Fit to Dive
June 9, 2025 · 9 min read
Managing Chronic Conditions and Building Real-World Dive Readiness
DAN epidemiological data spanning decades consistently identifies cardiac events as a leading cause of diver fatalities, accounting for roughly 25% of deaths — with the proportion highest in divers over 45. The 2021 DAN Annual Diving Report recorded 104 recreational compressed gas fatalities worldwide; cardiac events featured in a significant fraction. This matters for fitness-to-dive evaluation because the diver who dies from DCS is rare. The diver who dies from a first cardiac event underwater, misclassified as drowning, is common.
Medical Contraindications and Their Physiological Basis
Patent Foramen Ovale
The foramen ovale is a fetal cardiac opening between the right and left atria that normally closes at birth. It remains patent in approximately 25–30% of the adult population — a normal anatomical variant, not a disease. In diving, the risk is specific: venous gas emboli in venous blood are ordinarily filtered by the pulmonary capillaries. If a PFO is open, bubbles can cross from the right atrium directly into the left, bypassing that filter and entering arterial circulation. These arterial bubbles can lodge in cerebral, spinal, or coronary vessels.
Clinical data from the 2015 DAN/UHMS PFO Consensus Workshop found that divers with PFO had significantly higher rates of neurological decompression illness. Multiple studies also identified elevated rates of white matter hyperintensities in divers, attributed to repeated subclinical paradoxical embolism across a diving career.
PFO is not an automatic disqualification. Divers with known PFO should use conservative decompression profiles, avoid ascent schedules that generate high venous bubble loads, and discuss risk-benefit with a dive medicine physician. PFO closure is discussed in the literature as an option for divers with recurrent, unexplained neurological DCI attributable to confirmed shunting — but is not universally recommended as a preventive measure.
Asthma
The core concern with asthma and diving is gas trapping. During ascent, expanding gas in the lungs must vent freely. In a diver with bronchospasm or airways that close under cold, exercise, or allergen triggers, gas cannot escape fast enough. The result is pulmonary overdistension, which can progress to pneumothorax, mediastinal emphysema, or arterial gas embolism — the most serious form of pulmonary barotrauma.
Exercise-induced bronchoconstriction is particularly relevant because diving is physically demanding: cold water, breathing resistance from dense gas, and exertion all interact to provoke symptoms in susceptible divers.
Current medical consensus: well-controlled mild asthma, with normal spirometry (FEV₁ ≥75% predicted, FEV₁/FVC ratio normal) and no exercise-induced symptoms, may be compatible with recreational diving after specialist evaluation. Moderate or severe asthma, asthma requiring frequent rescue inhaler use, or asthma with exercise-induced bronchospasm should be considered a contraindication until controlled. The physiological test that matters is bronchoprovocation challenge (e.g., methacholine), which can reveal occult airway hyper-reactivity not apparent on resting spirometry.
ADHD Medications
A concern in dive medicine was whether stimulant use (methylphenidate, Ritalin, Concerta) might lower the threshold for CNS oxygen toxicity — the convulsive response to high partial pressures of oxygen that can cause sudden loss of consciousness underwater.
Research published in 2024 (Gur et al.) found that therapeutic doses of methylphenidate did not increase CNS oxygen toxicity risk in controlled hyperbaric exposure. This does not mean stimulant use is trivially safe in all diving contexts — cardiovascular effects (elevated heart rate, blood pressure) still warrant evaluation — but the specific CNS O₂ toxicity concern is not supported by the available evidence.
Divers on any stimulant medication should obtain dive medical clearance, particularly for technical or CCR diving where oxygen partial pressures are managed more precisely.
Diabetes
The 2005 DAN/UHMS Diabetes and Recreational Diving Workshop reversed the historical absolute prohibition on insulin-requiring diabetics diving. The workshop was based on over a decade of UK and DAN data showing that well-managed diabetic divers were already diving without disproportionate incident rates.
The key risks underwater are hypoglycaemia causing impaired judgment or loss of consciousness, and the inability to self-rescue. Cold water, exercise, and stress all accelerate glucose consumption. Peripheral vascular disease — a complication of poorly controlled diabetes — alters inert gas washout in limb tissues and may increase the risk of limb-site DCS.
The DAN 2005 consensus protocol: pre-dive blood glucose target ≥150 mg/dL (8.3 mmol/L), confirmed stable or rising. Three measurements at 60 min, 30 min, and immediately before the dive — cancel if any reading falls below 150 mg/dL or if glucose is falling between readings. Postpone if glucose exceeds 300 mg/dL (16.7 mmol/L). Maximum depth: 30 msw. Maximum duration: 60 minutes. No mandatory decompression stops, no overhead environments. Buddy must be non-diabetic, trained, and briefed; oral glucose must be carried on every dive. HbA1c ≤9% within 1 month of medical assessment. Monitor glucose for 12–15 hours post-dive.
Well-controlled diabetes with no significant vascular, retinal, renal, or neurological complications is manageable for recreational diving under these conditions. Poorly controlled diabetes or diabetes with significant end-organ complications remains a contraindication.
Cardiac Conditions and Exercise Tolerance
The cardiovascular demands of diving are routinely underestimated. Immersion alone increases central blood volume. Cold-water vasoconstriction raises cardiac afterload. Exertion while breathing dense gas under pressure imposes significant respiratory load. In emergencies — surface swims against current, pulling an unconscious buddy — peak demand can reach 10–12 METs.
For recreational diving, exercise tolerance of at least 6–8 METs without symptoms (angina, arrhythmia, dyspnoea) is recommended. For technical diving, a target of ≥10 METs on formal exercise testing is widely cited in dive medicine literature as appropriate.
Conditions requiring medical evaluation before diving include any history of angina, MI, or coronary artery disease; cardiac arrhythmias; heart failure; hypertrophic cardiomyopathy; and uncontrolled hypertension (systolic >160 mmHg at rest). Well-controlled hypertension on stable medication is generally compatible with recreational diving, but the specific medication matters — beta-blockers impair exercise tolerance and blunt the heart rate response to exertion, which can mask early cardiovascular decompensation.
COVID-19 and Return to Diving
The 2021 DAN/UHMS Cardiopulmonary Considerations for Divers Recovered from COVID-19 Workshop established tiered return-to-diving criteria based on illness severity.
COVID-19 can cause pulmonary parenchymal injury (including fibrosis and gas trapping), myocarditis, coagulopathy with microthrombi, and persistent endothelial dysfunction — all of which interact poorly with diving physiology.
Tiered return criteria (Sadler UCSD algorithm, reviewed at the 2021 workshop): asymptomatic or mild COVID warrants 10–14 day isolation, then gradual return if fully asymptomatic and exercise tolerance is restored. Moderate illness (hospitalised, not ICU) requires formal cardiopulmonary evaluation — ECG, echocardiogram, and spirometry at minimum. Severe illness (ICU admission or mechanical ventilation) requires extensive evaluation including cardiac MRI to rule out myocarditis, full pulmonary function testing, exercise stress testing, and chest CT to assess for residual parenchymal changes.
The pulmonary concern is directly relevant to diving: COVID pneumonia can produce air-trapping lesions that behave like asthma pathologically during ascent. Chest CT is recommended for any diver who had significant COVID pneumonia before returning to diving. Long COVID neuropsychological sequelae — cognitive impairment, anxiety, impaired decision-making — are also relevant to safety-critical dive decisions.
Cardiovascular Fitness
Aerobic capacity determines how much work you can sustain before generating anaerobic metabolic byproducts, accumulating CO₂, and fatiguing. Fitter divers breathe less gas for the same work rate, tolerate current and emergencies with less physiological stress, clear post-dive nitrogen and inflammatory load faster, and are less likely to make hypercapnia-driven decisions at depth.
Recommended training: 150–240 minutes per week of moderate aerobic exercise (swimming, cycling, rowing, walking with load); 75 minutes per week of vigorous activity for divers who regularly encounter demanding conditions. Avoid intense training within 24 hours before diving; avoid strenuous exercise for at least 4–6 hours post-dive to allow inert gas elimination to proceed without elevated cardiac output driving additional bubble formation.
Strength and Functional Training
Strength training supports gear transport, equipment management, and control underwater. Useful targets: planks, bird-dogs, and deadbugs for core trim and stability; squats, lunges, and loaded carries for finning power and boarding ladders in swell; hangs and farmer's carries for tank and stage cylinder management.
Mobility, Balance, and Breathing Control
Mobility work reduces injury risk and improves control underwater. Single-leg balance training transfers directly to boat work in swell and confined-space manoeuvring. Breathing exercises — diaphragmatic breathing, slow exhalation drills — improve air consumption and reduce the hyperventilation-driven CO₂ flushing that worsens narcosis tolerance and destabilises buoyancy.
Medical Screening
All divers — particularly those over 45 or with any listed condition — should complete a dive medical form and obtain evaluation from a physician with dive medicine training (UHMS, DAN, or equivalent) when indicated. A resting ECG is appropriate for any diver over 45 with cardiovascular risk factors. A formal exercise stress test adds significant information for anyone with borderline exercise tolerance or a history of exertional symptoms.
Related Reading
References
Pollock NW, Uguccioni DM, Dear GdeL (eds.). Diabetes and Recreational Diving: Guidelines for the Future. DAN/UHMS Workshop Proceedings, Las Vegas, June 2005.
Denoble PJ, Holm JR (eds.); Bove AA, Moon RE (chairs). Patent Foramen Ovale and Fitness to Dive Consensus Workshop Proceedings. DAN/UHMS, Montreal, June 2015.
Chimiak J, Moon R (eds.). Cardiopulmonary Considerations for Divers Recovered from COVID-19 Infections Workshop Proceedings. DAN/UHMS, June 2021.
DAN Annual Diving Report 2021 Edition (reporting 2019 data). Blogg SL, Tillmans F (eds.). Durham NC: Divers Alert Network.
Gur et al. Methylphenidate and CNS oxygen toxicity risk in hyperbaric conditions. Journal of Diving and Hyperbaric Medicine, 2024.
UHMS. Fitness to Dive. Undersea and Hyperbaric Medical Society position statement.
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