History

The important things to elicit from a history are:

• What are their symptoms? DCI can present with almost any clinical picture
• Dive profile: How deep? For how long? Any missed stops? What was their surface interval between dives? Decompression sickness is more likely with higher nitrogen loads. Nitrogen loads will be higher following long or deep dives.
• Multiple dives: How many dives, over how many days and how may consecutive days diving? Multiple dives or multi-day diving increase risk as there is an accumulation of nitrogen.
• Closed or open circuit and which gases used? This is explained below
• Were there any problems during the dive e.g. Did the diver make a rapid ascent? Arterial gas embolism is more likely following rapid ascent (occurring with pulmonary barotrauma).
• Has there been any improvement with oxygen? Symptoms tend to improve with oxygen, although this is not diagnostic
• Timing of symptoms: Before the dive, during descent, at depth, during ascent, or after surfacing? Exactly how long after surfacing? Arterial gas embolism generally presents very quickly after surfacing, and decompression sickness usually presents within 6 hours of surfacing though can present late. It would be unusual for a patient to develop DCI before a dive (unless they have done a recent previous dive which may have caused DCI), on descent or while at depth. If a patient has carried on diving despite symptoms they may report an improvement in symptoms when returning to depth for subsequent dives.

Which circuit?

Divers use many different forms of equipment to deliver air/gas to them to breathe underwater. These are known as circuits and there are three main types:

1. Open Circuit -The most common type of circuit used among recreational divers. Gas from the tank is supplied via the hose and regulator for a diver to breathe. When the diver exhales the gas in their lungs is lost to the environment (seen as bubbles). This means that depth time can be greatly limited by the amount of gas in the tank as most of it is wasted when the diver exhales.
2. Closed Circuit Rebreather gas waste is limited in this system as a scrubber allows exhaled waste gases to be partially removed from the exhaled air_. The air is then recycled through the loop and topped up by a small pony tank with a diluent gas (usually with oxygen but sometimes mixed gas). Therefore few or no gas bubbles are produced while diving. These systems are usually used by military or commercial divers but can be used by recreational divers with specialist training.
3. Semi-closed Circuit Rebreather these are rarely used by recreational divers.

Closed or semi-closed circuits means a diver will inhale less inert gas throughout their dive meaning that their inert gas load will be less for a comparable diver on an open circuit.