Response to CO2 in novice closed-circuit apparatus divers and after 1 year of active oxygen diving at shallow depths

Symptoms of central nervous system oxygen toxicity during 100% oxygen breathing at normobaric pressure with increasing inspired levels of carbon dioxide: a case report

The greatest danger faced by divers who use oxygen-enriched gas mixtures is central nervous system oxygen toxicity (CNS-OT). CNS-OT is characterised by convulsions resembling grand-mal epileptic seizures, which may terminate in drowning and death. Elevated arterial levels of carbon dioxide (CO₂) (hypercapnia) represent a major risk factor for CNS-OT when breathing hyperoxic gas mixtures. To reduce the risk of a diver being involved in a CNS-OT incident due to hypercapnia, candidates for combat diving are examined at our institute using a routine physiological training procedure, in which they are tested for CO₂ detection and retention. We present the case of a candidate for combat diving, who unexpectedly exhibited signs typical of CNS-OT while breathing pure oxygen under normobaric conditions with > 3 kPa inspired CO₂. Severe headache and nausea, as well as facial muscle twitching, appeared during one of these routine tests. Subsequent medical examination including neurological tests, magnetic resonance imaging and an electroencephalogram were unremarkable. To the best of our knowledge, an event such as this has never previously been published in the medical literature. We present a discussion of the case, and a review of the relevant literature regarding CO₂ as a risk factor for the development of CNS-OT.

Oxygen Toxicity and Special Operations Forces Diving: Hidden and Dangerous

In Special Operations Forces (SOF) closed-circuit rebreathers with 100% oxygen are commonly utilized for covert diving operations. Exposure to high partial pressures of oxygen (PO₂) could cause damage to the central nervous system (CNS) and pulmonary system. Longer exposure time and higher PO₂ leads to faster development of more serious pathology. Exposure to a PO₂ above 1.4 ATA can cause CNS toxicity, leading to a wide range of neurologic complaints including convulsions. Pulmonary oxygen toxicity develops over time when exposed to a PO₂ above 0.5 ATA and can lead to inflammation and fibrosis of lung tissue. Oxygen can also be toxic for the ocular system and may have systemic effects on the inflammatory system. Moreover, some of the effects of oxygen toxicity are irreversible. This paper describes the pathophysiology, epidemiology, signs and symptoms, risk factors and prediction models of oxygen toxicity, and their limitations on SOF diving.

Women candidates for diving with oxygen-enriched gas mixtures have a lower end tidal CO2 than men during moderate exercise

We have previously determined the thresholds for CO2 detection (conscious recognition of elevated CO2) and retention in male divers, beyond which a diving candidate should not continue his diving activity due to an increased risk of CNS oxygen toxicity. The purpose of the present study was to establish whether there is a difference in end tidal PCO2 between male and female divers who use oxygen-enriched gas mixtures. Ventilatory and perceptual responses to variations in inspired CO2 (range 0-42 mm Hg) were assessed during moderate exercise in 18 males and 18 females. End tidal PCO2 was lower in the female divers when breathing oxygen with 42 mm Hg CO2 (58.2±3.0 mm Hg vs. 61.5±4.5 mm Hg, P<0.03). These results suggest that female divers have a lower end tidal CO2 than males when breathing a hyperoxic gas mixture during exercise, which might imply that women are less susceptible to CNS oxygen toxicity than men.

An innovative anaesthesia machine: the closed system

PURPOSE OF REVIEW

Closed system anaesthesia allows economic use of medical gases and volatile anaesthetics, maintenance of airway climatization and reduction of anaesthetic gas loss into the environment. In this context we reviewed papers addressing recent technical and clinical advances of closed system anaesthesia.

RECENT FINDINGS

New anaesthesia ventilators with computer-aided control of inspiratory oxygen, nitrous oxide and expiratory volatile anaesthetic concentrations have been developed. When compared with low flow or minimal flow techniques applied to classical anaesthesia ventilators with out-of-circle vaporizers these new ventilators provide fast and reliable achievement of the targeted volatile agent concentrations. The profile of desired end-tidal concentrations are stable with minimal or no operator intervention and the times to washout volatile anaesthetics are short.

SUMMARY

Modern feedback controlled ventilators allow the application of closed system anaesthesia as a safe and economic technique for routine clinical practice.