Respiration-related renal motion may reduce targeting stability during flexible ureteroscopic laser lithotripsy and thereby decrease lithotripsy efficiency. This single-center, three-arm randomized controlled trial evaluated whether transient apnea during active lithotripsy could improve lithotripsy efficiency while maintaining short-term physiologic safety. A total of 150 patients undergoing flexible ureteroscopic lithotripsy for renal stones were randomized in a 1:1:1 ratio to regular mechanical ventilation, small tidal-volume ventilation, or transient apnea. The primary outcome was active lithotripsy efficiency, defined as CT-based stone volume divided by active lithotripsy time. Physiologic safety was assessed using serial arterial blood gas measurements at baseline, 3, 6, and 9 minutes during the lithotripsy phase and 3 minutes after resumption of ventilation, together with continuous intraoperative cardiopulmonary monitoring.

Respiration-related renal motion is a common technical challenge during flexible ureteroscopic laser lithotripsy. Movement of the kidney and collecting system may reduce laser targeting stability, interrupt continuous lithotripsy, and decrease active lithotripsy efficiency. Ventilation strategies that reduce respiratory motion may therefore improve operative stability, but they must be evaluated together with physiologic safety.

This study was designed as a single-center, prospective, three-arm, parallel-group randomized controlled trial. Adult patients scheduled for elective flexible ureteroscopic laser lithotripsy for renal stones under general anesthesia were screened for eligibility. After informed consent and eligibility confirmation, participants were randomized in a 1:1:1 ratio to one of three intraoperative ventilation strategies: regular mechanical ventilation, small tidal-volume ventilation, or transient apnea during active laser lithotripsy.

In the regular mechanical ventilation group, standard controlled ventilation was maintained during lithotripsy. In the small tidal-volume ventilation group, a reduced tidal-volume strategy was used during the lithotripsy phase to decrease respiration-related renal motion while maintaining clinically acceptable oxygenation and ventilation. In the transient apnea group, apnea was initiated during active laser lithotripsy after adequate preoxygenation and confirmation of hemodynamic stability by the anesthesiologist. Apnea was discontinued if any prespecified safety criterion occurred, including SpO₂ <90%, systolic blood pressure >160 mmHg or <80 mmHg, heart rate <50 beats/min, obvious arrhythmia, hemodynamic instability, or any safety concern from the attending anesthesiologist.

The primary efficacy outcome was active lithotripsy efficiency, defined as CT-based stone volume divided by active lithotripsy time. Secondary outcomes included active lithotripsy time, total operative time, postoperative stone-free status, residual stone burden, physiologic changes on arterial blood gas analysis, protocol-defined apnea interruption, and postoperative complications. Stone-free status was assessed by CT, and non-stone-free status was defined as any residual fragment >2 mm. Arterial blood gas measurements were obtained at baseline, 3 minutes, 6 minutes, and 9 minutes during the lithotripsy phase and 3 minutes after resumption of ventilation.

Inclusion Criteria:

1. Age 18 to 80 years.
2. Scheduled for elective flexible ureteroscopic laser lithotripsy for renal stones under general anesthesia.
3. Preoperative CT confirming renal stones suitable for flexible ureteroscopic lithotripsy.
4. Stone size ≥10 mm and <20 mm.
5. American Society of Anesthesiologists physical status I to III.
6. Ability to provide written informed consent.

Exclusion Criteria:

1. American Society of Anesthesiologists physical status IV or V.
2. Stone size <10 mm or ≥20 mm.
3. Multiple renal stones requiring a combined or alternative surgical strategy.
4. Radiolucent stone or inadequate radiographic assessment.
5. Uncontrolled urinary tract infection.
6. Reduced pulmonary function, including FEV1/FVC <70%, or other clinically relevant pulmonary dysfunction judged unsuitable for transient apnea.
7. Significant cardiopulmonary instability or anesthetic contraindication to transient apnea.
8. Pregnancy or planned pregnancy during the study period.
9. Planned combined procedure or alternative surgical strategy.
10. Incomplete preoperative assessment.
11. Declined participation after counselling.
12. Withdrawal of consent before randomization.

Individual participant data will not be publicly shared because the dataset contains patient-level perioperative, imaging, and physiologic monitoring data, and public sharing was not included in the original informed consent. Deidentified data underlying the published results may be made available from the corresponding investigator upon reasonable request and after institutional ethics approval.

The surgeon and anesthesiology team were not masked because the assigned ventilation strategy was apparent during the procedure. Postoperative CT images were assessed by outcome assessors who were not involved in the intraoperative ventilation protocol and were masked to group allocation whenever feasible.