• Hyperbaric oxygen (HBO) therapy is defined as the breathing of 100% oxygen inside a pressure vessel at greater than one atmosphere absolute (ATA).
  • Application of 100% oxygen to isolated body parts or breathing oxygen at surface (sea level) pressure is not HBO.
• Terminology is often marine related due to the close relationship between hyperbarics and diving:
  • Dive = treatment
  • Depth = treatment pressure
  • Descent = compression of the chamber
  • Ascent = decompression of the chamber
• There are two general types of chambers:
  • Monoplace
    • Capacity limited to one patient at a time
    • Generally compressed with oxygen
    • Limited patient access. Monitors and IVs must enter chamber through a specially designed "pass-through."
    • Less expensive to purchase and run than multiplace units
  • Multiplace
    • Capacity for more than one patient at a time
    • Generally compressed with air; oxygen is delivered to the patient via hood, mask, or other airway (endotracheal tube, tracheostomy tube)
    • Most often have an inside attendant that is compressed with the patient(s). This allows for easier patient care and manipulation of ventilators, IVs, etc., but requires the use of dive tables to avoid decompression illness ("the bends") in the attendant.
    • More expensive to purchase (each is custom built for the specific location), maintain, and staff
• In general, a hyperbaric oxygen treatment consists of breathing 100% oxygen for 60–120 minutes at pressures of 2.0–2.4 ATA. The pressure, duration, and number of treatments are varied depending on the disease being treated and the chamber type used (multiplace chamber are often able to achieve higher pressures than monoplace chambers).

• Boyle's gas law: The volume of a gas is inversely proportional to its pressure when the temperature is held constant.
  • Increased pressure directly shrinks gas bubbles in the blood and tissues.
  • Patients must equalize the pressure in the middle ear to the external environment via the Eustachian tube or risk eardrum rupture.
  • An untreated pneumothorax may transform into a tension pneumothorax on decompression as the extrapleural gas in the chest cavity expands.
  • Implanted devices, such as pacemakers, must be certified for hyperbaric environments as the change in pressure can result in malfunction and/or damage.
  • Air must be eliminated from IV tubing and bags to avoid air embolism on decompression.
  • Drains must be open to air during compression and decompression to allow equalization of pressure.
  • Endotracheal tube cuffs must be filled with saline or constantly adjusted during descent and ascent.
• Henry's gas law: The partial pressure of a gas dissolved in a liquid is directly proportional to the pressure of the gas on the surface of the liquid.
  • Increased ambient pressure and 100% oxygen increase the amount of oxygen dissolved in the plasma.
    • Oxygenation of ischemic tissues
    • Promotes angiogenesis
    • Increased diffusion gradient to enhance elimination of inert gases
    • Bacterial control via direct killing of anaerobic species and enhanced leukocyte function
    • Hyperoxia causes arterial vasoconstriction; there can be a ~10% increase in systemic vascular resistance under hyperbaric conditions.
    • CNS oxygen toxicity can provoke seizures in susceptible patients.
    • Pulmonary oxygen toxicity can result in pulmonary fibrosis during prolonged exposures.

• Oxygen is delivered to tissues via the bloodstream, so a minimum capillary density is needed to observe a benefit.
• Tissue oxygenation decreases geometrically as distance from the capillary increases.

• Only certain diseases and conditions have been approved as appropriate for HBO therapy by the Undersea and Hyperbaric Medicine Society (1)[A]
• Air or gas embolism
  • Air is introduced into the vascular system, usually via medical devices or trauma.
  • Hyperbaric effects
    • Increased pressure directly shrinks gas bubbles in the bloodstream and tissues.
    • Elevated oxygen tension provides an enhanced diffusion gradient for the elimination of inert gas.
    • Therapy: Depending on severity, prolonged treatments at high pressures (3.0–6.0 ATA) may be needed to resolve symptoms.
• Carbon monoxide (CO) poisoning
  • CO is produced from incomplete combustion
  • CO has ~200 times the affinity for hemoglobin (Hgb) than O₂ and is also a direct cellular toxin (can cause acute cardiac and acute/delayed neurological dysfunction).
  • Hyperbaric effects
    • HBO greatly accelerates the elimination of CO from the blood.
    • Can help prevent delayed neurologic sequela (2)[A].
    • Therapy: A short series (1–3) of higher pressure (2.8–3.0 ATA) treatments.
• Clostridial myositis and myonecrosis (gas gangrene) is caused by organisms in the clostridia family.
  • Clostridia organisms secrete exotoxins that impair host defenses and produce the toxic host reaction.
  • Hyperbaric effects
    • Inhibits bacterial growth (clostridia is a facultative anaerobe and is not directly killed by oxygen).
    • Stops alpha-toxin production (the exotoxin produced by clostridia that is most associated with morbidity and mortality).
    • Therapy: A short series of treatments, the first being at higher pressure (3.0 ATA), until the patient stabilizes. The initial three treatments are delivered in 24 hours, then twice daily after that.
• Crush injury, compartment syndrome, and other types of acute ischemia
  • Trauma reduces blood flow to tissues, which then become ischemic, resulting in vasogenic edema, further compromising blood flow and tissue oxygenation.
  • Hyperbaric effects
    • Oxygenates ischemic tissues
    • Reduces compartment edema due to decreased inflow due to arteriolar constriction from hyperoxia and unchanged venous outflow.
    • Therapy: A short series of twice daily treatments at 2.0–2.4 ATA until the acute ischemia is resolved.
• Decompression sickness
  • Breathing air at increased ambient pressures (such as when scuba diving) causes increased levels of nitrogen to be dissolved in the tissues and blood.
  • During decompression/ascent, this nitrogen comes out of solution and forms bubbles in the tissues and bloodstream if adequate decompression time is not allowed (i.e., nitrogen comes out of solution faster than it can be eliminated through the lungs).
  • These bubbles lodge in capillaries, resulting in decreased blood flow (joint pain, myocardial and neurologic ischemia, V/Q mismatch, etc.), platelet aggregation, and activation of the complement cascade.
  • Hyperbaric effects
    • Shrinks bubbles in plasma and tissues.
    • Enhanced diffusion gradient for inert gases.
    • Therapy: Depending on severity, prolonged treatments at high pressures (3.0 ATA) may be needed to resolve symptoms.
• Enhancement of healing in selected problem wounds
  • Problem wounds often fail to heal due to inadequate oxygen delivery secondary to impaired microvascular injury, such as in diabetes.
  • Transcutaneous oxygen monitoring can be used to assess potential benefit of hyperbaric therapy (3)[B].
  • Hyperbaric effects
    • Promotes angiogenesis
    • Bacterial control
    • Oxygenate ischemic tissues
    • Enhanced fibroblast activity
    • Therapy: 20–30 daily treatments at 2.0–2.4 ATA.
• Exceptional anemia
  • Massive bleeding without the possibility of transfusion (religious beliefs, difficult cross-match).
  • Hyperbaric effects
    • At 3.0 ATA on FiO₂ 1.0, enough oxygen can be dissolved in plasma to support basic metabolic function without hemoglobin.
    • Therapy: Short series of high pressure (2.8–3.0 ATA) treatments until blood products are available or until Hgb has increased to acceptable levels. Interval between treatments should be guided by evidence of recurring ischemia (ECG change, acidosis, etc.).
• Intracranial abscess
  • Adjunctive HBO therapy indicated for patients with: Multiple abscess or those in deep tissues or dominant locations, immunocompromised, contraindications to surgery, or no response to standard treatment.
  • Hyperbaric effects
    • Bacterial control
    • Oxygenate ischemic tissues
    • Therapy: Once to twice daily treatments at 2.0–2.4 ATA. Optimal duration of treatments unknown.
• Necrotizing soft tissue infections
  • Multiorganism, rapidly spreading infections that often tract along fascial layers.
  • Hyperbaric effects
    • Bacterial control
    • Oxygenation of ischemic tissues
    • Therapy: Short series twice daily treatments at 2.0–2.4 ATA
• Osteomyelitis (refractory)
  • Infection of bone that is difficult to clear due to impaired blood flow and ischemic environment.
  • Hyperbaric effects
    • Bacterial control
    • Oxygenation of ischemic bone
    • Enhanced osteoclast function
    • Therapy: 20–30 (or more) daily treatments at 2.0–2.4 ATA.
• Delayed radiation injury (soft tissue and bony necrosis) (4)[B]
  • Radiation therapy causes endovascular injury that results in decreased capillary density.
  • Capillary density continues to decrease over time, even after radiation therapy ends, resulting in ischemic tissues in the irradiated field.
  • This ischemia can result in tissue breakdown either spontaneously or after trauma, including surgery.
  • Hyperbaric effects
    • Promotes angiogenesis
    • Bacterial control
    • Oxygenates ischemic tissues
    • Therapy: 20–30 daily treatments at 2.0–2.4 ATA
    • Marx protocol: Used in the case of osteoradionecrosis of the jaw when a tooth extraction is planned in a previously irradiated area. The patient is given 20 treatments prior to the extraction and 10 treatments postextraction.
• Skin grafts and flaps
  • Compromised blood flow to grafts and flaps can result in ischemia and loss of the graft.
  • Hyperbaric effects
    • Oxygenation of ischemic tissues
    • Promotes angiogenesis
    • Therapy: A short series of twice daily treatments at 2.0–2.4 ATA until the acute ischemia is resolved.
• Thermal burns
  • Thermal injury impairs oxygenation via vascular coagulation and edema.
  • Hyperbaric effects
    • Oxygenation of ischemic tissues
    • Therapy: A series of treatments at 2.0–2.4 ATA.

• Hyperbaric treatment performed immediately prior to debridement can help differentiate viable from dead tissue.
• Myringotomies will need to be performed on intubated patients and others who are unable to clear their ears (equalize the pressure in the inner ear with the external environment).

• Boyle's law: P₁V₁ = P₂V₂; where P is pressure and V is volume
• Alveolar oxygen equation
  • PAO₂ = FiO₂(P_B – P_H2O) – (PaCO₂/RQ)
    • PaO₂ = alveolar oxygen pressure.
    • P_B = barometric pressure (mm Hg).
    • P_H2O = water vapor pressure (usually 48 mm Hg).
    • RQ = respiratory quotient (usually 0.8)
    • 21% O₂ at 1 ATA: PaO₂ = 100 mm Hg
    • 100% O₂ at 3 ATA: PaO₂ = 1,500 mm Hg
• Arterial oxygen content
  • CaO₂ = 1.34(Hgb)(%sat) + (0.003)(PaO₂)
    • 1.34(Hgb)(%sat) = oxygen attached to Hgb
    • (0.003)(PaO₂) = oxygen dissolved in plasma.
    • At 3 ATA on 100% oxygen, 5 mL/dL of oxygen can be dissolved in the plasma, which can support resting metabolism.
• 1 ATA = 33 feet of seawater = 14.2 psi

• Pao2
• Partial Pressure
• Carbon Monoxide Poisoning

Shawn T. Simmons , MD