Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2000-05-15
2001-10-16
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S529000, C600S532000
Reexamination Certificate
active
06302851
ABSTRACT:
This invention relates to a method and apparatus for determining a pulmonary function parameter, here abbreviated EVG, which is indicative of a living subject's effective lung volume, that is, the volume of the part-of the lungs which participates efficiently in the gas exchange between the respiratory gas and the pulmonary blood.
When a person is breathing spontaneously or is connected to an apparatus that provides artificial respiration, the ventilation of the lungs is characterised by the volume of the inspired gas, the so called tidal volume VT, and the breathing rate.
However, all of the gas making up the VT does not reach the finest structures, the alveoli, of the lungs where the exchange of gases takes place (oxygen being taken up by the pulmonary blood and carbon dioxide being released from the pulmonary blood into the gas phase). Moreover, part of VT gas may reach unperfused parts of the lungs and so will be unutilized because this part of the VT gas will have no contact with the pulmonary blood consequently will not take part in the gas-exchange process.
An important therapeutic goal in modern clinical care is to arrange for the best possible conditions for gas exchange in the lungs. The present invention addresses the problem how to assess the efficiency of the gas exchange. According to the invention an apparatus and a method are provided to measure this efficiency and a measure of the matching of ventilation with perfusion is given that can be used to guide therapy.
That portion of the VT gas which does not take part in the gas exchange process is called the dead space volume, VD, and the ratio of the dead space volume to the tidal volume is called the VD:VT fraction. The VD:VT fraction is routinely estimated in the clinical setting using measurements of the carbon dioxide partial pressures of the expired gas and the arterial blood. A description of the state of the art and a historical review of these methods are found in R. Fletcher “The single breath test for carbon dioxide” (1980) Thesis University of Lund Department of Anaesthesia and Clinical Physiology, Lund, Sweden. However, these methods are less than satisfactory because they rely only on measurements of carbon dioxide partial pressure and cannot therefore take into account the effects of both ventilation and circulation in the lungs. Methods based on the calculation of the VD:VT fraction therefore are not specific enough to guide therapy.
An alternative technique for estimating the effectiveness of the gas exchange process is to determine the fraction of the lung volume that participates fully in the gas-exchange process. Since carbon dioxide is soluble in lung tissue, the total volume of carbon dioxide in the lungs is made up of three parts, the gas volume in the alveoli of the lungs, V
g
, and the gas volumes dissolved in the lung tissue, V
t
, and in the pulmonary blood, V
b
, respectively. The total volume is called the equivalent lung volume for carbon dioxide, abbreviated ELV, and can be estimated using carbon dioxide rebreathing techniques. (A Dubois: J Applied Physiology 5, 1, 1952; M Winsborough et al: Clinical Science 58,263 1980).
Although this approach does not have the same limitations as the VD:VT fraction techniques, the equivalent lung volume concept does not provide a sensitive measure of the effectiveness of the gas exchange process. This is so because of the inevitable gas mixing that takes place in the lungs of a person who breathes back and forth. This gas mixing tends to diminish the distinction between effective and ineffective lung areas. (M Petrini et al: J Appl. Phys. 53(4) 930 1982). In addition, the resulting distortion of information depends on the characteristics of the breathing pattern and/or the conditions of the rebreathing setup. Both systematic and random errors of measurement are introduced in this way.
The present invention provides a solution to these problems whereby a sensitive and specific assessment of the effective gas exchange volume in the lungs is possible. In addition, the measurement is simple and quick and can be performed non-invasively.
REFERENCES:
patent: 5072737 (1991-12-01), Goulding
Mallari Patricia C.
Nasser Robert L.
Schiff & Hardin & Waite
Siemens-Elema AB
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