The Evolution of US Combat Diving. Part 5, the Physiology.

The limitations of physiology, and the workarounds.

This is Part 5 of 7 taken from my Submarine Medical Officer Thesis “A Review of the Combat Diving Evolution of US Naval Special Warfare with a Focus on the Necessity of Disruptive Technologies and Innovative Scientists.” Stay tuned for part 6…

The Physiology of Combat Divers

Solving the many complex logistical and technological issues associated with combat diving allowed the SEALs to dive further, faster, deeper, and quieter than ever before by the 1990s. But one cold, dark, pressurized, unforgiving barrier remained – the harsh aquatic operational environment of the combat diver. And this alien environment wreaks havoc on the combat diver’s humanness – his physiology. The underwater environment produces unique effects that can retard, confuse, and even kill a diver; and the extended dive profiles of SDV operations provide the perfect milieu for physiologic sabotage. Because of this harsh reality, combat diving has always needed and will always need a dedicated core of diving medicine trained researchers, doctors, and technicians.

Dr. Christian Lambertsen. A dedicated medical doctor, researcher, inventor, and patriot, Dr. Christian Lambertsen (Fig. 15) was a visionary in the world of combat diving. He nearly single-handedly created and refined the original US combat diver capability by personally inventing his own re-breather and adapting the British “SDV” technologies to the needs of the US military. He designed, built, and personally tested the LARU – the first US closed-circuit re-breather SCUBA. Also, despite multiple setbacks and initial rejections by the US military combat swimmer community, he persisted in the belief that combat diving was a necessary capability, and that his LARU would eventually revolutionize that capability. He continued to present his ideas and technologies to various communities during and after WWII and eventually won the hearts and minds of the UDTs – who were conveniently mandated to develop a combat diving capability by their leadership. Dr. Lambertsen was among the first US divers to lock out and then lock back into a submarine, and he was the first US diver to dock an early SDV on the deck of a sub. He also helped design the “Emerson-Lambertsen Oxygen Rebreather” – the re-breather that replaced the LARU and was used by the SEALs for twenty years before it was replaced by the Draeger LAR V in 1982.1,2

Figure 15: Dr. Christian Lambertsen.3

In addition to his many technological and visionary contributions, Dr. Lambertsen contributed immensely to the oxygen toxicity research that protected future combat divers from the potentially deadly effects of 100% oxygen re-breathers. In fact, Lambertsen himself experienced four separate episodes of oxygen toxicity, including one underwater seizure during his personal testing and evaluation of his LARUs. He also taught many OSS MU and UDT combat swimmers in the art of re-breather diving, each time emphasizing the necessity of safety precautions. Because re-breather diving was so new and unknown, he established his own subjective dive depth and time limits derived from his own diving experiences. He also requested on multiple occasions that NEDU develop more objective experimental data on oxygen toxicity and publish scientifically proven dive depth and time limits for future re-breather dives. In 1948, Lambertsen and LCDR Fane conducted a study at the Naval Submarine Medical Research Laboratory (NSMRL) escape training tower that “was the first time that fin-swimming, LARU oxygen divers were observed in a study under controlled conditions.” Dr. Lambertsen continued investing himself in the US military’s combat diving efforts for many years until his death in 2011. His contributions not only pioneered the US combat diver capability, but also undoubtedly saved the lives of numerous combat divers. In 2000, in honor of his incredible contributions, the Naval Special Warfare community recognized him as “The Father of US Combat Swimming.”4,5,6

UMOs and DMTs. Although Dr. Lambertsen’s genius and expertise could never be replaced, the US Navy needed more medical professionals specially trained in diving and undersea medicine to ensure the continued health and safety of the growing dive communities. Currently, Undersea Medical Officers (UMOs) are a part of the Navy’s solution to this need. UMOs are US Navy medical doctors who have successfully completed a six month diving and submarine medicine course conducted at the Naval Undersea Medical Institute (NUMI) in Groton, Connecticut, and the Naval Diving and Salvage Training Center (NDSTC) in Panama City, Florida (Fig. 16). After graduating from this arduous course, UMOs are typically billeted at a diving command, a submarine command, or a research command. Interestingly, whereas most UMOs practice either dive medicine or submarine medicine, the UMOs assigned to the SDV team have the unique privilege of practicing in both fields simultaneously. The SDV Team UMOs are medically responsible for all DDS operations; but they occasionally train, qualify, and then utilize the support of other UMOs from outside the command when the need arises.

Prior to DDS operations commencing, the UMO is actively involved in the medical evacuation and contingency planning, and also contributes to the evaluation of proposed dive profiles. During DDS diving, the UMO tracks all of the divers’ profiles in real time using the Navy Dive Planner, and is immediately available for medical advice if a diver is injured. Also, Dive Medical Technicians (DMTs) and Dive Independent Duty Corpsmen (IDCs) – corpsmen with special training in dive medicine – man the DDS hyperbaric chamber during dives for the immediate triage and treatment of injured divers. Together, UMOs and DMTs/IDCs form the DDS operations medical team.

Figure 16: The author using open-circuit SCUBA during dive school at NDSTC.

Interestingly, the most common injuries in DDS operations are traumatic injuries from the multiple moving parts of the DDS and SDV being tossed around by unpredictable currents and turbulence from the DDS. But the potential for decompression illness is also high during DDS operations if the submarine is unable to maintain strict depth control. The potential for trauma, the constant depth variance, and the lengthy exposures to the austere maritime environment combine to make DDS diving some of the riskiest diving in the Navy.

UMOs also participate heavily in diving related research that is continuously ongoing at NEDU and NSMRL. The operational application of the research performed at these institutions has undoubtedly saved many in the dive community from significant injury or death.  UMOs, researchers, and the research staff continue to fulfill requests for research into specific ventures and technologies, but a few unique problems arise in DDS and SDV operations that still need further research and delineation.

Hypothermia and Hypovolemia. A physiologic dilemma that has plagued the SDV Teams since their inception is the inevitable development of varying degrees of hypothermia and hypovolemia in the colder waters of operational SDV diving. For humans, air is our natural environment but being surrounded by water is alien; and while both water and air are fluids, the similarities stop there. Human normothermia, or normal human body temperature, is maintained when a person’s average core temperature remains at approximately 98.6°F. Whereas, hypothermia occurs if a human’s core temperature drops to 95°F or less. In water, a diver’s thermoneutral temperature is approximately 95°F;7 which means that a naked diver will remain normothermic indefinitely while resting submerged in 95°F water. If the water temperature drops below 95°F, that diver will eventually develop hypothermia; and the colder the water, the more rapidly this will occur.8,9

In air, body heat is lost through the physical mechanisms of conduction, convection, radiation, and evaporation, but when submerged, a diver cools primarily by conduction and convection. Despite the fewer means of cooling, a diver loses body heat much faster in water10 than in air due to the high thermal conductivity of water (Table 3) – “the rate at which heat is transferred between objects of different temperatures.”11 Also, as dive depth increases, a diver loses heat faster due to evaporative heat loss through the respiratory tract. So, diving colder, deeper, or longer dramatically increases the risk of hypothermia.12

Thermal Conductivity(kcal/hr per cm°C)
Foam neoprene4.6
Table 3: Thermal Conductivity Values (compare air to seawater)13

The clinical manifestations of hypothermia are often described in the medical literature as occurring in discrete stages at specific ranges of temperature, but these signs and symptoms are more accurately imagined as a continuum (Table 4) that depends on a variety of factors specific to the individual diver. Generally, upon immersion or submersion in cold water, peripheral vasoconstriction occurs immediately causing the central hording of warm blood which protectively preserves the warmth of the core. This is soon followed by a variety of physiologic compensations to preserve core temperature – sporadic shivering is eventually followed by uncontrollable shivering. Mental and physical impairments occur – speech slurring, confusion, decreased motivation, and diminished fine and then gross motor function. Finally, the extreme of the spectrum occurs – impaired shivering, hallucinations, cardiac dysrhythmias, loss of consciousness, drowning, and death. These signs and symptoms and the temperature at which they occur vary considerably dependent upon many variables unique to each individual diver – “age, gender, morphology, fitness, illness and injury, motion illness, nitrogen narcosis, nutritional state, blood sugar concentration, ambient CO2 levels, ambient O2 levels, cold habituation,14 environmental pressure.”15,16

Table 4: A generalized hypothermia continuum.17

Submersion in cold water not only causes hypothermia, but it also produces other unique effects. Due to the increased density of water compared to air, relatively small vertical excursions in the water column cause huge pressure changes, and these pressure changes cause a variety of detrimental physiologic changes in the diver. The increased pressure – combined with peripheral vasoconstriction due to cold water immersion – redistributes blood from the extremities to the thorax causing significant increases in cardiac preload and cardiac output. Cardiac baroreceptors, sensing this change as an increase in intravascular volume, signal the kidneys to diurese. The immersion in cold water also alters hormonal function and leads to intrinsic kidney dysfunction. These effects combine to cause massive diuresis during prolonged dives thus reducing a diver’s plasma volume by up to twenty percent. This causes significant hypovolemia and decreases human performance upon emerging from the water after long, cold dives.18,19 As RADM “Irish” Flynn – retired SEAL – explained, “You have considerable diuresis, and people in extended, multi-hour SDV operations…can have very low blood pressures at the time that they emerge from the water. Your first action on emergence from the water can be to faint.”20

While the severity of hypothermia and hypovolemia is dependent upon the extremis of the environment, their effects are also synergistic. Hypothermia exacerbates hypovolemia, and hypovolemia exacerbates hypothermia. In DDS operations, fatalities are avoided with proper dive planning and appropriate operational risk management (ORM), but the human performance detriments are difficult to overcome.21,22 While these effects do not fully manifest in warm, shallow training environments, the cold, deep waters of future operational environments will elicit a much more profound negative physiologic response.

NEDU and NMRI. Since the 1960s, two centers of naval research have contributed prolifically to the safety and evolution of the combat diver – the Navy Experimental Dive Unit (NEDU) in Panama City, Florida, and the Naval Medical Research Institute (NMRI) in Bethesda, Maryland. Many studies originating in these labs explored the physiological effects of cold water immersion and various methods of mitigating the associated detrimental effects. A study performed for the Office of Naval Research and published in 1973 simulated a cold SDV dive, insertion on land, and exfiltration by SDV. The UDT/SEAL participants showed decrements in both “manual and mental tasks” due to the cold water immersion.23 A 1985 NMRI study involving members of SDV Team 2 concluded that post-dive exercise should be discouraged due to a potentially dangerous “after-drop” in core temperature. It also showed decreased strength and aerobic capacity after cold water immersion.24 NEDU evaluated two Active Thermal Systems (ATS) for SDV operations in 1990, and made many insightful conclusions and recommendations for future research.25 Additional studies during the 1990s further characterized fluid and electrolyte changes associated with lengthy immersion in cold water,26 showed that glycerol ingestion was ineffective for reducing cold water immersion diuresis,27 provided post-dive rehydration guidelines and showed a potential benefit to administering DDAVP pre-dive to reduce diuresis.28

In 1991, NSW and NMRI hosted a thermal workshop in Virginia Beach with representatives from the SDV teams, most of the SEAL teams, NAVSEA, WARCOM, and other commands heavily involved in combat diving. The representatives comprised a wide variety of expertise that included operators, command leadership, research scientists and medical doctors. The workshop discussed operational thermal concerns, contemporary related technologies, previous and ongoing research in hypothermia, and the necessity of future research and information sharing. A 150 page transcript published by NMRI details the entirety of the two day discussion and is thoroughly enlightening with respect to hypothermia related issues in SDV diving. An important result of the workshop was the establishment of “standard thermal indices” and “standard performance indices” that outlined specific measures to be included in all future NSW diving related research.29

In 1996, Daniel Valaik from NMRI published “A Review of Manned Thermal Garment Diving Studies with Lessons Learned for the SDV Operator and Combat Swimmer.” The report reviewed eleven “of the most relevant manned thermal studies applicable to SDV and combat-swimmer diving” and made recommendations for future SDV diving based on the results of the compiled studies.30 Then in 1997, Valaik et al published “Thermal Protection and Diver Performance in Special Operations Forces Combat Swimmers (Resting Diver Phase).” In this study, they investigated the thermal protection properties of the wet suit, Viking dry suit, British Royal Navy Special Boat Service dry suit, and the Exotemp electric suit by having thirteen “healthy male volunteers” with extensive diving experience perform resting dives in (36.5°F-55°F) water wearing the provided protection. The wet suit performed well due to its “flexibility and reliability,” but suffered in the coldest temperatures with “an average dive duration of 2h in 36.5°F water.” As suspected, the dry suits offered improved thermal protection but at the expense of bulk and “manual dexterity and mobility.” Also, the divers’ post-dive performance tests suffered due to the exposure of their hands and feet to the elements. The active warming of the electric suit combined with the passive thermal protection of the dry suit offered the best in water performance by extending dive times significantly and improving the post-dive dexterity performance measures. The study recommended modifications and further improvements to the electric suit for future use in SDV diving to allow the best performance in the worst conditions.31

Hypothermia and hypovolemia in combat diving continued to be the subject of scientific investigation into the mid-2000s with further refinements to active thermal systems and explorations into pharmaceuticals that mitigate the profound diuresis related to cold water immersion.32,33,34 Further research is currently ongoing in search of the optimal thermal protection solution. “The current suit approach to operating in cold water consists of several layers. The first is an undergarment, such as polypropylene knit fabric. Over the undergarment is the tube suit, made of flexible material that incorporates tubing which warm water runs through to maintain the diver’s body temperature. On top of the tube suit is an insulation layer to reduce heat loss from the tube suit to the cold water. The outermost layer is a diver dry suit.”35 This combination allows for lengthy dives in cold waters, but unfortunately is incredibly bulky and cumbersome – sometimes requiring nearly an hour to get out of the suit and to continue on mission.36 While this system is a significant improvement over what was available previously, it is still not ideal.

The ASDS Concept. Another idea entered the realm of SDV diving in the late 1990s and consumed immense scientific, technological, and monetary investment over the following decade. Some wondered whether the physiological dilemma could be avoided altogether rather than pursuing only incremental improvements in physiologic optimization. What if the combat divers were never required to enter the chilling waters? Or if they did, what if the duration of the freezing dive was minimal? These questions birthed new efforts and technologies that interestingly were previously explored and rejected by NSW half a century earlier.

The Advanced SEAL Delivery System (ASDS) was a miniature, dry, one-atmosphere submarine designed in the 1990s to address these physiologic issues. The ASDS fastened to a submarine’s aft deck (Fig. 17), detached from the sub when within range, provided a warm and dry environment during transit, and allowed SEALs to lock-out underwater near their objective. The ASDS decreased the SEALs’ exposure time drastically therefore making them much more effective on mission.37,38,39 Interestingly, this same idea was explored after WWII with a British mini-submersible but then abandoned. While the ASDS project was a worthy concept, it was canceled in 2006 after encountering numerous design problems, massive budget overruns, and finally the accidental destruction of the one ASDS in existence.40,41 But the ASDS concept survived: SOCOM42 recently awarded a “three-year, $44 million contract to General Dynamics Electric Boat” to develop another Dry Combat Submersible (DCS) – the User Operational Evaluation System 3 (UOES 3). The contract is slated to continue through 2015, but many of the details and design objectives are still unknown to the public.43

The ASDS fiasco prompted questions about the utility of various technologies, especially such costly investments. When evaluating certain missions RADM Flynn queried, “How much of this can be done by other sensors?”44 Some suggested that instead of focusing on human physiologic optimization that NSW should focus on utilizing modern technological advancements to perform much of the combat divers’ tasks without ever exposing a human diver to the elements.45 But this prompted more questions, “Can a machine ever replace or adequately perform the work of the combat diver?”

Figure 17: ASDS on a SSGN.46

UUVs and the Role of Technology. Unmanned Underwater Vehicles (UUVs, see Fig. 18) have been in development for years with some showing promise for use in Naval Special Warfare.47 Eventually, the classic NSW missions of beach reconnaissance/demolition and mine detection/destruction may be performed by UUVs like the SAHRV (Semi-Autonomous Hydrographic Reconnaissance Vehicle), CETUS48 (Composite Endoskeleton Testbed Untethered Underwater Vehicle System), REMUS (Remote Environmental Monitoring Units), or by an aerial reconnaissance tool – AROSS49 (Airborne Remote Optical Spotlighting System). In testing, these advanced machines have proven very successful for specific missions when compared to manned operations.50,51

“Exploiting this technological advantage”52 not only improves the odds of mission success, but it also reserves the men of SOF for more vital missions rather than occupying and endangering them with mundane but necessary, high-risk evolutions. RADM Eric Olson – retired SEAL – explained, “People jokingly say, ‘Sure, we have the capability of one man, one mine.’ But it’s really more one mine, one platoon, and then you’ve got to replace that platoon when you find the next mine. If we have a program that will help keep the men out of the minefield, we ought to pursue it with vigor.”53 By combining the best aspects of man and machine, NSW can potentially perform more effectively and also preserve its most valuable asset – the men in uniform.

Figure 18: Unmanned Maritime Systems with UUV examples.54

Continued in Part 6…


  1. Butler, “Closed-circuit Diving,” 3-20.
  2. Vann RD, “Lambertsen and O2: Beginnings of operational physiology,” Undersea Hyperbaric Medicine 2004; 31, 21-31
  3. Dr. Christian Lambertsen, picture, accessed December 28, 2015, lambertsen.jpg
  4. Butler, “Closed-circuit Diving,” 3-20.
  5. Vann, “Lambersten and O2,” 21-31
  6. “Dr. Christian Lambertsen: Father of U.S. Combat Swimming and SCUBA,” accessed December 20, 2015, FeaturedLegends/christian_lambertsen_bio.htm
  7. Some sources give a range of 91-95°F.
  8. Bove, Diving Medicine, 19, 261-273
  9. Navy Diving Manual, 3-10
  10. A diver cools approximately five times faster in water than in air.
  11. Ibid.
  12. Ibid.
  13. Bove, Diving Medicine, Table 2-6, 19.
  14. Previous repeated cold exposures.
  15. Bove, Diving Medicine, 261-273
  16. Navy Diving Manual, 3-10
  17. Navy Diving Manual, Table 3-1
  18. Bove, Diving Medicine, 261-273
  19. Navy Diving Manual, 3-10
  20. Joiner, Naval Forces: A Look Back, 2-47, 48, 52
  21. Bove, Diving Medicine, 261-273, 551-562
  22. Arena AV, Birkler J, MacKinnon M, Rushworth D, Advanced SEAL Delivery System: Perspectives and Options, RAND: National Defense Research Institute 2006; 1-2.
  23. Vaughan WS, Anderson BG, Effects of Long-Duration Cold Exposure on Performance of Tasks in Naval Inshore Warfare Operations, Oceanautics, Inc. 1973.
  24. Doubt TJ, Weinberg RP, Baker CD, Flynn ET, Preliminary Studies of Exercise Capacity in Combat Swimmers After Cold Water Training Operations, Naval Medical Research Institute, NMRI Report 85-04, July 1985.
  25. Sterba JA, Physiological Evaluation of Two Diver Active Thermal Systems (ATS): S-TRON and ILC-DOVER, Navy Experimental Diving Unit, NEDU Report 3-90, March 1990.
  26. Deuster PA, Smith DJ, Smoak BL, Montgomery LC, Doubt TJ, Coldex-86: Fluid and Electrolyte Changes During Prolonged Cold Water Immersion, Naval Medical Research Institute, NMRI Report 90-133, December 1990.
  27. Goforth HW, Arnall DA, Effectiveness of Glycerol Ingestion for Enhanced Body Water Retention During Cold Water Immersion, Naval Health Research Center, Report 90-33, May 1990.
  28. Doubt TJ, Thorp JW, Weight Loss After AM and PM SDV Dives and Use of DDAVP, Naval Medical Research Institute, NMRI Report 92-75, September 1992.
  29. Doubt TJ, Curley MD, Proceedings of the 1991 NSW Thermal Workshop, Naval Medical Research Institute, NMRI Report 92-84, January 1993.
  30. Valaik DJ, A Review of Manned Thermal Garment Diving Studies With Lessons Learned for the SDV Operator and Combat Swimmer, Naval Medical Research Institute, NMRI Report 96-47, July 1996.
  31. Valaik DJ, Hyde DE, Schrot JF, Thomas JR, Thermal Protection and Diver Performance in Special Operations Forces Combat Swimmers (Resting Diver Phase), Naval Medical Research Institute, NMRI Report 97-41, November 1997.
  32. Nyquist PA, Schrot J, Thomas JR, Hyde D, Taylor WR, Desmopressin Prevents Immersion Diuresis and Improves Physical Performance After Long Duration Dives, Naval Medical Research Institute, NMRC Report 2005-001, March 2005
  33. Nuckols ML, Chao JC, Swiergosz MJ, Manned Evaluation of a Diver Heater for SDV Applications Using Hydrogen Catalytic Reactions, Navy Experimental Diving Unit, NEDU Report 05-08, June 2005
  34. Neste CH, Fredrickson KE, ANU Testing of Steadfast Technologies 15 VDC Resistive Heating System (RHS) Dry Suit Liner, Navy Experimental Diving Unit, NEDU Report 11-01, October 2001.
  35. Lore-Anne Ponirakis, “Naval Special Warfare (NSW) Diver Thermal Human Interface,” (January 15, 2015) accessed December 20, 2015,
  36. Ibid.
  37. RAND, ASDS
  38. Arena, ASDS, 1-2
  39. John Whipple, “ASDS: The Future of Submarine-Based Special Operations,” Undersea Warfare, (Winter/Spring 2002), accessed February 1, 2015,
  40. “First Advanced SEAL Delivery System Sub Accepted by Navy Despite Development Problems, Critics,” The Nav Log, accessed February 1, 2015,;
  41. Kris Osborn, “SOCOM Develops Dry Submersible Mini-sub for SEALs,” Defense Tech, (January 30, 2014), accessed February 1, 2015,
  42. SOCOM = US Special Operations Command
  43. Osborn, “SOCOM Develops Dry Submersible.”
  44. Joiner, Naval Forces: A Look Back, 2-52.
  45. Ibid., 2-23, 24, 48-57.
  46. ASDS on a SSGN, picture, accessed December 28, 2015,
  47. Button RW, Kamp J, Curtin TB, Dryden J, A Survey of Mission for Unmanned Undersea Vehicles, RAND: National Defense Research Institute 2009, xvii.
  48. “CETUS,” accessed February 1, 2015,
  49. David Hamrick, “Navy SBIR/STTR Success,” (2013), accessed February 1, 2015,
  50. Joiner, Naval Forces: A Look Back, 2-24, 66, 67
  51. Edward C. Whitman, “Unmanned Underwater Vehicles: Beneath the Wave of the Future,” Undersea Warfare, accessed December 20, 2015,
  52. Commander “Chuck” Hayes’ – SEAL – Tactical Expectations: Be competent in your Craft. In the absence of leadership, take charge. Always improve your fighting position. Push situational awareness up and down the chain of command. Always exploit your tactical and technological advantages.
  53. Joiner, Naval Forces: A Look Back, 2-66, 67.
  54. Unmanned Systems Integrated Roadmap FY2013-2038, Department of Defense 2013, 8. Accessed December 30, 2015,

1 comment

Leave a Reply

%d bloggers like this: