Bi-directional partial re-breathing method

Details Bi-directional partial re-breathing method Bi-directional partial re-breathing method

1998-09-09

2001-03-13

O'Connor, Cary (Department: 3736)

Surgery

Diagnostic testing

Respiratory

C600S529000, C600S500000, C128S204120

Reexamination Certificate

active

06200271

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of non-invasively determining the pulmonary capillary blood flow of a patient. Particularly, the present invention relates to re-breathing techniques for measuring the pulmonary capillary blood flow of a patient. More particularly, the methods of the present invention account for changes in the carbon dioxide content of the venous blood of a patient that may occur during re-breathing.
2. Background of Related Art
Cardiac output, the volume of blood that is pumped by the heart over a set period of time, includes two components, pulmonary capillary blood flow (Q
pcbf
) and intrapulmonary shunt (Q
s
). Pulmonary capillary blood flow is the volume of blood (typically measured in liters) that participates in the exchange of blood gases over a set period of time (typically one minute). Cardiac output is typically measured during surgery or while a patient is under intensive care and indicates the cardiovascular condition of the patient and the patient's response to medical intervention. Conventionally, cardiac output has been measured by both invasive and non-invasive techniques.
Indicator dilution, an exemplary invasive, typically intermittent technique for measuring cardiac output, includes introducing a predetermined amount of an indicator into a single point of the bloodstream of a patient and analyzing blood downstream from the point of introduction to obtain a time vs. dilution curve. Thermodilution, in which 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 an 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. Thermodilution and other indicator dilution techniques are, however, somewhat undesirable due to the potential for harm to the patient that is associated by inserting and maintaining such catheters in place.
Conventional, so-called “non-invasive” techniques for determining the cardiac output of a patient typically include a pulmonary capillary blood flow measurement according to the Fick principle: the rate of uptake of a substance by or release of a substance from blood at the lung is equal to the blood flow past the lung and the content difference of the substance at each side of the lung. The Fick principle may be represented in terms of oxygen (O
2
) by the following formula:
Q=V
O
2
/(
Ca
O
2
−Cv
O
2
),
where Q is the cardiac output of the patient, VO
2
is the volume of oxygen consumed by the patient per unit of time, CaO
2
is the O
2
content of the arterial, or oxygenated, blood of the patient, and CvO
2
is the O
2
content of the venous, or de-oxygenated, blood of the patient. The oxygen Fick principle may be employed in calculating the cardiac output of a patient either intermittently or continuously. The intrapulmonary shunt flow may also be estimated and subtracted from the cardiac output to determine the pulmonary capillary blood flow of the patient.
An exemplary method of determining the cardiac output of a patient by monitoring VO
2
is disclosed in Davies et al., Continuous Fick cardiac output compared to thermodilution cardiac output,
Crit. Care Med.
1986; 14:881-885 (“Davies”). The method of Davies includes continually measuring the O
2
content of samples of gas inspired and expired by a patient, the oxygen saturation (SvO
2
) of the patient's venous blood, and oxygen saturation (SaO
2
) of the patient's arterial blood. The O
2
measurements are made by a metabolic gas monitor, and VO
2
calculated from these measurements. SaO
2
is measured by pulse oximetry. SvO
2
is directly measured by a pulmonary artery (“PA”) catheter. Each of these values is then incorporated into the oxygen Fick equation to determine the cardiac output of the patient. Although the method of Davies may be employed to intermittently or continuously determine the cardiac output of a patient, it is somewhat undesirable from the standpoint that accurate VO
2
measurements are typically difficult to obtain, especially when the patient requires an elevated fraction of inspired oxygen (FiO
2
). Moreover, since the method disclosed in Davies requires continual measurement of SvO
2
with a pulmonary artery catheter, it is an invasive technique.
Due in part to the ease with which the carbon dioxide elimination (VCO
2
) of a patient may be accurately measured, VCO
2
measurements are widely employed in methods of non-invasively determining the pulmonary capillary blood flow of a patient. Since the respiratory quotient (RQ) is the ratio of carbon dioxide elimination to the amount of oxygen inhaled, VCO
2
may be substituted for VO
2
according to the following equation:
V
O
2
=V
CO
2
/RQ.
Alternatively, a modification of the Fick principle, which is based on the exchange of carbon dioxide (CO
2
) in the lungs of a patient, has been employed to calculate the pulmonary capillary blood flow of the patient. The carbon dioxide Fick equation, which represents the Fick principle in terms of CO
2
elimination and exchange, follows:
Q=V
CO
2
/(
Cv
CO
2
−Ca
CO
2
),
where VCO
2
is the carbon dioxide elimination of the patient, CvCO
2
is the content, or concentration, of CO
2
in the venous blood of the patient, and CaCO
2
is the content, or concentration, of CO
2
in the arterial blood of the patient. The difference between CvCO
2
and CaCO
2
is typically referred to as the arterial-venous gradient, or “AV gradient”.
The carbon dioxide Fick equation has been employed to non-invasively determine the pulmonary capillary blood flow and cardiac output of a patient on an intermittent basis. 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.
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, 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
).
Re-breathing 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) or determine the partial pressure of carbon dioxide in the patient's venous blood (PvCO
2
). Re-breathing processes typically include the inhalation of a gas mixture which includes carbon dioxide. During re-breathing, the CO
2
elimination of the patient is less than during normal breathing. Re-breathing during which the CO
2
elimination decreases to near zero is typically referred to as total re-breathing. Re-breathing that causes some decrease, but not a total cessation of CO
2
elimination, is typically referred to as partial 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 ai

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