Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
2003-01-21
2004-09-07
Patel, Mital B. (Department: 3743)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204220, C128S098100, C128S204180
Reexamination Certificate
active
06786217
ABSTRACT:
The invention generally relates to an apparatus and method for measurement, mixing, monitoring, and delivery of gases to a patient, including nitric oxide (“NO”) and oxygen. More specifically, the invention relates to an apparatus and method of delivering gaseous NO to spontaneous-breathing patients as well as to patients connected to a mechanical ventilator.
BACKGROUND OF THE INVENTION
NO is an environmental pollutant produced as a byproduct of combustion. At high concentrations (generally at or above 1000 ppm), NO is toxic. NO also is a naturally occurring gas that is produced by the endothelium tissue of the respiratory system. In the 1980's, it was discovered by researchers that the endothelium tissue of the human body produced NO, and that NO is an endogenous vasodilator, namely, an agent that widens the internal diameter of blood vessels.
With this discovery, numerous researchers have investigated the use of low concentrations of inhaled NO to treat various pulmonary diseases in human patients. See Higenbottam et al., Am. Rev. Resp. Dis. Suppl. 137:107, 1988. It was determined, for example, that PPH can be treated by inhalation of low concentrations of NO. With respect to pulmonary hypertension, inhaled NO has been found to decrease pulmonary artery pressure (PAP) as well as pulmonary vascular resistance (PVR).
Prior to the advent of NO inhalation therapy, pulmonary hypertension was treated by the administration of drugs known as systemic vasodilators. These drugs, such as prostacyclin, nitroprusside, hydroalazine, and calcium channel blockers suffered from the limitation that the drugs, by their nature, produced systemic effects. For example, the drugs not only decreased PAP levels, but also systemic blood pressure.
Unlike systemic vasodilators, inhaled NO acts as a selective pulmonary vasodilator, acting primarily on the endothelium tissue of the lung. Upon inhalation, NO is absorbed into the capillary blood in the precapillary airspaces and alveolar capillaries. Inhaled NO has negligible action beyond the site of its uptake since NO is rapidly inactivated by the reaction with hemoglobin to form methemoglobin.
The use of inhaled NO for PPH patients was quickly followed by the use of inhaled NO for other respiratory diseases. For example, NO has been investigated for the treatment of patients with increased airway resistance as a result of emphysema, chronic bronchitis, asthma, adult respiratory distress syndrome (ARDS), and chronic obstructive pulmonary disease, (COPD). Still other respiratory diseases where NO inhalation therapy is thought to be beneficial include, by way of illustration and not by way of limitation: allograft lung transplantation, ischemia-reperfusion injury, congestive heart failure, septic shock, and high-altitude pulmonary edema.
While NO has shown promising preliminary results with respect to the treatment and prevention of the diseases mentioned above, delivery methods and devices must cope with certain problems inherent with gaseous NO delivery. First, exposure to high concentrations of NO is toxic. NO is toxic in high concentrations, especially over 1000 ppm. Even lower levels of NO can be harmful if the time exposure is relatively high. For example, the Occupational Safety and Health Administration (OSHA) has set exposure limits for NO in the workplace at 25 ppm time-weighted average for eight (8) hours. Typically, NO is administrated to patients in the concentration range of about 1 ppm to about 100 ppm.
Another problem with the delivery of NO is that NO rapidly oxidizes in the presence of oxygen to form NO
2
, which is highly toxic, even at low levels. For example, OSHA has set exposure limits for NO
2
at 5 ppm. In any NO delivery device it is thus desirous to reduce, to the largest extent possible, the conversion of NO to NO
2
. The rate of oxidation of NO to NO
2
is dependent on numerous factors, including the concentration of NO, the concentration of O
2
, and the time available for reaction. One problem with the inhalation of NO is that when NO is therapeutically inhaled, it is often mixed with high concentrations of O
2
. Consequently, this increases the conversion rate of NO to NO
2
. It is thus preferable to minimize the contact time between NO and O
2
when the NO is combined with a source of oxygen gas.
Methods and devices for delivering NO to a patient have been developed to minimize the conversion of NO to NO
2
. For example, with respect to the delivery of NO to patients connected to a mechanical ventilator, the NO/NO
2
stream has been introduced directly into the respiratory limb of a patient. See Martin Francoe, et al., “
Inhaled Nitric Oxide: Technical Aspects of Administration and Monitoring
,” Critical Care Medicine, Vol. 26, No. 4, pp. 785-87 April 1998. This arrangement has the advantage over other designs that combine and mix NO/NO
2
and Air/O
2
prior to their input to the ventilator since the contact time between NO and O
2
is reduced.
Another delivery method and device that reduces the exposure to O
2
and to a certain extent NO is disclosed in the U.S. Pat. No. 5,839,433 issued to Higenbottam. The '433 patent discloses a method and apparatus for supplying NO to a patient. According to the '433 patent, a very short pulse of NO is delivered intermittently, either at the start or end of inspiration. The '433 patent thus teaches the delivery of a bolus or plug of nitric oxide to the patient by administering a very short pulse of NO during inspiration. The timing of the delivery (beginning vs. late) is altered depending on the disease that is to be treated. When NO is desired in the lowermost depths of the lungs, for example, during treatment of pulmonary hypertension where NO acts on the small pulmonary arteries and capillaries, a short pulse is given at the beginning of inspiration. On the other hand, for asthma-like airway diseases, a very short pulse is administered near the end of inspiration. This method attempts to deliver NO to the desired location of the lungs. The method reduces the total exposure of the lungs to NO as well as reduces the total amount of NO available to react with O
2
to form toxic NO
2
.
The pulses of NO delivered according to the '433 patent are of a predetermined width, which can be altered by changing the amount of time that a control valve is left open. The '433 patent, however, fails to disclose the proportional delivery of NO gas to the patient having a flow profile that tracks or is proportional or quasi-proportional to the flow profile of an oxygen-containing gas. Rather, the valve mechanism provides a bolus, or square wave-type “plug” of NO to the patient, the length of which, is altered by adjusting its width (i.e., holding the valve in the open position for a longer period of time). In this regard, the pulse has the flow profile of a square wave regardless of the profile of the patient's inspiration profile.
Generally, NO is administered to patients that are either spontaneously breathing or connected to a mechanical ventilator. In spontaneously breathing patients, a patient typically wears a tight fitting mask, transtracheal O
2
catheter, nasal cannula, or other tubing passing directly into the airway of a patient. NO is typically mixed with O
2
and air prior to introduction into the patient airway. See Dean Hess, Ph.D., et al., “
Delivery Systems for Inhaled Nitric Oxide
,” Respiratory Care Clinics of North America, Vol. 3, No. 3, pp. 402-404 Sep. 1997. These spontaneous systems, however, suffer from the limitation that the NO concentration can fluctuate within a relatively wide range. The dose of NO varies with the patient's ventilatory pattern due to the fact that the patient's inspiration profile changes on a breath-by-breath basis. The delivered dose of NO is thus approximated from assumptions regarding the patient's ventilatory pattern.
There are several different methods of delivering NO to a mechanically-ventilated patient. In one method, the NO/N
2
stream is premixed with Air/O
2
prior to entering the ventilator.
Patel Mital B.
Sensormedics Corporation
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