Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-02-28
2002-05-28
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S323000
Reexamination Certificate
active
06397093
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a non-invasive device and method for detecting possible carbon monoxide poisoning by determining the percentage of carboxyhemoglobin (CO-Hgb) in the subject's blood. The non-invasive device for determining the percentage of carboxyhemoglobin in blood is a pulse oximeter modified to discriminate between oxy- and carboxyhemoglobin. Preferably the device works in two modes. The first mode is a conventional pulse oximeter capable of determining the level of oxy-hemoglobin (hemoglobin saturated with oxygen) in the subject's arterial blood. Upon the achievement of certain criteria, described below, the preferred embodiment of the inventive device would switch to a second mode, in which mode the device would be capable of determining carboxyhemoglobin levels.
The device is used in a method for measuring carboxyhemoglobin which includes having the subject breathe oxygen to convert reduced hemoglobin to oxy-hemoglobin, thereby removing reduced hemoglobin as a blood constituent, determining the concentration of the principle two remaining hemoglobin constituents in the blood (oxy- and carboxy) and measuring carboxyhemoglobin by the modified pulse oximeter.
2. Description of the Prior Art
Carbon monoxide (CO) poisoning is an important cause of morbidity and mortality in the United States that often goes unsuspected and therefore is not promptly treated. Sadovnikoff N, Varon J, Sternbach G L, Carbon monoxide poisoning: an occult epidemic,
Postgraduate Medicine,
92:86-96 (1992)(incorporated by reference); Kales S N, Carbon monoxide intoxication,
Am Fam Phys,
48:1100-4 (1993)(incorporated by reference). CO intoxication is the leading cause of death by poisoning in the U.S. and accounts for approximately 3,800 accidental and suicidal deaths annually. Nonlethal CO poisoning occurs as well, but statistics are not available on the number of incidents of such occurrences. Occult CO poisoning is a type of subacute poisoning caused by an unrecognized source of CO in the home or other indoor environment. Many nonlethal exposures go undetected.
Smoke inhalation from fires accounts for the majority of CO exposure. Firefighters are at high risk. Other sources include furnaces, gas-powered engines, pool heaters and wood stoves.
CO combines preferentially with hemoglobin to produce carboxyhemoglobin, displacing oxygen and reducing systemic arterial oxygen content. CO binds reversibly to hemoglobin with an affinity more than 200 times that of oxygen. Inhaled CO rapidly diffuses across the alveolar-capillary membranes into the bloodstream, where the reversible binding with hemoglobin occurs and carboxyhemoglobin is formed. Carboxyhemoglobin decreases the amount of hemoglobin available for oxygen transport and also results in decreased release of oxygen to tissues.
Symptoms of acute CO poisoning are more dramatic than those of chronic exposure. Subacute or chronic CO poisoning may present less characteristic symptoms and patients may initially be misdiagnosed. The most common misdiagnosis is “flu-like” syndrome. At low carboxyhemoglobin levels in chronic CO poisoning, chronic cardiopulmonary problems may be exacerbated. Therefore, chest pains caused by reduced myocardial oxygen delivery due to CO poisoning may be misdiagnosed.
Acute exposure to CO correlates to various symptoms. At carboxyhemoglobin levels of above 10% the victim may be asymptomatic or have a headache. At 20%, CO exposure causes dizziness, confusion and nausea. Between 20 and 50% carboxyhemoglobin levels, the subject experiences visual disturbances, confusion and syncope. At levels above 50% the subject experiences seizures and coma, and death is likely at levels of carboxyhemoglobin above 60%. Sublethal acute exposure leaves some victims with permanent neurologic sequelae.
When CO poisoning is suspected, the diagnosis is usually established either by detection of abnormally high CO in expired air or by analysis of arterial or venous blood for carboxyhemoglobin. Both of these techniques require instrumentation that is not readily available to paramedics or emergency rooms. In addition, because of the lapse of time between the exposure and the test, confirming the diagnosis may be difficult in some patients. Carboxyhemoglobin levels as tested by the lab may be low or undetectable because of the time elapsed between the exposure and taking of the sample.
The analysis of arterial and venous blood samples requires taking a blood sample by arterial or veni puncture or by finger prick, which raises small, but important concerns regarding pain and the potential for transmission of infectious disease, such as vial hepatitis and human immunodeficiency virus (HIV) infection. In addition, analysis of the arterial or venous blood sample is usually done by spectrophotometric means, as disclosed in U.S. Pat. Nos. 4,997,769 and 5,491,341. Such analytic methods require bulky instrumentation.
Tests done after the exposure must be treated with caution. While elevated carboxyhemoglobin levels found by testing blood samples will confirm the diagnosis of CO intoxication, low and moderately increased values must be interpreted with caution. The half-life of carboxyhemoglobin is about four hours when breathing room air and about one hour when breathing pure oxygen. Thus, the carboxyhemoglobin level obtained upon taking the blood sample must be used to extrapolate to the patient's peak level.
The recent marketing of inexpensive home ambient air CO monitors has increased the frequency with which CO poisoning is reported, many times inaccurately. The increase of such reports requires an increase in EMS visits to investigate. For example, during two cold spells in 1994, the Chicago Fire Department logged over 50,000 calls for suspected CO poisoning, most of which were not corroborated, and many of which resulted in Emergency Room visits to exclude CO intoxication.
The subject must be removed from the source of CO if exposure is suspected. Supplemental oxygen, ventilatory support and monitoring for cardiac arrhythmias are the mainstays of therapy for CO poisoning. Administration of 100% oxygen is usually done as soon as CO poisoning is suspected and before laboratory confirmation is obtained. Since many hospitals send blood samples to distant labs for analysis of carboxyhemoglobin levels, the treating physician must initiate treatment empirically.
Transportation to an appropriate center for hyperbaric oxygen therapy is a method of treatment in severe cases. Hyperbaric oxygen at a pressure of 3 atmospheres reduces the elimination half-life of carboxyhemoglobin to less than 30 minutes and can dissolve enough oxygen to sustain life even in the absence of hemoglobin. In such severe cases of CO poisoning, time is of the essence. There are a limited number of hyperbaric oxygen centers available. The location of a suitable center and transportation of the subject can take some time. Therefore, waiting for lab analysis can be a severe limitation on the treatment of such a subject.
Typical lab analyses include analysis by CO-oximeters and/or gas chromatography. However, as stated above, the drawbacks of relying on these methods are the need to draw arterial blood and the prolonged delay between sample acquisition and the availability of the laboratory results. CO-oximeters are laboratory instruments that measure oxy-, reduced-, carboxy-, and met-hemoglobins (and sometimes also sulf-hemoglobin) in blood samples by analyzing absorbance at multiple wavelengths, chosen to optimally separate the various types of hemoglobin. Mahoney J J, Vreman H J, Stevenson D K, Van Kessel A L. Measurement of carboxyhemoglobin and total hemoglobin by five specialized spectrophotometers (CO-oximeters) in comparison with reference methods,
Clinical Chemistry,
39:1693-1700 (1993)(incorporated by reference); Steinke J M, Shepherd A P, Effects of temperature on optical absorbance spectra of oxy-, carboxy-, and deoxyhemoglobin,
Clinical Chemistry,
38:1360-4 (1992)(incorporated by reference
Essential Medical Devices, Inc.
Patterson Belknap Webb & Tyler LLP
Winakur Eric F.
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