Hyperbaric Oxygen Therapy: Mechanisms, Indications and Dose
The HBO domain tests whether you can justify a treatment, choose a dose, and recognize that the dose is more than a pressure. Expect stems about indications, mechanism, table selection and complications.
Mechanisms worth naming
- Hyperoxygenation: at 2-3 ATA, dissolved oxygen in plasma alone approaches resting tissue demand, so ischemic tissue is oxygenated even with poor perfusion.
- Bubble reduction: increased ambient pressure shrinks bubbles by Boyle's law - the reason DCI is treated at 2.8 ATA (60 fsw).
- Vasoconstriction with preserved oxygen delivery reduces edema; HBO also blunts reperfusion injury, enhances fibroblast and collagen activity, promotes angiogenesis and osteoclast activation, and improves oxidative killing by neutrophils (including aminoglycoside uptake).
Dose
- Routine wound, radiation and infection protocols: 2.0-2.4 ATA, 90-120 minutes, once daily, five to six days per week, with air breaks.
- Air breaks (typically 5 minutes every 30 minutes at treatment pressures) reduce both CNS and pulmonary oxygen toxicity.
- 2.8 ATA is a DCI treatment pressure; it is not a "stronger" wound-healing dose.
Indications to know cold
Decompression illness, arterial gas embolism, carbon monoxide and cyanide poisoning, clostridial myonecrosis, necrotizing soft-tissue infection, crush injury and compartment syndrome, compromised flaps and grafts, refractory osteomyelitis, osteoradionecrosis and soft-tissue radiation injury, delayed radiation injury, idiopathic sudden sensorineural hearing loss, central retinal artery occlusion, intracranial abscess, and non-healing diabetic foot ulcers. Traumatic brain injury and PTSD are conditional indications.
Worked example
A patient with a Wagner grade 3 diabetic foot ulcer after debridement has periwound transcutaneous oximetry of 22 mmHg on room air. Values below roughly 30-40 mmHg predict failure to heal, so adjunctive HBO is reasonable: 2.0-2.4 ATA for 90 minutes daily, with TcPO2 and wound measurements repeated around 20-30 treatments. Stop if there is no objective improvement after an adequate trial.
Traps
- Treating "more pressure" as better; 2.8 ATA outside DCI adds toxicity without benefit.
- Calling bleomycin an absolute contraindication - it is a relative risk, whereas untreated pneumothorax is absolute.
- Believing a carboxyhemoglobin level decides CO treatment; neurologic findings and pregnancy drive the decision.
- Forgetting that HBO accelerates COHb clearance from hours to roughly 20-30 minutes at treatment pressure.