FUNDAMENTALS OF ANESTHESIA MACHINES AND V ENTILATORS

The decrease of functional residual capacity during anaesthesia makes mechanical ventilation mandatory. Volume- and pressure-controlled mode s should both be possible. Ventilator-assisted spontaneous respiration is rarely indicated during general anaesthesia, and this mode is ther efore unnecessary for anaesthesia machines. From ecological and econom ical points of view, modern anaesthesia machines should be equipped wi th a circle system enabling administration of anaesthesia under rebrea thing conditions with reduced fresh gas flow. Basic requirements are l ow gas leakage of the system, precise gas-flow dosage, especially at l ow flow rates, and integrated monitoring of in-and expiratory gases. I n principle, older machines may also comply with these requirements if they are appropriately upgraded and properly maintained. There is rea sonable doubt whether a further reduction of flow to less than 1 1/min fresh gas is of any benefit. To maintain patient safety without compr omising practicability, a tremendous surplus of technical efforts is n ecessary. For easier management, fresh-gas-flow-compensated circle sys tems facilitating adjustment of tidal volume after changes of fresh ga s flow are desirable. Precautions aimed at the prevention of ventilati on with hypoxic gas mixtures are still insufficient: the oxygen failur e devices (nitrous oxide now shutoff valve) are only effective if oxyg en pressure from the gas supply is low. Distinct improvements have bee n achieved with oxygen ratio systems, preventing the administration of hypoxic gas mixtures at fresh gas flows higher than 1 1/min.