Letters to the Editor: Letter to the Editor
We thank Dr. Drummond for his comment on our study recently published in Anesthesia & Analgesia.1 He is completely right regarding his concern about the confusion between the area under the curve of the receiver operating characteristic (ROC) plot and the area under the volumetric capnogram that appeared in our article. This is an editorial erratum that was not included in our original submitted manuscript that we must have unfortunately missed when checking the manuscript proofs. In that last submitted version of the manuscript, the questioned sentence could be read as: “The device provides the CO2 eliminated per minute (Vco2 in milliliter per minute) multiplying the area under the curve of the volumetric capnogram by the respiratory rate.”
Thus, in the original manuscript, it was impossible to confuse the meaning of the area under the volumetric capnogram with the area under the receiver operating characteristic curve (AUROC).2 In our study, the AUROC represents the accuracy of ΔVco2 to detect fluid responsiveness according to the Δcardiac index measured by the reference method of thermodilution. Along the manuscript, tables, and figures, those “areas under the curves” were differentiated clearly between them, confirming that what Dr. Drummond highlighted was an erratum.
The area under the curve of the volumetric capnogram represents the volume of CO2 eliminated in one breath. This is the main volumetric capnographic parameter from which all derived calculations come from, such as the mixed expired of CO2 fraction/partial pressure (Feco2/Peco2), mean alveolar CO2 fraction/partial pressure (FAco2/PAco2), dead space, and alveolar ventilation.3 As Dr. Drummond correctly points out, multiplying the area under the volumetric capnogram by the respiratory rate gives the elimination of CO2 by the lungs, Vco2, expressed in milliliter per minute.
Vco2 is a well-known parameter that can be calculated from the expired CO2 volume as described by many authors using different clinical devices.4–8 Badal et al8 (Ref. 25 in the article) used the NICO calculation of Vco2 as a reference value to compare with the Vco2 derived from the volume of CO2 within the anesthesia ventilator bellows. Our intention was to quote a study performed by other research group using the same sensor and capnography derived calculations that we used in our study to highlight that the NICO monitor (now NM3) is the most common and accepted device to measure volumetric capnography and Vco2 in the clinical field.
We are grateful to Dr. Drummond to have drawn the attention to this error and hope that the journal allows us to correct it in a corrigendum.
Gerardo Tusman, MDDepartment of AnesthesiaHospital Privado de ComunidadMar del Plata, Buenos Aires, Argentinagtusman@hotmail.com
Fernando Suarez-Sipmann, PhDDepartment of Surgical Sciences section of Anesthesiologyand Critical CareHedenstierna LaboratoryUniversity HospitalUppsala, SwedenCIBER de Enfermedades RespiratoriasInstituto de Salud Carlos IIIMadrid, Spain
1. Tusman G, Groisman I, Maidana GA, et al. The sensitivity and specificity of pulmonary carbon dioxide elimination for noninvasive assessment of fluid responsiveness. Anesth Analg. 2016;122:14041411.
2. Hanley JA, McNeil BJ. A method of comparing the areas under receiver operating characteristic curves derived from the same cases. Radiology. 1983;148:839843.
3. Tusman G, Suarez FS, Bohm SH. Rationale of dead space measurement by volumetric capnography. Anesth Analg. 2012;114:866874.
4. Fletcher R, Jonson B, Cumming G, et al. The concept of deadspace with special references to the single breath test for carbon dioxide. Br J Anaesth. 1981;53:7788.
5. Breen PH, Isserles SA, Harrison BA, et al. Simple computer measurement of pulmonary VCO2 per breath. J Appl Physiol. 1992;72:20292035.
6. Kallet RH, Daniel BM, Garcia O, et al. Accuracy of physiologic dead space measurements in patients with ARDS using volumetric capnography: comparison with the metabolic monitor method. Respir Care. 2005;50:462467.
7. Tusman G, Suarez FS, Borges JB, et al. Validation of Bohr dead space measured by volumetric capnography. Intensive Care Med. 2011;37:870874.
8. Badal JJ, Loeb RG, Trujillo DK. A simple method to determine mixed exhaled CO2 using a standard circle breathing circuit. Anesth Analg. 2007;105:10481052.