Memory Masters : Anesthesia Equipment Part Two – Flashcards

The volatile agents that are delivered from variable-bypass vaporizers are isoflurane, enflurane, halothane, and sevoflurane. Modern day variable-bypass vaporizers are: (1) agent specific, (2) temperature-compensated, and (3) flow over (carrier gas flows over anesthetic liquid in the vaporizing chamber). The gas delivered to the va riable·bypass vaporizer is divided into carrier gas, which flows over liquid anesthetic in the vaporizing chamber, and non·carrier gas, which leaves the vaporizer unchanged. Because the gas flowing into the modern day vaporizer is divided into two streams, it is also known as a variable-bypass vaporizer.

Variable bypass vaporizers should be located outside the circle system, between the flow meters and common gas outlet. This placement lessens the likelihood of concentration surges during use of the oxygen flush valve.

The Drager Vapor 20n gas machine has a transport ("T") dial setting the helps prevent tipping·related problems. This function is provided by the vaporizer cassette systems of other modern gas machines.

The keyed filling port system on modern vaporizers prevents filling with the incorrect agent.

Unlike most contemporary vaporizers, the Tec 6 desflurane vaporizer is not a variable-bypass vaporizer. The Tec 6 vaporizer is best called a dual gas blender vaporizer.

Desflurane is delivered from a heated (39 degrees C) and pressurized (1300 mmHg) vaporizer. The desflurane vaporizer is not a flow-over vaporizer.

1) Desflurane's vapor pressure of 669 mm Hg is near atmospheric pressure, so it almost boils at sea level; (2) desflurane is only one-fifth as potent as the other volatile agents, so a relatively large volume of vapor must be delivered to the patient.

The Tec 6 vaporizer is an electrically heated, thermostatically controlled, constant-temperature, pressurized, electromechanically coupled dual circuit, gas-vapor blender. The pressure in the vapor circuit is electronically regulated to equal the pressure in the fresh gas circuit. At a constant fresh gas flow rate, the operator regulates vapor flow using a conventional concentration control dial. When the fresh gas flow rate increases, the working pressure increases proportionally. For a given concentration setting even when varying the fresh gas flow rate, the vaporizer output is constant because the amount of flow through each circuit remains proportional.

Because the Tec 6 vaporizer is a dual-gas blender, the Tee 6 will maintain a constant concentration of vapor output (% v/v), not a constant partial pressure, regardless of ambient pressure. This means that at high altitudes, the partial pressure of desflurane (P des) will be decreased in proportion to the atmospheric pressure. The Tec 6 vaporizer requires manual adjustment of the concentration control dial at altitudes other than sea level to maintain a constant (P des).

In the TEC 6 vaporizer, desflurane is electrically heated in a sump that is upstream of both the common outlet and shut-off valve. Heating of desflurane to 39°C creates a saturated vapor pressure of 2 atmospheres, which drives the agent towards the fresh gas flow. In contrast to other vaporizers, no fresh gas flow goes through the desflurane pump, i.e., fresh gas flow never comes in contact with liquid desflurane.

The Tec 6 desflurane vaporizer does not automatically compensate for changes in elevation. The concentration of desflurane is unaffected by elevation, but the partial pressure decreases. At high elevations, the anesthetist must increase the concentration on the control dial to raise the partial pressure to the desired level. Since the partial pressure of agent delivered to the patient is what counts, you would need to increase the concentration setting when you are in the mountains so as to deliver an equivalent partial pressure of desflurane to the patient.

If you set the dial on the Tec 6 at 5% and you are at sea level, the partial pressure of desflurane going to the patient is 0.05 x 760 mm Hg = 38 mm Hg (Dalton's law of partial pressures permits this calculation); if you go up into the mountains where the total atmospheric pressure is. let's say. 600 mm Hg, the partial pressure of desflurane delivered to the patient when you set the Tec 6 dial at 5% is 0.05 x 600 mm Hg = 30 mm Hg. These calculations explain why setting the dial at 5% results in lighter anesthesia in the mountains than it does at sea level The partial pressure going to the patient for a given dial setting is lower in the mountains.

Early vaporizer designs were susceptible to a pumping effect: intermittently fluctuating pressure in the breathing system, such as that generated by positive pressure ventilation or by intermittent pressing and releasing of the O2 flush valve, caused fluctuating back pressure to be transmitted into the low-pressure system. The "pumping effect" is more pronounced at low flow rates. The result of this pumping effect was an increased concentration of anesthetic delivered.

New anesthetic vaporizers incorporate mechanisms that decrease the size of the vaporizing chamber relative to the bypass channel and increase the volume of the inflow channel. Vapor-saturated gas cannot make its way back into the bypass channel. and thus the pumping effect is prevented.

(l) Vaporizer filled with incorrect anesthetic. (2) bypass chamber contaminated with liquid anesthetic if the vaporizer is tipped, (3) simultaneous administration of two anesthetics if the interlock mechanism is not operative, (4) development ofleaks, (5) contaminated liquid anesthetic placed into the vaporizer, and (6) overfilling the vaporizer.

Contemporary vaporizers are secured to the anesthesia machine in manifolds that hold 2-3 units. The interlock system, also known as thevaporizer exclusion system. prevents more than one vaporizer from being turned on at a time. In other words, the operator is prevented from delivering more than anyone agent simultaneously. The interlock system also ensures that all vaporizers are locked in such that leaks are decreased. and trace vapor output is minimal when the vaporizer is off.

Most Ohmeda machines have machine outlet check valves.

Check valves, also called unidirectional or one-way valves. prevent retrograde flow (back flow) during positive pressure ventilation, therefore minimizing the effects of downstream intermittent pressure fluctuations on inhaled anesthetic concentration.

The anesthesia machine check valve: (1) prevents "back-flow" from high pressure to low-pressure sides (prevents "pumping action" of gases). (2) allows for an empty cylinder to be exchanged for a full one with minimal loss of gas, and (3) minimizes leakage from an open cylinder to the atmosphere.

The machine outlet check valve is located downstream from the vaporizers and upstream from the oxygen flush. The machine outlet check valve is open in the absence of back pressure. Gas flows freely to the common outlet. The valve closes when back pressure is exerted on it. Back pressure is created by intermittent positive pressure and oxygen flushing.

In the case of cylinders that are double-yoked, the check valve prevents transfer of gas from the cylinder with higher pressure to the cylinder with lower pressure. In addition , one of the double-yoked cylinders can be changed while the other is in use.

Negative and positive pressure relief valves protect the patient from the negative pressure of the vacuum system or positive pressure from an obstruction in the disposal tubing.

The fail-safe valve prevents the delivery of hypoxic gas mixtures from the machine in the event of failure of the oxygen supply. The fail-safe valve goes by many other names- the oxygen failure safety valve, oxygen failure safety device, low-pressure guard ian system, oxygen failure protection device. pressure sensor shutoff system or valve, pressure sensor system, and nitrous oxide shut off valve.

Line pressures ofless than 30 psi will usually close the flow of all gases, except oxygen, to the common gas outlet.

Oxygen.

The oxygen pressure fell below 25-30 psi. When oxygen pressure falls below 25-30 psi (roughly 50% of normal), a fail-safe valve automatically closes the nitrous oxide and other gas lines to prevent accidental delivery of a hypoxic gas mixture to the patient. A gas whistle or electric alarm sounds to alert the anesthetist to this occurrence.

When the oxygen low-pressure alarm sounds-indicating profound loss of O2 pipeline pressure-fully open the E cylinder, disconnect the pipeline, and consider use oflow fresh gas flows.

The scavenging system needs to be checked daily as outlined by the FDA in 1993.

1) The gas-collecting system; (2) the transfer tubing; (3) the scavenging interface; (4) the gas disposal tubing; and (5) an active or passive gas disposal assembly.

Waste gas scavengers dispose of gases that have been vented from the breathing circuit by a pressure release valve located in the breathing circuit or the ventilator. Either of these valves is connected to hoses leading to the scavenger system.

The outlet of the scavenging system can be a direct line to the outside (passive scavenging), or a connection to the hospital's vacuum system (active scavenging).

When a scavenging system malfunctions or is misused, positive or negative pressure can be transmitted to the breathing system. This is more likely to occur with dosed interfaces.

10-15 liters/minute.

One relief valve is for negative pressure and one is for positive pressure.

If flow of waste gases into the vacuum source is insufficient and the reservoir bag distends. the positive pressure valve opens and vents some of the exhaled gases into room.

If the flow of waste gases into the vacuum system is too high and the bag collapses, the negative valve opens and lets in room air.

Excessive suction manifests as collapse of the gas reservoir bag. Inadequate suction allows excessive pressure in the breathing system leading to barotrauma.

A proportioning system on the anesthesia workstation is a hypoxia prevention safety device. Manufacturers equip anesthesia workstations with proportioning systems in an attempt to prevent creation and delivery of a hypoxic mixture. Nitrous oxide and oxygen are mechanically and/or pneumatically linked so that the minimum oxygen concentration at the common gas outlet is between 23 to 25% depending on manufacturer.

The Link-25 system is found on conventional Datex-Ohmeda machines. The heart of the system is the mechanical integration of the nitrous oxide and oxygen flow control valves. It allows independent adjustment of either valve. yet automatically intercedes to maintain a minimum 25% oxygen concentration with a maximum nitrous oxide-oxygen flow ratio of 3:1. The Link-25 automatically increases oxygen flow to prevent delivery of a hypoxic mixture. A 14-tooth sprocket is attached to the nitrous oxide flow control valve and a 28-tooth sprocket is attached to the oxygen flow control valve. A chain physically links the sprockets. When the nitrous oxide flow control valve is turned through two revolutions. or 28 teeth, the oxygen flow control valve will revolve once because of the 2: 1 gear ratio. The final 3:1 flow ratio results because the nitrous oxide flow control valve is supplied by approximately 26 psig, whereas the oxygen flow control valve is supplied by 14 psig. Thus, the combination of the mechanical and pneumatic aspects of the system yields the final oxygen concentration. The Link-25 proportioning system can be thought of as a system that increases oxygen flow when necessary to prevent delivery of a fresh gas mixture with oxygen concentration ofless than 25%.

The following five situations can lead to delivery of hypoxic gas mixtures on workstations equipped with proportioning systems: (I) wrong supply gas, (2) defective pneumatics or mechanics. (3) leaks downstream, (4)inert gas administration, and (5) dilution of inspired oxygen concentration by volatile inhaled anesthetics.

An inert third gas, such as He, N₂or C0₂, can cause delivery of a hypoxic mixture because contemporary proportioning systems link only nitrous oxide and oxygen.Use of an oxygen analyzer is mandatory (or referentially a multigas analyzer) if the operator uses a third gas.

Components found in the low pressure sys tem include: (1) flow indicators, (2)vaporizers, (3)vaporizer circuit control valves, (4) back pressure safety devices, (5) low-pressure safety devices, and (6) the common gas outlet.

To comply with ASTM FIBSO-DO (2000), newly manufactured anesthesia machines must have monitors that display the following parameters: (1) continuous breathing system pressure, (2) exhaled tidal volume, (3) ventilatory carbon dioxide concentration, (4) anesthetic vapor concentration, (5) inspired oxygen concentration, (6) arterial oxygen concentration, (7)oxygen supply pressure, (8) arterial blood pressure, and (9) continuous electrocardiogram.

Anesthesia machines with check valves should be tested with a negative pressure leak test. In 1993, the FDA implemented a universal negative pressure leak test that can be used on all contemporary anesthesia machines, regardless of the presence or absence of check valves.

To perform a negative-pressure leak test, a suction bulb is attached to the common fresh gas outlet and squeezed repeatedly until the bulb is fullycollapsed. The anesthesia machine is leak-free is the bulb remains collapsed for at least 10 seconds.

35-75 Iiters/min.

At least each day before the first case.

A damaged or defective flush valve can Stick in the fully open position, causing barotrauma.

The concern in this situation is the presence of trace amounts of volatile agents in Ihe rubber and plastic components of the gas machine and in the ventilator and CO2 absorber. The following 3 actions should be taken to prepare the gas machine for the patient with a known susceptibility to malignant hyperthermia. (1) The gas machine should be thoroughly flushed with 100% oxygen for at least IO minutes (to remove residual traces of volalile agents from rubber and plastic components in the machine. (2) The breathing circuits and CO₂canister should be replaced. (3) Vaporizers should be drained, inactivated, or removed.