Chapter 21

### INTRODUCTION

Anesthesia has been used in conjunction with hyperbaric oxygen therapy (HBOT) since the 1960s. Hyperbaric medicine was first found to be useful in divers suffering from decompression sickness. Since this time, increased ambient pressure has been used to treat a variety of clinical conditions, including carbon monoxide poisoning, arterial gas embolism, and decompression sickness.

### PHYSIOLOGIC EFFECTS

1. Increased barometric pressure—According to Boyle’s law, pressure and volume are indirectly related. Therefore, an increase in ambient pressure will result in a concurrent contraction of gas in the same chamber. For example, pockets of gas in the body (middle ear, paranasal sinuses, intestines, and pneumothorax) decrease in volume when exposed to increased altitude, increased water depth, or a hyperbaric oxygen chamber. This principle also explains how hyperbaric oxygen can be used to treat an arterial gas embolism or decompression sickness.

2. Increased partial pressure of oxygen—HBOT facilitates breathing oxygen at increased ambient pressure and results in an increased in arterial and tissue oxygenation (Po2). This increased oxygenation results in a reduction in cardiac output, an increase in systemic vascular resistance, and a decrease in pulmonary vascular resistance. The resulting systemic vasoconstriction allows for the treatment of traumatic edema that occurs in crush injuries, but is inhibited by the presence of nitric oxide. HBOT is typically delivered at 2–3 ATA (absolute atmospheres). At these levels, renal, mesenteric, and hepatic blood flows are unchanged; however, cerebral blood flow decreases.

3. Increased partial pressure of nitrogen—According to the Meyer–Overton hypothesis, increasing the partial pressure of an inert gas (e.g., nitrogen, hydrogen, argon) in a mixture of inhaled gases results in a narcotic effect. This effect is thought to be the result of increased GABAA receptors in the nigrostriatal pathway, resulting in the release of dopamine. At 3 ATA, N2 has a mild euphoric effect. At 6 ATA, individuals may experience memory loss. Unconsciousness results at 10 ATA.

4. High-pressure nervous syndrome—At ambient pressures greater than 15–20 ATA, high-pressure nervous syndrome (HPNS) can present as tremor, ataxia, nausea, and vomiting. HPNS can be prevented by slow compression and increasing the partial pressure of nitrogen or another narcotic gas to the breathing mix.

5. Pressure reversal of anesthesia—High pressures, in the absence of increased partial pressure of a narcotic gas, will decrease the effectiveness of both inhaled and intravenous anesthetics. The 50% effective dose (ED50) for most inhaled anesthetics is increased by 20% at 50 ATA. Similarly, the ED50 for barbiturates, propofol, and dexmedetomidine has been proven to increase in high pressure conditions. This effect is not seen at elevations used in HBOT (3–6 ATA). Regional anesthesia and intravenous agents are recommended rather than inhaled anesthetics. Gaseous anesthetics can infiltrate the chamber, potentially affecting medical personnel.

6. Effects of hyperbaric exposure on drug disposition—There is no evidence of pharmacokinetic or pharmacodynamic alteration of anesthetics under hyperbaric conditions under 6 ATA. Thus, conventional dosing of parenteral anesthetics ...

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