Noll et al.1 remind the reader that intraperitoneal (IP) gas composition may be significantly affected by alveolar gases.2 Concerns for both combustion and embolic phenomena dictate safe laparoscopic practice. Regardless of the inspired gas mixture, the IP gases reflect those delivered from the alveoli through the blood to the peritoneum as well as the continuous replacement of the insufflation gas delivered by a pressure limit device. The ideal composition of IP gases remains to be determined. A recent Cochrane database3 could not establish the relative safety of carbon dioxide (CO2), helium, and nitrous oxide (N2O) in laparoscopic gases. Helium afforded greater hemodynamic stability in low-risk patients, and N2O appeared to confer a degree of analgesia over CO2. In this study, no mention was made of the potential for combustion, which exists most markedly for gas mixtures containing high concentrations of oxygen (O2). Although combustion is possible in the presence of bowel gases such as hydrogen and methane when N2O is present, the clinical significance of this finding is yet to be established. It is to be noted that the time to equilibrium of IP fN2O in the study by Diemunsch et al.2 in 2000 was >8 hours during ventilation with inspired fN2O = 0.66 and that fN2O at 2 hours was 0.29, suggesting that in most cases, the fN2O in peritoneal gases is insufficient to increase ignition hazards. It is the normative practice at our institution to use an inspired gas mixture of air and O2 during laparoscopic procedures, which explains why N2O was not detected in any of our samples. This is not necessarily the case in other institutions, and in many parts of the world, O2 is the only gas available, although nitrogen remains present when an extractor is used as the O2 source. In circumstances in which compressed air is not available as a component of inspired gas, using N2O as a diluent for O2 reduces, but does not eliminate, the potential for combustion, especially during electrocautery and inadvertent release of bowel gases. As noted by Noll et al.,1 in the presence of hydrogen and methane, N2O facilitates combustion.4 Both our study and those referenced by Noll et al. inform the practitioner that the mixture of gases present during laparoscopy represent a balance created by those delivered from the body and the insufflation device and reflect their physical properties, notably perfusion and blood-gas solubility characteristics. The composition of peritoneal gases is dynamic, and efforts to maintain a high level of CO2 during procedures are critical to patient safety. Both gas analysis and venting of the pneumoperitoneum5 will contribute to achieving IP gas concentrations delivered by the insufflation apparatus as intended by the practitioner. Susan P. Taylor, MD, MPH Department of Anesthesiology Children’s Hospital of Wisconsin Medical College of Wisconsin Milwaukee, Wisconsin [email protected] George M. Hoffman, MD Anesthesiology and Pediatrics Children’s Hospital of Wisconsin Medical College of Wisconsin Milwaukee, Wisconsin