Br. J. Anaesth. (1986), 58, 1048-1054
EXCESSIVE WORK OF BREATHING DURING
INTERMITTENT MANDATORY VENTILATION
J. S. MECKLENBURGH, I. P. LATTO, T. A. A. AL-OBAIDI, E. A. SWAI
AND W. W. MAPLESON
Ventilators with facilities for intermittent mandatory ventilation (IMV) are intended to assist the
weaning of patients from controlled ventilation
(Downs et al., 1973; Margand and Chodoff, 1975;
Browne, 1984). In a perfect IMV ventilator,
breathing spontaneously through the breathing
system would be indistinguishable from breathing
directly from atmosphere — apart from the
different gas mixture, and assuming that no
positive end-expiratory pressure (PEEP) or
continuous positive airway pressure (CPAP) has
been set. A less than perfect IMV ventilator may
hinder spontaneous breathing by imposing additional resistance, or it may assist it if the supply
mechanism is such that mouth pressure is more
positive during inspiration than during expiration.
In the former, weaning may be delayed unnecessarily; in the latter the anaesthetist may be misled
into disconnecting the patient from the ventilator
prematurely. The present report analyses an
instance in which the resistance in the breathing
system resulted in difficulty in weaning.
The investigation was prompted by our experience with a 72-year-old female patient who had
had a mitral valve replacement. Attempts to wean
her from the ventilator over a period of 4 days
were unsuccessful. When she was allowed to
breathe spontaneously, or when her lungs were
ventilated at a low IMV rate, an increase in
arterial carbon dioxide tension and a decrease in
oxygen tension occurred, and the patient quickly
became distressed. Eventually an alternative IMV
J. S. MECKLENBURGH, B.SC., MJC.J I. P. LATTO, M.B., B.S.,
F.F.A.R.C.S.J E . A . SWAI, M.B., CH.B, F.FJV.R.C.S.; W . W.
MAPLESON, D . S C , F.INST.P.; Department of Anaesthetics,
University of Wales College of Medicine, Heath Park, Cardiff
CF44XW.T. A. A. AL-OBAmi,B^c.,PHJ).,M.rNST.p.,College
of Medicine, Al-Mustansiriyah University, Baghdad (presently
Visiting Colleague, University of Wales College of Medicine).
SUMMARY
Difficulties were experienced in weaning a
patient from a ventilator by means of intermittent
mandatory ventilation (IMV). The difficulty was
overcome by installing an alternative IMV system
(Hudson "disposable IMV valve") through
which the patient drew her spontaneous breaths.
Laboratory measurements showed that the resistance of the ventilator breathing system was
much higher than that of the alternative system,
mainly as a result of the resistance of the
humidifier. It was calculated from measurements
with a preset pattern of simulated breathing that
the extra, external, work of breathing through the
ventilator breathing system was approximately
1.5 times the normal internal mechanical work of
breathing for a normal patient; with the alternative
system, the extra work was only 0.5 times the
normal. It is stressed that the breathing systems
of IMV ventilators should be judged with the
same rigour as other systems through which the
patient is required to breathe spontaneously. It is
recommended that manufacturers should pro vide
the necessary information.
system (Hudson "disposable IMV valve") was
connected into the breathing system so that, in
spontaneous respiration, gas was drawn from this
alternative system instead of through the breathing
system of the ventilator. With this arrangement
the patient did not become distressed, and the
carbon dioxide and oxygen tensions were acceptable. Weaning from mechanical ventilation was
accomplished and the trachea extubated.
Since it was suspected that the original
difficulty in weaning was caused by a high
resistance in the ventilator breathing system, a
IMV: WORK OF BREATHING
1049
Gas inlet
Reservoir tub*
Alternative
One-way valve
IMV
system
Pneumotachograph
/
Pressure relief valve
Filter
Ventilator
Humidifier
Flow |
output
Modified Starling pump
Pressure
tapping
FIG. 1. Diagram showing ventilator breathing system, alternative IMV system and method of imitating
spontaneous breathing with a modified Starling pump. The alternative IMV system was connected into
the ventilator breathing system at the point indicated, when required.
laboratory study was undertaken to measure the
resistance of both breathing systems. In addition,
the work of breathing through the two systems
was estimated, because this is a more relevant
indicator of the burden imposed on the patient.
For instance, if a patient is made to breathe
through an external resistance equal to his own
respiratory system resistance, this doubles the
total resistance. However, during inspiration, the
patient has to do work not only on the total
resistance, but also on his own compliance.
Therefore, a more realistic representation of the
burden placed on the patient's respiratory muscles
is the percentage increase in the total work of
breathing.
Incidentally, it should be noted that, although
it is customary to speak loosely of the "work of
breathing", strictly, the terms should be the
"work of one breath" (or one inspiration) or the
"mean power of breathing" (the mean rate of
working averaged over one or more complete
breaths). The "work per litre of total ventilation"
has also been used (Mcllroy, Marshall and
Christie, 1954).
MATERIALS AND METHODS
The laboratory experiments were performed on
the same ventilator and breathing system as had
been used with the patient. The ventilator was a
Kontron 3100 with Varicontrol, a Kontron Pearl
3150 humidifier was included in the inspiratory
line, and a new dry PALL bacterial filter was
attached to the patient connection of the Y-piece
(ng. 1).
Spontaneous ventilation was simulated using a
modified Starling pump, set to deliver a tidal
volume of 500 ml at a frequency of 18.75 b.p.m.
and with an I:E ratio of 1:1.7. This produced an
approximately sine-wave flow pattern in each
half-cycle of respiration. Mandatory breaths
delivered by the ventilator were allowed to escape
to atmosphere through a pressure relief valve, set
to open at approximately 7 kPa, since the Starling
pump could not imitate lung compliance. The
humidifier was dry and unheated during the test,
so that the difficulties of measuring the flow of
humidified and heated gas were avoided. Careful
consideration of the design of the humidifier, the
alternative IMV valve and the expiratory valve of
the ventilator indicated that the use of warm
humidified air would not alter the performance of
these components to any great extent. The filter,
however, would show an increase in resistance
with time as water condensed within it and the use
of warm air in this study would, if anything,
diminish the difference between the Kontron and
alternative systems. When the alternative IMV
system (Hudson disposable IMV valve) was
connected into the breathing system (fig. 1),
inspired gas was drawn from this system instead
1050
BRITISH OF JOURNAL ANAESTHESIA
of from the ventilator, but expiration was still via
the normal expiratory pathway of the ventilator.
Although oxygen-enriched, humidified air had
been supplied to the alternative IMV system in
clinical use, room air was used for the laboratory
study.
Pressure and flow were measured at the
"mouth" using a strain gauge transducer for
pressure, and a screen pneumotachograph connected to a Greer differential pressure transducer
(Mercury Electronics) for flow, and displayed on
a chart recorder (Lectromed). The transducers
were calibrated before, and the calibration verified
after, the recording using a "series calibrated"
flow meter (Fisher Controls) and a liquid-filled
manometer.
The pressure-flow characteristics of the breathing systems were determined using a range of
steady inspiratory and expiratory flows. Pressureflow characteristics were also obtained "dynamically", when spontaneous ventilation was
imitated with the Starling pump, by digitizing the
flow and pressure signals at 0.04-s intervals and
plotting one against the other.
When pressure (P) and flow rate (P) are
monitored at the mouth the product, P. V, is the
instantaneous power ifr at any moment during the
respiratory cycle. This was computed at 0.04-s
intervals over a complete breath. The work done
in any defined time interval r0 to t1 is then given
by:
W
8 -0.4 -
-0.8 -
-1
-O.5
Flow (litre s-1)
FIG. 2. Pressure-flow characteristics using steady flows for the
different breathing system configurations. The curve in the
upper right quadrant is for the expiratory limb which was
common to all systems. The lower left quadrant shows the
characteristics of the inspiratory limb of: A = the complete
IMV system (Kontron ventilator, humidifier and filter);
B = as in A, but with the humidifier removed; C ™ as in B, but
with the bacterial filter removed; D «• the alternative IMV
system; E = the recommended upper limit of breathing
system resistance (Nunn, 1977).
Wdt
Jt-u,
The period over which work is calculated can be
the whole or part of the respiratory cycle. Work
calculated in this way (using P as the difference
between "mouth" and atmospheric pressure) is
the work done by the patient on the attached
breathing system during spontaneous breathing.
This method of determining work done is the
one used by Engstrom and Norlander (1962); it
avoids the difficulties frequently encountered in
interpreting pressure-volume loops and also
provides information on the waveform of power.
Computation was performed by a BBC microcomputer running BBC Basic and graphical
output was to a Hewlett-Packard 7470A graph
plotter.
In order to determine the sites of power
dissipation, experiments were repeated following
the removal of individual components of the
breathing system.
RESULTS
Pressure-flow characteristics for various configurations of the breathing systems, using steady flows,
are shown in figure 2. The slope of each
characteristic indicates the resistance. The inspiratory limb of the complete ventilator breathing
system (ventilator, humidifier and filter) exhibited
the highest resistance (steepest curve, curve A): at
a flow of 0.5 litre s"1 the pressure at the patient
connection was 0.6 kPa, that is, a resistance of 1.2
kPa litre"1 s. The total resistance at 0.5 litre s"1 was
reduced to 0.6 kPa litre"1 s on the removal of the
humidifier (curve B), and to 0.5 kPa litre"1 s on
removal of the filter (curve C). The alternative
IMV system had an inspiratory resistance of only
0.4 kPa litre"1 s (curve D) at 0.5 litre s"1. The upper
limit for resistance recommended by Nunn (1977)
is shown for comparison (curve E).
1051
IMV: WORK OF BREATHING
1
-1
FIG. 3. Flow, pressure, power and accumulated work during a simulated spontaneous breath (500 ml,
18.75 b.p.m.) for the different breathing system configurations. A = Complete IMV system; B = as in
A, but without humidifier; C — as in B, but without filter; D — alternative IMV system.
TABLE I. Work done (mj) on tht two IMV systems during a
simulated spontaneous breath of 500 ml tidal volume at 18.75
b.p.m.
Kontron breathing
system
Kontron with the
alternative IMV
breathing system
Inspiratory
phase
Expiratory
phase
Total
272
90
362
90
88
178
The expiratory pathway was the same for all
systems and, therefore, a single pressure-flow
curve was obtained. The resistance at aflowof 0.5
litre s"1 was 0.6 kPa litre"1 s.
Digitized values of pressure and flow obtained
during simulated spontaneous breathing gave
essentially the same pressure-flow characteristics
as from steady flows.
The flow, pressure, power and accumulated
work during a simulated spontaneous breath are
displayed against time for the different breathing
system configurations in figure 3. The flow
waveforms generated by the Starling pump were
similar for all four configurations (fig. 3 a).
Pressure waveforms, on the other hand, were
different (fig. 3 b) and, therefore, so were the
waveforms of power (fig. 3 c) and of accumulated
work (fig. 3d). (Accumulated work is the integral
of power from the start of inspiratory flow to the
current time.) The peak inspiratory power
required during a simulated breath was reduced
from 0.64 W with the ventilator IMV system, to
0.19 W with the alternative IMV system.
Table I gives the calculated work for the
inspiratory and expiratory phases for the two main
breathing systems. With the alternative IMV
system, calculated work in the inspiratory phase
was reduced to one-third of that with the original
system; expiratory work remained almost the
same and total work done (over a whole cycle) was
approximately halved.
DISCUSSION
IMV has potential advantages when weaning a
patient from controlled ventilation, but problems
may occur with patients whose lung mechanics are
BRITISH OF JOURNAL ANAESTHESIA
1052
0.8
0.6
0.6
c+m-A
0.5
0.4
C+R+D
0.3
0.2
0.1
0
1
2
3
4
Time (s)
FIG. 4. A: Power required to ventilate a lung with a total
compliance of 1 litre kPa"1 and a total resistance of 0.6 kPa
litre"' 8 derived theoretically using the same flow pattern as
during the simulated breaths shown in figure 3. Curves R and
C show the power dissipated in the resistance and absorbed and
released by the compliance, respectively. B : Accumulated work
over simulated spontaneous breaths. Curve C + R shows the
work calculated from the power given in A and represents the
total work done against the natural compliance and resistance
without any additional load. Curves C + R + D and C + R + A
show the total work of breathing (see text) when the alternative
(D) and ventilator (A) IMV systems are connected to the above
compliance and resistance.
abnormal (Gilston, 1977). It is clear from the
present study that a low breathing system
resistance is also of considerable importance
during weaning with IMV.
The humidifier was the major contributor to the
inspiratory resistance of the original IMV system.
The Draft International Standard 8185, "Humidifiers for Medical Use", does not specify a
maximum resistance to flow, but defines a test for
measuring such resistance and states that the
resistance toflowmust be quoted if the humidifier
is stated to be suitable for use within a breathing
system attached to a spontaneously breathing
patient.
The Draft International Standard 5369,
"Breathing Machines for Medical Use", is
equally unhelpful; it defines a maximum expiratory resistance for the breathing system of a
ventilator, but it does not mention a value for
inspiratory resistance. However, the Draft International Standard 8382, " Resuscitators Intended
for Use with Humans", does define a maximum
inspiratory resistance as a maximum pressure
below ambient of 0.5 kPa at the patient connection
when an inspiratoryflowof 50 litre min"1 is drawn
from the resuscitator (equivalent to a resistance of
0.6 kPa litre"1 s). In comparison, normal human
respiratory tract resistance is approximately 0.2
kPa litre"1 s at a flow rate of 0.5 litre s"1 (Nunn,
1977) and for anaesthetized patients it may be in
the range of 0.4-0.6 kPa litre"1 s at the same flow
rate (Bergman, 1969).
The ventilator breathing system studied in this
communication exhibited a total resistance of 1.2
kPa litre"1 s at a flow rate of 0.5 litre s"1, the
humidifier contributing about one-half of this
(0.64 kPa litre"1 s). This resistance was about twice
the upper limit recommended by Nunn (1977)—
compare curves A and E in figure 2—whereas the
alternative IMV system exhibited an inspiratory
resistance well below the limit. However, when
the ventilator breathing system, with the humidifier and filter removed, was compared with the
alternative IMV system, then there was only a
small difference in resistance—curves C and D.
Therefore, if the alternative IMV system is
"teed " into the ventilator breathing system on the
ventilator side of the humidifier, as is occasionally
done, the reduction in resistance will be
negligible.
The patient has to do work against his compliance as well as against resistance; therefore,
rather than compare just the added resistance
with the natural resistance, it is more relevant to
compare added work with natural work. Accordingly, the theoretical instantaneous power required to ventilate a lung with characteristics
representative of those of a conscious, intubated
patient (compliance = 1 litre kPa"1; resistance =
0.6 kPa litre"1 s) was calculated for theflowpattern
used in the experimental measurements. The
results are shown in figure 4A where curve R
indicates the power dissipated in the resistance and
curve C the power associated with the compliance.
IMV: WORK OF BREATHING
The fact that curve C is positive in inspiration and
negative in expiration reflects the fact that power
is absorbed by the compliance during inspiration
and released during expiration, that is, the energy
stored within the compliance during inspiration is
available to do work during expiration. The
accumulated work (on compliance plus resistance)
is the sum of the integrals of curves C and R in
figure 4A and, for a complete breath, is shown by
curve C + R in figure 4B. This "internal" work is
the work which the patient must do on his own
respiratory system and the tracheal tube when
breathing from atmosphere. The reasons for the
dip during expiration is that the energy released
by the compliance is greater than that required to
overcome resistance. This is discussed further
below. The accumulated work over the complete
respiratory cycle is of the order of 0.2 J and is done
mainly during inspiration. When such a lung is
connected to a breathing system, the work
required to overcome the impedance of the
breathing system must be added to the " internal"
work. In these circumstances, with the complete
ventilator breathing system (curve C + R + A in
figure 4B), the total work required was about 0.5
J, an increase of approximately 150% of that
done in breathing directly from atmosphere; with
the alternative IMV breathing system (curve
C + R + D) the work required was about 0.3 J, an
increase of only 50 %. Note that curve C + R + D
is the sum of curve C + R in figure 4B and curve
D in figure 3d. Similarly, curve C + R + A is the
sum of curve C + R and curve A. (The expiratory
portions of the curves in figure 4B are a little
artificial, because the Starling pump produces a
somewhat unphysiological waveform of expiratory
flow. However, this does not materially affect the
above argument, because the main differences
between the three curves lie in the inspiratory
portion).
1053
do this work. However, an additional complication
is that the patient's respiratory muscles normally
oppose expiration initially, because the inspiratory
muscles relax only gradually during the first part
of expiration. Therefore, mechanically, work is
then being done by the compliance, not only on
the resistance, but also on the respiratory muscles.
This would produce a decline in accumulated
work in the first part of expiration, much as in
curve C + R of figure 4B (between 1 and 2 s)—
although, there, the decline can be attributed to
the model compliance doing work on the model
pump. However, it seems most unlikely that the
muscles can in any way make use of the energy
coming from the compliance and they are almost
certainly still consuming oxygen during this
gradual relaxation as a result of being in a state of
contraction.
(2) When a load is imposed on a patient, the
waveform of flow during a respiratory cycle may
change.
(3) In addition, the tidal volume and frequency
may change even though the total, or at least
alveolar, ventilation may remain much the same.
(4) The respiratory muscles also do mechanical
work on the circulation—the "thoracic pump".
Therefore, if the magnitude or pattern of the
intrapleural pressure changes with external load,
the amount of work done on the circulation may
change.
(5) Any change in the magnitude or pattern of
mechanical work may affect the efficiency of the
respiratory muscles and, hence, their oxygen
consumption which, rather than the mechanical
work done, may be the limiting factor in some
patients being weaned from a ventilator.
Despite all these caveats, it remains true that
estimates of the change in the mechanical work of
one breath, offixedtidal volume, duration and I: E
ratio (on breathing through an IMV system
Although we have argued in favour of expressing instead of from the atmosphere) are more
the added burden of a breathing system in terms of informative than changes of total resistance—
work instead of resistance, the calculations which especially since some IMV systems may impose a
we have made for a fixed flow pattern through compliance load or may actually assist the
different loads still do not give a completely fair patient's breathing (Bingham, Hatch and Helms,
picture of energy consumed by the muscles 1986).
because of the following five factors:
(1) During expiration, work must be done on
CONCLUSION
respiratory resistance and any external load, but
only in extreme circumstances will the expiratory During controlled ventilation all the work of
muscles be called into play to provide the breathing is done by the ventilator and, hence, the
necessary energy. Normally, energy stored in the inspiratory resistance of the breathing system is
compliance during inspiration will be adequate to relatively unimportant. However, as soon as IMV
1054
is introduced, the patient has to do the work
during the spontaneous breaths; thus, the characteristics of the IMV system are just as important
as with any breathing system for spontaneous
ventilation. Therefore, before using the IMV
mode of a ventilator, the anaesthetist should
consider the characteristics of its breathing system
for spontaneous inspiration and, in particular,
note that a humidifier which is perfectly satisfactory during controlled ventilation may be
unacceptable for IMV use. It is recommended
that manufacturers should publish resistance
values of breathing system components if they are
intended for use with both controlled and IMV
modes of ventilation.
REFERENCES
Bergman, N. A. (1969). Properties of passive exhalations in
ancsthetised subjects. Anesthesiology, 30, 378.
BRITISH OF JOURNAL ANAESTHESIA
Bingham, R. M., Hatch, D. J., and Helms, P. J. (1986).
Assisted ventilation and the Servo ventilator in infants. An
assessment of three systems used for CPAP/IMV. Anaesthesia, 41, 168.
Browne, D. R. G. (1984). Review article, weaning patients
from mechanical ventilation. Intent. Care Med., 10, 55.
Downs, J. B., Klein, E. F., Desautels, D., Modell, J. H., and
Kirby, R. R. (1973). Intermittent mandatory ventilation: A
new approach to weaning patients from mechanical
ventilators. Chest, 64, 331.
Engstrom, G. G., and Norlander, O. P. (1962). A new method
for analysis of respiratory work by measurement of the
actual power as a function of gas flow, pressure and time.
Acta Anaesthesiol. Scand., 6, 49.
Gilston, A. (1977). Intermittent mandatory ventilation: Are
IMV, MMV, PEEP or sighing advantageous? Anaesthesia,
32, 665.
Mcllroy, M. B., Marshall, R., and Christie, R. V. (1954). The
work of breathing in normal subjects. Clin. Sci. 13, 127.
Margand, P. M. S., and Chodoff, P. (1975). Intermittent
mandatory ventilation. An alternative winning technic, a
case report. Anesth. Analg., 54, 41.
Nunn, J. F. (1977). Applied Respiratory Physiology, 2nd Edn,
p. 103. London: Butterworths.