Letters to the Editor: Letters & Announcements
Moens et al.1 are correct and our statement that “Dead space was calculated using the Bohr equation (alveolar dead space [VDS]/VT = [PaCO2 − PECO2]/PaCO2)” should have read “Dead space was calculated using a modification of the Bohr equation (VD/VT = [PaCO2 − PECO2]/PaCO2).”2 VD is total dead space rather than alveolar dead space.This incorrect equation also appears in the abstract; again, it was a modification of the Bohr equation to measure total dead space. We also used the term pulmonary dead space, which can be found in the literature but is more frequently referred to as total dead space.
Other than the mistake above, the terminology was used correctly. To be clear, physiologic dead space, also known as total dead space, can be divided into anatomic dead space (volume of gas in conducting airways) and alveolar dead space (volume of gas ventilating nonperfused alveoli).3 Anatomic dead space can be modified by tracheal intubation, and the addition of tubing between the endotracheal tube and the circuit's Y-piece. This modification of anatomic dead space is referred to as apparatus dead space. Using the term apparatus dead space in our article was important specifically because it was the parameter that we manipulated.
The abbreviations commented on (except for VDS mentioned above) were clear. In Figure 2, VDc is explained to be the subject's baseline measurement of dead space, to differentiate it from the dead space measurements after the apparatus dead space was added. It was labeled VDc in the figure, but VDC in the figure legend. We agree that it is indeed difficult to judge the results unless one is very familiar with the subject—true for many complicated studies.
Moens et al.1 comment that a randomized study with a larger, more uniform population with true measured alveolar dead space using volumetric capnography would provide more insights to the real clinical value of our method is a reasonable suggestion for future research. The intent of the current study was to simply discover whether a change in dead space could be detected using our simple technique. We agree that the study population was small, but the effect was large so the sample size was adequate.
We disagree that our technique requires an additional capnograph and significant modification of the anesthesia circuit with a potential for leaks. Although we used 2 capnometers in the study, we have also used a single capnograph by placing a 4-way stopcock at its sample inlet. In this situation, we continuously sample airway gasses and intermittently switch to sampling bellows gasses. Furthermore, the modification of the circuit takes seconds to complete, and we never had a noticeable leak in this study, or the previous study.4
John J. Badal, MD
Department of Anesthesiology
University of Arizona
1. Moens YPS, Tusman G, Mosing M, Böhm S. Measurement of dead space during anesthesia. Anesth Analg 2011;113:665
2. Badal JJ, Chen KJ, Loeb RG. Measurement of dead space in subjects under general anesthesia using standard anesthesia equipment. Anesth Analg 2011;112:375–7
3. Wilson WC, Benumof JL Respiratory physiology and respiratory function during anesthesia. In: Miller RD ed. Miller's Anesthesia. 6th ed. Philadelphia: Elsevier Churchill Livingstone, 2005: 697
4. Badal JJ, Loeb RG, Trujillo DK. A simple method to determine mixed exhaled CO2 using a standard circle breathing circuit. Anesth Analg 2007;105:1048–52