in normal subjects breathing withand without each endo- tracheal tube plus the demand CPAP circuit. Work per liter of minute ventilation due to the endotracheal.
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
Downloaded from chestjournal.chestpubs.org by guest on July 11, 2011 © 1988 American College of Chest Physicians
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
Downloaded from chestjournal.chestpubs.org by guest on July 11, 2011 © 1988 American College of Chest Physicians
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
Downloaded from chestjournal.chestpubs.org by guest on July 11, 2011 © 1988 American College of Chest Physicians
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