Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Patent
1997-12-05
1999-11-09
Weiss, John G.
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
12820426, 12820513, 12820514, 12820515, A61M 1600, A62B 704, F16K 3126
Patent
active
059794437
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to an inhalation apparatus which may be used in the measurement of the volume of a patient's blood, or which may be used as a closed anaesthetic system.
2. Description of the Prior Art
It is often necessary, particularly in the case of critically ill patients in an intensive care unit, to measure the patient's blood volume in order to assess the need for a blood transfusion. One of two generally-used methods involves the measurement of red-cell volume and subsequent calculation of total blood volume from the haematocrit, the other involves the measurement of plasma volume and again subsequent calculation of total blood volume from the haematocrit. Often both methods are used and total blood volume is then obtained by adding the measured red-cell and plasma volumes.
An established method of measuring red-cell volume involves the steps of removing a small sample of blood from the patient, labelling the red cells in the sample with radioactive tracers, and re-injecting the radio-labelled sample into the patient. After a predetermined period of time, the sample will have mixed evenly with all of the blood in the patient's body. Another sample of blood is then removed and the "dilution" of the radioactive tracers is determined, which enables the volume of blood into which the sample was re-injected to be calculated, and hence the blood volume of the patient may be determined.
The above method has been established as a sufficiently accurate method of indicating blood volume, involving relatively simple procedures and using standard equipment. However, this method is time consuming and expensive and cannot be repeated at frequent intervals: therefore, if an error occurs, it may be some time before the test can be performed again. This also means that it is not possible to monitor a patient's blood volume at frequent intervals, which may be required under certain circumstances. Another drawback of the method is that it involves radioactivity, which can be hazardous.
Another known method of measuring red-cell volume again involves the steps of removing a small sample of blood from the patient, labelling the red cells in the sample, and re-injecting the labelled sample into the patient. However, in this case, the red cells are labelled with a small quantity of carbon monoxide (CO) which is injected into the sample of blood. Carbon monoxide binds to haemoglobin in red blood cells to form carboxyhaemoglobin. Thus, once the re-injected sample has been allowed sufficient time to mix evenly with the blood in the patient's body, the "dilution" of the carboxyhaemoglobin in the patient's blood, and subsequently the blood volume, may be determined.
Although the above-described method eliminates the hazards of using radioactivity, it is still time-consuming and cannot be repeated at frequent intervals: it moreover involves the risk of infection to the medical staff handling the blood samples at the bedside. A more convenient method of measuring blood volume has been proposed wherein carbon monoxide saturation of a patient's haemoglobin or carboxyhaemoglobin is first measured and the patient then inhales a small known quantity of carbon monoxide which crosses to the red blood cells through the lungs. The carboxyhaemoglobin is again measured and the total quantity of haemoglobin may thereby be calculated which, in turn, allows calculation of the patient's blood volume. Patients in intensive care units are usually unable to breathe unaided and are thus ventilated artificially by means of a tube passed into the trachea (endotracheal tube). It is therefore necessary to provide apparatus for ventilating the patient artificially whilst administering the required dose of carbon monoxide. Such an apparatus is shown in FIG. 1 of the drawings and comprises an elongate pipe 100 connected at one end via a connector 102 to the endotracheal tube, through which the patient is ventilated. A carbon dioxide (CO.sub.2) absorber 104 is provided in the pipe 100
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Schindler Edwin D.
University of Wales College of Medicine
Weiss John G.
Weiss, Jr. Joseph F.
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