Special Communication

RESPIRATORY OF CARBON DIOXIDE Weingarten M. Respiratory monitoring of carbon dioxide and oxy- gen: a ten-year perspective. AND OXYGEN: A TEN-YEAR PERSPECTIVE J Clin Monit 1990;6:217-225 Maxwell Weingarten, MD ABSTRACT.During the past 10 years, instrumentation has been developed that can continuously and noninvasively measure changes in carbon dioxide and oxygen. The information gen- erated, which cannot be obtained through the human senses, provides vital clinical data regarding the effectiveness of intu- bation, ventilation, circulation, oxygenation, and the circuit. This instrumentation plays a major role in decision making both in the safe conduct of and mechanical ventila- tion as well as in the detection and prevention of potentially catastrophic mishaps. For these reasons, a review of what has been learned regarding the instrumentation, collection, and interpretation of the clinical data, and the clinical value of the information is timely. The clinical significance of the carbon dioxide and oxygen waveforms, inspired to expired carbon dioxide and oxygen differences, alveolar-arterial gradients, and global supply-to-demand oxygen relationships measured by , oxygraphy, and pulse oximetry are ad- dressed in this essay. KEY WORDS.Measurement techniques: capnography; oxyg- raphy; pulse oximetry.

During the past 10 years major advances have been made in our ability to monitor and quantify carbon dioxide (CO2) and oxygen (O2) exchange continuously and noninvasively. Since evidence is accumulating that monitoring improves outcome, it may be useful to give a perspective gained from 10 years of monitoring CO2 and 02 during anesthesia and in ventilated patients.

MEASUREMENT OF C02

The introduction of capnography by Smalhout and Kalenda [1] defined the relationship of exhaled CO2 to metabolism, circulation, respiration, and the circuit. Under their direction, capnography survived a stormy gestational period as it reached maturity in The Nether- lands. It was introduced in the United States at a small private meeting sponsored by a major instrument man- ufacturer held in conjunction with the World Congress on in Washington, DC, in May 1978. Five anesthesiologists attended the meeting, two of whom concluded that capnography would prove to be of very little value. Since then, capnography has come to be recognized as an extremely valuable method for the continuous monitoring of respiration and circu- lation in unconscious patients. Recognition of the value From the Department of , St. Francis Hospital, Mil- ofcapnography for the detection and prevention of mis- waukee, WI 53215. haps has grown to the extent that some states (New Received Sep 15, 1988, and in revised formjan 29, 1990. Accepted for York, New Jersey) have mandated its use on all intubated publication Feb 7, 1990. patients.

Copyright 1990 by Little, Brown and Company 217 218 Journal of Clinical Monitoring Vol 6 No 3 July 1990

Capnography refers to the graphic portrayal of the placed in one of the prongs of a nasal 0 2 catheter, or changing concentration of exhaled CO2 during the en- shielded inside a catheter placed in the nose or pharynx. tire respiratory cycle. It also may refer to the interpreta- tion of the waveforms. It should not be confused with INFECTION CONTROL. Sidestream sampling is simpler be- capnometry, which refers to only the digital presentation cause the water trap, connectors, and sampling catheter of the concentration without a waveform. are all disposable. With mainstream detectors the cu- The basis of capnography is rooted in the fact that all vette can be sterilized. mammalian cells, irrespective of their diversity of func- tion, have one common denominator, namely, that the