Inhaled Anesthetic Delivery Systems

Golparvar M. MD

Introduction: the anesthesia delivery system has evolved

from a simple pneumatic device to a complex multisystem workstation

For anesthesia care providers understanding of its operation is essential.

Caplan and coworkers found that although claims related to the medical gas delivery system were rare, when they did occur, they were usually severe and death or permanent brain injury frequently resulted.

In this Lecture:

we examine the anesthesia workstation piece by piece.

We will describe the normal operation, function, and integration of major anesthesia workstation subsystems.

we illustrate potential problems and hazards associated with the various components of the anesthesia delivery system

We overview appropriate preoperative checks that may help detect and prevent such problems.

Standards and Pre-use Procedures

Anesthesia Workstation Standards and Pre-use Procedures

Standards for Anesthesia Machines and Workstations› 1979: American National Standards

Institute (ANSI) Z79.8-1979[2]› 1988: American Society for Testing and

Materials (ASTM) F1161-88[3]› 1994: ASTM F1161-94[4] (reapproved in

1994 and discontinued in 2000)› 2000: ASTM F1850-00[5]

2000 ASTM F1850-00 standards: Newly manufactured workstations must

have monitors that measure the following parameters: continuous breathing system pressure, exhaled tidal volume, ventilatory carbon dioxide concentration, anesthetic vapor concentration, FiO2, oxygen supply pressure, SpO2, BP, and EKG.

The anesthesia workstation must have a

prioritized alarm system that groups the alarms into three categories: high, medium, and low priority

Checking Your Anesthesia Workstation

A complete anesthesia apparatus checkout procedure must be performed each day before the anesthesia workstation is first used.

An abbreviated version should be performed before each subsequent case.

The user must always refer to the original equipment manufacturer's operator's manual for special procedures or precautions related to particular workstations.

Responsibility:

Every anesthesia care provider must be aware that the ultimate responsibility for proper machine pre-use safety checks rests on the provider using the machine to deliver anesthetic care.

Testing Specific Components of the Anesthesia Delivery System

The three most important preoperative checks are:

› (1) calibration of the oxygen analyzer

› (2) the low-pressure circuit leak test

› (3) the circle system tests

Calibration of the Oxygen Analyzer

It is the only machine safety device that evaluates the integrity of the low-pressure circuit in an ongoing fashion

The only machine monitor that detects problems downstream from the flow control valves is the oxygen analyzer

sensing element must be exposed to room air for calibration to 21%

Low-Pressure Circuit Leak Test

checks the integrity of the anesthesia machine from the flow control valves to the common gas outlet

Leaks in the low-pressure circuit can cause hypoxia and/or patient awareness

Loose filler caps on vaporizers are a common source of leaks Several different methods have been used:

› the oxygen flush test, › the common gas outlet occlusion test, › the traditional positive-pressure leak test, › the North American Dräger positive-pressure leak test, › the Ohmeda 8000 internal positive-pressure leak test, › the Ohmeda negative-pressure leak test, › the 1993 FDA universal negative-pressure leak test, › and others.

Oxygen Flush Positive-Pressure Leak Test

Inappropriate use of the oxygen flush valve to check the low-pressure circuit

1993 FDA Negative-Pressure Leak Test

Left, A negative-pressure leak-testing device is attached directly to the machine outlet. Squeezing the bulb creates a vacuum in the low-pressure circuit and opens the check valve.

Right, When a leak is present in the low-pressure circuit, room air is entrained through the leak and the suction bulb inflates

1993 FDA Negative-Pressure Leak Test

if the bulb re inflates in less than 10 seconds, a leak is present somewhere in the low-pressure circuit.

The “universal” negative-pressure leak test is the most sensitive of all contemporary leak tests because it is not dependent on volume

It can detect leaks as small as 30 mL/min

Circle System Tests The leak test is performed by closing the

pop-off valve, occluding the Y-piece, and pressurizing the circuit to 30 cm H2O with the oxygen flush valve. The value on the pressure gauge will not decline if the circle system is leak free

The flow test checks the integrity of the unidirectional valves: The operator should be able to inhale but not exhale through the inspiratory limb and able to exhale but not inhale through the expiratory limb.

Workstation Self-Tests

self-diagnostic tests varies from one model and manufacturer to another

to detect internal vaporizer leaks on this type of system, the “leak test” portion of the self-diagnostic must be repeated separately with each individual vaporizer turned to the “on” position

Anatomy of an Anesthesia Workstation

Diagram of a generic two-gas anesthesia machine

Supply Sources

Pipeline Supply Source

The pipeline supply: primary gas source

must supply: correct gases at appropriate pressure

In a survey: 31% reported difficulties with pipeline systems (The most common problem was inadequate oxygen pressure)

The most devastating reported hazard: accidental crossing of oxygen and nitrous oxide pipelines, (which has led to many deaths)

In the event that a pipeline crossover is ever suspected immediately:

› First, turned on the backup oxygen cylinder

› Second, disconnect pipeline supply

The pipeline inlet fittings are gas-specific Diameter Index Safety System (DISS)

Diameter Index Safety System (DISS)

Cylinder Supply Source

Cylinder Supply Source

Medical gases attached to the anesthesia machine via the hanger yoke assembly.

Each hanger yoke is equipped with the Pin Index Safety System (PISS).

The PISS is a safeguard introduced to eliminate cylinder interchanging

Pin-Indexed Yoke Assemblies and

Cylinder Valve Connections

Pin Index Safety System (PISS)

A check valve is located downstream from each cylinder and serves several functions. › First, it minimizes transfer of gas from a cylinder

at high pressure to one with lower pressure.› Second, it allows an empty cylinder to be

exchanged for a full one while gas flow continues from the other cylinder into the machine

› Third, it minimizes leakage from an open cylinder to the atmosphere if one cylinder is absent.

Each cylinder supply source has a pressure-reducing valve known as the cylinder pressure regulator

The oxygen cylinder pressure regulator reduces the oxygen cylinder pressure from a high of 2200 psig to approximately 45 psig.

The nitrous oxide cylinder pressure regulator receives pressure of up to 745 psig and reduces it to approximately 45 psig.

psig and bar

The gas supply cylinder valves should be turned off when not in use, except during the preoperative machine checkout period.

the volume of gas remaining in the cylinder is proportional to cylinder pressure.

use of a pneumatically driven mechanical ventilator will dramatically increase oxygen utilization rates,

Hand ventilation at low fresh gas flow rates may consume less than 5% of the amount of oxygen consumed by intermediate flow meter settings coupled with the use of pneumatically powered mechanical ventilation

Nitrous oxide cylinder ??

Oxygen Supply Pressure Failure Safety Devices

abrupt or insidious oxygen pressure failure had the potential to lead to the delivery of a hypoxic mixture.

The 2000 ASTM F1850-00 standard states that “The anesthesia gas supply device shall be designed so that whenever oxygen supply pressure is reduced to below the manufacturer specified minimum, the delivered oxygen concentration shall not decrease below 19% at the common gas outlet.”

Fail-safe valve

A fail-safe valve is present in the gas line supplying each of the flow meters except oxygen. Controlled by oxygen supply pressure

the valve shuts off or proportionally decreases the supply pressure of all other gases as the oxygen supply pressure decreases.

Pressure sensor shut-off valve

An oxygen failure protection device that responds proportionally

Pneumatic and Electronic Alarm Devices

(oxygen supply failure alarm)

Many older anesthesia machines have a pneumatic alarm device that sounds a warning when the oxygen supply pressure decreases to a predetermined threshold value, such as 30 psig

Electronic alarm devices are now used to meet this guideline.

Second-Stage Oxygen Pressure Regulator

Most contemporary Datex-Ohmeda workstations have a second-stage oxygen pressure regulator set at a specific value ranging from 12 to 19 psig

Flow Meter Assemblies

precisely controls and measures gas flow to the common gas outlet:

› Traditional glass flow meter

› Electronic flow sensors numerical graphic combination