Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2000-02-22
2003-04-01
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S532000, C600S504000
Reexamination Certificate
active
06540689
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for accurately, noninvasively measuring the pulmonary capillary blood flow (PCBF), cardiac output, and mixed venous carbon dioxide content of the blood of a patient. Particularly, the present invention relates to a method for noninvasively measuring pulmonary capillary blood flow or cardiac output that employs an algorithm to increase the accuracy of data upon which the pulmonary capillary blood flow or cardiac output measurement is based.
2. Background of Related Art
Carbon dioxide elimination (V
CO
2
) is the volume of carbon dioxide (CO
2
) excreted from the body of a patient during respiration. Conventionally, carbon dioxide elimination has been employed as an indicator of metabolic activity. Carbon dioxide elimination has also been used in rebreathing methods of determining pulmonary capillary blood flow and cardiac output.
The carbon dioxide Fick equation:
Q=V
CO
2
/(CvCO
2
−CaCO
2
), (1)
where Q is cardiac output, CvCO
2
is 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, has been employed to noninvasively determine the pulmonary capillary blood flow or cardiac output of a patient. The carbon dioxide elimination of the patient may be noninvasively 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, or the fraction of respiratory gases that comprises carbon dioxide, or “carbon dioxide fraction”, times the rate of flow over an entire breath.
The partial pressure of end-tidal carbon dioxide (PetCO
2
or etCO
2
) is also measured in rebreathing processes. The partial pressure of end-tidal carbon dioxide, after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PACO
2
) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (PaCO
2
).
Rebreathing is typically employed either to noninvasively estimate the carbon dioxide content of mixed venous blood (as in total rebreathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial rebreathing). Rebreathing processes typically include the inhalation of a gas mixture that includes carbon dioxide. During rebreathing, the carbon dioxide elimination of the patient decreases to a level less than during normal breathing. Rebreathing during which the carbon dioxide elimination decreases to near zero is typically referred to as total rebreathing. Rebreathing that causes some decrease, but not a total cessation of carbon dioxide elimination, is typically referred to as partial rebreathing.
Rebreathing is typically conducted with a rebreathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide.
FIG. 1
schematically illustrates an exemplary rebreathing 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 rebreathing 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 rebreathing 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 rebreathing, valve
68
is positioned to direct the flow of exhaled and inhaled gases through deadspace
70
.
The rebreathed air, which is inhaled from deadspace
70
during rebreathing, includes air that has been exhaled by the patient (i.e., carbon dioxide-rich air).
During total rebreathing, substantially all of the gas inhaled by the patient was expired during the previous breath. Thus, during total rebreathing, the partial pressure of end-tidal carbon dioxide (PetCO
2
or etCO
2
)is typically assumed to be equal to or closely related to the partial pressure of carbon dioxide in the arterial (PaCO
2
), venous (PvCO
2
), or alveolar (PACO
2
) blood of the patient. Total rebreathing processes are based on the assumption that neither pulmonary capillary blood flow nor the content of carbon dioxide in the venous blood of the patient (CvCO
2
) changes substantially during the rebreathing process. 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, where the change in the carbon dioxide content of the blood (CvCO
2
−CaCO
2
)is equal to the slope (s) of the carbon dioxide dissociation curve multiplied by the measured change in end-tidal carbon dioxide (PetCO
2
)as effected by a change in effective ventilation, such as rebreathing.
In partial rebreathing, the patient inhales a mixture of “fresh” gases and gases exhaled during the previous breath. Thus, the patient does not inhale a volume of carbon dioxide as large as the volume of carbon dioxide that would be inhaled during a total rebreathing process. Conventional partial rebreathing processes typically employ a differential form of the carbon dioxide Fick equation to determine the pulmonary capillary blood flow or cardiac output of the patient, which do not require knowledge of the carbon dioxide content of the mixed venous blood. This differential form of the carbon dioxide Fick equation considers measurements of carbon dioxide elimination, CvCO
2
, and the content of carbon dioxide in the alveolar blood of the patient (CACO
2
)during both normal breathing and the rebreathing process as follows:
Q
pcbfBD
=
V
CO
2
B
-
V
CO
2
D
(
Cv
CO
2
B
-
Cv
CO
2
D
)
-
(
Ca
CO
2
B
-
Ca
CO
2
D
)
,
(
2
)
where V
CO
2B
and V
CO
2D
are the carbon dioxide production of the patient before rebreathing and during the rebreathing process, respectively, CvCO
2B
and CvCO
2D
are the content of CO
2
of the venous blood of the patient before rebreathing and during the rebreathing process, respectively, and CaCO
2B
and CaCO
2D
are the content of CO
2
in the arterial blood of the patient before rebreathing and during rebreathing, respectively.
Again, with a carbon dioxide dissociation curve, the measured PetCO
2
can be used to determine the change in content of carbon dioxide in the blood before and during the rebreathing process. Accordingly, the following equation can be used to determine pulmonary capillary blood flow or cardiac output when partial rebreathing is conducted:
Q=&Dgr;V
CO
2
/s&Dgr;PetCO
2
. (3)
Alternative differential Fick methods of measuring pulmonary capillary blood flow or cardiac output have also been employed. Such differential Fick methods typically include a brief change of PetCO
2
and V
CO
2
in response to a change in effective ventilation. This brief change can be accomplished by adjusting the
Kück Kai
Orr Joseph A.
Nasser Robert L.
NTC Technology Inc.
TraskBritt
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