The term "decompression illness" (DCI) is used to refer to both decompression sickness (DCS) and arterial gas embolism (AGE).
For the novice getting certified, it's unimportant to be able to distinguish between the two. In fact, based on symptomatology alone, a well-trained hyperbaric physician might not be able to tell the difference either, particularly when symptom onset occurs shortly after the diver surfaces after a dive. If more time has passed between the end of the dive and symptomatic onset, then the differential heavily favors DCS over AGE.
You should know that first aid treatment is the same for both.
To address your question...
Bubbles in the bloodstream occur in both conditions (and also in subclinical DCS).
Consider where those bubbles are going in the body.
Bubbles in the venous system are routed back to the heart and lungs and then subjected to the "pulmonary filter."
Bubbles in the arterial system go straight to the various organs in the body and can interrupt bloodflow (directly/indirectly) to those organs.
For what it's worth, the two conditions are not mutually exclusive. For example, it's certainly possible to have DCS giving rise to venous bubbles which cross into the arterial system (via a patent foramen ovale?) and then cause AGE.
AGE is feared because of its sudden onset and the targeting of the central nervous system (CNS; brain and spinal cord) which can result in death or serious morbidity (long-term paralysis, etc.). In many cases, it's precipitated by lung-overexpansion injury (breath-holding during a diver's ascent?) which introduces gas into the arterial bloodstream and is routed directly to the CNS. This can cause serious neurological symptoms: loss of consciousness, lack of breathing, paralysis, convulsions, etc. Nerve cells can only survive for a few minutes when deprived of oxygen. Nerve cells, particularly those in the CNS, have a limited potential for self-renewal.
It's worth mentioning that the diver should pay attention to gas management during the dive. It's easy to see how an out-of-gas emergency can lead to a rapid, uncontrolled ascent to the surface.
I can see how you might get the impression that DCS is "less risky" than AGE.
Know that both can be deadly. Perhaps you have that impression because a fair amount of time is spent in an OW course teaching students about the symptoms of "minor" DCS: skin mottling, joint pain, fatigue, and other symptoms that may not be life-threatening.
The take-away messages from all of this are:
- Stick to your training when it comes to planning/executing your dive. Plan your dive conservatively, particularly with respect to no-decompression limits (NDLs) and gas supplies. It's good practice to conduct an optional 3-minute (or more) safety stop in the 10-20 fsw range at the end of a dive.
- Check your remaining gas supplies frequently during the course of a dive.
- Don't hold your breath while ascending in the water column. Doing so can cause a lung-overexpansion injury along with other serious complications.
- Pay attention to any abnormal symptoms after a dive. Be familiar with the list of DCS and AGE symptoms.
- The first aid treatment for both conditions is the same: allow the patient to lie down on his/her back (on side if drowsy, nauseous, unconscious), monitor for responsiveness (ABCs; perform CPR if necessary), administer 100% O2, make fluids available to the patient if responsive/stable/not complaining of nausea (since dehydration may be a factor), and transport the patient ASAP to a hyperbaric chamber/ER for evaluation.
- Have an emergency action plan in place no matter where you are diving. If you are diving off of a boat with a dive op, ask where the oxygen tank is, whether it's been tested recently, and the location of the nearest hyperbaric chamber and ER. All of this should be covered in the safety briefing. If you are shore diving on your own, make sure that you review the emergency action plan with your dive team during the pre-dive briefing.
Hope this helps...
