Hyperbaric oxygen therapy as short note in md anaesthesia exam from standard textbook like millers

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Hyperbaric Oxygen Therapy (HBOT)

Based primarily on Miller's Anesthesia, 10th Edition (Chapter 71)

Definition

Hyperbaric oxygen therapy (HBOT) is the administration of 100% oxygen at ambient pressures greater than 1 atmosphere absolute (ATA). It can be delivered in:

Monoplace chambers: Single patient; pressurized with 100% O2 directly
Multiplace chambers: Multiple patients + medical personnel; pressurized with compressed air while patients breathe 100% O2 via mask, hood, or endotracheal tube

Physiological Basis / Mechanism

Increased Oxygen Delivery

By Henry's Law, the amount of gas dissolved in a liquid is proportional to its partial pressure. At 3 ATA with 100% O2, dissolved O2 in plasma rises to ~6 mL/dL (vs. ~0.3 mL/dL at 1 ATA) -- enough to meet tissue O2 requirements even without hemoglobin.

Key Mechanisms

Mass action effect: Elevated PaO2 accelerates CO dissociation from hemoglobin. HbCO half-life at room air (~5 hrs) falls to ~90 min on 100% O2 at 1 ATA, and ~20 min at 2.5 ATA (Miller's, Fig. 71.3)
Vasoconstriction: HBOT causes ~20% reduction in blood flow via vasoconstriction, but tissue PO2 still rises due to dissolved O2 -- this reduces edema in crush injuries and compartment syndromes
Antimicrobial effect: Tissue PO2 >70 mmHg inhibits growth of anaerobes (e.g., Clostridium). Also enhances leukocyte oxidative killing of bacteria
Angiogenesis and wound healing: HBOT stimulates fibroblast proliferation and neovascularization in hypoxic wounds
Anti-inflammatory: Inhibits neutrophil-endothelium adhesion (relevant in CO poisoning and reperfusion injury)
Bubble compression: Reduces volume of gas bubbles in decompression sickness and gas embolism (Boyle's Law)

Indications

UHMS-Approved / Evidence-Based Indications (Miller's)

Condition	Rationale
Carbon monoxide (CO) poisoning	Accelerates HbCO dissociation; reduces delayed neurologic sequelae
Decompression sickness ("bends")	Compresses nitrogen bubbles; accelerates nitrogen elimination
Arterial gas embolism	Mechanical bubble reduction + promotes reabsorption
Clostridial myonecrosis (gas gangrene)	Inhibits anaerobe growth; reduces toxin production
Necrotizing soft tissue infections	Adjunct: reduces mortality
Crush injury / compartment syndrome	Reduces edema; preserves marginal tissue
Compromised flaps and grafts	Enhances neovascularization and O2 delivery
Osteoradionecrosis / radiation tissue injury	Promotes angiogenesis in hypoxic irradiated tissue
Chronic refractory osteomyelitis	Enhances leukocyte bactericidal activity
Thermal burns	Reduces edema; enhances wound healing
Diabetic foot ulcers / problem wounds	Stimulates angiogenesis in hypoxic wound bed
Acute anemia	Bridge when transfusion refused (e.g., Jehovah's Witnesses)
Air/gas embolism	Bubble compression and resolution

Treatment Schedules (Miller's, Fig. 71.9)

Chronic conditions (e.g., osteoradionecrosis): 100% O2 at 2 ATA for 2 hours, repeated daily
Clostridial myonecrosis / acute conditions: 100% O2 at 2.8-3 ATA for 85 minutes with two 5-minute "air breaks" to reduce O2 toxicity
CO poisoning: 2.8-3 ATA for 1-3 treatments or until clinically stable (consensus guidelines)
Decompression sickness: US Navy Treatment Table 6 (most commonly used)

Contraindications

Absolute

Untreated pneumothorax (tension pneumothorax on decompression)

Relative

Chronic obstructive pulmonary disease with CO2 retention (risk of apnea)
Uncontrolled seizure disorders (CNS O2 toxicity risk)
Claustrophobia
High fever (lowers CNS O2 toxicity threshold)
Certain chemotherapeutic agents (e.g., bleomycin - pulmonary toxicity; cisplatin - neuropathy; doxorubicin - cardiac toxicity; disulfiram - blocks superoxide dismutase)
Recent ophthalmic surgery (air/gas bubble in eye)
Active viral infection (theoretical)
Uncontrolled congestive heart failure

Complications / Adverse Effects

1. Barotrauma

Middle ear squeeze (most common) -- tympanic membrane rupture
Sinus squeeze
Pulmonary barotrauma (on ascent if airway obstruction)
Arterial gas embolism (rare, with pulmonary barotrauma)

2. Oxygen Toxicity

CNS O2 toxicity ("Paul Bert effect"):

Occurs at high PO2 (>1.6 ATA), especially with elevated CO2, fever, or exercise
Manifests as visual symptoms (tunnel vision), nausea, tinnitus, facial twitching, and grand mal seizures (the "VENTID" mnemonic - Vision, Ears, Nausea, Twitching, Irritability, Dizziness)
Self-limiting on removal of O2; no residual damage
Prevented by air breaks during treatment

Pulmonary O2 toxicity ("Lorrain Smith effect"):

Tracheobronchial irritation, cough, burning chest pain
Decreased vital capacity with prolonged exposure
Progressive to ARDS if continued
Quantified by Unit Pulmonary Toxic Dose (UPTD): 1425 UPTD = ~10% VC reduction
Mitigated by intermittent air breaks

Ocular O2 toxicity:

Myopia (reversible) from lens changes with long-term treatment
Cataracts with very prolonged treatment courses

3. Decompression Sickness in Attendants (multiplace chambers)

Personnel inside multiplace chambers breathe compressed air and are at risk for decompression sickness on ascent

4. Fire Hazard

Oxygen-enriched environment is highly combustible; strict protocols required

CO Poisoning - Special Emphasis (Exam Favourite)

Guidelines for HBOT in CO poisoning (Miller's):

Neurologic impairment (dizziness, loss of consciousness), even if apparently recovered
Cardiac abnormalities (ischemia, arrhythmias, ventricular failure)
Metabolic acidosis
HbCO > 25%
Pregnancy (fetal Hb has higher CO affinity; fetus at greater risk)

HBOT vs. normobaric O2: Four RCTs with adequate pressure (2.5-3 ATA) showed improved neurologic outcomes with HBOT; persistent/delayed neurologic sequelae are significantly reduced.

Anesthetic Considerations During HBOT

Standard monitoring is modified for the hyperbaric environment
Drugs: parenteral drug dosing schedules are safe up to 6 ATA (no significant pressure-related changes in pharmacokinetics)
Cuffed endotracheal tubes: inflate cuffs with water or saline (not air) to prevent cuff volume changes during pressurization/depressurization
Ventilators must be hyperbaric-compatible
IV fluid bags must be vented to prevent volume changes
Pacemakers/implanted devices: check device compatibility with pressure
Anesthetic agents: nitrous oxide is avoided (expands gas spaces)

Summary Table

Parameter	Details
Standard pressure	2-3 ATA for most conditions
Duration	60-120 min per session
Dissolved O2 at 3 ATA	~6 mL/dL (vs. 0.3 normal)
HbCO half-life at 2.5 ATA	~20 minutes
Absolute contraindication	Untreated pneumothorax
Most common complication	Middle ear barotrauma
CNS toxicity threshold	>1.6 ATA PO2

Sources: Miller's Anesthesia 10e (Chapter 71, Hyperbaric Oxygen Therapy) | Morgan & Mikhail's Clinical Anesthesiology 7e | Guyton & Hall Medical Physiology

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