Physiological effects of rapid reduction in carbon dioxide partial pressure in submarine tower escape

The objective of this study was to determine whether adverse effects from a rapid drop in inspired carbon dioxide partial pressure (PiCO₂) in the breathing gas could hinder or prevent submarine tower escape.

A total of 34 male volunteers, mean (SD) age 33.8 (7.5) years, completed the trial. They breathed air for five minutes then 5% CO₂/16% O₂, 79% N₂ (5CO₂/16O₂) for 60 minutes before switching to breathing 100% O₂ for 15 minutes and then returned to air breathing. Breathing gases were supplied from cylinders via scuba regulators and mouthpieces. Blood pressure, cerebral blood flow velocity, electrocardiogram and end-tidal CO₂ and end-tidal O₂ were monitored throughout. Subjects were asked at intervals to indicate symptom type and severity.

Symptoms whilst breathing 5CO₂/16O₂ included breathlessness and headache. Following the switch to 100% O₂ seven subjects reported mild to moderate faintness, which was associated with a significant drop in cerebral blood flow compared to those who did not feel faint (P < 0.02). No subject vomited or fainted following this breathing-gas switch.

This study shows that the risk of fainting, sudden collapse or vomiting on switching to 100% O₂ following acute exposures to hypercapnia at a PiCO₂ of up to 5.0 kPa is less than 8%.