To the Editor:-When using closed circle system anesthesia and an anesthesia gas monitor, one may decide to return the exhaust gases of the monitor to the circuit to maintain a totally closed system. When we did this as a part of a study of closed system oxygen and isoflurane or desflurane using POET II or IQ anesthetic gas analyzers (Criticare Systems, Waukesha, WI), we found that the circuit nitrogen concentration gradually increased more than we would have expected from excretion of nitrogen from the body stores.

We learned that a major source of the nitrogen was the air added to the sample gas before it is exhausted from the anesthetic gas analyzer. The vacuum pump maintains fine regulation of the desired 50 or 150 ml/min aspiration rate by drawing largely from the sample port and partly from air inside the monitor. The air is added after the sample is analyzed. When the "Auto Cal in Progress" is activated, circuit gas is sampled more rapidly for 15-20 s, and a higher exhaust gas flow also occurs.

Exhaust gas volume also exceeds sample gas volume with the Raman scattering analyzer (RASCAL II, Ohmeda, The BOC Healthcare Group, Edison, NJ). The exhaust gas contains air during regular use and argon during the calibration procedure. Sampling from the patient is interrupted during calibration. The RASCAL II has a single nominal aspiration rate of 210 ml/min.

In a small survey of 3 POET II, 3 POET IQ, and 2 RASCAL II monitors used daily in our operating rooms, we obtained the following data. With the POET monitors, exhaust exceeded sample flow rate by 4-19 ml/min at the 50 ml/min setting and 9-19 ml/min at the 150 ml/min setting. With the two RASCAL II monitors, the exhaust exceeded sample flow rate by 16 and 23 ml/min. When sampling oxygen only with the POET analyzers, nitrogen concentration of the exhaust gas ranged between 9.3% and 19% at the 50 ml/min setting and 11% and 18% at the 150 ml/min setting. With the RASCAL IIs, the exhaust gas nitrogen values were 5.9% and 11%.

The monitors functioned at or very near manufacturer's specifications. However, returning the monitor exhaust to the circuit constantly adds air to the circuit. With the RASCAL II, a small amount of argon also is added during its calibration cycle.

Wendell C. Stevens, M.D., J. A. Nash, M.B., Ch.B., Richard Bunney, A.B.E.T., Department of Anesthesiology, Oregon Health Sciences University, Portland, Oregon 97201.

(Accepted for publication August 29, 1996.)