To the Editor:
We congratulate Tautz and colleagues1on an insightful case presentation of malignant hyperthermia and systematic analysis of increased end-tidal carbon dioxide. We write to clarify a detail in their analysis that may be misunderstood.
The capnograph/meter is an essential tool for deciphering the etiology of increased carbon dioxide during anesthesia. As a point of clarification, inspiratory and expiratory valve malfunctions in anesthesia breathing circuits do not result in identical capnograms, as shown in figure 1 of the article. The capnogram in the upper left panel of this figure shows increased carbon dioxide with increased inspiratory baseline. Although this is accurate for a stuck expiratory valve, the capnogram of a stuck inspiratory valve is actually quite different, because there is a dampening of the inspiratory downstroke on the capnogram, which does in fact get to zero.2
Consider a circuit with the inspiratory valve removed. In this scenario, the exhaled breath with carbon dioxide-rich gas is exhaled about equally into both limbs of the breathing circuit; therefore, about half of the exhaled tidal volume partially fills the inspiratory limb. With the next breath, the carbon dioxide–rich gas from the inspiratory limb is reinspired first, followed by fresh gas without carbon dioxide. The capnometer thus displays a sluggish inspiratory downstroke (or a β angle greater than 90°).2The inspiratory baseline will therefore return to zero during the second half of inspiration. These capnogram differences may seem subtle but can be critical in the identification of machine fault etiologies.
*University of Florida College of Medicine, Gainesville, Florida. email@example.com