Normal vs Low Tidal Volume Ventilation in Retrograde Intrarenal Surgery

Retrograde intrarenal surgery (RIRS) is a minimally invasive endourological procedure widely used for the treatment of renal stones. Because the procedure is performed within the narrow collecting system of the kidney, endoscopic image stability is important for accurate laser targeting, efficient lithotripsy, and procedural safety. During general anesthesia, positive-pressure ventilation may cause diaphragmatic and respiratory-related renal movement, resulting in micro-movements of the endoscopic image. These movements may interfere with continuous laser targeting and may reduce procedural efficiency.

This study is designed as a single-center, prospective, randomized, parallel-group, assessor-blinded controlled trial. Adult patients scheduled for elective RIRS under general anesthesia will be enrolled after obtaining written informed consent. Participants will be randomized in a 1:1 ratio to one of two intraoperative ventilation strategies: normal tidal volume ventilation or low tidal volume ventilation. Randomization will be performed using a computer-generated randomization list, and allocation concealment will be ensured with sequentially numbered, opaque, sealed envelopes. The video assessors will be blinded to group allocation.

After induction of general anesthesia and endotracheal intubation, mechanical ventilation will be standardized according to the assigned group. In the normal ventilation group, tidal volume will be set at 6-8 mL/kg predicted body weight with 5 cmH₂O PEEP. In the low tidal volume ventilation group, tidal volume will be set at 4-6 mL/kg predicted body weight with 5 cmH₂O PEEP. In both groups, respiratory rate will be titrated to maintain EtCO₂ within the target range of 35-45 mmHg, and FiO₂ will be adjusted to maintain SpO₂ above 94%. Patient safety will take priority throughout the study, and ventilation settings may be modified if clinically required. Any protocol deviations will be recorded.

Intraoperative endoscopic videos obtained during routine RIRS will be used for the video-based analysis. After the procedure, video files will be anonymized and coded in a way that does not reveal group allocation. A standardized 60-second segment in which the stone is clearly visible and/or laser lithotripsy is active will be selected according to predefined criteria. The selected segments will be analyzed using a phase-correlation-based image registration method. Sequential video frames will be converted to grayscale, mild noise reduction will be applied, and horizontal and vertical image displacement between consecutive frames will be calculated. The Motion Index will be derived from these displacement components for each frame pair, generating a time series for each patient.

The primary outcome of the study is the mean Motion Index value calculated from the standardized intraoperative video segment. Secondary outcomes include additional Motion Index parameters, including median, 95th percentile, and maximum Motion Index values; laser activation time; anesthesia duration; surgical duration; intraoperative respiratory and hemodynamic parameters such as EtCO₂, SpO₂, mean arterial pressure, heart rate, airway pressures when available, and vasopressor requirement; perioperative complications; and stone-free status at routine postoperative follow-up, planned at approximately 4 weeks.

The study aims to determine whether low tidal volume ventilation improves endoscopic image stability during RIRS without compromising respiratory or hemodynamic safety. The findings may contribute to the standardization of anesthetic ventilation strategies during endourological stone surgery and may provide an objective, reproducible method for evaluating respiratory-related image motion during RIRS.

Inclusion criteria

• Adult patients aged 18 years or older
• Patients scheduled for elective retrograde intrarenal surgery under general anesthesia
• ASA physical status I-III
• Patients who are able to read and understand the informed consent form
• Patients who provide written informed consent

Exclusion criteria

• Patients younger than 18 years
• Pregnancy
• Emergency surgery
• Severe chronic obstructive pulmonary disease, severe restrictive pulmonary disease, or preoperative hypercapnia
• Severe uncontrolled cardiovascular disease
• Active urinary tract infection, sepsis, or untreated infectious focus
• Cases in whom the standardized ventilation protocol is difficult to apply because of morbid obesity
• Patients scheduled for an additional major urological procedure in the same session
• Cases in which an intraoperative video recording of sufficient quality cannot be obtained
• Patients in whom the study protocol cannot be safely continued according to the investigator's clinical judgment