The Effects of Inflow, Overflow and Valve Placement on Economy of the Circle System

Abstract

Several arrange -merits of inspiratory and expiratory valves and of overflow location in a circle absorber system were tested to determine which arrangement best conserved fresh gas and preferentially eliminated alveolar gas. The influences of inflow rate, deadspace, tidal volume and alveolar ventilation were also examined. During spontaneous ventilation, the most economical arrangements (most alveolar gas eliminated at a given inflow rate) were those with the overflow close to the patient. However, with 1 exception, when ventilation was controlled, the arrangements with overflow near the patient became least economical. The exception was the arrangement with both inspiratory and expiratory valves close to the patient and the overflow valve immediately downstream from the expiratory valve. This was the most economical of all the arrangements tested. Economy was directly related to inflow rate and indirectly related to alveolar ventilation in all cases. When the overflow valve was distant from the patient, concomitant increases in deadspace and tidal volume (alveolar ventilation unchanged) reduced economy. However, these increases of deadspace and tidal volume had no effect on economy when the overflow was close to the patient.