Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
1999-12-01
2003-11-25
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S322000, C600S353000
Reexamination Certificate
active
06654622
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for the combined measurement of the arterial oxygen saturation and the transcutaneous carbon dioxide partial pressure on the ear.
BACKGROUND OF THE INVENTION
It is known to measure the oxygen saturation of the haemo-globin in arterial blood (arterial oxygen saturation) by means of a noninvasive optical method which is referred to as pulse oximetry. The principle of this method is based on measuring and evaluating changes in the absorption of light caused by the pulsatile inflow of arterial blood into a well-perfused part of the body (e.g. finger pad or ear lobe). The SpO
2
measured in this way normally provides reliable information about the patient's oxygenation. Pulse oximetry is routinely employed in various medical fields, in particular for intra- and postoperative patient monitoring.
However, information about oxygenation is not always sufficient on its own. It is frequently necessary also to know the arterial carbon dioxide partial pressure (paCO
2
) in order to be able to assess the patient's respiratory functions. The methods currently available for measuring the paCO
2
are essentially the three described below:
1. Removal and analysis of an arterial blood sample. Although this method allows direct measurement of the paCO
2
, it has the disadvantage that it is invasive and requires access to an artery. In addition, the measurement is not continuous and therefore does not allow changes in the paCO
2
to be monitored continuously. The method has the further disadvantage that the analytical result is usually available only after a delay of several minutes.
2. Capnometry. This is an optical absorption measurement in the infrared region used to determine the concentration of CO
2
in the expired gas mixture. The paCO
2
can be calculated from the CO
2
concentration in the end-expiratory phase. However, as indirect method, capnometry has the disadvantage that it does not always correctly reflect the paCO
2
. Thus, it is known that this value is often an underestimate to varying extents. It is also possible for other parameters, e.g. a change in the cardiac output, to result in a change in the end-expiratory CO
2
concentration and thus cause an incorrect estimate of the paCO
2
.
Furthermore, the possible applications of capnometry are restricted by the fact that it can be employed in general only for intubative, artificially ventilated patients. It is therefore in general impossible to determine the paCO
2
by capnometry during operations on nonventilated patients. Nor is capnometry suitable for monitoring the transition phase from artificial ventilation to spontaneous breathing. It is precisely during such a transition that continuous measurement of the paCO
2
is often required.
3. Transcutaneous PCO
2
measurement. This method is likewise indirect and makes use of the fact that carbon dioxide is able easily to diffuse through body tissue and skin. The gas is measured with a sensor attached to the surface of the skin. When a sensor of this type is warmed to a temperature of about 41° C. to about 45° C., this produces local dilatation and arterialization of the capillary bed at the measurement site. Under these conditions, the transcutaneous carbon dioxide partial pressure (tcpCO
2
) measured there shows a good correlation with the arterial value. This makes it possible, with certain restrictions, to determine the paCO
2
with an accuracy which is sufficient for most applications.
Detailed information about the measurement methods mentioned and their clinical applications may be found, for example, in the review article “Noninvasive Assessment of Blood Gases, State of the Art” by J. S. Clark et al., Am. Rev. Resp. Dis., Vol. 145, 1992, pp. 220-232.
Of the abovementioned methods for paCO
2
measurement, at first sight the transcutaneous method appears to have the most advantages: this measurement is noninvasive, continuous and can also be employed for nonintubated patients. Nevertheless, transcutaneous PCO
2
measurement has not to date become widely used for intra- and postoperative patient monitoring. It is employed for this only extremely rarely, whereas it has long been established as a routine method in other medical fields, for example in intensive monitoring in neonatology.
One of the reasons for this is that the sensors currently available for tcpCO
2
measurement are suitable for application only to sites on the body to which access by the anaesthetist during an operation is usually difficult: a tcpCO
2
sensor must be applied by means of an adhesive ring which is adhesive on both sides to a well-perfused, hairless site of low convexity on the skin, with the diameter of the area of skin covered by the sensor and adhesive ring being about two to three centimetres. Particularly suitable measurement sites are therefore the thorax region, the abdominal regions and the inside of the upper arm or thigh. These sites are, however, not directly accessible for the anaesthetist, and can usually not be inspected either if they are covered. Thus, for example, it is difficult to check whether the sensor is adhering well or has become detached. Possible repositioning of the sensor during the operation is also difficult. In addition, on these sites on the body, the sensor may impede the surgeon or conflicts with the requirements for sterility in the vicinity of the operative field may occur. In addition to these difficulties which derive from the measurement site, application of a tcpCO
2
sensor is often regarded as complicated because a contact gel must be applied in order to avoid inclusions of air between sensor and skin. The dosage of this gel is critical because if the amount is too large the adhesion area of the adhesive ring would be wetted and, in this case, satisfactory attachment of the sensor would no longer be ensured. On the other hand, too small an amount of the gel would be ineffective.
There are no difficulties of this nature on application of a pulse oximeter sensor. This is usually attached by means of a clamp or an adhesive strip to a finger or an ear lobe. No special complexity is required for this. In contrast to a transcutaneous sensor, no contact gel is required. The measurement site on the ear is in particular usually easily accessible and easy to inspect by the anaesthetist. It is extremely rare for the surgeon to be impeded or problems to arise with the requirements for sterility there.
However, the disadvantage of pulse oximetric measurement on the ear lobe is that the signal measured there is often very weak. On the one hand, this derives from the fact that the thickness of the tissue from which the signal is obtained is relatively small by comparison with the finger pad. On the other hand, the ear lobe is often cold, as a result of the frequently low temperature in the operating theatre, and therefore poorly perfused. This may result in the signal measured on the ear being so weak that pulse oximetric measurement is no longer possible there. The anaesthetist is then forced to carry out the measurement on a finger. Although the finger is in principle also easily accessible as measurement site, its location is less favourable from the anaesthetist's point of view, who normally does his work near the patients head. An additional factor is that an arterial catheter or a cuff for measuring blood pressure is frequently attached to the patient's arm. Such an arm must not be used for pulse oximetric measurements because the latter would be impaired otherwise. The other arm is frequently less accessible, depending on the patient's position.
It may be stated in summary that the problems of monitoring the arterial oxygen saturation and the arterial PCO
2
in patients during and after surgical operations have by no means been satisfactorily solved yet.
The object of the present invention is therefore to provide a device which makes simple and reliable measurement of these two parameters possible on the measurement site preferred by the anaesthetist, the ear. It is additionally in
Eberhard Patrick
Palma Jean-Pierre
Kremer Matthew
Linde Medical Sensors AG
Selitto Behr & Kim
Winakur Eric F.
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