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BACKGROUND OF THE INVENTION
The employment of gas activated intra-aortic balloons is at the present
time an accepted therapeutic method for mechanically assisting the heart.
The balloon is inserted into the proximal portion of the descending
thoracic aorta and is inflated and deflated with gas in synchoronism with
heart action. Therapeutic benefit is derived from decreasing the workload
of the heart and from augmenting perfusion of the coronary arteries during
diastole. The number of applications, in the United States alone, is in
excess of 18,000 per year. With the advent of the so-called "percutaneous"
type balloon, the membrane of the balloon is significantly thinner than in
the older types. This thinness and the necessity of wrapping the balloon
by winding it up on itself for percutaneous insertion and thereafter
unwinding it in the aorta have increased the risks of balloon failure
caused by any weaknesses introduced during manufacture, by damage caused
by intra-aortic plaques, or by inappropriate medical techniques. However,
a balloon failure such as a leak, and in particular a rupture, which
allows the inflating gas to enter the bloodstream, can cause a disastrous
gas embolism. The prior art, the detection means, which are incorporated
into existing intra-aortic balloon drive and control systems, are based on
detecting with varying degrees of sensitivity--a loss of drive gas, i.e.,
after gas has escaped. Equally unsatisfactory is "detection" by personnel
observing blood appearing in the pneumatic drive line.
The present invention is directed to a method of and an apparatus for
providing a direct, intra-corporeal detection of intra-aortic balloon
leaks which when used in conjunction with an automatic shutdown and an
alarm system is designed to prevent gas escape and consequent gas
embolism. In addition, the present method and apparatus also provides the
means for direct monitoring of the inflation and deflation of the balloon
operation, a feature presently not available in balloon drive systems.
SUMMARY
The present invention is directed to a method of and an apparatus for
intra-aortic balloon monitoring and leak detection. One object is to
provide a method and apparatus for a direct sensitive detection of balloon
failure and for a consequent automatic shutdown and an alarm. The present
apparatus and method is based upon the continuous, on-line measurement,
preferably by means of an alternating current, although direct current may
be used, the electrical impedance across the balloon membrane thereby
providing a signal for automatic shutdown when the measured impedance is
reduced below a predetermined level by the shunting effect of a
transmembrane fluid conduction path. A further object is the provision of
using an alternating current impedance measurement of the balloon to
monitor the inflation and deflation of the intact balloon.
A still further object of the present invention is the provision of a
method of and an apparatus for monitoring the operation and detection of
leaks of an intra-aortic balloon inserted into the aorta of a body in
which the balloon includes internally an electrical conductor that is
accessible outside the body. The method includes attaching an electrode to
the exterior surface of the body and applying an alternating current
source between the electrical conductor and the electrode, thereby
transmitting an alternating current through the body and across the
balloon. An impedance measuring means is electrically connected to the
electrode and the electrical conductor for measuring the impedance across
the balloon membrane and the impedance between balloon membrane and the
electrical conductor. As the intact balloon is expanded and contracted the
impedance across the gas space will change, and balloon membrane position
and motion can thus be monitored.
A still further object of the present invention is the provision of a
shutdown means connected to the impedance measuring means for
automatically stopping the operation of the balloon when the impedance
falls below a predetermined level. In the event of a leak or rupture in
the membrane of the balloon, the resulting transmembrane fluid path,
between the blood outside the balloon and the electrical conductor inside
the balloon, will reduce the measured impedance and thereby, by means of
appropriate circuitry, trigger automatic shutdown and alarm. Thus, escape
of gas into the bloodstream will in most cases be prevented.
A still further object of the present invention is the provision of a
signal means connected to the measuring means for actuating a signal when
the impedance exceeds a predetermined level which is an indication that
the electrical measuring circuit is improperly operating such as would
occur if the external electrode was not making a good contact with the
body.
Still a further object of the present invention is the improvements of
various safety means including means for limiting the electrical current
flowing between the electrode and the electrical conductor and passing
through the body and wherein the alternating current is sinusoidal and
wherein the frequency of the alternating current is above the range of
cardiac susceptibility.
Still a further object of the present invention is the provision of a
method and apparatus for monitoring the operation of and detecting a leak
in an intra-aortic balloon inserted into the aorta of a body in which the
balloon is inflated and deflated with gas by a control system and in which
the balloon includes an electrical conductor therein which extends or can
be extended out of the body and which includes an electrode or electrode
assembly adapted to be affixed to the exterior of the body. An electrical
subassembly including an alternating current impedance measuring circuit
is connected to the external electrode(s) and the internal electrical
conductor for providing a current path through the body across the
balloon. The subassembly is powered with direct current from an
electrically isolated power source. A sinusoidal oscillator provides an
alternating current signal to the measuring circuit but is isolated from
the subassembly by a first transformer. An alternating current to direct
current converter is connected to the output of the measuring circuit but
is isolated from the subassembly by a second transformer. A signal
processor is connected to the output of the converter for providing an
output proportional to the measured impedance and the output of the
processor is connected to the gas control system for deactivating the
control system when the measured impedance falls below a predetermined
level.
Other and further objects, features and advantages will be apparent from
the following description of a presently preferred embodiment of the
invention, given for the purpose of disclosure and taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic elevational view of a conventional intra-aortic
balloon system in use with the present invention,
FIG. 2 is a schematic representing in analog form the electrical
characteristics of the system being measured,
FIG. 3 is a block diagram of the principal features of the electronic
design employed in implementation of the present invention,
FIG. 4 is a schematic of the impedance measuring circuit of the present
invention,
FIG. 5 is a block diagram of the monitor output and the shutdown and alarm
circuitry of the present invention,
FIG. 6 includes graphs A and B illustrating the output measurement of the
impedance change as a function of balloon operation for different balloon
filling volumes,
FIG. 7 includes output graphs A, B and C illustrating impedance changes
caused by a small membrane leak and the consequently triggered shutdown
signal,
FIG. 8 is an elevational schematic of a direct current sensing circuit, and
FIG. 9 is an electrical block diagram of a system for direct current
sensing combined with telemetry.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and particularly to FIG. 1, the reference
numeral 10 generally indicates a conventional intra-aortic balloon system
in combination with the monitoring and leak detection system of the
present invention, generally indicated by the reference numeral 30 of the
present invention. The system 10 generally includes a flexible balloon 12
mounted on a catheter 14 which is connected to a handle 16 and is also in
communication with a gas line 18 which in turn is connected to the control
system 20. The balloon 12 is also supported by a metal rod 22 which is
rotatably attached to the handle 16 and affixed to turning knob 24. In
use, the knob 24 is rotated to wrap the balloon 12 around the rod 22 to
form it into a compact roll for insertion through a prepositioned 4 mm ID
sleeve into the aorta 26 in a body 128. After the balloon 12 is inserted
into the aorta 26, it is unwrapped by reversing the rotation of the
turning knob 24. The control system 20 then transmits gas to and from the
balloon 12 through the line 18 and catheter 14. The cycling is precisely
synchronized with the heart action whereby the balloon assists the heart.
However, the balloon 12 is a thin membrane and is subject to wear and tear
by twisting it into a roll, inserting it, unrolling it, and continuously
inflating and deflating it wherewith repeated contact with a calcified
plaque protruding from the aortic wall may cause localized membrane
damage. A leak or rupture in the balloon 12 could lead to disastrous
consequences as the gas in the balloon could escape into the blood 27 in
the aorta 26 and thus create gas embolism in the patient.
The present invention is directed to monitoring the operation of the
balloon 12 and the detection of any leak therein by measuring the
impedance across the balloon membrane 12 and the gas space 29 between the
membrane 12 and the metal rod 22. The impedance measurement is made
practical by using the metal rod 22, which is normally incorporated for
the purpose of supporting the balloon 12 and controlling the wrapping and
unwrapping of the balloon 12, as an electrical electrode. The metal rod 22
provides the means for passing an alternating current across the balloon
membrane 12 while the balloon 12 is in the aorta 26 in the body 28. The
current path is provided by affixing a skin electrode 32 to the exterior
of the body 28 and passing the current through the body tissues and
fluids, blood, across the balloon membrane 12 and gas space 29, and then
via the metal conductor 22 back to the current source. Thus, the present
invention 30 includes an alternating current source which is connected to
an electrode 32 affixed to the human body and by a conductor 34 to the
electrical conductor 22 incorporated in knob 24.
Monitoring of the operation of the balloon 12 and the detection of a
transmembrane defect are accomplished by measuring the impedance by means
of an electrically isolated (floating) constant current source which is
connected to the electrode 32, such as a silver-silver-chloride skin
electrode, and to the metal conductor 22 by the electrical conductor 34.
Since the impedances of the electrode 32 interface, tissues and blood of
the body 28 are relatively low, the main impedances are those across the
balloon membrane 12 and across the gas space 29. That is, in effect, the
measured impedance is of two in-series capacitors of which one, that of
the membrane 12, is for practical purposes constant. The various
components which make up the overall impedance Z are shown in analog form
in FIG. 2 in which an alternating current from source 40 is applied
between the electrode 32 and the electrical conductor 22. While the total
impedance Z consists of the various listed factors, the main variable
factors may be used to monitor the operation of the intact balloon 12 as
well as to detect a membrane leak or rupture. For example, the variation
in the total capacitance across the balloon is approximately 200 pf as a
standard 40 cc balloon is inflated and deflated. With the high impedance
across the membrane 12 and the gas space 29, even a minute defect in the
membrane 12, causing a tiny fluid channel across the membrane 12, can
easily be detected by the appearance of the factor R.sub.x. The present
invention utilizes a drop in the measured impedance Z, such as when the
blood 27 makes contact with the electrical conductor 22 during balloon
deflation, to trigger an alarm and an immediate shutdown of the control
system 20 of the balloon 12. Extensive benchtop testing, under conditions
analogous to those encountered clinically, has demonstrated that a pin
hole defect in the membrane 12 can be detected before any gas escapes. A
very dangerous balloon failure, that can occur, is a partial detachment of
the balloon 12 from the catheter 14 during insertion, an event that could
lead to the disastrous consequences of 40 cc of gas being pumped into the
bloodstream 27. However, the present invention, by measuring the lowering
of the impedance between the electrode 32 and the electrical conductor rod
22, has the capability of triggering an immediate shutdown and thus
preventing even a single inflation cycle.
The intra-aortic balloon system 10 shown in FIG. 1 is a type exemplified by
the "Percor 10.5 F" model from the Datascope Corporation. However, the
present invention is applicable to various other types of intra-aortic
balloon models. In the device of FIG. 1, the metal rod 22 is a thin solid
wire which is normally electrically insulated by the balloon 12 from the
blood 27 in the aorta 26. In another conventional system, sold by Kontron
Corporation, the metal conductor consists of a small caliber stainless
steel tubing, the lumen of which communicates with blood at the end of the
balloon for permitting balloon insertion directly over a guide-wire, blood
pressure measurement and intra-aortic injections. However, for ease of
insertion, a flexible hollow 5 centimeter long segment is at the distal
end of the balloon. Since this flexible segment is electrically insulated,
the impedance of the inside fluid column (saline perfusate) is high
enough, approximately 30 Kohm (cf. R.sub.s in FIG. 2), to make the present
system 30 applicable. In invitro experiments with this "dual lumen" type
balloon, the balloon inflation and deflation pattern was readily
discernible and with the appropriate threshold setting, a small pin hole
in the balloon membrane, equivalent to a R.sub.x of about 50 Kohm, was
readily detected.
The monitoring of balloon operation and the detection of a balloon membrane
leak are effective for balloons with and without the second lumen. The
sensitivity of detection of increased impedance, caused typically by poor
contact of the external electrode 32, is straightforward with the dual
lumen type, but is, however, low for balloons 12 containing only the
pneumatic channel, as depicted in FIG. 1. In conjunction with the latter
type of balloon 12, the sensing of an impedance increase is significantly
improved by introducing a second external electrode 33 which, via a 30
Kohm resistor 35 (cf. R.sub.s in FIG. 2) is connected to the internal
conductor lead 34. As presented schematically in FIG. 1, by this
arrangement a portion of the alternating current will flow through the
thus created shunt, and an increase in impedance caused by poor continuity
of electrodes, connectors or leads, can readily be detected. Additionally,
since 30 Kohm matches the effective resistance of the fluid shunt in the
dual lumen balloon (Kontron), uniformity of signal amplitude is attained.
Impedance and changes thereof may be measured in a number of ways. However,
in making electrical measurements on the human body, important system
specifications are safety, efficacy, reliability and ease of clinical
implementation. In the present system 30, safety is ensured (1) by using a
sinusoidal alternating current signal with a frequency beyond the range of
cardiac susceptibility, for example at a frequency of 10 KHz or higher,
(2) by limiting the current passing through the human body, for example by
limiting the current to 10 microamperes, (3) by providing a high degree of
electrical isolation, particularly from power ground, and (4) by excluding
possibilities of interference with the normal operation of the balloon
system 10. While passing an electrical current through the human body for
the measurement of impedance is used clinically for other purposes and in
different configurations, the present purpose, method and configuration
are novel.
Referring now to FIG. 3, a block diagram of the present system 30 is best
seen connected to the control system 20 of the intra-aortic balloon system
10. One feature of the system 30 is the electrical isolation of the
subassembly 40 which contains the electronic components connected to the
body 28 of the patient. For safety reasons, the subassembly 40
incorporates a minimum of total circuitry. The circuitry in the
subassembly 40 is powered by means of an isolation dc to dc converter 42.
If desired, the subassembly 40 could be battery powered for further
isolation. The alternating current signal is provided by a conventional
oscillator 44 which is coupled by a first transformer 46 to the isolated
subassembly 40. The impedance (amplitude) modulated signal from the
isolated subassembly is coupled by a second transformer 48, and is
tranmitted to an ac to dc rms converter 50. If desired, optical coupling
could be substituted for the transformers 46 and 48. The resulting dc
voltage, which corresponds to the amplitude (rms) of the impedance
measuring ac signal, is then transmitted to a signal processor 52 for
providing an automatic shutdown signal 54 to the balloon control system
20, a signal 56 to an alarm output 58 and a signal 60 to a monitor and/or
recorder 36. It is noted that patient 28 is by means of subassembly 40
isolated from main circuit components and earth ground. Preferably,
electrical power is applied to and discontinued from the human body 28
gradually.
Referring now to FIG. 4, a schematic diagram of the circuitry of the
subassembly 40 is best seen. A first amplifier 64 is arranged in a Howland
configuration for receiving the ac signal through the transformer 46 and
for providing a constant, alternating current through the circuit ground
referenced measured impedance Z without any dc component. Output from the
first amplifier 64 is amplified by a second amplifier 66 and transmitted
through the transformer 48 to the ac to dc converter 50 (FIG. 3).
The signal processor is best seen in the simplified block diagram in FIG.
5. The processor 52 receives the impedance measuring signal through the
transformer 48 and converts the ac signal to its rms dc equivalent in
converter 50. The dc signal is transmitted to a buffer amplifier 70 to
provide a signal on line 60 to any suitable monitor or recorder 36 (FIG.
3). The incoming dc signal which is used for monitoring the balloon
operation is fed into a first comparator 72 and a second comparator 74.
The first comparator 72 is triggered when the dc level exceeds a preset
reference level, as may be caused by the patient lead or electrode 32
malfunction which causes an increase in the measured impedance. Comparator
74 is triggered when the dc level falls below a preset reference level
such as caused by a leak in the balloon membrane 12 creating a fluid
conductive path across the balloon membrane 12 causing a decrease in the
measured impedance. Actuation of the first comparator 72 is transmitted to
an alarm 76 such as a visual or an audible alarm. However, actuation of
the second comparator 74 triggers a latching circuit such as provided by a
silicon controlled rectifier (SCR) 80 which in turn causes a current to
flow through a coil 82 of a relay for opening a normally closed switch 84
of the relay which shuts down the control system 20 (FIG. 3) which
operates the balloon. Simultaneously, current through the SCR 80 enables a
timer oscillator 86 which provides an interrupted sounding alarm 76 and a
flashing light. The SCR 80 has the desirable feature of having an inherent
latching property. That is, the latch 80 remains ON even if the original
trigger signal from the comparator 74 is removed from the gate of the SCR.
Consequently, a resumption of the balloon 12 operation requires a reset
action on the part of the human operator by actuating the reset switch 88.
The SCR 80 may be connected to a substitute resistor 90, with a switch 92,
for allowing the system to be tested without interrupting the operation of
the balloon 12.
During normal pumping with the intact balloon 12, the inflation and
deflation of the balloon 12 may be monitored as it is reflected in the dc
output from the converter 50. The tracings reproduced in FIG. 6 illustrate
the correlation of the balloon sensing output with the pressure variations
in the drive gas line to the balloon 12. In FIG. 6 the tracing 94
illustrates the impedance change across the balloon 12 as a function of
balloon operation while the lower tracing 96 indicates the line pressure
of the gas actuating the balloon. It is to be noted that as the gas
pressure increases and the balloon 12 is inflated, the impedance across
the balloon increases. In FIG. 6 in the graphs A the 40 cc balloon was
inflated only partially with 25 cc of gas. Similar tracings are provided
in FIG. 6 in the graphs B wherein tracing 98 indicates the impedance
change as a function of balloon operation and tracing 100 indicates the
gas line pressure. However, in the graphs B of FIG. 6 the balloon was
fully inflated with 40 cc of gas. By means of appropriate circuitry, the
monitored output may be linearized for attaining proportionality with
balloon volume.
FIG. 7 includes graphs A, B and C which are tracings of the measured
impedance reading 102 relative to a shutdown level of 104 and a tracing of
the shutdown signal 106. In to graph A of FIG. 7 the tracings illustrate
normal balloon operation wherein the impedance tracing 102 is at all times
above the predetermined reference 104. In graph B a small puncture has
occurred in the membrane of the balloon 12 causing the impedance value 102
to fall, during balloon deflation, below the predetermined set value 104
and consequently causing the shutdown signal 106 to fall to a shutdown
value (as it appears at the anode of the SCR). In continous operation in
graph C, after some fluid (saline) had accumulated in the balloon 12
through the small puncture, causing the measured impedance to be reduced
even during balloon inflation.
As indicated above, it is the preferred embodiment of the present invention
to utilize alternating current which can both determine a balloon leak and
monitor the static and dynamic characteristics of the balloon operation.
However, under certain circumstances, if leak detection is the sole
objective, then this may be accomplished, in principle, by utilizing a
direct current (dc). For example, referring to FIG. 1, a dc source may be
connected to a reference skin electrode such as electrode 32 and to the
electrical metal conductor 22 within the balloon 12. As long as the
balloon 12 is intact, no electrical current will flow since the balloon 12
and catheter 14 will represent, for practical purposes, an infinite
resistance to direct current. If, however, a leak develops, typically
across the membrane of the balloon 12 or its attachment to the catheter
14, then a fluid conductive path is established that permits a measurable
current to flow, typically, when the balloon is deflated. Thus, an
electrical signal is obtained which can be similarly utilized for alarm
and automatic shutdown as previously described.
In the "dual lumen" type of intra-aortic balloon, as described on pages 8
and 9, the incorporation of a fluid channel constitutes in effect an
electrical shunt relative to the balloon membrane. In this case, a current
is flowing through the shunt and the measurement is likely to be affected
by the problems associated with the use of dc under the circumstances
concerned. Phenomena, such as electrical polarization and galvanic
potentials are likely to be present and may vary considerably with changes
at the electrode interfaces, typically as caused by electrode motions.
Consequently, the dc detection of a balloon membrane leak, particularly a
small one, would be significantly degraded by the extraneous potentials
and, therefore, the use of dc measurements with the "dual lumen" type of
balloon is inferior to measurements with alternating current.
In short, the employment of an alternating current is the preferred method
as it permits continuous on-line monitoring of balloon operation and
provides sensitive and reliable detection of intra-aortic balloon leaks,
regardless of whether the balloon has a fluid channel or not. However, in
fulfilling the sole objective of leak detection in a "single lumen"
balloon, the direct current method may be employed. Since under these
circumstances no current will flow across the intact balloon, the dc
approach lends itself for a sensing system design that permits long-term
battery operation. Furthermore, if desired, a dc sensing module may
advantageously incorporate a wireless transmitter which, similarly, is
activated and powered only during testing and in response to a balloon
leak.
Safety requirements are similar to those pertaining to the use of
alternating current, as previously stated. Patient connected circuitry
needs to be isolated from earth ground; in the system here described,
isolation is achieved by means of battery operation combined with optical
coupler interfacing (alternatively telemetry). Further, if a balloon leak
should occur, the direct current flowing through the body should be
limited (for instance a constant current of 10 to 20 microamp.) and its
onset should be gradual.
Referring now to FIG. 8, an electronic circuit implementation is shown. In
this design, a reference skin electrode 32 and the intra-balloon conductor
22 are connected between the positive side of a constant current source
112 and the plus side 114 of the battery 116. Thus, with an intact balloon
no dc flows through transistor 118 and transistor 120 is consequently
biased in the non-conducting state. Should a balloon membrane leak occur,
an electrical conduction path from 114 to 112 is established, as
symbolized by R.sub.x. Via the constant current source, current flows
through resistor 122 and builds up a charge on capacitor 124 and thus a
gradual onset of conduction through transistor 118 is effected. This in
turn activates transistor 120 and the LED 126 in the optical coupler 128,
resulting in a coupler output signal 130 for triggering a latching silicon
controlled rectifier (SRC) such as the SRC 80 in FIG. 5. In this circuit,
in the standby non-triggered mode, the current through transistor 120 is
one to two nanoamp.; alternately, an FET may be used. Because of the
negligible current drain, battery life is likely to be determined by the
frequency and duration of system testing, at which time the current is
appoximately 0.3 milliamp. A suitable time for transistor 118 to reach a
steady level of conduction is about 150 milliseconds. Operationally, the
battery voltage (of a standard, small nine volt alkaline battery) is not
critical. Dependent upon values of the resistors, the sensing function can
be preserved at a drop of battery voltage down to about five volt.
Further, the circuit will effect an output signal for any value of R.sub.x
, including a value significantly higher than that encountered in the
event of a tiny pinhole in the membrane in the balloon 12 (50 to 100
Kohm).
Referring now to FIG. 9, a block diagram is shown of a system that includes
telemetry, which, as noted, lends itself to an extension of the dc
approach to balloon leak detection. Using a conventional design, a low
powered transmitter section 134 may be combined with the sensing section
132 of FIG. 8 in a common battery powered unit for transmitting to a
receiver 136 and to a signal processor 138 for effecting alarm and
shutdown, as previously disclosed. Again, the current drain is negligible
in the standby, non-triggered mode; and maximum electrical isolation from
earth ground is inherent in this configuration.
The present invention, therefore, is well adapted to carry out the objects
and attain the ends and advantages mentioned as well as others inherent
therein. While presently preferred embodiments of the invention have been
given for the purpose of disclosure, numerous changes in the details of
construction and arrangement of parts will be readily apparent to those
skilled in the art and which are encompassed within the spirit of the
invention and the scope of the appended claims.
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