Diving at Altitude

Function

Professional divers frequently conduct dives at elevations exceeding 1,000 feet for occupational purposes, including bridge construction, maintenance, or search and rescue operations. Recreational diving typically occurs in mountain lakes or rivers for training or sport. Military operations have documented dives at altitudes reaching 14,200 ft.[3][4][5]

Issues of Concern

In most cases, approaches to and departures from dive sites occur at lower altitudes. In mountainous regions, divers frequently traverse passes, exposing them to higher elevations and lower atmospheric pressures, which may induce off-gassing of residual nitrogen. This risk can be mitigated using the same precautions applied to flying after sea-level dives. Commercial aircraft are typically pressurized to approximately 1,830 to 2,440 m (6,000 to 8,000 ft) above sea level. The Centers for Disease Control and Prevention recommends a minimum interval of 6 hours after a single no-decompression dive, 18 hours after multiple dives or multiple days of diving, and 24 to 48 hours after dives requiring decompression stops.

Upon arrival at altitude, divers remain saturated with nitrogen, necessitating sufficient off-gassing time. Approximately 98% of decompression sickness cases occur within 24 hours of surfacing. In addition to site-specific risk mitigation, the elevation of transit routes must be considered, as altitude gain constitutes a significant risk factor.

Acclimatization to new elevations reduces overall risk. A minimum of 12 hours at a higher altitude is recommended before diving. A 3-day acclimatization period may be required at elevations above 3,000 m (10,000 ft). Travel above 3,000 m should limit daily increases in sleeping elevation to no more than 500 m, with a rest day every 3 to 4 days.

Residual nitrogen and elapsed time must be considered when diving before the recommended postdive interval has elapsed. Adjustments to standard dive tables, incorporated into bottom times, are required to calculate decompression obligations accurately.

High-altitude environmental conditions further influence risk. Temperature extremes and reduced ambient oxygen increase with elevation, while colder and drier air is common. These conditions can affect respiratory function, producing bronchial irritation ranging from a minor cough to asthmatic wheeze. Reduced oxygen availability may exacerbate hypoxia. Compensatory increases in respiratory rate initially facilitate carbon dioxide elimination in response to lower arterial partial pressure of oxygen, simultaneously increasing insensible fluid losses and promoting dehydration. These physiologic changes elevate oxygen consumption, potentially resulting in unanticipated depletion of air in the diver’s breathing apparatus.

Higher altitudes (8,000 to 10,000 ft) may expose individuals to cold injuries, AMS, high-altitude pulmonary edema, or high-altitude cerebral edema. Measurement of individual apnea responses at sea level has been hypothesized to assist in risk stratification for AMS prior to ascent. Prophylactic administration of acetazolamide is not indicated in all high-altitude exposures but should be considered in moderate- to high-risk scenarios, including day-1 sleep elevations above 2,800 m, a history of AMS, or ascent rates exceeding 500 m per day above 3,000 m.

Water density differences must also be accounted for when calculating standardized equivalent sea depth. Most mountain lakes contain freshwater with a density of 1,000 kg/m³, whereas seawater is standardized to 1,033 kg/m³. The relative density ratio of freshwater to seawater (1,000/1,033) affects nitrogen pressure generated during inspiration. Therefore, divers must convert depth calculations from seawater to freshwater or utilize freshwater-specific charts or dive computers capable of this conversion.[6][7][8][9][10][11][12]

Clinical Significance

Specialized dive tables and procedures are required for all dives conducted at elevations exceeding 1,000 ft (305 m). These tables are indicated for dives above 300 ft (91 m) when planned depths exceed 145 ft. No correction is necessary for dives between sea level and 299 ft (90 m).[13]

Standard depth gauges are calibrated at sea level and register negative values at altitude. Certain gauges may be recalibrated or zeroed at altitude prior to the dive. Alternatively, altitude correction may be approximated by adding approximately 1 fsw per 1,000 ft of altitude, with conversion to ffw if indicated.

Dive computers employ decompression algorithms to reduce the risk of decompression illness by continuously updating depth profiles and recalculating decompression variables, including maximum depth, diver age, surface intervals, and total breathing gas consumption. Most modern dive computers automatically adjust for starting altitude and freshwater versus seawater conditions. Some devices require manual input.

Many dive computers calculate additional risk based on temperature. However, these tools cannot account for individual risk factors such as age, body weight or habitus, presence of a patent foramen ovale, alcohol consumption, or dehydration. Many formally tested dive computers demonstrate significant errors and are unreliable at altitude. Consequently, meticulous manual dive profile planning is essential, even when using dive computers. In addition to careful planning, studies suggest that extended periods in shallow water effectively reduce bubble formation, highlighting the importance of slow ascents and safety stops in decreasing the risk of decompression illness.

Adequate acclimatization periods should be incorporated into dive profiles. Additional postdive surface intervals at the dive altitude are recommended to allow nitrogen off-gassing before travel involving altitude changes greater than 1,000 ft (305 m).

Hemoconcentration may develop as a protective acclimatization response, with elevations in erythropoietin manifesting within 2 hours of arrival at altitude and red cell mass increases occurring within 2 to 4 days. Prior cold exposure enhances cold adaptation and may reduce the risk of hypothermia. Surface-level swims, with or without a wetsuit, performed within days of a dive can improve thermal tolerance.

Prophylaxis with acetazolamide is recommended in moderate- to high-altitude scenarios. Ibuprofen may serve as an alternative for individuals who cannot tolerate acetazolamide. Deployment of a portable 1-man hyperbaric chamber or Gamow bag on-site is advised for higher-risk altitude dive profiles. Management of high-altitude cerebral edema or AMS should include immediate descent or administration of dexamethasone.[14]

Other Issues

Historically, Jacques Cousteau is credited with the first extreme-altitude dive in 1968, during an expedition to Bolivia and Peru’s Lake Titicaca at 12,507 ft (3,812 m), searching for submerged Inca artifacts. In 1982, a team led by Charles Brush and Johan Reinhard established a new record at the Lago Licanbur volcano in Chile at 19,400 ft (5,900 m), followed by another team led by Nathalie Cabrol at the same site in 2006. As of this publication, the highest recorded scuba dive was performed by Erno Tósoki at Ojos del Salado, the tallest volcano on Earth, at 6,382 m (20,965 ft) on February 21, 2016. This dive lasted only 10 minutes due to the physiologic effects of extreme altitude. The highest scuba dive in the continental U.S. was conducted in September 2013 by John Bali at Pacific Tarn Lake, Colorado, at 13,420 ft (4,090 m).

The increasing use of closed-circuit rebreather diving introduces additional complexity at altitude, including cold exposure, AMS, dehydration, and more intricate decompression considerations. Exceptional caution, acclimatization, and preparation are required to operate rebreather systems safely under high-altitude conditions.[15]

Enhancing Healthcare Team Outcomes

The incidence of decompression illness is rising with increasing participation in water-based activities. Management of decompression illness requires an interprofessional approach due to the diverse clinical presentation and potential for high morbidity. Emergency department physicians and nurse practitioners should promptly refer all symptomatic patients to a hyperbaric chamber. Public education on scuba diving and the critical importance of gradual ascent is essential to reduce risk.