Surgery – Diagnostic testing – Cardiovascular
Patent
1997-05-29
1999-09-28
Kamm, William E.
Surgery
Diagnostic testing
Cardiovascular
600523, 600483, 600408, A61B 50444
Patent
active
059578556
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Fetal monitors of various types are widely used in the obstetrics field. Most of the devices provide to the clinician an indication of fetal heart rate (HR) as one item of data used by the clinician to evaluate overall fetal well-being. Other more sophisticated devices perform frequency analysis on heart rate time data to produce indications of heart rate variability (HRV), another very important item of data used to monitor fetal condition.
Typically, these frequency analysis devices collect fetal heart rate time data over relatively long periods of time. At the end of each period, a linear time-frequency representation, such as a short-term Fourier transform or a fast Fourier transform, is computed on the time data to obtain a frequency distribution for the data. In one such prior system, each time data window is 30-60 seconds in duration.
A significant drawback to these devices is that, because of the long time data window duration, or, equivalently, the low time resolution of the device, short term or transient variations in heart rate and/or heart rate variability cannot be detected. These transient changes can be indicators of significant fetal characteristics such as fetal breathing and should be considered in evaluating overall fetal well-being. Because these prior systems have long time windows, transient changes are not detected, and, therefore the often critically important fetal characteristics associated with the transient changes are not taken into consideration in the overall fetal evaluation.
SUMMARY OF THE INVENTION
The present invention is directed to a fetal data processing system and method and a fetal monitor and method for monitoring fetal condition which overcomes the drawbacks of prior art monitors. Specifically, the invention analyzes fetal heart rate data in both time and frequency domains with sufficient time resolution to detect transient changes in fetal heart rate and fetal heart rate variability while also detecting other important heart rate and heart rate variability characteristics. The invention also has sufficient frequency resolution to provide a substantial improvement in accuracy of frequency analysis and in diagnostic capabilities over prior systems. This is particularly true during the human birthing process and/or when the mother or fetus are at risk due to medication, disease, injury, or other reason.
The system or monitor of the invention receives fetal heart rate time data and samples the data at preferably periodic intervals. The invention transforms the time data into the frequency domain by computing a time-frequency representation (TFR), in one embodiment, a non-linear TFR, for the time data. In one embodiment, a non-linear TFR is computed for time data covering a time interval of no more than ten seconds; the time interval is preferably in the range between 0.1 and 1.0 second. The system then analyzes the non-linear TFR to indicate a condition of the fetus.
In one embodiment, the time data is sampled at 0.25 second intervals. For each sample in this embodiment, the invention computes a non-linear time-frequency representation (TFR) of the data. It then analyzes the non-linear TFR to indicate the condition of the fetus and to indicate fetal well-being. In other embodiments, the data is sampled at the same 0.25 second interval, but the TFR may not be calculated on every data sample. Instead, it may be calculated at greater intervals. Preferably, the interval between recalculation of the TFR will not exceed ten seconds.
As noted above, the sampling interval is preferably 0.25 second or, equivalently, the sampling frequency is preferably 4 Hz. Other sampling frequencies can be used. However, the frequency will preferably never be below 2 Hz to meet the Nyquist criterion for certain signal frequencies in heart rate data near 1 Hz, as will be described below in detail. Lower sampling frequencies can be used if anti-aliasing filters are used.
In the system of the invention, the fetal heart rate time series is obtained from either a
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Bennett Frederick M.
Oriol Nancy E.
Beth Israel Deaconess Medical Center
Kamm William E.
Schaetzle Kennedy J.
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