Non-invasive Fetal Heart Signature Monitor
Regardless of the monitoring technique, critical difficulties frequently arise when there is an emergent need to transfer the monitored patient from the labor area to the operating room. The monitors are usually detached during this critical interval with the mother and her fetus unmonitored during the transfer. Reattachment to monitors in the operating room (if at all) requires additional, possibly precious time and attention. Doppler transducers, if used, are inevitably in the operative field for an emergency cesarean section. Likewise, scalp electrodes must be removed or cut and withdrawn with the baby through the abdominal incision, again increasing the risk of infection.
The present invention provides a method of monitoring a fetal biopotential waveform. More particularly, the present invention provides a method for generating a fetal biopotential waveform and using the waveform components to monitor many variables including, but not limited to, the fetal heart rate, the fetal heart rate variability, and/or the fetal heart vector orientation of a fetus in a pregnant mother. The method includes the steps of measuring at least one biopotential waveform indicative of the mother's heart beat to form a maternal waveform, measuring at least one biopotential waveform indicative of the combined maternal and fetal heart beats to form a combined biopotential waveform, and using signal processing to cancel the maternal waveform from the combined waveform to derive a fetal waveform indicative of the fetus's biopotential electrocardiographic waveform.
The present invention also provides an apparatus for monitoring a fetal biopotential waveform. The present invention also provides an apparatus for generating a fetal biopotential waveform and using the waveform to monitor the fetal heart rate, the fetal heart rate variability, and/or the fetal heart vector orientation of a fetus in a pregnant mother. The apparatus includes at least one sensor, e.g., an electrode, for measuring at least one biopotential waveform indicative of a maternal heart beat, at least one sensor for measuring at least one biopotential waveform indicative of the combined maternal and fetal heart beats taken from a pregnant mother, and signal processing hardware, software, or hybrid mixes that can cancel the maternal waveform from the combined waveform to form a waveform indicative of the EKG.sub.f.
The present invention non-invasively and passively measures fetal and maternal electrocardiographic and maternal electromyographic waveforms by using traditional surface electrode electrocardiographic and electromyographic techniques combined with adaptive signal processing methods to solve the problems associated with the devices/techniques described above. The invention provides patient information (e.g., fetal heart rate/variability, taking into account noise artifacts attributable to uterine contractions) that at least duplicates current clinical standards.
In particular, the invention uses, for example, suitable skin contact electrodes connected to amplifiers to acquire biopotential waveforms and form signals, preferably differential signals, indicative of the mother's heart beat from sensors, e.g., electrodes, placed on her chest, and indicative of the combined maternal and fetal heart beats from sensors placed on the mother's abdomen, lower back, or both, as well as electromyographic signatures indicative of noise artifacts attributable to changes in uterine tone. Maternal heart rate, heart rate variability, and respiration rate are derived from the chest signals; standard maternal EKG is derived from planar leads. Instead of differential signals, more vectors may be formed by collecting single-ended signals and creating "differential pairs" therefrom.
The sensors placed on the mother's abdomen, lower back, or both, are preferably placed to form pairs of sensors wherein each sensor of the pair is spaced from the other and each pair is positioned in a substantially criss-crossed pattern with respect to other sensor pairs. Substantial spacing between the sensors of each sensor pair and between pairs of sensors is preferred so as to achieve a three-dimensional processing of the fetal biopotential waveform. As mentioned above, the sensors are preferably positioned to avoid blocking any surgical fields, for example, the abdominal area. By sensing the combined fetal and maternal waveforms with a multiplicity of sensors, the uniqueness of the vectors can be used to establish the vector orientation of the fetus. Preferably, the number of vectors used is sufficient to achieve a clear signal indicative of the combined fetal and maternal waveforms. If a clear enough combined signal is obtained from a single sensor, the present invention can operate using a single sensor to obtain the combined waveform.
The signals from the abdominal electrodes are divided into a plurality of channels. After data validation, an adaptive signal processing filter (ASPF) algorithm or other suitable algorithm is used to cancel the estimated maternal waveform from each channel in the abdominal electrodes, using chest signals as references. The system then selects from at least one of the resulting waveforms to serve as the reference fetal waveform, for example, the waveform with the highest peak-to-peak amplitude. Using another ASPF or other suitable algorithm, the reference waveform is then processed against the other abdominal waveforms with the maternal waveforms canceled to form an enhanced fetal signal that is a representation of the EKG.sub.f. The EKG.sub.f can subsequently be used to measure fetal heart rate and other biophysical profile parameters. Surface electromyogram (EMG) signals allow for concurrent monitoring of uterine contractions and afford improved cancellation of motion artifacts including noise attributable to skeletal muscles and uterine contractions.
The present invention provides a device that is totally non-invasive, passive and will supplant the fetal scalp electrode and, therefore, eliminate those risks of infection. In one embodiment, all signals are derived from standard EKG electrodes applied to the patient's skin.
The present invention also provides a device with sensor placement, e.g., probe electrode placement, that is universal across the patient population. Furthermore, in embodiments of the present invention wherein sensor strips or other free floating sensors, e.g., non-adhesive, are used to contact the mother's chest, abdomen, and/or back, the patient's position can be rotated or reorientated relative to the sensor field. In such an embodiment, the sensors must be capable of sensing a respective waveform without the need to be adhered to the patient's body.
The present invention also provides a device where the placement of the electrodes maintains a clear surgical field, thereby facilitating operative procedures such as cesarean section deliveries, and will not interfere with resuscitation of the mother, should either become necessary.
The present invention also provides a device that overcomes the signal loss anomaly of ultrasound devices resulting from fetal movement. There is no need to tend to the device and reposition electrodes as the fetus moves, thereby allowing health professional time and attention to be directed toward more productive patient care activities.
The present invention also provides a device that will achieve a full representation of the fetal EKG.sub.f waveform which may provide useful information about the fetal condition.
The present invention also provides a device which upon interpretation of the fetal EKG.sub.f waveform makes the subject device capable of determining the instantaneous orientation of the fetal heart vector, thereby indicating the orientation of the fetus and permitting prediction of delivery complications associated with atypical presentation.
The present invention also provides a device that routinely collects maternal EKG signals. Thus, collateral information about the well-being of the mother and possible maternal-fetal interactions are immediately available.
The present invention also provides a device that will function for an ambulatory patient, either pre-term or during prolonged labors where the patient wishes to ambulate.
The present invention also provides a device that can be used in the case of non-imminent deliveries, for example, pre-term patients who may have high risk pregnancies.
The present invention also provides a device that computes and displays a unique monitoring reading that provides a measure of the instantaneous processing performance.
The present invention also provides a device that computes and displays heart rate variability information in at least two forms: i) long term variability trend, as is available with current commercial systems; and ii) a unique measure of instantaneous variability.
The present invention also provides a means to monitor multiple gestations with no additional sensors and/or processing techniques being required.
The present invention also provides a device that routinely collects electromyographic (EMG) signals as a means for monitoring maternal uterine contractions and for providing an additional signal input for noise cancellation. In addition, the device also permits the identification and characterization of active (maternal movement) and passive (surgical manipulation, uterine contraction) maternal signals from EMG inputs useful for canceling noise artifacts to even further enhance the EKG.sub.f.
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate several embodiments of the present invention and together with the description serve to explain the principles of the present invention.
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