient's airway. A mechanical ventilator attaches to the breathing circuit but can be excluded with a switch during spontaneous or manual (bag) ventilation.

cylinder component parts - describe the check valve

The check valve in the cylinder yoke functions to: minimize trans-filling, allow change of cylinders during use, and minimize leaks to atmosphere if a yoke is empty.

describe the circle system

The circle is the most popular breathing system in the US. It cleanses carbon dioxide from the patient's exhalations chemically, which allows rebreathing of all other exhaled gases (a unique breathing arrangement in health care, but used extensively in other environments e.g. space, submarine). Circle components: fresh gas inflow source, inspiratory & expiratory unidirectional valves, inspiratory and expiratory corrugated tubing, Y connector, overflow (called popoff, adjustable pressure-limiting valve, or APL valve), reservoir bag, carbon dioxide absorbent canister and granules. Resistance of circle systems is less than 3 cm H2O (less than the resistance imposed by the endotracheal tube). Dead space is increased (by all respiratory apparatus). VD/VT= 0.33 normally, 0.46 if intubated and 0.65 if mask case. Mechanical dead space ends at the point where inspired and expired gas streams diverge (the Y-connector).

Effect of altitude on variable bypass vaporizers

The effect of altitude on vaporizer performance is controversial. Some sources state that variable bypass types need not be adjusted for moderate changes in barometric pressure, but the Tec 6 must be dialed up beyond the desired dose at higher altitudes. Note that nitrous oxide is less useful as altitude increases, since it becomes more difficult to supply adequate pO2 using N2O when total atmospheric presure declines.

The fraction of inspired oxygen (FIO2) delivered by a resuscitator breathing system to the patient is _______ proportional to the oxygen concentration and flow rate of the gas mixture supplied to the resuscitator (usually 100% oxygen) and _______ proportional to the minute ventilation delivered to the patient.

The fraction of inspired oxygen (FIO2) delivered by a resuscitator breathing system to the patient is directly proportional to the oxygen concentration and flow rate of the gas mixture supplied to the resuscitator (usually 100% oxygen) and inversely proportional to the minute ventilation delivered to the patient.

cylinder component parts - describe the hanger yoke

The hanger yoke: orients cylinders, provides unidirectional flow, and ensures gas-tight seal.

describe the pipeline gas source

The hospital medical gas pipeline source is the primary source for the anesthesia gas machine. Oxygen is produced by fractional distillation of liquid air. It is stored as a liquid at -150 to -175 degrees C in a large flask (because the liquid occupies 1/860 of the space the gas would occupy). Safety systems and regulators send oxygen to the hospital pipeline at approximately 50 psi; which is therefore the "normal working pressure" of the anesthesia delivery system. Nitrous oxide is stored as a liquid, at ambient temperature, in large tanks (745 psi- H tank) connected to a manifold which regulates the pipeline pressure to approximately 50 psi. Pipeline inlets (near the yoke blocks for cylinders) are connected with DISS (diameter index safety system) non-interchangeable connections. The check valve, located down stream from the pipeline inlet, prevents reverse flow of gases (from machine to pipeline, or to atmosphere), which allows use of the gas machine when pipeline gas sources are unavailable.

important numbers to remember: psi for gas pipline oxygen cylinder psi oxygen flush L/min tubing sizes

The hospital pipeline is the primary gas source at 50 psi, which is the normal working pressure of most machines. Cylinders - Oxygen is supplied at around 2000 psi (regulated to approximately 45 psi after it enters the machine). Oxygen flush is a "straight shot" from pipeline to common gas outlet (bypassing vaprizers and flowmeters), 35-75 L/min. OSHA links on Waste Anesthetic Gases (WAGs) gives the NIOSH recommendation to OSHA - occupational exposure should be limited to (an eight hour time-weighted average of) not more than 2 ppm halogenated agents (0.5 ppm if nitrous oxide in use), and not more than 25 ppm nitrous oxide. Read the 1977 NIOSH recommendation on waste gases. Tubing sizes- scavenger 19 or 30 mm, ETT or common gas outlet (CGO) 15 mm, breathing circuit 22 mm.

what is the normal leakage current in the OR? what happens when leakage current exceeds threshold?

The magnitude of a leakage current is normally imperceptible to touch (<1 mA, and well below the fibrillation threshold of 100 mA). If the current bypasses the high resistance offered by skin, however, and is applied directly to the heart (microshock), current as low as 100 μA may be fatal. The maximum leakage allowed in operating room equipment is 10 μA.

what is the major advantage of a piston ventilator?

The major advantage of a piston ventilator is its ability to deliver accurate tidal volumes to patients with very poor lung compliance and to very small patients. In a piston design, the ventilator substitutes an electrically driven piston for the bellows; the ventilator requires either minimal or no pneumatic (oxygen) power. During volume-controlled ventilation the piston moves at a constant velocity whereas during pressure- controlled ventilation the piston moves with decreasing velocity. As with the bellows, the piston fills with gas from the breathing circuit. To prevent generation of significant negative pressure during the downstroke of the piston the circle system configuration has to be modified The ventilator must also incorporate a negative-pressure relief valve or be capable of terminating the piston's downstroke if negative pressure is detected. Introduction of a negative-pressure relief valve to the breathing circuit may introduce the risk of air entrainment and the potential for dilution of oxygen and volatile anesthetic concentrations if the patient breathes during mechanical ventilation and low fresh gas flows.

what is the only reliable way to determine the residual volume in the N2O cylinder? why?

The only reliable way to determine residual volume of nitrous oxide is to weigh the cylinder. If liquid nitrous oxide is kept at a constant temperature (20°C), it will vaporize at the same rate at which it is consumed and will maintain a constant pressure (745 psig) until the liquid is exhausted

effect of flow rate on variable bypass vaporizers

The output of all current variable-bypass vaporizers is relatively constant over the range of fresh gas flows from approximately 250 mL/min to at least 10 L/min, due to extensive wick and baffle system that effectively increases surface area of vaporizing chamber. All sevoflurane vaporizers are less accurate (due to the low vapor pressure of the agent) at high fresh gas flows (> 10 L/min) and high vaporizer concentration settings typical after induction, where they deliver less than the dial setting (Anesth Analg 2000;91:834-6 notes that this tendency is accentuated if the vaporizer is nearly empty). Clinically this is relatively unimportant, since we titrate to effect (end tidal agent concentration) and use overpressure.

effect of ambient temperature on variable bypass vaporizers

The output of modern vaporizers is linear from 20-35 degrees C, due to Automatic temperature compensating devices that increase carrier gas flow as liquid volatile agent temperature decreases Wicks in direct contact with vaporizing chamber walls Constructed of metals with high specific heat and thermal conductivity

at 20 C, a N2O tank should weigh ____ PSI. If it is higher than this, what does it imply?

The pressure gauge of a nitrous oxide cylinder should not exceed 745 psig at 20°C. A higher reading implies gauge malfunction, tank overfill (liquid fill), or a cylinder containing a gas other than nitrous oxide.

cylinder component parts - describe the safety relief device

The safety relief device is composed of at least one of frangible disc (bursts under extreme pressure), fusible plug (Wood's metal, which has a low melting point), or safety relief valve (opens at extreme pressure).

what is the fail safe?

What happens inside the workstation when oxygen pipeline pressure drops? The fail safe device ensures that whenever oxygen pressure is reduced and until flow ceases, the set oxygen concentration shall not decrease at the common gas outlet. In addition, the loss of oxygen pressure results in alarms, audible and visible, at 30 psi pipeline pressure. Fail-safe systems don't prevent hypoxic mixtures. For example, as long as there is pressure in the oxygen line, nothing in the fail safe system prevents you from turning on a gas mixture of 100% nitrous oxide (however, this should be prevented by the hypoxic guard system, see below) or 100% helium (which wouldn't be prevented by the hypoxic guard, since the hypoxic guard only connects oxygen and nitrous oxide flowmeters).

what is a disconnect alarm? why are they so important?

Whenever a ventilator is used "disconnect alarms" must be passively activated. Anesthesia workstations should have at least three disconnect alarms: low pressure, low exhaled tidal volume, and low exhaled carbon dioxide. Most modern anesthesia ventilators also have integrated spirometers and oxygen analyzers that provide additional alarms.

Fresh gas decoupling versus compensation

A final factor adding to modern ventilator accuracy is that they compensate delivered tidal volume for changes in fresh gas flow (FGF). In traditional ventilators, which are not fresh gas decoupled, the delivered tidal volume is the sum of the volume delivered from the ventilator bellows, and the fresh gas flow delivered during the inspiratory phase of each breath. Thus, delivered tidal volume may change as FGF is changed. For example, consider a patient with a FGF of 4 L/min, a respiratory rate of 10, inspiratory:expiratory (I:E) ratio of 1:2, and a tidal volume of 700 mL. During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 1,320 mL (4000 mL/min FGF times 1/3). Each of the 10 breaths of 700 mL is augmented by 132 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 832 mL/breath. This 19% increase is reasonably unimportant. But what happens if we decrease the fresh gas flow? Assume the same parameters, but a FGF of 1,000 mL/min. During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 330 mL (1000 mL/min FGF times 1/3). Each of the 10 breaths of 700 mL is augmented by 33 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 733 mL/breath. This means that changing FGF from 4,000 mL/min to 1,000 mL/min, without changing ventilator settings, has resulted in a 14% decrease in delivered tidal volume (832 to 733 mL). It would not be surprising if the end tidal carbon dioxide rose as a result. The situation is more acute with a traditional anesthesia ventilator in children. Assume a 20 kg patient with a FGF of 4 L/min, a respiratory rate of 20, inspiratory:expiratory ratio of 1:2, and a tidal volume of 200 mL. During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 1,320 mL (4000 mL/min FGF times 1/3). Each of the 20 breaths of 200 mL is augmented by 66 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 266 mL/breath. This is a 33% increase above what is set on the ventilator. We now decrease the FGF from 4 to 1 L/min for the same 20 kg child, (RR 20, I:E ratio of 1:2, and VT 200 mL). During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 333 mL (1000 mL/min FGF times 1/3). Each of the 20 breaths of 200 mL is augmented by 16.5 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 216 mL/breath. This is a 23% decrease in VT (266 to 216 mL/breath) caused solely by changing FGF, and without altering vent settings.

A pressure of _______ psig indicates an E-cylinder that is approximately half full and represents ______ L of oxygen.

A pressure of 1000 psig indicates an E-cylinder that is approximately half full and represents 330 L of oxygen.

what would cause an increase in airway pressure? a decrease?

A rise in airway pressure may signal worsening pulmonary compliance, an increase in tidal volume, or an obstruction in the breathing circuit, tracheal tube, or the patient's airway. A drop in pressure may indicate an improvement in compliance, a decrease in tidal volume, or a leak in the circuit.

accurate detection of Vt descrepencies depends on what?

Accurate detection of tidal volume discrepancies is dependent on where the spirometer is placed. Sophisticated ventilators measure both inspiratory and expiratory tidal volumes. It is important to note that unless the spirometer is placed at the Y-connector in the breathing circuit, compliance and compression losses will not be apparent. Several mechanisms have been built into newer anesthesia machines to reduce tidal volume discrepancies.

Typical ventilator alarms

All current gas machines have VPO (volume, pressure, oxygen) monitoring built in the breathing circuit. Most have agent monitoring as well. Some have spirometry and capnography. High pressure Pressure below threshold for 15 to 30 seconds (apnea or disconnect) Continuing high pressure Subatmospheric pressure Low tidal or minute volume High respiratory rate Reverse flow (may indicate incompetence of expiratory unidirectional valve in the breathing circuit) Apnea/disconnect alarms may be based on Chemical monitoring (lack of end tidal carbon dioxide) Mechanical monitoring Failure to reach normal inspiratory peak pressure, or Failure to sense return of tidal volume on a spirometer Visual monitoring Failure of standing bellows to fill during mechanical ventilator exhalation Failure of manual breathing bag to fill during mechanical ventilation (machines with fresh gas decoupling- the Apollo, Fabius GS) Auditory monitoring - lack of breath sounds in precordial, lack of sound from ventilator cycling, etc

describe the modern day vaporizer

All modern vaporizers are agent specific and temperature corrected, capable of delivering a constant concentration of agent regardless of temperature changes or flow through the vaporizer.

cylinder component parts - describe the cylinder valve

Cylinder valve - is the most fragile part, so protect during transport. Consists of body port (where the gas exits) stem (shaft) handle or hand wheel (to open the valve) safety relief device conical depression (opposite the port, it accepts the tip of the screw which secures the cylinder in the yoke) PISS pins (Pin Index Safety System)

Effect of intermittent back pressure on variable bypass vaporizers

Effect of intermittent back pressure transmitted from breathing circuit: The pumping effect is due to positive pressure ventilation or use of the oxygen flush valve. It can increase vaporizer output. Modern vaporizers are relatively immune (older vaporizers were certainly not immune) due to check valves between the vaporizer outlet and the common gas outlet, smaller vaporizing chambers, or tortuous inlet chambers. Any of these design features prevent gas which has left the vaporizers from re-entering it.

what happens if excessive tilting of the vaporizer occurs?

Excessive tilting of older vaporizers (Tec 4, Tec 5, and Vapor 19.n) during transport may flood the bypass area and lead to dangerously high anesthetic concentrations. In the event of tilting and spill-age, high flow of oxygen with the vaporizer turned off should be used to vaporize and flush the liquid anesthetic from the bypass area.

what is the pumping effect?

Fluctuations in pressure from positive-pressure ventilation in older anesthesia machines may cause a transient reversal of flow through the vaporizer, unpredictably changing agent delivery. T his "pumping effect" is more pronounced with low gas flows. A one-way check valve between the vaporizers and the oxygen flush valve (Datex-Ohmeda) together with some design modifications in newer units limit the occurrence of some of these problems.

what should you do if there is a fire on the pt?

For fires on the patient, the flow of oxidizing gases should be stopped, the surgical drapes removed, and the fire extinguished by water or smothering. The patient should be assessed for injury. If the fire is not immediately extinguished by first attempts, then a carbon dioxide (CO2) fire extinguisher may be used. Further actions may include evacuation of the patient and activation of the nearest pull station

how do you figure out how many minutes you have left in your oxygen tank if you're running oxygen at 3 LPM and you're tank is half full?

If the oxygen is exhausted at a rate of 3 L/min, a cylinder that is half full will be empty in 110 min. full tank pressure/ L = what you read on the gauge/X x/LPM = min left 2000 psi/660 L = 1000psi/ x x = 330 330/3 = 110 min

describe the thorpe tube

In a traditional flowmeter, flow is mechanically controlled (needle valve) and displayed (glass tube with bobbin). A Thorpe tube is an older term for flowmeters. The components are- needle valve, indicator float, knobs, valve stops. Flow increases when the knob is turned counterclockwise (same as traditional vaporizers). At low flows, the annular-shaped orifice around the float is (relatively) tubular so (according to Poiseuille's Law) flow is governed by viscosity. At high flows (indicated on the wider top part of the float tube), the annular opening is more like an orifice, and density governs flows.

what causes large discrepancies between the set and actual tidal volume during volume controlled ventilation?

Large discrepancies between the set and actual tidal volume are often observed in the operating room during volume-controlled ventilation. Causes include breathing circuit compliance, gas compression, ventilator-fresh gas flow coupling, and leaks in the anesthesia machine, the breathing circuit, or the patient's airway. The use, of peak inspiratory pressure is 20 cm H2O, about 100 mL of set tidal volume is lost to expanding the circuit. For this reason breathing circuits for pediatric patients are designed to be much stiffer, with compliances as small as 1.5-2.5 mL/cmH2O. Compression losses, normally about 3%, are due to gas compression within the ventilator bellows and may be dependent on breathing-circuit volume. Thus if tidal volume is 500 mL another 15 mL of the set tidal gas may be lost. Gas sampling for capnography and anesthetic gas measurements represent additional losses in the form of gas leaks unless the sampled gas is returned to the breathing circuit, as occurs in some machines. During the initial electronic self-checkout, some machines measure total system compliance and subsequently use this measurement to adjust the excursion of the ventilator bellows or piston; leaks may also be measured but are usually not compensated. The actual method of tidal volume compensation or modulation varies according to manufacturer and model. In one design a flow sensor measures the tidal volume delivered at the inspiratory valve for the first few breaths and adjusts subsequent metered drive gas flow volumes to compensate for tidal volume losses (feedback adjustment). Another design continually measures fresh gas and vaporizer fl ow and subtracts this amount from the metered drive gas flow (preemptive adjustment). Alternately, machines that use electronic control of gas flow can decouple fresh gas flow from the tidal volume by delivery of fresh gas flow only during exhalation. Lastly, the inspiratory phase of the ventilator-fresh gas flow may be diverted through a decoupling valve into the breathing bag, which is excluded from the circle system during ventilation. During exhalation the decoupling valve opens, allowing the fresh gas that was temporarily stored in the bag to enter the breathing circuit.

what is the most common risk factor for surgical fires? what are other potential contributors to OR fires and explosions?

Likely the most common risk factor for surgical fire relates to the open delivery of oxygen. other risks = flammable agnets -alcohol, chlorhexadine, surgical drapes and gowns, sutures, mesh, latex and plastic products, ETTs, masks, cannulas, tubing, interstitial gases, hair gases supporting combustion - oxygen, N2O ignition sources - lasers, bovies, fiberoptic light sources, drills and burrs, external defibrillators

what is the problem with long breathing tubes with a high compliance?

Long breathing tubes with high compliance increase the difference between the volume of gas delivered to a circuit by a reservoir bag or ventilator and the volume actually delivered to the patient. The compliance of the breathing tubes largely determines the compliance of the circuit. (Compliance is defined as the change of volume produced by a change in pressure.) For example, if a breathing circuit with a compliance of 8 mL gas/cmH2O is pressurized during delivery of a tidal volume to 20 cm H2O, 160 mL of the tidal volume will be lost to the circuit. The 160 mL represent a combination of gas compression and breathing-tube expansion. This is an important consideration in any circuit delivering positive-pressure ventilation through breathing tubes (eg, circle systems).

The disadvantages of the piston design include:

Loss of the familiar visible behavior of a standing bellows during disconnects, or when the patient is breathing over and above the ventilator settings. Quiet (less easy to hear regular cycling) The piston ventilator design cannot easily accommodate nonrebreathing circuits (although the same can be validly argued in regards to any newer model breathing circuit). Potential for NEEP

what is the difference between the flow of oxygen versus the flow of other gases through the anesthesia machine?

Whereas the oxygen supply can pass directly to its flow control valve, nitrous oxide, air, and other gases must first pass through safety devices before reaching their respective flow control valves. These devices permit the flow of other gases only if there is sufficient oxygen pressure in the safety device and help prevent accidental delivery of a hypoxic mixture in the event of oxygen supply failure. Thus in addition to supplying the oxygen flow control valve, oxygen from the common inlet pathway is used to pressurize safety devices, oxygen flush valves, and ventilator power outlets (in some models). Safety devices sense oxygen pressure via a small "piloting pressure" line that may be derived from the gas inlet or secondary regulator. In some anesthesia machine designs (eg, Datex-Ohmeda Excel), if the piloting pressure line falls below a threshold (eg, 20 psig), the shut-off valves close, preventing the administration of any other gases. The terms fail-safe and nitrous cut-off were previously used for the nitrous oxide shut-off valve.

what prevents rebreathing of CO2 at low flows in the circle system?

With an absorber, the circle system prevents rebreathing of carbon dioxide at fresh gas flows that are considered low (fresh gas flow ≤ 1 L) or even fresh gas flows equal to the uptake of anesthetic gases and oxygen by the patient and the circuit itself (closed-system anesthesia). At fresh gas flows greater than 5 L/min, rebreathing is so minimal that a CO2 absorber is usually unnecessary. With low fresh gas flows, concentrations of oxygen and inhalation anesthetics can vary markedly between fresh gas (ie, gas in the fresh gas inlet) and inspired gas (ie, gas in the inspiratory limb of the breathing tubes). The latter is a mixture of fresh gas and exhaled gas that has passed through the absorber. The greater the fresh gas flow rate, the less time it will take for a change in fresh gas anesthetic concentration to be reflected in a change in inspired gas anesthetic concentration. Higher flows speed induction and recovery, compensate for leaks in the circuit, and decrease the risks of unanticipated gas mixtures.

describe the PSI and L of a full tank and the proper pin position air N2O O2

air - 1900 PSI, 625L, 1-5 O2 - 1900 PSI, 660L, 2-5 N2O - 745 PSI, 1590 L, 3-5

circle system advantages

constant inspired concentrations conserve respiratory heat and humidity useful for all ages (may use down to 10 kg, about one year of age, or less with a pediatric disposable circuit) useful for closed system or low-flow low resistance (less than tracheal tube, but more than a NRB circuit)

The _____ the soda lime, the more likely it will absorb and degrade volatile anesthetics.

drier Soda lime is the more common absorbent and is capable of absorbing up to 23 L of CO2 per 100 g of adsorbent. It consists primarily of calcium hydroxide (80%), along with sodium hydroxide, water, and a small amount of potassium hydroxide. Absorbent granules can absorb and later release medically important amounts of volatile anesthetic. This property can be responsible for modest delays of induction or emergence.

circle system disadvantages

increased dead space malfunctions of unidirectional valves

Factors contributing to a discrepancy between set and delivered tidal volumes (during volume controlled ventilation) are especially acute in pediatrics and include

large compression volume of the circle system relative to the infant's lung volume leaks around uncuffed endotracheal tubes effects of fresh gas flow on delivered tidal volume mechanical difficulty of setting a small tidal volume using an adult bellows asembly

how can a hypoxic guard system create a hypoxic mixture?

remember the alternate term proportioning system, and hypoxic guard systems CAN permit hypoxic breathing mixtures IF: wrong supply gas in oxygen pipeline or cylinder, defective pneumatic, mechanical, or electronic components, leaks exist downstream of flow control valves, or if third inert gas (such as helium) is used (not true of electronic gas mixers).

if the surgical site is close to the airway and a flammable tube is present, what should be done before use of an ignition device such as a bovie or laser?

the oxygen concentration should be reduced for a sufficient period of time before use of an ignition device (eg, laser or cautery) to allow reduction of oxygen concentration at the site. Laser airway surgery should incorporate either jet ventilation without an endotracheal tube or the appropriate protective tube specific for the wavelength of the laser.

vaporizers are agent specific. what would happen if the vaporizer was filled with the wrong agent?

unintentionally filling a sevoflurane-specific vaporizer with halothane could lead to an anesthetic overdose. First, halothane higher vapor pressure (243 mm Hg versus 157 mm Hg) will cause a 40% greater amount of anesthetic vapor to be released. Second, halothane is more than twice as potent as sevoflurane (MAC 0.75 versus. 2.0). Conversely, filling a halothane vaporizer with sevoflurane will cause an anesthetic underdosage

Administration of oxygen to concentrations of greater than ______% should be guided by clinical presentation of the patient and not solely by protocols or habits. This is of prime importance when the surgical site is above the level of the _______.

30% If oxygen is being delivered via nasal cannula or face mask, and if increased oxygen levels are needed, then the airway should be secured by either endotracheal tube or supraglottic device. This is of prime importance when the surgical site is above the level of the xiphoid.

describe the variable bypass vaporizer how do they combat temperature changes?

Turning a single calibrated control knob counter-clockwise to the desired percentage diverts an appropriate small fraction of the total gas flow into the carrier gas, which flows over the liquid anesthetic in a vaporizing chamber, leaving the balance to exit the vaporizer unchanged Because some of the entering gas is never exposed to anesthetic liquid, this type of agent-specific vaporizer is also known as a variable-bypass vaporizer. Temperature compensation is achieved by a strip composed of two different metals welded together. The metal strips expand and contract differently in response to temperature changes. When the temperature decreases, differential contraction causes the strip to bend allowing more gas to pass through the vaporizer. Such bimetallic strips are also used in home thermostats. As the temperature rises differential expansion causes the strip to bend the other way restricting gas flow into the vaporizer. Altering total fresh gas flow rates within a wide range does not significantly affect anesthetic concentration because the same proportion of gas is exposed to the liquid. However, the real output of an agent would be lower than the dial setting at extremely high flow (>15 L/min); the converse is true when the flow rate is less than 250 mL/min.

when should the oxygen flush valve not be used and why?

Use of the oxygen flush valve during the inspiratory cycle of a ventilator must be avoided because the ventilator spill valve will be closed and the adjustable pressure-limiting (APL) valve is excluded; the surge of oxygen (600-1200 mL/s) and circuit pressure will be transferred to the patient's lungs. Intermittent or sustained high inspiratory pressures (>30 mm Hg) during positive-pressure ventilation increase the risk of pulmonary barotrauma (eg, pneumothorax) or hemodynamic compromise, or both, during anesthesia. Excessively high pressures may arise from incorrect settings on the ventilator, ventilator malfunction, fresh gas flow coupling (above), or activation of the oxygen flush during the inspiratory phase of the ventilator.

what is hypoxic guard system?

"Proportioning Systems" is the board exam terminology for the hypoxic guard system. These systems link nitrous oxide and oxygen flows (mechanically, pneumatically, or electronically) to prevent final inspired oxygen concentration less than 0.25. The older Ohmeda Link 25 is a purely mechanical system: A chain links nitrous oxide and oxygen flow control knobs, allows either to be adjusted independently, yet automatically intercedes to maintain a minimum 1:3 ratio of oxygen to nitrous oxide. Also, older Ohmeda machines (Modulus, Excel) supply nitrous oxide to its flow control valve at 26 psi, via a second-stage pressure regulator. Therefore, the system has pneumatic and mechanical components in its control of gas mixture. Dräger S-ORC (newer hypoxic guard system as found on Fabius GS). Resistors generate back pressure on a control diaphragm, in proportion to oxygen and nitrous oxide flows. S-ORC guarantees a minimum FIO2 of 21% by limiting nitrous oxide flow. Newest workstations with electronic gas mixers (Aisys, Avance, Perseus) supply a minimum FIO2 of 25% with nitrous oxide, but will allow 21% if the carrier gas is air only. The hypoxic guard system includes desflurane on workstations with electronic gas mixers. These sytems usually include a backup knob and glass flowtube, because they will not function if electric power fails.

What should you do if you lose oxygen pipeline pressure?

1. Open the emergency oxygen cylinder fully (not just the three or four quick turns used for checking) 2. Disconnect the pipeline connection at the wall Why? Something is wrong with the oxygen pipeline. What if the supply problem evolves into a non-oxygen gas in the oxygen pipeline? If so, it will flow (pipeline pressure 50 psi) rather than your oxygen cylinder source (down-regulated to 45 psi). If you are lucky, the oxygen alarm will sound to warn you of the change (you do leave your alarms enabled, don't you?). If for some reason the oxygen analyzer does not warn of the crossover, the pulse oximeter will- but only after the oxygen has been washed out by ventilation of the patient's functional residual capacity and vessel-rich group. So disconnect the pipeline connection at the wall if oxygen pipeline pressure is lost. It's also easier to remember one strategy which works for any problem with the pipeline, than to remember that sometimes you must, and sometimes it is optional, to disconnect. And use that oxygen analyzer always! 3.Ventilate by hand with the anesthesia breathing circuit, rather than with the mechanical ventilator (which uses cylinder oxygen for the driving gas if the pipeline is unavailable) or a bag-valve-mask (Ambu), because staying with the anesthesia breathing circuit means you can still deliver volatile agent.

describe the mapleson breathing circuit

All non-rebreathing (NRB) circuits lack unidirectional valves and soda lime carbon dioxide absorption: thus, the amount of rebreathing is highly dependent on fresh gas flow (FGF) in all. Work of breathing is low in all (no unidirectional valves or soda lime granules to create resistance). Minimum FGF In practice, most anesthetists will provide a minimum 5 L/min for children on up to adults to prevent rebreathing (or 2-3 x minute ventilation [VE], whichever is greater). The Bain circuit is a "coaxial" Mapleson D- the same components, but the fresh gas flow tubing is directed within the inspiratory limb, with fresh gas entering the circuit near the mask. Fresh gas flow requirements are similar to other NRB circuits. The Bain has been shown to add more heat and humidity to inhaled gases than other Mapleson circuits. How do NRB's work? During expiration, fresh gas flow (FGF) pushes exhaled gas down the expiratory limb, where it collects in the reservoir (breathing) bag and opens the expiratory valve (pop-off or APL). The next inspiration draws on the gas in the expiratory limb. The expiratory limb will have less carbon dioxide (less rebreathing) if FGF inflow is high, tidal volume (VT) is low, and the duration of the expiratory pause is long (a long expiratory pause is desirable as exhaled gas will be flushed more thoroughly). All NRB circuits are convenient, lightweight, easily scavenged. One objection is that the circuit must be reconfigured between cases, with the possibility of error.

how is linking nitrous oxide flow gas and oxygen a safety feature of the anesthesia machine?

Another safety feature of anesthesia machines is a linkage of the nitrous oxide gas flow to the oxygen gas flow; this arrangement helps ensure a minimum oxygen concentration of 25%. The oxygen/nitrous oxide ratio controller links the two flow valves either pneumatically or mechanically. It should be noted that this safety device does not affect the flow of a third gas (eg, air, helium, or carbon dioxide).

what is wood's metal and how does it work?

As a safety feature of oxygen E-cylinders, the yoke has integral components made from Wood's metal. This metallurgic alloy has a low melting point, which allows dissipation of pressure that might otherwise heat the bottle to the point of ballistic explosion.

which mapleson circuit is the best for spontaneous ventialtion? why?

Because a fresh gas flow equal to minute ventilation is sufficient to prevent rebreathing, the Mapleson A design is the most efficient Mapleson circuit for spontaneous ventilation. Mapleson circuits are lightweight, inexpensive, and simple. Breathing-circuit efficiency is measured by the fresh gas flow required to reduce CO2 rebreathing to a negligible value. Because there are no unidirectional valves for CO2 absorption in Mapleson circuits, rebreathing is prevented by adequate fresh gas flow into the circuit and venting exhaled gas through the APL valve before inspiration. There is usually some rebreathing in any Mapleson circuit. The total fresh gas flow into the circuit controls the amount. To attenuate rebreathing, high fresh gas flows are required.

why might a N2O tank freeze?

Because energy is consumed in the conversion of a liquid to a gas (the latent heat of vaporization), the liquid nitrous oxide cools. The drop in temperature results in a lower vapor pressure and lower cylinder pressure. The cooling is so pronounced at high flow rates that there is often frost on the tank, and pressure regulators may freeze.

where does apparatus dead space begin in a circle system? why?

Because of the unidirectional valves, apparatus dead space in a circle system is limited to the area distal to the point of inspiratory and expiratory gas mixing at the Y-piece. Unlike Mapleson circuits, the circle system tube length does not directly affect dead space. That part of a tidal volume that does not undergo alveolar ventilation is referred to as dead space. Mechanical dead space ends at the point where inspired and expired gas streams diverge (the Y-connector). Unlike Mapleson circuits, the circle system tube length does not affect dead space. Like Mapleson circuits, length does affect circuit compliance and thus the amount of tidal volume lost to the circuit during positive-pressure ventilation.

what contributes to the tidal volume delivered to the pt?

Because the ventilator spill valve is closed during inspiration, fresh gas flow from the machine's common gas outlet normally contributes to the tidal volume delivered to the patient. it is important to appreciate that because the ventilator spill valve is closed during inspiration, fresh gas flow from the machine's common gas outlet normally contributes to the tidal volume delivered to the patient. For example, if the fresh gas flow is 6 L/min, the I:E ratio is 1:2, and the respiratory rate is 10 breaths/min, each tidal volume will include an extra 200 mL in addition to the ventilator's output: 6000 ml/min x 33%/10 breaths per min = 200 ml/breath

what happens if there is malfunction of either unidirectional valve in a circle system?

Malfunction of either unidirectional valve in a circle system may allow rebreathing of carbon dioxide, resulting in hypercapnia. Unidirectional valves, which function as check valves, contain a ceramic or mica disk resting horizontally on an annular valve seat Forward flow displaces the disk upward, permitting the gas to proceed through the circuit. Reverse flow pushes the disk against its seat, preventing reflux. Valve incompetence is usually due to a warped disk or seat irregularities. The expiratory valve is exposed to the humidity of alveolar gas. Condensation and resultant moisture formation may prevent upward displacement of the disks, resulting in incomplete escape of expired gases and rebreathing. Inhalation opens the inspiratory valve, allowing the patient to breathe a mixture of fresh and exhaled gas that has passed through the CO2 absorber. Simultaneously, the expiratory valve closes to prevent rebreathing of exhaled gas that still contains CO2. The subsequent flow of gas away from the patient during exhalation opens the expiratory valve. This gas is vented through the APL valve or re breathed by the patient after passing through the absorber. Closure of the inspiratory valve during exhalation prevents expiratory gas from mixing with fresh gas in the inspiratory limb.

what are the most frequent causes of equipment related adverse outcomes?

Misuse of anesthesia gas delivery systems is three times more likely than failure of the device to cause equipment-related adverse outcomes. An operator's lack of familiarity with the equipment or a failure to check machine function, or both, are the most frequent causes. These mishaps account for only about 2% of cases in the ASA Closed Claims Project database. The breathing circuit was the most common single source of injury (39%); nearly all damaging events were related to misconnects or disconnects.

what factors can make the PISS ineffective?

Multiple washers placed between the cylinder and yoke, which prevent proper engagement of the pins and holes, have unintentionally defeated this system. The pin index safety system is also ineffective if yoke pins are damaged or the cylinder is filled with the wrong gas.

Storage, handling and installation of the cylinder

Never stand upright without support. Use only aluminum cylinders in an MRI suite. Use only one washer, or PISS system will be defeated. Do not oil valve. Protect the valve when transporting. To install Check & remove labels Hold valve away from face, and "crack" valve Place in hanger yoke Observe for appropriate pressure and lack of audible leak. Leave cylinders on machine closed. Don't leave empty cylinders on the machine.

what are the five tasks of oxygen in the anesthesia machine?

Oxygen has five "tasks" in the AGM; it powers the ventilator driving gas flush valve oxygen pressure failure alarm oxygen pressure sensor shut-off valve ("fail-safe") flowmeters. It also plays a role in the hypoxic guard system, which maintains the correct proportion between flows of oxygen and nitrous oxide.

describe the power source for a piston vent

Piston ventilators (Apollo, Fabius GS) do not require driving gas. They are driven by compression from an electric motor to move a piston, creating pressure which moves the gas within the piston to the patient's lungs. Thus, piston or turbine ventilators are economical of wall oxygen (only using it for metabolic needs, and not to compress a bellows). Piston ventilators use an electric motor to drive a piston, which compresses gas in the breathing circuit, creating the motive force for mechanical ventilator inspiration to proceed. The motor's force compresses the gas within the piston, raising the pressure within it, which causes gas to flow into the patient's lungs. Thus a piston ventilator uses no driving gas, and may be used without depleting the oxygen cylinder in case of oxygen pipeline failure. Compliance losses are significantly decreased by omitting the compressible bellows. If the corrugated limbs are expanded before the morning electronic self-test of compliance and leaks, the compliance losses out to the Y-piece are known. Thereafter, all that is required is a pressure sensor anywhere in the breathing circuit to deliver accurate VT. There is no longer a need for flow sensors close to the patient- where they are susceptible to errors due to humidity (e.g. ADU D-lite sensor).

why are Mapleson A circuits not ideal for controlled ventilation?

Positive pressure during controlled ventilation, however, requires a partially closed APL valve. Although some alveolar and fresh gas exits through the valve during inspiration, no gas is vented during expiration, since the exhaled gas stagnates during the expiratory phase of positive pressure ventilation. As a result, very high fresh gas flows (greater than three times minute ventilation) are required to prevent rebreathing with a Mapleson A circuit during controlled ventilation.

piston advantages

Quiet No PEEP (2-3 cm water are mandatory on standing bellows ventilators due to the design of the ventilator spill valve) Greater precision in delivered tidal volume due to compliance and leak compensation, fresh gas decoupling, and the rigid piston design. There are less compliance losses with a piston as compared to a flexible standing bellows compressed by driving gas. Measuring compliance and leaks with a transducer near the piston eliminates a bulky, costly sensor close to the patient's airway (such as the D-Lite sensor on the ADU). Electricity is the driving force for the piston, so if oxygen pipeline pressure fails and one must rely on oxygen from the emergency cylinder, mechanical ventilation may continue (without exhausting the cylinder oxygen simply to drive the bellows).

what is the fire triad? what situations are considered high fire risk?

Situations classified as carrying a high risk for a surgical fire are those that involve an ignition source used in close proximity to an oxidizer. The simple chemical combination required for any fire is commonly referred to as the fire triad or fire triangle. The triad is composed of fuel, oxidizer, and ignition source

what should you do if there is an airway fire?

The sequence of stopping gas flow and removal of the endotracheal tube when fire occurs in the airway is not as important as ensuring that both actions are performed quickly. should a fire occur in the operating room it is important to determine whether the fire is located on the patient, in the airway, or elsewhere in the operating room. For fires occurring in the airway, the delivery of fresh gases to the patient must be stopped. Effective means of stopping fresh gases to the patient can be accomplished by turning off flowmeters, disconnecting the circuit from the machine, or disconnecting the circuit from the endotracheal tube. The endotracheal tube should be removed and either sterile water or saline should be poured into the airway to extinguish any burning embers After these actions are carried out, ventilation may be resumed, preferably using room air and avoiding oxygen or nitrous oxide- enriched gases. The tube should be examined for missing pieces. The airway should be reestablished and, if indicated, examined with a bronchoscope. Treatment for smoke inhalation and possible transfer to a burn center should also be considered.

describe insufflation

The term insufflation usually denotes the blowing of anesthetic gases across a patient's face. Although insufflation is categorized as a breathing system, it is perhaps better considered a technique that avoids direct connection between a breathing circuit and a patient's airway. Because children often resist the placement of a face mask (or an intravenous line), insufflation is particularly valuable during inductions with inhalation anesthetics in children It is useful in other situations as well. Carbon dioxide accumulation under head and neck draping is a hazard of ophthalmic surgery performed with local anesthesia. Insufflation of air across the patient's face at a high flow rate (>10 L/min) avoids this problem, while not increasing the risk of fire from accumulation of oxygen insufflation can also be used to maintain arterial oxygenation during brief periods of apnea (eg, during bronchoscopy). Instead of blowing gases across the face, oxygen is directed into the lungs through a device placed in the trachea.

what is the pressure relief valve on the cylinder?

This pressure-relief "valve" is designed to rupture at 3300 psig, well below the pressure E-cylinder walls should be able to withstand (more than 5000 psig).

what is the name of the safety system to avoid misconnections of cylinders?

To discourage incorrect cylinder attachments, cylinder manufacturers have adopted a pin index safety system.

what is the line isolation monitor?

To reduce the chance of two coexisting faults, a line isolation monitor measures the potential for current flow from the isolated power supply to the ground. Basically, the line isolation monitor determines the degree of isolation between the two power wires and the ground and predicts the amount of current that could flow if a second short circuit were to develop. An alarm is activated if an unacceptably high current flow to the ground becomes possible (usually 2 mA or 5 mA), but power is not interrupted unless a ground-fault circuit interrupter is also activated --> usually not installed in locations such as operating rooms, where discontinuation of life support systems (eg, cardiopulmonary bypass machine) is more hazardous than the risk of electrical shock. Since the line isolation monitor alarms when the sum of leakage current exceeds the set threshold, the last piece of equipment is usually the defective one; however, if this item is life-sustaining, other equipment can be removed from the circuit to evaluate whether the life safety item is truly at fault.

Operating principles of variable bypass vaporizers

Total fresh gas flow (FGF) enters and splits into carrier gas (much less than 20%, which becomes enriched- saturated, actually- with vapor) and bypass gas (more than 80%). These two flows rejoin at the vaporizer outlet. The splitting ratio of these two flows depends on the ratio of resistances to their flow, which is controlled by the concentration control dial, and the automatic temperature compensation valve.

describe the ventilators found on the anesthesia machines

Traditionally ventilators on anesthesia machines have a double-circuit system design and are pneumatically powered and electronically controlled Newer machines also incorporate microprocessor control that relies on sophisticated pressure and flow sensors. This Feature allows multiple ventilatory modes, electronic PEEP, tidal volume modulation, and enhanced safety features. Some anesthesia machines have ventilators that use a single-circuit piston design

describe the physical principles of the vaporizer

Vapor pressure Molecules escape from a volatile liquid to the vapor phase, creating a "saturated vapor pressure" at equilibrium. Vapor pressure (VP) increases with temperature. VP is independent of atmospheric pressure, it depends only on the physical characteristics of the liquid, and its temperature. So, even although evaporation proceeds at a rate governed by liquid temperature and is independent of altitude (barometric pressure), individual vaporizer types may or may not function the same at altitude. Latent heat of vaporization is the number of calories needed to convert 1 g of liquid to vapor, without temperature change in the remaining liquid. Thus, the temperature of the remaining liquid will drop as vaporization proceeds, lowering VP, unless this is prevented. Specific heat is the number of calories needed to increase the temperature of 1 g of a substance by 1 degree C. Manufacturers select materials for vaporizer construction with high specific heats to minimize temperature changes associated with vaporization. Thermal conductivity - a measure of how fast a substance transmits heat. High thermal conductivity is desirable in vaporizer construction.

describe the following terms variable bypass flow over temp compensated agent specific out of circuit

Variable bypass Fresh gas flow from the flowmeters enters the inlet of any vaporizer which is on. The concentration control dial setting splits this stream into bypass gas (which does not enter the vaporizing chamber), and carrier gas (also called chamber flow, which flows over the liquid agent). Flow over Carrier gas flows over the surface of the liquid volatile agent in the vaporizing chamber, as opposed to bubbling up through it (as in the older copper kettle and Vernitrol) Temperature compensated Equipped with automatic devices that ensure steady vaporizer output over a wide range of ambient temperatures Agent-specific Only calibrated for a single gas, usually with keyed fillers that decrease the likelihood of filling the vaporizer with the wrong agent Out of circuit Out of the breathing circuit, as opposed to (much) older models such as the Ohio #8 (Boyle's bottle) which were inserted within the circle system.

Volatile anesthetics can be broken down to ________ by dry absorbent

Volatile anesthetics can be broken down to carbon monoxide by dry absorbent (eg, sodium or potassium hydroxide) to such a degree that it is capable of causing clinically significant carbon monoxide poisoning. The formation of carbon canisters permit more complete CO2 absorption, less frequent absorbent changes, and lower gas flow resistance.

what is the point of waste gas scavengers

Waste-gas scavengers dispose of gases that have been vented from the breathing circuit by the APL valve and ventilator spill valve. Pollution of the operating room environment with anesthetic gases may pose a health hazard to surgical personnel.