Capnograph Questioned

Joseph Antognini, M.D.

To the Editor

The review of the hazards of older equipment by Ms. Nichols in December’s issue was very good. However, I feel that an error was made regarding the effect of high gas flows on the capnographs from a machine with an incompetent expiratory valve if flows are high enough, the inspiratory value for carbon dioxide will return to the baseline. This can be confirmed in everyday practice by preventing the proper seating of the expiratory valve with a small piece of folding paper, or, in the extreme, by removing the valve altogether. When the flow rate is varied, one can observe the changing capnograph.

The above findings should not be surprising, since a circle system will behave much like a Mapleson D system, the latter of course having a capnograph highly dependent on gas flow.

Joseph Antognini, M. D.

Sacremento, CA