Introduction

Surface decompression (Sur-D) is a decompression technique used primarily in military and commercial surface-supplied diving. On a Sur-D schedule, the first part of a diver's decompression is completed in the water. The remainder is completed in a deck decompression chamber (DDC) after the diver surfaces.

Indications

Surface decompression requires a DDC and therefore is beyond the scope of most recreational and public safety diving. Surface decompression is mainly used during time-sensitive diving operations where divers must be cycled in and out of the water quickly and when sea conditions are unfavorable to minimize the diver's time spent decompressing in the water. Surface decompression can be used in air or mixed gas (helium and oxygen; HeO2) diving operations.

Equipment

Surface decompression is almost always associated with surface-supplied diving, a technique in which the compressed gas supply is transferred from the surface to the diver's life-support equipment via a flexible hose. Surface decompression requires using a deck decompression chamber (DDC), pictured below.

Technique or Treatment

A brief explanation of the decompression process helps one understand surface decompression.

Seawater weighs approximately 64 pounds per cubic foot or 1024 kilograms per cubic meter. While freshwater weighs slightly less, the 2 are considered equivalent when determining the diving depth and calculating decompression schedules. As a diver descends into the water, the pressure around them increases as a function of the weight of the surrounding water. Assuming 1 ATA = 1.01325 bar = 760 mm Hg, this translates to:

• At 33 feet of seawater (FSW) or 10 meters of seawater (MSW), the pressure is twice atmospheric pressure or 2 atmospheres absolute (ATA)
• At 66 FSW or 20 MSW, the pressure is three times atmospheric pressure (3 ATA)
• At 99 FSW or 30 MSW, the pressure is four times (4 ATA)

For the diver to inflate their lungs, breathing gas must be supplied at a pressure equivalent to the ambient water pressure. This is the function of diving equipment, whether self-contained underwater breathing apparatus (SCUBA) or surface-supplied.

Commercial and military dives at depths less than 200 FSW (60 MSW) are generally made using compressed air, roughly 78% nitrogen. However, nitrogen produces measurable decrements in cognitive performance beginning at a depth of about 66 FSW (20 MSW). This effect, known as nitrogen narcosis, becomes debilitating beyond approximately 200 FSW (60 MSW) [1][2]. Military and commercial dives at depths greater than this are typically made using a mixture of helium and oxygen since helium has almost no narcotic properties. Many technical, recreational divers use a combination of helium, nitrogen, and oxygen (trimix) at shallower depths to help decrease the effects of nitrogen narcosis and the considerable cost of using only helium as a diluent. Of note, though nitrogen is not chemically inert, it is often referred to by divers as an "inert" gas compared to oxygen, an active metabolic gas.

At atmospheric pressure, the dissolved inert gas in the body is in equilibrium with that of the atmosphere. As a diver descends to greater depths, the pressure of the breathing gas increases, as does the partial pressure of inert gas in the breathing mix. This creates a positive pressure gradient between the inert gas in the lungs and the gas dissolved in the blood and body tissues. Inert gas molecules in the lungs then pass through the alveolar-capillary interface and dissolve in the body as a function of partial pressure and time. In other words, the farther a diver descends and the longer they stay at depth, the more inert gas dissolves in the blood and body tissues.

As a diver ascends toward the surface, the inert gas pressure in the lungs decreases, and the pressure gradient between the lungs and the body equilibrates and then reverses. The tissues become supersaturated with inert gas when the partial pressure of the dissolved inert gas in the body is higher than the partial pressure of inert gas in the lungs. The gas molecules in the body then pass through the alveolar-capillary membrane into the lungs and are exhaled. This is a simplified description of the process known as decompression. Detailed decompression algorithms and techniques are designed to control this process and allow the diver to return safely to the surface; surface decompression is one of these techniques.

Body tissues, including blood, will tolerate some supersaturation. However, excess supersaturation will result in bubble formation within the tissues. Some bubble formation after long, deep dives is expected and tolerated; however, if bubbles form sufficiently to impede circulation or displace tissue, the diver may develop symptoms of decompression sickness (DCS).[3]

A diver on a Sur-D schedule accomplishes part of their decompression in the water. This usually includes timed stops at successively shallower depths. Surface decompression schedules plan for enough in-water decompression to prevent the development of DCS symptoms during the time spent on the surface before recompression in the DDC, known as the surface interval. Some tissues are intolerably supersaturated after the in-water phase of surface decompression. Therefore, the diver must be transferred to the DDC quickly once they reach the surface to avoid decompression sickness.

For example, surface decompression procedures in the United States Navy stipulate that it should take no more than 5 minutes for a diver to ascend from the last water decompression stop at 40 feet, board the diving vessel, undress, and be compressed to 50 feet in the DDC. The remainder of the decompression procedure occurs at the pressure in the DDC, typically breathing oxygen to facilitate inert gas washout. In practice, this requires a well-trained and experienced crew. Typically, one or two diver tenders will assist the diver in removing the diving apparatus and exposure suit.

In commercial diving operations, the inner lock of the DDC is often pressurized beforehand to a predetermined depth deeper than the planned chamber decompression depth. After undressing, the diver enters the outer lock, the outer lock door is held closed, and the diver opens the crossover valve (see photo below) to equalize the pressure between the inner and outer locks. The 2 locks will equilibrate at the planned decompression depth if performed correctly. The diver enters the inner lock (see photo below), begins breathing oxygen through the built-in breathing system (BIBS) mask, and closes the inner lock door. The DDC operator surfaces the outer lock via the external exhaust valve. This procedure allows the diver to be pressurized to decompression depth in the DDC even if the air supply to the DDC is inadvertently lost before the diver enters the chamber.

The technique used by the US Navy requires the diver to enter the inner lock after undressing, close the inner lock door, and the DDC operator pressurizes the inner lock to the decompression depth.

Complications

If the surface interval is exceeded, Sur-D procedures usually call for extended decompression time in the chamber; this may slow diving operations. Diving in contaminated water can increase the surface interval as the diver must be decontaminated before entering the chamber. If the diver develops DCS during the surface interval or in the chamber, they must be treated in the DDC on an established DCS treatment protocol; this may also slow or stop diving operations depending on the company policy and availability of another DDC on the job site.

Clinical Significance

Healthcare practitioners who evaluate commercial or military divers or are directly involved in commercial or military diving operations must know the decompression procedures used. Surface decompression is a complex procedure that requires a well-trained and experienced dive crew. In addition, practitioners must be aware of potential complications during this procedure, be prepared to consult with dive teams, and treat divers who suffer injury due to those complications.

Enhancing Healthcare Team Outcomes

Ideally, surface decompression should not require the intervention of a healthcare team. A diver on a commercial or military surface-supplied dive station who presents with DCS symptoms during or after surface decompression is typically treated in the DDC on-site, in consultation with the dive team's medical advisory team. If the medical advisory team determines that the diver requires evacuation to the shore, careful coordination between on-site personnel, evacuation crews, and the receiving facility is crucial. Evacuation can be an arduous and lengthy process, depending on the location of the dive site.