continuous positive airway pressure. work due to

Pressure support compensation for inspiratory work due to endotracheal tubes and demand continuous positive airway pressure. J F Fiastro, M P Habib and S F Quan Chest 1988;93;499-505 DOI 10.1378/chest.93.3.499 The online version of this article, along with updated information and services can be found online on the World Wide Web at: http://chestjournal.chestpubs.org/content/93/3/499

Chest is the official journal of the American College of Chest Physicians. It has been published monthly since 1935. Copyright1988by the American College of Chest Physicians, 3300 Dundee Road, Northbrook, IL 60062. All rights reserved. No part of this article or PDF may be reproduced or distributed without the prior written permission of the copyright holder. (http://chestjournal.chestpubs.org/site/misc/reprints.xhtml) ISSN:0012-3692

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Pressure Support Compensation for Inspiratory Work due to Endotracheal Tubes and Demand Continuous Positive Airway Pressure* I.

Ferdinand

andStuartF

Fiastro,

M.D.,

Quan,

M.D.,

F.C.C.P.;t

Michael

P. Habib,

tubes

with

and

without

a ventilator

demand

CPA!’ circuit. Added work was measured as the integral of the product of airway pressure and volume during inspiration. Additional work was a function of the tube’s size, and each 1-mm decrease in the tube’s diameter resulted in a 67 to 100 percent increase in work. Adding the ventilator CPAP circuit further increased work and was responsible for 30 to 50 percent ofthe total work resulting from a tube and CPAP circuit together. Pressure support was added to a level at which net work on the airway was zero, and a relationship

W

hen

an intubated

through an (!MY) or demand (CPAP)

ventilator

patient

intermittent continuous circuit,

breathes

both

the

resistance

Pressure which

muscle fatigue and interfere with from mechanical ventilation.78 support is a mechanical ventilator

applies

a predetermined

airway

pressure to a patient’s phase ofany spontaneous IMV may

amount

breath

mode. It has been proposed be used to compensate

creased inspiratory endotracheal tube

guidelines are not

work and

of the

The

success-

support

option

of positive

the inspiratory in the CPAP or

for this purpose

of this

study

was

to

5From the Division ofRespiratory Sciences, Department of Internal Medicine, University ofArizonaCollege ofMedicine, and Veterans Administration Medical Center, Tucson. Presented in part at the 52nd Annual Scientific Assembly, American College ofChest Physicians, San Francisco, Sept 23, 1986. tClinical tAssoclate

Assistant

Profossor

for

of Medicine.

Professor of Medicine. Manuscript received May 26; revision accepted August 26. Reprint requests: D. Habib, VA Medical CenterllU, Thcson

system

support can compensate work due to breathing and a ventilator demandand

to establish

guidelines

use.

MATERIALS Mechanical

Model

ofLung pump

reciprocating model

to

AND

METHODS

and Computation Apparatus

(Harvard

simulate

a patient’s

Work

ofinspiratory

Co) was used spontaneous

as a

breathing

effort (Fig IA). The “mouth” of this model was connected to an endotracheal tube alone or to endotracheal tube and a ventilator demand CPAP circuit. Air flow (V) was measured proximal to the endotracheal tube using a screen pneumotachygraph (Electronics for Medicine, adult-size) which was calibrated to flows ofO to 1.5 us. Volume (V) was integrated from the airway flow signal using a respiratory integrator (Hewlett-Packard 88L5A). Airway pressure (Paw) was measured between the pneumotachygraph and endotracheal tube with a pressure transducer (Validyne DP45). Inspiratory work was computed as the integral of the product of Paw and V during inspiration. Inspiratory work measured when Paw was negative represented work done by the simulated respiratory system (Was). Inspiratory work measured during times of positive Paw represented work done on the respiratory system by the Ventilator (Wv). Net airway vork (Waw) was defined as Was minus Wv. Airway pressure, flow, volume, and work were recorded on a four-channel

time-based

strip-chart

recorder

(Hewlett-Packard

work per liter of ventilation (WilL) and mean inspiratory flow (VTPfl) were calculated as the mean of ten cyded breaths during each condition of measurement. 7754A).

85723

whether pressure added inspiratory endotracheal tubes

gas delivery its

A

that pressure support selectively for the in-

intention

valve

mechanical

due to breathing through an ventilator circuit; however,

for use ofpressure available.

during while

mean inspiratory fiow(VTPfl) and the optimal level of pressure support was established for each endotracheal tube. The inspiratory work ofbreathing was then measured in normal subjects breathing with and without each endotracheal tube plus the demand CPAP circuit. Work per liter of minute ventilation due to the endotracheal tube and CPAI circuit was increased from 54 to 240 percent over levels measured while breathing through an open airway. For each endotracheal tube and VTPfl, a level of pressure support(range, 2 to 20 cm HO)was found which eliminated added work in the spontaneously breathing subject. This level correlated well with that predicted from the data derived using the mechanical model. We conclude that when adjusting for an endotracheal tube’s diameter and Vs-I Ti, pressure support can be used to compensate for the added inspiratory work due to artifical airway resistances. between

determine for the through

spontaneously

mandatory ventilation positive airway pressure

endotracheal tube and the ventilator circuit can contribute to the inspiratory work ofbreathing.’4 The level ofthis additional work increases as the diameter of the endotracheal tube decreases and minute ventilation increases. Increased inspiratory work can contribute to respiratory ful weaning

F.C.C.P.;t

F.C.C.P.

We evaluated the use ofpressure support to compensate for the added inspiratory work of breathing due to the resistances of endotracheal tubes and a ventilator demand-valve system for continuous positive airway pressure (CPAP). A mechanical modeiwas used to simulate spontaneous breathing at five respiratory rates through 7-mm, 8-mm, and 9-mm

endotracheal

M.D.,

Inspiratory

CHEST


I 93

I 3 I MARCH, 1988

499

A

Pneumotach

(;f)

Pressure

Transducer

of approximately

1:2 (actual

L’s,

At each

respectively).

VT/TI=0.38,

respiratory

0.54, rate,

0.64,

work

0.74,

0.83

was measured

with

(1) an open airway, (2) each size ofendotracheal tube alone, size ofendotracheal tube plus the ventilator circuit.

(PAW)

and (3)

each

For each pressure

condition

sure-support

spontaneous

Transducer

is

breath. ofPaw,

of the

pressure Pressure

added

option

deflection

B

was

selected

When

ventilator.

with

the Puritan-Bennett inspiratory phase

the

inspiratory support

eflbrt

Pressure

The

pressure

support

tube

size,

the

pres-

When

is sensed

is activated.

end-expiratory

support.

endotracheal

each

during

pressure

positive

rate and

with

it is operative

ventilator,

sum

ofrespiratory

support

as a negative

target

(PEEP)

is terminated

7200 of any Pawis

and

ifPaw

the

the

set

exceeds

the

target Paw by 1.5 cmH2O or when inspiratory flow is 5 L/min or less. Pressure support can be applied over a range of 0 to 30 cmH,O in

(PES)

increments

ofl

Without

cmH2O.

pressure

system

(Waw

circuit

resistance and

Wv

pressure

support

2A).

As pressure

increases.

decreased

negative),

is added

and

during

to the

increased,

inspiration

tube

support

We reasoned

Wv

as in standard

work

to the endotracheal

was that level at which 2B). With higherlevels

(Was=Wv)(Fig on the airway

there

due

(Fig

decreases,

further

support,

is positive)

respiratory

and ventilator

is increased,

that

the

Was

optimal

level

of

the Waw was equal to zero ofpressure

support,

resulting

beingdone

pressure-cycled

in most

by the ventilator mechanical

is

Was

of the

work

(Wawis

ventilation

(Fig

2C). With

each

condition

ofinspiratory

support

was

increased

until

linear

relationship

pressure Ficuan 1. Apparatus for measurement. A. Mechanical system used to simulate spontaneous breathing effort; Paw and V were recorded. B. Trained subjects breathed through same system while airway flow and Pes were recorded. All measurements were displayed on multichannel strip chart.

Pressure

Support

Three

different

were

calibrated

Compensationfor

Added

Irispiratory

7200 ventilators

Puritan-Bennett to the manufacturer’s

specification,

All with

patient’s circuit and humidifier(Cascade I, Puritan Co.), and set in the CPAP=0 mode with a sensitivity of0.5

Bennett

(fractional Shiley

concentration low-pressure

ofoxygen cuffed

internal

diameters

The

mechanical

model

pattern

with

volume

breaths

per

a tidal minute

of 7, 8, and

with

Airway

in the

endotracheal was

9 mm

were

cycled

ofSOO

inspired tubes

rates

an inspiration-to-expiration

signal.

0.21).

ofa

ofl5,

was a sinusoidal time

Respiratory used

as in the pressure

of

and

of

in four

trans-

normal

subjects.

rates

ofl5,

25,

of approximately

monitored displayed

pressure

model,

volume the

1:2. using

an

integrated

flow

held constantwith the aid through the same circuit as of the study (Fig 1B). In addition, were

breathed

part

(Pes)

A

level tube

ataVTof500mlat

which

subject first

as zero.

optimal

relationship mechanical

I:E ratios

rate and I:E ratio Each

endotracheal

the

subjects 5113)

each

measured

with

study,

the pressure

was

measured

using

an

esophageal

balloon.’A 10-cm latex balloon attached to aperforated polyethylene catheter was positioned transnasally into the esophagus and filled

20, 25, 30, and 35 (I:E)

the

this

the

to breathe

minute

(Tectronix

metronome.

esophageal

flow

per

of

was

size,

calculated

squares.

from

work

trained

breaths

oscilloscope

used.

to generate

mlat

=

35

usefulness derived

inspiratory

Throughout

30 cm) with

Support

was

between for

and Pressure

subjectwas

and

cmH2O

gas [FIoJ (length,

Each

a

VT/TI

ofleast

VT/TI

Waw

established the

the

and

net

the method

determine

pulmonary

tested.

standard

Work

support

equipped

and

using

Subject’s

Work

were

support

ventilator

To

was

rate and tube

the

ratio

Pressure

e

LVTJ Ps=o

A

Ventilation

Mechanical

PS=”optimal”

C

B

FIGURE 2. Airway pressure-volume curves with increasing pressure support. Schematic shows airway pressure-volume curves generated by mechanical breathing model with endotracheal tube and ventilator circuit. Shadedareas represent Was; and hatchedareas represent Wv without pressure support(A) and with “optimal” pressure support (that level at which Was-Wv and Waw=0) (B). With higher levels of pressure support (C), Waw is negative (1#{128}, performed mostly by pressure-support system).

500

Pressure

Support

Compensation

for Inspiratory


Work (Flastro,

Habib,

Quan)

-

.120

.110 E .100-

0) -J

.090-

0

.080-

.070

0

I

-

.060

a

U) C

.050

Cu C 0

.030

z

ii1 ii

.020

.010

EU

Diameter

(mm)=9

f(breaths/min) V1/T1(L/s) 3. Net

FIGURE

the

with

upright

8-mm,

and

circuit

(CPAP

appropriate position

8

7

=

through

9

(Waw) tubes

support

were

a mouthpiece

respiratory

seated in an into the circuit

integral

of the

area

enclosed

by the

plot

The

At each

mouthpiece plus

the

alone

respiratory

and through

ventilator

CPAP

rate,

subjects

each ofthe

of Pes and

circuit

breathed

three

(CPAP

0).

through

endotracheal

For

each

tube

model

net

breathing

additional

three

through 7-mm, and ventilator

in Waw = 0) was tube size for each

tested. A linear regression was points for each tube size (Fig 5). level of correlation between optimal and VT/TI. In each condition of

these

support

tracheal

(resulting for each

progressively 4). The optimal

ventilators

was a high

pressure

tube

size

than a 2-cm H20 among the three

a

7

Waw decreased support (Fig

pressure

fitted through There

8 35 .83

Support

increasing

ofthe

tubes

and

there

VTiTI,

variation ventilators

was

in optimal tested.

never

pressure

more support

and

size

level of mean inspiratory flow, pressure support was increased zero in 2-cm H,O increments up to 6 cm H,O above the predicted by the data from the mechanical lung.

9

level of pressure support plotted as a function ofVT/TI

pulmonary volume during inspiration. The beginning of inspiration was defined as the point at which either inspiratory flow began (when Paw = 0) or when the sign of the Paw became negative prior to inspiratory flow (during resistive breathing). Inspiratory work per liter of ventilation (WilL) was calculated from the pressure-volume curves of ten representative breaths during each condition of measurement.

system

with

I

7

30 .74

Pressure

an open glottis and occluded flares. Esophageal pressure, airway flow, and volume were recorded simultaneously, and VT/TI was calculated as the mean often breaths. Inspiratory work done by the subject on the lung and airway was as the

8

(open bars) and through endotracheal tubes (hatched bars) at five respiratory rates.

alone

0) together

=

9

.64

to mechanical (ETT)

7

25

with

measured

8

.54

work

0; pressure

7

20

of a’#{176}Subjects

breathed

8

.38

endotracheal

=

9

15

additional

9-mm

volume

and

1I

.040

0 #{149}0

from

level

.030 .

Err 7 mm 8 mm 9 mm

#{163}

RESULTS

.

S

.020W

0

Mechanical The

Model net

added

progressively tube

with

and

in the

percent

increase

increased

the

due

tube’s

size VT/TI

diameter

in work. work

to the

work

decreasing

increasing

decrease

work

inspiratory

with

(Fig

increased

endotracheal Each in a 67

the ventilator

The

endotracheal

3).

resulted

Adding

further.

(Waw) of the

tube

proportion

alone,

1-mm to 100

circuit of added

when

0

pared to the added work for the tube and ventilator together, was a function ofthe tube’s size. On average, a 9-mm

work, 70

tube

with

percent,

contributed 8-mm

and

respectively.

50 percent

7-mm

tubes

ofthe

contributing

total

added

60 and

t

_j

corn-

2 Pressure

FIGURE

4.

Net

additional

I

4

work

Support

by

(positive Waw) due to endotracheal with increasing pressure support endotracheal tubes at respiratory

6

(ETT) 9-mm,

rate of 20/mm

CHEST


1 93

10

(cm H20)

mechanical

tube for

8

respiratory

system

and ventilator 8-mm, and

and

1 3 I MARCH,

VT

circuit 7-mm

of 0. 5 L.

1988

501

*

.110

A 20

.100 *

.090 15

Ventilator

E

1 21

0)

3.

* a)

10

-J

Ps= 5

39.9

(VT IT1 )-10.9

.060h.

*

0

r0.99 p