Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-07-28
2003-08-19
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S324000, C600S529000
Reexamination Certificate
active
06609016
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to methods and devices for use in clinical medicine particularly to provide the integration of the measurement of oxygen saturation of arterial blood and the associated minute ventilation from which the oxygen saturation is derived to enhance the clinical utility of the oxygen saturation measurement.
The brief measurement of oxygen saturation by pulse oximetry in the assessment of shortness of breath is a standard medical practice. This is commonly known as “spot (oximetry,” as is differentiated from continuous oximetry monitoring in that the oxygen saturation measurement is made for a brief period of time (such as 30 seconds to 5 minutes) and then generally reported as a single value (such as the average oxygen saturation or lowest oxygen saturation). There has been a trend toward smaller, more compact oximeters, which do not record or print out the saturation values but rather provide a digital reading of the number (such as “96%”). Unfortunately, this trend toward using a single spot “number,” while more simple and convenient, does not take into account the dynamic interrelated mechanisms operative to generate the oximetry number.
Spot oximetry provides important information, is useful as a screening test, and is often employed as an additional “routine 5
th
vital sign” along with temperature, pulse, respiration, and blood pressure. Unfortunately, spot oximetry measurement has serious limitations which are often not considered by physicians and respiratory therapist is ordering them. To understand the clinical significance of these limitations and their potential to adversely affect patient care, it is important to understand how spot oximetry is used in clinical medicine on the hospital wards.
Spot oximetry provides important information, is useful as a screening test, and is often employed as an additional “routine 5
th
vital sign” along with temperature, pulse, respiration, and blood pressure. Unfortunately spot oximetry measurement has serious limitations which are often not consider by the physicians and respiratory therapist ordering them. To understand the clinical significance of these limitations and their potential to adversely affect patient care, it is important to understand how spot oximetry is used in clinical medicine on the hospital wards.
Commonly, indeed thousands of times each day, a respiratory therapist, intern, resident or other physician is called to see a hospitalized patient for the evaluation shortness of breath. Often the first diagnostic test ordered is a spot oximetry test. Many medical personnel think of this measurement as pivotal. The thinking goes like this, “if the spot oximetry measurement is within the normal range then the shortness of breath is much less likely to reflect a problem which is life threatening.” This is a common misconception and leads to the general and widespread use of spot oximetry on hospital wards as a test to assess the immediate clinical significance of shortness of breath. Unfortunately, as will be discussed, spot oximetry is a poor indicator of the immediate clinical significance of shortness of breath and a normal spot oximetry value commonly provides the health care worker with a false sense of security which can lead to a poor patient outcome due to delayed diagnosis.
The present inventor has long attempted to instruct interns and medical residents of the hazards of using spot oximetry in this way. But teaching alone cannot correct this ubiquitous problem. Even within the hospital of the present inventor, each year the stability of the respiratory state of many patients is misconstrued by interns and respiratory therapists by the false sense of security provided by a normal spot oximetry value, and timely patient treatment is delayed. Given the frequency of this problem within a hospital wherein active teaching of the hazards of spot oximetry is provided, it is likely that nationwide, thousands of patients have delayed intervention every year due to the improper interpretation of the significance of a normal spot oximetry measurement.
When considering the management of critically ill patients, the timing of intervention is of paramount importance. The value of any diagnostic test is in part determined by the ability of the test to predict adverse physiologic events early in the course of the disease process wherein intervention is more likely to be effective. This is particularly true of the diseases associated with infections such as toxic shock, sepsis, and pneumonia or with thromboembolic disease. Regrettably, with respect to early identification of impending respiratory failure or severe respiratory dysfunction, spot oximetry is very poor.
The lack of utility of spot oximetry as an early indicator of impending respiratory failure has as it's physiologic basis the position the overall significance arterial oxygen saturation has to human cellular respiration and survival. The arterial saturation, as determined by the spot oximetry test, is one of the most important parameters defining the sufficiency of lung function and oxygen delivery to the tissues. This is one of the reasons that physicians give a normal reading such significance, but it is also the reason that the human body protects oxygen delivery by keeping the value of arterial oxygen saturation close to the normal value till death is very near. In humans, a primary protective mechanism is immediately operative to keep the arterial oxygen saturation value high until late in the course of critical illness such that the patient will increase the volume of air breathed per minute which will “blow off carbondioxide” and which will raise the oxygen saturation. (With a normal respiratory quotient the partial pressure of oxygen increases by 1.2 mm for each 1 mm the CO2 falls, while the associated rise in oxygen saturation in response to a fall in CO2 is dependent on the position of the oxygen saturation on the oxyhemoglobin dissociation curve). This often occurs early in the course of the illness (such as sepsis) before any fall in oxygen saturation has occurred and can be seen as a “preventive” survival mechanism protecting the patient against a fall in oxygen saturation before it occurs. The fall in CO2 caused by the increased minute ventilation also increases the affinity of hemoglobin for oxygen, this means that at any given level of partial pressure of oxygen, the arterial oxygen saturation as measured by a spot oximetry device is higher. In other words, despite a decline in the lung's ability to keep the dissolved arterial oxygen in the normal range the spot oximetry reading can still be entirely normal due to this compensatory mechanism.
A final confounding factor is anxiety. Anxiety causes an increase in minute ventilation, which raises the spot oximetry value due to the mechanisms noted supra. Unfortunately, in some patients, the increased minute ventilation can be associated to a greater extent with larger volume breaths than with a marked increase in respiratory rate. For this reason, increased minute ventilation may be subtle and not recognized during clinical assessment. The classic case is the anxious young woman with a vaginal discharge. The spot oximetry value may be normal even as she is dying of toxic shock. Her body is protecting the oxygen delivery to the tissues and the anxiety is driving the CO2 lower further increasing the spot oximetry value. But the intern, or emergency room physician, may misconstrue the anxiety and the increased minute ventilation and fail to recognize the impact it has on the spot oximetry value. The physician seeing a perfect spot oximetry value of 97% for example may feel reassured that the patient is not in danger of respiratory failure during the night. However, with toxic shock there is no time to lose. When the compensating mechanisms are exhausted, such patients deteriorate rapidly, often over 6-12 hours, too rapidly for the physician seeing the patient the next day to intervene.
In addition to the problems noted above there is a
Kremer Matthew
Nixon & Vanderhye P.C.
Winakur Eric F.
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