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REFERENCE TO RELATED APPLICATION
This application is related to an application, Ser. No. 531,621 filed By
Frederic M. Hulett, III, on even date herewith entitled "Microprocessor
Implemented Electrosurgical Generator, now abandoned.
BACKGROUND OF INVENTION
This invention relates to electrosurgical generators and in particular to
such generators including means for controlling the output power level
thereof. The invention further relates to the use of such generators with
bipolar forceps or handpieces.
Bipolar electrosurgery refers to the use of a handpiece having two small
electrodes to apply electrosurgical current instead of a single small
active and a large return electrode as is used in monopolar
electrosurgery.
The advantages of bipolar electrosurgery over monopolar are:
(1) A lower power level is used which translates directly to less tissue
destruction.
(2) The only tissue destroyed is that located between the bipolar
electrodes so that there is virtually no danger of alternate site burns.
(3) The applied voltage can be much lower. This prevents tissue charring
and scarring due to sparks at the electrodes.
Bipolar electrosurgery is used extensively in surgical procedures on the
eye and brain where the delicate tissue can be easily damaged by excessive
heat or sparking.
SUMMARY OF INVENTION
In an effort to improve the effects of bipolar electrosurgery, studies have
been conducted on the variation of tissue impedance during desiccation.
The results showed two phases of desiccation. The first phase begins as
soon as electrosurgical power is applied. The tissue temperature rises,
cell walls are ruptured and the impedance of the tissue shows a decrease.
The temperature continues to rise and the water is driven off as steam. As
the tissue dries out, the resistance rises. The output voltage goes up
somewhat as the resistance rises and at this point sparking, excessive
heating of the forceps, or sticking of forceps to tissue may occur.
Accordingly, it is a primary object of this invention to provide an
improved electrosurgical generator whose power decreases rapidly with
increasing impedance so that the power diminishes towards the end of the
desiccation phase.
It is a further object of this invention to provide an improved generator
of the foregoing type where the power decreases as the inverse of the
square of the tissue impedance.
It is a further object of this invention to provide an improved generator
of the foregoing type for use in bipolar electrosurgery.
Other objects and advantages of this invention will be apparent from a
reading of the following specification and claims taken with the drawing.
DISCUSSION OF PRIOR ART
U.S. Pat. Nos. 3,946,487; 3,980,085; 4,188,927, and 4,092,986 disclose
means for reducing the output current in accordance with increasing load
impedance. In particular, these patents teach the use of constant voltage
outputs whereby the current is decreased with increasing load impedance.
However, there is no disclosure in these patents that output power be
inversely varied in accordance with the square of the load impedance. In
general, there is no disclosure the output power be decreased at a rate
which is substantially greater than that which results in the constant
voltage outputs of these patents.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram of an illustrated electrosurgical generator in
accordance with the invention.
FIG. 2 is a graph illustrating different modes of operation of the
generator of the present invention.
FIG. 3 is a flow chart of the program executed by and stored in the
microprocessor controller of FIG. 1.
FIGS. 4A and 4B is a schematic diagram of the switching power supply, the
generator and the voltage and current sensing circuitry of FIG. 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION
Reference should be made to the drawing where like reference numerals refer
to like parts.
Referring to FIG. 1, an illustrative electrosurgical generator in
accordance with the invention includes a switching regulated power supply
10, an unregulated DC voltage source 11, an RF generator 12, voltage and
current sensors 14, a microprocessor controller 16, a nominal power
indicator 18, and a footswitch 20. Electrosurgical current is applied from
the generator to forceps 22, which are diagramatically shown in contact
with a patient 24.
The power delivered to the load is a function of the voltage from DC supply
10 and the load impedance. As will be described in more detail
hereinafter, sensors 14 develop sensing signals proportional to the load
voltage and current which are respectively applied over lines 15 and 17 to
controller 16. The controller digitizes the sensing signals and computes
the load impedance and the actual power being delivered to the load.
A required load power is also calculated which is a function of the
calculated load impedance and the nominal power selected by the operator
at 18. A graph of the required load power for a nominal power of 50 watts
is illustrated in FIG. 2.
At low load impedances less than typically 70 ohms, the computed required
power is reduced to prevent damage or overheating of the RF output. In
particular, the control voltage applied over line 19 to supply 10 controls
the output power in such a manner as to maintain the load current constant
over this low value impedance range. Over a mid range extending from about
70 to 100 ohms, the computed required power is held constant at the
selected nominal power.
At impedances above 100 ohms, one of two modes may be chosen by the
operator. As indicated in FIG. 2, the computed required power is reduced
in the first of these modes with the voltage being held constant, this
effect being similar to that of known generators. In the second mode the
computed required power is reduced at a rate which is substantially
greater than that which occurs when the voltage is held constant.
Preferably, the computed required power is reduced, in the second mode, as
the square of the load impedance. Thus, for example, the computed required
power for a load having an impedance of 200 ohms is one-fourth that
required for a load impedance of 100 ohms. When the impedance is increased
to about 800 ohms, for example, in the second mode of operation, a
constant voltage characteristic may be implemented since at this level,
the power levels may become impractically small.
After the required power has been calculated for a given nominal power and
load impedance range, a comparison is then effectively made between the
required power and the actual power to adjust the control voltage applied
to line 19 by the correct amount to cause the actual power to match the
required power.
The load impedance varies continuously during electrosurgery due to
differing amounts of tissue contact, tissue heating, etc. The
microprocessor controller 16 accordingly repeats the measurement,
calculation and correction process approximately every 20 milliseconds as
long as the foot switch 20 or bipolar handswitch (not shown) is closed.
Reference should now be made to FIG. 3, which is a generalized flow chart
of the program executed by microprocessor controller 16 where the
microprocessor may be an INTEL 8039, a member of the 8048 family of
singlechip microcomputers and where the program may be stored on an INTEL
2716 programmable memory. After certain initialization routines (not
shown), program control passes to routine COLDST 26 (COLD START), which
calculates certain parameters and initiates certain functions. First it
calculates I.sub.MAX, which is the current value which should occur at 70
ohms for a given nominal power selected by the operator. For example, if
the operator selects a nominal power of 70 watts, the current delivered to
a 70 ohm load will be 1 ampere and thus I.sub.MAX is 1 ampere in this
case. The nominal power range available to the operator typically extends
from 0 to 70 watts. The nominal power selected by the operator is termed
P.sub.MAX and occurs over the mid range impedance extending from 70 to
100 ohms. Since the maximum power occurs over the 70 to 100 ohm range, and
since the largest current which will occur in this range occurs at 70
ohms, I.sub.MAX is calculated at 70 ohms as described above. In general,
I.sub.MAX =(P.sub.MAX /70).sup.1/2 where P.sub.MAX equals the nominal
power selected by the operator and 70 corresponds to a 70 ohm load.
COLDST also calculates the initial parameter V.sub.MAX in the following
manner. Assume the operator selects a nominal power of 25 watts. The
voltage occuring across a 100 ohm load for this wattage corresponds to
V.sub.MAX, thus in this case V.sub.MAX would be 50 volts. As stated above
P.sub.MAX occurs over the 70 to 100 ohm range. Further, the maximum
voltage will occur with a 100 ohm load. Accordingly, V.sub.MAX is
calculated in the foregoing manner. In general, V.sub.MAX =(100
P.sub.MAX).sup.1/2 =10(P.sub.MAX).sup.1/2.
After the calculations of V.sub.MAX and I.sub.MAX COLDST applies a small
control voltage AFB over line 19 to supply 10 to turn on RF generator 12
then waits for about 0.01 second to allow the RF output to become stable
before transferring a control to the control routines, GETPWR 28 and CNTLP
30. GETPWR detects the load voltage signal V.sub.SEN occuring at line 15
and the load current signal I.sub.SEN occuring at line 17. The load power
P.sub.LOAD is calculated as V.sub.SEN .multidot.I.sub.SEN. Furthermore,
the load impedance Z.sub.LOAD is calculated as V.sub.SEN /I.sub.SEN. A
determination is also made as to which impedance range the load impedance
occurs in. Thus, a first impedance range flag (not shown) is set if the
load impedance range is less than 70 ohms, a second flag is set if it is
between 70 and 100 ohms, and a third flag is set if it is above 100 ohms.
It should be understood a substantial amount of tolerance may be employed
in the selection of the impedance ranges. Thus, if desireable, the mid
impedance range may extend from 60 to 115 ohms, for example.
As stated above the operator may select between one of two different modes
in the high impedance range. Depending upon the selected mode, a further
flag will be set by COLDST indicating the selected mode. The routine CNTLP
inspects the above flags and determines which of the following algorithms
should be executed. These algorithms are LOZ 32, MIDZ 36, ZOL 38, and
HIZR2 40 as can be seen in FIG. 3. All of these algorithms eventually pass
control to a routine AFBADJ 34. Each of these routines will now be
individually discussed.
Assuming the load impedance is measured as 40 ohms, CNTLP will transfer
control to LOZ 32. A correction factor will be computed which will
implement the constant current characteristic described above for the low
impedance range. In particular, if I.sub.SEN=1.2 amps and if I.sub.MAX =1
amp (for example), then the correction factor is computed as I.sub.MAX
/I.sub.SEN. Thus, the calculated factor equals 1.0/1.2. After calculation
of the correction factor control is passed to AFBADJ 34 which raises or
lowers the control voltage AFB based on the computed correction factor.
Thus, if the value of AFB is 10 volts, it will be reduced by the
correction factor of 1.0/1.2. In this manner the current is maintained
constant at the value of I.sub.MAX (1.0 amp) over the lower impedance
range. Note in this example that I.sub.SEN exceeds I.sub.MAX. Whether it
exceeded it or not, the correction factor is calculated as I.sub.MAX
/I.sub.SEN over the entire low impedance range by the LOZ routine.
After the corrected control voltage is applied over line 19 to supply 10, a
test is made to determine if the operator is still enabling the generator
to apply electrosurgical power to the forceps. This test is conducted at
42. If the generator is so enabled, control is returned to GETPWR.
Execution of the routines from GETPWR through AFBADJ requires at most
about 20 milliseconds and thus the power delivered to the load is
constantly being adjusted depending upon the sensed load conditions. If
there are no more enables, the routine may return to an initialization
mode where the output is set off and certain self-test routines are
executed while waiting for the next enable. This feature is not shown in
the flow chart of FIG. 3.
As stated above the impedance of the tissue will tend to increase as
desiccation proceeds. Hence, assume the next load measured by the GETPWR
routine is 80 ohms. The MIDZ flag would be set and the load power
calculated. The routine CNTLP would again determine the actuated flags and
transfer control to the MIDZ routine 36. There a correction factor is
calculated in accordance with the expression (P.sub.MAX
/P.sub.LOAD).sup.1/2 where P.sub.MAX equals the nominal power selected by
the operator and P.sub.LOAD =V.sub.SEN .multidot.I.sub.SEN as calculated
in GETPWR. Thus, if P.sub.LOAD, the actual load power, is 60 watts and
P.sub.MAX the nominal power is 70 watts, the calculated correction factor
is (7/6).sup.1/2. Once this correction factor has been determined, control
is passed to the AFBADJ routine 34 where the correction factor is
multiplied by the previous value of control voltage AFB applied to line
19. The adjustment of the control voltage and the application over line 19
is executed in the same manner as described above for the LOZ routine.
With the passage of further time the impedance continues to increase. Thus,
assuming the desiccation mode is still enabled by the operator, control
will be returned to GETPWR and CNTLP. Further, assuming the operator has
selected mode one for the high range impedances, CNTLP 30 will transfer
control to ZOL 38 after having sensed the appropriate flags. If the
impedance calculated by GETPWR is now 200 ohms, the power delivered to it
will have a constant voltage characteristic as described above with
respect to FIG. 2. In particular, the constant voltage will be V.sub.MAX
as calculated by the COLDST routine. In order to implement this constant
voltage characteristic ZOL calculates a correction factor determined by
the expression V.sub.MAX /V.sub.SEN where V.sub.SEN equals the load
voltage sensed by GETPWR. If the sensed voltage is 70 volts while
V.sub.MAX is 50 volts, the correction factor will be 5/7. AFBADJ utilizes
this correction factor in the same manner as discussed above for LOZ and
MIDZ to adjust the control voltage applied to line 19.
If the operator has selected mode 2, this will be sensed by the CNTLP
together with the fact that the impedance is now in the high impedance
range. Assume that the impedance at this time is 200 ohms while P.sub.MAX
is 50 watts and P.sub.LOAD =60 watts. The correction factor is computed as
[(10,000 P.sub.MAX /Z.sup.2.sub.LOAD)/P.sub.LOAD ].sup.1/2. Thus, in the
above instance, it equals [(10,000)(50)/(40,000)/60].sup.1/2
=(5/24).sup.1/2. Again, after computation of the correction, control is
passed to AFBADJ to adjust the line 19 control voltage. Attached hereto as
Appendix I is a program listing for the FIG. 3 flowchart where the listing
also includes various initialization, testing, and error programs.
Reference should now be made to FIGS. 4A and 4B which is a schematic
diagram of switching power supply 10, high efficiency RF generator 12, and
current sensors 14 of FIG. 1. The switching power supply includes a
standard switching supply circuit 44 made by Silicon General. The
remaining integrated circuits of the invention are also made by Silicon
General and other such companies. The switching power supply is of
conventional configuration and includes a switching transistor 46 and
inductor 47, and an output line 48 which applies regulated high voltage to
output transistor 52 of generator 12. The power supply voltage applied to
line 48 originates from terminal 50, this voltage typically being about
100 volts. The voltage at terminal 50 is regulated by switching transistor
46, inductor 47 and capacitor 49 where, in particular, the longer
transistor 46 is switched off, the less the DC voltage applied to line 48
will be in magnitude. A voltage DC SENSE proportional to the line 48
voltage is applied to terminal 2 of circuit 44 via a voltage divider
comprising resistors 54 and 56. This voltage is compared to and follows
AFB on line 19.
On-off pulses are applied to switching transistor 46 from circuit 44 via
transistors 58 and 60. The repetition rate of these pulses is determined
by resistor 62 and capacitor 64 connected to the R.sub.T and C.sub.T
terminals of circuit 44. The width of these pulses and thus the on/off
time of transistor 46 is a function of the difference between AFB and DC
SENSE. Thus, in this manner the power supply voltage follows AFB as the
impedance changes during a desiccation procedure.
The output power is delivered to the load via lines 66 and 68 which in turn
are connected to output transformer 70. The generator 12 is driven by a
750 KHz signal applied to terminal 72 where it is amplified and applied to
output transistor 52 which in turn drives primary winding 74 of
transformer 70.
The load current is sensed at transformer 76 and converted to a voltage
representative of the current by the conversion circuitry indicated at 78.
This voltage is applied over line 17 as the I.sub.SEN signal to the
microprocessor 16. Furthermore, a voltage representative of the load
voltage is derived from winding 80 of transformer 70. This signal is
converted by signal conversion circuitry 82 to develop the load voltage at
V.sub.SEN on line 19.
The values given for the resistors are in ohms while those for the
capacitances are microfarads unless otherwise specified, these values
being illustrative of the embodiment of the invention.
It should be noted that overall operation of the present invention is such
that a very small heat sink is needed to dissipate heat, thus, a 3-4 watt
sink connected to transistor 52 is suitable, such a sink being very small
in size. This follows from the fact that the amplifier is essentially
matched to a 100 ohm load and only in a limited impedance range around
that value is the maximum delivered to the load.
With the load impedance substantially removed from 100 ohms, the power is
reduced as discussed above and shown in FIG. 2. There is no need to
generate substantial wattage at impedances removed from the impedance to
which the amplifier is matched. Accordingly, even at impedances removed
from 100 ohms only a small amount of power is involved whereby the small
heat sink mentioned above is suitable for these smaller powers.
It is to be understood that the above detailed description of an embodiment
of the invention is provided by way of example only. Thus, for example,
the above principles are also applicable to monopolar surgery. Various
details of design and construction may be modified without departing from
the true spirit and scope of the invention as set forth in the appended
claims.
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