Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-06-25
2001-07-10
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S494000, C600S490000
Reexamination Certificate
active
06258037
ABSTRACT:
BACKGROUND
This invention relates to measuring blood pressure.
Systems for measuring blood pressure are generally known. During a measurement cycle, a blood pressure cuff secured around the patient's limb is inflated to a sufficiently high pressure to cut off arterial blood flow beneath the cuff, and the cuff is incrementally deflated to allow the artery to slowly open. As the cuff is deflated, biological signals indicative of blood pressure (such as sounds, known as Korotkoff sounds, caused by the blood forcing its way through the artery) are detected by a transducer on the cuff and converted to electrical signals that are processed to determine the systolic and diastolic blood pressures. This type of measurement technique is known as auscultation.
Other blood pressure measurement techniques are known. In oscillometry, small pressure changes in an inflated cuff induced by flowing blood are detected by a transducer (disposed either on the cuff or at a remote monitor) and used as a basis for determining blood pressure. Another procedure involves using multiple transducers to detect the times of occurrence of heart pulses at different locations along the artery, and determining the blood pressure based on the pulse propagation time between the transducers.
Often it is clinically useful to measure blood pressure (by any of the techniques described above, or possibly by other techniques) during critical care periods (for example, while the patient is undergoing surgery or being treating in an intensive care unit), or as the patient exercises to, for example, monitor how blood pressure changes with variations in heart rate. But activity, by the patient or by others, during these times generates noise (i.e., signals that are not indicative of blood pressure) that may be incorrectly interpreted, resulting in inaccurate blood pressure measurement.
In some systems that measure blood pressure by auscultation, a threshold based on the level of previously received, valid Korotkoff sounds is applied to all signals produced by the transducer as a discriminant to remove noise. Some systems use two transducers that are spaced on the cuff to provide a half-period delay between the Korotkoff sounds and take the difference between the signals produced by the sensors to reinforce the Korotkoff sounds; because noise appears generally the same at each transducer, the noise level in the difference signal is reduced.
U.S. Pat. No. 5,392,781, entitled “Blood Pressure Monitoring in Noisy Environments” and assigned to the present assignee (the “'781 patent”), describes several techniques for aiding in the discrimination of blood pressure signals from noise.
SUMMARY
This invention, in general, provides enhancements to the techniques described in the '781 patent, and additional techniques used during the measurement of blood pressure for assisting the discrimination of biological signals (such as Korotkoff sounds) indicative of blood pressure from other signals not indicative of blood pressure. Examples of these other signals not indicative of blood pressure include noise generated by the movement of the subject or by the activity of others that occur in all but the quietest measurement environments and which, if not discounted, may result in erroneous measurements. The use of the techniques of this invention, separately or combined, leads to blood pressure measurements that are accurate and, equally as important, highly reliable even in high noise environments (such as those encountered with an exercising subject or when monitoring the subject in an operating room or an intensive care unit).
In one aspect of the invention, mixed signals which include biological signals indicative of blood pressure and noise signals not indicative of blood pressure are detected during time periods, and are evaluated over a plurality of the time periods to aid in discriminating the biological signals from the noise signals.
Preferred embodiments may include one or more of the following features.
The evaluation comprises assembling the mixed signals detected at corresponding times during the plurality of time periods, and analyzing the assembled mixed signals to aid in discriminating the biological signals from the noise signals. Preferably, the assembling includes accumulating the mixed signals detected at the corresponding times during the plurality of time periods.
The time periods are cardiac cycle intervals, and the accumulation is performed on samples of the mixed signals that are taken at corresponding time delays after a start of each of the cardiac cycle intervals. The start of each cardiac cycle interval is detected based on an occurrence of a timing signal that is synchronous with the cardiac cycle. Preferably, the timing signal is an R-wave signal.
Evaluating the mixed signals over multiple time periods increases the signal-to-noise ratio between the biological signals (e.g., Korotkoff sounds) and the noise signals, thereby improving the detection of the biological signals. Korotkoff sounds are rhythmic with heartbeat (that is, they occur at the same frequency as the heartbeat rhythm) while motion-induced noise (such as sounds generated by the patient's stride on a treadmill or by swinging his or her arms) generally do not occur at the heartbeat frequency. Thus, accumulating the mixed signals detected at corresponding times during the time periods reinforces the Korotkoff sound components in the mixed signals but does not reinforce the noise components in these signals. As a result, the Korotkoff sound components are emphasized with respect to noise, and thus can be detected more accurately.
The assembled mixed signals are analyzed by developing a candidate blood pressure signal based on the biological signals indicative of blood pressure in the assembled mixed signals, and determining whether the candidate blood pressure signal should be used to measure blood pressure. In general, the determination is based on whether the candidate blood pressure signal exceeds a threshold. In a preferred embodiment, the determination is based on whether the candidate blood pressure signal exceeds a plurality of combinations of different thresholds. In this approach, a score is assigned to the candidate blood pressure signal that indicates a likelihood that the candidate blood pressure signal is a valid blood pressure signal, the score being based on which thresholds are exceeded. Blood pressure is measured based on the scores of the candidate blood pressure signals.
One of the thresholds includes a history threshold. The history threshold is developed based on previously detected biological signals indicative of blood pressure in the assembled mixed signals that have exceeded the threshold. Another threshold includes a noise threshold, which is developed based on the noise signals not indicative of blood pressure in the assembled mixed signals.
In a preferred embodiment, the candidate blood pressure signal is developed from the biological signals indicative of blood pressure in the assembled mixed signals only during a selected portion of one of the time periods (e.g, cardiac cycle intervals). The selected portion is determined based on levels of the biological signals indicative of blood pressure in the assembled mixed signals and times during the cardiac cycle intervals during which they are detected.
Preferably, the mixed signals are detected with a one or more transducers on a blood pressure cuff. If a plurality of transducers are used, the mixed signals detected at corresponding times during the plurality of time periods by each transducer are assembled (e.g., by an accumulator associated with each transducer) and analyzed by processing circuitry to aid in discriminating the biological signals from the noise signals.
The assembled mixed signals from each of the transducers are combined with each other in one way to develop the candidate blood pressure signal, and are combined with each other in another way to develop the noise threshold for the candidate blood pressure signal. For example, the candida
Cardiodyne Division of Luxtec Corporation
Fish & Richardson P.C.
Nasser Robert L.
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