Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2001-07-11
2003-04-08
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S529000
Reexamination Certificate
active
06544191
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a process for determining the functional residual capacity (FRC) of the lungs during respiration. The determination of the FRC during respiration therapy in general and during the diagnosis of the maturity of the lungs of respirated premature and newborn babies in particular may be helpful in the initiation and the monitoring of the necessary therapeutic measures. At the same time, disturbances in the intrapulmonary gas distribution can be quantified by such a process.
BACKGROUND OF THE INVENTION
The functional residual capacity (FRC) is considered to be an important, informative variable for the respiration of a patient; however, its measurement has not yet found acceptance in routine clinical practice because of the rather complicated apparatus required. The measurement of the FRC has only been known from the area of clinical experiments so far.
The determination of the FRC can be carried out in a patient in various ways. A method known for healthy patients in the lung function laboratory is body plethysmography, in which the patient is sitting in an air-tightly closed chamber and the lung volume can be determined on the basis of the breath-dependent variations in the air pressure in the chamber. This method is very complicated and cannot be used in intensive care patients.
Another method is the so-called wash-out method. In an open breathing circuit, either the nitrogen contained in the lungs or an inert gas washed in before, usually a gas that is poorly soluble in blood, is washed out by another gas, i.e., replaced by that gas. Nitrogen wash-out methods, noble gas wash-out methods, and sulfur hexafluoride wash-out methods operate according to this principle. The FRC is calculated by measuring the amount of indicator gas washed out and its concentration in the lungs before and after the wash-out, the FRC being obtained as the quotient of the indicator gas volume washed out and the difference between the indicator gas concentration before the wash-out and the indicator gas concentration after the wash-out. The indicator gases used shall not induce any physiological, toxic or metabolic reactions in the patient's lungs. Moreover, they shall not be possibly soluble in the blood in order not to distort the mass balance, on which the calculation of the FRC according to the wash-out method is based.
To determine the FRC by means of nitrogen wash-out, an increased oxygen concentration, e.g., 100 vol. %, is supplied for the patient for a short time or part of the nitrogen is replaced with a noble gas at constant oxygen concentration. Contrary to the nitrogen wash-out by means of noble gases, no additional gas is needed in a respirator for the nitrogen wash-out by means of an increased oxygen concentration because oxygen and normal breathing air are normally available there. However, there are reservations against increasing the oxygen concentration in the case of use for premature and newborn babies because of the suspicion that retrolental fibroplasia may develop in this case, which may subsequently lead to loss of eyesight. Conversely, the oxygen concentration cannot be reduced without problems in the case of intensive care patients, who need a high oxygen concentration, without running the risk of inadequate oxygen supply. Moreover, the measurement of the nitrogen concentration during the wash-out is technically very complicated, because nitrogen is difficult to detect. It can be carried out, e.g., with a mass spectrometer. Laughing gas, N
2
O, has been used as an inhalation anesthetic for a long time. Due to its light absorption behavior, it can be detected relatively rapidly and accurately by infrared optical measurement methods. It is therefore logical to use laughing gas at low concentrations as a trace gas for determining the FRC. However, laughing gas is rapidly absorbed by the blood and a considerable percentage of laughing gas, about 40%, cannot therefore be washed out directly, as a result of which the mass balance necessary for the determination of the FRC is distorted.
The wash-out method can also be carried out with so-called trace gases, e.g., noble gases, instead of nitrogen. Noble gases, e.g., helium or argon, are well suited for the determination of the FRC for physiological reasons. They are inert, they have neither toxic effect nor, at low concentrations, anesthetic effect and only insignificant quantities of these gases are dissolved in the blood. They are not combustible and are thermally stable. With such a wash-out method, the noble gas is first administered at a low concentration, e.g., 1 vol. %, during the respiration of the patient, until an equilibrium becomes established in the lung. The metering is then switched off and the noble gas is subsequently washed out with normal breathing air. The concentration and the volume flow of the expired air are now measured continuously. If noble gases are used as a trace gas, mainly mass spectrometers, which are unsuitable for routine clinical use, are likewise used for the concentration measurement.
Methane and butane may also be used as trace gases to determine the FRC. They are physiologically harmless at low concentrations and they can be readily detected according to infrared optical methods. The drawback is that methane and butane are combustible and are explosive at certain mixing ratios. An explosive mixing ratio is formed, e.g., by 4.1 vol. % of methane in usual breathing air. Contrary to the determination of the FRC in spontaneously breathing patients, a higher oxygen concentration is often necessary in intensive care patients than the concentration of about 21 vol. % normally present in the air. However, the risk of explosion also increases markedly with increasing oxygen concentration in the case of methane and butane, so that the concentration of these gases must be very low to avoid an explosive gas mixture. In light of such risks, the use of methane and butane is not indicated, especially in intensive care medicine.
As an alternative to wash-out, it is also possible to analyze the wash-in of trace gas, i.e., to set up the net balance of the trace gas flowing into the patient's lung, by means of concentration and volume flow measurements.
To determine the FRC according to a wash-out method as well as a corresponding wash-in method, inert trace gases that are harmless for the patient are therefore sought, which can be measured with a small sensor near the patient. The sensor itself must have a very rapid time response. The response must take place in less than 25 msec in the case of the respiration of newborn babies, because a sufficient resolution of the curve showing the trace gas concentrations over time, which is necessary for the determination of the FRC according to the wash-out method, is only possible under these conditions. If the sensor is arranged in the main stream of the breathing circuit, it should be as small as possible in order not to needlessly increase the dead space, which would lead to an impairment of the quality of respiration. An increased dead space leads to the less satisfactory wash-out of carbon dioxide and therefore to hypercapnia, especially in premature and newborn babies.
Sulfur hexafluoride is mentioned as a trace gas in EP 0 653 183 B1. Sulfur hexafluoride, SF
6
, has been known for a long time as a trace gas for the determination of the FRC. Sulfur hexafluoride is considered to be inert and can be easily detected by means of infrared optical gas sensors; it absorbs in the wavelength range of 10.6 &mgr;m.
One drawback of sulfur hexafluoride is that it can lead to very high absorption of sunlight at high altitudes of the atmosphere and thus to warming of the environment. Sulfur hexafluoride is therefore discussed in connection with the “greenhouse effect.” Whenever possible, it should be replaced with more environmentally friendly gases. Moreover, a TLV (Threshold Limit Value) of 0.1 vol. % is specified for sulfur hexafluoride.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the present invention is t
Ankerhold Georg
Hattendorff Horst-Dieter
Koch Jochim
Weismann Dieter
Dräger Medizintechnik GmbH
Mallari Patricia C
McGlew and Tuttle , P.C.
Nasser Robert L.
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