Accuracy of isoflurane delivery by halothane-specific vaporizers

The precision of isoflurane delivery from 4 vaporizers designed and calibrated for halothane was studied. Isoflurane concentration in oxygen (O2) was determined for various vaporizer dial settings (0 to maximum) and O2 flow rates (0.375 to 15 L/min). The effects on vaporizer output of time (settings constant for up to 15 minutes), ambient temperature (15, 22, 30 C), and conditions simulating intermittent positive-pressure ventilation were also studied. The performance of the 4 halothane-specific vaporizers with isoflurane was qualitatively similar and was within the range expected as normal accuracy for halothane delivery in these same vaporizers. At room temperature, isoflurane concentrations were generally greater than dialed percentages with low carrier gas flows and were less than dialed percentages at higher gas flows for most vaporizers. The relationship of halothane output relative to isoflurane measured during similar conditions fits arithmetic coordinates well. Compared with concentrations of halothane, slightly higher concentrations of isoflurane were delivered from 3 halothane vaporizers studied (av 14%, 15%, and 8%). Temporal changes in delivered isoflurane concentration at room temperature were relatively small at low dial and carrier gas-flow rate settings. However, as gas-flow rate and vaporizer dial settings increased, vaporizer output decreased with time. Alterations in accuracy of isoflurane delivery by 1 vaporizer with temperature changes were most pronounced at the higher gas-flow rates, higher vaporizer dial settings, and lowest temperature. There was little effect of simulated positive-pressure breathing on vaporizer output at peak inspired circuit pressures less than 12 mm of Hg. Use of halothane-specific vaporizers to deliver isoflurane has advantages and disadvantages which must be evaluated individually. The delivery of isoflurane by halothane-specific vaporizers is not recommended by anesthetic and vaporizer manufacturers.