CHAPTER 201 Hyperbaric Oxygen Therapy
Hyperbaric oxygen (HBO) is treatment with high-concentration oxygen achieved by having the patient breathe 100% oxygen inside a pressurized hyperbaric chamber. Oxygen is delivered to the tissues through respiration because absorption of oxygen through the skin is insufficient. HBO treatment is based on two physical factors related to the hyperbaric environment: mechanical effects of pressure and increased oxygenation of tissues. This chapter reviews the scientific and clinical literature regarding HBO therapy in laboratory animals and in humans and introduces the practitioner to the potential use of this treatment modality for the equine patient.
HISTORY OF HYPERBARIC CHAMBERS
Dr. J.L. Corning first introduced compressed air therapy into the United States in 1871. In the early 1900s, University of Kansas anesthesia professor Dr. Orville Cunningham noted that patients with heart disease and other circulatory disorders had difficulties acclimating to high altitudes. He postulated that exposure to increased atmospheric pressure would be beneficial for their heart disease. To test his hypothesis (1918) he used 2 atmospheres of pressure in a chamber designed for animal studies to successfully treat a young resident physician undergoing a hypoxic crisis during the flu. Cunningham then built an 88-foot-long chamber, 10 feet in diameter, in Kansas City and began treating a multitude of diseases, most of them without scientific rationale. The American Medical Association and the Cleveland Medical Society, failing to find any scientific evidence for his rationale, forced him to close his facility in 1930.
It is frequently stated that the history of “hyperbaric oxygenation” goes back “over 300 years,” probably referring to the work of Henshaw. This is incorrect, however, because oxygen was not discovered until 1775 by Priestly. All the early chambers were pressurized with compressed air, and oxygen was not a consideration. The advent of HBO treatment in modern clinical medicine began in 1955, when Dr. Churchhill-Davis used oxygen-rich environments to help attenuate the effects of radiation therapy in cancer patients. That same year Dr. Ite Boerma, in the Netherlands, proposed using HBO in cardiac surgery to help prolong the patient’s tolerance to circulatory arrest. He conducted surgical operations under hyperbaric conditions, including correction of transposition of the great vessels, tetralogy of Fallot, and pulmonic stenosis. Boerma also noted that pigs deprived of erythrocytes survived when exposed to 3 atmospheres of oxygen. Treated pigs had sufficient oxygen dissolved in their plasma to sustain life.
PHYSICAL PRINCIPLES OF HYPERBARIC THERAPY
Pressure is measured as force per unit area. One atmosphere of pressure (ATA) is equal to 14.7 pounds per square inch (PSI), which results from the weight of the air on the surface of the earth at sea level. Weathermen usually refer to this pressure as barometric pressure, and quantify it in inches or millimeters of mercury or kilopascals (1 ATA = 29.9 inches Hg = 760 mm Hg = 10 kPa) (Table 201-1). The term atmosphere refers to atmospheres absolute. Absolute pressure equals the gauge pressure plus the ambient air pressure on the surface at sea level (1 ATA). For example, if one descends 33 feet in seawater, one is at an absolute pressure of 2 ATA. This is because 33 feet of water exerts a pressure of 14.7 PSI as read on the gauge. Absolute pressure at this depth equals gauge pressure plus atmospheric pressure (i.e., 1 ATA + 1 ATA = 2 ATA).
Boyle’s law describes the pressure-volume relationship of gases. The volume of a gas is inversely proportional to the pressure exerted on it. For example, doubling the pressure reduces the gas volume by about one half, and tripling the pressure reduces it by one third. Hyperbaric therapy involves pumping a volume of air or oxygen into a sealed chamber. Because an increased volume is enclosed in a fixed space, the gas exerts a higher pressure; basically the gas molecules are more tightly packed within the enclosed space. When this pressure is exerted on the body of the patient, gas within the body cavities such as intestinal gases decreases in volume, and when pressure is removed, the volume increases again. These mechanical effects are responsible for unwanted barotrauma that may result in a ruptured lung if the patient breath holds during decompression. If a patient suffers from gaseous distension of the bowel, compression in the chamber eases discomfort, and inhalation of oxygen forms a high gradient for the removal of nitrogen from the distended intestine. Gas trapped in the bowel decreases by about 50% when a patient breathes oxygen over a 6-hour period at 2 atmospheres.
In the case of body cavities that normally communicate with air in the chamber, for example, the inner ear, sinuses, and lungs, the compressed gas simply enters those cavities, and there is no change in their volume as long as there is no obstruction to air flow. However, if the eustachian tube is blocked, for example, pressure changes can cause considerable discomfort, as air travelers with colds know.
Dalton’s law states that the pressure exerted by a mixture of gases is equal to the sum of the pressure exerted by each individual gas in the mixture. The pressure exerted by a gas such as oxygen (PO2) is equal to the total pressure (Ptot) multiplied by the fraction of that gas (FO2) expressed as a decimal (PO2 = Ptot × FO2). When a horse breathes air (FO2 = 0.21) at sea level (1 atmosphere pressure), the PO2 in the air is