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Respiration in closed environments and space


Key points

  • Environments in which a closed atmosphere suitable for breathing is maintained include closed-circuit anaesthesia, submarines and space vehicles.

  • Problems of maintaining acceptably low carbon dioxide concentrations and low levels of inhaled contaminants are common to all these environments.

  • In the microgravity of space static lung volumes are reduced, ventilation and perfusion are better matched, and airway obstruction during sleep is uncommon.

  • For atmospheric regeneration in long-term space missions of the future, a combination of physicochemical and biological systems is likely to be needed.

The fascination of the human race with exploration has taken humans well beyond the high altitude and underwater environments described in Chapters 16 and 17 . Our ability to maintain life in space, the most hostile of environments yet explored, was developed as a result of techniques used to sustain breathing in other seemingly unrelated environments on Earth. All these environments share problems common to maintaining respiration while separated from the Earth’s atmosphere.

Closed-system anaesthesia

This may not represent the most dramatic example of closed-environment breathing but it is the most common. Careful control of the composition of respired gas is the hallmark of inhalational anaesthesia. The anaesthetist must maintain safe concentrations of oxygen and CO 2 in the patient’s lungs while controlling with great precision the dose of inhaled anaesthetic. It was recognized over 100 years ago that anaesthesia could be prolonged by allowing patients to rebreathe some of their expired gas, including the anaesthetic vapour. Provided oxygen is added and CO 2 removed, other gases can be circulated round a breathing system many times, providing beneficial effects such as warm and humid inspired gas. Rebreathing systems are also useful as a method for reducing both the amount of anaesthetic used and the pollution of the operating theatre environment.

A totally closed system during anaesthesia means that all expired gases are recirculated to the patient, with oxygen added only to replace that consumed and anaesthetic agent added to replace that absorbed by the patient. In practice, low-flow anaesthesia, in which over half of the patient’s expired gases are recirculated, is much more commonly used. In each case, CO 2 is absorbed by chemical reaction with combinations of calcium, sodium, potassium or barium hydroxides, resulting in the formation of the respective carbonate and water. The reaction cannot be reversed, and the absorbent must be discarded after use.

Widespread use of closed-system anaesthesia is limited by perceived difficulties with maintaining adequate concentrations of gases that the patient is consuming, such as oxygen and anaesthetic agent. However, gas-monitoring systems are now universally used with low-flow anaesthesia, allowing accurate control of breathing system gas composition.

Accumulation of other gases in closed circuits

Closed breathing systems with a constant inflow and consumption of oxygen will allow retention of other gases entering the system either with the fresh gas or from the patient. This affects the patient in two quite distinct ways. First, inert gases such as nitrogen and argon may accumulate to such an extent that they dilute the oxygen in the system. Second, small concentrations of more toxic gases may develop within the breathing system.

Nitrogen enters the system from the patient at the start of anaesthesia. Body stores of dissolved nitrogen are small, but air present in the lungs may contain 2 to 3 L of nitrogen, which will be transferred to the system in the first few minutes. If nitrogen is not intended to be part of the closed-system gas mixture, the patient must ‘denitrogenate’ by breathing high concentrations of oxygen before being anaesthetized, or higher fresh gas flow rates must be used initially to flush the nitrogen from the closed system.

Argon is normally present in air at a concentration of 0.93%. Oxygen concentrators effectively remove nitrogen from air, concentrating argon in similar proportions to oxygen, resulting in argon concentrations of around 5%. In a study of closed-system breathing in volunteers using oxygen from an oxygen concentrator, argon levels in the breathing system reached 40% after only 80 minutes. Cylinders of medical-grade oxygen and hospital supplies from liquid oxygen evaporators contain negligible argon, so the risk of accumulation is low.

Methane is produced in the distal colon by anaerobic bacterial fermentation and is mostly excreted directly from the alimentary tract. Some methane is, however, absorbed into the blood, where it has low solubility and is rapidly excreted by the lung, following which it will accumulate in the closed system. There is a large variation between subjects in methane production, and, therefore, the concentrations seen during closed-system anaesthesia. Mean levels in a circle system in healthy patients reached over 900 parts per million (ppm), well below levels regarded as unacceptable in other closed environments, but sufficient to cause interference with some anaesthetic gas analysers.

Acetone, ethanol and carbon monoxide all have high blood solubility, so concentrations in the breathing system remain low, but rebreathing causes accumulation in the blood. Levels achieved are generally low, but acetone accumulation may be associated with postoperative nausea. Closed-system anaesthesia is not recommended in patients with increased excretion of acetone or alcohol, such as in uncontrolled diabetes mellitus, after recent alcohol ingestion or during prolonged starvation.

Submarines

Submersible ships have been used for almost 100 years, almost exclusively for military purposes until the last few decades when they have become more widespread for undersea exploration and industrial use. Atmospheric pressure in the submarine remains approximately the same as at surface level during a dive, the duration of which is limited by the maintenance of adequate oxygen and CO 2 levels for the crew in the ship.

Diesel-powered

Submarines were used extensively during both world wars and were powered by diesel engines like surface-based warships. Clearly, the oxygen requirement of the engines precluded them from use during dives, and battery-powered engines were used, thus limiting the duration of dives to just a few hours. A more significant limitation to dive duration was atmospheric regulation. No attempt was made to control the internal atmosphere, and, after ventilation at the surface, the submarine dived with only the air contained within. After approximately 12 hours the atmosphere contained 15% oxygen, 5% CO 2 and a multitude of odours and contaminants. The need to return to the surface was apparent when the submariners became short of breath and were unable to light their cigarettes because of low levels of oxygen.

Nuclear-powered

Short dive duration severely limited the use of diesel-powered submarines. The development of nuclear power allowed submarines to generate an ample supply of heat and electricity completely independent of oxygen supply, allowing prolonged activity underwater. Atmospheric regeneration was therefore needed, and current nuclear-powered submarines routinely remain submerged for weeks.

Atmosphere regeneration

The plentiful supply of seawater and electricity make hydrolysis of water the obvious method for oxygen generation. Seawater must first have all electrolytes removed by a combination of evaporation and deionization. Theoretically, 1 L of water can yield 620 L of oxygen, so, even with less than 100% efficient electrolysis, large volumes of oxygen are easily produced. Submarine atmosphere oxygen concentration is maintained at 21% ± 2%.

Atmospheric CO 2 in submarines is absorbed by passage through monoethanolamine, which chemically combines with CO 2 to produce carbonates. When fully saturated, the absorber can either be replaced or be regenerated by heating with steam, when the CO 2 is released and can be vented into the sea. This method maintains the CO 2 concentration in submarines at 0.5% to 1.5%, and although further reduction is possible, the energy cost of doing so is prohibitive.

Atmospheric contamination during prolonged submarine patrols is well recognized, with many hundreds of substances entering the atmosphere, originating from both machinery and crew. These substances include volatile hydrocarbons such as benzene, oil droplets, carbon monoxide, cadmium and microbial organisms, with varying concentrations in different parts of the submarine. Continuous monitoring of many compounds is now performed, and maximum allowable levels during prolonged patrols are defined. Submarine air-conditioning units include catalytic burners that oxidize carbon monoxide, hydrogen and other hydrocarbons to CO 2 and water, and charcoal absorbers to absorb any remaining contaminants. The health risks from submarine occupation are therefore believed to be extremely small.

Physiological effects of prolonged hypercapnia

Definition of a ‘safe’ level of atmospheric CO 2 over long periods has concerned submarine designers for some years. Symptoms caused by elevated inhaled CO 2 are common, and include respiratory symptoms, flushing, sweating, dizziness and feeling faint, but the levels normally seen in submarines are not associated with impaired cognition. The respiratory response to inhalation of low concentrations of CO 2 (<3%) is similar to that at higher levels (page 48), but compensatory acid-base changes seem to be quite different.

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