Hyperbaric oxygen therapy

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Hyperbaric oxygen therapy

Klaus D. Torp, MD, Neil G. Feinglass, MD, FASE, FCCP and Timothy S.J. Shine, MD

Hyperbaric O2 therapy (HBOT) refers to the inhalation of 100% O2 in an environment in which the barometric pressure is greater than 1 atmosphere. Note that 1 atmosphere absolute (ATA) is the pressure at sea level. For every increase in ambient pressure of 760 mm Hg, or 14.7 psi or 33 feet of seawater, the pressure increases by 1 ATA. Exposure to increased gas pressures can occur in other situations, such as breathing compressed gas mixtures while diving (scuba) or working in underground tunnels (caisson workers). For HBOT, the pressure in a hyperbaric chamber is typically at least 1.4 ATA.

All gases follow fundamental gas laws:

In clinical medicine, the liquid of interest is blood and the dissolved gas is O2. The driving pressure of O2 into blood is the partial pressure of O2 in alveoli (PAO2). Note that it is the partial pressure of O2 and not the percentage of O2 that is responsible for its effects (Table 224-1). As the PaO2 increases in arterial blood, the saturation of hemoglobin approaches 100% (at PaO2 ∼100 mm Hg). Above this level, all additional O2-carrying capacity of blood comes from the oxygen dissolved in the plasma. HBOT can therefore increase O2 content in the face of severe anemia and increase O2 delivery in areas of partial obstruction to blood flow. In addition, the increased barometric pressure can reduce intravascular air bubbles in patients with decompression sickness or air embolism, improving perfusion and increasing the removal of N from the blood (Figure 224-1).

Table 224-1

Expected Gas Tensions and Arterial Blood O2 Content at Various Ambient Pressures in a Normal Individual*

Atm FIO2 Inspired PO2 (mm Hg) PAO2 (mm Hg) PaO2 (mm Hg) CaO2 (mL/dL) PaCO2 (mm Hg)
          Total Dissolved  
1 0.21  150  102   87 18.7 0.3 40
1 1.0  713  673  572 21.2 1.7 40
2 1.0 1473 1433 1218 23.1 3.7 40
3 1.0 2233 2193 1864 25.1 5.6 40
6 0.21  898  848 >750 21.8 2.3 40

image

*Hemoglobin = 14 g/dL.

Modified, with permission, from Moon RE, Camporesi EM. Clinical care in altered environments: At high and low pressure and in space. In: Miller RD, ed. Anesthesia. 6th ed. Philadelphia: Churchill Livingstone; 2005:2665-2701.

image
Figure 224-1 Gas volume (Vol) and bubble size as a function of depth: Boyle’s law. ATA, Atmosphere(s) absolute pressure; Dia, diameter; FSW, feet of sea water; PSIA, pounds per square inch absolute.

The effectiveness of HBOT has been established for several indications, and the basic mechanisms for its effect on the body have been demonstrated. Table 224-2 lists conditions that are recommended for HBOT by the Undersea and Hyperbaric Medical Society, as well as those that are reimbursed by the Centers for Medicare and Medicaid Services. For any other indications, one should first examine how the basic mechanisms of HBOT (Box 224-1) will affect the underlying pathophysiology of the disease.

Box 224-1   Basic Mechanisms of Hyperbaric O2 Therapy

Table 224-2

Recommended (UHMS) and Reimbursed (CMS) Indications for Hyperbaric O2 Therapy

Indication UHMS CMS
Air or gas embolism X X
Carbon monoxide poisoning X X
Carbon monoxide poisoning complicated by cyanide poisoning X X
Clostridial myositis and myonecrosis (gas gangrene) X X
Crush injury, compartment syndrome, and other acute traumatic ischemias X X
Decompression sickness X X
Enhancement of healing in selected problem wounds X  
Exceptional blood loss (anemia) X  
Intracranial abscess X  
Necrotizing soft tissue infections X X
Osteomyelitis (refractory) X X
Delayed radiation injury (soft tissue and bony necrosis) X X
Skin grafts and flaps (compromised) X X
Thermal burns X  
Acute peripheral arterial insufficiency X X
Refractory actinomycosis   X
Diabetic wounds of the lower extremities in patients who meet certain criteria   X
Idiopathic Sudden Sensorineural Hearing Loss X  
Central Retinal Artery Occlusion X  

CMS, Centers for Medicare and Medicaid Services; UHMS, Undersea and Hyperbaric Medical Society.

Effects of hyperbaric oxygen

Pulmonary effects

A high PO2 is thought to overcome the body’s scavenging system for free radicals, resulting in the formation of reactive O2 species, such as superoxides, hydrogen peroxide, and hydroxyl radicals. Reactive O2 species can cause pulmonary O2 toxicity symptoms, such as retrosternal burning, coughing, and fibrosis and can lead to a measurable decrease in vital capacity. The pulmonary toxicity is dependent upon duration and PO2, and its effect is cumulative. Most HBOT protocols reduce toxicity by introducing room air breaks between O2 treatment periods.

Anesthetic management in an HBOT chamber

Provision of general anesthesia in a hyperbaric chamber is rare. Potential indications include double-lung lavage and emergent surgical procedures in patients who cannot be brought out of the chamber in a timely manner. The anesthesia provider may, however, be called for airway management or to sedate and provide support for critically ill patients. All personnel have to be pressure tested and properly trained before entering an HBOT chamber. The induction of anesthesia by inhalation agents depends on the partial pressure of those agents in the brain, not on the concentration that is inhaled. If 1.1 minimum alveolar concentration (MAC) of isoflurane (about 8 mm Hg) produces anesthesia at sea level (1 ATA), then the same effect will be produced by 0.33 MAC of isoflurane at 3 ATA because partial pressure of the drug in the alveoli and brain will still be 8 mm Hg.

Increasing pressure on variable bypass vaporizers leads to a decrease in the concentration of anesthetic agent leaving the vaporizer, but, because nearly the same partial pressure of agent in the CNS is produced, the clinical changes are imperceptible. A vaporizer with a heating element should not be taken into an HBOT chamber. Nitrous oxide has been used successfully in a hyperbaric chamber before, but should be avoided, because of it’s increased solubility and the complex changes in pressure and breathing gases in a hyperbaric chamber. Total intravenous anesthesia is preferred over the use of inhalation anesthetic agents in an HBOT chamber because total intravenous anesthesia requires less equipment and eliminates pollution of the HBOT chamber with anesthetic gases. Regional anesthesia is also a very good choice, but the local anesthetic agent should be devoid of any air bubbles when injected.

Increased gas density caused by the increase in atmospheric pressure decreases flow through rotameter flowmeters, leading to falsely high readings under hyperbaric conditions. Gas cylinders should function normally but are usually not found inside an HBOT chamber.

Anesthesia equipment needs to be rated for use in an HBOT chamber because it may not function normally. Cuffs on tracheal tubes and intravenous and bladder catheters should be filled with fluid. Air-filled cuffs will undergo large volume changes with changes in pressure, as will drip chambers on intravenous lines, which require frequent observation during pressure changes to rapidly identify air and avoid an intravenous air embolus. Mechanical ventilators should be rated for use in an HBOT chamber or, at the least, tested for accuracy and safety under pressure. Petroleum-based lubricants and alcohol must be avoided because they are a fire hazard in an O2-enriched environment.

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