Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2001-02-06
2003-11-18
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C128S204210, C128S204230, C128S205170
Reexamination Certificate
active
06648832
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to non-invasive means of determining cardiac output or pulmonary capillary blood flow in patients and, more specifically, to partial re-breathing systems and methods for determining cardiac output or pulmonary capillary blood flow in patients.
2. Statement of the Art
It is important in many medical procedures to determine or monitor the cardiac output or the pulmonary capillary blood flow of a patient. Cardiac output as the volume of blood pumped by the heart over a given period of time. Pulmonary capillary blood flow is the volume of blood that participates in gas exchange in the lungs. Techniques are known and used in the art which employ the use of catheters inserted into blood vessels at certain points (e.g., into the femoral artery, the jugular vein, etc.) to monitor blood temperature and pressure and to thereby determine the cardiac output or pulmonary capillary blood flow of the patient. Although such techniques can produce a reasonably accurate result, the invasive nature of these procedures has a high potential for causing morbidity or mortality.
Adolph Fick's formula for calculating cardiac output, which was first proposed in 1870, has served as the standard by which other means of determining cardiac output and pulmonary capillary blood flow have since been evaluated. Fick's well-known equation, which is also referred to as the Fick Equation, written for carbon dioxide (CO
2
), is:
Q
=
V
CO
2
(
C
v
CO
2
-
C
a
CO
2
)
,
where Q is cardiac output, V
CO
2
is the amount of CO
2
excreted by the lungs, or “CO
2
elimination,” and Ca
CO
2
and Cv
CO
2
are the CO
2
contents of arterial blood and venous blood, respectively. Notably, the Fick Equation presumes an invasive method (i.e., catheterization) of calculating cardiac output or pulmonary capillary blood flow because the arterial blood and mixed venous blood must be sampled in order to directly determine the CO
2
contents of arterial blood and venous blood.
It has been shown, however, that by using the principles embodied in the Fick Equation, non-invasive means may be employed to determine cardiac output or pulmonary capillary blood flow. That is, expired CO
2
levels, measured in terms of fraction of expired gases that comprise CO
2
(f
CO
2
) or in terms of partial pressure of CO
2
(P
CO
2
), can be monitored and employed to estimate the content of CO
2
in the arterial blood. Thus, a varied form of the Fick Equation may be employed to estimate cardiac output or pulmonary capillary blood flow based on observed changes in f
CO
2
or P
CO
2
.
An exemplary use of the Fick Equation to non-invasively determine cardiac output or pulmonary capillary blood flow includes comparing a “standard” ventilation event to a change in expired CO
2
values and a change in excreted volume of CO
2
, which is referred to as carbon dioxide elimination or CO
2
elimination (V
CO
2
), which may be caused by a sudden change in ventilation. Conventionally, a sudden change in effective ventilation has been caused by having a patient inhale or breathe a volume of previously exhaled air. This technique is typically referred to as “re-breathing.”
Some re-breathing techniques have used the partial pressure of end-tidal CO
2
(Pet
CO
2
or et
CO
2
) to approximate the content of CO
2
in the arterial blood of a patient while the patient's lungs act as a tonometer to facilitate the measurement of the CO
2
content of the venous blood of the patient.
By further modification of the Fick Equation, it may be assumed that the CO
2
content of the patient's venous blood does not change within the time period of the perturbation. Thus, the need to directly calculate the CO
2
content of venous blood was eliminated by employing the so-called “partial re-breathing” method. (See, Capek et al., “Noninvasive Measurement of Cardiac Output Using Partial CO
2
Rebreathing”,
IEEE Transactions on
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 volume of carbon dioxide inhaled and exhaled may each be corrected for any deadspace or for any intrapulmonary shunt.
The partial pressure of end tidal carbon dioxide is also measured in re-breathing 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 (P
A
CO
2
) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (Pa
CO
2
). Conventionally employed Fick methods of determining cardiac output or pulmonary capillary blood flow typically include a direct, invasive determination of Cv
CO
2
by analyzing a sample of the patient's mixed venous blood. The re-breathing process is typically employed to either 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) or determine the partial pressure of carbon dioxide in the patient's venous blood (Pv
CO
2
).
Re-breathing processes typically include the inhalation of a gas mixture that includes carbon dioxide. During re-breathing, the carbon dioxide elimination of a patient typically decreases. In total re-breathing, carbon dioxide elimination decreases to near zero. In partial re-breathing, carbon dioxide elimination does not cease. Thus, in partial re-breathing, the decrease in carbon dioxide elimination is not as large as that of total re-breathing.
Re-breathing can be conducted with a re-breathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide.
FIG. 1
schematically illustrates a conventional ventilation system that is typically used with patients who require assisted breathing during an illness, during a surgical procedure, or during recovery from a surgical procedure. The conventional ventilator system
10
includes a tubular portion
12
that may be inserted into the trachea of a patient by known intubation procedures. The end
14
(i.e., the end most distant from the patient) of the tubular portion
12
may be fitted with a Y-piece
16
that interconnects an inspiratory hose
18
and an expiratory hose
20
. Both the inspiratory hose
18
and expiratory hose
20
may be connected to a ventilator machine (not shown), which delivers air into the breathing circuit through the inspiratory hose
18
. A one-way valve
22
is positioned on the inspiratory hose
18
to prevent exhaled gas from entering the inspiratory hose
18
beyond the valve
22
. A similar one-way valve
24
on the expiratory hose
20
limits movement of inspiratory gas into the expiratory hose
20
. Exhaled air flows passively into the expiratory hose
20
.
With reference to
FIG. 2
, an exemplary known re-breathing ventilation circuit
30
is shown. Re-breathing circuit
30
includes a tubular portion
32
insertable into the trachea of a patient by known intubation procedures. Gases may be provided to the patient from a ventilator machine (not shown) via an inspiratory hose
34
interconnected with tubular portion
32
by a Y-piece
36
. Tubular portion
32
and an expiratory hose
38
are also interconnected by Y-piece
36
. An additional length of hose
40
is provided in flow communication with the tubular portion
32
, between the tubular portion
32
and the Y-piece
36
, and acts as a deadspace for receiving exhaled gas. A three-way valve
42
, generally positioned between the Y-piece
36
and the opening to the additional length of hose
40
, is constructed for intermittent actuation to .selectively direct the flow of gas into or from the additional length of hose
40
. That is, at one setting, the v
Jaffe Michael B.
Kofoed Scott A.
Orr Joseph A.
Westenskow Dwayne
Nasser Robert L.
NTC Technology Inc.
TraskBritt
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