Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2002-04-09
2004-02-03
Bockelman, Mark (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C600S519000
Reexamination Certificate
active
06685649
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for monitoring a condition of a patient under anaesthesia or sedation, the method comprising the steps of: acquiring a signal representing temporally a cardiovascular activity of the patient; detecting periodically repeated waveforms in said signal; calculating at least one parameter selected from: time intervals between, or pressures from, or temporal rates from successive waveforms; and forming respectively at least one of the following: a mathematical time series of said time intervals, or a mathematical pressure series of said pressures, or a mathematical rate series of said temporal rates. The invention also relates to a use of the method, in which a signal representing a cardiovascular activity of a patient is analyzed for detection of periodically repeated waveforms, at least one of the following: time intervals between, or pressures from, or temporal rates from successive waveforms are calculated to provide at least one of the following: a mathematical time series of said time intervals, or a mathematical pressure series of said pressures, or a mathematical rate series of said temporal rates, as well as to an apparatus for monitoring a condition of a patient under anaesthesia or sedation, the apparatus comprising: detection means for receiving a substantially continuous electrical signal representing a cardiovascular activity of the patient; a voltage/current dependent circuit detecting each predetermined waveform in said signal; and first calculation means providing at least one of the following: a series of time intervals between successive waveforms, or a series of pressures, or a series of temporal rates from successive waveforms.
RELATED STATE OF THE ART
Concept of the depth of anaesthesia has been of interest for recent decades, and several measures have been proposed to assess the depth of anaesthesia. Recently, however, this unitary anaesthesia theory of the existence of one-dimensional concept called “depth of anaesthesia” has been criticised as oversimplified. Instead it has been suggested that the anaesthesia has not one but three main components: hypnosis, analgesia and muscle relaxation. Different anaesthetic regimens have different effect on these three components. Furthermore, they have effects on both cortical and subcortical levels. An adequate anaesthesia means unresponsiveness to both noxious and non-noxious stimuli. The former may be defined by means of hemodynamic, motor and endocrine stability, while the latter is related to the loss of consciousness and recall and amnesia. In practice the adequate anaesthesia is administered by using a combination of drugs with different effects on brain, spinal cord, autonomic nervous system and neuro-muscular junction. The combination of these effects hence creates the hypnotic, analgesic and muscle relaxing effects.
Heart rate (=HR) is controlled by autonomic nervous system. Especially sympathetic activation to the heart causes heart rate acceleration. Sudden pain causes a stress reaction, which is associated with sympathetic activation leading to sudden increase in HR and blood pressure. Hence, in clinical practise HR and blood pressure responses have been used for decades by experienced anaesthesiologists to heuristically detect inadequacy of anaesthesia. However, not only pain but also various other issues may cause variations in HR and blood pressure, and consequently the HR and blood pressure are continuously varying. Hence, monitoring just the mean level of HR and blood pressure is not sufficient.
Document U.S. Pat. No. 5,372,140 proposes a method and an apparatus for providing a measure of the depth of anaesthesia based on analyzing beat-to-beat heart rate together with respiration. For that purpose a series of so-called R-waves from the cardiac signal is analyzed for determining the position in time of each R-wave relative to the respiratory cycle within which it occurs, and a measurement value representing the degree of clustering is derived. Further a so-called circular statistics is utilized with a test for randomness, and finally the measurement value is compared with a reference value to find the depth of anaesthesia. The proposed measure is related to respiratory sinus arrhythmia, which is primarily controlled by parasympathetic nervous system. The measure is poorly related to the functioning of the sympathetic nervous system, and hence may not measure sympathetic reactions to pain. As a conclusion, the disclosed method and apparatus does not provide results, which could be considered as an objective measure for the level of analgesia of a patient.
Document U.S. Pat. No. 6,120,443 proposes another method and device for determining a depth of anaesthesia by acquiring a plurality of successive signals representing a cardiac activity of the patient, detecting a periodic wave therefrom, calculating time intervals between successive waves, determining a digital series of the time intervals, and calculating a fractal dimension of said series as well as calculating a depth of anaesthesia as a function of the fractal dimension. According to the document the signal is filtered by calculating the maximum correlation between the sampled signal and a theoretical signal, whereupon a signal period between 20 and 80 intervals is suggested, then regrouping the time intervals to form several digital series, and finally the fractal dimension is approximated by determining a dimension of correlation between said digital series, whereupon two to ten series is used. The disclosed method so is mathematically based on computing correlation dimension for beat-to-beat heart rate signal. Theoretically this kind of calculations requires a very long data sequence, leading to large delays in real-time monitoring. Though the inventor proposes to use relatively short data sequences to minimise the acquisition delay this makes the theoretical basis of the method questionable. As above, also this disclosed method and apparatus does not provide results, which could be considered as an objective or reliable measure for the level of analgesia of a patient.
Recently, methods for assessing the hypnotic component especially by monitoring electroencephalographic (EEG) signal have been introduced. The most well-known is the Bispectral Index (BIS), but also fractal spectrum, Lempel-Ziv complexity, Kolgomorov-Sinai entropy and spectral entropy etc. has been studied, e.g. Ira J. Rampil: “A Primer for EEG Signal Processing in Anesthesia”—Anesthesiology, Vol. 87, No. 4, October 1998, and evidently some of them can provide a data describing reliably the level of consciousness independent of the individual. Muscle relaxation may be measured e.g. by monitoring neuro-muscular transmission time and/or excited force, and one method and instrument for this purpose is disclosed in publication EP-0 691 105. Measuring muscle relaxation can be considered to be an established and reliable technology, and it is already a standard practice, because various devices are commercially available, too.
SUMMARY OF THE INVENTION
Accordingly the main object of the invention is to achieve a method and an apparatus for monitoring the anaesthesia or sedation of a patient so that a reliable data about level or depth of analgesia would be available to an anaesthetist or to other purposes. The second object of the invention is to achieve a method and an apparatus for monitoring the anaesthesia or sedation capable of using measured signals derived from various sources of the patient, which means that the method should not be dependent on any single type of detector. The third object of the invention is to achieve a method and an apparatus for monitoring the anaesthesia or sedation capable to deliver such results as an output, with the basis of which the adequacy of analgesia could be reliably enough assessed by inexperienced anaesthetists or other operators, too. The fourth object of the invention is to achieve a method and an apparatus for monitoring the anaesthesia or sedation functioning with an
Andrus Sceales Starke & Sawall LLP
Bockelman Mark
Instrumentarium Corp.
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