1944: Shallow Water Black Out Royal Naval Physiological Laboratory Report; 44/125 UPS 48A

By Rik Rösken

Many of the rapports of the Royal Naval Research Centers where secret during the second world war. This year many of those rapports have been become public after 50 years. In one of those reports was the term of Shallow Water Black Out used for the first time.

The report was written by H.B. Barlow & F.C. MacIntosh. It was written for the "Royal Naval Personnel Research Committee". Both physicians worked at the "National Institute for Medical Research" which was based in Hamstead.

The research presents several cases in between 1943 and 1944 with so-called "self contained breathing sets". This materials aren't like any of the current used compressed air systems, but is more similar to the rebreather type of diving apparatus. These rebreathers works by breathing on 100% oxygen in combination with a filter system that removes carbon dioxide from the breathing gas.

Most of the incidents happened after a short stay at depth of less than 25 feet or 7.6 meters. Often havy work was involved. Most of the divers experienced subjective symptoms as confusion and tiredness. At that moment they often where out of breath, and weren't able to control this by infusing more oxygen trough a bypass system. Other divers experienced a feeling of euphoria. In some cases the symptoms increased after being exposed to ambient air at the surface.

A few divers where unconscious at the surface or became unconscious after breathing ambient air. More often it happened that the diver was conscious but seemed disoriented and react on haling. Speech was sometimes slurred and breathing was sometimes fast and superficial.

The recovery of the loss of consciousness was often quick. The only symptoms remaining where nausea and headache, disappearing within several hours after the dive.

Besides this more severe cases, also smaller cases where known with a change of consciousness. The symptoms of slurred speech and disorientation is most similar to drunkenness.

To find a the cause of this occurrence the researchers tried to determine if the effects had any relation with problems in the blood circulation or disrupting blood gasses (oxygen shortage, oxygen poisoning, carbon dioxide shortage and carbon dioxide poisoning).

The outcome was that the poisoning with carbon dioxide probably caused the change in consciousness. The poisoning was caused by filters who where not able to remove the gas sufficiently when the divers worked hard.

Shallow Water Black Out was back then something completely different. Currently the term is used for three situations: carbon dioxide poisoning, ascent blackout and blackout under hyperbaric situations due of hypoxia.

All three can happen with freediving. Another article will discuss the specifics of shallow water black-out how to avoid confusion between them.