Parameters influencing the response time of volatile anesthetics monitors

In a given inhalational anesthetic analyzer, response (RT) is usually thought to be a constant value, however, several factors may influence RT. RT's measured under ideal conditions for the Beckman LB 2, the Normac (Datex), the Servo S 120 (Siemens) and the Irina (Dräger) were 107 +/- 5, 589 +/- 14, 538 +/- 30, and 166 +/- 15 msec, respectively. In addition, we investigated the RT of a Beckman LB 2 analyzer under conditions which may occur in clinical practice (non ideal conditions). Increasing aspirating flow (AF) resulted in shorter RT's, the effect being most pronounced when AF was below 200 ml/min. Interposing a filter prolonged RT by 80%. The type of the inhalational anesthetic (halothane or isoflurane), humidity and temperature of the carrier gas as well as size and direction of the concentration step change did not influence RT. Increasing length or internal diameter (ID) of the sample tube resulted in longer RT's. Changing the sample tube material from glass to Teflon or polyethylene resulted in a slight increase of RT, but the increase was dramatic when rubber or silicone tubes were used. The partition coefficient of halothane in the material of a particular sample tube was directly correlated to the corresponding RT in this sample tube. The influence of different sampling places was studied by interposing copper or corrugated rubber tubing between the place where the concentration step change occurred and the place where the gas was sampled, the measured time was called total response time (TRT). Using corrugated rubber tubes instead of copper tubes increased TRT two to four times. More distal gas sampling and/or lower flow rates caused longer TRT's. Compared with sampling in its center, gas sampling near the walls of the tube resulted in an increase in TRT of 13-45%. It is concluded that the response time of an infrared inhalational anesthetic analyzer is not a constant parameter, but varies between 100 and 4000 msec depending on the characteristics of the analyzer, the sample line, and the place of gas sampling.