Hyperbaric Oxygen Chamber
HBOT- An overview
Over the past 40 years hyperbaric oxygen therapy (HBO) has been recommended and used in a wide variety of medical conditions but the use of mild oxygen therapy had been always trivial. Consequently a high degree of medical skepticism had developed regarding its use. Over the last two decades,animal studies, clinical trials have produced reasonablescientific evidence or well validated clinical experience for Mild oxygen therapy. This has now produced a set of indications for which Mild HBOT is beneficial such as Traumatic brain injury, cerebral palsy, autism, sports rehabilitation etc.
The physics behind Hyperbaric Oxygen Therapy (HBOT) lies within the ideal gas laws. The application of Boyle’s law (p1 v1 = p2 v2) is seen in many aspects of Hyperbaric Medicine. This can be useful with embolic phenomena such as decompression sickness (DCS) or arterial gas emboli (AGE). As the pressure is increased, the volume of the concerning bubbles decreases. This also becomes important with chamber decompression; if a patient holds her breath, the volume of the gas trapped in the lungs over expands and may cause a pneumothorax. Charles law ([p1 v1]/T1 = [p2 v2]/T2) explains the temperature increase when the vessel is pressurized and the decrease in temperature with depressurization. This is important to remember when treating children or patients who are very sick or are intubated. Henry’s law states that the amount of gas dissolved in a liquid is equal to the partial pressure of the gas exerted on the surface of the liquid. By increasing the atmospheric pressure in the chamber, more oxygen can be dissolved into the plasma than would be seen at surface pressure. The clinician must be able to calculate how much oxygen a patient is receiving. In order to standardize this amount, atmospheres absolute (ATA) are used. This can be calculated from the percentage of oxygen in the gas mixture (usually 100% in Oxygen Therapy; 21% if using air) and multiplied by the pressure. The pressure is expressed in feet of seawater , which is the pressure experienced if one were descending to that depth while in seawater. Depth and pressure can be measured in many ways. Some common conversions are 1 atmosphere = 33 feet of seawater = 10 meters of sea water = 14.7 pounds per square inch (psi) = 1.01 bar.
Hyperbaric oxygen therapy describes a person breathing 100 percent oxygen at a pressure greater than sea level for a prescribed amount of time—usually 60 to 90 minutes. Atmospheric Pressure – The air we breathe is made up of 20.9 percent oxygen, 79 percent nitrogen, and 0.1 percent inert gases. Normal air exerts pressure because it has weight and this weight is pulled toward the earth’s center of gravity. The pressure experienced is expressed as atmospheric pressure. Atmospheric pressure at sea level is 14.7 pounds per square inch (psi). Hydrostatic Pressure – As you climb above sea level, the atmospheric pressure decreases because the amount of air above you weighs less. If you dive below sea level, the opposite occurs (the pressure increases) because water has weight that is greater than air. Thus, the deeper one descends under water the greater the pressure. This pressure is called hydrostatic pressure. Atmospheres Absolute (ATA) – ATA refers to gauge pressure that is true regardless of location. This way, a standard depth can be reached whether located above or below sea level. There are various terms for measuring pressure. HBO therapy uses pressure greater than that found at the earth’s surface at sea level, which is called hyperbaric pressure. The terms or units used to express hyperbaric pressure include millimeters or inches of mercury (mmHg, inHg), pounds per square inch (psi), feet or meters of sea water (fsw, msw), and atmospheres absolute (ATA). One atmosphere absolute, or 1 ATA, is the average atmospheric pressure exerted at sea level, or 14.7 psi. Two atmosphere absolute, or 2 ATA, is twice the atmospheric pressure exerted at sea level. If a physician prescribes one hour of HBOT treatment at 2 ATA, the patient breathes 100 percent oxygen for one hour while at two times the atmospheric pressure at sea level.
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