Hyperbaric Oxygen Therapy is the application of Oxygen in the environment that the atmospheric pressure is greater than 1 ATM (760 mmHg, 14.7 psi, 1.013 bars). To remind the basic knowledge, the Alveolar air equation is 760-47 (Pb of H2O) x FiO2 - PaCO2/RR. When the situation that 100% FiO2 cannot solve the problem, then the only way to increase the PAO2 is to increase the barometric pressure. The PaO2 can rise upto over 2000 mmHg at 2-3 ATM + 100%O2. The dissolved portion of the oxygen in the blood rises significantly from 0.3 ml/dL while breathing room air at 1 ATM to 1.5 ml/dl while breathing 100%O2 at 1 ATM and to 6 ml/dl if breathing 100% O2 at 3 ATM.

Besides than the improvement in tissue oxygenation, the HBO can also reduces the size of the inert gas bubbles by hastens their dissolution and replacing the inert gas with O2. This principle explains why the HBO is also used in the treatment for decompression sickness.

The HBO unit can be a single container(Monoplace chamber), filled with 100% O2, that can enclose the single patient in the supine position or a larger room (multiplace chamber), filled with room air, that can hold multiple patients (who breathes 100% O2) and/or the healthcare workers. The single treatment can vary from 45 min (CO poisoning) to 5 hours (severe decompression disorder). Chronic treatment is also used in the vascular insufficiency, gangrene, necrotic wounds, etc.

Multiplace hyperbaric chamber, which is large enough for one or more patients and tenders. Chamber atmosphere is compressed air. The patient receives 100% O2 via mask, head tent, or ET. Monitors are usually kept outside the chamber because of electrical safety considerations. Monitoring is possible through a porthole. Personal lock and transfer lock allow physicians, nurses or other personnel, in additional to medications, food, and blood samples, to be moved into and out of the chamber without repeated compression and decompression of the patient.

Monoplace chamber. This type of chamber has room for one patient or a tender with a small child. Chamber atmosphere is 100% O2. The chamber is constructed of transparent plastic to allow easy observation. Through-hull penetrators (not shown) near the head of the patient allow monitoring and IV fluid administration and control of a ventilator inside the chamber.

Indications for hyperbaric O2 therapy

Side effects of Hyperbaric Oxygen

1. Oxygen toxicity

Pulmonary : Lung tissues is the most susceptible organ to oxygen toxicity. The pathophysiology includes the epithelial thinning and vacuolization which leads to increased permeability and interstitial fluid accumulation. Subsequently, the large areas of epithelium are lost, the type II alveolar cell proliferates which combines with the accumulated fluid, leads to the thickening of alveolar/capillary membrane.
   The symptoms started with substernal distress (about 10 hour after 100% O2 at 1 ATM), occasional and paroxysmal coughing and fibrosis. Most of the HBO chambers deliver the air breaks between the oxygen treatment periods.

CNS : because the HBO reduces the cerebral blood flow which may lead to the vasoconstriction and retinal hypoxia (retrolental fibroplasia, mainly occurred in the premature infants who have immature retinal vascular bed). The O2 convulsion is believed to be related to a reduction in a concentration of GABA and possibly the inactivation of sulfhydryl-containing enzymes by O2 free radicals. The CNS symptoms include nausea, facial numbness, facial twitching, unpleasant taste or smell and finally tonic/clonic seizure which can be fatal.

2. Inert Gas Uptake

This applies to the multiplace chamber which employ the pressurized room air. The toxic effects are due to the nitrogen which can cause "Nitrogen Narcosis". Symptoms include an impairment in fine motor control e.g. IV placement, difficulty in rapid decision-making. These effects can be minimized by using Helium-O2 (heliox) which is more expensive and produce "Donald Duck voice"

Another major adverse effect of nitrogen is the risk of decompression sickness which manifested by joint pains or neurologic symptoms. Decompression schedule must be designed to minimize this complication in tenders.

3. Exposure to trace gas

Only in the multiplace chamber when the unit is tight close and no ventilation. The waste gas e.g. CO2 (from exhalation), other inert gases e.g. hydrogen, sulfur dioxide (from batteries/medical equipment) can build up. Most hyperbaric facilities require the PCO2 not rise > 4 mmHg. This complication is nonexistent in the monoplace chamber which is ventilated continually with 100% O2.

4. Barotrauma

The gas-containing space in the body must equalize the pressure with the ambient pressure. The possible complications in the non-compliant space e.g. lung, sinuses, middle ears include tissue disruption and hemorrhage.

4. CVS : the PaO2 of above 1500 mmHg causes systemic vasoconstriction and reduces the peripheral blood flow which will be manifested with hypertension and bradycardia. For O2 transport, the final outcome is attenuated with the markedly increase in tissue O2 delivery.

Anesthetic consideration

Providing anesthesia care of MAC during HBO therapy is quite challenging. There are many cautions to be exercised. The more details about this topic can be read further in the ref. no. 1. Generally, many areas of consideration which include :

   -Pressure related problems e.g. middle ear perforation, hemorrhage, barotrauma to air trapping pocket in the lungs, decompression sickness, etc. The ET's cuff should be checked and the air must be released if necessary.

   -The IV fluid administration : usually not an issue in multiplace chamber. But for monoplace chamber, the IV fluid pump must work against 2-3 ATM and only certain pumps can provide that. The IV tubing system should include the anti-reflux valve to prevent blood flow retrogradely back to the IV bag. In multiplace chamber, the glass container should be avoided or at least, used with the air vent device.

    -Fire hazard : rare but lethal. Both hyperbaric and hyperoxia environment enhance to the burning process. Caution should be exercised on any sparking materials even with patient's cloth and static electricity. Even with the lubricant used for the patient's transportation must be a nonflammable fluorocarbon. Humidification can reduce the static electricity build up. If the fire has started, it will be very fast and devastating that the extinguisher system may not be activated until all occupants of the chamber have died.

    -Anesthetic gases : perhaps the TIVA is the best technique because of anesthetic waste gas build up in the close chamber (multiplace chamber). However, halothane and other potent anesthetic agents has been used successfully. At hyperbaric O2 environment, the N2O can be used exceeding it's MAC and theoretically can be used as a sole anesthetic agent. This thought is hampered by the risk of developing of gas bubbles (decompression sickness) at faster rate, and the waste gas concern. For other volatile anesthetic agent, there are concerns about partial pressure of anesthetic gas at hyperbaric environment. This pressure reversal theory is very rare and non-problematic at the therapeutic HBO (no more than 6 ATA). However, the vaporizer including the flow meter which are calibrated at 1 ATM will not work properly. The gas density also increased. For IV anesthetic, even with the fact that hyperbaric environment can cause drug disposition, these effects are very rare in the clinical settings.

    -Mechanical Ventilator, not all ventilators can be used in this environment. For further details please read more in the ref no.1

    -Patient Monitoring : aneroid or electronic BP device is preferred than a mercury sphygmomanometer (concerns about mercury contaminant). The direct pressure monitoring system must be carefully zeroed again after the treatment started. The PA catheter's balloon should be left open (detach the syringe)