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Hyperbaric Oxygen Therapy (HBOT) - Emergency Medicine Resident Guide

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1. What Is HBOT?

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2. The Equipment

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3. Key Physics & Physiology

Pressure Units

How Oxygen Delivery Changes Under Pressure

Two Mechanisms of Effect

• Reduces bubble volume (Boyle's Law: PV = constant) - relevant in decompression sickness, arterial gas embolism
• Increases nitrogen washout from bubbles
• Reduces edema in post-traumatic tissue beds (vasoconstriction reduces inflow without reducing outflow)

• Reverses tissue ischemia
• Induces reactive oxygen species (ROS) - this is BOTH the therapeutic mechanism AND the toxicity mechanism
• Stimulates the body's antioxidant enzyme system
• Upregulates endothelial NO synthase
• Induces angiogenesis (elevated O2 levels persist 2-4 hours post-treatment)

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4. Beneficial Effects Table (Tintinalli's)

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5. UHMS-Approved Indications (14 Official)

Emergency/Acute Indications (HIGH PRIORITY for EM residents):

• Bubbles in arterial or venous circulation from trauma, iatrogenic causes (central line placement, surgery, lung biopsy), or diving
• HBOT reduces bubble size via Boyle's Law AND promotes nitrogen reabsorption
• HBO protocol: 6 ATA initially (Navy Treatment Table 6) to crush bubbles, then step down
• Time-critical - initiate as soon as possible; delay worsens outcomes

• CO impairs hemoglobin function AND directly poisons the mitochondrial electron transport chain, inhibiting ATP formation
• HBO accelerates CO elimination: CO half-life on room air = ~5 hours, on 100% O2 NRB = ~60-90 min, on HBO = ~20-30 min
• HBO directly competes with CO at cytochrome oxidase
• Two studied protocols:
  • Salt Lake City protocol: 3 ATA x 60 min → 2 ATA x 65 min; repeat x2 treatments at 6-12h intervals
  • Philadelphia protocol: 2.8 ATA x 30 min → 2.0 ATA x 90 min (single treatment)
• Indications for HBO in CO poisoning (professional society consensus):
  • Loss of consciousness at any point
  • Neurologic signs/symptoms (confusion, ataxia, focal deficits)
  • Cardiovascular compromise (ischemia, arrhythmia)
  • COHb >25-30% (some centers use >25%, pregnant patients >15-20%)
  • Severe metabolic acidosis
  • Extremes of age (elderly, pediatric)
• Goal: reduce delayed neurologic sequelae (which develop 2-28 days post-exposure and affect up to 30% of moderate-severe cases)
• The landmark Weaver RCT (2002) showed HBO reduced neurologic sequelae from 46% to 25% at 6 weeks

• Seen in closed-space fires burning synthetic materials
• CN and CO produce synergistic toxicity
• Standard CN antidote (hydroxocobalamin) remains primary; HBO is adjunctive
• HBO may directly reduce CN toxicity and augment antidote therapy

• Occurs when dissolved inert gas (nitrogen) comes out of solution and forms bubbles in tissues/blood after rapid ascent from depth (diving), or altitude ascent, or rapid decompression
• Type I DCS: musculoskeletal pain ("the bends"), skin ("the creeps"), lymphedema
• Type II DCS: neurologic (spinal cord or cerebral), inner ear ("the staggers"), cardiopulmonary ("the chokes")
• Mechanism: nitrogen bubbles obstruct microcirculation, activate complement, trigger inflammation
• HBOT is definitive treatment - recompression physically compresses bubbles, HBO promotes nitrogen elimination and reduces inflammation
• Contact the Divers Alert Network (DAN) at 1-919-684-9111 for 24/7 guidance on nearest chamber and treatment protocol
• Do NOT delay; field oxygen pre-treatment (100% NRB) while arranging transfer is appropriate

• Clostridium perfringens produces alpha-toxin and is an obligate anaerobe
• Growth is inhibited at tissue PO2 >70 mmHg; HBO directly kills the organism at high O2 tensions (Guyton & Hall, p. 563)
• HBO inhibits toxin production, limits spread, and may convert a previously near-100% fatal condition to a survivable one
• HBO is adjunctive to aggressive surgical debridement and antibiotics - never delays surgery
• Typical protocol: 3 ATA x 90 min, three times daily initially

• HBO reduces edema, reverses ischemia, and blunts ischemia-reperfusion injury
• For established compartment syndrome: HBO is adjunctive after fasciotomy to prevent wound complications and accelerate recovery (~3-5 treatments needed)
• For threatened fasciotomy cases: HBO can reduce need for fasciotomy by reducing edema
• Also used in: threatened replants/digit amputations, frostbite, electrical injury, snake envenomation, high-pressure injection injuries

• Mixed aerobic-anaerobic infections (Fournier's, necrotizing fasciitis)
• HBO kills obligate anaerobes, reduces toxin production, enhances leukocyte killing of aerobic organisms (oxygen-dependent oxidative burst)
• Always adjunctive to aggressive surgical debridement - this is the primary treatment
• HBO may reduce mortality and limit spread

Wound/Subacute Indications (EM should know these for referral):

• Adjunctive to surgical debridement and antibiotics
• 14/15 studies in systematic review showed positive results; median remission rate 89% (Rockwood & Green)
• Evidence quality is moderate; UHMS gives "weak recommendation"

• Central retinal artery occlusion (CRAO) - most recently added UHMS indication; time-critical (treat within 24h)
• Diabetic foot ulcers with wound PO2 <35 mmHg (transcutaneous O2 pressure; in-chamber TcPO2 >200 mmHg predicts benefit)
• 90-120 min sessions at 1.9-2.5 ATA, once daily 5-6x/week; clinical improvement expected after 15-20 treatments

• Soft tissue and bony necrosis (osteoradionecrosis)
• Promotes angiogenesis in hypoxic irradiated tissue
• Standard wound protocol; 20-40 treatments typical

• HBO mobilizes stem cells, promotes angiogenesis, reduces edema in jeopardized tissue

• When transfusion is impossible (religious objection, unavailability, hemolytic reaction)
• At 3 ATA on 100% O2, dissolved plasma O2 alone can meet basal metabolic needs

• Adjunctive to surgical drainage and antibiotics

• Protocol: 2.4 ATA x 100 min with two air breaks; starts 3x/day, tapers to 2x, then 1x/day
• Benefits: reduced fluid requirements (30-35% less), decreased mortality, fewer surgical procedures, shorter LOS (Tintinalli's, p. 181)

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6. Emerging/Off-Label Indications (not UHMS-approved but in use)

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7. Transcutaneous Oxygen Pressure (TcPO2) - Key Monitoring Tool

• TcPO2 <35 mmHg in room air = hypoxic wound (candidate for HBO)
• In-chamber TcPO2 >200 mmHg = predicts likely benefit from HBO
• If in-chamber TcPO2 does NOT rise above 200, patient is unlikely to respond; evaluate for vascular disease (Sabiston, p. 917)

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8. Contraindications

Absolute Contraindications

Relative Contraindications

• Claustrophobia (may require anxiolytics)
• Untreated URI/sinusitis - inability to equalize barotrauma risk
• Seizure disorder - oxygen toxicity lowers seizure threshold
• COPD with CO2 retention - may suppress hypoxic drive
• Eustachian tube dysfunction
• High fever - increases seizure risk from O2 toxicity
• Implanted cardiac defibrillator or certain pacemakers (check with device manufacturer)
• Pregnancy (relative; used when benefit outweighs risk, e.g., CO poisoning)
• Asymptomatic pulmonary blebs - risk of pneumothorax

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9. Complications (from the drug and the device)

Pressure-Related (Barotrauma)

Oxygen Toxicity

Fire Risk

• High-pressure O2 environment dramatically increases fire risk
• Strict protocols: no petroleum-based products, no combustible materials, no electronics not approved for hyperbaric use
• This is why multiplace chambers are pressurized with air (not O2) despite patients breathing O2 through masks

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10. Practical EM Workflow: When to Call for HBOT

EM Scenario → Consider HBOT consultation if:

CO Poisoning:
  → LOC, neuro sx, COHb >25%, CVS compromise, pregnancy, age extremes

Diving emergency:
  → Any DCS or arterial gas embolism → call DAN (1-919-684-9111) immediately
  → Start 100% O2 NRB during transfer

Gas gangrene (Clostridial):
  → After OR debridement and antibiotics → adjunctive HBO

Necrotizing fasciitis:
  → After OR debridement → adjunctive HBO

Crush injury / compartment syndrome:
  → Post-fasciotomy or threatened fasciotomy → adjunctive HBO

Acute thermal burn:
  → Significant burns → HBO consultation for early treatment

CRAO:
  → Within 24h of onset → urgent HBO (time-critical)

Severe anemia (no transfusion possible):
  → HBO as bridge therapy

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11. Quick Reference Summary Card

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12. High-Yield EM Board/Clinical Pearls

1. HBO is a drug - it has dose-dependent toxicity, contraindications, and drug interactions (bleomycin, doxorubicin, disulfiram are absolute contraindications).
2. Never delay surgery for HBO in gas gangrene or necrotizing fasciitis - HBO is always adjunctive, never primary.
3. Pneumothorax must be treated before HBO - it is the only absolute contraindication related to the patient's acute condition in the ED.
4. CO poisoning is your most common EM-HBOT indication - know the indications for HBO vs. normobaric O2, the two protocols, and the goal of preventing delayed neurologic sequelae.
5. DCS and gas embolism are the two most pressure-responsive conditions - the physical effect of bubble compression is as important as the oxygenation.
6. Middle ear barotrauma is the most common complication - intubated/sedated patients cannot equalize; myringotomy may be needed before treatment.
7. At 3 ATA on 100% O2, dissolved plasma O2 alone sustains metabolism - this is why HBO works even in severe anemia or CO poisoning where hemoglobin is non-functional.
8. Air breaks are protective - ask why a protocol includes them (answer: limits cumulative O2 dose to prevent CNS and pulmonary toxicity).
9. TcPO2 guides wound patient selection - in-chamber TcPO2 >200 mmHg predicts response; <200 mmHg suggests vascular disease needs addressing first.
10. Know your local chamber - find out if your hospital has a hyperbaric unit, and if not, which facility is your regional referral center. DAN (1-919-684-9111) provides 24/7 guidance for diving emergencies nationwide.

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