[Anesthesia equipment and respirators. Fundamentals]

Abstract

The decrease of functional residual capacity during anaesthesia makes mechanical ventilation mandatory. Volume- and pressure-controlled modes should both be possible. Ventilator-assisted spontaneous respiration is rarely indicated during general anaesthesia, and this mode is therefore unnecessary for anaesthesia machines. From ecological and economical points of view, modern anaesthesia machines should be equipped with a circle system enabling administration of anaesthesia under rebreathing conditions with reduced fresh gas flow. Basic requirements are low gas leakage of the system, precise gas-flow dosage, especially at low flow rates, and integrated monitoring of in- and expiratory gases. In principle, older machines may also comply with these requirements if they are appropriately upgraded and properly maintained. There is reasonable doubt whether a further reduction of flow to less than 1 l/min fresh gas is of any benefit. To maintain patient safety without compromising practicability, a tremendous surplus of technical efforts is necessary. For easier management, fresh-gas-flow-compensated circle systems facilitating adjustment of tidal volume after changes of fresh gas flow are desirable. Precautions aimed at the prevention of ventilation with hypoxic gas mixtures are still insufficient: the oxygen failure devices (nitrous oxide flow shutoff valve) are only effective if oxygen pressure from the gas supply is low. Distinct improvements have been achieved with oxygen ratio systems, preventing the administration of hypoxic gas mixtures at fresh gas flows higher than 1 l/min.