Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
1998-09-09
2001-05-29
O'Connor, Cary (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S529000, C600S538000, C128S200240
Reexamination Certificate
active
06238351
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of compensating for non-metabolic changes in one or more respiratory or blood gas profile parameters of a patient, such as non-metabolic changes caused by changes in ventilation or breathing. Particularly, the present invention relates to methods of compensating for non-metabolic changes in one or more respiratory profile parameters that may be continuously, non-invasively measured. Specifically, the method of the present invention is useful during unstable breathing events for compensating for non-metabolically altered carbon dioxide elimination measurements.
2. Background of Related Art
Many conventional techniques for determining respiratory and cardiac profile parameters may only be performed on an intermittent basis. For example, conventional methods of measuring the cardiac output of a patient, such as indicator dilution and re-breathing techniques, are performed intermittently. Both indicator dilution and re-breathing are useful for determining the cardiac output of a patient.
Indicator dilution, an exemplary invasive, typically intermittent technique for measuring cardiac output, includes introducing a predetermined amount of an indicator into the bloodstream through the heart of a patient and analyzing blood downstream from the point of introduction to obtain a time vs. dilution curve. Thermodilution, in which a room temperature or colder saline solution, which may also be referred to as “cold” saline, is employed as the indicator, is a widely employed type of indicator dilution. Typically, the cold saline is introduced into the right heart bloodstream of a patient through a thermodilution catheter, which includes a thermistor at the end thereof. The thermistor is employed to measure the temperature of the blood after it has passed through the right heart, or downstream from the point at which the cold saline is introduced. A thermodilution curve is then generated from the data, from which the cardiac output of the patient may be derived. Such invasive measurement of cardiac output is, however, somewhat undesirable due to the potential for harming the patient that is typically associated with the introduction and maintenance of a catheter in the pulmonary artery.
Thus, non-invasive techniques for determining cardiac output and pulmonary capillary blood flow have been developed. Cardiac output includes the flow of blood that participates in gas exchange, which is typically referred to as pulmonary capillary blood flow, and the flow of blood that does not participate in the gas exchange, which is typically referred to as intrapulmonary shunt flow, or venous admixture.
The pulmonary capillary blood flow of a patient has been non-invasively determined by employing various respiratory, blood, and blood gas profile parameters in a derivation of the Fick equation, typically either the oxygen (O
2
) Fick equation or the carbon dioxide (CO
2
) Fick equation, such as by the use of total or partial re-breathing.
The carbon dioxide Fick equation, which may be employed to non-invasively determine the cardiac output of a patient, follows:
Q
t
=V
CO
2
/(C
V
CO
2
—CaCO
2
),
where Q
t
is the cardiac output of the patient, V
CO
2
is the carbon dioxide elimination of the patient, C
V
CO
2
is the carbon dioxide content of the venous blood of the patient, and CaCO
2
is the carbon dioxide content of the arterial blood of the patient.
The carbon dioxide elimination of the patient may be non-invasively measured as the difference per breath between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration, and is typically calculated as the integral of the carbon dioxide signal times the rate of flow over an entire breath. The volumes of carbon dioxide inhaled and exhaled may each be corrected for any deadspace or intrapulmonary shunt flow.
The partial pressure of end-tidal carbon dioxide (PetCO
2
or etCO
2
) is also measured in re-breathing processes. The partial pressure of end-tidal carbon dioxide during normal breathing, after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveolar blood (P
A
CO
2
) of the patient or, if there is no intrapulmonary shunt flow or parallel deadspace, the partial pressure of carbon dioxide in the arterial blood (PaCO
2
) of the patient.
Conventionally employed Fick methods of determining cardiac output typically include a direct, invasive determination of C
V
CO
2
by analyzing a sample of the patient's mixed venous blood. A re-breathing process is typically employed either to non-invasively estimate the carbon dioxide content of mixed venous blood (in total re-breathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial re-breathing). Re-breathing processes typically include the inhalation of a gas mixture which includes carbon dioxide. During re-breathing, the carbon dioxide elimination of the patient decreases. In total re-breathing, the carbon dioxide elimination of the patient decreases to near zero. In partial re-breathing, the carbon dioxide elimination of the patient does not cease. Thus, the decrease of carbon dioxide elimination in partial re-breathing is not as significant as that in total re-breathing.
Re-breathing is typically conducted with a re-breathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide.
FIG. 1
schematically illustrates an exemplary re-breathing circuit
50
that includes a tubular airway
52
that communicates air flow to and from the lungs of a patient. Tubular airway
52
may be placed in communication with the trachea of the patient by known intubation processes, or by connection to a breathing mask positioned over the nose and/or mouth of the patient. A flow meter
72
, which is typically referred to as a pneumotachometer, and a carbon dioxide sensor
74
, which is typically referred to as a capnometer, are disposed between tubular airway
52
and a length of hose
60
, and are exposed to any air that flows through re-breathing circuit
50
. Both ends of another length of hose, which is referred to as deadspace
70
, communicate with hose
60
. The two ends of deadspace
70
are separated from one another by a two-way valve
68
, which may be positioned to direct the flow of air through deadspace
70
. Deadspace
70
may also include an expandable section
62
. A Y-piece
58
, disposed on hose
60
opposite flow meter
72
and carbon dioxide sensor
74
, facilitates the connection of an inspiratory hose
54
and an expiratory hose
56
to re-breathing circuit
50
and the flow communication of the inspiratory hose
54
and expiratory hose
56
with hose
60
. During inhalation, gas flows into inspiratory hose
54
from the atmosphere or a ventilator (not shown). During normal breathing, valve
68
is positioned to prevent inhaled and exhaled air from flowing through deadspace
70
. During re-breathing, valve
68
is positioned to direct the flow of exhaled and inhaled gases through deadspace
70
. During total re-breathing, the partial pressure of end-tidal carbon dioxide (PetCO
2
) is typically assumed to be equal to the partial pressure of carbon dioxide in the venous blood (P
V
CO
2
) of the patient, as well as to the partial pressures of carbon dioxide in the alveolar blood (P
A
CO
2
) and arterial blood (PaCO
2
) of the patient. The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve.
In partial re-breathing, measurements during normal breathing and subsequent re-breathing are substituted into the carbon dioxide Fick equation. This results in a system of two equations and two unknowns (carbon dioxide content in the mixed venous blood and cardiac output), from which pulmonary capillary blood flow can be determined without knowing the carbon dioxide content of the mixed venous blood.
The inabilit
Haryadi Dinesh G.
Jaffe Michael B.
Kuck Kai
Orr Joseph A.
Astorino Michael
Britt Trask
NTC Technology Inc.
O'Connor Cary
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