Fetal pulse oximetry sensor

Details Fetal pulse oximetry sensor Fetal pulse oximetry sensor

1999-07-07

2001-09-04

Winakur, Eric F. (Department: 3736)

Surgery

Diagnostic testing

Measuring or detecting nonradioactive constituent of body...

C600S310000, C600S323000

Reexamination Certificate

active

06285896

ABSTRACT:

BACKGROUND OF THE INVENTION
Physicians have long relied on intrapartum fetal surveillance for an early warning of complications arising during labor. The ultimate goal of fetal monitoring is to prevent damage to the most vital and sensitive organs, such as the brain and the heart, by detecting a decreased oxygen supply to these organs before the onset of cell damage. Some causes of fetal hypoxia are umbilical cord compression, placental insufficiency or hypertonia of the uterus. Early examples of fetal monitoring are intermittent auscultation of fetal heartbeat, electronic monitoring of fetal ECG and heart rate, and scalp blood pH. These techniques are based on the assumption that fetal hypoxia, leads to fetal acidemia and also to specific pathologic fetal ECG and heart rate patterns. These indirect techniques, however, are unsatisfactory because it is only after hypoxia has occurred for some time that it is reflected in adverse changes in the heart rate or blood pH.
More recently, fetal assessment has evolved to the direct measurement of fetal oxygen status using pulse oximetry. Pulse oximetry instrumentation, which provides a real-time measurement of arterial oxygen saturation, has become the standard of care for patient vital sign monitoring during anesthesia and in neonatal and adult critical care. A pulse oximetry system consists of a sensor attached to a patient, a monitor, and a cable connecting the sensor and monitor. The sensor typically has red and infrared light emitting diodes that illuminate a tissue site and a photodiode detector that measures the intensity of that light after absorption by the pulsatile vascular bed at the tissue site. From these measurements, the oxygen saturation of arterial blood can be calculated.
SUMMARY OF THE INVENTION
Pulse oximetry as applied to fetal intrapartum monitoring must overcome several significant and interrelated obstacles not faced by pulse oximetry as applied to adults, children, infants and neonates. These obstacles include attaching the sensor to a readily accessible tissue site, obtaining a representative measurement of central arterial oxygen saturation at that site, and calibrating the sensor. Pulse oximetry sensors are conventionally attached, for example, to an adult finger or a neonate foot using a self-adhesive mechanism that wraps around the tissue site. Sensor attachment to a fetus in this manner is impractical if not impossible. The uterine environment is fluid filled and the skin of the fetus is coated with vernix, an oily substance. Further, the presenting portion of the fetus is typically the crown of the head, which yields only the fetal scalp as a readily accessible tissue site. A number of mechanisms have been developed to overcome these impediments to attachment of a pulse oximetry sensor to the fetus. These include suction cups, clamps and vacuum devices for scalp attachment. There are also devices that slide beyond the fetus presenting portion, wedging between the uterine wall and the fetus.
FIG. 1
illustrates a scalp attachment mechanism used in conjunction with a fetal ECG sensor but also applicable to fetal pulse oximetry. The sensor assembly
100
consists of a fetal sensor
110
, a drive tube
120
, a guide tube
130
, and interconnecting conductors
140
. The fetal sensor
110
has a spiral probe
112
attached to a sensor base
114
. The probe
112
is utilized to attach the sensor
110
to the fetal scalp and also functions as an ECG electrode. The sensor base
114
is removably connected to the drive tube
120
by a fin
116
that fits within slots
122
of the drive tube
120
. The sensor
110
and connected drive tube
120
are movably contained within the guide tube
130
. The interconnecting wires
140
are attached at one end to the sensor base
114
, and one of the conductors
140
is electrically connected to the probe
112
. The other end of the conductors
140
are threaded through the inside of the drive tube
120
and the guide tube
130
, extending from the end of the drive tube
120
opposite the sensor
110
.
When using the sensor assembly
100
to attach the sensor
110
to a fetus, a physician first inserts the guide tube
130
into the mother's birth canal toward the cervix until the guide tube forward end
132
makes contact with the fetal head. Holding the forward end
132
stationary, the physician then inserts the drive tube
120
and attached sensor
110
into the guide tube
130
, pushing the rear end of the drive tube
120
forward until the spiral probe
112
makes contact with the fetal scalp. The physician then rotates the drive tube
120
causing the spiral probe
112
to embed into the fetal epidermis. Thereafter, the physician removes the guide tube
130
and drive tube
120
from the mother, sliding these tubes off the conductors
140
, leaving the sensor attached to the fetus with the wires
140
extending from the mother. The conductors
140
are then attached to a heart rate monitor.
Mere attachment of a pulse oximetry sensor to the fetal scalp, however, does not insure that the sensor can measure a representative value of central arterial oxygen saturation from that site. There are many potential tissue sites for scalp attachment of a fetal sensor, but conventional fetal sensors are prone to inconsistent, site-dependent saturation readings. Further, conventional fetal sensors are prone to measurements of oxygen saturation that are dependent on localized oxygen consumption and, therefore, may not be representative of central arterial oxygen supply. These problems are the result of the nonuniform vascularization of the scalp, as explained with respect to
FIG. 2A
, below. Further, based upon the various presentations of the fetal head, uterine, cervical and vaginal pressures to the head may be unequally applied, resulting in portions of the scalp having compromised perfusion.
FIG. 2A
depicts the large arteries of the scalp, which are located in the deeper tissue layers. Unlike an adult fingertip or a neonatal foot, the fetal scalp does not provide a specific tissue site with a readily located large artery from which to take pulse oximetry measurements. As shown in
FIG. 2A
, the scalp contains a web of large arteries separated by significant areas perfused only by branching small arteries, arterioles and capillaries. Because arterial vascularization of the scalp is not uniform, different scalp sites yield measurements taken from various sized arteries and under conditions of differing blood volumes with respect to tissue volume (blood fraction). This, in turn, affects the measured saturation, as described below with respect to FIG.
2
B.
FIG. 2B
is adapted from
Microvascular and Tissue Oxygen Distribution,
M. Intaglietta, P. Johnson, and R. Winslow, Cardiovascular Research, Elsevier Science 1996, which depicts the distribution of oxygen in the arterioles starting from the larger A1 vessels to the smallest A4 precapillary vessels and capillaries.
FIG. 2B
is composed of interconnected graphs
210
,
260
. The graph
210
has an x-axis
212
that corresponds to pO
2
and a y-axis
214
that corresponds blood vessel type. The graph
260
has an x-axis
262
that also corresponds to pO
2
and is aligned with the x-axis
212
of graph
210
. The y-axis
264
of graph
260
corresponds to oxygen saturation. The length of the bars
218
of graph
210
indicate the pO
2
of the blood according to vessel size. The oxygen dissociation curve
268
in the graph
260
illustrates the oxygen binding characteristics of blood hemoglobin.
FIG. 2B
shows that the oxygen saturation of blood in the microcirculation is dependent on vessel size, indicating the role of the various vessels with respect to tissue oxygenation. In particular, blood flowing through the smaller arterioles and capillaries has been partially desaturated by vessel and localized tissue oxygen consumption. Whereas larger arterioles contain more highly saturated blood reflective of the central oxygen supply.
FIGS. 2A and 2B
demonstrate that a fetal pulse oximetry sensor that measures a relatively sm

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