Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2002-05-29
2004-04-20
Hindenburg, Max F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S322000
Reexamination Certificate
active
06725074
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the determination of a quantitative statement concerning the quality of a measurement signal, preferably a medical measurement signal, such as in pulsoximetry.
Measuring of signals usually comprises a process in several stages, typically with the steps of signal recording, signal processing, and signal evaluation. In the simplest case of a measurement, the mere signal recording suffices, but depending on the application the signal processing and/or evaluation are also regularly necessary.
In signal recording, signals representing the quantity to be measured are recorded as basic signal values, for example by means of a sensor or some other suitable recording device. In some cases, for example in electrocardiography (ECG), these recorded basic signal values already directly represent the measurement quantity to be determined (also called parameter). In other cases, for example the determination of oxygen saturation (SpO
2
), the recorded basic signals indirectly represent the desired measurement quantity, and a signal evaluation is still necessary in each case so as to derive the desired measurement quantity from the basic signals.
Depending on the measurement process, the measurement accuracy, and influences on the measurement, it is necessary for the basic signals (both those directly and those indirectly representing the measurement quantities) to undergo a signal processing, i.e. the basic signals must be suitably adapted, for example through an improvement in the signal quality such as the signal to noise ratio, or through filtering or suppression of undesirable measurement influences.
Similarly, a signal evaluation is also necessary, depending on the measurement process and the measurement quantity, so as to obtain from the recorded basic signals or the processed signals the desired measurement value of the measurement quantity. As was noted above, the quantities representing the measurement quantity indirectly only are to be evaluated, because the obtained basic signals by themselves are not conclusive.
Given the fact that the obtained measurement values represent the result of the measurement process, or that further quantities, conclusions, or consequences are derived from these measurement values, the question often arises in how far the obtained measurement values can be relied on, i.e. how well or how badly these measurement values represent the actual values of the quantity measured. The reliability or quality of the measurement values is of major importance in particular in the field of medical applications, such as patient monitoring, because a measurement value incorrectly representing the quantity to be measured, while this incorrectness is not observable, may have serious consequences for a patient's life and health, for example owing to an incorrectly prescribed or omitted therapy, or owing to a suppressed alarm function.
A good example of the importance of the quality of measurement values is found in pulsoximetry, where this has frequently led to problems in the past, while at the same time the requirements were set higher and higher. Pulsoximetry is a non-invasive, continuous determination of the oxygen content of the blood (oximetry) based on an analysis of the photospectrometrically measured pulse. It is necessary in this field that a pulse curve (plethysmogram) should be available at several wavelengths. Practically all appliances operate at only two wavelengths, which renders possible inexpensive, compact solutions. The principle of photometry is based on the fact that the quantity of absorbed light is determined by the degree of absorption of a substance and by the wavelength. Pulsoximeters utilize this in that the arterial blood volume, and only the arterial blood volume, pulsates in the rhythm of the heartbeat. The basic principle and the application possibilities of pulsoximetry are generally known and have frequently been described, in particular in EP-A-262778 (with a good summary of the theory), U.S. Pat. No. 4,167,331, or Kästle et al. in “A New Family of Sensors for Pulsoximetry”, Hewlett-Packard Journal, vol. 48, no. 1, pp. 39-53, February 1997.
Although ever more difficult cases, as regards the signal quality of the pulsoximetric measurement, are still used for deriving measurement values, there has until now not been a conclusive indicator for the clinical user which renders it possible to evaluate conclusively the reliability and quality of the measurement values obtained. Such an evaluation, however, is important because the pulsoximeters with their plethysmographic basic signals contain too little information for deciding with full certainty in boundary cases whether a measurement value can be indicated. A doctor, for example, regularly has substantially more information available to him for deciding whether the oxygen supply of his patient is actually critical or whether there is merely an artifact of the pulsoximeter.
The aids frequently available to the user of the pulsoximetrical measurement are a representation of the basic signal in the form of a curve (plethysmogram) or the one-dimensional representation as a pulsating histogram bar. In addition, there are warning signals such as “motion”, “noise”, “low signal” in text fields on displays, or flashing displays (cf. Maurice et al., A Comparison of Fifteen Pulsoximeters, Anesth. Intens. Care, vol. 17, pp. 62-82, 1989).
A first step towards achieving a quality indicator for the pulsoximetrical measurement values can be found in EP-A-587009. A bar display is described therein which is controlled by the transmission (light transmission of the tissue), i.e. by the absolute signal strength (DC component) of the measurement signal. This system, indeed, is certainly capable of indicating an unsuitable measuring location, for example because the received luminous intensity is too low. Nevertheless, the AC component (perfusion) and the interference level remain unconsidered, so that an indication with a high rating for the quality level does not give any guarantee for a reliable measurement.
The user may indeed get a certain indication as to the quality of the signal through judging the size and shape of the plethysmogram and the other displayed quantities. The conclusiveness thereof, however, is limited. A problem is, for example, that only one basic signal is often displayed as the plethysmogram, while the pulsoximeter evaluates two basic curves.
The N-400 Fetal Oxygen Saturation Monitor from the Nellcor Puritan Benett Company has at its front a triangular rod display serving as a multi-parameter reliability indicator for the average signal quality. This signal quality indicator represents the quality of the signal which is used for calculating the SpO
2
value. When the signal quality drops to below a required threshold value, an acoustic alarm signal is triggered upon the loss of the signal.
A further method of testing the obtained pulsoximetric measurement value is described in EP-A-904727 and is based on a comparison of the pulse frequency of the pulsoximeter with the heartbeat frequency of the ECG signal. If, for example, the deviation between the two lies within a range of only a few beats per minute, the pulsoximeter is considered credible. If this difference is greater, however, the pulsoximeter readings are considered doubtful.
It is a disadvantage in most of the methods of determining the credibility of the pulsoximetrical measurement value mentioned above that at most only half of the measurement signals available is actually used. Thus, for example, only one of the basic signals suffices for deriving the pulse rate, so that the function of the other channel usually remains fully untested. Alternatively, the periodicity of the signal only is evaluated, while, for example, the amplitude of the basic curve remains substantially unconsidered. This results in numerous potential errors, which are often not even noticeable. If erroneous measurement values or erroneous quality indicators for the measurement values lead to, for e
Hindenburg Max F.
Kremer Matthew
Vodopia John
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