Abstract
First, I wish to express my gratitude to Dr. Linjie Peng for his letter to the editor that brings our study to the readers' attention once more. 1 We believe that all of the issues raised in that letter had been well addressed in the discussion section of the study. However, it appears that there are some misunderstandings regarding the clinical significance of mildly elevated values of end-tidal CO2 (EtCO2) that led to Dr. Linjie's reservations. Therefore, these issues merit some clarifications and extended discussion. The standard limits for EtCO2 are 35–45 mmHg. Although EtCO2 is used as a marker of ventilation, mildly elevated levels do not necessarily reflect hypo-oxygenation and hypercapnia, nor do they increase the risk of related deleterious side effects. To the contrary, well-monitored permissive hypercapnia approaches can be beneficial by improving tissue oxygenation, skin blood flow velocity, cardiac index, and muscle oximeter saturation, all of which are effects that may increase resistance to surgical wound infections. 2 In their analysis of hypercapnia, Akça and colleagues 2 assessed a group of subjects exposed to EtCO2 of 60 mmHg for 45 minutes and none sustained any adverse effects. Those authors enumerated many other beneficial effects associated with “permissive hypercapnia” during general anesthesia.
In the setting of respiratory kidney movements (RKMs), there is no doubt that treating a stable target is more efficient than a moving target. One-lung intubation allows for a stable intrathoracic surgical field in complex lung surgery; jet ventilation is used for difficult bronchoscopic procedures to reduce lung movement. Moreover, even the heart is stopped by means of a cardiovascular pump to facilitate completion of coronary bypasses and other open heart procedures. In endourology, the efforts to reduce the movement of an intrarenal target were twofold. One was by interfering with the RKMs 1,3,4 and the other was by reducing retropulsion during holmium laser lithotripsy (e.g., Moses technology by Lumenis, Vapor Tunnel™ and Master Pulse™ by Quanta, and thulium fiber SuperPulse by Olympus, etc.). Although retrograde intrarenal surgery (RIRS) is well established, there is still room for refinement and improvement. The surgeon is free to decide whether to adopt novel approaches or to continue using a traditional approach. I strongly believe that the aims of an endourologic procedure are stone free, no intraoperative complications, short operating time, and short hospitalization with no complications. There are various ways to achieve these goals, but we should continuously aspire to progress, improve, and evolve.
When it comes to analyzing studies, we should take into consideration not only their content but also the level of evidence (LE)-based medicine. 5 The article published by Emiliani et al. 3 on the induction of apnea up to 5 minutes to stop RKMs is intriguing. It is designed as a point-of-technique article in the section “Surgeon's Workshop” without a formal assessment of a study group, and does not describe any complications related to increased levels of EtCO2 that reach up to 50 mmHg. As such, it is classified as LE 5 (“expert opinion without explicit critical appraisal”). The study by Gadzhiev and colleagues 4 analyzed the feasibility of combining HFJV with small-volume mechanical ventilation on a group of 38 patients treated by RIRS. Although proving feasibility and reporting an EtCO2 of 42 mmHg, the study is limited by subjective assessment of RKMs by means of a nonvalidated survey and lacking a control group (LE 4: case series). This approach implies the use of two systems, specifically, a standard ventilation machine and a HFJV machine, which are resources that are not available in real life, even in the most advanced institutions. Gadzhiev et al.'s 4 work was published after our study, precluding our ability to refer to it. Importantly, our report 1 describes a prospective randomized controlled study and can be categorized as LE 1B (“individual RCT with narrow confidence intervals”). It provides outcomes of a ventilation technique that is well monitored, proven to be safe, and employs a standard ventilation machine that is available in any operating room. We did not encounter any complications related to elevation of the EtCO2 level up to 50 mmHg along a mean period of 54 minutes. This mode of general anesthesia became our anesthetic choice and it has been implemented in all endourologic cases, including retrograde and antegrade percutaneous procedures, since the end of the study.
In conclusion, my opinion is that research for novel approaches should continue even when traditional and established treatments are effectively in use. This is the essence of the continuing evolution of medicine. Since reproducibility is one of the most important processes to confirm the quality of a treatment, I invite Dr. Linjie to implement our approach in his practice and to report his experience with it.
Thank you