Current issues and trends in Respiratory therapy

Using Anesthesia Machines as Critical Care Ventilators During the COVID-19 Pandemic

Paul N Austin and Richard D Branson

Somewhere between 30% and 89% of patients with COVID-19 admitted to a critical care unit require

invasive mechanical ventilation. Concern over the lack of adequate numbers of critical care ventilators

to meet this demand led the U.S. Food and Drug Administration to authorize the use of anesthesia

machines as critical care ventilators. The use of anesthesia machines for ventilating patients with

COVID-19 is overseen by an anesthesia provider, but respiratory therapists may encounter their use.

This article reviews the fundamental differences between anesthesia machines and critical care ventila-

tors, as well as some common problems encountered when using an anesthesia machine to ventilate a

patient with COVID-19 and steps to mitigate these problems. Key words: COVID-19; anesthesia; me- chanical ventilation; critical care. [Respir Care 2021;66(7):1184–1195. © 2021 Daedalus Enterprises]

Introduction

Somewhere between 30% and 89% of patients with

COVID-19 who are admitted to a critical care unit require

invasive mechanical ventilation.1 Concern over the lack of

adequate numbers of critical care ventilators to meet this

demand led the U.S. Food and Drug Administration to author-

ize the use of anesthesia machines (sometimes termed anesthe-

sia workstations) as critical care ventilators.2 This may occur

by repurposing operating rooms as intensive care areas or by

relocating anesthesia ventilators to the ICU. In both cases, the

stated simplicity belies the important technical differences in

devices and skills required for safe and effective operation.

Anesthesia machines are multi-component devices in-

tended to deliver oxygen (O2) and other gases (eg, nitrous ox-

ide and air) along with volatile inhaled anesthetic agents. The

anesthesia machine often includes a physiologic monitor, cap-

nograph, anesthetic gas monitor, and additional monitors. A

mechanical ventilator is integrated into the anesthesia machine

as one of these components. Anesthesia machines may effec-

tively ventilate critically ill patients but differ significantly

from critical care ventilators both in design and operation.3,4

Anesthesia providers (eg, certified registered nurse anesthetists

or physician anesthesiologists) should oversee the use of anes-

thesia machines for patients with COVID-19,3 but respiratory

therapists are likely to be involved in their monitoring and use

in the critical care unit.

This article provides critical care respiratory therapists

with a review of the fundamental differences between anesthe-

sia machines and critical care ventilators. Also examined are

the common problems encountered when using an anesthesia

machine to ventilate a COVID-19 patient and steps to mitigate

these problems. Volatile anesthetics are sometimes adminis-

tered to patients with severe asthma5 and seizure disorder6 in

critical care units. A discussion of this practice is beyond the

scope of this article. This information is not intended to

replace formal training or manufacturer instructions. Like crit-

ical care ventilators, there are many manufacturers and models

of anesthesia machines, and readers must follow manufacturer

instructions and other guidelines. Readers are referred else-

where for detailed directions on how to use an anesthesia

machine as a critical care ventilator.2-4

Fundamental Differences Between Critical Care

Ventilators and Anesthesia Machines

Critical care ventilators deliver breaths containing a vari-

able O2 concentration (FIO2), typically set from 0.21 to 1.0.

Depending on the ventilation mode selected, the flow of gas

Dr Austin is affiliated with Texas Wesleyan University, Fort Worth,

Texas. Mr Branson is affiliated with the Division of Trauma and Critical

Care, Department of Surgery, University of Cincinnati Medical Center,

Cincinnati, Ohio.

Mr Branson is Editor-in-Chief of RESPIRATORY CARE. He discloses

relationships with Mallickrodt Pharmaceuticals, Pfizer, Ventec Life

Systems, Vyaire, and Zoll Medical. Dr Austin has no conflicts to

disclose.

Correspondence: Paul N Austin PhD CRNA, 14311 Harvest Moon Rd,

Boyds, MD 20841. E-mail: [email protected].

DOI: 10.4187/respcare.08799

1184 RESPIRATORY CARE � JULY 2021 VOL 66 NO 7

from the ventilator can be triggered by patient effort, by set-

tings controlled by the operator (eg, delivering a specified

number of breaths per minute), or a combination of these 2

options. A critical care ventilator is capable of a number of

modes of ventilation as well as volume, pressure, and adapt-

ive pressure breaths. The critical care ventilator also delivers

PEEP and allows control of inspiratory flow, inspiratory

time, rise time, and flow termination criteria. Critical care

ventilators include a dizzying array of alarms and displays of

monitored variables based on airway pressure and flow.

An anesthesia machine delivers oxygen and other gases

such as air and nitrous oxide (an analgesic that has some an-

esthetic properties) along with volatile (also called “inhala-

tional”) anesthetics (Fig. 1). Patients may also breathe

spontaneously with no ventilatory support. A positive pres-

sure breath can be delivered either manually by squeezing a

breathing bag that is part of the machine or by using the

integrated mechanical ventilator. The ventilator may range

from a simple bellows-in-a-box device to one approaching

the sophistication of a critical care ventilator and deliver

patient-triggered breaths. The anesthesia machine may con-

tain a gas and anesthetic monitor, analyzing the inspiratory

and expiratory concentrations of O2, CO2, and the volatile

anesthetic as well as a physiologic vital signs monitor.

Unlike the critical care ventilator, the anesthesia machine is

intended to be operated with an anesthesia provider in

attendance at all times.

Two of the 5 major differences between critical care and

anesthesia machine ventilators are related: potential for

rebreathing of exhaled gases and CO2 absorption. The third

major difference is the use of a scavenging system to pre-

vent pollution of the room with inhaled anesthetics. The

fourth major difference is that FIO2 may be set using gas

flow meters on older anesthesia machine, while FIO2 is

directly set on a critical care ventilator or newer anesthesia

ventilator. The final major difference is that manual and

mechanical ventilation can be delivered using the anesthe-

sia machine.7 Table 1 contains a summary of these and

other differences between an anesthesia machine and a crit-

ical care ventilator. These 5 major differences are discussed

further below.

Non-, Partial, and Complete Rebreathing

The critical care ventilator operates as a non-rebreathing

system where exhaled gases are vented to the atmosphere.

The anesthesia machine can operate as a non-rebreathing

(also called an open) system where all of the exhaled gases

are vented to the scavenging system, as a partial rebreathing

(also called semi-closed) system where a portion of the

exhaled gases are recycled, or a complete rebreathing (also

called a closed) system where all of the exhaled gases are

recycled. The anesthesia machine is rarely used as a com-

plete rebreathing system in the operating room; in that

Fig. 1. A modern anesthesia machine (Aisys, GE Healthcare). There are many design variations.

ANESTHESIA MACHINES FOR CRITICAL CARE

RESPIRATORY CARE � JULY 2021 VOL 66 NO 7 1185

setting, the anesthesia machine is often used as a partial

rebreathing system to help conserve volatile anesthetics, to

reduce costs, and to conserve heat and moisture.7 When

used as a partial rebreathing system, the fresh gas flow (ie,

the amount of gas in L/min continuously entering the

breathing circuit set by the operator, not the flow of gas

during inhalation) is less than the patient’s minute ventila-

tion, and the CO2 contained in the portion of the patient’s

exhaled breath is chemically removed by the CO2 absorb-

ent.7 In contrast, the anesthesia machine is used as a non-

rebreathing system when used with patients with COVID-

19. This is primarily done to mitigate problems resulting

from excess moisture buildup in the inspiratory limb of the

breathing circuit when the anesthesia machine is used with

these patients.3,4 This is discussed further below.

Factors or controls determining whether the anesthesia

machine is operating as a complete, partial, or non-

rebreathing system include the fresh gas flow, adjustment

of the adjustable pressure-limiting valve if the patient is

breathing spontaneously or being manually ventilated (or

analogous valve located with the integrated mechanical

ventilator), use of one-way inspiratory and expiratory

valves, and the presence of the CO2 absorber. 7 Table 2

contains an explanation of terms used when discussing the

anesthesia machine and rebreathing. Fresh gas flow and the

circle system are discussed further below.

Fresh Gas Flow. Oxygen and other gases such as air and ni-

trous oxide are supplied from central pipeline sources or

tanks mounted on the anesthesia machine. The anesthesia

machine is disabled if there is a loss of the O2 supply pres-

sure. This is a safety system to prevent hypoxic gas mixtures

being delivered in the absence of an O2 supply. Regulators in

the anesthesia machine reduce the pressure of the supplied

gases prior to delivery. The flow of gases (O2, nitrous oxide,

air) to the patient are regulated by the anesthesia provider

directing setting the flows of the gases on flow meters or by

setting variables such as total flow and FIO2. 7 This gas flow

is termed the fresh gas flow and is the amount of new gas

added to the breathing circuit each minute.

High fresh gas flow is associated with minimal rebreath-

ing. High fresh gas flows (ie, greater than the patient’s mi-

nute ventilation) are used when the anesthesia machine is

operated as a non-rebreathing system, which is recom-

mended for patients with COVID-19 to help minimize ex-

cessive moisture production in the breathing circuit.3,4

Table 1. Important Differences Between an Anesthesia Machine and a Critical Care Ventilator

Anesthesia Machine Critical Care Ventilator

Major differences

Can operate as a non-rebreathing, a partial rebreathing, or a complete

rebreathing system by using a circle breathing system where exhaled gases

can be recycled and reintroduced with fresh gas flow

Operates as a non-rebreathing system; does not use a circle system

Equipped with a CO2 absorber (if depleted, may result in increased inspira-

tory CO2 and hypercapnia)

No need for a CO2 absorber

Can deliver manual or mechanical ventilation Delivers only mechanical ventilation

FIO2 determined by settings on the gas flow meters or an FIO2 control The operator directly sets the FIO2 Equipped with a scavenger system that prevents pollution of room with

anesthetics

No need for a scavenger system

Other differences

Operated by a continuously present anesthesia provider, maintained by an

anesthesia provider or anesthesia technician

Operated and maintained by a respiratory therapist

Delivers O2 and other gases such as air and nitrous oxide as well as volatile

(inhaled) anesthetics (eg, isoflurane, sevoflurane, desflurane)

Typically delivers only O2 and air

Requires an operator in attendance at all times Does not require an operator to be with the device at all times

Designed for intermittent use during a single day with multiple patients Designed for continuous use for days with the same patient

Alarm volume may not be loud enough to be heard in the critical care unit Alarm volume designed to be heard in a critical care unit

Alarms do not interface with the hospital nurse call alarm system Alarms may generate alerts in the hospital nurse call alarm system

May contain residual amounts of anesthetic agents in the breathing system

(remote risk of malignant hyperthermia)

Does not contain residual amounts of anesthetic agents

Often has an integrated gas and anesthetic monitor May contain or interface with a gas monitor

May have a physiologic monitor Typically does not have an integrated physiologic monitor

May interface with an electronic anesthesia record (that ultimately

becomes part of the patient record) but not directly with the patient elec-

tronic medical record

May interface directly with the patient electronic medical record

Rarely used with a heated humidifier Commonly used with a heated humidifier

From References 2-4,7.

ANESTHESIA MACHINES FOR CRITICAL CARE

1186 RESPIRATORY CARE � JULY 2021 VOL 66 NO 7

T a b le

2 .

T h re e O p e ra ti n g M o d e s A n e st h e si a M a c h in e s

C o m p le te R e b re a th in g (C lo se d ) S y st e m

P a rt ia l R e b re a th in g (S e m i- C lo se d ) S y st e m

N o n -R e b re a th in g (O

p e n ) S y st e m

F G F a n d m in u te v e n ti la ti o n

* F G F c o m p o se d o n ly

o f O 2 w it h th e fl o w

m e e ti n g m e ta b o li c d e m a n d † w it h fl o w

m u c h lo w e r th a n m in u te v e n ti la ti o n

F G F d o e s n o t e q u a l o r e x c e e d m in u te v e n ti la -

ti o n , g a se s o th e r th a n O 2 m a y b e u se d su c h

a s a ir o r n it ro u s o x id e

F G F e q u a ls o r e x c e e d s m in u te v e n ti la ti o n

A P L v a lv e ‡

C lo se d

P a rt ia ll y o p e n

F u ll y o p e n

R o le o f C O 2 a b so rb e n t

F u ll y d e p e n d e n t o n C O 2 a b so rb e n t to

re m o v e

e x h a le d C O 2

S o m e p o rt io n o f th e e x h a le d b re a th

is

re b re a th e d ; d e p e n d e n t o n C O 2 a b so rb e n t to

re m o v e e x h a le d C O 2

L it tl e d e p e n d e n c e o n C O 2 a b so rb e n t to

re m o v e d e x h a le d C O 2

P o rt io n o f e x h a le d b re a th

sh u n te d to

a tm

o sp h e re

§

A ll u n u se d p o rt io n o f th e in h a le d b re a th

is

re b re a th e d w it h n o p o rt io n o f th e e x h a le d

b re a th

sh u n te d to

th e a tm

o sp h e re

v ia th e

sc a v e n g in g sy st e m

S o m e o f th e e x h a le d b re a th

is sh u n te d to

th e

a tm

o sp h e re

v ia th e sc a v e n g in g sy st e m

A lm

o st a ll o r a ll o f th e e x h a le d b re a th

is

sh u n te d to

th e a tm

o sp h e re

v ia th e sc a v -

e n g in g sy st e m

A d v a n ta g e s

C o n se rv a ti o n o f O 2 a n d v o la ti le a n e st h e ti c s;

le ss

h e a t a n d w a te r lo ss

fr o m

th e re sp ir a to ry

sy st e m

C a n u se

o th e r g a se s w it h O 2 ; so m e c o n se rv a -

ti o n o f v o la ti le a n e st h e ti c s a n d so m e c o n -

se rv a ti o n o f h e a t a n d w a te r lo ss

fr o m

th e

re sp ir a to ry

sy st e m ; so m e c o n se rv a ti o n o f

C O 2 a b so rb e n t, le ss

b u il d u p o f m o is tu re

in

b re a th in g c ir c u it fr o m

th e C O 2 a b so rb e n t

C a n u se

o th e r g a se s w it h O 2 , li tt le d e p e n d -

e n c e o n C O 2 a b so rb e n t to re m o v e C O 2

fr o m

e x h a le d b re a th s, li tt le b u il d u p o f

m o is tu re

in b re a th in g c ir c u it fr o m

th e C O 2

a b so rb e n t

D is a d v a n ta g e s

R e q u ir e s u se

o f F IO

2 ¼

1 .0 , te c h n ic a ll y d if fi -

c u lt , li tt le ro o m

fo r e rr o r if p a ti e n t’ s O 2

re q u ir e m e n t in c re a se s; e x c e ss

m o is tu re

c a n

b u il d in

th e c ir c u it d u e to

w a te r p ro d u c ti o n

b y th e C O 2 a b so rb e n t3 ; to x ic a c c u m u la ti o n

o f m e ta b o li te s is p o ss ib le ; m a y se e q u ic k

e x h a u st io n o f th e C O 2 a b so rb e n t w it h

re su lt in g h y p e rc a p n ia

E x c e ss

m o is tu re

c a n b u il d in

th e c ir c u it d u e to

w a te r p ro d u c ti o n b y th e C O 2 a b so rb e n t,

so m e w h a t q u ic k e x h a u st io n o f th e C O 2 a b -

so rb e n t c o m p a re d w it h th e n o n -r e b re a th in g

m o d e w it h re su lt in g h y p e rc a p n ia ; sh o u ld

e n su re

a n H M E F is u se d to

h e lp

c o n se rv e

h e a t a n d re d u c e m o is tu re

lo ss

fr o m

th e

lu n g s

Q u ic k e r d e p le ti o n o f g a s a n d v o la ti le a n e s-

th e ti c s, m u st u se

a n H M E F to

h e lp

c o n -

se rv e h e a t a n d re d u c e m o is tu re

lo ss

fr o m

th e re sp ir a to ry

sy st e m ,|| m o re

ra p id

d e p le -

ti o n o f g a s su p p li e s

F ro m

R e fe re n c e s 3 ,4 ,7 .

* T y p ic a ll y d o n e w it h o n ly

F IO

2 ¼

1 .0 .

† M e ta b o li c d e m a n d fo r o x y g e n o f a n o rm

a l si z e a n e st h e ti z e d a d u lt is �

0 .2

L /m

in .

‡ A n a n a lo g o u s v a lv e is p re se n t o n th e in te g ra te d m e c h a n ic a l v e n ti la to r.

§ W a te r is p ro d u c e d a s a b y -p ro d u c t o f th e C O 2 a b so rb e n t re a c ti o n to

re m o v e C O 2 fr o m

e x h a le d b re a th s. A s m o re

C O 2 is re m o v e d , m o re

w a te r is p ro d u c e d .

|| H M E F m a y n o t p re v e n t th ic k a ir w a y se c re ti o n s, e sp e c ia ll y w it h h ig h m in u te v o lu m e s a n d th ic k o r b lo o d y se c re ti o n s.

A P L ¼

a d ju st a b le p re ss u re -l im

it in g

F G F ¼

fr e sh

g a s fl o w

H M E F ¼

h e a t- a n d -m

o is tu re

e x c h a n g in g fi lt e r

ANESTHESIA MACHINES FOR CRITICAL CARE

RESPIRATORY CARE � JULY 2021 VOL 66 NO 7 1187

More rebreathing occurs as the fresh gas flow is decreased.7

To emphasize, fresh gas flow is the amount of gas in L/min

continuously entering the breathing circuit set by the opera-

tor and not the flow of gas mixture provided by the me-

chanical ventilator during inspiration. The flow of the gas

mixture from the mechanical ventilator during inspiration

is determined by the settings on the ventilator.

If desired by the anesthesia provider, the fresh gas flow

may pass through vaporizers before entering the breathing

circuit. These vaporizers convert the liquid volatile anes-

thetics to a vapor. The vaporizers allow delivery of the an-

esthetic at the desired concentration. Vaporizers should be

removed or drained on anesthesia machines repurposed as

critical care ventilators for patients with COVID-19.3,4 The

fresh gas flow then flows directly into the circle system.7

Circle System. The circle system is designed to permit

rebreathing of exhaled gases while chemically absorbing

exhaled CO2. The flow of gas to the patient circuit is con-

tinuous (ie, the fresh gas flow). The simplified patient cir-

cuit or circle system includes the adjustable pressure-

limiting valve, breathing bag, CO2 absorber, one-way

inspiratory and expiratory valves, breathing circuit (usually

22-mm corrugated tubing), and a Y-connector with a heat

and moisture exchanging filter (HMEF) (Fig. 2). The ad-

justable pressure-limiting valve prevents pressure buildup

in the system if fresh gas flow significantly exceeds the ox-

ygen consumption of the patient. An analogous valve on

the integrated mechanical ventilator performs this function

if the patient is mechanically ventilated.7

CO2 Absorption

There is no need for CO2 absorption with a critical care

ventilator as no portion of the exhaled gases are recycled.

Exhaled gases containing CO2 can be recycled using an anes-

thesia machine. The CO2 in the exhaled breath must be

removed to prevent hypercapnia. CO2 is removed using an ab-

sorbent material often soda lime. This commonly used CO2 absorbent contains primarily calcium hydroxide along with

small amounts of additional chemicals such as sodium hy-

droxide. The exhaled breath passes through the granular ab-

sorbent before returning to the patient.7

The absorbent does not soak up CO2 like a sponge.

Rather, CO2 is chemically removed by converting it to cal-

cium carbonate in a series of chemical reactions. These

reactions produce heat and water (Table 3). This water pro-

duction may be excessive in patients with COVID-19 due,

High pressure gas source

Flow meters O2, air, N2O or FIO2 control

Fresh gas flow inlet

Anesthetic vaporizer

CO2 absorber

Inspiratory limb

Inspiratory one-way valve

Expiratory one- way valve

To scavenger

APL valve

Breathing bag

Bag/vent selector

Ventilator

Expiratory limb

Ventilator outflow valve

To scavenger

Y-piece

Gas monitor

Gas sample tubing

HMEF

ETT and patient

Fig. 2. A simplified drawing of the anesthesia machine circle system. APL ¼ adjustable pressure-limiting; ETT ¼ endotracheal tube; HMEF ¼ heat-and-moisture exchanging filter.

Table 3. Series of Chemical Reactions to Remove CO2 From

Exhaled Breaths by Soda Lime

CO2 + H2O ! H2CO3 H2CO3 + NaOH ! NaHCO3 + H2O NaHCO3 + Ca(OH)2 ! CaCO3 + H2O + NaOH + Heat

Note that water and heat are by-products of these reactions. From Reference 8.

H2CO3 ¼ carbonic acid NaOH ¼ sodium hydroxide NaHCO3 ¼ sodium bicarbonate Ca(OH)2 ¼ calcium hydroxide CaCO3 ¼ calcium carbonate

ANESTHESIA MACHINES FOR CRITICAL CARE

1188 RESPIRATORY CARE � JULY 2021 VOL 66 NO 7

in part, to an increased minute ventilation and elevated CO2 production. The granules of the CO2 absorbent typically

contain an indicator. This enables the granules to change

color (such as from white to blue or purple) when the

absorbent’s capacity has been exhausted. The life of the ab-

sorbent is highly variable, depending on factors including

the type of absorbent, manufacturer, minute ventilation,

patient CO2 production, and the fresh gas flow. 7,8

Ability to Deliver Manual and Mechanical Ventilation

The critical care ventilator only delivers mechanical

breaths. If required, manual ventilation is accomplished

using a separate, manual resuscitator. A non-self-inflating

bag is part of the anesthesia machine. This bag is kept

inflated by the fresh gas flow. Closing the adjustable pres-

sure-limiting valve and squeezing the bag delivers a manual

breath to the patient via endotracheal tube or face mask. An

O2 flush valve is present in the anesthesia machine and,

when depressed, delivers 100% O2 into the patient circuit at

a flow of 35–70 L/min. This high flow of O2 helps when

manually ventilating a patient using the anesthesia machine

with a face mask in the presence of leaks. The anesthesia

machine does not initiate mechanical ventilation automati-

cally.7 The breathing bag/ventilator switch and ventilator

controls must be set correctly before mechanical ventilation

will begin.

Setting the FIO2

Anesthesia machines used as critical care ventilators

must be able to deliver air due to the consequences of pro-

longed breathing of 100% O2. The FIO2 is directly set on

the critical care ventilator. This may be done directly on

more modern anesthesia machines, but some anesthesia

machines require the use of settings on the flow meters to

determine the FIO2. For instance, if 1 L/min each of O2 and

nitrous oxide are delivered, the FIO2 is 0.5. If there is a flow

meter for air, then setting the O2 and air flow meters will

control the FIO2 (Table 4, Table 5).

FIO2 is continuously monitored using an O2 analyzer

with high and low alarms. Modern anesthesia machines are

designed to make it difficult to deliver hypoxic gas mix-

tures if the system is properly functioning.7 It is neverthe-

less possible to unknowingly change the FIO2 setting. This

underscores the need for the operator to be near the anesthe-

sia machine.

Scavenging System

There is no need for a scavenging system with a critical

care ventilator as the device does not deliver inhaled anes-

thetics. With an anesthesia machine, exhaled gases that are

not rebreathed when operating as a partial or open system

exit the circle system. These exhaled gases exit the circle

system via the adjustable pressure-limiting valve or another

analogous valve in the integrated mechanical ventilator

through a system that directs these gases out of the room, as

chronic exposure to personnel may be toxic.7 The scaveng-

ing hose leading from the anesthesia machine to the wall

should be removed if the anesthesia machine is used to ven-

tilate a patient with COVID-19 unless the machine will not

function properly without it attached to a wall suction

source.3

Excessive Water Production With Rebreathing

Partial rebreathing with lower fresh gas flows, often

done in the operating room, requires the CO2 absorbent to

chemically remove CO2 from the portion of the exhaled

breath that is rebreathed. In the operating room, this side

effect of CO2 absorption increases the temperature and hu-

midity of inspired gases. Water and heat are by-products of

the reactions of the chemicals in the absorbent and CO2. 7,8

This water production is a major problem in patients with

COVID-19 because they are often hypermetabolic (ie, high

CO2 production) and thus require a high minute ventilation.

Large amounts of water are produced, which can occlude

the patient circuit and interfere with flow sensors in the

Table 4. Basic Principles for Determining FIO2 Using Air and O2 Flow Meter Settings on an Anesthesia Machine

Basic Principles

Flow meter settings are used on some anesthesia machines to determine

the FIO2 .*

Fresh gas flow is the total flow per min of all gases (eg, O2 and N2O, or

O2 and air).

21% of air is O2.

If only air is used, the FIO2 will be 0.21.

FIO2 is calculated as follows when using air and O2:

FIO2 ¼ Air flow � 0:21 ð Þ þ O2 flow � 1:0ð Þ

Fresh gas flow .

* For an adult patient with COVID-19, fresh gas flow should be equal to or higher than the

patient’s minute ventilation. See the manufacturer recommendations.9 Some have reported using

a fresh gas flow of 150% of minute ventilation or 10 L/min with these patients.3

N2O ¼ nitrous oxide

Table 5. Air and O2 Flow Meter Settings With Resultant FIO2 Using

a Fresh Gas Flow of 10 L/min

O2 Flow Meter

Setting, L/min

Air Flow Meter

Setting, L/min

O2 Content of Air

at This Flow Meter

Setting, L/min

FIO2

9 1 0.21 0.92

5 5 1.05 0.61

4 6 1.26 0.53

1 9 1.89 0.29

ANESTHESIA MACHINES FOR CRITICAL CARE

RESPIRATORY CARE � JULY 2021 VOL 66 NO 7 1189

anesthesia machine.3 The anesthesia machine should be

operated as a non-rebreathing (open) system to mitigate

the problem of excess water production from the CO2 absorbent.3,4

Fresh gas flows greater than the patient’s minute ventila-

tion (such as 1.5 times higher) are recommended for

patients with COVID-19. Manufacturers and others offer

guidance on setting the fresh gas flow (Fig. 3). Some

rebreathing may occur even with high fresh gas flows, and

the CO2 absorbent should be monitored for color change

indicating depletion and replacement as needed. Using a

capnograph to detect excess inspiratory CO2 may help

determine whether there is unwanted CO2 rebreathing. 3,4,7

Heated Humidifiers, HMEFs, and the

Anesthesia Machine

Heated humidifiers are rarely used with anesthesia

machines. It may not be possible to attach a heated humidi-

fier to an anesthesia machine because the moisture produced

may interfere with its operation. Therefore, an HMEF is

placed between the endotracheal tube and the patient connec-

tor to help conserve heat and moisture (Fig. 4). Importantly,

when using an anesthesia machine with a COVID-19 patient,

a heat-and-moisture exchanger with an integrated bacterial

and viral filter should be used. This positioning helps protect

the anesthesia machine from contamination with COVID-19

and from excessive moisture that can affect flow sensors in

the machine. This positioning also protects the small-bore

sample tubing leading to the gas monitor, thus helping pro-

tect the monitor from contamination. The HMEF may

become occluded due to copious secretions or by the added

moisture from the process of CO2 absorption. This may

result in a slow increase in resistance. The HMEF should be

routinely inspected and replaced as needed. Protection of the

anesthesia machine is also facilitated by placing a filter on

the end of expiratory limb where it connects to the anesthesia

machine. Decontamination of the anesthesia machine is

described in the manufacturer’s instructions.3,4

United States Food and Drug Administration Ventilator Supply Mitigation Strategies: Letter to Health Care Providers https://www.fda.gov/medical-devices/coronavirus-disease-2019-covid-19-emergency use-authorizations-medical-devices/ventila tors-and-ve ntilator-accessories-euas

Manufacturers Dräger Medical Letter https://www.draeger.com/Library/Content/Draeger Customer Letter-COVID-19- Usage of Anesthesia devices for long term ventilation-2020-03-18.pdf

COVID-19 general information https://www.draeger.com/en-us us/Home/novel-coronavirus-outbreak#anesthesia

GE Healthcare Letter https://www.gehealthcare.com/- nssmedia/3c655c83bd6b427 e9824994c12be0da5.pdf?la=en-us

COVID-19 general information https://www.gehealthcare.com/corporate/covid-19

Anesthesia machines https://www.gehealthcare.com/products/Anesthesia-Delivery-Systems-User-Resources

Mindray COVID-19 general information https://www.mi nd raynorthameri ca.com/covid-19-response/

Getinge General information https://www.getinge.com/dam/hospitaVdocuments/markeling-sales/customer letters/enqlish/mcv00103387 reva covid- 19 customer letter long term ventilation with flow-en-us.pd!

Professional Organizations American Association for Respiratory Care COVID-19 News & Resources https://www.aarc.org/nn20-covid-19-news-resources/

American Association of Nurse Anesthetists Resources https://www.aana.com/aana-covid-19-resources

SARS CoV-2 Guidance document https://www.aarc.org/wp-content/uploads/2020/03/guidance-document-SARS COVID19.pdf

American Society of Anesthesiologists COVID-19 Resources https://www.asahg.org/in-the-spotliqht/corona virus-covid-19-information

Anesthesia Patient Safety Foundation Novel Coronavirus (COVID-19) Anesthesia Resource Center https://www.apsf.orq/novel-coronavirus-covid-19-resource-center/

Society of Critical Care Medicine Emergency Resources: COVID-19 https://www.sccm.org/Disaster/COVID19

Fig. 3. Resources for using anesthesia machines as critical care ventilators for use with patients with COVID-19.

Fig. 4. Proper location of heat-and-moisture exchanger protecting the anesthesia machine and gas monitor.

ANESTHESIA MACHINES FOR CRITICAL CARE

1190 RESPIRATORY CARE � JULY 2021 VOL 66 NO 7

The Anesthesia Machine Ventilator

Older Anesthesia Machines

Older anesthesia machines were equipped with simple

ventilators that solely delivered controlled ventilation.

These ventilators were pneumatically powered and con-

trolled. These devices used compressed gas (usually oxy-

gen) to squeeze the bellows to deliver the gas mixture to

the patient. These are commonly referred to as bag-in-the-

box or bellows-in-the-box ventilators. Older anesthesia

ventilators were not capable of delivering spontaneous

breathing ventilation modes.7 Many of these anesthesia

machines are still in use today and perform well in the oper-

ating room with anesthetized patients.7

Newer Anesthesia Machines

Newer anesthesia machines are often equipped with so-

phisticated, electronically controlled ventilators capable of

delivering many ventilation modes. Like older anesthesia

machine ventilators, some of these devices use compressed

O2 to squeeze a bellows, so demand on the compressed O2 supply is a consideration.7,9-14 Others use an electrically

powered piston or turbine to generate inspiratory flow.7,15-17

There is tremendous variability in the features included on

these devices, and manufacturers are constantly updating

the machines, so the user must consult the manufacturer’s

description and directions for use for a specific anesthesia

machine (Table 6).

Reports suggest that anesthesia machine ventilators may,

however, not perform as well as critical care ventilators for

patients with COVID-19. For example, asynchrony and its

accompanying problems, including excess airway pressure

and patient discomfort, may occur; the asynchrony resolved

when the anesthesia machine was replaced with a critical

care ventilator.18,19 This evidence suggests that anesthesia

machines should be used only if critical care ventilators are

not available and should not be used with complicated

patients. In addition, experts and experienced providers

suggest a critical care ventilator may have to be used if

there is difficulty ventilating the patient with an anesthesia

machine.3,19

Considerations When Using an Anesthesia Machines

For Patients With COVID-19

Stakeholders should consider triage planning such as

using anesthesia machines with patients suffering from

non-respiratory conditions (eg, trauma, neurologic condi-

tions) and using critical care ventilators with patients with

challenging respiratory conditions such as COVID-19.

Providers should consider replacing an anesthesia machine

with a critical care ventilator if the patient with COVID-19

is ineffectively ventilated when steps such as adjusting ven-

tilator settings (eg, inspiratory time), increasing sedation,

and paralysis are not effective.3,4,18,19

Discussed below are general considerations when using

an anesthesia machine to ventilate a patient with COVID-

19 in a critical care unit. Many agencies, manufacturers,

and organizations provide detailed instructions for repur-

posing an anesthesia machine as a critical care ventilator

for such scenarios (Fig. 3). Information includes specific

details for determining proper fresh gas flow, preventing

contamination, and disinfection of the anesthesia machine.

This list is not all inclusive and, like many aspects of this

pandemic, resources are frequently revised and new resour-

ces become available.

General Planning

All stakeholders, including respiratory therapists, nursing

personnel, critical care providers, and anesthesia providers

must work together to formulate the policy for using an an-

esthesia machine for patients with COVID-19 in the critical

care unit. Their use must be overseen by an anesthesia pro-

vider.2 The policy should include such things as how often

the anesthesia provider rounds on the patients, how often an

anesthesia machine checkout procedure is required by anes-

thesia provider, key alarms the non-anesthesia providers

must be aware of, and methods of communication with the

anesthesia provider.

Patients ventilated using an anesthesia machine should

be located in the same unit to facilitate the anesthesia pro-

vider’s supervision of their use.2 Room size is a considera-

tion as anesthesia machines are often larger than critical

care ventilators. Central O2, air, and suction outlets must be

at the bedside. While there usually are 1–2 small O2 cylin-

ders for emergency use on the anesthesia machine, these

will become quickly exhausted. Oxygen use is dramatically

increased if the anesthesia machine ventilator is pneumati-

cally powered by compressed O2. 3,4

Selection and Preparation of the Anesthesia Machine

The anesthesia machine must be capable of delivering

patient-triggered breathing modes and air/O2 mixtures, as

well as a constant level of PEEP. Personnel from the anes-

thesia department should prepare the anesthesia machine,

including removing or disabling all vaporizers and remov-

ing the high-pressure nitrous oxide hose. Flushing the anes-

thesia machine with high-flow O2 or air helps ensure

removal of any residual anesthetic.3,4,20 If the anesthesia

machine has an integrated monitor, including a capnograph

and anesthetic gas monitor, these monitors should remain

intact.

ANESTHESIA MACHINES FOR CRITICAL CARE

RESPIRATORY CARE � JULY 2021 VOL 66 NO 7 1191

T a b le

6 .

F e a tu re s o f V a ri o u s M o d e rn

E le c tr o n ic a ll y C o n tr o ll e d A n e st h e si a V e n ti la to rs *

D rä g e r F a b iu s G S

P re m iu m

1 5

D rä g e r A p o ll o 1 6

D rä g e r P e rs e u s A 5 0 0 1 7

D a te x -O

h m e d a

A e st iv a W it h 7 9 0 0

V e n ti la to r1

2

G E A v a n c e C S 9

G E A e sp ir e 7 9 0 0 1 0

G E A is y s

C a re st a ti o n C S 1 1

M in d ra y A 5 a n d A 7

A d v a n ta g e † 1 3 ,1 4

D ri v e m e c h a n is m

P is to n

P is to n

T u rb in e

A sc e n d in g b e ll o w s

A sc e n d in g b e ll o w s

A sc e n d in g b e ll o w s

A sc e n d in g

b e ll o w s

A sc e n d in g b e ll o w s

P o w e re d

E le c tr ic

E le c tr ic

E le c tr ic

P n e u m a ti c

P n e u m a ti c

P n e u m a ti c

P n e u m a ti c

P n e u m a ti c

S ta n d a rd

v e n ti la ti o n

m o d e s

M a n u a l/

sp o n ta n e o u s

V o lu m e c o n tr o l

M a n u a l/ sp o n ta n e -

o u s

V o lu m e m o d e

P re ss u re

m o d e

M a n u a l/ sp o n ta n e o u s

P C -C M V , P C -S IM

V +

V C -C M V , V C -

C M V /a u to fl o w ,

V C -S IM

V /a u to fl o w

V o lu m e c o n tr o l

P re ss u re

c o n tr o l

S IM

V

P S V P ro

V o lu m e c o n tr o l

w it h ti d a l

v o lu m e

c o m p e n sa ti o n

V o lu m e c o n tr o l

P re ss u re

c o n tr o l

S IM

V

P S V P ro

V o lu m e c o n tr o l

w it h ti d a l

v o lu m e

c o m p e n sa ti o n

M a n u a l/ sp o n ta n e o u s

V o lu m e c o n tr o l

C P A P /P S P C -V

G

P C V -V

C

S IM

V -P C

O p ti o n a l

v e n ti la ti o n

m o d e s

P re ss u re

c o n tr o l

P re ss u re

su p p o rt

S V

C V

P S

S y n c h ro n iz a ti o n

P re ss u re

su p p o rt

V C -a u to fl o w

P S -C P A P P C -S IM

V

+ P S a n d V C -S IM

V /

a u to fl o w /P S P C -

A P R V

P re ss u re

c o n tr o l

P C V -V

G

S IM

V P S V P ro

C P A P + P S V

S IM

V

P re ss u re

c o n tr o l

P C V -V

G

S IM

V

P S V P ro

C P A P + P S V

S IM

V -V

G

A P R V

L u n g re c ru it m e n t

P E E P , c m

H 2 O

0 – 2 0

2 – 3 5

O ff ; 2 – 3 5

E le c tr o n ic a ll y c o n -

tr o ll e d O ff ; 4 – 3 0

E le c tr o n ic a ll y

c o n tr o ll e d O ff ;

4 – 3 0

E le c tr o n ic a ll y

c o n tr o ll e d O ff ;

4 – 3 0

E le c tr o n ic a ll y

c o n tr o ll e d

O ff ; 4 – 3 0

O ff , 0 – 3 0

M a x im

u m

in sp ir a to ry

fl o w , L /m

in

7 5 in

v o lu m e a n d

p re ss u re

c o n tr o l

m o d e s; 8 5 in

p re ss u re

su p p o rt

m o d e

M a x im

u m

o f 1 5 0

in p re ss u re

m o d e

1 8 0

1 2 0

1 2 0

1 2 0

1 2 0

1 2 0 p lu s fr e sh

g a s fl o w

* In fo rm

a ti o n in

th is ta b le se rv e s a s a g u id e . C o n su lt th e m a n u fa c tu re r.

† A 4 A d v a n ta g e c a p a b le o f d e li v e ri n g sp o n ta n e o u s v e n ti la ti o n m o d e s w it h fe w e r st a n d a rd

v e n ti la ti o n m o d e s.

P C ¼

p re ss u re

c o n tr o l

V C ¼

v o lu m e c o n tr o l

C M V ¼

c o n ti n u o u s m a n d a to ry

v e n ti la ti o n

S IM

V ¼

sy n c h ro n iz e d in te rm

it te n t m a n d a to ry

v e n ti la ti o n

P S V P ro

¼ p re ss u re -s u p p o rt v e n ti la ti o n w it h a p n e a b a c k u p

P S ¼

p re ss u re

su p p o rt

V G ¼

v o lu m e g u a ra n te e

P C V ¼

p re ss u re

c o n tr o l v e n ti la ti o n

S V C V -P S ¼

sy n c h ro n iz e d v o lu m e c o n tr o l v e n ti la ti o n w it h p re ss u re

su p p o rt

A P R V ¼

a ir w a y p re ss u re -r e le a se

v e n ti la ti o n

ANESTHESIA MACHINES FOR CRITICAL CARE

1192 RESPIRATORY CARE � JULY 2021 VOL 66 NO 7

T a b le

7 .

P o te n ti a l P ro b le m s W h e n U si n g a n A n e st h e si a M a c h in e to

V e n ti la te

a P a ti e n t W it h C O V ID

-1 9

P ro b le m

P o ss ib le C a u se

S te p s to

H e lp

M it ig a te th e P ro b le m

R e sp ir a to ry

th e ra p is t a n d c ri ti c a l c a re

p h y si c ia n la c k o f

fa m il ia ri ty

w it h a n e st h e si a m a c h in e s

It is n o t e x p e c te d fo r re sp ir a to ry

th e ra p is ts to

b e

fa m il ia r w it h a n e st h e si a m a c h in e s

A ll st a k e h o ld e rs sh o u ld

b e in v o lv e d w it h th e p o li c y a n d p ro c e d u re

o f

u si n g th e a n e st h e si a m a c h in e a s a v e n ti la to r, in c lu d in g th e ir re sp o n -

si b il it ie s a n d m e th o d s o f c o m m u n ic a ti o n

E x c e ss

w a te r in

th e b re a th in g c ir c u it

W a te r p ro d u c ti o n fr o m

C O 2 re m o v a l b y th e C O 2

a b so rb e n t

F G F sh o u ld

b e h ig h e r th a n m in u te v e n ti la ti o n (i e , $

1 .5

ti m e s m in u te

v e n ti la ti o n ) to

d e c re a se

th e n e e d fo r th e a b so rb e n t to

re m o v e C O 2

a n d d e c re a se

th e re su lt a n t w a te r b y -p ro d u c t* ; m o n it o r c lo se ly

O c c lu si o n o f H M E F lo c a te d b e tw e e n th e e n d o tr a c h e a l

tu b e a n d p a ti e n t

P a ti e n t se c re ti o n s a n d w a te r

M o n it o r th e c o n d it io n o f th e H M E F ; c h a n g e p e ri o d ic a ll y a n d a s

n e e d e d , k e e p in g in

m in d th e p o ss ib le n e e d to

c o n se rv e su p p li e s

F re q u e n t e x h a u st io n o f th e C O 2 a b so rb e n t

In su ff ic ie n t F G F

F G F sh o u ld

b e h ig h e r th a n m in u te v e n ti la ti o n (i e , $

1 .5

ti m e s m in u te

v e n ti la ti o n ) to

d e c re a se

th e n e e d fo r th e a b so rb e n t to

re m o v e C O 2

fr o m

th e e x h a le d b re a th *

A lv e o la r d e -r e c ru it m e n t a n d p o ss ib le h y p o x e m ia w h e n

c h a n g in g H M E F ; b re a th in g c ir c u it o r re m o v in g th e

p a ti e n t fr o m

th e a n e st h e si a m a c h in e fo r a n e st h e si a

m a c h in e c h e c k o u t p ro c e d u re

D e c re a se

in a ir w a y p re ss u re , F IO

2 , o r m in u te v e n ti la ti o n

P la n a n d re h e a rs e p ro c e d u re s w h e n th e p a ti e n t w il l b e se p a ra te d fr o m

th e a n e st h e si a m a c h in e

E x c e ss iv e a ir w a y p re ss u re

a n d p a ti e n t d is c o m fo rt

P a ti e n t- v e n ti la to r a sy n c h ro n y

A d ju st th e v e n ti la ti o n m o d e ; m a y h a v e to

sw it c h to

a c ri ti c a l c a re

v e n ti -

la to r a s it m a y n o t b e p o ss ib le to

se t th e d e si re d in sp ir a to ry

ti m e in

a

h ig h m in u te v e n ti la ti o n sc e n a ri o

G a s m o n it o ri n g sa m p le li n e tu b in g o r w a te r tr a p c o n -

ta m in a ti o n o r o c c lu si o n

H M E F n o t in

a p o si ti o n to

p ro te c t th e g a s m o n it o ri n g

sa m p le li n e

R e p o si ti o n H M E F a p p ro p ri a te ly ; m o n it o r c lo se ly

A n e st h e si a m a c h in e fl o w se n so r m a lf u n c ti o n

E x c e ss

m o is tu re

in th e p a ti e n t c ir c u it

B e su re

th e H M E F is p la c e d b e tw e e n th e e n d o tr a c h e a l tu b e a n d th e

p a ti e n t c o n n e c to r o n th e b re a th in g c ir c u it ; u se

a d e q u a te F G F to

li m it

th e a m o u n t o f w a te r p ro d u c ti o n fr o m

th e C O 2 a b so rb e n t

U se

w it h in li n e n e b u li z e r

N o t d e si g n e d fo r u se

w it h a n in li n e n e b u li z e r

C lo se ly

m o n it o r if a n e b u li z e r is u se d

L e a k c o m p e n sa ti o n

L ik e ly

n o le a k c o m p e n sa ti o n

M o n it o r c lo se ly

if a b re a th in g c ir c u it le a k is su sp e c te d ; N IV

w it h th e

a n e st h e si a v e n ti la to r is n o t re c o m m e n d e d

Q u ic k e m e rg e n c y a c ti v a ti o n to

d e li v e r F IO

2 ¼

1 .0

N o t p re se n t o n a n a n e st h e si a m a c h in e

M u st a d ju st th e fl o w m e te rs to

d e li v e r F IO

2 ¼

1 .0

U n w a n te d a c ti v a ti o n o f th e o x y g e n fl u sh

v a lv e w it h

re su lt in g in c re a se d a ir w a y p re ss u re

A c c id e n ta l a c ti v a ti o n o f th e v a lv e b y p e rs o n n e l

O ri e n ta ti o n o f n o n -a n e st h e si a p ro v id e rs to

th e b a si c c o m p o n e n ts a n d

fu n c ti o n o f th e a n e st h e si a m a c h in e

L o ss

o f e le c tr ic a l p o w e r to

th e a n e st h e si a m a c h in e

O v e rl o a d e d c ir c u it b re a k e r d e d ic a te d to

th e p a ti e n t

ro o m

C a re fu ll y m o n it o r th e d e m a n d s o f th e d e v ic e s in

th e p a ti e n t ro o m

L a c k o f p ro c e d u re s fo r d o c u m e n ta ti o n o f a n e st h e si a

m a c h in e se tt in g s a n d o u tp u t fr o m

m a c h in e a n d p h y si -

o lo g ic m o n it o rs

C u rr e n t d o c u m e n ta ti o n p ro c e d u re s m a y b e p ro b le m a ti c ;

a n e st h e si a m a c h in e d o e s n o t in te rf a c e d ir e c tl y w it h

th e p a ti e n t e le c tr o n ic m e d ic a l re c o rd

C o n su lt in fo rm

a ti o n te c h n o lo g y p e rs o n n e l to

d e te rm

in e if th e a n e st h e -

si a m a c h in e a n d p h y si o lo g ic m o n it o rs c a n in te rf a c e d ir e c tl y w it h th e

p a ti e n t re c o rd ; id e n ti fy

a d o c u m e n ta ti o n p ro c e d u re

F ro m

R e fe re n c e s 1 8 ,1 9 ,2 4 -2 7 .

* S e e th e m a n u fa c tu re r a n d o th e r re so u rc e s in

F ig u re

3 fo r g u id a n c e o n se tt in g th e F G F . R e p o rt s o f u si n g F G F o f 1 0 L /m

in h a s b e e n e ff e c ti v e in

li m it in g w a te r p ro d u c ti o n fr o m

th e C O 2 a b so rb e n t. 1 8

F G F ¼

fr e sh

g a s fl o w (f lo w p e r m in u te o f O 2 a n d o th e r g a se s su c h a s n it ro u s o x id e )

H M E F ¼

h e a t- a n d -m

o is tu re

e x c h a n g in g fi lt e r

ANESTHESIA MACHINES FOR CRITICAL CARE

RESPIRATORY CARE � JULY 2021 VOL 66 NO 7 1193

Anesthesia Machine Checkout Procedure

The anesthesia machine may require completion of a

checkout procedure prior to use and periodically during

use, perhaps as frequently as every 24–72 h. The device

may malfunction if this checkout procedure is not per-

formed, and the anesthesia provider should monitor the

need for this checkout and perform the checkout preemp-

tively. An alternative ventilation method (eg, another venti-

lator, a self-inflating bag) should be available because the

anesthesia machine cannot be used during the checkout

procedure, which may last a few minutes. Considerations

include preventing alveolar de-recruitment while using an

alternative ventilation method.3,4,18,21

Alarms

Alarms on anesthesia machines typically include alerts

for disconnect, high pressure, and high and low FIO2, among

others. These should be tested and set appropriately, and all

personnel should be aware that these audible alarms are not

as loud as those on a critical care ventilator. The alarms

will likely not interface with the nurse call system or con-

nect with the electronic health record. Personnel must

remain close enough to patients ventilated with an anesthe-

sia machine to hear the alarms.2-4 Care of critically ill

patients in an operating room repurposed as an ICU

presents significant logistical issues. If the patient is pro-

vided critical care in an operating room, the ability for care-

givers to hear alarms is further diminished.

Setting the Fresh Gas Flow and the FIO2

As previously mentioned, an anesthesia machine should

be used as a non-rebreathing system. This helps prevent the

CO2 absorbent from having to remove as much CO2 and

reduces the resultant water by-product. Use as a non-

rebreathing system will also extend the supply of CO2 ab-

sorbent. Some rebreathing can occur even with high fresh

gas flows, so the CO2 absorbent must be left in place and

monitored. Fresh gas flow (ie, a combined amount of O2 and air) will determine the FIO2, which should be higher

than the patient’s minute ventilation. This difference may

be as much as 1.5 times the minute ventilation, but recom-

mendations vary between anesthesia machine manufac-

turers (refer to the manufacturer resources in Fig. 3).3,4

Others report using a fresh gas flow of 10 L/min for adult

patients with COVID-19.18 FIO2 is determined by the flow

meter setting, or it can be set directly (Fig. 2). All personnel

must take care not to accidentally alter the desired settings,

underscoring the need for a qualified operator to be with an-

esthesia machine at all times.

HMEF and Breathing Circuit Filter

As discussed above, an HMEF should be placed between

the endotracheal tube and breathing circuit patient connec-

tor (Fig. 4). A filter should be placed between the exhala-

tion limb of the breathing circuit where it attaches to the

anesthesia machine.3,4 In the presence of excess humidity,

this filter should be monitored for increases in resistance,

and it should be changed as needed. The importance of fre-

quent monitoring of this filter cannot be overstated.

Sharing Anesthesia Machines

Authors of a small case series described ventilating 3

groups of patients (2 patients with COVID-19 per group)

using a single critical care ventilator.22 The patients were

sedated and paralyzed. The authors concluded this could be

feasible as a stopgap measure, but safety requires careful

patient selection and patient monitoring.22 In one of these 3

pairs of patients, an anesthesia machine was used to provide

shared ventilation. In this case, the authors noted that

increased HME resistance due to moisture accumulation fur-

ther exacerbated volume maldistribution between patients.

In a 24-h period, they reported changing the HMEF 4 times

due to excess water accumulation, as well as issues related to

rapid exhaustion of the CO2 absorbent. 22 No large studies

have been conducted examining ventilator sharing, including

anesthesia machine sharing. A consensus statement authored

by multiple critical care professional organizations warns

against sharing mechanical ventilators. Reasons include vol-

umes preferentially going to the most compliant lung seg-

ments, monitoring difficulties, and risking life-threatening

treatment failure of the involved patients.23

Potential Problems and Mitigation Methods

Potential problems and those reported when using an an-

esthesia machine to ventilate patients with COVID-19 are

described in Table 7, along with possible causes and miti-

gation methods. All stakeholders must be keenly aware of

the potential for excess water in the breathing circuit. This

can lead to obstruction of the circuit and occlusion of the

HMEF. Other problems include frequent exhaustion of

the CO2 absorbent filter and resulting hypercapnia, ineffec-

tive ventilation, and alveolar de-recruitment when the anes-

thesia machine is disconnected from the patient for various

reasons, as well as inability to hear alarms.18,19,24-27

Summary

Anesthesia machines are being repurposed as critical

care ventilators during the COVID-19 pandemic.18,24-26,28,29

Their use should be overseen by an anesthesia provider;

however, respiratory therapists will be involved in their

ANESTHESIA MACHINES FOR CRITICAL CARE

1194 RESPIRATORY CARE � JULY 2021 VOL 66 NO 7

monitoring and use. Anesthesia machines are fundamentally

different than critical care ventilators, and respiratory thera-

pists should be familiar with these differences. All providers

must be aware of the potential problems and limitations

when using these devices with patients with COVID-19.

ACKNOWLEDGMENTS

Special thanks to Michael Dosch PhD CRNA and Shari Burns EdD

CRNA for their input and review of this manuscript.

REFERENCES

1. Wunsch H. Mechanical ventilation in COVID-19: interpreting the cur-

rent epidemiology. Am J Respir Crit Care Med 2020;202(1):1-4.

2. US Food and Drug Administration. Ventilator supply mitigation strat-

egies: letter to health care providers. March 22, 2020. Available at:

https://www.fda.gov/medical-devices/letters-health-care-providers/

ventilator-supply-mitigation-strategies-letter-health-care-providers.

Accessed December 20, 2020.

3. American Society of Anesthesiologists and Anesthesia Patient Safety

Foundation. APSF/ASA guidance on purposing anesthesia machines

as ICU ventilators. May 7, 2020. Available at: https://www.asahq.org/

in-the-spotlight/coronavirus-covid-19-information/purposing-anesthesia-

machines-for-ventilators. Accessed December 20, 2020.

4. Dosch M. Using the anesthesia workstation as a ventilator for critically

ill patients: technical considerations. AANA J 2020;83(3):13-17.

Available at: https://www.aana.com/docs/default-source/aana-journal-

web-documents-1/dosch-2.pdf?sfvrsn¼b2aa1640_6. Accessed November 22, 2020.

5. Louie S, Morrissey BM, Kenyon NJ, Albertson TE, Avdalovic M. The

critically ill asthmatic–from ICU to discharge. Clin Rev Allergy

Immunol 2012;43(1-2):30-44.

6. Wilkes R, Tasker RC. Intensive care treatment of uncontrolled status

epilepticus in children: systematic literature search of midazolam and

anesthetic therapies. Pediatr Crit Care Med 2014;15(7):632-639.

7. Riutort KT, Eisenkraft JB, et al. The anesthesia workstation and deliv-

ery systems for inhaled anesthetic. In Barash PG, Cullen BF, Stoelting

RK, Cahalan MK, Stock MC, Ortega R, editors. Clinical Anesthesia,

8th Edition. Philadelphia: Wolters Kluwer; 2017:644-705.

8. Feldman JM, Hendrickx J, Kennedy RR. Carbon dioxide absorption

during inhalation anesthesia: a modern practice. Anesth Analg

2021;132(4):993-1002.

9. GE Healthcare. Avance CS2 Brochure. Available at: https://www.

vingmed.dk/wp-content/uploads/sites/3/2020/06/ANE_EMEA_

English_Avance_CS2_Spec_sheet_102013.pdf. Accessed December 7,

2020.

10. Datex-Ohmeda, Inc. Aespire 7900 User’s Reference Manual - Part

1. 2004. Available at: https://www.medwrench.com/documents/view/

10907/datex-ohmeda-aespire-7900-user-manual-pdf. Accessed December

7, 2020.

11. GE Healthcare. Aisys CS2 Brochure. 2013. Available at: http://www3.

gehealthcareturkiye.com/�/media/documents/turkey/products/ anesthesia%20delivery/product%20specification%20sheets/aisys%

20cs2/aisys%20cs2%20spec%20sheet.pdf?Parent¼%7B4EAD3924- 0CD3-4A07-B9EF-FC1F9B92A68A%7D. Accessed December 7, 2020.

12. Datex-Ohmeda, Inc. Aestive/5 Operation Manual Part 1. 2003.

Available at: http://www.meditegic.com/wp-content/uploads/pdfs/

Aestiva_User_Manual_part1.pdf. Accessed December 7, 2020.

13. Mindray. A7 Workstation. 2017. Available at: https://www.

mindrayuk.com/wp-content/uploads/2020/07/A7-Specifications.pdf.

Accessed December 10, 2020. 14. Mindray North America. A Series Advantage Product Comparison.

2018. Available at: https://www.mindraynorthamerica.com/wp-content/

uploads/2018/11/A-Series-Comparison-Chart.pdf. Accessed December 7, 2020.

15. Draeger, Inc. Drager Fabius GS Premium (brochure). Available at:

https://www.draeger.com/Products/Content/fabius-gs-premium-br-

9066189-us.pdf. Accessed December 7, 2020.

16. Draeger Medical, Inc. Drager Apollo Anesthesia Workstation (bro-

chure). Available at: https://www.draeger.com/Products/Content/apollo-

br-9050390-us.pdf. Accessed December 7, 2020. 17. Draeger Medical, Inc. Drager Perseus A500 Brochure. 2015. Available

at: https://www.draeger.com/Products/Content/perseus-mechanical-pi-

9069004-en-1506-2.pdf. Accessed December 7, 2020. 18. Haina KMK. Use of anesthesia machines in a critical care setting dur-

ing the coronavirus disease 2019 pandemic. A A Pract 2020;14(7)

e01243.

19. Xi AS, Chang MG, Bittner EA. Rapid establishment of an ICU using

anesthesia ventilators during COVID-19 pandemic: lessons learned.

Crit Care 2020;24(1)388.

20. Austin PN, Dosch MP. Transporting anesthesia workstations to hospi-

tals for use as ventilators. AANA J 2020;83(3):24-28. Available at:

https://www.aana.com/docs/default-source/aana-journal-web-documents-

1/austin-r.pdf?sfvrsn¼d12b6728_6. Accessed December 7, 2020. 21. Greig PR, Dixson T, McCorkell S. A process for daily checks when

using anaesthetic machines to ventilate the lungs of COVID-19

patients: the ’domino switch’ technique. Anaesthesia 2020;75(7):973.

22. Beitler JR, Mittel AM, Kallet R, Kacmarek R, Hess D, Branson R, et al.

Ventilator sharing during an acute shortage caused by the COVID-19

pandemic. Am J Respir Crit Care Med 2020;202(4):600-604.

23. The Society of Critical Care Medicine, American Association for

Respiratory Care, American Society of Anesthesiologists, Anesthesia

Patient Safety Foundation, American Association of Critical-Care

Nurses, American College of Chest Physicians. Consensus statement

on multiple patients per ventilator. March 26, 2020. Available at:

https://www.sccm.org/Disaster/Joint-Statement-on-Multiple-Patients-

Per-Ventilato. Accessed December 7, 2020. 24. Canelli R, Spence N, Kumar N, Rodriguez G, Gonzalez M. The ventila-

tor management team: repurposing anesthesia workstations and person-

nel to combat COVID-19. J Intensive Care Med 2020;35(9):927-932.

25. Fullick J, Oliver M. Water, water, everywhere: a challenge to ventilators

in the COVID-19 pandemic. Br J Anaesth 2020;125(1):e188-e190.

26. Panchamia RK, Piracha MM, Samuels JD. Desperate times: repurpos-

ing anesthesia machines for mechanical ventilation in COVID-19. J

Clin Anesth 2020;66:109967.

27. Levin MA, Burnett G, Villar J, Hamburger J, Eisenkraft JB,

Leibowitz AB. Anesthesia machine as an ICU ventilator: a near

miss. APSF Newsletter. 2020. Available at: https://www.apsf.org/

article/anesthesia-machine-as-an-icu-ventilator-a-near-miss/. Accessed

December 20, 2020. 28. Orser BA, Byrick R, Cooper R, Henry E, Lau P, Rittenberg B, et al.

Locating and repurposing anesthetic machines as intensive care unit

ventilators during the COVID-19 pandemic. Can J Anaesth 2020;67

(8):1066-1067.

29. Gouel-Cheron A, Couffignal C, Elmaleh Y, Kantor E, Montravers P.

Preliminary observations of anaesthesia ventilators use for pro-

longed mechanical ventilation in intensive care unit patients dur-

ing the COVID-19 pandemic. Anaesth Crit Care Pain Med

2020;39(3):371-372.

ANESTHESIA MACHINES FOR CRITICAL CARE

RESPIRATORY CARE � JULY 2021 VOL 66 NO 7 1195