HIGH FI02 OXYGEN MASK WITH A SEQUENTIAL DILUTION FEATURE AND FILTER
 This application is a continuation-in-part application claiming priority to U.S. patent application Ser. No. 10/259,997, filed on Sep. 27, 2002, the complete specification of which is hereby incorporated by reference as if fully set forth herein.
FIELD OF THE INVENTION
 The field of the present invention relates to devices for delivery of oxygen and other gases, and in particular, it relates to gas delivery masks.
BACKGROUND OF THE INVENTION
 Hypoxemia is a deficient oxygenation of the blood. The deficiency of oxygen may result from acute respiratory failure stemming from conditions such as pneumonia, heart disease, trauma to the chest or other etiologies. Conditions causing acute respiratory distress are also often associated with hyperventilation (high levels of breathing). Paradoxically, the patient's high minute ventilations and associated high inspiratory flows during hyperventilation severely limit the fraction of inspired oxygen (FI02) that most oxygen (02) masks can deliver. When a sufficiently high arterial oxygen partial pressure (Pa02) cannot be provided by mask alone, the therapeutic option defaults to endotracheal intubation. Since endotracheal intubation is associated with considerable discomfort, morbidity and cost, this step is not to be undertaken without exhausting other options.
 The treatment objective for severe hypoxemia resulting from acute respiratory failure is to optimize the oxygen flowing to the alveoli and, thereby, increasing arterial P02. The effective delivery of high concentrations of 02 depends on the capability of a mask to match the 02 flow to the patient's minute ventilation and peak inspiratory flow without limiting the FI02. One common approach for delivering oxygen using a mask is to try to match the patient's peak inspiratory flows with 02 flowing to the mask. Peak inspiratory flows in breathless patients, however, can reach several hundred liters per minute; and most 02 flowmeters are calibrated to only 15 L/min. Even when set at "flush," the upper limit of the flowmeter is still far less than peak flow requirements. Hence, delivery of higher 02 flows into the mask in this situation requires a tandem set-up of multiple flowmeters, which increases the complexity and cost of the delivery system.
 Another approach for oxygen delivery is to use a mask with an 02 reservoir on the inspiratory side with or without a one-way valve between the reservoir and the mask. A mask with a valve between the reservoir and the mask is known as a non-rebreathing mask ("NRM"), while a mask without the one-way valve between the reservoir and the mask is known as a partial rebreathing mask ("PRM"). In theory, the reservoir fills with 02 during exhalation and is available to meet peak inspiratory flow demands during inspiration. In practice, however, FI02 is limited because there is an obligatory entrainment of room air throughout inspiration. Most conventional oxygen masks dilute the inspired oxygen with entrained room air because of the presence of ports on the mask through which the patient also
exhales. Oxygen is also diluted because of poor fit of the mask to the face. In this case, the gas filling the alveoli and airways is the average diluted concentration, rather than the concentration of the supplied oxygen. Hence, during inspiration, these entrainment pathways provide a large source of dilution of the oxygen and reduce the FI02.
 Furthermore, the volume of entrained air depends on the relative resistance to flow in the portholes of the mask and the 02 inlet. The difference in performance between the NRM and PRM may be small in this situation. On the one hand, the valve at the 02 inlet prevents expired gas with lower P02 from entering the reservoir; on the other hand, it increases the resistance to flow from the bag to the mask and thus results in entrainment of more air that further decreases the FI02. These considerations apply even when the mask may fit well on the face of the patient.
 Another consideration for delivery of oxygen using a mask is the risk of oxygen flow failure because either the oxygen supply is exhausted or the oxygen flow path may be blocked. In this situation, an anti-asphyxiation valve allows inhalation of room air. For example, a mask called the BLB mask was developed around the time of World War II for pilots who required supplemental oxygen while flying at higher altitude. It used an external oxygen source that filled an inspiratory reservoir. The pilots inhaled through a oneway valve between the mask and the reservoir. They exhaled through a second valve in the mask. The operation of the BLB mask called for supplying an oxygen flow sufficient to meet the pilot's ventilatory requirements. If the pilot required more oxygen than was present in the reservoir, the BLB mask contained an anti-asphyxiation valve to enable the pilot to inhale room air rather than asphyxiate in these situations. The anti-asphyxiation valve is acceptable only for emergency situations and for short periods of time. For extended periods of time, breathing through the anti-asphyxiation valve leads to fatigue. In the case of distress patients, this may not acceptable.
 Another concern that may be associated with patients who are receiving oxygen is the air that they exhale. Many of these patients have respiratory infections and the pathogens in their exhaled breath such as viruses or bacteria can contaminate the environment around them, potentially exposing other patients or healthcare workers to these organisms. Some of these patients may be receiving inhaled medications while receiving oxygen, and the inhaled medications may be toxic or harmful to others if exhaled into the environment. Thus, there exists a need for an improved mask for efficient and safe delivery of oxygen or other gases to a patient or subject.
SUMMARY OF THE INVENTION
 The present invention provides for a method and a device to deliver oxygen or other gases to a patient. According to one aspect of the invention, a method is provided for delivering oxygen or other gases to a patient by sequentially diluting room air to the oxygen flow during a respiratory cycle of the patient wherein the room air is inspired at the end of inspiration. In particular, high concentration of oxygen or other gases from the gas source is first delivered to the alveoli of the lung before substantial volume of room air is allowed to enter the oxygen or gas flow path and dilute the flow of oxygen or gas. Since the space between the nose and