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1. Field of the Invention
The present invention relates to a method and system for providing air with a lowered oxygen concentration to a human or other subject. Specifically, the invention relates to a method and system that creates at near sea level, oxygen partial pressure equivalents to a desired “simulated” above ground level altitude and creates hypoxia in a subject through the identical physiologic mechanisms as high altitude.
2. Altitude Physiology/Definition of Terms
Simulated altitude, or physiological altitude is defined as the partial pressure of oxygen that corresponds to a particular actual altitude. Upon ascention, the normal concentration of atmospheric gases does not change with altitude. However, total barometric pressure scales largely with altitude and temperature; the Babinet equation can be used to calculate the relationship between altitude, temperature and atmospheric pressure. It is shown below:
Z=C×(Bo−B)/(Bo+B)
A gas in a mixture exerts a “partial pressure” proportional to its' fraction of the total pressure as per Dalton's law of partial pressure. Accordingly, the partial pressure of oxygen is influenced both by its' concentration and the atmospheric pressure.
It has long been known that the partial pressure of oxygen (Po2) is what most living organisms especially humans are sensitive to and lowering the Po2 below a threshold value (hypoxia) will induce graded symptoms from whole organism down to cellular level. Chronic safe exposures to mild reductions in Po2 induce physiologic mechanisms of acclimatization which are known to benefit athletes, rock climbers, and any human endeavor at high altitudes. Short term exposure at simulated high altitudes can train a subject to recognize and respond to the motor skill and cognitive degradation of hypoxia before losing consciousness.
3. Description of Prior Art
There have been various attempts at providing systems for simulating different altitudes in order to study and recognize the debilitating effects of hypoxia, as well as obtain some of the advantages of simulating different altitudes for athletic training and hypoxia symptom recognition. The relevant embodiments of these are discussed immediately below.
Wartman, Vacchiano et al U.S. Pat. No. 6,871,645 Mar. 29, 2005 describes a reduced oxygen device in which nitrogen is injected into a reservoir which communicates with outside air creating a nitrogen enriched sub-environment or volume of gas from which a person can inspire. The concentration of oxygen within this reservoir is monitored by an oxygen sensor and requires considerable volume (between 150 and 500 cubic inches) to compensate for the relatively slow response (6 seconds or slower) of the oxygen sensor.
Vacchiano et al. US patent application # 20050247311 Nov. 10, 2005 describes a reduced oxygen device which similarly blends gas in a reservoir which is monitored by an oxygen sensor. Both nitrogen and air are fed under pressure via two “off the shelf” thermal mass flow controller units, one each for air and nitrogen, and the reservoir is maintained at constant pressure of 5 PSIG, slightly more than 350 cmH2O. According to the American Lung Association, even 45 cmH2O can cause lung injury. Since the magnitude of this reservoir pressure (which is in direct communication with the subject during inspiration) is sufficient to cause immediate lung injury to the subject, an overpressure valve and backup system is required to reduce the possibility of overdistention lung injury. A further concern with this method of blending gas is the closed nature of the pneumatic circuit. This device is attached to the human trainee via a closed tubing and airtight mask. In the event of gas supply or flow controller failure in the off position, the #20050247311 device does not allow the subject access to ambient air.
A necessary part of any prudent system that deliberately induces hypoxia is a means of providing immediate re-oxygenation of the subject. In the case of reduced oxygen systems with a reservoir, the resultant volume of high oxygen content gas within the reservoir can pose a significant combustion safety threat. Any vessel that contains both high oxygen concentrations and electromechanical devices such as mixing fans treated with petroleum lubricants pose an explosion threat. Accordingly, it is desirable to eliminate these safety concerns.
The present invention is referred to herein as an “Altitude Simulation Module II” (also denoted as ASMII herein) and encompasses both a method and a system for breathing at simulated altitudes. The ASMII creates desired oxygen partial pressures by the instantaneous ratiometric addition of nitrogen to spontaneous, uncontrolled inspired ambient air as a function of continuous and instantaneous measurement of inspiratory flow, not oxygen concentration, and therefore eliminates the need for a gas reservoir and associated devices. The inlet port of the ASMII is a continuously open conduit to atmosphere, without valves of any kind. These design elements improve upon prior art in terms of safety, packaged size, simplicity, and reliability.
The current invention/Altitude Simulation Module II creates at near sea level, oxygen partial pressure equivalents to a desired “simulated” above ground level altitude and creates hypoxia in a subject through the identical physiologic mechanisms as high altitude. The current invention differs from prior art aimed at this goal in the following ways: While other devices claim “near instant” and “breath by breath” responses, all rely on feedback from an unheated oxygen sensor, the best of which responds in slightly more than 6 seconds. Concomitantly, the relevant prior art also rely on a volume of stored gas mixed in a reservoir or “vessel” in which the oxygen sensor resides. This prevents quick changes to the oxygen concentration within this vessel and partially compensates for the slow response of the oxygen sensor relative to within breath flow changes. During normal breathing, typical inspirations last 0.5 to 1.5 seconds, begin and end at zero flow, and typically change rapidly to 25 to 50 liters per minute or more, depending on a persons' metabolic state, size, and many other factors. Each consecutive breath can vary considerably, as does flow within a breath To adequately describe the inspired flow profile requires a sampling frequency of at least 30 Hz. Breathing systems which rely on an oxygen sensor will obviously not be able to detect within breath changes in gas concentrations let alone make appropriate corrections and are thus reliant on a reservoir to provide damping to the system as described previously. Central to the function of the current invention is the ratiometric addition in real time of nitrogen to inspired room air which is unpressurized and inspired normally. The system accomplishes oxygen partial pressure changes as a function of continuously sampled inspired flow rate. The invention does not rely on an oxygen sensor nor a reservoir as these result in temporal responses at least an order of magnitude too slow to allow near real time performance.
The ASMII adds nitrogen to inspired gas as a ratio of instantaneously measured, subject determined inspiratory flow. There is no gas reservoir or “vessel” which is kept pressurized or at a programmed oxygen level. The nitrogen is added to each breath as a ratio based on measured spontaneous flow from each breath in real-time. Since there is no reservoir to potentially become overpressurized, the need for a backpressure valve is eliminated. When oxygen is added to the system to rapidly re-oxygenate a hypoxic subject, the lack of a volume reservoir of near 100% oxygen and concomitant explosion hazard of previous altitude simulation devices is a significant improvement in safety.
The inlet port of the ASMII is a continuously open path to atmosphere, without valves of any kind. This was designed specifically to provide inherent safety as compared to prior art. Valves can fail and either block the flow of inspired air, or over distend the lungs in the case of a failed backpressure relief valve. The continuously open large bore inlet port of the ASMII is of sufficient size as to eliminate the possibility of lung overdistension even in the case of a failed nitrogen enrichment valve and provide open and easy access to ambient unpressurized room air.
Referring to
US patent application #US 20050202374A1 filed Jan 6, 2005 Provisional application No. 60/534,628 filed Jan. 6, 2004