Hyperbaric Oxygen Therapy

1. THE PHYSIOLOGICAL FOUNDATION

• The Pressure-Volume Engine: Hyperbaric oxygen (HBO) therapy involves breathing 100% oxygen inside a chamber pressurized to an ambient pressure greater than sea level. The therapeutic effects are driven by both increased ambient pressure and supraphysiologic oxygen tension.
• Cellular and Mechanical Effects: Mechanically, the increased pressure shrinks the volume of obstructing gas bubbles (Boyle's Law) (External Knowledge) and facilitates the massive washout of inert nitrogen from the tissues. At the cellular level, the supraphysiologic oxygen terminates lipid peroxidation (halting tissue destruction/reperfusion injury), preserves intracellular adenosine triphosphate (ATP), and profoundly enhances leukocyte peroxidase bactericidal activity and antibiotic penetration against pathogens.

2. THE MATHEMATICAL / DIAGNOSTIC ENGINE

• Carbon Monoxide (CO) Poisoning Thresholds: HBO is strictly indicated for CO toxicity if the carboxyhemoglobin (COHb) level is >25% with normal clinical findings, or >15% in pregnancy/fetal distress.
• Clinical Triggers for CO Poisoning: Regardless of the absolute COHb level, HBO is mandated if the patient exhibits: syncope/loss of consciousness, coma, seizures, altered mental status (GCS <15), abnormal cerebellar function, or evidence of myocardial ischemia (abnormal ECG, angina, or positive troponin).
• The Dive Tables: The standard first-line hyperbaric protocol for decompression sickness (DCS) and arterial gas embolism (AGE) is the U.S. Navy Treatment Table 6. This begins at a pressure of 2.8 atmospheres absolute (ATA) with subsequent slow decompression to 1.8 ATA, for a total elapsed treatment time of 285 minutes.

3. THE CRASHING PATIENT PROTOCOL

• Step 1: The Normobaric Bridge: In the prehospital or initial ED setting, immediately administer 100% normobaric oxygen (15 L/min via non-rebreather mask). This is critical to begin washing inert nitrogen out of the lungs while awaiting chamber availability.
• Step 2: Hemodynamic Stabilization: Maintain the patient in a supine position and administer isotonic intravenous fluids (0.9% NaCl or Ringer's lactate); aggressively avoid hypothermia.
• Step 3: Pre-Chamber Airway Modifications: If the patient requires endotracheal intubation or a urinary catheter, you must ensure the ET tube cuff and Foley balloons are filled with sterile saline/water, rather than air, prior to chamber entry to prevent volume expansion and tissue rupture during decompression.
• Step 4: Aeromedical Transport Rules: If flying the patient to a hyperbaric facility, the aircraft cabin must be pressurized, or the pilot must fly at an altitude below 500 feet (152 meters) above the extraction point to prevent further expansion of gas bubbles.

4. THE PHARMACOLOGY MATRIX

• 100% Oxygen (Hyperbaric)
• Mechanism: Hyper-oxygenates hemoglobin (100% saturation) and forces massive amounts of oxygen to dissolve directly into the blood plasma, bypassing the need for functional red blood cells for tissue oxygen delivery (External Knowledge).
• Dosing (AGE/DCS): U.S. Navy Table 6 (2.8 to 1.8 ATA over 285 minutes). For severe or worsening symptoms, U.S. Navy Treatment Table 6A (initial pressurization up to 6 ATA) may be used.
• Clearance/Metabolism: Systemic utilization.
• Adjunctive Therapies: Sedating agents (e.g., benzodiazepines) are frequently required to manage severe confinement anxiety/claustrophobia inside the hyperbaric chamber.

5. THE TITRATION & MONITORING GRID

• Endpoint of Therapy: For patients with AGE or DCS, additional repetitive HBO treatments are recommended until the patient reaches a clinical plateau where there is no further neurologic improvement.
• Ear Pressurization (Barotrauma): Intubated patients, or those lacking the capacity to actively equalize middle ear pressures during the "dive," may require prophylactic tympanostomy prior to treatment to prevent tympanic membrane rupture.
• Oxygen Toxicity: High-pressure oxygen can cause central nervous system toxicity, classically manifesting as hyperoxic seizures inside the chamber. This requires immediate removal of the oxygen mask and lowering of the FiO2 (External Knowledge).

6. THE DANGER ZONE

• The Untreated Pneumothorax Trap: Placing a patient with an untreated or occult pneumothorax into a hyperbaric chamber is a lethal, "clean kill" mistake. Pressure changes will rapidly convert it into a fatal tension pneumothorax. A chest tube or needle decompression must be placed prior to HBO therapy.
• The Trendelenburg Trap: Historically, patients with suspected arterial gas embolism were placed in the Trendelenburg (head-down) position. Do not do this. It increases intracranial pressure and dangerously facilitates coronary gas embolization. Keep the patient strictly supine.
• The Cyanide Distractor: While HBO is highly effective for severe carbon monoxide poisoning, it provides no proven benefit for isolated cyanide victims. Do not delay the administration of standard cyanide antidotes (e.g., hydroxocobalamin) to arrange for HBO.

7. MCQ MASTERCLASS

• The "Pregnant CO Victim" Buzzword: A board question featuring an asymptomatic pregnant woman with a carboxyhemoglobin (COHb) level of 18% is testing your knowledge of specific thresholds. Because CO binds more strongly to fetal hemoglobin, causing severe fetal hypoxia, the absolute threshold for HBO in pregnancy is >15%.
• The "Scuba Surfacing Stroke" Pairing: A patient who rapidly ascends from a scuba dive and immediately loses consciousness or develops focal neurologic deficits (stroke-like symptoms) has an Arterial Gas Embolism (AGE). The definitive treatment is immediate recompression in a hyperbaric chamber.
• The Distractor: A question asking for the initial treatment of a diving casualty will often offer "hyperbaric therapy" as an answer choice. The correct initial bedside action is always 100% normobaric oxygen at 15 L/min via a non-rebreather mask.

8. THE ER RESIDENCY SCRIPT

"Critical Care / Hyperbaric team, I am consulting you for a 28-year-old G1P0 pregnant female who presents with acute carbon monoxide poisoning secondary to a faulty home generator. She had a transient loss of consciousness at the scene but is currently GCS 15. Her vital signs are stable, but her venous blood gas reveals a COHb level of 18%. Because she crosses the >15% threshold for pregnancy and had an episode of syncope, she strictly meets UHMS criteria for emergent hyperbaric oxygen therapy to prevent fetal hypoxic injury. We have her on 100% normobaric oxygen via non-rebreather. A screening chest X-ray confirms there is no pneumothorax, and her ECG shows no acute ischemic changes. She requires immediate transfer to your facility for a hyperbaric dive to clear the COHb load."