System and methods for measuring oxygenation parameters

Details System and methods for measuring oxygenation parameters System and methods for measuring oxygenation parameters

1996-12-11

2002-05-21

Nasser, Robert L. (Department: 3736)

Surgery

Diagnostic testing

C600S505000, C600S526000, C600S483000

Reexamination Certificate

active

06390977

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to systems and methods for non-invasively determining physiological parameters related to the oxygenation status of a patient. More specifically, the invention is directed to systems and methods for the real time determination of parameters associated with global tissue oxygenation in a subject.
BACKGROUND OF THE INVENTION
A problem that has long troubled physicians is how to accurately measure the oxygenation state of a patient's tissues without resorting to an invasive procedure. This is important during many medical procedures because the physician needs to know when to administer medicaments or transfuse more blood into a patient. When the oxygenation state of a patient's tissues is low, the physician may wish to transfuse more blood or other oxygen carriers to increase the oxygen transportation rate and maintain adequate cellular respiration.
In the surgical and postoperative settings, decisions regarding the need for blood transfusion normally are guided by hemoglobin (Hb) or hematocrit levels (Hct). Hematocrit is typically defined as the percentage by volume of packed red blood cells following centrifugation of a blood sample. If the hemoglobin level per deciliter of blood in the patient is high, the physician can infer that the patient has sufficient capacity to carry oxygen to the tissue. During an operation this value is often used as a trigger; i.e. if the value falls below a certain point, additional blood is given to the patient. While these parameters provide an indication of the arterial oxygen content of the blood, they provide no information on the total amount of oxygen transported (or “offered”) to the tissues, or on the oxygen content of blood coming from the tissues.
For example, it has been shown that low postoperative hematocrit may be associated with postoperative ischemia in patients with generalized atherosclerosis. Though a number of researchers have attempted to define a critical Hct level, most authorities would agree that an empirical automatic transfusion trigger, whether based on Hb or Hct, should be avoided and that red cell transfusions should be tailored to the individual patient. The transfusion trigger, therefore, should be activated by the patient's own response to anemia rather than any predetermined value.
This is, in part, due to the fact that a number of parameters are important in determining how well the patient's tissues are actually oxygenated. In this regard, the patient's cardiac output is also an important factor in correlating hemoglobin levels with tissue oxygenation states. Cardiac output or CO is defined as the volume of blood ejected by the left ventricle of the heart into the aorta per unit of time (ml/min) and can be measured with thermodilution techniques. For example, if a patient has internal bleeding, the concentration of hemoglobin in the blood might be normal, but the total volume of blood will be low. In this situation, due to the inadequate venous return of blood to the heart, the cardiac output decreases to provide better circulation to the tissues. Accordingly, simply measuring the amount of hemoglobin in the blood without measuring other parameters such as cardiac output is not always sufficient for estimating the actual oxygenation state of the patient.
More specifically the oxygenation status of the tissues is reflected by the oxygen supply/demand relationship of those tissues i.e., the relationship of total oxygen transport (DO
2
) to total oxygen consumption (VO
2
). Hemoglobin is oxygenated to oxyhemoglobin in the pulmonary capillaries and then carried by the cardiac output to the tissues, where the oxygen is consumed. As oxyhemoglobin releases oxygen to the tissues, the partial pressure of oxygen (PO
2
) decreases until sufficient oxygen has been released to meet the oxygen consumption (VO
2
). Although there have been advances in methods of determining the oxygenation status of certain organ beds (e.g., gut tonometry; near infrared spectroscopy) these methods are difficult to apply in the clinical setting. Therefore, the use of parameters that reflect the oxygenation status of the blood coming from the tissues i.e., the partial pressure of oxygen in the mixed venous blood (PvO
2
; also known as the mixed venous blood oxygen tension) or mixed venous blood oxyhemoglobin saturation (SvO
2
) has become a generally accepted practice for evaluating the global oxygenation status of the tissues.
Unfortunately, relatively invasive techniques are necessary to provide more accurate tissue oxygenation levels. In this respect, direct measurement of the oxygenation state of a patient's mixed venous blood during surgery may be made using pulmonary artery catheterization. To fully assess whole body oxygen transport and delivery, one catheter (a flow directed pulmonary artery [PA] catheter) is placed in the patient's pulmonary artery and another in a peripheral artery. Blood samples are then drawn from each catheter to determine the pulmonary artery and arterial blood oxygen levels. As previously discussed, cardiac output may also be determined using the PA catheter. The physician then infers how well the patient's tissue is oxygenated directly from the measured oxygen content of the blood samples.
While these procedures have proven to be relatively accurate, they are also extremely invasive. For example, use of devices such as the Swan-Ganz® thermodilution catheter (Baxter International, Santa Ana, Calif.) can lead to an increased risk of infection, pulmonary artery bleeding, pneumothorax and other complications. Further, because of the risk and cost associated with PA catheters, their use in surgical patients is restricted to high-risk or high-blood-loss procedures (e.g., cardiac surgery, liver transplant, radical surgery for malignancies) and high-risk patients (e.g., patients who are elderly, diabetic, or have atherosclerotic disease).
Among other variables, determination of the oxygenation status of the tissues should include assessment of the amount of blood being pumped toward the tissues (CO) and the oxygen content of that (arterial) blood (CaO
2
). The product of these variables may then be used to provide a measure of total oxygen transport (DO
2
). Currently, assessment of DO
2
requires the use of the invasive monitoring equipment described above. Accordingly, determination of DO
2
is not possible in the majority of surgical cases. However, in the intensive care unit (ICU), invasive monitoring tends to be a part of the routine management of patients; thus, DO
2
determinations are obtained more readily in this population.
Partial pressure of oxygen in the mixed venous blood or mixed venous blood oxygen tension (PvO
2
) is another important parameter that may be determined using a PA catheter. Because of the equilibrium that exists between the partial pressure of oxygen (PO
2
) in the venous blood and tissue, a physician can infer the tissue oxygenation state of the patient. More specifically, as arterial blood passes through the tissues, a partial pressure gradient exists between the PO
2
of the blood in the arteriole passing through the tissue and the tissue itself. Due to this oxygen pressure gradient, oxygen is released from hemoglobin in the red blood cells and also from solution in the plasma; the released O
2
then diffuses into the tissue. The PO
2
of the blood issuing from the venous end of the capillary cylinder (PvO
2
) will generally be a close reflection of the PO
2
at the distal (venous) end of the tissue through which the capillary passes.
Closely related to the mixed venous blood oxygen tension (PvO
2
) is the mixed venous blood oxyhemoglobin saturation (SvO
2
) which is expressed as the percentage of the available hemoglobin bound to oxygen. Typically, oxyhemoglobin disassociation curves are plotted using SO
2
values vs. PO
2
values. As the partial pressure of oxygen (PO
2
) decreases in the blood (i.e. as it goes through a capillary) there is a corresponding decrease in the oxygen saturation

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