Hyperbaric Exposure

SECTION TWO

Human Performance in Extreme Environments

CHAPTER 13 | HYPERBARIC EXPOSURE

Barotrauma, Gas Toxicity, Decompression Sickness & Arterial Gas Embolism — CAsP Unit 13 Objectives 1–11

Hyperbaric exposure — immersion in an environment of elevated pressure, as experienced during underwater diving — is the physiological mirror image of hypobaric exposure. Where altitude physiology is defined by the consequences of reduced pressure (hypoxia, DCS from dissolved gas supersaturation, trapped gas expansion), hyperbaric physiology is defined by the consequences of elevated pressure and the gas mixtures required to breathe in it: barotrauma from pressure differentials during ascent and descent, oxygen toxicity from elevated partial pressures of breathing gas, narcosis and other inert gas effects, and decompression sickness from the tissue nitrogen loaded at depth that must be carefully eliminated during ascent. Aviation physiology encompasses hyperbaric exposure because aviators are also divers, and because understanding both the hypo- and hyperbaric sides of pressure physiology is required for comprehensive aeromedical management of the diver-aviator population.

This chapter covers all 11 CAsP Unit 13 objectives: the physical gas laws in the hyperbaric context (13.1), barotrauma of the ear, sinuses, lungs, and brain (13.2), pulmonary oxygen toxicity (13.3), cerebral oxygen toxicity (13.4), carbon dioxide and carbon monoxide toxicity (13.5), nitrogen narcosis, helium, and other inert gas effects (13.6), hyperbaric DCS (13.7), flying after diving and diving at altitude (13.8), individual DCS susceptibility factors (13.9), DCS versus cerebral arterial gas embolism (13.10), and treatment of DCS and AGE (13.11).

The hyperbaric environment is governed by the same three physical gas laws that govern hypobaric physiology, but in the opposite direction: where altitude produces decreasing pressure with expanding gas volumes and falling partial pressures, diving produces increasing pressure with compressing gas volumes and rising partial pressures. Each gas law has specific hyperbaric clinical implications.

Boyle’s Law and Barotrauma

Boyle’s Law (P₁V₁ = P₂V₂ at constant temperature) governs the behavior of all gas-containing body spaces during pressure change. The key difference between hyperbaric and hypobaric barotrauma:

• Descent (increasing pressure): Ambient pressure increases → gas in body cavities COMPRESSED. If a gas-filled space cannot freely equalize (Eustachian tube blocked, lung gas trapped by breath-hold), the space shrinks below the ambient-equilibrated volume → the surrounding tissue is squeezed inward against the compressing gas space → SQUEEZE (negative pressure barotrauma). At 10 msw (2 ATA), gas volume is compressed to half its surface volume.
• Ascent (decreasing pressure): Ambient pressure decreases → any gas trapped in body cavities EXPANDS. If the gas cannot vent (breath-hold during ascent), the expanding gas overpressurizes the space → REVERSE SQUEEZE or pulmonary overinflation. Most dangerous on ascent from diving is breath-hold during ascent → pulmonary overinflation syndrome → pneumothorax, pneumomediastinum, and arterial gas embolism (AGE).

Worked Boyle’s Law Example

A diver descends from surface (1 ATA = 760 mmHg) to 30 msw (4 ATA). A gas pocket of 100 mL at surface is compressed to:

• V₂ = P₁V₁/P₂ = (1 × 100)/4 = 25 mL. Three-quarters of the original volume is lost.

On ascent from 30 msw to surface while breath-holding: the 25 mL pocket expands back to 100 mL — a 4-fold increase in volume that can rupture alveoli if the gas cannot vent through the airways.

Henry’s Law and Decompression Sickness

Henry’s Law (dissolved gas concentration ∝ partial pressure of gas in contact with liquid) explains why diving causes far greater tissue nitrogen loading than altitude exposure — and therefore why hyperbaric DCS is generally more severe than hypobaric DCS:

• Tissue nitrogen loading at depth: At 30 msw (4 ATA), a diver breathing compressed air breathes nitrogen at a partial pressure of 4 × 0.79 × 760 = 2,401 mmHg. By Henry’s Law, tissue nitrogen equilibrates to this pressure over time — approximately 4× the sea-level saturation value. When the diver ascends, all this dissolved nitrogen must be safely eliminated through carefully staged decompression stops.
• Tissue half-times and diving: The same five-compartment tissue model (Haldane half-times: 5, 10, 20, 40, 75 minutes) applies. At depth, ALL tissues are being nitrogen-loaded; fast tissues (blood, CNS) equilibrate rapidly, slow tissues (fat, tendons, bone marrow) continue loading throughout long dives. The total tissue nitrogen burden after a deep or long dive far exceeds anything achievable at altitude.

Altitude vs. Diving DCS Comparison

Table 13.1. Comparison of Altitude (Hypobaric) and Diving (Hyperbaric) DCS

Dalton’s Law and Partial Pressures at Depth

Dalton’s Law (P(total) = sum of partial pressures; P(gas) = P(total) × F(gas)) governs all gas partial pressure calculations in diving:

• P(O₂) at depth: At 30 msw (4 ATA) breathing air: P(O₂) = 4 × 0.21 × 760 = 638 mmHg. This approaches the threshold for pulmonary oxygen toxicity (>300–400 mmHg for prolonged exposure). At 60 msw (7 ATA) on air: P(O₂) = 1,117 mmHg — well within the range for acute CNS oxygen toxicity.
• Oxygen exposure limits: CNS toxicity threshold: P(O₂) > 1,400–1,600 mmHg (approximately 1.6–2.0 ATA partial pressure). Pulmonary toxicity: prolonged exposure to P(O₂) > 300–400 mmHg (0.5 ATA). These limits constrain the maximum safe diving depth on air and the depth of oxygen treatment tables.
• P(N₂) at depth: At 30 msw (4 ATA) breathing air: P(N₂) = 4 × 0.79 × 760 ≈ 2,400 mmHg. This is approximately 4× the sea-level saturation value — the tissue nitrogen loading driving hyperbaric DCS risk.

Barotrauma during diving results from the failure of gas-containing body spaces to equilibrate with ambient pressure during depth changes. The same Boyle’s Law pressure-volume relationship that governs hypobaric trapped gas disorders now operates in reverse (and with greater magnitudes) in the hyperbaric environment.

Middle Ear Barotrauma (Ear Squeeze)

The most common diving injury. During descent, increasing ambient pressure compresses the gas in the middle ear. If the Eustachian tube is blocked (by URI, allergic rhinitis, or mucosal swelling), the middle ear cannot equilibrate — the tympanic membrane is forced inward by the higher external pressure, and the middle ear mucosa becomes engorged and hemorrhagic:

• Mechanism: Descent → ↑ ambient pressure → if Eustachian tube blocked, middle ear gas cannot equalize → TM retracts inward → mucosal hemorrhage → pain → TM perforation (with sudden pain relief followed by water entry).
• Prevention: Equalize early and often during descent (Valsalva, Frenzel, Toynbee). Never dive with nasal congestion or URI. Descend feet-first to allow natural equalization. Never forcibly equalize below the threshold of initial discomfort — if equalization fails, ascend to reduce the differential.
• Reverse squeeze (ascent): Rarely, gas trapped in the middle ear on descent cannot vent during ascent → middle ear pressure exceeds ambient → TM pushed outward → pain on ascent. Less common than descent squeeze.

Inner ear barotrauma: Extreme equalization effort (Valsalva at high differential) can transmit pressure to the inner ear through the round and oval windows — rupturing the round window membrane (perilymph fistula) or causing hemorrhage in the inner ear structures. Presents as sudden severe vertigo, sensorineural hearing loss, and tinnitus during or immediately after an aggressive equalization attempt. A medical emergency requiring immediate ENT evaluation.

Sinus Barotrauma (Sinus Squeeze)

Parallels altitude sinus barotrauma but on descent. Descent → ↑ ambient pressure → if sinus ostia are blocked (URI, sinusitis, polyps), the sinus gas cannot equalize → sinus mucosa engorges and hemorrhages into the sinus → severe facial pain. Frontal sinus most commonly affected (tortuous fronto-nasal duct). Prevention: never dive with active sinusitis or URI. No volitional technique can open blocked sinuses; decongestants may help but are unreliable underwater.

Pulmonary Barotrauma and Arterial Gas Embolism (AGE)

The most dangerous diving barotrauma. Caused by gas expansion in the lungs during ascent while the breath is held (or when airway obstruction prevents gas from venting freely). As ambient pressure decreases during ascent, the expanding lung gas exceeds the structural limits of alveolar walls:

• Mechanism of alveolar rupture: Breath-hold or regional airway obstruction (bronchospasm, mucus plug, asthma) during ascent → expanding alveolar gas cannot vent through airways → trans-alveolar pressure exceeds ~80–100 cmH₂O → alveolar rupture → gas tracks along tissue planes.
• Consequences of alveolar rupture: (a) Pneumothorax (gas into pleural space → lung collapse; tension pneumothorax if gas continues to accumulate); (b) Pneumomediastinum (gas into mediastinum → substernal pain, voice change, subcutaneous emphysema); (c) Arterial Gas Embolism (AGE) — gas enters pulmonary veins → left heart → systemic arterial circulation → end-arterial bed lodging (brain, coronary arteries). AGE is the most feared consequence.
• Clinical presentation of AGE: Immediate neurological symptoms (usually within seconds to minutes of surfacing): loss of consciousness, seizure, focal neurological deficits (stroke-like), visual disturbance, confusion. AGE from pulmonary barotrauma is one of the most time-critical diving emergencies.
• Prevention: NEVER breath-hold during ascent on compressed gas. Continuous open-circuit breathing ensures gas can vent. Particular risk in divers with pulmonary blebs, bullae, or obstructive lung disease.

Brain Barotrauma (Cerebral AGE)

The brain is the primary target organ of AGE because cerebral arterial circulation is an end-arterial system — there is minimal collateral circulation around major cerebral arteries. Gas emboli lodging in cerebral arteries produce immediate ischemia in the vascular territory distal to the obstruction. The clinical picture mimics acute stroke: sudden unilateral or bilateral motor/sensory deficits, aphasia, visual field defects, altered consciousness, or seizure — all appearing within seconds to minutes of surfacing from a compressed-gas dive with breath-holding.

A separate ‘cerebral DCS’ category exists for bubble formation within brain tissue (see Objective 13.7 and 13.10), but the most immediately dramatic neurological presentation in diving is AGE from pulmonary barotrauma rather than from dissolved nitrogen bubble formation.

Pulmonary oxygen toxicity results from prolonged exposure to elevated partial pressures of oxygen. Unlike CNS oxygen toxicity (which presents acutely and dramatically), pulmonary toxicity develops insidiously over hours to days of sustained oxygen exposure and reflects cumulative oxidative damage to the delicate alveolar-capillary barrier.

Mechanism

Elevated P(O₂) → increased generation of reactive oxygen species (ROS) in pulmonary tissue, particularly superoxide radical (O₂•⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (•OH). These ROS overwhelm the pulmonary antioxidant defense systems (superoxide dismutase, catalase, glutathione) with prolonged exposure, causing:

• Lipid peroxidation of pulmonary cell membranes.
• Oxidative damage to type I and type II alveolar epithelial cells (type I cells cannot regenerate; type II cells, which produce surfactant, are particularly vulnerable).
• Endothelial cell damage → capillary leak → interstitial and alveolar edema.
• Progressive inflammation → alveolar membrane thickening → impaired diffusion.

Threshold for Pulmonary Oxygen Toxicity

Pulmonary toxicity is a cumulative, time-dose dependent phenomenon with a recognized threshold:

• P(O₂) < 0.5 ATA (< 380 mmHg): Generally considered safe for indefinite exposure in normobaric settings. Standard medical oxygen therapy at sea level (P(O₂) ≈ 713 mmHg − water vapor) can begin to produce pulmonary toxicity with very prolonged exposure (>6–12 hours at very high flow rates), but is practically a concern mainly in hyperbaric or intensive care settings.
• P(O₂) 0.5–1.0 ATA (380–760 mmHg): Symptoms may develop after 8–24 hours of continuous exposure. Relevant to commercial air travel oxygen users and some therapeutic applications.
• P(O₂) > 1.0 ATA (> 760 mmHg): Relevant threshold in hyperbaric medicine. HBO therapy at 2.0–2.8 ATA breathing 100% O₂ delivers P(O₂) of 1,400–2,100 mmHg — within the pulmonary toxicity range. Air breaks during USN Treatment Tables prevent accumulating a toxic oxygen dose during therapeutic HBO sessions.

Clinical Presentation

• Early symptoms: Substernal chest discomfort (tracheobronchitis) aggravated by deep inspiration; dry cough; nasal stuffiness.
• Progressive: Worsening chest tightness and dyspnea on exertion; reduced vital capacity (measurable reduction in VC is the earliest objective sign of pulmonary toxicity); bilateral inspiratory crackles.
• Severe: Acute respiratory distress syndrome (ARDS) pattern: diffuse alveolar damage, bilateral infiltrates, refractory hypoxemia. Rare in controlled diving or therapeutic settings.

The Unit Pulmonary Toxic Dose (UPTD) system provides a quantitative method for tracking cumulative pulmonary oxygen exposure across multiple dives or HBO sessions, allowing oxygen dose management to prevent toxicity.

Diving Context

In diving, pulmonary oxygen toxicity is primarily a concern with:

• Oxygen-enriched breathing gas mixtures (Nitrox: 32–36% O₂; EANx) at depth.
• 100% oxygen decompression stops at shallow depth (3–6 msw).
• Closed-circuit rebreathers using high O₂ fractions.
• Repetitive dives on oxygen-enriched gas without adequate surface intervals.

Cerebral (CNS) oxygen toxicity is the acute, life-threatening form of oxygen toxicity in diving, presenting with sudden and unpredictable convulsions (grand mal seizure without premonitory warning) in an underwater diver. Unlike pulmonary toxicity, which develops gradually over hours, CNS toxicity can occur without warning within minutes of exceeding the oxygen partial pressure threshold in a susceptible individual.

Mechanism

The mechanism of CNS oxygen toxicity is not completely understood but involves several proposed pathways:

• Excessive neuronal oxidative stress: High P(O₂) overwhelms antioxidant defenses in neuronal tissue, producing ROS that damage neuronal membranes, disrupt neurotransmitter systems, and impair neuronal energy metabolism.
• Vasoconstriction and paradoxical ischemia: High P(O₂) causes cerebral vasoconstriction (reducing cerebral blood flow), potentially producing paradoxical local ischemia in the context of hyperoxia. Interruption of oxidative phosphorylation efficiency.
• GABA inhibition: Proposed inhibition of GABA-ergic inhibitory neurons by ROS → increased net excitatory tone → seizure threshold lowered.

Threshold and Dose-Response

• P(O₂) threshold: CNS toxicity risk increases substantially above P(O₂) = 1.4–1.6 ATA (approximately 1,060–1,216 mmHg). The USN maximum allowable P(O₂) for in-water diving is 1.6 ATA; for closed-circuit rebreathers 1.2–1.4 ATA (allowing a safety margin for the added risk of exercise and immersion).
• Individual variation: CNS oxygen toxicity susceptibility varies markedly among individuals and even within the same individual on different days. Factors increasing risk: exercise (increases oxygen consumption and CNS metabolic rate), CO₂ retention (retained CO₂ increases cerebral blood flow and O₂ delivery), fever, hypoglycemia, anxiety, and certain medications.
• VENTID-C mnemonic (CNS toxicity prodromal symptoms): Visual disturbances (tunnel vision, flickering vision), Ears (tinnitus, ringing), Nausea, Twitching (facial or extremity muscle twitching), Irritability/abnormal behavior, Dizziness. However, these prodromal symptoms may not occur before the convulsion in many cases — seizure can be the first sign.

Operational Consequences

An underwater CNS oxygen toxicity seizure is potentially fatal because:

• Loss of consciousness underwater → drowning if the diver is not on a surface-supplied system.
• Loss of demand valve control → removal of regulator from mouth → inhalation of water.
• Breath-holding during the tonic phase of the convulsion during ascent → pulmonary barotrauma and AGE as the diver rises through decreasing pressure.

Management: Reduce inspired P(O₂) immediately. For in-water seizure: support the diver at the surface, maintain airway, remove from water, do not restrain during active convulsion. Most seizures self-terminate within 1–2 minutes with removal from high oxygen environment.

Carbon Dioxide Toxicity in Diving

Carbon dioxide (CO₂) toxicity in diving results from inadequate elimination of metabolic CO₂ from the breathing circuit, producing hypercapnia that compounds the physiological stresses of depth:

Causes of CO₂ Retention in Diving

• Skip breathing: Deliberate breath-holding between breaths to reduce air consumption. Slows CO₂ elimination — dangerous because CO₂ accumulates silently while the diver is unaware.
• Exhausted CO₂ scrubber (rebreather): Closed-circuit rebreathers recirculate exhaled gas through a CO₂ absorbent (typically soda lime or similar). When the scrubber is exhausted, CO₂ is rebreathed — rapidly producing hypercapnia. Scrubber failure is the most dangerous single-point failure in CCR diving.
• High work of breathing: Dense gas at depth increases the work of breathing (gas density ∝ pressure → at 40 msw [5 ATA], gas is 5× denser than at surface → airway resistance ↑ → ventilation limited → CO₂ retention).
• Inadequate ventilation / breath-holding: Any reduction in alveolar ventilation at depth elevates PACO₂ with the same consequences as at the surface, but with additional compound effects.

Effects of Hypercapnia in Diving

• Increased cerebral blood flow: CO₂ is a potent cerebral vasodilator. Elevated PACO₂ → increased CBF → increased O₂ delivery to the brain → significantly increases the risk of CNS oxygen toxicity at any given P(O₂). This CO₂–O₂ toxicity synergy is one of the most dangerous interactions in diving medicine.
• Headache, confusion, and narcosis amplification: Mild hypercapnia (PACO₂ > 50–55 mmHg) produces headache, confusion, and dizziness. Moderate hypercapnia worsens nitrogen narcosis effects.
• Loss of consciousness: Severe hypercapnia (PACO₂ > 80–90 mmHg) can directly produce loss of consciousness ('CO₂ convulsions'), which is rapidly fatal underwater.
• Increased DCS risk: Elevated CO₂ promotes bubble nucleation and may increase DCS risk during decompression.

Carbon Monoxide Toxicity in Diving

Carbon monoxide (CO) toxicity in diving can result from:

• Contaminated air supply: The most common cause. CO enters the compressed air supply from: (a) compressor exhaust gases drawn into the compressor intake (improper positioning); (b) lubricating oil breakdown within a faulty compressor cylinder producing CO; (c) other combustion sources near the intake. Even small concentrations of CO in compressed air become dangerous at depth because Dalton’s Law amplifies all partial pressures — 40 ppm CO at the surface becomes 200 ppm CO at 40 msw (5 ATA), rapidly producing acute CO toxicity.
• Mechanism: CO binds hemoglobin with approximately 250× the affinity of oxygen → carboxyhemoglobin (COHb) → functional anemia + left-shift of ODC (Bohr effect) → severe tissue hypoxia at the cellular level. At depth, the simultaneous hyperoxia (elevated P(O₂) from compressed air at depth) may mask the symptoms of CO toxicity (the high dissolved oxygen partially compensates for the COHb-mediated oxygen transport failure), delaying recognition until ascent when P(O₂) falls to surface levels and the full impact of CO poisoning becomes apparent.
• Clinical presentation: Headache, dizziness, nausea, confusion, loss of consciousness (classic CO poisoning picture). The cherry-red skin discoloration seen in severe CO poisoning may be less apparent in divers due to compression effects on skin vasculature.
• Treatment: 100% oxygen (or hyperbaric oxygen for severe cases). HBO therapy is the most effective treatment for significant CO poisoning — it displaces CO from hemoglobin approximately 3× faster than normobaric 100% O₂.

Nitrogen Narcosis (‘Rapture of the Deep’)

Nitrogen narcosis is a reversible, dose-dependent impairment of neurological function produced by high partial pressures of dissolved nitrogen in neural tissue. It is the hyperbaric equivalent of alcohol intoxication — affecting judgment, reaction time, manual dexterity, and anxiety management in a manner directly analogous to ethanol, but without the hangover (it reverses completely on ascent).

Mechanism

The ‘Meyer-Overton hypothesis’ proposes that nitrogen narcosis (and general anesthetic effects of inert gases) results from lipid solubility of the inert gas — gases with higher oil-to-water solubility coefficients are more narcotic (Bunsen solubility coefficient for N₂ in oil = 0.067; significantly narcotic at elevated pressures). The exact molecular mechanism likely involves: membrane expansion from dissolved inert gas (disrupting ion channel function), interference with synaptic transmission (particularly NMDA receptor modulation), and possibly direct protein binding at hydrophobic sites.

Depth and Severity

The narcotic potency of nitrogen is governed by the partial pressure of nitrogen dissolved in neural tissue, which increases proportionally with depth. The ‘Martini Law’ approximation: each 10 msw (33 fsw) of depth on air has an intoxicating effect equivalent to approximately one standard martini at the surface:

Table 13.2. Nitrogen Narcosis: Depth, PN₂, and Approximate Impairment

Operational Concerns

• Narcosis is compounding: Anxiety, cold, fatigue, and CO₂ retention all worsen narcosis at any given depth. The stressed, cold, fatigued diver is at higher narcotic risk than a rested, warm, calm diver at the same depth.
• Narcosis masks DCS symptoms: A narced diver may not recognize or report pain from joint DCS. This delayed recognition can allow DCS to progress to more serious forms during decompression.
• Reversal: Narcosis resolves completely and rapidly on ascent (within seconds to minutes as dissolved nitrogen partial pressure in neural tissue falls). No persistent neurological sequelae from nitrogen narcosis alone.

Helium and Other Inert Gases

Helium (He) is used in deep diving gas mixtures (Heliox: He + O₂; Trimix: He + N₂ + O₂) as a replacement for nitrogen specifically because it has much lower narcotic potency than nitrogen:

• Why helium is less narcotic: Helium has very low lipid solubility (Bunsen coefficient in oil much lower than N₂), meaning it dissolves poorly in lipid-rich neural membranes and causes minimal membrane disruption. Practical rule: helium has essentially NO narcotic effect at recreational or military diving depths.
• High-pressure nervous syndrome (HPNS): At very great depths (>150–200 msw; professional saturation diving), breathing helium at extreme pressure produces a distinct neurological syndrome: tremors, myoclonic jerking, electroencephalographic changes (theta wave intrusion), nausea, and dizziness. HPNS is distinct from narcosis and is caused by the direct effect of very high hydrostatic pressure on neural membrane function (not related to helium’s chemical properties specifically). Adding small amounts of N₂ to helium mixtures (Trimix) at extreme depths partially ameliorates HPNS.
• Helium speech distortion: The low density and high sound velocity of helium (4× the velocity of sound compared to air) distorts voice communications from the diver, producing a characteristic high-pitched ‘Donald Duck’ voice that can impair voice intelligibility underwater and at the surface. Special electronic unscramblers are used in saturation diving systems.
• Hydrogen: Used experimentally in very deep saturation diving (Hydrox: H₂ + O₂). Very light, low narcotic potential, but explosive risk limits its use. Not employed in standard diving.
• Argon: Highly narcotic (high lipid solubility) and dense — contraindicated as a breathing gas but sometimes used as a drysuit inflation gas (not breathed).

Hyperbaric (diving) DCS shares the same fundamental mechanism as hypobaric (altitude) DCS — dissolved inert gas supersaturation leading to bubble formation — but differs in important ways that make it generally more severe, more variable in presentation, and more operationally complex to prevent and treat.

Mechanism of Hyperbaric DCS

During a dive, compressed breathing gas (typically air or a helium-containing mixture) is breathed at the ambient pressure of depth. By Henry’s Law, tissues absorb inert gas (nitrogen and/or helium) proportional to the partial pressure of that gas in the breathing mix. A dive to 30 msw (4 ATA) breathing air saturates tissues with nitrogen at 4× the sea-level value. During ascent, ambient pressure decreases — tissue inert gas tensions now exceed the ambient pressure (supersaturation) and gas must be eliminated through the lungs. If ascent is too rapid, supersaturation exceeds the critical threshold and bubbles form in tissues and vasculature.

Key Differences Between Hyperbaric and Hypobaric DCS

Table 13.3. Hyperbaric vs. Hypobaric DCS: Key Differences

Clinical Manifestations of Hyperbaric DCS

The same Type I and Type II classification applies in hyperbaric DCS, but the distribution of manifestations differs from altitude DCS:

• Type I (Pain only): Joint pain (bends) most common, typically large joints (knees and shoulders most frequent in diving DCS). Skin manifestations (pruritus, cutis marmorata). Often resolves with recompression + HBO.
• Type II Neurological: More common in diving than altitude DCS. The spinal cord is the most commonly affected neurological structure in diving DCS, particularly the thoracic cord. Possible mechanisms: bubble formation in the white matter fat sheaths; venous infarction from spinal epidural venous engorgement under high pressure; paradoxical AGE through PFO. Presents as: girdle pain (belt-like sensation around chest/abdomen), ascending paresthesias, lower extremity weakness, bladder dysfunction, paraplegia. Requires immediate HBO.
• Type II Cerebral: Confusion, visual disturbances, focal neurological deficits mimicking stroke. Must be distinguished from arterial gas embolism (AGE) from pulmonary barotrauma (see Objective 13.10).
• Type II Pulmonary (Chokes): Massive venous bubble embolism to pulmonary vasculature → substernal chest pain + dyspnea + cardiovascular collapse. Less common than in altitude DCS proportionally, but potentially fatal.
• Dysbaric Osteonecrosis: A chronic complication unique to hyperbaric DCS (not seen after altitude DCS). Aseptic necrosis of bone (particularly humeral head and femoral head) results from vascular bubble obstruction and ischemia. Develops over weeks to months. Documented in commercial divers and tunnel workers with a history of DCS. Not typically seen in recreational divers making no-decompression dives.

Decompression Tables and Models

Prevention of hyperbaric DCS requires managing the rate of inert gas elimination to keep supersaturation below the bubble-formation threshold throughout ascent. The U.S. Navy Dive Tables and modern dive computers use decompression algorithms (based on Haldane’s compartmental model and its successors, including dissolved gas models and bubble models):

• Haldane compartment model: Five tissue half-times (5–75 minutes). Staged decompression stops allow slow tissues to off-gas at safe supersaturation ratios. The 2:1 rule (tissue PN₂/ambient pressure ≤ 2.0) remains the foundational constraint.
• U.S. Navy Standard Air Decompression Tables: Define bottom time and depth combinations requiring decompression stops, and specify the stop depth and duration. Mandatory use for all U.S. Navy dive operations.
• No-decompression limit (NDL): For shallower dives and shorter bottom times, ascent to the surface can be performed without decompression stops (within the no-decompression limit). Exceeding the NDL requires decompression stops.

The combination of diving and altitude exposure creates additive physiological risks because both hypobaric and hyperbaric exposures affect tissue inert gas levels and bubble formation thresholds. Both directions of this interaction matter: the diver who then flies (flying after diving) and the diver who dives at altitude above sea level (altitude diving).

Flying After Diving

As described in Chapter 9 (Hypobaric Exposure, Objective 9.8), tissue nitrogen levels remain elevated above sea-level baseline for hours after diving, even after a safe no-decompression dive that produces no DCS symptoms. When this diver then ascends to flight altitude — even commercial cabin altitude (8,000 ft equivalent; PB ≈ 565 mmHg) — the additional pressure reduction pushes tissue nitrogen supersaturation above the critical threshold, producing DCS at altitudes that would be safe for non-diver, sea-level-saturated individuals.

UHMS Minimum Surface Intervals Before Flying

Table 13.4. UHMS Minimum Pre-Flight Surface Intervals After Diving

Special concerns for aviation: Any aviator who has dived within the surface interval window must be excluded from altitude chamber training, high-altitude flights, or any flight in unpressurized aircraft. Pre-chamber screening must specifically ask about recent diving activity. The consequences of waiving this exclusion — DCS during an altitude chamber run in a recently dived trainee — represent a preventable medical emergency.

Diving at Altitude (Altitude Diving)

When diving at a lake or water body located at altitude (e.g., a mountain lake at 2,000–3,000 m / 6,500–10,000 ft elevation), the ambient surface pressure is already BELOW sea level. This creates two important physiological concerns:

• Reduced ambient pressure before diving: The diver’s tissue PN₂ at this altitude is ALREADY below sea-level saturation (they have been at altitude, exhaling nitrogen). When they enter the water and descend, the absolute tissue nitrogen loading at any given depth is calculated from the altitude-adjusted surface pressure, not from sea-level pressure. Standard sea-level dive tables overestimate safe bottom time for altitude dives because they assume sea-level starting tissue N₂. The diver should use altitude-corrected tables or an altitude-capable dive computer.
• Reduced surface pressure on ascent: After the dive, the diver surfaces to an ambient pressure that is BELOW sea level. The ascent decompression calculation must account for this reduced surface pressure. Standard sea-level decompression tables programmed for a surface pressure of 1 ATA are not valid at altitude — they would underestimate the supersaturation ratio at the reduced-pressure surface, potentially allowing DCS to develop despite technically following the table.
• Risk after altitude dive + further altitude ascent: An altitude diver who then travels to even higher altitude (e.g., driving from a 2,000 m lake to a 3,000 m mountain pass) faces compounded risk from two sequential pressure reductions. Any venous gas emboli from the dive will expand by Boyle’s Law during the further ascent.

DCS susceptibility varies markedly among individuals and even within the same individual on different dives, despite identical exposure profiles. This variability is one of the most operationally challenging aspects of DCS prevention — no dive table or computer can guarantee DCS protection for any individual on any dive. Understanding the modifying factors allows identification of high-risk scenarios and individuals.

Table 13.5. Factors Affecting Individual DCS Susceptibility in Hyperbaric Diving

Cerebral arterial gas embolism (CAGE, also called AGE — arterial gas embolism) and neurological DCS are both causes of acute neurological emergency in divers, and they require the same definitive treatment (HBO). However, they have fundamentally different mechanisms, different timing patterns, and different risk profiles. Distinguishing them clinically is important for understanding pathophysiology, though the practical immediate management is the same.

Table 13.6. Neurological DCS vs. Cerebral Arterial Gas Embolism (CAGE): Differential Features

The key clinical heuristic: AGE typically presents IMMEDIATELY at the surface with sudden, severe, maximal-at-onset neurological symptoms (LOC, convulsion, hemiplegia) in a diver who surfaced from a shallow dive — often after an uncontrolled or panicked ascent. Neurological DCS more typically presents progressively, over minutes to hours after surfacing, in a diver who exceeded their bottom time or ascent rate from depth.

However, this distinction is not absolute: (1) Both can occur simultaneously (a diver with DCS also develops AGE from PFO shunting). (2) Severe neurological DCS can present rapidly. (3) The same definitive treatment (HBO) is required regardless. The term Decompression Illness (DCI) is sometimes used to encompass both DCS and AGE when distinction is clinically impossible or irrelevant to immediate treatment decisions.

The treatment of DCS and AGE is one of the most time-critical interventions in diving medicine. Early treatment dramatically improves outcomes; delayed treatment allows progression of neurological injury, bubble enlargement, secondary inflammatory cascades, and potentially permanent disability. The treatment algorithm follows a stepwise escalation from first aid to definitive hyperbaric therapy.

Step 1: Field First Aid — Immediate 100% Oxygen

The single most important immediate field intervention is administration of 100% oxygen by demand mask or non-rebreather mask at the highest available flow rate. Benefits:

• Nitrogen window: Breathing 100% O₂ sets inspired PN₂ = 0, maximizing the diffusion gradient from bubble/tissue nitrogen toward the alveoli. Bubbles begin to shrink as nitrogen diffuses out, driven by the maximum nitrogen elimination gradient.
• Tissue oxygenation: Hyperoxygenated plasma (PaO₂ may reach 600+ mmHg on 100% O₂) can perfuse ischemic tissues around bubbles where normal red cell-carrying blood cannot penetrate.
• Reduces bubble growth: The nitrogen window prevents further nitrogen diffusion INTO bubbles, arresting bubble growth.

Do NOT administer analgesics: Pain is the primary treatment response monitor. Masking pain prevents recognition of treatment success or failure during HBO and may delay appropriate treatment escalation.

Step 2: Evacuation at Lowest Possible Altitude

Transport to a hyperbaric facility by the lowest-altitude route available (below 1,000 ft AGL by helicopter, or sea-level-pressurized fixed-wing aircraft). Any altitude increase expands existing bubbles by Boyle’s Law, worsening DCS or AGE. Continuous 100% oxygen during transport.

Step 3: Hyperbaric Oxygen Therapy (HBO) — Definitive Treatment

HBO is the definitive treatment for DCS and AGE. The standard USN Treatment Tables:

USN Treatment Table 5 (Pain-Only DCS — Type I)

Indicated for: Type I DCS (pain only, skin manifestations) that completely resolves with surface-level 100% O₂ before HBO initiation, or mild Type I that fails to resolve with surface O₂. Not used for Type II or AGE.

• Depth: 60 fsw (2.8 ATA) breathing 100% O₂ with air breaks.
• Total time at pressure: Approximately 135 minutes.
• If pain-only DCS symptoms resolve during Table 5, treatment is complete. If not, extend to Table 6.

USN Treatment Table 6 (Serious DCS and AGE — Primary Table)

Indicated for: ALL Type II DCS (neurological, pulmonary, vestibular), AGE, and any DCS not resolved by Table 5. The most commonly used treatment table in diving medicine worldwide.

• Initial depth: 60 fsw (2.8 ATA) on 100% O₂.
• If AGE or severe neurological DCS: May initially compress to 165 fsw (6 ATA) on air (Table 6A) to achieve maximum bubble compression (Boyle’s Law: bubble volume ≈ 17% of surface volume at 6 ATA) before surfacing to 60 fsw for oxygen breathing.
• Air breaks: Mandatory air-breathing intervals (5 minutes of air after each 20–25 minutes of O₂) to prevent oxygen toxicity (pulmonary and CNS) at 2.8 ATA.
• Total time: Approximately 4 hours 45 minutes at standard Table 6; extensions (additional 25-minute O₂ periods) added if symptoms do not fully resolve at planned ascent times.

Physiological Mechanisms of HBO in DCS/AGE Treatment

• Boyle’s Law (mechanical compression): At 2.8 ATA, bubble volume is reduced to ~36% of surface-level volume. This mechanical compression immediately reduces the obstruction caused by intravascular bubbles and the mechanical tissue distension from extravascular bubbles. At 6 ATA (Table 6A), bubble volume is reduced to ~17% — nearly complete mechanical elimination.
• Nitrogen elimination gradient (oxygen window): Breathing 100% O₂ at elevated pressure → zero inspired PN₂ → maximum diffusion gradient from bubble nitrogen to surrounding tissue and blood → bubble nitrogen content progressively decreases → bubble shrinks toward extinction.
• Tissue oxygenation through hyperoxygenated plasma: At 2.8 ATA breathing 100% O₂, PaO₂ reaches approximately 1,900–2,000 mmHg. The resulting hyperoxygenated plasma (dissolved O₂ exceeds normal without hemoglobin) can diffuse into ischemic areas around bubbles where red cells cannot pass, breaking the vicious cycle of hypoxia-induced edema and inflammation.
• Anti-inflammatory effects: HBO reduces neutrophil adherence to endothelium, platelet aggregation, and inflammatory mediator release triggered by bubble-endothelium contact — limiting secondary tissue injury from the inflammatory cascade.

Treatment Extensions and Tailing Dives

If neurological or pulmonary symptoms have not fully resolved at the scheduled ascent time, the Table 6 may be extended with additional 25-minute periods of 100% O₂ breathing (up to 2 extensions at 60 fsw). Following the initial treatment, additional HBO sessions (‘tailing dives’) at shallower pressures may be conducted daily until the patient plateaus or fully recovers. Typically 2.0 ATA for 90–120 minutes.

Special Considerations: Spinal Cord DCS and AGE

• Spinal cord DCS: Requires immediate HBO (Table 6). Delay is the primary preventable cause of permanent neurological deficit. Even mild initial symptoms (paresthesias only) should be treated urgently — spinal DCS can progress rapidly to paraplegia without treatment.
• AGE: May be initially treated at 165 fsw (6 ATA) on air for maximum bubble compression (Table 6A), then transitioned to 100% O₂ at 60 fsw (2.8 ATA) for the remainder of Table 6. Neurological recovery from AGE can be rapid with prompt HBO; outcomes worsen significantly with treatment delays >4–6 hours.

Post-Treatment Return to Diving and Flying

• Pain-only DCS complete resolution: No diving for a minimum of 72 hours; no flying for 72 hours after complete resolution.
• Neurological DCS: No diving for a minimum of 4 weeks after complete resolution; diving medical evaluation required. No flying for minimum 4 weeks; formal aeromedical evaluation.
• AGE: No diving for minimum 3–6 months; pulmonary evaluation (CT chest) to identify underlying blebs/bullae before return. Aeromedical evaluation required before return to flight.
• Recurrent DCS: Evaluation for PFO, pulmonary pathology, and individual risk factors before any return to diving or altitude.