Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
1998-08-27
2001-06-26
Lacyk, John P. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S529000, C600S538000, C600S543000
Reexamination Certificate
active
06251082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for assessing the ventilatory status of a patient. More particularly, the present invention provides a system for implementing a non-invasive procedure for estimating the amount or concentration of dissolved carbon dioxide within the arterial portion of the vasculature. The arterial carbon dioxide content, expressed as a partial pressure, i.e., pCO
2
, is an important measure of ventilatory status which ultimately reflects pulmonary health.
2. State of the Art
Physicians and other health care providers often use elevated arterial pCO
2
(PaCO
2
) as an indicator of incipient respiratory failure. In this regard, the determination of PaCO
2
is useful in optimizing the settings on ventilators and detecting life-threatening blood gas changes in an anesthetized patient undergoing surgery. The traditional method for obtaining arterial blood gas values is to extract a sample of arterial blood and measure the partial pressure of carbon dioxide using a blood gas analyzer (PaCO
2
ABG
). Arterial puncture has inherent limitations: 1) arterial puncture carries a degree of patient discomfort and risk, 2) handling of the blood is a potential health hazard to health care providers, 3) significant delays are often encountered before results are obtained and, 4) measurements can only be made intermittently.
Continuous invasive monitoring requires in-dwelling arterial lines which entail inherent problems. These include sepsis, slow response times, and signal decay. The nature of this monitoring system excludes its use under routine care and is generally restricted to intensive care units within a hospital facility.
There have been attempts to assess PaCO
2
levels indirectly, including a technique known as capnography. The approach utilized in capnography involves tracking patient exhalation and measuring expiratory gas CO
2
concentration against time during one or more respiratory cycles. The resulting relationship is plotted to create a graph depicting three distinct phases in breath CO
2
gas concentration during the patient's exhale cycle. (See, FIG.
1
). Typically, the three phases reflect the clearing of the conducting airways which do not normally participate in gas exchange (i.e., airway dead space) (Phase I) followed by the exhalation of air from conducting airways dynamically mixed with lung gases from the active (alveoli) membrane surfaces within the lung that have undergone gas exchange with arterial blood (Phase II). The final phase (Phase III) reflects the exhalation of unmixed gas from regions of the lung which are normally in active exchange with the alveoli tissue. Phase III thus closely resembles (in healthy patients) gas properties associated with arterial blood in contact with the lung for gas exchange (CO
2
release and O
2
absorption). In normal lungs, the graph line of Phase III is substantially level (slope=0) since ventilated and perfused alveolar regions are closely matched. In a diseased lung, the Phase III line may not appear level due to a mismatch in ventilation and perfusion of this lung region. See, Table I below:
TABLE I
Phase I
Represents CO
2
-free gas expired ftom the airway conduction
structures where gas exchange does not occur,
Phase II
The S-shaped upswing in CO
2
concentration (expressed as a
percent) represents the transition from airway to alveolar
gas, and
Phase III
The alveolar plateau representing CO
2
rich gas from the
alveoli.
In the past, capnography has utilized the peak or end-tidal (PetCO
2
) values as an estimate of PaCO
2
. PetCO
2
is a measure of the mean alveolar partial pressure of carbon dioxide from all functional gas exchange units of the lung. PetCO
2
obtained from capnography is a measure of mean alveolar pCO
2
, which value approximates PaCO
2
in normal lungs. Because CO
2
readily diffuses across the alveolar-capillary membrane, the PetCO
2
closely approximates the PaCO
2
with normal ventilation-perfusion. The difference between PetCO
2
and PaCo
2
is primarily a function of the proportion of the lung where gas exchange does not occur (Fletcher, R., Johnson, G., and Brew, J., The Concept of Deadspace with Special Reference to Single Breath Test for Carbon Dioxide.
Br. J. Anaesth
., 53, 77, 1981). In patients afflicted with a lung disease, there often exists a proportional increase in the region of the lungs where gas exchange does not occur. This increase in so-called “alveolar dead space” results in a significant difference between peak CO
2
(PetCO
2
) obtained from capnography and elevated arterial CO
2
(PaCO
2
).
Other techniques have been utilized for assessing patient blood gas levels with mixed results. Transcutaneous sensors measure tissue pCO
2
diffused through the heated skin but have practical and theoretical limitations. Oximetry is a widely used, non-invasive method for estimating the arterial oxygen carried on hemoglobin. For example, U.S. Pat. Nos. 4,759,369, 4,869,254 and 5,190,038 describe pulse oximeters which measure the percentage of hemoglobin which is oxygenated. However, neither of the aforementioned techniques measures the amount of dissolved oxygen present, nor the amount of oxygen carried when hemoglobin levels are reduced. Low hemoglobin levels are found when there is a significant blood loss or when there is insufficient red blood cell formation. Additionally, oximeter readings are specific to the point of attachment, which is typically the finger tip or ear lobe, and may not reflect the oxygen level of vital organs during conditions such as shock or hypothermia.
There remains a significant need in the art for an accurate, non-invasive, sensitive method for accurately determining the levels of arterial blood gases. As will be seen hereinafter, the instant invention sets forth a non-invasive system to overcome the problems of the prior art.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a system to rapidly and accurately derive a patient's arterial carbon dioxide concentration and employs a non-invasive method for monitoring arterial partial pressure of carbon dioxide in a patient as an indicator of ventilatory status.
The present invention also comprises a system for detecting expiratory CO
2
concentration and volumetric rate data and accurately deriving actual arterial pCO
2
based thereon. The system affords non-invasive, substantially real time determination of blood gas concentrations as derived from current expiratory data as correlated with processed data collected from past expiratory measurements.
The present invention converts expiratory data from a time domain to a volume domain or accumulates data in the volume domain in the first instance. The arterial CO
2
partial pressures of a patient can then be ascertained by selectively analyzing the slope and intercept values associated with Phase II and Phase III expiratory data in the volume domain. If time domain data is converted to the volume domain, the conversion may be said to “normalize” the data by placing it in the volume domain, wherein time-dependent respiratory differences between patients are eliminated and a standardized gas concentration-to-incremental breath volume relationship is achieved.
The present invention also provides for the accurate determination of arterial CO
2
partial pressures by measuring expiratory gas data and statistically filtering this data to ascertain readings having the highest correlation to actual pulmonary performance with a statistically significant level of confidence.
The above and other advantages of the present invention are realized in a specifically delineated gas analysis and data processing system operated in accordance with select data qualifying and enhancing procedures. In particular, the inventive system provides for the collection of concise expiratory data from a patient undergoing treatment. This data includes details on CO
2
gas partial pressure, concentration, and total gas volume sampled during a breath exhaust cycle. Multiple readi
Lacyk John P.
Natnithithadha Navin
NTC Technology Inc.
Trask & Britt
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