Predisposing Factors for Decompression Sickness
June 3, 2025 · 9 min read
It's Not Just the Dive Profile
For background on DCS itself, see the article on decompression sickness.
Across more than four decades of Diving & Hyperbaric Medicine literature, DCS is presented as a multifactorial condition influenced by both external exposure variables — depth, duration, ascent profile, ambient conditions, post-dive behaviour — and intrinsic biological factors: age, body composition, vascular and endothelial function, sex, and individual susceptibility to venous gas emboli. Two divers can run identical profiles on the same day and have completely different outcomes. DCS risk is not determined solely by what the algorithm permits.
DCS Type I vs Type II
Type I DCS affects musculoskeletal and lymphatic tissues. Symptoms include joint and limb pain (most commonly shoulders, elbows, knees), skin marbling or mottling, and lymphatic swelling. Painful, but not immediately life-threatening.
Type II DCS involves the central nervous system, spinal cord, inner ear, and pulmonary vasculature. Symptoms include paresthesias, weakness or paralysis, bladder or bowel dysfunction, vertigo, hearing loss, and the "chokes" — pulmonary DCS with chest pain and respiratory distress. These are serious presentations requiring urgent recompression.
Inner ear DCS is classified as Type II. It presents with true vertigo, nausea, and nystagmus — predominantly vestibular, typically after ascent — and is distinct from inner ear barotrauma (see the article on pulmonary barotrauma and AGE).
When Symptoms Appear
Published onset data compiled across multiple clinical series: approximately 42% of cases are symptomatic within 30 minutes of surfacing; 60% within 1 hour; 83% within 3 hours; 95% within 6 hours; 99% within 24 hours. Symptoms presenting more than 6 hours after surfacing should prompt consideration of alternative diagnoses. Beyond 24 hours, DCS becomes unlikely.
Physiological Risk Factors
Age
Age-related decline in cardiovascular efficiency and endothelial function appears to slow gas exchange and increase susceptibility. Several studies (McKenzie 1984; Skinner 1989; Berenji Ardestani 2015) identify increasing age as a risk modifier. Reduced vascular compliance and slower nitrogen elimination likely account for this tendency. Conservative profiles become more important, not less, as age increases.
Sex
Female divers show approximately a threefold increased incidence of DCS in some datasets (McKenzie 1984; Skinner 1989). Mechanisms are not fully established — hormonal effects on vascular compliance and adipose distribution have been proposed. Oral contraceptives may interact with decompression physiology, though evidence remains limited.
Body Composition
Nitrogen is approximately five times more soluble in fat than in aqueous tissues. Divers with higher adiposity carry a greater nitrogen burden for any given exposure. Weinmann (1991) identifies high body fat as increasing DCS risk through both greater nitrogen solubility and increased tissue mass. Tables and computers do not adjust for body composition.
Patent Foramen Ovale
PFO is present in approximately 25–30% of the general population — a normal anatomical variant, not a disease. During diving, the lungs function as a filter for venous gas emboli: small bubbles that form in venous blood are normally trapped and eliminated in the pulmonary capillaries before reaching arterial circulation. In divers with a significant PFO, a right-to-left shunt allows venous bubbles to cross directly into arterial circulation, bypassing the pulmonary filter entirely. These arterialised bubbles can then reach the brain, spinal cord, or coronary circulation.
Clinical data (Wilmshurst series; Germonpré prospective study, both cited in DAN/UHMS 2015 workshop) confirm that divers with right-to-left shunts have significantly higher rates of neurological DCI — often on profiles that would not ordinarily produce symptoms.
Unexplained neurological DCS on a conservative profile — particularly in a diver who followed tables or computer guidance and was not dehydrated, cold, or fatigued — warrants evaluation for PFO. The standard diagnostic test is bubble contrast echocardiography (saline "bubble study"), which reveals atrial-level shunting. Management options include more conservative decompression and, in divers with recurrent neurological DCI attributable to a large PFO, percutaneous closure — though this remains a specialist decision.
Hydration
Dehydration reduces plasma volume and impairs inert gas transport. Reduced circulating volume means slower nitrogen delivery to the lungs for elimination. Accepted as a risk modifier in multiple series (Gorman 1988). This includes alcohol-induced dehydration from the night before.
Fitness and Endothelial Function
Aerobic fitness supports cardiovascular efficiency and faster gas elimination. Endothelial function — the ability of vessel walls to produce nitric oxide and regulate perfusion — is impaired in unfit, obese, or older divers, and appears to modulate DCS susceptibility independently of cardiac output (Dugrenot 2022). ACE inhibition has been shown to modulate this susceptibility in animal models, suggesting the vascular endothelium is a meaningful variable, not just a correlate.
CO₂ Retention
Hypercapnia at depth — from skip-breathing, heavy work of breathing with dense gas, or inadequate ventilation — increases cerebral blood flow via vasodilation and may activate pre-existing bubble nuclei. French military investigation of cluster DCS events implicated mask-wearing pre-dive and CO₂ accumulation as contributing factors (Daubresse 2024).
Subclinical DCS: Bubbles Without Symptoms
Post-dive Doppler monitoring finds venous gas emboli in divers who feel completely fine after a dive. Grades of circulating bubbles on the Spencer scale (0–4) have been detected in significant proportions of asymptomatic divers following compliance with standard profiles. Alongside VGE detection, laboratory studies have found elevated endothelial microparticles and platelet activation markers in divers after recreational dives — even without reported symptoms.
This represents a spectrum from fully asymptomatic with subclinical bubble grades and endothelial markers, through mild fatigue and vague symptoms never attributed to DCS, through to frank symptomatic DCS. The subclinical findings provide the physiological rationale for conservative gradient factor settings — even clean dives involve bubble nuclei activity that accumulates across a dive series.
Environmental and Operational Factors
Cold Water Exposure
Cold causes peripheral vasoconstriction, reducing perfusion and off-gassing in the extremities. Rewarming after a cold dive may mobilise bubbles as circulation returns — a phenomenon noted in multiple series (McKenzie 1984; Gorman 1988; Weinmann 1991). Hot showers immediately post-dive increase vasodilation and may accelerate bubble growth at a time when nitrogen remains elevated in tissues (Blake 2018).
Exercise at Depth and Post-Dive
Physical exertion increases cardiac output and nitrogen uptake during the dive. Post-dive exercise — heavy lifting, running — increases circulation at a time when VGE are being eliminated, potentially mobilising bubbles and accelerating their growth.
Repetitive Dives and Short Surface Intervals
Residual nitrogen from previous dives adds to the loading of subsequent dives. Computers calculate residual nitrogen and extend no-decompression limits accordingly, but they cannot account for all variables: bubble nuclei from one dive may persist and be re-excited on the next. Repetitive diving appears in 55–64% of DCS cases in some series (Weinmann 1991; Knight 1988; Walker 1992).
Missed or Shortened Decompression
Profile violations were a factor in 42% of cases in a DAN report (Knight 1988) and 69% of cases in the Adelaide series (Gorman 1988). Skipping or shortening stops remains the most direct modifiable risk factor.
Previous DCS
Prior DCS episodes are associated with increased likelihood of recurrence (McIver 1991; Weinmann 1991; Gawthrope 2015). Whether this reflects persistent vascular damage, ongoing structural factors such as PFO or endothelial compromise, or behavioural patterns is not clearly established — likely a combination.
Flying After Diving
Commercial aircraft are pressurised to approximately 0.75–0.8 ATA — equivalent to about 2,000–2,500 m altitude. This reduced ambient pressure can trigger bubble growth if significant nitrogen remains in tissues. DAN minimum surface intervals before flying:
- Single no-decompression recreational dive: 12 hours minimum
- Repetitive no-decompression dives: 18 hours minimum
- Dives requiring decompression stops: 24 hours minimum
For technical dives or any dive with residual symptoms, the 24-hour figure should be treated as a floor, not a ceiling.
Practical Interpretation
DCS risk is driven by the interaction of gas loading, ascent profile, and a set of physiological and environmental modifiers. No algorithm accounts for all of these. A profile within limits is necessary but not sufficient — the diver's state on that day matters.
Factors within direct control: hydration, alcohol avoidance, pre- and post-dive exercise management, ascent discipline, post-dive thermal exposure, and surface interval compliance. Factors requiring medical awareness: PFO history, prior DCS, fitness-to-dive evaluation for significant comorbidities.
References
- McKenzie B (1984). Epidemiology of DCS. Cited in multiple DAN reports.
- Gorman DF (1988). Decompression sickness — Adelaide series.
- Knight R (1988). DCS in recreational divers. DAN report.
- Weinmann M (1991). DCS risk factors in recreational diving.
- McIver RG (1991). Recurrent DCS. US Navy series.
- Gawthrope IC (2015). Venous gas emboli and DCS history.
- Dugrenot E (2022). Endothelial function and decompression susceptibility.
- Blake DF (2018). Temperature, bubble growth, and DCS risk.
- Daubresse A et al (2024). CO₂ and decompression sickness in French military divers. Diving and Hyperbaric Medicine.
- Denoble PJ, Holm JR (eds) (2015). Patent Foramen Ovale and Fitness to Dive: Consensus Workshop Proceedings. DAN/UHMS.
- Denoble PJ, Marroni A (eds) (2019). Differential Diagnosis of Decompression Illness Workshop Proceedings. DAN/UHMS.
- Risberg J et al (2024). Surface decompression on air vs oxygen. Diving and Hyperbaric Medicine.
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