To the Editor

In response to the letter by Jurgen Link, “What is Value of FiO2 Monitoring Compared to Pulse Oximetry?” (APSF Newsletter, Summer 1992, P.19), we would like to make the following points:

Although pulse oximetry is a valuable tool in the operating room, it is less than ideal for the detection of hypoxic gas administration for the following reasons:

1. Pulse oximetry measures the patient’s response to hypoxia, whereas an oxygen analyzer allows the anesthesiologist to detect a low FiO2 before the patient is affected.

2. Pulse oximetry is very non-specific, and hypoxic gas delivery (as can occur with pipeline cross-connections of misfilled cylinders) is generally not the first thing one thinks of when the SpO2 starts to decrease. A variety of factors, most of which are far more common than problems with oxygen delivery, will cause a decrease in the SpO2. These include circulatory and pulmonary pathology, mechanical problems with gas delivery (obstructed endotracheal tubes or disconnections), methmoglobinemia or intravenous dyes, and numerous sources of artifact. By the time one has ruled out the more likely causes of a decrease in SpO2 prolonged, severe hypoxia may ensue.

3. An oxygen-nitrous proportioning system will work only if there is oxygen flowing through the oxygen pipes and nitrous flowing through the nitrous pipes. Case reports in the anesthesia literature describe how pipelines may be crossed in the bulk supply system or in the anesthesia machine itself,” or how cylinders can be misfilled or attached to the wrong yoke of the anesthesia machine.’,’ An in-line oxygen analyzer is gas specific instead of flow dependent, and can therefore identify potential disasters such as these.

Oxygen analyzers utilizing a Clark electrode are placed in the inspiratory limb because the moisture in expired gas can affect the accuracy of the monitor.

We sympathize with the financial problems, but do not believe that pulse oximetry is a replacement for oxygen analysis. Argument can be made that mass spectroscopy also being gas specific, could be a replacement, but that wouldn’t help with the financial problem. We believe that both an oxygen analyzer and pulse oximetry are essential for patient safety.

Judith T. Hutchinson, M.D. Elizabeth Lee, M.D.

Department of Anesthesia

University of Maryland Medical System Baltimore, MD

References

1. Tingay MG, et al. Gas identity hazards and major contamination of the medical gas system of a new hospital. Anesth Intensive Care 1978; 6:202-209.
2. Arrowsmith LWM. Medical gas pipelines. Eng Med 1979; 8: 247-249.
3. Lane GA. Medical gas outlets A Hazard from interchangeable ‘quick-connect’ couplers. Anesthesiology 1980; 52: W87.
4. Bonsu AK, Stead AL. Accidental cross-connection of oxygen and nitrous oxide in an anesthesia machine. Anesthesiology 1982; 38: 767-769.
5. Feeley TW, et al, Potential hazards of compressed gas cylinders. Anesthesiology 1978; 48: 72-74.
6. Holland R, Foreign correspondence: Another wrong gas incident in Hong Kong. APSF Newsletter 1991; Spring P.9.