 |
Description  |
|
|
TECHNICAL FIELD
This invention relates to an instrument for the noninvasive continuous
measurement of arterial blood pressure in which the actual blood pressure
waveform is reproduced. This is achieved by means of an inflatable
flexible finger cuff which incorporates a photoelectric infrared
transmitter and receiver, electronic circuitry connected to the
transmitter and receiver and controlling an electro-pneumatic transducer
which, in turn, is connected to the inflatable cuff by a flexible tube to
which is also attached an electronic pressure transducer.
BACKGROUND ART
A similar instrument is known in accordance with the prior art portion of
claim 1 (Czechoslovakian Pat. No. 133205).
However, the finger cuff used in that version of the instrument comprises a
number of inflatable sacks in a rigid cylinder with light source and
sensor mounted in the cylinder in such a way that the light must pass
through the sacks as well as through the finger.
In accordance with the prior art, the idea of placing the photoelectric
source-sensor pair directly against the skin, under the pressure cuff
(made possible with the advent of miniature photoelectric sources and
sensors), was put forward in 1975 by Reichenberger et al. (see Proceedings
"Colloque International sur les Capteurs Biomedicaux", Paris, 1975, A7.5).
Such a cuff was displayed in a demonstration of a valve and compressed air
version of the instrument in Leiden, The Netherlands, in 1978 (Wesseling,
K. H.: Niet invasieve vingerbloeddrukmeter, Boerhaave Lezingen,
Wetenschappelijk rapport afd. Cardiologie, Academisch Ziekenhuis, Leiden,
Oct. 6, 1978), and in Eindhoven, The Netherlands, in 1979 (Wesseling, K.
H.: Bloeddrukmeting en een prototype vingerbloeddrukmeter, Colloquium
Meten en Regelen, Technische Hogeschool Eindhoven, Afd. der
Elektrotechniek en Technische Natuurkunde, June 8, 1979).
Such a cuff is virtually a miniaturized version of the conventional
sphygmomanometer cuff, but incorporating an infrared light source and
sensor. (Infrared light absorption is insensitive to blood oxygen changes
or to changes in extravascular fluid volume resulting from the application
of the cuff pressure to the finger.) Owing to the direct skin contact with
the light source and sensor obtained in this type of cuff design, a much
larger plethysmogram (the signal from the photoelectric sensor) can be
obtained than with the original Czechoslovakian cuff design. Motion
artifact is substantially reduced also as a result of this intimate
contact with the finger plus the absence of the inertia (and thus motion
relative to the finger) of a rigid cylinder. The cuff fits a large range
of adult finger sizes (a smaller cuff is required for children). With this
cuff design, the cuff air space is minimized, this being a major
determinant of the size of the linear motor used in the present invention.
The cuff length is determined by theoretical considerations of the
longitudinal distribution of pressure transmitted from the cuff to the
arterial wall, as well as, though to a lesser extent, the light
source-sensor field pattern. A cuff length of minimally 4 cm allows
accurate measurements to be obtained.
Moreover, in that earlier version (Czechoslovakian Pat. No. 133205) the
electro-pneumatic transducer consists of an electrically controlled valve
which controls the amount of compressed air shunted to the inflatable
finger cuff or leaked off into the surrounding air. It has become apparent
that this form of electro-pneumatic transducer represents a severely
limiting factor in the operation of such an instrument, owing to the
necessary presence of a flow constriction in the pneumatic circuit which
thereby limits the speed at which the cuff can be inflated and thus the
fastest component in the blood pressure that can be reliably tracked by
the instrument. Such a restriction particularly degrades the performance
of the instrument at higher heart rates and higher pulse pressures,
besides severely limiting the maximum allowable length of flexible tubing
connecting cuff and instrument thereby restricting the freedom of movement
of the subject or patient. However, an even greater drawback of that
earlier version is that its operation relies on the availability of a
compressed air source. Whether that source be in metal bottle form or a
bulky compressor motor, that version of the instrument cannot be
considered to be self-contained.
Another instrument is known in accordance with the prior art employing the
same underlying principle, but using a hydraulic system (see Yamakoshi et
al., IEEE Transactions on Biomedical Engineering, vol. BME-27, no. 3,
March 1980, pp. 150-153). However, such a hydraulic system precludes the
use of a flexible finger cuff with all its inherent advantages, owing to
the required rapid displacement of a significant mass of water from the
prime mover to the cuff which would be required for this type of cuff in
order to follow the blood pressure waveform. The resonance frequency of
such a system is unacceptably low. Moreover, for this application, air is
a much safer medium to work with than water, since leakages in the
hydraulic system could be extremely hazardous for the patient.
DISCLOSURE OF INVENTION
The present invention as claimed is intended to remedy these drawbacks by
pneumatically generating the controlled pressure in the flexible finger
cuff by means of a dynamic compressor. Such a dynamic compressor comprises
an air-filled bellows whose degree of compression is controlled by a
linear motor. This version of electro-pneumatic transducer is almost an
order of magnitude faster than the valve and compressed air version.
Moreover, the resulting system dynamics are virtually independent of
finger size and arterial pressure level, unlike the earlier version, thus
considerably simplifying the electronic compensation required. The motor
is driven by a power amplifier which forms part of the electronics, the
instrument now being a self-contained unit requiring only connection to
the electrical mains supply.
BRIEF DESCRIPTION OF DRAWINGS
One way of carrying out the invention is described in detail below with
reference to drawings which illustrate only one specific embodiment, in
which:
FIG. 1 is a schematic block diagram of the instrument according to the
present invention. In addition to the functions described elsewhere in the
text, block (7) also incorporates the front panel controls "START",
"INITIALIZE", "MEASURE", and "GAIN", while block (16) contains the
controls "SYSTOLIC" and "DIASTOLIC" for the beat-to-beat digital display
of these values.
FIG. 2 shows how the instrument clamps the plethysmogram to a fixed
(reference) value by producing a cuff pressure, measured by the electronic
pressure transducer (15), equal to that in the finger arteries. The
reference level corresponds to the d.c. level of the plethysmogram (in
open-loop mode) for a cuff pressure which produces a maximum peak-to-peak
value in the plethysmogram waveform.
BEST MODE FOR CARRYING OUT THE INVENTION
The instrument uses the principle of the unloaded vascular wall. As FIG. 1
shows, the finger (1) is surrounded by an inflatable flexible cuff (2).
The cuff incorporates an infrared light-emitting diode (LED) (3) which is
activated by high frequency electrical impulses from a transmitter circuit
(4) and (when the cuff is wound around the finger), located diametrically
opposite, an infrared photoelectric sensor diode (5) which receives the
resulting impulses of light once they have passed through the finger. Note
that the cuff is normally wound around the middle phalanx of the middle
finger in such a way that the source and sensor lie against the finger
sides. After high-pass filtering (to remove any extraneous disturbances,
including mains supply hum and light flicker), the received signal is
processed by the demodulator circuit (6), which, after compensation and
power amplification (7), activates the linear motor (8). The motor (8),
via the plunger (22), controls the position of the plate (9) which is
fixed to one end of the bellows (10). The other end of the bellows is
fixed to a second plate (11) which, in turn, is mounted on the outer case
of the motor by means of four mounting posts (12). In order to utilize the
force of traction as well as the force of compression available from the
linear motor, three springs (13) are used to link the plate with a plate
(21) mounted at the point where the plunger (22) threads into the motor
drive spigot (23). In this way, the available uni-directional force of the
motor is almost doubled. It should be noted, however, that the power
amplifier and its associated power supply used to drive the linear motor
must be capable of supplying both positive and negative potential to the
motor. The bellows is then connected to the inflatable cuff with flexible
tubing (14), to which is also attached an electronic pressure transducer
(15), in such a way that the hollow spaces of the bellows, tubing,
electronic pressure transducer, and cuff, form one complete closed
air-filled space.
The operation of the instrument is then as follows:
The light-emitting diode (3) which is activated by high-frequency
electrical impulses from the electronic circuit (4) produces a stream of
infrared light impulses. The amplitude of these light impulses, detected
by the demodulator circuit (6) from the signal received from the
photoelectric sensor diode (5), will only be constant if the walls of the
finger arteries under the light beam do not move. It is this state,
specifically when this portion of the finger arteries are unstressed (both
radially and longitudinally), that one is striving for (see
"Initialization" description). The function of the electronics (6) and (7)
is to ensure that the electrical signal in the lead (17) which activates
the dynamic compressor (18) produces this situation (block (7) includes
the initialization circuitry and controls, power amplifier, and the
electronic compensation of the finger arteries and dynamic compressor/cuff
combination). The necessary consequence of this state prevailing is that
the instantaneous cuff pressure is very nearly equal to the pressure in
the finger arteries.
The dynamic compressor (18) comprising the the linear motor (8), plunger
(22), bellows end-plates (9) and (11), and the bellows (10) which is fixed
to the outer case of the motor by the end-plate (11) and the mounting
posts (12), generates a pressure in the hollow space (19) of cuff (2)
which is measured by the electronic pressure transducer (15). The signal
from this transducer is calibrated and the beat-to-beat diastolic and
systolic values detected and displayed by the electronic circuitry (16).
The springs linking plates (11) and (21) allow the force of traction
available from the motor (8) to effectively be transformed into an
additional force of compression, thereby almost doubling the available
force (and thus pressure) which would otherwise be available.
The dynamic compressor
The dynamic compressor comprises a bellows which is compressed by a linear
motor. Such motors are commercially available under the names "shakers",
"vibration exciters", and "vibrators". As their names suggest, they are
normally used in vibration testing of materials and structures.
The diameter of the bellows was chosen such that a stroke of 0.5 cm (the
standard stroke available from most of the smaller range of commercial
shakers) could produce a pressure of 300 Torr in the cuff loosely wound
around an adult middle finger. (It should be noted that winding the cuff
too tightly around the finger will produce incorrect instrument readings
due to "preloading" the finger.) Such a bellows has an outer diameter of
about 3.5 cm and requires a force of about 4 kg to produce this pressure.
The bellows is constructed in such a way that when it is fully compressed
its remaining air space (the dead space) is minimal, since the cuff
pressure attainable (in fractions of an atmosphere) is the ratio of the
compressor stroke volume to the sum of the cuff volume plus the dead space
of the bellows. This can be achieved either by inserting a solid block
into the bellows to occupy whatever dead space does exist, or by using a
very short bellows constructed of thin material. The latter technique was
used in the present version of the instrument.
In order to utilize the force of traction as well as the force of
compression available from such commercial shakers, three springs were
used to provide an additional force of compression across the bellows. It
should be noted from FIG. 1 that the mounting posts (12) and plunger (22)
were made long enough that the full motor stroke is a small fraction of
this length. In this way the force of the (preloaded) springs does not
appreciably change over the full motor stroke, a prerequisite for optimal
force transfer. The available uni-directional force of the shaker (and
thus attainable cuff pressure) is thereby almost doubled. Thus, a shaker
capable of delivering a vector force of at least 2 kg over a distance of
0.5 cm suffices as the prime mover for the system. It should be noted,
however, that the power amplifier and its power supply (contained in block
(7) of FIG. 1) used to drive the shaker must be capable of supplying both
positive and negative potential.
Electronic compensation
The electronic compensation, contained in block (7) of FIG. 1, comprises an
integrator (so that changes in mean blood pressure can be accurately
followed) combined with a band-limited first-order high-emphasis
d.c.-coupled amplifier to compensate for the visco-elastic wall properties
of the finger arteries, and a band-limited second-order high-emphasis
d.c.-coupled amplifier to compensate for the second-order resonance effect
of the compressor-cuff combination (resulting from the moving mass of the
motor, the stiffness of its suspension spring, and the compliance of the
air-filled bellows and cuff) the parameters of which are obtained from the
Bode plot of the compressor-cuff combination. The compressor-cuff
combination possesses much faster dynamics than a combination of cuff with
a valve and compressed air form of electro-pneumatic transducer, owing to
the necessary presence of a flow constriction in the latter which thereby
limits the speed at which the cuff can be inflated and thus the fastest
blood pressure component that can be reliably tracked. The present form of
electro-pneumatic transducer is almost an order of magnitude faster than
the earlier version. Moreover, the resulting system dynamics are virtually
independent of finger size and arterial pressure level, thus considerably
simplifying the electronic compensation required.
The corner frequency of the finger compensator was obtained from the
Lissajous figure resulting from the plethysmogram signal on the vertical
plates of an oscilloscope and the sinusoidally varied finger cuff pressure
on the horizontal plates. This was found to be between 5 and 10 Hz,
varying somewhat for different subjects, but within the range reported
from actual physiological measurements on the visco-elastic wall
properties of systemic arterial segments. In fact, the use of the
instrument in this mode of operation, viz., to estimate the visco-elastic
time constant of peripheral arteries, may prove to be of clinical value in
screening for such diseases as, e.g., arterioscleroses.
Band-limiting (of the high-frequency emphasis) is required in the
compensation networks so that the noise intrinsic to the photoelectric
sensor diode is not unduly amplified. This would limit the degree to which
the plethysmogram could be clamped (the error signal can be no smaller
than the noise). The higher the natural resonant frequency of the
mechanical pressure generating system, the less electronic high-frequency
emphasis required, which limits the noise amplification and thereby allows
greater clamping of the plethysmogram to be attained. The spring
suspension of the motor must be stiff enough that, when combined with the
compliance of the air-filled bellows and cuff, the resonant frequency of
the pressure generating system is high enough not to require too much
electronic high-frequency emphasis, but still allow a maximum cuff
pressure of 300 Torr to be attained. Since the moving mass of the motor
combined with only the compliance of the air-filled bellows and cuff
already gives an acceptably high resonant frequency, the suspension spring
need only be stiff enough to provide adequate mechanical support for the
moving mass.
As FIG. 2 shows, this form of compensation allows a high enough loop gain
to be attained that the plethysmogram can be clamped to a small fraction
of its open-loop pulsation, without the outbreak of large extraneous
oscillations which result if such compensation is not included. In fact,
the effective accuracy of the instrument in following the finger blood
pressure is even higher than that indicated by the (already) small
pulsations in the plethysmogram, owing to the fact that, in the unloaded
region of the finger arteries, even a small change in transmural pressure
results in a relatively large change in arterial cross-section. Thus, a
given reduction in plethysmogram pulsation in this region (by closing the
control loop) is indicative of an even smaller error between cuff and
arterial pressure. This can also be seen from the fact that above a
certain level of loop gain ("GAIN" on front panel) (the factor determining
the error and thus measured waveform in a feedback control system),
further increases in the loop gain do not alter the form of the measured
blood pressure signal but only serve to introduce extraneous noise into
the signal. The d.c. value to which the plethysmogram is clamped
corresponds to the value resulting from an open-loop cuff pressure which
produces a maximum peak-to-peak plethysmogram waveform, as explained in
the next section.
Adjustment of reference level ("initialization")
The reference level to which the plethysmogram is clamped is determined by
the following procedure ("initialization"), the circuitry and controls of
which are contained in block (7) of FIG. 1. The feedback loop is opened
(press button "INITIALIZE" on front panel), i.e., the dynamic compressor
is disconnected from the (compensated) plethysmogram and its driving
potential is varied manually by means of a potentiometer on the front
panel (also marked "INITIALIZE") until the peak-to-peak amplitude of the
plethysmogram (indicated by the front panel digital display) is maximum.
(With the abundance of inexpensive microprocessors currently commercially
available, this adjustment can be reduced to a push-button operation.) The
"reference adjust" circuit automatically adjusts to the d.c. value of the
plethysmogram and this value is stored and used as the reference value
when the loop is again closed (press button "MEASURE"). It can be shown
that the diameter of the finger arteries will now be maintained at a value
corresponding to their unloaded region. FIG. 2 shows that when the loop is
closed, any deviations in intravascular volume due to changes in
intravascular pressure are immediately compensated by an automatic
adjustment of cuff pressure which therefore instantaneously follows the
intravascular waveform. That the value to which the plethysmogram is
clamped corresponds to the unloaded region of an artery can be seen (in
the closed-loop mode) when the reference level is made to slowly decrease
linearly with time. A plateau is obtained in the cuff
pressure-plethysmogram relationship, characteristic of the unloaded region
of an artery. The range of reference level values corresponding to this
plateau is equal to the range of d.c. values of the plethysmogram which
occur when the cuff pressure (open-loop mode) is adjusted to give maximum
peak-to-peak plethysmogram waveforms.
Finger height correction
Unless the subject is lying down, the blood pressure in the finger differs
substantially from that measured by a conventional sphygmamanometer or a
catheter in the brachial artery, owing to the hydrostatic pressure
differential between heart and finger. This hydrostatic differential can
be corrected for in the instrument by fastening a small external
electronic pressure transducer to the patient at heart level (e.g., by
placing it in a shirt pocket over the heart) and connecting this
transducer to a thin water-filled flexible tube which runs to the measured
finger, taped at intervals along the arm. At the finger the tube is sealed
with a thin compliant membrane. The electrical signal from this transducer
is then electronically added to that of the internal (cuff pressure)
transducer and the resulting signal corresponds to finger blood pressure
corrected to heart level.
Leak correction
When the instrument is about to be used, the power is switched on and the
operator pushes the button marked "START" which opens an on-off air valve
(not shown in FIG. 1) in the normally closed pneumatic circuit as well as
causing the drive spigot of the linear motor to fully retract. The cuff is
then wound around the finger and the button marked "INITIALIZE" is pressed
which closes the valve and disconnects the dynamic compressor from the
(compensated) plethysmogram, leaving it connected only to a
potentiometer-controlled voltage. The operator then turns the
potentiometer knob (also marked "INITIALIZE"), until the digital display
shows a maximum. (As already mentioned, this initialize procedure can be
automated to a push-button operation with the aid of a small
microprocessor.) Finally, by pushing the button marked "MEASURE", the
control loop is closed, i.e., the dynamic compressor is again connected to
the (compensated) plethysmogram, the d.c. value of which has been made
zero by the reference adjust circuit, and the pressure transducer will
measure arterial blood pressure. Should there be a slow leak in the
pneumatic system the drive spigot of the motor will gradually move forward
in order to nontheless clamp the plethysmogram to zero. However, when the
drive spigot finally reaches its maximum outward displacement (as sensed
by a limit switch), the electronics produce a situation equivalent to that
of pressing "START", viz., the air valve is opened and the drive spigot
fully retracts for sufficient time to replenish the system with the air
that has leaked out, after which time the valve is again automatically
closed and the instrument is again automatically returned to the "MEASURE"
mode. In the normal course of events, this should only occur about once an
hour and require only a few seconds.
* * * * *
|
|
 |
|
 |
Description |
|
