Blood aspirator

A blood aspirator is composed of a suction circuit adapted to receive a flow of blood, a sensor associated with the suction circuit for generating a signal relating to the presence of bubbles in aspirated blood in the suction circuit, a variable speed pump coupled to the suction circuit and adapted to pump blood through the suction circuit, and a controller for controlling the speed of the pump. The controller causes the pump to adjust the blood flow through the suction circuit so that a predetermined, nonzero concentration of bubbles flows through the suction circuit.

BACKGROUND OF THE INVENTION 
The present invention is directed to a blood aspirator that reduces the 
intake of air bubbles during aspiration of blood to reduce damage to the 
aspirated blood due to aspiration of the bubbles. 
Vacuum-operated blood aspirators are conventional devices which are used in 
certain cases to remove blood from a patient, such as during a surgical 
procedure. The removed blood may be reinfused back into the patient at 
substantially the same time as it is removed to eliminate the need to 
obtain blood from an alternative blood source. Although re-use of 
vacuum-aspirated blood is generally advantageous, it has been recognized 
that aspiration may damage the blood due to air bubbles entrained in the 
aspirated blood. For example, in an article entitled "A Low-Hemolysis 
Blood Aspirator Conserves Blood During Surgery," Clague, et al. state that 
"Blood damage caused by traditional vacuum-operated suction tubes, 
particularly when air is aspirated along with the blood, usually exceeds 
damage from all other components. In addition to platelet injury, there is 
a high degree of hemolysis, which leads to high plasma hemoglobin levels 
and reduces the number of red blood cells available for reinfusion during 
cases of blood conservation, such as autologous transfusion and cardiac 
bypass." 
U.S. Pat. No. 4,976,682 to Lane, et al. discloses a blood recovery system 
that reduces blood damage by minimizing the intake of air bubbles in the 
aspirated blood. The Lane, et al. blood recovery system includes a suction 
pump for aspirating blood and a bubble detector for detecting the presence 
of bubbles in the aspirated blood. As described in column 13, lines 34-42 
of the Lane, et al. patent, as soon as a bubble is detected within the 
suction tip at the bubble detector, the speed of the suction pump is 
reduced. The pump speed continues to be reduced until no air is detected 
by the bubble detector. When the vacuum pump slows to the point where no 
air is detected by the bubble detector, the suction pump speeds up 
slightly until a small bubble appears at the suction tip, at which time 
the pump again slows. 
SUMMARY OF THE INVENTION 
The invention is directed to a blood aspirator having a suction circuit 
adapted to receive a flow of blood, sensor means associated with the 
suction circuit for generating a signal relating to the presence of 
bubbles in aspirated blood in the suction circuit, a variable speed pump 
coupled to the suction circuit and adapted to pump blood through the 
suction circuit, and control means for controlling the speed of the pump. 
The control means may include means for causing the pump to increase blood 
flow through the suction circuit when bubbles are present in the aspirated 
blood and/or means for causing the pump to adjust the blood flow through 
the suction circuit so that a predetermined, nonzero concentration of 
bubbles flows through the suction circuit. 
The control means may also include means for causing the pump to increase 
the blood flow through the suction circuit in response to a bubble 
concentration that is lower than a predetermined nonzero value and means 
for causing the pump to decrease the blood flow through the suction 
circuit in response to a bubble concentration that is higher than a 
predetermined nonzero value. The control means may control the pump based 
upon the bubble concentration and a flow signal relating to the magnitude 
of blood flow through the suction circuit, and the control means may also 
control the pump based on the rate of change over time of the bubble 
concentration. 
The blood aspirator may include memory means for storing a plurality of 
bubble concentration ranges, each of the ranges having an associated 
factor relating to the pump speed, means for determining which of the 
bubble concentration ranges the bubble concentration falls within, and 
means for controlling the pump speed based upon the factor associated with 
the bubble concentration determined by the determining means. 
These and other features and advantages of the present invention will be 
apparent to those of ordinary skill in the art in view of the detailed 
description of the preferred embodiment, which is made with reference to 
the drawings, a brief description of which is provided below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a preferred embodiment of a blood aspirator 10 in 
accordance with the invention which may be used to evacuate blood from a 
surgical area during surgery, for example. The blood aspirator 10 includes 
a blood suction circuit composed of a suction device 12, a liquid conduit 
14 connected to the suction device 12, a variable speed pump 16 connected 
to the liquid conduit 14, a second liquid conduit 18 connected to the pump 
16, a blood reservoir 20 connected to the conduit 18, and a blood return 
conduit 22 connected to a patient in a conventional manner such as via a 
catheter (not shown). The pump 16 may be a conventional pump, such as a 
roller pump. 
The suction device 12 includes a tip portion 12a and a handle portion 12b. 
The blood aspirator 10 also includes a sensor 24 coupled to the tip 
portion 12a of the suction device 12 and a controller 30 connected to the 
sensor 24 via a signal line 32 and to the pump 16 via a signal line 34. 
The controller 30 includes a microprocessor 40, a random-access memory 
(RAM) 42, a permanent memory in the form of a read-only memory (ROM) 44, 
and an input/output (I/O) circuit 46, all of which are interconnected via 
an address/data bus 48. The particular type of controller 30 used in not 
important to the invention. A display device 50 and an input device 52 may 
optionally be connected to the controller 30. 
The controller 30 controls the speed of the pump 16 in response to the 
concentration of bubbles detected in the suction device 12. The sensor 24 
may be an optical sensor that generates a beam of radiation (such as 
infrared radiation) that is passed through a transparent portion of the 
tip portion 12a and detects whether or not a bubble is present by 
detecting whether or not the radiation beam is interrupted, as sensed by a 
receiver portion of the sensor 24. If the radiation beam is detected, then 
the sensor 24 generates a bubble-absent signal (e.g. a five-volt signal), 
whereas if no radiation beam is detected, the sensor 24 generates a 
bubble-present signal (e.g. a zero-volt signal). 
In response to the concentration of bubbles detected in the suction device 
12, the controller 30 adjusts the speed of the pump 16 by transmitting a 
drive signal to the pump 16 via the line 34. For example, the drive signal 
could be an analog drive signal that varies between zero and five volts, 
with a zero-volt signal corresponding to the lowest pump speed and a 
five-volt signal corresponding to the highest pump speed. Alternatively, 
the drive signal could be a multi-bit digital signal. 
The manner in which the controller 30 controls the pump speed is described 
below in connection with FIG. 2, which is a flowchart of a main operating 
routine 100 stored in the ROM 44 and executed by the microprocessor 40. 
The main routine 100 is continuously performed while the blood aspirator 
10 is in use. 
Referring to FIG. 2, at step 102, a drive signal is transmitted to the pump 
16 via the line 34 to adjust the speed of the pump 16. When the blood 
aspirator 10 is first being used, the drive signal may be a predetermined 
signal that corresponds to a minimum level of pump speed or suction, such 
as 125 milliliters per minute. During subsequent performance of step 102, 
the magnitude of the drive signal is based on the bubble concentration, as 
described in more detail below. 
At step 104, the sensor 24 is read or sampled a relatively large number of 
times, such as in excess of 1,000, during a predetermined period of time, 
such as 1.5 seconds, referred to herein as a sampling window. During such 
sampling window, the sensor 24 will typically generate a plurality of 
bubble-present signals and a plurality of bubble-absent signals. The total 
number of such signals will be the same as the number of times the sensor 
24 was sampled. 
If a sampling window is used and where the pump 16 is a roller pump, the 
duration of the sampling window is selected to be relatively long so that 
any flow variations due to the use of the roller pump are averaged out. 
At step 106, the concentration of bubbles in the aspirated blood flowing 
through the suction device 12 is determined by dividing the number of 
bubble-present signals generated by the sensor 24 by the number of times 
the sensor 24 was sampled. For example, where the sensor 24 was sampled 
10,000 times during the sampling window and where it generated 800 
bubble-present signals, the bubble concentration determined at step 106 
would be 8%. 
At step 108, the change in bubble concentration is determined by 
calculating the difference between the bubble concentration just 
determined at step 106 and the bubble concentration determined during the 
previous performance of step 106. For example, if the bubble concentration 
just determined at step 106 is 8% and the previous bubble concentration 
determined at step 106 is 5%, the change in bubble concentration is 3%. 
At step 110, a speed factor is determined based upon the bubble 
concentration determined at step 106 and the change in bubble 
concentration determined at step 108. Referring to FIG. 3, the speed 
factor is retrieved from a speed factor table stored in the memory (e.g. 
the ROM 44) of the controller 30. The speed factor table is in the form of 
a two-dimensional array of numeric values (each numeric value being 
represented by a two-letter acronym such as "PS"), each column of the 
array corresponding to a particular bubble concentration and each row of 
the array corresponding to a particular change in bubble concentration. 
The table of FIG. 3 has seven possible bubble concentrations (1%, 2%, 3%, 
5%, 10%, 14% and 18%) and seven possible changes in bubble concentration 
(6%, 4%, 2%, 0%, -2%, -4%, -6%). The two letter acronyms of the speed 
factor table and their meanings and corresponding numeric values are set 
forth below: 
______________________________________ 
Acronym Meaning Value 
______________________________________ 
PL Positive Large 
.075 
PM Positive Medium 
.050 
PS Positive Small 
.025 
ZE Zero 0 
NX Negative Small 
-.025 
NM Negative Medium 
-.050 
NL Negative Large 
-.075 
______________________________________ 
A positive speed factor, such as 0.075, will cause the magnitude of the 
pump drive signal on the line 34 to increase, and will therefore cause the 
pump speed to increase, with an increased rate of suction. A negative 
speed factor will cause the magnitude of the drive signal to decrease, 
causing the pump speed to decrease. The numeric values set forth above may 
correspond to predetermined increments of flow. For example, a speed 
factor of 0.025 may correspond to a change in flow of 16 milliliters per 
minute. 
The above numeric values are selected to attempt to control the 
concentration of bubbles within the aspirated blood to 5% at all times. 
For example, if the bubble concentration is 5% and there is no change in 
the bubble concentration from the previous bubble concentration, the 
corresponding speed factor (at the intersection of the "0" row and "5" 
column) is ZE, which has a corresponding numeric value of zero. If the 
bubble concentration is 10% and the change in the bubble concentration 
from the previous bubble concentration is 2%, the corresponding speed 
factor (at the intersection of the "2" row and "10" column) is NS, which 
has a corresponding numeric value of -0.025, which will cause the suction 
rate to be decreased. 
Controlling the bubble concentration to a relatively small, nonzero value 
is desirable in that a relatively high degree of suction can be 
maintained, while at the same time preventing the bubble concentration 
from increasing enough to cause significant blood damage. A relatively 
high degree of suction is desirable to keep the surgical area dry, or free 
of relatively large amounts of accumulated blood. 
Since bubbles would not be generated if the end of the suction tip 12a were 
totally submerged in a pool of blood, the presence of some bubbles is 
desirable since it indicates that at least a portion of the end of the 
suction tip 12a is not submerged. Thus, the presence of some bubbles 
indicates that there is not a large pool of blood which needs to be 
evacuated (assuming that the suction tip 12a is being used properly by 
placing the end of the tip 12a at the bottom of the surgical area where 
blood is accumulating). 
If the bubble concentration and change in bubble concentration do not 
exactly correspond to the numeric values used in the table, various 
approaches could be used to determine a speed factor. For example, if the 
bubble concentration was 4% and the change in bubble concentration was 1%, 
a weighted average of the speed factors could be used. In this case, the 
speed factor would be determined in accordance with the following 
equation: 
EQU Speed Factor=(1/2ZE+1/2NS+1/2PS+1/2ZE)/2 
Alternatively, the speed factor could be selected based upon the bubble 
concentration and change in bubble concentration set forth in the table 
that were the closest to the actual bubble concentration and the actual 
change in bubble concentration. For example, if the actual bubble 
concentration was 7% and the actual change in bubble concentration was 
1.5%, the speed factor corresponding to the "2" row and the "5" column 
could be selected. 
Another method of determining the speed factor which incorporates fuzzy 
logic could used. This fuzzy logic method is described below in connection 
with FIGS. 4 and 5. FIG. 4 illustrates a graph of bubble concentration 
versus percent level that conceptually illustrates the interpolation of an 
actual bubble concentration between the bubble concentrations of the speed 
factor table of FIG. 3. In FIG. 4, each bubble concentration in the speed 
factor table is provided with an associated triangle which is used for 
interpolation purposes. For example, if the actual bubble concentration is 
9% as represented by a vertical line in FIG. 4, the vertical line would 
intersect the triangle (in solid lines) for the 10% concentration at the 
80% level and the triangle (in dotted lines) for the 5% concentration at 
the 20% level. 
FIG. 5 illustrates a graph, similar to that of FIG. 4, of change in bubble 
concentration versus percent level that conceptually illustrates the 
interpolation of an actual change in bubble concentration between the 
changes in bubble concentrations of the speed factor table of FIG. 3. If 
the actual change in bubble concentration was 1.4%, the vertical line 
shown in FIG. 5 would intersect the triangle for 0% (shown in solid lines) 
at 30% and would intersect the triangle for 2% (shown in dotted lines) at 
the 70% level. 
To determine the speed factor in accordance with this method, the four 
speed factors in the speed factor table which correspond to the triangles 
in FIGS. 4 and 5 that were intersected by the two vertical lines are used. 
For the example set forth above (9% bubble concentration and 1.4% change 
in bubble concentration), these four speed factors are shown in a darkened 
box 109 in FIG. 3. Then, the two rows and two columns which intersect 
those four speed factors are assigned the level percentages determined in 
connection with FIGS. 4 and 5. For this example, row "2" of the speed 
factor table has been assigned a 70% percentage; row "0" of the table has 
been assigned a 30% percentage; column "5" of the table has been assigned 
a 20% percentage; and column "10" of the table has been assigned an 80% 
percentage. 
Each speed factor in the box 109 is then multiplied by the smaller of the 
two percentages which are associated with that speed factor. For example, 
the upper left-hand speed factor NS in the box 109 would be multiplied by 
20%, since the 20% percentage associated with the "5" column is less than 
the 70% percentage associated with the "2" row. 
After each speed factor in the box 109 is multiplied by the smaller 
corresponding percentage, the resultant values are added together and 
divided by the sum of the percentages used. For this example, the 
resultant speed factor would be equal to (0.20NS+0.70NS+0.30NS+0.20ZE)/1.4 
(1.4 is the sum of 0.20, 0.70, 0.30, and 0.20). Various other methods of 
determining the speed factor at step 110 could be used. 
Referring back to FIG. 2, a gain factor is then determined at step 112. One 
example of how the gain factor could be determined is shown in FIGS. 
6A-6C, which are a flowchart of a gain determination routine. A gain 
factor is assigned at step 112 based upon which of a plurality of 
predetermined bubble concentration ranges the actual bubble concentration 
falls within and the speed of the pump 16, which corresponds to the flow 
rate through the suction device 12 and the conduit 14. 
Referring to FIG. 6A, at step 130, if the bubble concentration determined 
at step 106 (FIG. 2) is greater than 60%, the program branches to step 132 
where the gain is set to 0.04 times the percentage bubble concentration 
(expressed as a whole number, not as a percentage). For example, if the 
bubble concentration was 70%, the gain would be set to 2.8. At step 134, 
if the bubble concentration determined at step 106 is between 40% and 60%, 
the program branches to step 136 where the gain is set to 0.02 times the 
percentage bubble concentration. 
Referring to FIG. 6B, at step 138, if the bubble concentration determined 
at step 106 is between 20% and 40%, the program branches to step 140, 
where the current speed of the pump 16 is compared with a predetermined 
threshold value, such as 250 milliliters per minute. If the pump speed, 
and thus the flow through the suction circuit, is smaller than the 
threshold value, it is more likely that there is some statistical error in 
the bubble concentration determined at step 106 because a relatively low 
blood volume was inspected by the bubble sensor 24 (due to the relatively 
low blood flow). In this case, the sensitivity of the aspirator 10 is 
somewhat reduced by using a smaller gain value so that the aspirator 10 
does not "overreact" to the bubble concentration. If the pump speed, and 
thus the blood flow, is greater than the threshold value, then a larger 
gain value is used. 
At step 140, if the pump speed is less than the threshold value, the 
program branches to step 142 where the gain is set to 0.005 times the 
percentage bubble concentration. If the pump speed is not less than the 
threshold, the program branches to step 144 where the gain is set to 0.010 
times the percentage bubble concentration. 
At step 146, if the bubble concentration is between 10% and 20%, the 
program branches to step 148, where the pump speed is compared with the 
threshold value. If the pump speed is less than that value, the program 
branches to step 150 where the gain is set to 0.075. If not, the gain is 
set to 0.100 at step 152. 
Referring to FIG. 6C, if step 154 is reached, then the bubble concentration 
is less than 10%. At step 154, if the pump speed is less than 250, the 
program branches to step 156 where the gain is set to 0.075. If not, the 
gain is set to 0.100 at step 158. At step 160, if the bubble concentration 
determined at step 106 and the change in bubble concentration determined 
at step 108 are not both zero, the program branches to step 162 where a 
count variable is set to zero. At step 160, if the bubble concentration 
and the change in bubble concentration are both zero, the program branches 
to step 164 where the count variable is incremented by one. 
The count variable is used to measure any continuous period of time for 
which the aspirator 10 has detected no bubbles. This time corresponds to 
the duration for which the bubble concentration is zero and the rate of 
change of bubble concentration is zero. A significant time period for 
which no bubbles have been detected may correspond to a situation where 
blood has increased at the surgical area at a relatively large rate, due 
to the cutting of an artery, for example. In this case, it is desirable to 
increase the suction rate more quickly than usual if the suction rate is 
below a given threshold, such as 250 milliliters per minute. 
At step 166, if the pump speed is less than 250, the program branches to 
step 168 where the gain is determined by adding the gain value determined 
at step 156 to the cube of the count determined at step 164. For example, 
if the current count is two, the gain value determined at step 168 would 
be 0.075+8, or 8.75. If the pump speed was not less than 250 as determined 
at step 166, then the gain is set equal to a predetermined value, such as 
10. 
The particular gain values, bubble percentage ranges, pump speed values, 
and threshold values shown in FIGS. 6A-6C are not considered important to 
the invention, and other values could be used. 
Referring back to FIG. 2, at step 114 the new pump speed is determined 
based upon the current pump speed, the speed factor determined at step 110 
and the gain factor determined at step 112, in accordance with the 
following equation: 
EQU New speed=current speed+(speed factor.times.gain factor) 
As stated above, the pump speed may be represented by an analog value 
between zero and five volts (which is transmitted to the pump 16 via the 
line 34). 
After the new speed is determined, at step 116 the new speed is compared to 
determine whether it is greater than a predetermined upper limit. The 
upper limit may be used for safety reasons to prevent the aspirator 10 
from removing blood from a patient at very large rates, such as two liters 
per minute. If the new speed determined at step 116 is greater than the 
upper limit, the program branches to step 118 where the new speed is set 
equal to the numeric value of the upper limit. 
At step 120 the new speed is compared to determine whether it is less than 
a predetermined lower limit. A lower speed limit may be used to guarantee 
that the aspirator 10 maintains a minimum suction rate so that the amount 
of blood sampled by the bubble sensor 24 is large enough to guarantee 
statistically significant data relating to the bubble concentration. At 
step 120, if the new speed is less than the predetermined lower limit, the 
program branches to step 122 where the new speed is set equal to the 
numeric value of the lower limit. The numeric values of the upper and 
lower limits may be selected to correspond to the desired upper and lower 
flow limits of the pump 16, such as a lower flow limit of 125 milliliters 
per minute and an upper flow limit of one liter per minute. 
After the new pump speed is set at one of steps 114, 118, or 122, the 
program branches back to step 102 where the signal corresponding to the 
new pump speed is transmitted to the pump 16 via the line 34 to drive the 
pump 16 at its new speed. Steps 104-122 are continuously repeated while 
the aspirator 10 is in use to control the pump speed as described above. 
The handle 12b of the suction device 12 could be provided with a switch to 
override the output of the bubble sensor 24, e.g. a switch that provided a 
predetermined voltage on the line 32, so that a continuous bubble-absent 
signal was generated. The purpose of the override switch would be to cause 
the aspirator 10 to increase the suction rate at its maximum rate of 
increase, which could be used, for example, where a surgeon was about to 
make an incision which would cause a large amount of bleeding. 
Although the new pump speed is described above as being determined based 
upon a speed factor and a gain factor, it is not considered necessary to 
the invention that both factors be used to determine the new pump speed. 
For example, instead of separately determining a speed factor and a gain 
factor, a single speed adjustment factor could be used. Such a factor 
could be stored in a memory table which has numerous such factors, one of 
which is selected based upon the bubble concentration, the change in 
bubble concentration, and/or the current pump speed. 
Instead of using values stored in a memory table to determine the new pump 
speed, the new speed could be determined in accordance with one or more 
equations which take into account the bubble concentration, the flow rate 
through the suction circuit, and/or the change in bubble concentration 
over time. 
Although the sensor 24 is disclosed above as being an optical sensor, other 
types of sensors could be utilized. For example, a conventional impedance 
sensor for sensing the impedance at a point across the suction device 
could be used to generate the bubble-present and bubble-absent signals. 
Since air bubbles have a different impedance than blood, if a bubble was 
present, the impedance across the suction device 12 would be different 
than if a bubble were not present. 
Instead of a sensor which detects whether or not one or more bubbles are 
present at a particular point, a sensor that generates a signal indicative 
of the bubble concentration along a length of the suction device could be 
used. For example, an impedance sensor in the form of a pair of 
impedance-sensing devices spaced along a length of the suction device 12 
could be utilized to generate a signal indicative of the impedance between 
them, and thus indicative of the bubble concentration. In such case, step 
106 of FIG. 2 would be unnecessary since the signal generated by such an 
impedance sensor would already be indicative of the bubble concentration. 
Modifications and alternative embodiments of the invention will be apparent 
to those skilled in the art in view of the foregoing description. This 
description is to be construed as illustrative only, and is for the 
purpose of teaching those skilled in the art the best mode of carrying out 
the invention. The details of the structure and method may be varied 
substantially without departing from the spirit of the invention, and the 
exclusive use of all modifications which come within the scope of the 
appended claims is reserved.