A collection reservoir 54 includes a rigid-walled cassette adapted
for connection to aspiration tubing 50. The cassette also includes
a pair of electrodes 36 positioned so that the electrodes are electrically
connectable to a flow meter 34. In operation, the electrodes are
exposed to the fluid and tissue being aspirated from the surgical
site so that the flow meter indicates a flow rate of the fluid and
tissue from the surgical site.
1. A collection reservoir for use in a surgical pump system comprising:
a rigid-walled cassette adapted for connection to an aspiration
tube and for receiving fluid and tissue aspirated from a surgical
site; and a pair of electrodes molded into the cassette and positioned
so that the electrodes are electrically connectable to a flow meter
wherein the electrodes, in operation, are exposed to the fluid and
tissue such that the flow meter indicates a flow rate of the fluid
and tissue from the surgical site without the electrodes interfering
with the flow of fluid and tissue through the cassette.
2. The reservoir of claim 1 wherein the electrodes are positioned
so that, in operation, the electrodes align with an electromagnet
thereby forming a Hall-Effect flow meter.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to sensing an aspiration flow
rate in a surgical pump system. More particularly, the present invention
is directed towards a flow meter and control system for use with
surgical pump systems.
2. Description of Related Art
The flow and flow rate of tissue and fluids through an aspiration
tube is of interest during operations, including ophthalmic operations.
However, direct measurement of the flow rate is typically impractical.
Flow rates are generally inferred for positive displacement pumps,
e.g., flow-based pumps that are based on pump rotation or other
in-direct periodic measurements, these pumps are also commonly referred
to as peristaltic pumps. Flow rates for venturi-based pumps have
generally not been measured nor has an indirect measurement been
Measurement of the surgical aspiration flow rate may be valuable
in that it can provide for safe control of the ophthalmic surgical
equipment. In most positive displacement-based systems, flow has
been known to be inferred from the cycle frequency, i.e., the rotation
rate, of the aspiration pump. However, this inference may be invalid
in situations where there are varying pressure differentials within
the pump system. The pressure variations may occur as a result of
changes in the irrigation-fluid bottle height, changes in the viscosity
of the aspirant, and changing occlusion conditions at the distal
end of the aspiration tube. For known venturi-based aspiration systems
no flow measurement has previously been feasible, nor can flow be
accurately inferred from the vacuum level. This is because the actual
flow rate varies with the viscosity of the aspirant and the occlusion
state of the aspiration tube.
In the prior art, it is possible to measure the flow rate in the
aspiration tube with a positive displacement or venturi-based system
or any other type of pump system using traditional flow sensors.
These traditional flow sensors include paddle-wheel, hot-wire, or
other devices which are deflected in the presence of fluid flow.
However, these devices become contaminated or closed by the aspirant
and cannot be reused on a different patient, thereby making the
use of such sensors expensive.
Therefore, it would be desirable to have a low-cost flow sensor
that could be inexpensively incorporated into a disposable or reusable
system to directly measure flow rate. Such a flow measurement can
enable new modes of operation, particularly for vacuum-based systems.
One such application is the emulation of a flow-based pump by a
vacuum-based pump using an additional control loop.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a prior art circuit diagram of a Hall-effect flow meter;
FIG. 2 is a partial cut-away view of a pump system in accordance
with the present invention;
FIG. 3 is a front view of a surgical cassette inserted in a console
in accordance with the present invention;
FIG. 4. is a surgical cassette in accordance with one aspect of
the present invention;
FIG. 5 is an illustration of an aspiration tube for connection
to a surgical cassette in accordance with an alternative embodiment
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Hall-effect flow meters for sensing the flow of conductive fluids
are known. Such a prior art Hall-effect flow meter is shown in FIG.
1. The flow meter 10 includes a conduit 12 including electrodes
14 that are in communication with the inner diameter of conduit
12 such that the electrodes are in contact with conductive fluids
16 flowing through conduit 12. A magnetic core 18 is placed around
conduit 12 so as to induce an electromagnetic field perpendicular
to a line-drawn through electrodes 14. Transformer 20 is connected
to driver 22 and oscillator 24. Signals from electrodes 14 are amplified
by amplifier 26 and the amplified signals are fed to synchronous
demodulator 28. A signal representative of the flow rate of the
conductive fluid 16 is outputted to node 30.
Ophthalmic surgical systems can be broadly categorized as flow-based
or vacuum-based. Flow-based pump systems attempt to maintain a constant
or controlled rate of flow within specific vacuum ranges. A feedback
or control loop may be used to ensure the constancy of the drive
system under differing loads conditions. An additional feedback
control loop may exist between a vacuum sensor in the aspiration
line in the motor, to limit the amount of vacuum in the aspiration
Vacuum-based systems also have feedback control loops, where the
signal from a vacuum sensor in the aspiration path is compared to
the pre-set desired vacuum level. Error signals are then sent to
a vacuum generator, such as a proportional value and venturi chamber,
to increase or decrease the vacuum level.
In certain situations, the emulation of a flow-based pump system
by a vacuum-based pump system may be desirable. Such emulation has
not been practical before the present invention, because there has
been no practical means to measure flow rate in the vacuum-based
The present invention solution for ophthalmic aspiration flow rate
measurement preferably utilizes an isolated Hall-effect electromagnetic
flow meter, such as described above in FIG. 1. The present invention,
shown in FIG. 2 takes advantage of the fact that the saline solution
commonly used in ophthalmic surgery is electrically conductive.
Thus, a Hall-voltage can be induced across an aspiration tube if
a magnetic field is applied. The flow meter 34 (preferably similar
to the flow meter 10 of FIG. 1) in the ophthalmic surgical pump
system 32 includes of a magnetic field source or electromagnetic
magnet and meter 34 in use is connected to a disposable electrode
assembly 36 (as shown in FIGS. 3-5). Control electronics assembly
38 preferably responds to the output of flow meter 34 to control
a proportional value or venturi chamber (not shown) of a venturi
pump 56 to emulate a peristaltic pump by maintaining a constant
flow rate of fluids and tissues through path or tube 50. The preferred
embodiment shows an aspiration tube 50 but the tube 50 could also
be other pathways that allow fluids and tissues to be carried away
from the surgical site.
FIG. 2. further shows an irrigation-fluid bottle 40 connected to
an irrigation line 42 with arrow 44 showing the direction of travel
of the saline fluid into handpiece 46. Surgical handpiece 46 performs
a surgical operation on eye 48. Fluids from bottle 40 and excised
surgical tissue are aspirated from eye 48 through aspiration path
50 (which is preferably standard surgical tubing) in the direction
shown by arrow 52. The aspirated fluid and tissue is received by
collection reservoir 54 which is contained within pump 56 (preferably
a venturi pump though a peristaltic or other pump may be used as
well). The venturi pump is preferably the same pump sold with Bausch
& Lomb's Millennium.RTM. Ophthalmic Surgical System. Venturi
pump 56 creates a vacuum level for aspirating fluid and tissue from
the surgical site at eye 48 to the collection reservoir 54. The
flow meter 34 is electrically connected (connection shown at 58)
to control circuitry 38 as well as being electrically connected
to one of the aspiration tubing 50 or the collection reservoir 54
as further described below. Control circuitry 38 is connected to
the flow meter 34 and to venturi pump 56 for varying the vacuum
level of the pump 56 and thereby maintaining a desired flow rate
of the fluid and tissue being aspirated from the surgical site.
Preferably, collection reservoir 54 is a rigid-walled cassette
so that the cassette will be operable and not collapse during operation
when a vacuum level is applied by the venturi pump 56. Further collection
reservoir 54 is similar to cassettes currently sold by Bausch &
Lomb except as modified and described in this invention. Electrodes
36 are not visible in the view of FIG. 2 though may be seen in alternative
embodiments in FIGS. 3-5.
FIG. 3 shows a partial front view of a venturi pump 56 including
a collection reservoir 54 for use in surgical pump system 32. Collection
reservoir 54 is preferably a rigid-walled cassette adapted for connection
to the aspiration tubing 50 and receives fluid and tissue aspirated
from a surgical site. Collection reservoir 54 also includes a pair
of electrodes 36 positioned so that the electrodes are electrically
connectable to flow meter 34. During operation, the electrodes 36
are exposed to the fluid and tissue such that the flow meter 34
shall indicate a flow rate of the fluid and tissue from the surgical
site. Preferably, electrodes 36 are positioned so that, in operation,
electrodes 36 align perpendicular to an electromagnet 60 thereby
forming a Hall-effect flow meter. FIG. 4 shows a side elevation
view of a collection reservoir 54 as described above. Electrodes
36 may be molded into the reservoir 54 or inserted by any other
known method, but in any case, the electrodes should form a sufficient
seal with reservoir 54 to prevent fluids from leaking.
FIG. 5 is an alternate embodiment of the present invention, wherein
surgical tubing 62 is essentially the same as tube 50 except that
tube 62 includes a pair of electrodes 64 for cooperation with electromagnet
60 and flow meter circuitry 34 and is shown to be connectable with
a collection reservoir 66. Surgical tubing 62 carries fluid to or
from a surgical site and includes a pair of electrodes positioned
within the tubing such that the electrodes, in operation, are exposed
to the fluids and wherein the electrodes are electrically connectable
to a flow meter for indicating a flow rate of the fluids through
the tubing. Preferably, the electrodes 64 are positioned so that,
in operation the electrodes are perpendicular to an electromagnet,
such as that shown in FIG. 3 to form a Hall-effect flow meter.
Electrodes 64 may be molded into tubing 62 or may be press fit and
should form a liquid tight seal with tubing 62.
The flow meter 34 provides a magnetic field required to produce
the Hall-effect voltage. The magnetic driver can be constructed
of a permanent or preferably an electromagnet. An electromagnet
is preferred so that the magnetic field may be oscillated. Alternatively,
the field may be oscillated by rotation of a fixed cylindrical magnet
(not shown). In either configuration, an air gap is required so
that the aspiration tube may be inserted within the magnetic field.
The disposable electrodes 64 or 36 must be in contact with the aspirant.
These electrodes may be molded into an aspiration tube 60 as shown
in FIG. 5 into a cassette 54 as shown in 3 and 4 or into a reflux
bulb 68 at low cost. It is also noted that a Hall-effect flow meter
as described, can be connected to irrigation tubing 42 to provide
an accurate flow rate of the saline solution into the eye.
The metal electrodes 36 or 60 are in contact with the aspirated
saline solution resulting in an electrochemical reaction, such as
corrosion, which, in turn, produces electrical signals. The use
of an alternating magnetic field from the Hall-effect flow meter
induces alternating voltages. The amplitude of this alternating
field is then correlated to a flow rate. Electrochemical voltages
not associated with the flow rate are filtered and eliminated easily
because they are steady currents.
Thus, there has been shown an inventive flow meter for an ophthalmic
surgical pump system. This flow meter could be used with flow-based
pumps or vacuum-based pumps as described above.
Important application using the flow meter 34 in connection with
the venturi-based pump system is the emulation of a positive displacement
pump. The flow rate output of meter 34 can be used in a feedback
control loop to adjust the vacuum level. This feedback control loop
is preferably part of control circuitry 38. This control loop consists
of measuring the flow rate with meter 34 and comparing that rate
to the commanded flow rate. If the sense flow rate is lower than
commanded, a vacuum generator level is increased to generate additional
vacuum. This in turn increases the flow rate. Conversely, if the
flow rate is too high, the vacuum generator level is decreased resulting
in a decreased flow rate. In this way, using control system design
the characteristics of a flow-based pump may be emulated using venturi
pump 56. With or without a feedback control loop, it is preferred
that the flow rate detected by flow meter 34 be displayed (not shown)
by pump system 32.
An additional application of flow sensor 34 is occlusion detection.
Essentially, occlusion detection is simply another flow rate detection
scheme where the flow rate detected approaches zero (0) when tissue
blocks the tip of a surgical device or the aspiration tube. When
the flow rate approaches zero (0) dangerous conditions result such
as overheating of the surgical device or occlusion of the aspiration
pathway and quick detection of such a condition is highly desirable.
Therefore, detection of a flow rate approaching zero (0) could be
used to warn the user (audibly or visually) that the system has
an occlusion or upon detection the aspiration could be slowed or
stopped. As those skilled in the art will appreciate, the detection
and warning of a flow rate need not be at zero (0), but could occur
at some flow rate above zero (0), but where overheating may still
be of concern.
Thus, there has been shown an inventive ophthalmic surgical pump
system providing a low-cost aspirant flow meter. In addition, such
a system may be used in other surgical pump systems, such as endoscopic