Apparatus and method for vascular examination of a limb

An apparatus and a method are provided for vascular examination of a limb by inflation of a flexible-walled pneumatic chamber mounted to the limb and calibration of the pressure within the chamber in units of volume. Calibration is accomplished by inflating the chamber to a preselected reference pressure, altering the volume of the chamber, and measuring the calibration pressure at the altered volume. The calibration pressure is stored for use off-line in scaling subsequent pressure fluctuations as changes in limb volume.

BACKGROUND OF THE INVENTION 
The present invention relates to an air-plethysmograph for vascular 
examination of limbs. More particularly, it pertains to a calibrated 
apparatus and method for measuring limb volume changes through changes in 
pressure in a closed pneumatic system. 
Reconstructive surgery of deep veins, and a continuing controversy about 
the mechanism of the effect of elastic compression on venous hemodynamics 
in the lower limb, have created a need for a noninvasive measurement of 
reflux and calf muscle pump ejection. Plethysmographs have been used to 
study limb muscle pump function by determining pressure changes in a cuff 
surrounding a limb due to postural changes and exercise. 
A prior form of plethysmograph, disclosed by Christopoulos et al. in 
"Air-plethysmography and the effect of elastic compression on venous 
hemodynamics of the leg", Journal of Vascular Surgery, Vol. 5, No. 1, pg. 
148-159, January 1987, consists of an air chamber which surrounds a limb 
and is inflated to a pressure sufficient to ensure good contact with the 
limb. Because pressure and volume are inversely related in a closed 
system, and be the volume of the air chamber is inversel related to the 
volume of the limb that it surrounds, pressure in the air chamber is 
directly proportional to the volume of blood in the limb. Problems arise 
in calibrating the pressure to volume units, however, as attempted by 
Christopoulos et al., with a second chamber between the air chamber and 
the limb. The air chamber is initially inflated and closed off from the 
atmosphere while the limb is kept in an elevated position. After a 
preselected waiting period, the pressure in the air chamber is measured. A 
calibration is then performed by injections of known volumes of water into 
the second chamber. The pressure in the air chamber is measured after each 
injection, yielding a water volume calibration curve which correlates air 
chamber pressure with increases in water volume. The water is then removed 
from the second chamber and vascular examination is conducted. During 
examination the patient is asked to perform various tests, such as 
arterial inflow and venous obstruction tests, and the pressure within the 
air chamber is recorded by a chart recorder. Attempts are made to match 
recorded pressures with volumes using the volume calibration curve. 
Unfortunately, the use of water in the second chamber of Christopoulos et 
al. complicates the procedure and disrupts the temperature gradient 
between the air chamber and the limb. This can cause significant output 
errors. The arbitrary units of the pressure signal also make it difficult 
to read in terms of volume. Volumes are known at specific points, but 
interpolation is difficult. 
Other devices of this type have a small syringe (.about.1 cc) in 
communication with an air chamber for injecting a known amount of air into 
a cuff. The injection of air acts as a reduction in system volume which 
calibrates the chart for a 1 cc increase in limb volume. However, these 
devices are calibrated "on line", like the Christopoulos et al. device, 
and do not produce a one-to-one correspondence between divisions on the 
chart and units of displayed volume. 
Therefore, it is desirable in many applications to provide an apparatus and 
a method for easily calibrating the output of a plethysmograph to limb 
volume over a range of volumes and reducing output errors. 
SUMMARY OF THE INVENTION 
The present invention simplifies calibration of an air-plethysmograph to 
reflect limb volume and permits calibration over a range of operation so 
that each increment of distance on the chart corresponds to a known volume 
change. Calibration takes place "off-line" using pressure measurements 
taken at a first reference volume and a second calibration volume 
resulting from a controlled alteration of system volume. Ongoing volume 
fluctuations due to changing conditions in the cuff and the patient's limb 
do not affect calibration. This eliminates significant errors during 
calibration and allows the gain and offset of the output signal to be 
adjusted accurately. 
Another distinct advantage of the present invention over prior 
plethysmographs is that the reference pressure level is maintained 
dynamically for a period long enough for temperature, arterial inflow and 
relaxation of the cuff to stabilize. Air is added and released in the air 
chamber during this period, as necessary, to achieve an accurate base line 
from which calibration and subsequent volume measurements can be made. 
Specifically, the present invention comprises an apparatus and a method for 
vascular examination of a limb in which: at least one flexible-walled 
chamber of a pneumatic system is mounted to a limb for monitoring limb 
volume; the pneumatic system, and thus the flexible-walled chamber, are 
inflated to a preselected reference pressure at a stable reference volume; 
the pneumatic system is closed to the atmosphere; the volume of the 
pneumatic system is altered by a preselected amount to achieve a 
calibration volume and a corresponding calibration pressure; the 
calibration pressure is measured and stored; and, fluctuations in system 
pressure relative to the reference pressure are displayed as changes in 
limb volume. 
In a preferred embodiment, the reference pressure is stored for use with 
the calibration pressure in calibrating subsequent fluctuations. In 
another embodiment, the fluctuations are displayed on a graphical output 
device calibrated to reflect changes in limb volume by: sending signals 
representative of the reference pressure and the calibration pressure to 
the output device for display; and, adjusting the gain of those signals so 
that the reference pressure and the calibration pressure are displayed a 
preselected distance apart in a direction representing limb volume. The 
signals can then also be adjusted so that the reference pressure is 
displayed at a preselected baseline level corresponding to the reference 
volume. This is accomplished by sending the reference pressure and the 
calibration pressure signals alternately to the output device for display 
during calibration in the absence of any other pressure signal. Thus, 
calibration takes place while the output device is "off line". 
Yet another embodiment of the invention involves sensing the pressure in 
the pneumatic system and maintaining it at the reference pressure for a 
preselected stabilization period to permit conditions within the 
flexible-walled chamber and the limb to stabilize. The pneumatic system is 
maintained during the stabilization period by introducing and withdrawing 
air, as necessary, to achieve stable temperature and volume conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a preferred form of apparatus constructed according to 
the present invention for vascular examination of a patient's limb has a 
microcomputer 10 which monitors the pressure of air in a flexible-walled 
chamber or cuff 12 of a pneumatic system 14 through a pressure sensor 16. 
The microcomputer 10 displays the pressure of the pneumatic system 14 on a 
chart recorder or other suitable output device 18 as a signal calibrated 
to the volume of a limb surrounded by the cuff 12. 
Calibration is accomplished by reducing the volume of the system from a 
reference volume at which the pressure of the pneumatic system 14 is a 
known, stabilized value to an altered volume which differs from the 
reference volume by a known amount, preferably 100 ml. Storage of system 
pressure at both the reference volume and the altered volume, and 
knowledge of the difference between the volumes, permit the chart recorder 
18 to be calibrated by adjusting the pressure signal so that subsequent 
fluctuations in pressure are displayed on a graph having a known number of 
divisions for each unit of limb volume. Calibration is performed by 
outputting the reference and calibration pressure levels alternately to 
the chart recorder 18 and making appropriate adjustments to either the 
pressure signal or the gain and offset of the recorder. 
The cuff 12 is part of the pneumatic system 14 and may be any suitable 
flexible-walled chamber designed for placement around a limb. Typically, 
it is an air cuff such as a 14-inch long toroidally-shaped polyurethane 
bag capable of surrounding a lower leg from the ankle to just below the 
knee so that a change in leg volume due to vascular activity causes an 
equal and opposite change in volume of the cuff. For this purpose, the 
outer wall of the cuff should be relatively insensitive to changes in cuff 
pressure within the range encountered in use. 
Pressure in the cuff 12 is maintained by an air pump 20 acting through a 
cuff inflate valve 22. The air pump 20 and the cuff inflate valve 22 are 
both part of the pneumatic system 14. The microcomputer 10 controls the 
inflation process with a pump driver 24. Inflation begins with the 
microcomputer 10 signalling the pump driver 24 to start the air pump 20 
and open the cuff inflate valve 22. When a desired pressure is achieved, 
the microcomputer 10 closes the pneumatic system 14 by signalling the pump 
driver 24 to stop the air pump 20 and close the cuff inflate valve 22. 
Pressure in the cuff 12 can be released through a bleed-off solenoid valve 
26 which communicates with the cuff 12 and is controlled by the 
microcomputer 10. As described more fully below, the air pump 20 and the 
bleed-off valve 26 may be actuated alternately, as needed, in a pressure 
stabilization mode of the apparatus to achieve a stable reference volume 
at a preset reference pressure. 
Pressure in the cuff 12 is constantly monitored by an "overpressure" switch 
28 of the pneumatic system 14. When the pressure in the cuff 12 exceeds a 
maximum preset limit, the overpressure switch 28 signals the pump driver 
24 to deactivate the air pump 20. The overpressure switch 28 also opens 
the bleed-off valve 26 to release air from the cuff 12 and triggers an 
overpressure LED 30 to signal that the pressure has exceeded its limit. 
The microcomputer 10 is also monitored during inflation by a watch dog or 
pump timer 32. The microcomputer 10 starts the pump timer 32 each time it 
activates the pump driver 24. The pump timer 32 times out at a preselected 
time-out interval. If the microcomputer 10 fails to stop and restart the 
pump driver 24 at intervals less than the time-out interval of the pump 
timer 32, the pump timer 32 turns off the pump driver 24, and resets the 
microcomputer 10. This ensures that the air pump 20 will not stay on in 
the event of a microcomputer 10 failure. 
Each time the microcomputer 10 activates the pump driver 32, the 
microcomputer 10 checks the pressure level to determine if there is an 
appropriate rise in the pneumatic system pressure. If the microcomputer 10 
detects no rise in pressure, or a decrease in pressure shortly after 
activating the pump driver 24, the microcomputer 10 goes into a "fault" 
condition, indicating either a pneumatic system leak or a faulty air pump 
20 or pump driver 24. 
The pneumatic system 14 also includes a calibration syringe or other 
mechanism 34 having a piston or plunger 36 which moves axially within a 
cylinder 38 to enlarge or reduce the volume of the system. In a preferred 
embodiment, the plunger 36 of the calibration syringe 34 is actuable by a 
solenoid or motor device 40 in response to signals from the microcomputer 
10. Thus, the microcomputer 10 can be used to control the volume of the 
pneumatic system 14 between an initial reference volume corresponding to a 
retracted condition of the plunger 36 and a smaller calibration volume 
corresponding to a bottomed-out condition of the plunger 36. The plunger 
36 is preferably provided with a mechanical stop 42 in order to accurately 
fix the reference volume. In another embodiment, the calibration syringe 
34 may be a simple syringe which is controlled manually by an operator. 
The pressure transducer or sensor 16 may be connected to the cuff 12 by a 
length of tubing, or may be placed on or near the cuff 12. In either case, 
it is exposed at a sensing surface thereof to the pressure within the 
pneumatic system 14. The pressure sensor 16 therefore generates an analog 
pressure signal proportional to the system pressure. 
The pressure signal is conditioned by an amplifier 44, after which it is 
applied directly to an analog switch 46 and through a low pass filter 48 
to an analog switch 50. The analog switches 46 and 50 are both controlled 
by the microcomputer 10 to selectively apply the pressure signal to an 
analog-to-digital (A/D) converter 52 along a common output line 54. The 
only difference between these paths is that the signal applied through the 
analog switch 50 is filtered while that applied through the analog switch 
46 is not. The filter 48 decreases sensed pressure variations known as 
"pump artifacts" which are inherent in reciprocating air pumps and enables 
a stable pressure to be sensed while the system is pumping. Thus, the 
microcomputer 10 can selectively apply a filtered or unfiltered pressure 
signal to the A/D converter 52. 
The digitized pressure signal from the A/D converter 52 is applied to the 
microcomputer 10 along a line 56 for use in controlling the apparatus and 
for calculating the magnitude of changes in limb volume. Such calculated 
volumes are applied to an output device 58, which may be any form of 
conventional readout mechanism, including a digital display, a graphical 
display or a mechanical or electrical bar graph. 
In addition to being applied to the microcomputer 10, the analog pressure 
signal along the line 54 can be applied to the chart recorder 18 through a 
third analog switch 60 and a buffer 62. This provides a direct graphical 
readout of system pressure, and therefore volume, whenever the analog 
switch 60 is energized. This output can be calibrated and scaled 
automatically, if desired, by the microcomputer 10 which controls the gain 
and offset of the chart recorder 18 along a control line 64. 
The microcomputer 10 also has a fault output 66. The microcomputer 10 
activates the fault output 66 when an error, such as a ROM or RAM error, 
occurs in the microcomputer 10. 
FIGS. 2A and 2B are flow charts of a method performed by the 
air-plethysmograph apparatus of FIG. 1. The procedure begins with 
initialization at step 200, which is begun by depressing a start switch 70 
(see FIG. 1). This powers up the microcomputer 10 and causes it to perform 
an internal self-test routine. Failure of any of the self-tests causes the 
microcomputer 10 to deactivate the system and trigger the fault output 66. 
Thus, the pump driver 24 is de-energized and the bleed-off valve 26 is 
opened to deflate the cuff 12. 
If the self-tests are all passed, a reference pressure is selected in step 
202 by moving a thumb wheel or other mechanism of a reference pressure 
selector switch 72. The reference pressure is chosen to be high enough to 
ensure good contact between the cuff and the limb but not so high that it 
significantly alters the hemodynamics within the limb. A pressure of 6 mm 
Hg is typically used. 
Once the reference pressure is selected, the cuff 12 is inflated and 
maintained at the reference pressure to permit the temperature gradient 
between the limb, the cuff 12 and the surrounding air to stabilize and 
ensure a resting arterial inflow to the limb. The limb is typically 
elevated to empty the veins and the patient is asked to keep as still as 
possible. 
In the first step of the inflation phase, step 204, the microcomputer 10 
transmits an activation signal to the pump timer 32 and the pump driver 
24. In turn, the pump driver 24 opens the cuff inflate valve 22, signals 
the air pump 20 to begin pumping air into the cuff 12, and activates the 
pump timer 32. If the time to attain a stable reference pressure ever 
exceeds a preselected maximum time for stabilization of the cuff pressure 
(step 206), indicating a malfunction in the pneumatic system 14, the pump 
timer 32 signals the microcomputer 10 that an error has occurred (step 
208). The microcomputer 10 then deactivates the pump driver 24 and opens 
the bleed-off valve 26 to release the cuff pressure. 
In step 210 the microcomputer 10 monitors the digital representation of the 
pressure signal received from the A/D converter 52 to identify when the 
pressure signal becomes stable over a preselected period of time 
sufficient to reach the temperature and arterial flow conditions described 
above. The preselected period is preferably approximately 5 minutes. Any 
deviation from the reference pressure signal during this period causes the 
processor to return to step 204 along a path 212 to correct for the 
deviations. The pneumatic system 14 is thus returned to the reference 
pressure by activating the pump driver 24 to add air or activating the 
bleed-off valve 26 to release air from the system, as required. 
While the exact sequence of steps by which system pressure is maintained at 
the reference level may vary from one embodiment to the other, it has been 
found desirable to operate the air pump 20 and the cuff inflate valve 22 
in a two-phase manner. When the sensed pressure falls short of the 
reference pressure by more than a preselected amount, the filtered signal 
is sensed while the air pump 20 is on. After the pressure increases to a 
level within the preselected amount of the reference pressure, the system 
enters a second phase in which the air pump 20 stops momentarily and the 
system senses the unfiltered signal while the air pump 20 is stopped to 
obtain more accurate pressure readings. 
When the cuff pressure remains at the reference level for the preselected 
period of time without requiring the addition or withdrawal of air, the 
system is stabilized. The microcomputer 10 then passes to step 214 and 
stores the reference pressure in memory. 
After the reference pressure is stored, the volume of the pneumatic system 
14 is altered by a preselected amount (step 216) and the pressure at the 
altered volume is stored in memory (step 218). This provides a second 
point at which system pressure is known in relation to system volume which 
itself is inversely related to changes in volume of the limb within the 
cuff 12. The two calibration pressure points therefore define a linear 
relationship between system pressure (P) and limb volume (V) which allows 
subsequent pressure changes to be expressed as changes in limb volume. 
Although the volume of the pneumatic system can be either increased or 
decreased in step 216, it is typically decreased by approximately 100 ml. 
This provides a second data point which, in calibration with the reference 
pressure, defines a range of pressures within which the system can operate 
to measure increases in volume of a patient's limb. The decrease in volume 
is accomplished in the present embodiment by fully depressing the plunger 
36 of the calibration syringe 34. 
After the pneumatic system 14 is restored to its reference volume by 
withdrawal of the plunger 36 (step 220) the system should return to its 
original reference pressure. However, the actual pressure may differ from 
the reference pressure due to changes in temperature, deformation of the 
cuff material, and moderation of arterial inflow during calibration. The 
microcomputer 10 therefore examines the pressure in step 222 and, if 
necessary, acts in step 224 to return the system to its original reference 
pressure by adding or releasing air. Step 224 is identical to step 204 
described above. 
After the reference volume is restored, the stored values of reference and 
altered volume pressure are used to scale the output of the apparatus so 
that it reflects changes in volume of the pneumatic system 14 (step 226). 
This can be done by altering the signal from the pressure sensor 16 within 
the microcomputer 10 or by adjusting the gain and offset of the recording 
instrument (18 or 58). In either case, the goal is to adjust the output so 
that the distance between the reference and altered volume signals 
represent a convenient number of unit divisions on the recording 
instrument and the reference volume signal occurs at an appropriate 
baseline position. Exemplary methods of accomplishing these goals are 
discussed below in connection with FIGS. 3A and 3B. The ordinate of the 
recording instrument can then be marked to represent increments of volume. 
After the pressure signal is scaled, the output of the pressure sensor 16 
is sent on-line to the recording instrument for display in calibrated 
volume units (step 228) while patient tests are performed (step 230). The 
resulting output, an example of which is shown in FIG. 4B, is an accurate 
measure of changes in blood volume in the limb of the patient. Referring 
specifically to FIG. 4B, which is an example of an output resulting from 
filling of the vascular volume of a leg, an output curve 80 rises from a 
reference volume level 82 (denoted "zero ml") to a level 84 which is close 
to the calibrated "100 ml" level. 
Except for the initialization step 200 and the pressure selection step 202, 
all of the steps described above are preferably performed automatically by 
the microcomputer 10 in the sequence described. The plunger 36 of the 
calibration syringe 34 is then operated automatically by the microcomputer 
10 through the solenoid device 40. In some instances, however, it may be 
desirable to perform one or more of the steps manually. Thus, steps 214 
and 218 can be performed by sequentially depressing a store switch 74 (See 
FIG. 1) and steps 216 and 220 can be accomplished by depressing and 
withdrawing, respectively, the plunger 36. The scaling step 226 can also 
be performed either automatically or manually by depressing a send switch 
76 to begin scaling and depressing it again to stop. 
FIGS. 3A and 3B illustrate alternative "subroutines" for scaling the 
pressure signal as described in step 226. The principal difference is that 
the subroutine of FIG. 3A is performed manually while that of FIG. 3B is 
performed automatically. Referring first to FIG. 3A, the manual subroutine 
begins by alternately outputting the stored reference and altered volume 
pressure signals to the chart recorder 18 by depressing the send switch 
76, both of FIG. 1 (step 232). This gives rise to the calibration waveform 
of FIG. 4A, wherein 90, 92, 94 and 96 represent the reference pressure 
signal (P.sub.ref), and 98, 100 and 102 represent the altered volume 
pressure signal (P.sub.alt). 
While the calibration waveform is displayed, the operator manually adjusts 
the "gain" and "offset" of the chart recorder 18 (step 234). The gain is 
adjusted so that the pen of the chart recorder 18 is displaced vertically 
a distance corresponding to a convenient number (e.g., four) of divisions 
of chart recorder paper. The offset is then adjusted to place the 
reference pressure signal at an appropriate major division of the paper. 
The calibration waveform is then terminated by depressing the send switch 
76 of FIG. 1 (step 236), leaving the apparatus calibrated for on-line 
operation in steps 228 and 230 of FIG. 2B. 
The foregoing description is intended specifically for the case in which 
the signal is displayed on a chart recorder having a moving pen mechanism. 
The routine of FIG. 3A is suitable, however, for use with any form of 
graphical display device which provides a hard copy. For example, a chart 
recorder having an array of independently addressable thermal heads may be 
used to print the scale and the output data simultaneously. In such a 
case, it is necessary only to adjust the gain of the recorder so that the 
pressure signal output is proportioned to the size of the paper. 
Referring to FIG. 3B, automatic scaling of the output signal may be 
accomplished internally by the microcomputer 10. The microcomputer first 
calculates a scale factor in step 238 according to the expression: 
##EQU1## 
and then calculates scaled volume information (V) from on-line pressure 
information (P) provided by the sensor 16 using the relationship: 
V=P.times.SF+OFFSET (step 240). The scale factor is a constant for any set 
of measurements because P.sub.alt and P.sub.ref are stored in the 
microcomputer 10 and the desired pen deflection for calibration pressure 
values is known from the characteristics of the chart recorder or other 
recording instrument 18. The desired location of the baseline pressure 
signal is selectable in advance and the output required to achieve it is 
also known from the characteristics of the recording instrument. Thus, the 
"offset" signal can be calculated automatically, as well. 
From the above, it can be seen that a non-invasive pneumatic apparatus and 
corresponding method have been provided for generating a calibrated 
display of limb volume over time for vascular examination. 
Although described for purposes of clarity with regard to specific 
preferred embodiments, the present invention is not limited to those 
embodiments but rather is applicable broadly to all versions falling 
within the scope and spirit of the appended claims. For example, the 
calibration syringe described herein can take the form of a piston and 
cylinder device but may be a rotary device or other mechanism having an 
internal volume which can be altered repeatedly and reproducibly by a 
preselected amount. The calibration pressure values also need not be 
displayed as the square waveform shown in FIG. 4A, but may be a staircase 
function which would be repeatable at the discretion of the operator 
during the scaling method. This is particularly useful when a plurality of 
altered volumes are used to provide more than two calibration data points.