Modern Anesthesia Machines: What You Should Know

The closed claims analysis by Caplan, et.al. in 1997 remains the most definitive description of gas delivery equipment morbidity. Although relatively few claims arise (1.2% since 1985), the rate of misuse is three times higher than pure equipment failure, and better monitoring could have prevented 78%. This point is emphasized by Caplan in the 2004 APSF workshop on use of audible alarms 2 , and recent FDA MAUDE database reports continue to implicate misuse as a significant factor 3 In 76% of claims, death or brain damage occurred. By far, the breathing circuit was the major culprit (39%), with a misconnection or disconnection leading to hypoventilation or barotrauma. In 17% the ventilator was not activated correctly, or it delivered an excess tidal volume (Vt) or inspiratory pressure (Pi). The vaporizer (21%) contributed to an overdose, or to cases of awareness, while high-pressure gas supplies (11%) injured patients if connected directly to the patient's respiratory system. This lecture will include those modifications of various machine components, which might attenuate equipment or human failure. II. OBSOLESCENCE and LIMITATIONS OF CONVENTIONAL ANESTHESIA MACHINES Definitions. " Modern " machines might simply be defined as those with computerized feedback modulation (or warnings) of fresh gas (FG) supply, agent delivery, ventilation, and machine checkout. Obsolescence. A panel of experts within the ASA Committee on Equipment and Facilities has recently published guidelines for determining obsolescence of conventional machines 4. Protection against barotrauma. Users may neglect to manually pre-set breathing circuit pressure limiters, while some devices will only generate an alarm. Activating the oxygen flush during inspiration might cause barotrauma. A true limiter will end the inspiratory (I) phase, or it will complete the I phase at the pressure limited value, perhaps limiting Vt. Pressure sensors not located near the patient's trachea might give misleading airway pressure (Paw) data. Some APL valves are designed as variable resistors, instead of regulators, with Paw proportional to flow and they cannot be fully opened instantaneously. Standing bellows and scavengers with dirty valves cause PEEP. Vaporizer risks. Variable bypass vaporizers are either fixed-mounted or removable. If tilted, liquid agent could enter the bypass chamber, and deliver an overdose. Inadequate detection of leaks 6 around the vaporizer o-rings might decrease volatile agent delivery and cause awareness. Advanced ventilation features. There may not be adequate flow generation, integrated, variable PEEP, inverse ratio ventilation (IRV), nor the sensitivity to ventilate neonates; No pressure controlled ventilation …