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Claims  |
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I claim:
1. A method of determining the physical posture of a patient's body, having
a superior-inferior body axis, an anterior-posterior body axis and a
lateral-medial body axis, in relation to earth's gravitational field
comprising the steps of:
implanting a multi-axis, solid state sensor, comprising first and second DC
accelerometers having first and second sensitive axes, respectively, which
respond to earth's gravitational field to provide first and second
respective DC accelerometer signals of a magnitude and polarity dependent
on the degree of alignment therewith, in the patient's body so that said
first and second sensitive axes are generally aligned with a respective
first and second one of said superior-inferior, anterior-posterior or
lateral-medial body axes;
defining a first characteristic magnitude and polarity of said first and
second DC accelerometer signals on alignment of the sensitive axes of said
first and second DC accelerometers with earth's gravitational field, a
second characteristic magnitude and polarity of said first and second DC
accelerometer signals on alignment against earth's gravitational field,
and a third characteristic magnitude and polarity of said first and second
DC accelerometer signals on alignment normal to earth's gravitational
field;
deriving first and second DC accelerometer signals from said first and
second DC accelerometers as the patient assumes various body positions
moving said first or second sensitive axes generally into alignment with
earth's gravitational field; and
determining the body posture of the patient through comparison of the
magnitudes and polarities of said derived first and second DC
accelerometer signals with the magnitudes and polarities of said first,
second and third characteristic magnitudes and polarities.
2. The method of claim 1 further comprising the steps of:
defining a characteristic activity magnitude of said first and second DC
accelerometer signals effected by body movement occurring within a
predetermined frequency range signifying a threshold patient activity
level; and
deriving an activity level signal from said first or second DC
accelerometer signals exceeding said characteristic activity magnitude
over a predetermined time period.
3. The method of claim 2 further comprising the step of:
delivering a treatment therapy to the patient having a treatment parameter
dependent on the determined body posture and the activity level signal of
the patient.
4. The method of claim 2 further comprising the step of:
storing said determined body posture and activity level of the patient.
5. The method of claim 1 wherein said implanting step further comprises:
implanting said multi-axis, solid state sensor, comprising said first and
second DC accelerometers having first and second sensitive axes,
respectively, which respond to earth's gravitational field to provide
first and second respective DC accelerometer signals of a magnitude and
polarity dependent on the degree of alignment therewith, in the patient's
body so that said first and second sensitive axes are generally aligned
with said superior-inferior and one of said anterior-posterior and
lateral-medial body axes, respectively.
6. The method of claim 1 wherein said implanting step further comprises:
implanting said multi-axis, solid state sensor, comprising said first and
second DC accelerometers having first and second sensitive axes,
respectively, which respond to earth's gravitational field to provide
first and second respective DC accelerometer signals of a magnitude and
polarity dependent on the degree of alignment therewith, in the patient's
body so that said first and second sensitive axes are generally aligned
with said anterior-posterior and lateral-medial body axes, respectively.
7. A method of determining the physical posture of a patient's body, having
a superior-inferior body axis, an anterior-posterior body axis and a
lateral-medial body axis, in relation to earth's gravitational field
comprising the steps of:
implanting a multi-axis, solid state sensor, comprising first, second, and
third DC accelerometers having first, second, and third sensitive axes,
respectively, which respond to earth's gravitational field to provide
first, second, and third respective DC accelerometer signals of a
magnitude and polarity dependent on the degree of alignment therewith, in
the patient's body so that said first, second and third sensitive axes are
generally aligned with said superior-inferior, anterior-posterior and
lateral-medial body axes, respectively;
defining a first characteristic magnitude and polarity of said first,
second and third DC accelerometer signals on alignment of the sensitive
axes of said first, second and third DC accelerometers with earth's
gravitational field, a second characteristic magnitude and polarity of
said first, second, and third DC accelerometer signals on alignment
against earth's gravitational field, and a third characteristic magnitude
and polarity of said first, second, and third DC accelerometer signals on
alignment normal to earth's gravitational field;
deriving first, second, and third DC accelerometer signals from said first,
second, and third DC accelerometers, respectively, as the patient assumes
various body positions moving said first or second or third sensitive axes
generally into alignment with earth's gravitational field; and
determining the body posture of the patient through comparison of the
magnitudes and polarities of said derived first, second, and third DC
accelerometer signals with the magnitudes and polarities of said first,
second and third characteristic magnitudes and polarities.
8. The method of claim 7 further comprising the steps of:
defining a characteristic activity magnitude of said first, second, and
third DC accelerometer signals effected by body movement occurring within
a predetermined frequency range signifying a threshold patient activity
level; and
deriving an activity level signal from said first or second or third DC
accelerometer signals exceeding said characteristic activity magnitude
over a predetermined time period.
9. The method of claim 8 further comprising the step of:
delivering a treatment therapy to the patient having a treatment parameter
dependent on the determined body posture and the activity level signal of
the patient.
10. The method of claim 8 further comprising the step of:
storing said determined body posture and activity level of the patient.
11. A method of pacing a patient's heart at a pacing rate dependent on
patient activity and the physical posture of a patient's body, having a
superior-inferior body axis, an anterior-posterior body axis and a
lateral-medial body axis, in relation to earth's gravitational field,
comprising the steps of:
measuring the constant acceleration of gravity on the patient's body in at
least two of the superior-inferior, anterior-posterior, and lateral-medial
body axes with first and second solid state DC accelerometer means aligned
thereto for providing first and second DC accelerometer signals therefrom
having a characteristic magnitude and polarity on alignment with earth's
gravitational field and varying magnitude and polarity depending on the
degree of mis-alignment of said first and second solid state DC
accelerometer means with earth's gravitational field;
determining a body position signal related to the posture of the patient
through comparison of the magnitudes and polarities of the first and
second DC accelerometer signals with said characteristic magnitudes and
polarities;
determining a patient activity signal from the frequency of body movements
recurring over a time unit;
deriving a rate control signal from the body position and patient activity
signals correlated to the physiologic demand on the patient's heart in the
determined body posture and level of activity;
defining physiologic escape intervals as a function of the rate control
signal to establish a physiologic pacing rate;
generating pacing pulses at the physiologic pacing rate; and
applying the pacing pulses to a chamber of a patient's heart.
12. The method of claim 11 wherein said measuring step further comprises:
generally aligning the first and second sensitive axes of said first and
second solid state DC accelerometers, respectively, with said
superior-inferior body axis and one of said anterior-posterior and said
lateral-medial body axes, respectively, and deriving said first and second
DC accelerometer signals therefrom.
13. The method of claim 11 wherein said measuring step further comprises:
generally aligning the first and second sensitive axes of said first and
second solid state DC accelerometers, respectively, with said
lateral-medial and anterior-posterior body axes, respectively, and
deriving said first and second DC accelerometer signals therefrom.
14. The method of claim 11 wherein said measuring step further comprises
the steps of:
mounting a first solid state DC accelerometer
having an first sensitive axis of deflection providing a first output
signal varying in magnitude in response to the DC force of earth's
gravitational field and AC forces of acceleration applied along the first
sensitive axis in a pacemaker pulse generator housing so that the pulse
generator may be implanted with the first sensitive axis generally aligned
to a patient's superior-inferior body direction while in an upright
posture;
mounting a second solid state DC accelerometer having an second sensitive
axis of deflection providing a second output signal varying in magnitude
in response to the DC force of earth's gravitational field and AC forces
of acceleration applied along the second sensitive axis in said pacemaker
pulse generator at a generally orthogonal angle to said first sensitive
axis;
implanting said pulse generator in a patient's body such that said first
sensitive axis is generally aligned with the patient's superior-inferior
axis and said second axis is generally aligned with the patient's
anterior-posterior or lateral-medial body direction while in the upright
position; and
deriving the first and second DC accelerometer signals from DC components
of the first and second output signals; and
said step of determining a body position signal related to the posture of
the patient further comprises the steps of:
providing a reference DC accelerometer signal magnitude representative of
the DC component of the first and second output signals generated by
alignment of said first and second sensitive axes of said first and second
DC accelerometers with the force of earth's gravitational field;
comparing the magnitudes of the first and second DC accelerometer signals
to the reference DC accelerometer signal magnitude and determining from
the comparison the position of the patient's body with respect to the
force of earth's gravitational field; and
providing the body position signal representative of the determined
physical posture of the patient.
15. The method of claim 11 wherein said measuring step further comprises
the steps of:
mounting a first DC accelerometer having an first sensitive axis of
deflection providing a first output signal varying in magnitude in
response to the DC force of earth's gravitational field and AC forces of
acceleration applied along the first sensitive axis in a pacemaker pulse
generator so that the pulse generator may be implanted with the first
sensitive axis generally aligned to a patient's superior-inferior body
axis while in an upright posture;
mounting a second DC accelerometer having an second sensitive axis of
deflection providing a second output signal varying in magnitude in
response to the DC force of earth's gravitational field and AC forces of
acceleration applied along the second sensitive axis in said pacemaker
pulse generator at a generally orthogonal angle to said first sensitive
axis so that the pulse generator may be implanted with the second
sensitive axis generally aligned to the patient's anterior-posterior body
axis while in said upright posture;
mounting a third DC accelerometer having a third sensitive axis of
deflection providing a third output signal varying in magnitude in
response to the DC force of earth's gravitational field and AC forces of
acceleration applied along the third sensitive axis in said pacemaker
pulse generator at a generally orthogonal angle to said first and second
sensitive axes so that the pulse generator may be implanted with the third
sensitive axis generally aligned to the patient's lateral-medial body axis
while in said upright posture;
implanting said pulse generator in a patient's body such that said first
sensitive axis is generally aligned with the patient's superior-inferior
axis and said second and third axes are generally aligned with the
patient's anterior-posterior and lateral-medial body axes, respectively,
while in said upright posture; and
deriving the first, second and third DC accelerometer signals from DC
components of the first, second, and third output signals; and
said step of determining a body position signal related to the posture of
the patient further comprises the steps of:
providing a reference DC accelerometer signal magnitude representative of
the DC component of the first, second, and third output signal magnitudes
generated by alignment of said first, second and third sensitive axes of
said first, second and third DC accelerometers, respectively, with the
force of earth's gravitational field;
comparing the magnitudes of the first, second and third DC accelerometer
signals to the reference DC accelerometer signal magnitude and determining
from the comparison the position of the patient's body with respect to the
force of earth's gravitational field; and
providing the body position signal representative of the determined posture
of the patient.
16. Apparatus for determining the physical posture of a patient's body
having patient body axes including a superior-inferior axis, an
anterior-posterior axis and a lateral-medial axis by reference of the
patient body axes to earth's gravitational field in an assumed body
position comprising:
an implantable housing having first, second and third positional axes
adapted to be implanted in a patient's body in a generally predetermined
alignment relationship of said first, second and third positional axes
with said superior-inferior, anterior-posterior, and lateral-medial body
axes, respectively;
a first DC accelerometer mounted within said implantable housing having a
first sensitive axis aligned with one of said first, second and third
positional axes of said implantable housing for providing a first DC
accelerometer signal varying in magnitude and polarity as a function of
the degree of alignment of earth's gravitational field with or against
said first sensitive axis in the body position assumed by the patient;
a second DC accelerometer mounted within said implantable housing having a
second sensitive axis aligned with one other of said first, second and
third positional axes of said implantable housing for providing a second
DC accelerometer signal varying in magnitude and polarity as a function of
the degree of alignment of earth's gravitational field with or against
said second sensitive axis in the body position assumed by the patient;
and
means for determining the physical posture of the patient through a
comparison of the magnitudes and polarities of said first and second DC
accelerometer signals.
17. The apparatus of claim 16 further comprising
means for determining the activity level of the patient from the frequency
of body movements recurring over a time unit; and
means for storing the determined physical posture and the activity level of
the patient.
18. The apparatus of claim 16 further comprising:
means for delivering a treatment to the patient having a treatment
parameter dependent on the body posture and the activity level of the
patient.
19. The apparatus of claim 16 wherein said DC accelerometer means further
comprises:
a first solid state DC accelerometer for measuring the constant
acceleration of earth's gravitational field on the patient's body in the
superior-inferior body axis and deriving said first DC accelerometer
signal therefrom as the patient assumes various body positions moving said
first or second or third sensitive axes generally into alignment with
earth's gravitational field; and
a second solid state DC accelerometer for measuring the constant
acceleration of earth's gravitational field on the patient's body in one
of the anterior-posterior and the lateral-medial body axes and deriving
said second DC accelerometer signal therefrom as the patient assumes
various body positions moving said first or second or third sensitive axes
generally into alignment with earth's gravitational field.
20. The apparatus of claim 19 wherein said DC accelerometer means further
comprises:
a third solid state DC accelerometer for measuring the constant
acceleration of earth's gravitational field on the patient's body in the
lateral-medial body axis and deriving a third DC accelerometer signal
therefrom as the patient assumes various body positions moving said first
or second or third sensitive axes generally into alignment with earth's
gravitational field; and wherein:
said means for determining the posture of the patient is responsive to a
comparison of a parameter of the first, second and third DC accelerometer
signals.
21. The apparatus of claim 20 further comprising:
means for defining a first characteristic magnitude and polarity of said
first, second and third DC accelerometer signals on alignment of the
sensitive axes of said first, second and third DC accelerometers with
earth's gravitational field, a second characteristic magnitude and
polarity of said first, second, and third DC accelerometer signals on
alignment against earth's gravitational field, and a third characteristic
magnitude and polarity of said first, second, and third DC accelerometer
signals on alignment normal to earth's gravitational field; and wherein:
said means for determining the posture of the patient is responsive to a
comparison of the magnitudes and polarities of said derived first, second,
and third DC accelerometer signals with the magnitudes and polarities of
said first, second and third characteristic magnitudes and polarities.
22. The apparatus of claim 21 further comprising:
means for defining a characteristic activity magnitude of said first,
second, and third DC accelerometer signals effected by body movement
occurring within a predetermined frequency range signifying a threshold
patient activity level; and
means for deriving an activity level signal from said first or second or
third DC accelerometer signals exceeding said characteristic activity
magnitude over a predetermined time period.
23. The apparatus of claim 22 further comprising:
means for delivering a treatment therapy to the patient having a treatment
parameter dependent on the determined body posture and the activity level
signal of the patient.
24. The apparatus of claim 22 further comprising:
means for storing said determined body posture and activity level of the
patient.
25. Apparatus for pacing a patient's heart at a pacing rate dependent on
patient activity and the physical posture of a patient's body, having a
superior-inferior body axis, an anterior-posterior body axis and a
lateral-medial body axis, in relation to earth's gravitational field,
comprising:
first and second solid state DC accelerometer means for measuring the
constant acceleration of gravity on the patient's body in at least two of
the superior-inferior, anterior-posterior, and lateral-medial body axes
for providing first and second DC accelerometer signals therefrom having a
characteristic magnitude and polarity on alignment with earth's
gravitational field and varying magnitude and polarity depending on the
degree of misalignment of said first and second solid state DC
accelerometer means with earth's gravitational field;
means for determining a body position signal related to the posture of the
patient through comparison of the magnitudes and polarities of the first
and second DC accelerometer signals with said characteristic magnitudes
and polarities;
means for determining a patient activity signal from the frequency of body
movements recurring over a time unit;
means for deriving a rate control signal from the body position and patient
activity signals correlated to the physiologic demand on the patient's
heart in the determined body posture and level of activity;
means for defining physiologic escape intervals as a function of the rate
control signal to establish a physiologic pacing rate;
means for generating pacing pulses at the physiologic pacing rate; and
means for applying the pacing pulses to a chamber of a patient's heart.
26. The apparatus of claim 25 further comprising:
means for generally aligning the first and second sensitive axes of said
first and second solid state DC accelerometers, respectively, with said
superior-inferior body axis and one of said anterior-posterior and said
lateral-medial body axes, respectively, and deriving said first and second
DC accelerometer signals therefrom.
27. The apparatus of claim 25 further comprising:
means for generally aligning the first and second sensitive axes of said
first and second solid state DC accelerometers, respectively, with said
lateral-medial and anterior-posterior body axes, respectively, and
deriving said first and second DC accelerometer signals therefrom.
28. The apparatus of claim 25 further comprising:
a pacemaker pulse generator housing;
means for mounting a first solid state DC accelerometer having a first
sensitive axis of deflection providing a first output signal varying in
magnitude in response to the DC force of earth's gravitational field and
AC forces of acceleration applied along the first sensitive axis in said
pacemaker pulse generator housing so that the pulse generator may be
implanted with the first sensitive axis generally aligned to one of said
patient's body axes while in an upright posture;
means for mounting a second solid state DC accelerometer having an second
sensitive axis of deflection providing a second output signal varying in
magnitude in response to the DC force of earth's gravitational field and
AC forces of acceleration applied along the second sensitive axis in said
pacemaker pulse generator at a generally orthogonal angle to said first
sensitive axis;
said pacemaker pulse generator housing adapted to be implanted in the
patients body such that said first sensitive axis is generally aligned
with the selected one of the patient's body axes and said second axis is
generally aligned with one of the other of the patient's body axes; and
wherein:
said means for determining a body position signal related to the posture of
the patient further comprises:
means for providing a reference DC accelerometer signal magnitude
representative of the DC component of the first and second output signals
generated by alignment of said first and second sensitive axes of said
first and second DC accelerometers with the force of earth's gravitational
field;
means for comparing the magnitudes of the first and second DC accelerometer
signals to the reference DC accelerometer signal magnitude and determining
from the comparison the position of the patient's body with respect to the
force of earth's gravitational field; and
means for providing the body position signal representative of the
determined physical posture of the patient.
29. The apparatus of claim 25 further comprising:
a pacemaker pulse generator housing;
means for mounting a first DC accelerometer having an first sensitive axis
of deflection providing a first output signal varying in magnitude in
response to the DC force of earth's gravitational field and AC forces of
acceleration applied along the first sensitive axis in a pacemaker pulse
generator so that the pulse generator may be implanted with the first
sensitive axis generally aligned to a patient's superior-inferior body
axis while in an upright posture;
means for mounting a second DC accelerometer having an second sensitive
axis of deflection providing a second output signal varying in magnitude
in response to the DC force of earth's gravitational field and AC forces
of acceleration applied along the second sensitive axis in said pacemaker
pulse generator at a generally orthogonal angle to said first sensitive
axis so that the pulse generator may be implanted with the second
sensitive axis generally aligned to the patient's anterior-posterior body
axis while in said upright posture;
means for mounting a third DC accelerometer having a third sensitive axis
of deflection providing a third output signal varying in magnitude in
response to the DC force of earth's gravitational field and AC forces of
acceleration applied along the third sensitive axis in said pacemaker
pulse generator at a generally orthogonal angle to said first and second
sensitive axes so that the pulse generator may be implanted with the third
sensitive axis generally aligned to the patient's lateral-medial body axis
while in said upright posture;
said pacemaker pulse generator housing adapted to be implanted in a
patient's body such that said first sensitive axis is generally aligned
with the patient's superior-inferior axis and said second and third axes
are generally aligned with the patient's anterior-posterior and
lateral-medial body axes, respectively, while in said upright posture; and
wherein:
said means for determining a body position signal related to the posture of
the patient further comprises:
means for providing a reference DC accelerometer signal magnitude
representative of the DC component of the first, second, and third output
signal magnitudes generated by alignment of said first, second and third
sensitive axes of said first, second and third DC accelerometers,
respectively, with the force of earth's gravitational field;
means for comparing the magnitudes of the first, second and third DC
accelerometer signals to the reference DC accelerometer signal magnitude
and determining from the comparison the position of the patient's body
with respect to the force of earth's gravitational field; and
means for providing the body position signal representative of the
determined posture of the patient. |
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Claims |
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Description |
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REFERENCE TO RELATED APPLICATION
Reference is made to commonly assigned co-pending U.S. patent application
Doceket No. P-3270 entitled RATE RESPONSIVE CARDIAC PACEMAKER FOR
DISCRIMINATING STAIR CLIMBING FROM OTHER ACTIVITIES filed on even date
herewith.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of an array of DC accelerometers
for detection of patient posture and activity level for medical monitoring
and/or the delivery of therapies, including cardiac pacing.
2. Description of the Prior Art
In the field of medical device technology, patient monitoring of
physiologic parameters e.g. heart rate, temperature, blood pressure and
gases and the like are well known. In addition, the delivery of various
therapies including drugs and electrical stimulation by implanted or
invasive medical devices is well known. Factors that may be appropriately
taken into account during monitoring or delivery of therapies include
patient position or posture and activity level. Both may have an effect on
the other parameters monitored and in the decision process for setting an
appropriate therapy. Particularly in the field of cardiac pacing, patient
activity level can be correlated to the need for cardiac output.
Rate responsive pacing has been widely adopted for adjusting pacing rate to
the physiologic needs of the patient in relatively recent years. Early
single chamber patient in relatively recent years. Early single chamber
cardiac pacemakers provided a fixed rate stimulation pulse generator that
could be reset, on demand, by sensed atrial or ventricular contractions
recurring at a rate above the fixed rate. Later, dual chamber demand
pacemakers became available for implantation in patients having an intact
atrial sinus rate but no AV conduction, so that ventricular pacing could
be synchronized with the atrial sinus rate, and backup fixed rate
ventricular pacing could be provided on failure to sense atrial
depolarizations. In addition, rate programmable pacemakers became
available wherein the base pacing rate could be selected by a physician to
provide a compromise fixed rate that did not interfere with patient rest
and provided adequate cardiac output at moderate levels of exercise.
Such fixed rate pacing, particularly for patients not having an adequate
atrial sinus rate to allow synchronous pacing, left most patients without
the ability to exercise, lift objects or even walk up stairs without
suffering loss of breath due to insufficient cardiac output. However, the
introduction of the Medtronic.RTM. Activitrax.RTM. pacemaker provided
patients with the a pulse generator having a rate responsive capability
dependent on the level of patient activity. A piezoelectric crystal bonded
to the interior of the implantable pulse generator can or case is employed
in that pacemaker and successor models to provide a pulse output signal
related to the pressure wave generated by a patient's footfall and
conducted through the body to the crystal. Thus, low frequency activity
signals recurring at the patient's rate of walking or running could be
sensed and processed to derive a pacing rate appropriate to the level of
activity. The activity sensor and its operation is described in commonly
assigned U.S. Pat. No. 4,428,378 to Anderson.
Since the introduction of the Activitrax.RTM. pacemaker, a great many rate
responsive pacemakers employing a wide variety of activity sensors and
other physiologic sensors have been proposed and marketed. A comprehensive
listing of such rate responsive pacemakers, sensors and sensed physiologic
parameters is set forth in commonly assigned U.S. Pat. No. 5,226,413 to
Bennett et al., incorporated herein by reference. However, the activity
sensor of the type employed in the Activitrax.RTM. pacemaker continues to
be used in successor single and dual chamber, rate responsive pacemaker
models and remains the most widely used physiologic sensor.
As mentioned above, this piezoelectric crystal sensor is responsive to
pressure waves generated by patient footfalls striking the exterior of the
pulse generator case. Activity sensor configurations employing integrated
circuit, AC accelerometers on an IC chip inside the pacemaker are also
being employed in the EXCEL"VR pacemaker sold by Cardiac Pacemakers, Inc.,
and in similar rate responsive pacemakers sold by other manufacturers. The
AC accelerometer is formed of a silicon beam mass suspended on the IC that
swings or moves in response to shock waves caused by body motion and
provides an output signal having a magnitude dependent on the rate of
movement.
Like the piezoelectric crystal sensor, there is no signal output from the
AC accelerometer in the absence of body motion and related to body
position or attitude. In other words, when a patient is at rest, neither
activity sensor provides any indication as to whether the patient is
upright and awake and resting or lying down and presumably sleeping or
resting. A lower sleep pacing rate than the rest pacing rate while awake
and upright may be desirable for a given patient. Other sensors for
sensing physiologic parameters induced by high levels of exercise have
been proposed to detect the physiologic changes accompanying exercise,
rest and sleep to trigger appropriate rates. Particularly, to lower the
pacing rate during sleep, the inclusion of a real time clock to establish
a Circadian rhythm pacing rate have also been proposed. None of these
proposed sensors or systems are capable of determining a patient's
position or posture.
A mechanical sensor has been proposed in the article "A New Mechanical
Sensor for Detecting Body Activity and Posture, Suitable for Rate
Responsive Pacing" by Alt et al. (PACE, Vol. 11, pp. 1875-81, November,
1988, Part II) and in Alt U.S. Pat. No. 4,846,195 that involves use of a
multi-contact, tilt switch. This switch employs a mercury ball within a
container that is proposed to be fixed in the pulse generator case, so
that if the pulse generator is implanted at a certain orientation, and
stays in that orientation, certain contacts are closed by the mercury ball
when the patient is upright and others are closed or none are closed when
the patient is prostrate, i.e., either prone or supine. During movement of
the body, the mercury ball is expected to jiggle randomly and the number
of contacts made per unit of time may be used as a measure of the level of
activity. Similar sensors have been proposed in U.S. Pat. Nos. 4,869,251,
5,010,893, 5,031,618 and 5,233,984.
In the commonly assigned '984 patent, a cubic shaped multi-axis position
and activity sensor is employed in rate responsive paci | | |
