Brit. J. Anaesih. (1967), 39, 659
THE HIGH OXYGEN MIXTURES DELIVERED BY THE AIR-MEX CONTROL
OF THE BIRD MARK 7 VENTILATOR
BY
G. A. HARRISON
Department of Anaesthetics, St. Vincent's Hospital, Sydney, Australia
SUMMARY
The reasons for the high oxygen mixtures delivered by the Bird Mark 7 ventilator were
investigated by measuring the flow of gases through various parts of the ventilator. The
major causes were the constant flow of 100 per cent oxygen from the nebulizer, the
decrease and eventual cessation of flow through the venturi as the airway pressure rose,
and the decrease in the proportion of entrained air at low flow settings.
When the Bird Mark 7 automatic ventilator is
driven by a source of compressed oxygen, an "airmix" control permits the administration of either
100 per cent oxygen or a mixture of oxygen and
air. This oxygen-air mixture is obtained when the
oxygen passes through an injector, entraining air
through a filter (fig. 1). The mixture then passes
through a spring-loaded one-way valve into the
righthand chamber of the ventilator and on to the
patient via a length of wide-bore tubing. Before
reaching the patient the mixture also passes
through a humidifier and through a nebulizer to
FIG. 1
Simplified diagram of the Bird Mark 7 ventilator. The arrows indicate the flow of gases during
the inspiratory phase.
1. Flow control
6. Oxygen supply for non8. Humidifier
2. Inspiratory pressure control
return valve and
9. Nebulizer
3. Filter for entrained air
nebulizer
10. Non-rebreathing valve
4. Venturi
7. Line for oxygen-air
11. Patient attachment
5. Spring-loaded valve
mixture
BRITISH JOURNAL OF ANAESTHESIA
660
which 100 per cent oxygen is added from the
ventilator by a length of small-bore pressure
tubing. This oxygen also exerts pressure on a nonrebreathing valve, keeping it closed during inflation. When the pressure in the righthand chamber
reaches a figure set by the operator, a diaphragm
snaps over to the left, the supply of oxygen is
interrupted, and the machine cycles to expiration.
Fairley and Britt (1964) investigated the mean
inspired oxygen percentages when the air-mix
control was used and found that they varied
directly with the cycling pressure and inversely
with the inspiratory flow rate. The oxygen concentrations ranged from 46 to 99 per cent but, at
any fixed flow or pressure, there was considerable
variation from one Mark 7 to another. Using a
technique similar to Fairley and Britt, the author
has confirmed their findings and has demonstrated
that a decrease in compliance or an increase in
airway resistance causes a further rise in the mean
inspired oxygen percentage at any particular flow
or pressure setting. This paper describes an investigation into the causes of the high inspired
oxygen mixtures and the reason for the increase
with changes in the compliance and airway resistance.
lator and the nebulizer. This demonstrated
the flow from the Bird ventilator of the oxygen-air mixture which had arisen from the
injector system.
(<f) In the wide-bore tubing between the nebulizer and the non-rebreathing valve. This
demonstrated the total inspiratory flow (mixture from ventilator plus nebulizer oxygen).
(e) Between the non-rebreathing valve and the
endotracheal tube. This demonstrated the
overall inspiratory and expiratory flow patterns. As the inspiratory flow pattern was
expected to be the same as found in (d), this
pneumotachometer acted as a check against
any leaks or additional sources of oxygen
around the non-rebreathing valve.
The pneumotachometer head was connected to
a Sanborn Model 270 differential transducer and
the output recorded on a Sanborn two-channel
recorder Model 321. On the other channel was
recorded the airway pressure measured in the
endotracheal tube by a Sanborn Model 267B pressure transducer.
The ventilator was set to a pressure setting of
25 and recordings taken at flow settings of 6, 10,
METHOD
Three Bird Mark 7 ventilators in regular use in
an intensive therapy ward were studied. The
ventilator was connected by a size 10 Magill endotracheal rube and non-distensible plastic tubing to
a rigid drum with a compliance of 23.2 ml/cm
KLO (fig. 2). Two one-way valves ensured the
mixing of the inflating gases which could be
collected and sampled as desired in a Douglas bag
en the expiratory side of the non-rebreathing
valve. The flow of gases in various parts of the
ventilator was measured by inserting a Fleisch
pneumotachometer head in one of five positions
in turn (fig. 2):
(a) Connected to ± e lefthand side of the ventilator containing the air filter by a surrounding layer of thick plasticine which was free
of leaks up to a pressure of 20 mm Hg. This
measured the flow of air which was entrained
into the Bird ventilator.
(6) In the small-bore tubing leading to the nonrebreathing valve and nebulizer.
(c) In the wide-bore tubing between the venti-
FIG. 2
The arrangement of the Bird Mark 7 ventilator and
the artificial lung (rigid drum). The flow of gases
through different parts of the circuit was measured
in turn by a pneumotachometer inserted in the positions shown. Airway pressure was measured at the
ventilator end of the endotracheal tube.
a. Pneumotachometer: entrained air filter
b. Pneumotachometer: oxygen to valve and nebulizer
c. Pneumotachometer: Bird side of nebulizer
d. Pneumotachometer: patient side of nebulizer
e. Pneumotachometer: between valve and "patient"
f. Oxygen analyzer
g. Douglas bag
h. Endotracheal tube
i. Rigid drum
HIGH OXYGEN MIXTURES FROM BIRD MARK 7 VENTILATOR
20, 30 and 40. The flow setting was then set at
20 and recordings taken at pressure settings of
15, 25, 40 and at a setting slightly in excess of 40
(>40). Recordings were repeated three times for
each of the three ventilators. The sensitivity was
fixed at 15 and the rate at 16 b.p.m. at intermediate flows and pressures. The rate then varied
with the duration of the inspiratory period which
depended on the particular flow or pressure setting. Tracings of flow from positions (a) (entrained air), (c) (air and oxygen from injector) and
(d) (air + oxygen from injector + nebulizer oxygen) were superimposed. It was possible to check
that no unsuspected leaks or obstructions had
occurred in the circuit during the change of site
of the pneumotachometer by comparing the pressure curve and the flow pattern from the same
site during different runs. The contribution of the
entrained air to the total air-oxygen mixture was
established by measurement of the areas under
the curves from positions (a) and (c) by planimetry. Finally the experiments were repeated at
a pressure setting of 20 and flow settings of 6, 10,
20, 30, and 40, (1) with the drum of compliance
23.2 ml/cm H2O and a size 10 Magill tube, (2)
with the drum of compliance 23.2 ml/cm H,O
and a size 3 Magill tube, and (3) with the drum of
compliance 12.5 ml/cm H2O and a size 10 Magill
tube.
661
ward the "patient" ceased, oxygen actually passed
in the reverse direction from the injector through
the filter to the atmosphere.
When the flow was increased to a setting of 10
the total flow into the "patient" increased to 37
l./min but again there was a constant contribution
of 8 l./min of oxygen from the nebulizer. The
flow from the Bird eventually stopped and reverse
flow occurred through the air filter. The period of
infiltration of the "patient" by the nebulizer oxygen only was much shorter.
At flow setting 20 the total flow had risen but
the contribution of the nebulizer remained a
RESULTS
Compliance 23.2 ml/cm H2O with size 10 Magill
tube.
When the pressure was set at 25 the flow pattern at flow setting 6 showed that the total flow
into the "patient" rose rapidly to 25 l./min but
quickly fell to a constant flow of 8 l./min which
was continued for a long inspiratory period. The
flow from the Bird rose to a level below the total
flow, the difference being due to the contribution
from the nebulizer oxygen (fig. 3). In addition the
flow from the Bird (oxygen-air mixture) quickly
fell to zero and in Bird No. 1 there was actually
a small flow towards the ventilator from the
nebulizer.
Once flow from the Bird had stopped, the
constant flow from the nebulizer continued until
the pressure in the system rose to that set for
cycling. The entrainment of air followed the flow
from the Bird but when flow from the Bird to-
TIME
3
Flow (l./min) and pressure (mm Hg) produced by the
Bird Mark 7 ventilator No. 1 at a fixed pressure setting
of 25 and flow settings (F) of 6, 10, 20, 30 and 40. The
compliance of the rigid drum was 23.2 ml/cm H,O and
the airway represented by a size 10 Magill tube. The
flow patterns at each flow setting consist of three superimposed tracings from above down.
Total flow into "patient" (nebulizer oxygen + airoxygen mixture) from position (d) (fig. 2).
Flow from ventilator (air-oxygen mixture only)
from position (c) (fig. 2).
Flow through air filter (entrained air) from position (a) (fig. 2).
The area of vertical hatching represents the contribution of the nebulizer oxygen, and the horizontal
hatching the contribution of atmospheric air.
FIG.
662
FIG. 4
Flow and pressure produced by the Bird Mark 7 ventilator No. 2 with a fixed flow setting of 20 and pressure
settings (P) of 15, 25, 40 and a pressure setting slightly
greater than 40 (P>40). The tracings are represented
in the same manner as figure 3.
steady 8 l./min. The flow from the nebulizer and
Bird stopped as the cycling pressure was reached.
At flow setting 30, as the total flow increased the
nebulizer contribution remained the same but the
contribution from the Bird, including the entrained air, had only just begun to fall as the
cycling pressure was reached. At flow setting 40
the situation was similar but the inspiratory period
had become very shon and the cycling pressure
fell. Apparently the balance of the magnets within
the machine was upset by the high flow of gases
with resultant early cycling.
With the flow setting kept constant at 20 and
the pressure setting raised in stages to 15, 25, 40,
> 4 0 , a picture similar to the above was seen (fig.
4). As the pressure was raised the contribution of
the nebulizer remained constant but flow from the
Bird with the entrained air fell to zero and the
"patient" was inflated with nebulizer oxygen only
and oxygen flowed out the air filter. Increasing
the pressure only increased the duration of this
state as the pressure in the system rose slowly due
to the constant oxygen flow until cycling occurred.
BRITISH JOURNAL OF ANAESTHESIA
6
10
30
30
40
Flow setting
FIG. 5
The percentage of atmospheric air in the air-oxygen
mixture delivered by three Bird Mark 7 ventilators at
flow settings of 6, 10, 20, 30 and 40. The volumes were
measured from the areas under the flow-time tracings
from positions (c) and (a) (fig. 2).
These findings on the contribution of the
nebulizer system were confirmed by the flow
through pneumotachometer in position (b) (fig. 2).
Entrained air.
In all of the ventilators the percentage of air
in the air-oxygen mixture leaving the Bird before
reaching the nebulizer rose as the flow rate was
increased (fig. 5) but there was considerable variation in the percentage amongst the three machines.
Bird 3 in particular had a low percentage of entrained air with a higher mean inspired oxygen
concentration than Birds 1 and 2.
Effect of a decreased compliance (compliance
12.5 ml/cm H2O with size 10 Magill tube).
At flow settings 6 and 10 flow from the ventilator stopped as the pressure rose and the "patient"
was then inflated with 100 per cent oxygen from
the nebulizer while the oxygen flowing through
the injector passed out through the air filter (fig.
6). The duration of this state was shorter than
HIGH OXYGEN MIXTURES FROM BIRD MARK 7 VENTILATOR
663
8 l./min. The pressure and flow patterns otherwise were similar to the patterns produced by a
size 10 Magill tube (fig. 6).
Entrained air.
In all three ventilators there was a fall of 1-3
per cent in the contribution of the entrained air
to the air-oxygen mixture at any particular flow
setting when the compliance was reduced or the
resistance increased. This probably resulted from
the decreased flow through the injector.
TMElMoordU
6
Comparative flow and pressure tracings from the Bird
Mark 7 No. 3 for a compliance of 23.2 ml/cm HjO and
size 10 Magill tube (C23 RIO), a compliance of 23.2
ml/cm H,O and a size 3 Magill tube (C23 R3), and a
compliance of 12.5 ml/cm H,O and a size 10 Magill
tube (C12 RIO). The pressure setting was fixed at 20
and flow setting varied. The results at flow settings 6
and 30 only are shown.
FIG.
Validity of some of the observations in patients.
To confirm in patients the occurrence of the
flow patterns seen in the experiments with models,
Fleisch pneumotachometers were inserted on the
ventilator side and patient side of the nebulizer
in patients being ventilated by Mark 7 Bird ventilators after cardiac surgery. Similar patterns were
seen to those occurring in the experiments with
models (fig. 7).
DISCUSSION
It appears that there are several reasons for the
high oxygen mixtures delivered by the Bird Mark
7 when the air-mix control is used.
As the oxygen-air mixture from the Bird and
the oxygen flow from the nebulizer into the
patient's lungs the pressure in the righthand
with the higher compliance because the pressure chamber of the ventilator rises and flow through
in the system rose more rapidly and the cycling the venturi falls. As the flow of oxygen into the
pressure was reached sooner. However, as the patient from the nebulizer quickly becomes conpeak flow and duration of the flow of the oxygen- stant the concentration of oxygen flowing into the
air mixture were markedly decreased the nebulizer patient rises as inflation proceeds. The lower the
oxygen represented a higher percentage of the flow setting, and hence the flow through the
total inspired mixture. At higher flow settings, the injector, the greater the percentage contribution
flow from the Bird did not stop until the cycling of the nebulizer oxygen to the inhaled mixture.
pressure was reached but the nebulizer oxygen There is some variation in the flow through the
flow at 8 l./min represented a large part of the injector at any given flow setting in different
reduced total flow and smaller tidal volume when Mark 7s which will contribute to the difference
compared with the same flow setting with the from one Bird to another.
higher compliance.
When a graph of airway pressure and flow
from the Bird through the venturi is drawn for
Effect of an increased airway resistance (com- various flow settings (fig. 8) it is evident that the
pliance 23.2 ml/cm H2O with size 3 Magill lower the flow setting the lower the pressure at
which the flow from the venturi ceases as the
tube).
There was decrease in the total flow and tidal one-way valve in front of it shuts. If the cycling
volume at all flow settings due to a decrease in the pressure is above that at which the injector flow
oxygen-air flow from the ventilator associated with stops then inflation of the lungs can only continue
a slightly more rapid rise in pressure in the system from the nebulizer oxygen. Similarly at a fixed
but the flow of oxygen in the nebulizer remained pressure setting the flow from the Bird falls to
P?0F6
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OF NEBULISER
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FIG. 7
Pneumotachometer tracings from a
patient who was being ventilated by a
Bird Mark 7 through a size 10 oral
Oxford tube after open heart surgery.
Tracings A and C were taken on the
patient's side of the nebulizer and
tracings B and D on the Bird side. The
pressure setting was fixed at 20 (P20).
At low flow settings (F6) flow from the
Bird stopped early in inspiration, leaving only th; oxygen from the nebulizer
to inflate th: lungs. Flows at F6 and
F30 were measured at different attenuations.
BIRD SIDE
OF NEBULISER
TIME (seconds)
HIGH OXYGEN MIXTURES FROM BIRD MARK 7 VENTILATOR
665
70 -i
P40
60 -
P>40
Flow
(l/min)
50-
4020
Ftow
(L/min)
30
Pressure (mm Hg)
FIG. 9
The relationship between the flow of the air-oxygen
mixture from the Bird Mark 7 No. 2 ventilator and the
airway pressure at a fixed flow setting of 20. The
pressure settings and the cycling pressure they produced are represented by the series of vertical lines
(PI5, 25, etc.). At this flow setting, flow from the Bird
stopped at a pressure setting of 25. At higher pressure
settings, the cycling pressure was reached because of
the flow of the nebulizer oxygen.
30-
20-
10
20
30
Pressure (mm Hg)
FIG. 8
The relationship between the flow of the air-oxygen
mixture from the Bird Mark 7 No. 1 and the airway
pressure at flow settings of 6, 10, 20, 30 and 40. When
the cycling pressure was set at 25 mm Hg (represented
by the vertical line on the right), flow from the ventilator stopped at lower pressures at flow settings 6 and
10. The cycling pressure was reached because of the
flow of oxygen from the nebulizer.
zero as the pressure rises (fig. 9). Unless the
cycling pressure is equal to or less than the pressure at which flow stops (P25 and P15), the
nebulizer oxygen continues until the cycling
pressure is reached (P40 and P>40). There is
some difference in the pressure at which flow
stops in different Birds.
The lower the flow of oxygen through the
injector the lower the percentage of air entrained
into the air-oxygen mixture. The percentage at
any flow varies in different Birds as suggested by
Fairley and Britt (1964).
When there is an increase in ±e airway
resistance, the flow of the air-oxygen mixture is
decreased for any given pressure reached in the
airway (fig. 10). As a result, the nebulizer oxygen
contributes a higher proportion of the reduced
tidal volume. At any given flow setting the flow
through the venturi stops at the same pressure
as occurs with a lower airway resistance. At low
flow settings a period of inflation by the nebulizer
oxygen alone is likely to occur. When there is a
decrease in compliance of the lungs, the results
are similar (fig. 10), but because of the rapid rate
of rise of the airway pressure the duration of the
changes is shorter and the tidal volume is considerably reduced.
Besides the possibility of oxygen toxicity in the
respiratory tract, the ventilator-nebulizer characteristics of the Mark 7 can probably lead to an
inspired mixture low in water vapour content.
This appears likely to occur at low flow and high
pressure settings when the common practice is
adopted of humidifying the air-oxygen mixture
but leaving the nebulizer empty (as in fig. 1).
Attempts to increase the tidal volume in patients
BRITISH JOURNAL OF ANAESTHESIA
666
C23R3
3
W
C12RK)
15
PRESSURE (mm Hg)
FIG. 10
Relationship between the flow of the air-oxygen mixture from the Bird Mark 7 ventilator and
the airway pressure with (a) a compliance of 23.2 ml/cm H , 0 and a size 10 Magill tube (C23 RIO),
(b) a compliance of 23.2 ml/cm H,O and a size 3 Magill tube (C23 R3), (c) a compliance of
12.5 ml/cm H,O and a size 10 Magill tube (C12 RIO).
with a high airway resistance or diminished compliance by decreasing the flow setting or increasing the pressure setting will increase the
contribution of the nebulizer oxygen until the
patient may be inflated with completely dry and
undiluted oxygen.
ont £t£ drudie'es par la mesure du flux gazeux dans
diffirentes parties du ventilateur. Les principales causes
sont les suivantes: le flux constant de 100% d'oxygene
a partir du ne'buliseur, la diminution et eventuellement
rarre't du flux a travers l'injecteur a mesure que la
pression montait dans les voies aeriennes et enfin la
diminution de la proportion d'air entrain^ pour les
flux faibles.
ACKNOWLEDGEMENTS
This work was supported by a grant from the Research
Committee of St. Vincent's Hospital. My thanks are
due to Mrs. M. McCutcheon for the preparation of the
illustrations.
REFERENCE
Fairley, H. B., and Britt, B. A. (1964). The adequacy
of the air-mix control in ventilators operated from
an oxygen sourc:. Canad. med. Ass. J., 90, 1394.
LES MELANGES A FORT POURCENTAGE
D'OXYGENE FOURNIS PAR LE OONTROLE
AIR-MELANGE DES VENTILATEURS DU
TYPE BIRD MARK 7
SOMMAIRE
Les raisons pour lesquelles un ventilateur du type Bird
Mark 7 foumit des melanges a forte teneur en oxygene
DIE HOHEN SAUERSTOFF-GEMISCHE, DIE
AUS DER LUFTGEMISCHKONTROLLE VON
BIRD MARK 7—BEATMERN FREIGEGEBEN
WERDEN
ZUSAMMENFASSUNG
Die Griinde fur die hohen Sauerstoff-Gemische, die
aus dtm Bird Mark 7—Beatmungsgerat freigegeben
werden, wurden durch Messung der Gasstrdmungen
in den verschiedenen Teilen des GerStes untersucht.
Die Hauptursachen waren die standige Stromung von
lOOprozentigem Sauerstoff aus dem Vernebler, die
Verminderung und das schliefihche Aufhfiren des
Stromes durch den Injektor, wahrend der Druck in
den Luftwegen anstieg, und die Abnahme des Anteils
an aufgeladener Luft bei niedrigen Stromungseinstellungen.