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Description  |
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FIELD OF THE INVENTION
This invention pertains to pneumatic tourniquets for maintaining occluded
or restricted blood flow into a patient's limb while surgical procedures
are performed on the limb. In particular, the invention pertains to
pneumatic tourniquets having means for automatically sensing and
controlling the pressure in an inflatable cuff which encircles the limb.
BACKGROUND OF THE INVENTION
Conventional pneumatic tourniquets typically provide an inflatable cuff
which may be wrapped around a proximal portion of a patient's limb, a
source of compressed gas for inflating the cuff, a pressure gauge for
measuring the cuff pressure, and a pressure regulating mechanism.
Typically, the cuff is wrapped around the patient's limb and inflated with
compressed gas to a supra-systolic pressure as high as 650 millimetres of
mercury ("mmHg") in order to stop the flow of blood into the distal
portion of the limb. A surgeon is thus provided with a "bloodless field"
in which surgical procedures may be performed on the limb. Maintaining a
bloodless surgical field makes dissection easier, renders surgical
procedures less traumatic, and generally shortens the time required to
complete the surgical procedure. The pressure gauge provides the operator
with an indication of cuff pressure. The pressure regulating mechanism in
such conventional devices is intended to maintain the cuff pressure
relatively constant.
It has been estimated that about 10,000 conventional pneumatic tourniquets
are currently used in about 1,000,000 surgical procedures performed
annually in North America. Regrettably, the widespread use of pneumatic
(and non-pneumatic) tourniquets in surgery has been accompanied by
continuing reports of limb paralysis, nerve damage and other injuries
believed to be attributable to tourniquets. A survey of the literature
indicates that such complications may result from many factors, including:
1. Excessive cuff pressure (which may lead to nerve compression and other
damage at the cuff site).
2. Insufficient cuff pressure (which may lead to passive congestion or
hemorrhagic infiltration of the nerve).
3. Excessive periods of application of an inflated tourniquet to the limb.
4. Application of the tourniquet cuff without
consideration of the local limb anatomy.
Many reported cases of preventable nerve damage, limb paralysis and other
injuries are believed to have resulted from the factors listed above, the
most common of which appears to be overpressurization of the cuff [see,
for example: D. K. Wheeler and P. R. Lipscomb, A Safety Device for a
Pneumatic Tourniquet, J. Bone Joint Surg., 45A:870, 1964; W. K. Hamilton
and M. D. Sokoll, Tourniquet Paralysis, Journal of the American Medical
Association, 199:37, 1967; S. J. Prevoznik, Injury from Use of Pneumatic
Tourniquets, Anesthesiology, 32:177, 1970; J. M. Bruner, Time, Pressure
and Temperature Factors in the Safe Use of the Tourniquet, Hand, 2:39-42,
1970; D. Fry, Inaccurate Tourniquet Gauges, Br. Med. J., 1:511, 1972; A.
E. Flatt, Tourniquet Time in Hand Surgery, Arch. Surg., 104:190-192, 1972;
G. Burchell and G. Stack, Exsanguination of the Arm and Hand, Hand,
5:124-126, 1973]. Unfortunately, the actual incidence of
tourniquet-induced complications in surgery may not be reliably estimated
because the "tourniquet paralysis syndrome" (to borrow a phrase from J.
Moldaver, Tourniquet Paralysis Syndrome, Arch, Surg. 68:136-144, 1954) may
be difficult to detect or may be masked by the effects of surgery, because
the damage is generally transient and reversible to a large extent and
because such incidents may not be consistently reported due to concern
over potential legal liability. (A hospital was recently found liable for
nerve injuries suffered by a patient as a result of excessive pressure
applied to her arm by a tourniquet ["Hospital Liable to Patient for
Tourniquet Paralysis", Citation, 38:5, Oct. 15, 1978]).
Conventional tourniquets examined by the inventor which have been linked to
possible nerve injuries or paralysis associated with cuff
over-pressurization have been found to have malfunctioning pressure
regulating mechanisms or inherent hysteresis in the pressure regulating
mechanism which permitted the cuff pressure to rise about 150-400 mmHg
above the desired cuff pressure (which is typically in the 200-650 mmHg
range). Similar findings have been made by other investigators [see, for
example: D. L. Johnson, P. D. Neufeld and R. G. Hussey, Hazards in
Single-Stage Regulation of Pressure Cuffs; J. Clin. Eng., Vol. 5, pp.
59-62, 1980.] Other tourniquets have been found to have aneroid pressure
gauges which produced readings inaccurate by about 200 mmHg.
Ideally, a pneumatic tourniquet should be inflated to the minimum
supra-systolic pressure required to maintain a bloodless surgical field
distal to the cuff. Simultaneous maintenance of a bloodless surgical field
and minimization of tourniquet cuff pressure should help to minimize the
likelihood of pressure related injuries [see: R. Sanders, the Tourniquet:
Instrument or Weapon?, Hand, 5:119-123, 1973; and, J. C. Adams, Standard
Orthopaedic Operations, Churchill P. Livingston, New York, 1976, pp. 4-5].
Theoretically, the minimum cuff pressure required to maintain a bloodless
surgical field distal to the cuff should be equal to or slightly greater
than the patient's systolic blood pressure, which is the maximum blood
pressure produced during each cycle of the heart. However, a patient's
systolic blood pressure may continually change (particularly when surgical
procedures are being performed on the patient). Thus, one practical
approach would be to pressurize the cuff to a supra-systolic pressure
which is known to exceed, by a reasonable safety margin, the maximum value
which the patient's intra-operative systolic blood pressure might reach.
One difficulty with this approach is that, because the tourniquet cuff
pressure is held constant throughout the surgical procedure at a pressure
selected to account for a possible rise in the patient's systolic blood
pressure to a "worst case" high pressure, the cuff may, for a substantial
period of time, be pressurized well above the minimum pressure required to
maintain a bloodless surgical field. This is an unnecessary hazard, and
may be of particular concern in the case of some patients such as infants,
small children, or adults with thin limbs having little protective
musculature who may be particularly susceptible to injury caused by cuff
over-pressurization.
A preferred approach, which overcomes the foregoing difficulty, is to vary
the cuff pressure in response to variations in the patient's
intra-operative systolic blood pressure, thereby maintaining a
substantially constant pressure difference between the cuff pressure and
the patient's systolic blood pressure. The pressure difference is selected
so that the cuff is pressurized above the patient's systolic blood
pressure but near the minimum supra-systolic pressure required to maintain
a bloodless surgical field.
In implementing this preferred approach, the present invention provides a
pneumatic tourniquet which senses the patient's systolic blood pressure
during surgical procedures and which regulates the tourniquet cuff
pressure as a function of the patient's intra-operative systolic blood
pressure to maintain the cuff pressure near the minimum supra-systolic
pressure required to maintain a bloodless surgical field. In other words,
the cuff pressure is "adapted" to the patient's systolic blood pressure so
as to maintain cuff pressure approximately near the minimum pressure
required to provide a bloodless surgical field.
Ideally, cuff pressure is regulated as a function of the patient's
intra-operative systolic blood pressure in accordance with the "preferred
approach" described above. However, if the patient's blood pressure cannot
reliably be sensed with accuracy then it would be undesirable to rely upon
a sensed value of systolic blood pressure as a guide to regulation of
tourniquet cuff pressure.
If the sensed blood pressure is unreliable, then an alternative to
"adaptive" cuff pressure regulation is to fall back to the first approach
described above and to maintain the cuff pressure relatively constant near
a selected pressure (for example, within about 4 mmHg of a pressure in the
200-400 mmHg range). Thus, the present invention also provides a pneumatic
tourniquet capable of automatically sensing and regulating cuff pressure
to maintain the cuff pressure near a selected pressure.
SUMMARY OF THE INVENTION
The invention is directed to a pneumatic tourniquet for maintaining
occluded or restricted blood flow into a patient's limb. The tourniquet
comprises an inflatable cuff, pressurizing means for pressurizing the
cuff, pressure relief means for depressurizing the cuff, blood pressure
sensing means for sensing the patient's systolic blood pressure and for
producing a blood pressure output signal representative of the systolic
blood pressure, and pressure regulator means responsive to the blood
pressure output signal for selectably activating ("adapting") the
pressurizing means and the pressure relief means to maintain the cuff
pressure above the patient's systolic blood pressure.
Preferably, the pneumatic tourniquet further comprises cuff pressure
sensing means for sensing the cuff pressure and for producing an output
signal representative thereof. The pressure regulator means may comprise
electronic sensing and control apparatus for comparing the blood pressure
and cuff pressure output signals, for producing a pressure decrease output
signal to actuate the pressure relief means and depressurize the cuff if
the cuff pressure exceeds an upper pressure a limit, and for producing a
pressure increase output signal to actuate the pressurizing means to
pressurize the cuff if the cuff pressure is below a lower pressure limit.
In the "adaptive" mode of operation, the pressure regulator means varies
the cuff pressure in response to variations in the patient's systolic
blood pressure, thereby maintaining a substantially constant pressure
difference between the cuff pressure and the patient's systolic blood
pressure. In the "constant" mode of operation, the pressure regulator
means holds the cuff pressure near a pressure which the operator may
pre-select. The patient's systolic blood pressure is not used to regulate
cuff pressure in the constant mode.
In the preferred embodiment, if the cuff is to occlude blood flow into an
arm of the patient, the upper pressure limit is about 94 mmHg above the
patient's systolic blood pressure and the lower pressure limit is about 86
mmHg above the patient's systolic blood pressure. If the cuff is to
occlude blood flow into a leg of the patient, the upper pressure limit is
about 129 mmHg above the patient's systolic blood pressure, and the lower
pressure limit is about 121 mmHg above the patient's systolic blood
pressure. Advantageously, limb indicator means may be provided to indicate
whether the cuff is to occlude blood flow into an arm or into a leg of the
patient. The upper and lower cuff pressure limits may then be selected by
operation of the limb indicator means.
Preferably, the pneumatic tourniquet also comprises blood pressure alarm
means for producing a blood pressure alarm signal if the patient's
systolic blood pressure falls below 80 mmHg or rises above 160 mmHg. An
inhibit alarm means is preferably provided for producing an inhibit alarm
signal if the blood pressure sensing means is unable to successively sense
the patient's systolic blood pressure for more than about 3 minutes.
The blood pressure sensing means should sense the patient's systolic blood
pressure at periodic intervals. Blood pressure fluctuation alarm means may
then be provided for producing a blood pressure fluctuation alarm signal
if the systolic blood pressure sensed during a particular time interval
differs, by more than a selected amount, from the systolic blood pressure
sensed during the immediately preceding time interval. The selected amount
may be about 32 mmHg.
Upon production of any one of the blood pressure alarm signal, the inhibit
alarm signal, or the blood pressure fluctuation alarm signal, the pressure
regulator means is preferably rendered non-responsive to the blood
pressure output signal and responsive to the cuff pressure output signal,
thereby selectably activating the pressurizing means and the pressure
relief means to maintain the cuff pressure near a selected pressure
without regard to the patient's systolic blood pressure (i.e. the
"adaptive" mode of operation is discontinued in favour of the "constant"
mode of operation).
Advantageously, the cuff includes a tourniquet cuff segment for occluding
blood flow into the patient's limb, and a blood pressure cuff segment for
coupling to the blood pressure sensing means for sensing the patient's
systolic blood pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the preferred embodiment.
FIG. 2 is a pictorial representation of a control/display panel for the
preferred embodiment.
FIG. 3 is a block diagram of a power supply for the preferred embodiment.
FIG. 4 is an electronic circuit schematic diagram of a power supply, backup
battery and battery charging circuit for the preferred embodiment.
FIGS. 5A through 5E are an electronic circuit schematic diagram for the
microprocessor and related circuitry which controls the preferred
embodiment. FIG. 5F illustrates the interconnection of those portions of
the circuitry which are shown separately in FIGS. 5A through 5E.
FIGS. 6A and 6B are an electronic circuit schematic diagram for the
control/display circuitry of the preferred embodiment. FIG. 6C illustrates
the interconnection of those portions of the circuitry which are shown
separately in FIGS. 6A and 6B.
FIGS. 7A through 7C depict a dual function cuff having a tourniquet cuff
segment and a blood pressure sensing cuff segment.
cl DESCRIPTION OF THE PREFERRED EMBODIMENT
I. Introduction
FIG. 1 is a block diagram which illustrates the operation of the preferred
embodiment. An inflatable tourniquet cuff 10 which may be wrapped around a
patient's limb is coupled via hose 12 to a pressurizing means 14 such as
an electric air pump for pressurizing cuff 10. Hose 12 is also coupled to
a pressure relief means 16 such as a normally closed valve which may be
electronically opened to depressurize cuff 10. A cuff pressure sensing
means 18 such as an electronic pressure transducer is coupled via hose 20
to a second port of cuff 10.
Blood pressure sensing cuff 11 is coupled via hoses 13 to blood pressure
sensing means 15 which may be a commercially available blood pressure
monitoring device such as a DINAMAP.TM. Model 845 non-invasive blood
pressure monitor. Blood pressure sensing means 15 senses the patient's
systolic blood pressure at periodic time intervals and produces a blood
pressure output signal representative of the sensed systolic blood
pressure. [The DINAMAP.TM. Model 845 blood pressure monitoring device does
not directly "sense" the patient's systolic blood pressure. The device
uses oscillometry techniques to periodically estimate the patient's mean
arterial blood pressure, and employs an algorithm to extrapolate values of
the patient's systolic and diastolic blood pressure. Other commercially
available blood pressure monitoring devices (such as the VITASTAT.TM.
Model 900-S blood pressure monitor--which may use an auscultatory method
to estimate systolic blood pressure, or other devices using infrasonde
techniques) could be used as blood pressure sensing means 15. As used
herein, the term "sensing" includes devices which estimate systolic blood
pressure indirectly. Devices which measure blood pressure directly, for
example, by insertion of a catheter in an arterial blood vessel, could
also be used.]
Ideally, the pressure in cuff 10 is varied as a function of the patient's
systolic blood pressure at the site where cuff 10 is applied. This is
because the sensed value of the patient's systolic blood pressure may vary
with a number of factors, including the distance from the heart at which
the blood pressure measurement is made, the width of blood pressure
sensing cuff 11 as compared with the limb circumference at the site where
cuff 11 is applied, and the local limb geometry. Thus, the systolic blood
pressure sensed in a patient's arm may differ significantly from the
systolic blood pressure that might be sensed at the same time in the same
patient's leg. Accordingly, it is desirable that blood pressure sensing
cuff 11 be applied as closely as possible to tourniquet cuff 10 in order
to minimize differences between the sensed value of the patient's systolic
blood pressure (i.e. the value which will be used to adaptively vary the
pressure in tourniquet cuff 10) and the value of the patient's systolic
blood pressure at the site where tourniquet cuff 10 is applied (i.e. the
value with respect to which the pressure in cuff 10 is ideally varied).
For a variety of clinical reasons, this may not always be possible.
Advantageously, however, in many instances tourniquet cuff 10 and blood
pressure sensing cuff 11 may be combined in a "dual function cuff" having
two cuff segments arranged as shown in FIGS. 7A through 7C. Hoses 12 and
20 may be connected to ports 23 in tourniquet cuff segment 17. Hoses 13
would be connected to ports 21 in blood pressure sensing cuff segment 19.
Tourniquet cuff segment 17 and blood pressure sensing cuff segment 19 are
stitched together over only about one third of their total length (see
FIG. 7A). Thus, the two segments may be independently fitted about a
tapered limb to more closely conform the cuff fit to the limb geometry
than would be possible if both segments were stitched together so that
they had to be wrapped around the limb in a substantially cylindrical
configuration.
FIG. 7B shows how each segment of the dual function cuff may be assembled.
A plastic liner is fitted over an inflatable bladder and the two are then
slipped inside a cuff envelope. Holes in the plastic liner and in the cuff
envelope enable the cuff ports to protrude from the cuff segment. The
plastic liner is positioned away from the side of the cuff segment which
will be against the patient's skin. This is because the liner acts as a
stiffener, tending to direct the pressure exerted by the inflated bladder
inward to the patient's limb, rather than outward against the cuff
envelope. The cuff envelope includes a strap which is folded back over the
cuff ports (to the right in FIG. 7B) to the position shown in FIG. 7A. The
strap and the outer surface of the cuff are lined with VELCRO.TM. so that
the two may be releasably fastened together around the patient's limb.
Ties are also provided to assist in fastening the cuff segment around the
limb.
The dual function cuff reduces the labour intensive operations required to
fit the patient with two separate cuffs and also reduces the total space
obstructed by the cuffs. The latter advantage may be of particular
significance if there is only one limb available for cuff application
(i.e. since other limbs might be obstructed by intravenous lines, etc.).
Pressurizing means 14, blood pressure sensing means 15, pressure relief
means 16 and cuff pressure sensing means 18 are electronically coupled to
microprocessor 22 which has an associated memory 24.
A user control panel 26 is provided to enable the selection of various
operating parameters. For example, a user may, with the aid of control
panel 26, define a selected pressure to which cuff 10 is to be inflated
when operating, as hereinafter described, in the "constant" mode, and a
time period for which cuff 10 is to be pressurized.
Cuff pressure sensing means 18 produces a cuff pressure output signal which
is representative of the pressure in cuff 10. Microprocessor 22 is
pre-programmed (as hereinafter described) to compare the cuff pressure and
blood pressure output signals and to produce a pressure decrease output
signal for actuating pressure relief means 16 to depressurize cuff 10 if
the cuff pressure exceeds an upper pressure limit, or to produce a
pressure increase output signal for actuating pressurizing means 14 to
pressurize cuff 10 if the cuff pressure falls below a lower pressure
limit. For example, in the preferred embodiment, if cuff 10 is to occlude
blood flow into a patient's arm, the upper pressure limit is about 94 mmHg
above the patient's systolic blood pressure and the lower pressure limit
is about 86 mmHg above the patient's systolic blood pressure (i.e. the
cuff pressure is maintained 90 mmHg .+-.4 mmHg above the patient's
systolic blood pressure). If cuff 10 is to occlude blood flow into a
patient's leg, then the upper pressure limit is preferably about 129 mmHg
above the patient's systolic blood pressure and the lower pressure limit
is about 121 mmHg above the patient's systolic blood pressure (i.e. the
cuff pressure is maintained 125 mmHg .+-.4 mmHg above the patient's
systolic blood pressure). Microprocessor 22 is also pre-programmed to
monitor the cuff pressurization time period.
A number of alarm/status indicators 28 provide the operator with
information respecting the operating status of the pneumatic tourniquet as
well as visual and audible alarms to warn the operator of hazardous
conditions such as pressurization of cuff 10 for, or in excess of the
selected cuff pressurization time period. The operator is provided with a
digital readout of the instantaneous pressure in cuff 10 at cuff pressure
display 38 and of the elapsed time during which cuff 10 has been
pressurized at elapsed time display 40.
Printer 27 is electronically coupled to microprocessor 22. Printer 27
serves as a recorder means for periodically recording the operational
status of the pneumatic tourniquet. For example, periodic, contemporaneous
records of the sensed value of the patient's systolic blood pressure and
of the pressure in cuff 10 may be printed on printer 27. The records may
also include an indication of the time at which each periodic
contemporaneous record is made, a message to indicate whether the pressure
regulator means is operating in response to the blood pressure output
signal (i.e. in the "adaptive" mode) or in response to the cuff pressure
output signal (i.e. in the "constant" mode), and messages to indicate
whether any alarms have been triggered. In the preferred embodiment,
printer 27 is a DINAMAP.TM. Model 950 trend recorder.
The preferred embodiment will first be described from the point of view of
a typical user such as an operating room nurse or technician. A technical
description of the construction and operation of the preferred embodiment
will then be provided, followed by a discussion of the software
programming for the microprocessor used in the preferred embodiment.
II. Operation by Typical User
FIG. 2 shows a control/display panel for the pneumatic tourniquet. The AC
power plug (not shown) of the device is connected to an AC power
receptacle and the pneumatic tourniquet is activated by moving switch 34
from the "off" position to either the "on-arm" position (if cuff 10 is to
occlude blood flow into the patient's arm) or the "on-leg" position (if
cuff 10 is to occlude blood flow into the patient's leg). Switch 34 serves
as a "limb indicator means" for indicating whether cuff 10 is to occlude
blood flow into an arm or into a leg of the patient. In the preferred
embodiment, the aforementioned upper and lower cuff pressure limits are
automatically selected by operation of switch 34. For example, if switch
34 is turned to the "on-arm" position the upper and lower pressure limits
are set, respectively, to 94 and 86 mmHg above the patient's systolic
blood pressure.
Instead of a limb indicator means such as switch 34, different tourniquet
cuff couplers might be provided to uniquely identify whether cuff 10 is to
be used on an arm or on a leg. Each coupler could have a characteristic,
detectable by microprocessor 22, which would indicate whether cuff 10 was
to occlude blood flow into an arm or a leg of the patient. For example,
separate leg cuff couplers and arm cuff couplers could be provided on user
control panel 26. The leg cuff couplers on the user control panel would be
capable of mechanical coupling only with mating couplers on a "leg cuff".
Similarly, the arm cuff couplers on the user control panel could be
mechanically coupled only with mating couplers on an "arm cuff".
When switch 34 is turned to either "on" position, the pneumatic tourniquet
automatically enters a "self-test" mode of operation which is indicated by
the illumination of indicator light 36. The self-test mode of operation
enables the operator to verify that the device is operating properly.
(a) Self Test Mode of Operation
In the self-test mode, pressure display 38 and time display 40 (which are
each three digit 7-segment light emitting diode displays) are each caused
to display the numerals "888" so that the operator may verify that all
display segments are functioning. An audible alarm (not shown in FIG. 2)
is also sounded so that the operator may verify that it is working
properly.
The operator should then disconnect the AC power plug from the receptacle
and ensure that power fail indicator light 58 is thereby illuminated. The
AC plug is then reconnected, which should extinguish indicator light 58.
Once the operator has verified the correct operation of displays 38 and 40,
power fail indicator light 58 and the audible alarm, he may momentarily
depress switch 42 to the "reset" position to terminate the self-test
sequence and enter the "normal" mode of operation (switch 42 normally
remains in the "on" position depicted in FIG. 2). When the normal mode of
operation is entered self-test mode indicator light 36 is extinguished and
normal mode indicator light 44 is illuminated.
(b) Normal Mode of Operation
The normal mode of operation is divided into two sub-modes--the "adaptive"
mode of operation, and the "constant" mode of operation. In the adaptive
mode, the pressure in cuff 10 is regulated as a function of the patient's
intra-operative systolic blood pressure in response to the blood pressure
output signal produced by blood pressure sensing means 15. In the constant
mode, the pressure in cuff 10 is maintained at or near a pressure which is
pre-selected by the operator, and which is not varied in response to
intra-operative changes in the patient's systolic blood pressure. In the
constant mode, the pressure in cuff 10 is changed only in response to
variations in the cuff pressure output signal produced by pressure sensing
means 18.
Operation of the pneumatic tourniquet always begins in the constant mode.
Once cuff 10 has been pressurized to the pressure pre-selected by the
operator, the adaptive mode may be entered by manually moving switch 30
momentarily from its normal centre position to the "adaptive" position.
The pneumatic tourniquet may be caused to revert from the adaptive to the
constant mode of operation either automatically (by microprocessor 22, as
described hereinafter) or manually (by momentarily moving switch 30 from
its normal centre position to the "constant" position).
Indicator light 32 is illuminated when the pneumatic tourniquet is
operating in the adaptive mode. If operation automatically reverts to the
constant mode (by action of microprocessor 22), indicator light 32 is
flashed on and off, indicator light 33 is illuminated, the audible alarm
sounds and a message is printed on recorder 27 to indicate the change of
mode. Alternatively, if operation reverts manually to the constant mode
(by operation of switch 30) indicator light 32 is extinguished, indicator
light 33 is illuminated, and a message is printed on recorded 27 to
indicate the change of mode, but the audible alarm does not sound.
The operator first selects a nominal pressure to which tourniquet cuff 10
is to be initially pressurized before the adaptive mode of operation is
entered. This is the pressure near which cuff 10 will be held in the
constant mode of operation.
To select the initial cuff pressure, the operator depresses switch 46 to
the "set" position shown in FIG. 2. A pre-selected nominal pressure of 200
mmHg will appear in display 38 if switch 34 is in the "on-arm" position. A
pre-selected nominal pressure of 275 mmHg will appear in display 38 if
switch 34 is in the "on-leg" position. While continuing to depress switch
46, the operator may then either raise or lower the selected cuff pressure
with respect to the pre-selected nominal level by moving switch 48 to the
"increase" position (if a selected pressure greater than the nomimal level
is desired) or to the "decrease" position (if a selected pressure less
than the nominal level is desired). When switch 48 is in the "increase"
position, the pressure appearing in display 38 will gradually increase to
a maximum of 400 mmHg. When switch 48 is in the "decrease" position, the
pressure appearing in display 38 will gradually decrease to a minimum of 0
mmHg. When the selected cuff pressure appears in display 38, switch 46 is
released. Note that two separate switches must be operated to select the
cuff pressure. This is a safety feature intended to prevent inadvertent
alteration of the selected cuff pressure. When switch 46 is released it
returns to the "sensed" position, and display 38 provides the operator
with a continual digital readout of the pressure to which cuff 10 is
inflated (initially, this will be "0").
The operator then selects, in similar fashion, the cuff pressurization time
period--the estimated time for which cuff 10 is to be pressurized. Switch
50 is depressed to the "set" position shown in FIG. 2 and a pre-selected
nominal time period of 60 minutes appears in display 40. While continuing
to depress switch 50, the operator may either increase or decrease the
cuff pressurization time period with respect to the nominal 60 minute time
period by moving switch 52 to the "increase" position (if it is desired
that cuff 10 be pressurized for a period in excess of 60 minutes) or to
the "decrease" position (if it is desired to pressurize cuff 10 for a
period of time less than 60 minutes). In either case, the time presented
in minutes at display 40 will gradually increase (to a maximum of 180
minutes) or decrease (to a minimum of 0 minutes). When the selected time
period appears in display 40, the operator releases switches 50 and 52.
Again, as a safety feature, two separate switches are required to set the
cuff pressurization time period to avoid inadvertent alteration thereof.
When switch 50 is released it returns to the "elapsed" position and
display 40 provides the operator with a continual digital readout of the
time period during which cuff 10 has been pressurized (initially, a time
period of "0" is displayed).
The patient's limb is prepared and cuffs 10 and 11 applied thereto in
accordance with established medical procedures. Blood pressure cuff 11 is
applied to the limb in a position which is proximal to tourniquet cuff 10.
(This assumes that both cuffs 10 and 11 are applied to the same limb.
Blood pressure sensing cuff 11 may be applied to a limb other than that to
which tourniquet cuff 10 is applied although, as previously discussed, it
is desirable that blood pressure sensing cuff 11 be applied as closely as
possible to tourniquet cuff 10.)
Hose 12 couples an air inlet port of cuff 10 to pressurizing means 14 and
to pressure relief means 16 via port 54 shown in FIG. 2. Hose 20 couples
an air outlet port of cuff 10 to pressure sensing means 18 via port 56
shown in FIG. 2. Preferably, separate supply and return hoses are used to
convey pressurized air to and from cuff 10. Such a "dual-line" cuff may
facilitate the detection of "kinks" or occlusions in the hoses. However,
if a conventional single-port cuff must be used then an appropriate "Y"
type adaptor should be used to couple a single hose from the cuff to ports
54 and 56.
Once the initial cuff pressure and the cuff pressurization time period have
been selected, switch 60 is momentarily depressed to the "start" position
to actuate pressurizing means 14 and pressurize tourniquet cuff 10.
Instantaneous values of the pressure in cuff 10 (expressed in mmHg) appear
in display 38. When switch 60 is depressed to the "start" position, an
elapsed-time clock is automatically activated to "count" the cuff
pressurization time period, and instantaneous values of elapsed time (in
minutes) appear in display 40. At this point, the device is in the
constant mode of operation and automatically regulates the pressure in
cuff 10 (as hereinafter described) to maintain it within about 4 mmHg of
the selected cuff pressure. The operator should then activate blood
pressure sensing means 15 and ensure that reasonable measurements of
systolic blood pressure are obtained (the DINAMAP.TM. Model 845 blood
pressure monitor used in the preferred embodiment prints these
measurements directly on recorder 27). To change from the constant to the
adaptive mode of operation, switch 30 is moved from its normal centre
position to the "adaptive" position and the device then regulates the
pressure in cuff 10 as a function of the patient's intra-operative
systolic blood pressure as determined by blood pressure cuff 11 and blood
pressure sensing means 15. In the adaptive mode, the pressure in cuff 10
is preferably maintained at about 86-94 mmHg above the patient's systolic
blood pressure if cuff 10 is to occlude blood flow into an arm (as
determined by the setting of switch 34). A somewhat higher pressure above
systolic may be required to effectively occlude blood flow into a leg.
Thus, in the adaptive mode, the pressure in cuff 10 is preferably
maintained at about 121-129 mmHg above the patient's systolic blood
pressure if cuff 10 is to occlude blood flow into a leg.
To deflate cuff 10 upon completion of the surgical procedure switches 46
and 48 are used to set the selected cuff pressure to a "zero" value. Cuff
10 then deflates to zero pressure as soon as switch 46 is released. Once
cuff 10 has deflated switch 34 should be moved to the "off" position and
cuffs 10 and 11 removed from the patient.
(c) Alarms
Three alarms, namely, a "blood pressure alarm", an "inhibit alarm" and a
"blood pressure fluctuation alarm" may be triggered while the device is
operating in the adaptive mode. If any of these three alarms are
triggered, operation of the pneumatic tourniquet is automatically switched
by microprocessor 22 from the adaptive to the constant mode, resulting in
the alarm indications noted previously. Momentarily depressing switch 42
to the "reset" position will clear the alarm condition.
The blood pressure alarm is triggered if the patient's systolic blood
pressure, as sensed by blood pressure sensing means 15, is either below 80
mmHg or above 160 mmHg. Either extreme may represent an abnormal condition
in the patient requiring medical attention. These limits however are
somewhat arbitrary at present and may be revised as clinical experience is
gained.
The inhibit alarm is triggered if blood pressure sensing means 15 is unable
to make successive measurements of the patient's systolic blood pressure
for more than about 3 minutes, which may be indicative of a malfunction of
blood pressure sensing means 15, or the presence of physiologic or
environmental conditions which prevent blood pressure sensing means 15
from functioning properly.
The blood pressure fluctuation alarm is triggered if the systolic blood
pressure sensed by blood pressure sensing means 15 during a particular
time interval differs, by more than a selected amount, (preferably about
32 mmHg) from the systolic blood pressure sensed during the immediately
preceeding time interval. Since blood pressure sensing means 15 is
programmed, in the preferred embodiment, to normally sense the patient's
systolic blood pressure about once every minute, a 32 mmHg change
represents a significant alteration and may be indicative of a change in
the patient's physiologic status, or a malfunction of blood pressure
sensing means 15, either of which may require attention.
If blood pressure sensing means 15 becomes incapable of providing reliable
measurements of the patient's systolic blood pressure then, as indicated
above, the device automatically reverts from the adaptive to the constant
mode of operation and holds the pressure of cuff 10 near the pre-selected
pressure. This is accomplished by discontinuing the regulation of the
pressure in cuff 10 in response to the blood pressure output signal
produced by blood pressure sensing means 15 (i.e. the adaptive mode is
terminated) and by regulating the cuff pressure only in response to
changes in the cuff pressure output signal produced by pressure sensing
means 18 (i.e. the constant mode is activated).
The remaining alarms may be triggered whether the device is operating in
the adaptive or constant modes.
Microprocessor 22 periodically activates pressurizing means 14 or pressure
relief means 16 to minutely increase or decrease the pressure in cuff 10.
Such minute variations have no significant effect upon the occlusion of
blood flow into the patient's limb, but they are detectable by pressure
sensing means 18. A characteristic change in the pressure of cuff 10
should occur upon activation of either pressurizing means 14 or pressure
relief means 16, depending upon the pre-activation pressure of cuff 10 and
the time during which pressurizing means 14 or pressure relief means 16 is
activated. If the expected characteristic change is not detected by
microprocessor 22 (via pressure sensing means 18), a cuff pressure alarm
is triggered by sounding the audible alarm and by flashing on and off the
cuff pressure which appears in display 38. The operator should examine
hoses 12 and 20 for kinks or occlusions which may prevent free passage of
pressurized air to or from cuff 10. Cuff 10, hoses 12 and 20, and the
various connectors should also be checked for damage, leaks or
obstructions. The cu | | |
