Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2000-06-08
2002-08-06
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S529000
Reexamination Certificate
active
06428483
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of capnography in general, and in particular to instrumentation and methods for respiratory status determination by means of analyzing and interpreting the waveforms obtained in capnographic measurements.
BACKGROUND OF THE INVENTION
Capnography, as is well known in the art, is the measurement of the level of carbon dioxide (CO
2
) in exhaled or inhaled breath. The information obtained from a capnographic measurement is generally presented on the display of the capnograph as a series of waveforms, representing the partial pressure of CO
2
in the patient's exhaled breath as a function of time. The information can be displayed at a fast rate, in which case the fine detail of each breath can be evaluated, or at a slow rate, in which case the trend in the end tidal value of exhaled CO
2
(EtCO
2
) can be evaluated.
Clinicians use capnography in order to assess a patient's respiratory status Respiratory arrest and shunt can be speedily diagnosed, and a whole range of other respiratory problems and conditions can be determined by the shape and trends of the waveforms. In conjunction with other measurements, pulmonary blood flow, embolism and cardiac arrest can be assessed. Capnography is considered to be a prerequisite both for safe intubation and general anesthetic, and for correct ventilation management. Furthermore, recently proposed standards will mandate the use of capnographic monitoring of patients under sedation, even in simple everyday situations such as minor cosmetic surgical procedures, or pediatric dental surgery, or pediatric sedation for the performance of imaging procedures such as MRI.
Up to now, the interpretation of capnographic waveforms has generally been performed by the respiratory specialist or the anesthetist. Such personnel have sufficient experience to be able to perform an accurate diagnosis from an inspection of the waveforms on the monitor screen of the capnograph. However, with the increased use of capnography both by medica specialists in other fields and by paramedical personnel in emergency service environments, the capnograph operator is not always able to make a safe diagnosis of the patient's respiratory condition by observing the waveform graphs. The waveforms typically have a large number of different parameters, each of which taken by itself can indicate a different condition, and combinations of which can indicate yet further different conditions. These conditions relate not only to the physiological state of the patient, but also to the correct operation and connection of respiratory support apparatus
There therefore exists an important need for instrumentation which will provide diagnostic support for the capnograph operator, beyond the display of the capnographic waveforms and trends on a monitor screen.
SUMMARY OF THE INVENTION
The present invention seeks to provide a new capnographic instrument which performs an analysis of the breath waveforms measured by the carbon dioxide sensor, interprets the results of this analysis and outputs to the operator a suggested diagnosis or diagnoses of the respiratory clinical state of the patient, or advises of possible malfunctions of the ventilation equipment being used to assist the patient's breathing.
There is thus provided in accordance with a preferred embodiment of the present invention, a capnograph, which provides to the operator interpretation of the capnographic waveforms measured, in the form of diagnostic information about the respiratory status of the patient, or about the adequacy of the breathing support provided by the ventilation equipment to which the patient is attached. The instrument provides this information by comparing a number of parameters characteristic of the waveforms of the patient's breath with an internal library of the values of those parameters expected from normal waveforms stored in its memory. These parameters may either relate to specific features of the waveform in the time domain, or may characterize spectral components of the waveform in the frequency domain. The differences detected are interpreted as an indication of respiratory malfunction in the patient, or as a measure of the effectiveness of the ventilation equipment, or of the patient's connection thereto. The diagnosis suggested by the system is dependent on which of the particular parameters are deviant from the normal values, and by how much. According to a preferred embodiment of the present invention, several parameters which characterize the form, frequency and magnitude of the waveform are determined for each waveform, and the values input to the processor unit of the instrument for comparison with values of these parameters expected in normal waveforms.
In addition to the analysis of individual waveforms, any long term trend in the values of the parameters is also detected, thus providing an indication of changes occurring in the respiratory state of the patient, whether as a result of his clinical state or of the performance of the ventilation equipment to which he is attached.
There is also provided in accordance with a preferred embodiment of the present invention, a method whereby the analysis and interpretation of capnographic waveforms and waveform trends are performed to provide possible diagnoses of a patient's clinical respiratory state, or indications of the effectiveness of the patient's ventilation equipment.
A useful feature of capnographic interpretation would be the determination of the value of the arterial level of carbon dioxide (PaCO
2
) from the measured capnographic value of EtCO
2
. It is often assumed that the value of PaCO
2
is close to that of EtCO
2
or only a few mm. of Hg higher. In effect, this is only so for subjects in good respiratory health, with a normal perfusion/ventilation ratio of pulmonary activity. For subjects with any form of dead space ventilation, or with defective perfusion mechanisms, the two values can be widely different. Since wasted ventilation is so common among a large percentage of respiratory ailments, such as severe respiratory failure, pulmonary hypoperfusion, pulmonary thromboembolism, systemic hypoperfusion and air embolism, the need to be able to determine the true value of PaCO
2
from the measured value of EtCO
2
is of great importance. Alternative methods of determining PaCO
2
involve invasive procedures, such as blood gas analysis.
According to a further preferred embodiment of the present invention, there is thus provided a capnograph, which, as well as measuring the usual carbon dioxide partial pressure. In the patient's breath, simultaneously measures the partial pressure of oxygen. A high oxygen level in the patient's exhaled breath is indicative of poor perfusion. According to this embodiment of the present invention, the instrument utilizes this indication of the perfusion efficiency to provide an indication of discrepancy between the values of EtCO
2
obtained from the carbon dioxide capnographic values, and the value of the arterial PaCO
2
In addition, to the determination of oxygen utilization, the ventilated volume of gas may be used in order to estimate the discrepancy between the measured values of EtCO
2
and the arterial PaCO
2
level. A low ventilation volume, which may be indicative of a perfusion shunt condition, causes the measured values of EtCO
2
to be less than the values of PaCO
2
. In accordance with a further preferred embodiment of the present invention, there is therefore provided a capnographic measurement instrument, which incorporates a measurement of the ventilated volume and provides an indication of divergence of the measured values of EtCO
2
from the PaCO
2
level. The ventilated volume measurement can be obtained from a flow meter sensor section incorporated into the instrument according to the present invention, or, according to a flatter preferred embodiment, it may be obtained from a remote measurement and incorporated into a computation of the EtCO
2
levels.
According to yet
Mallari Patricia
Morgan & Finnegan L.L.P.
Nasser Robert L.
Oridion Medical (1987) Ltd.
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