Perforated appendicitis accounts for 15% - 20% of all cases of pediatric appendicitis [1] [2] . Perforated appendicitis is itself a significant risk factor for postoperative intra-abdominal abscess (PO-IAA), which occurs in 5% - 10% of pediatric patients with perforated appendicitis [2] - [5] . PO-IAA prolongs hospitalization due to long-term antibiotic use, drainage, and reoperation [6] [7] . Therefore, many surgeons have attempted to improve the surgical technique and perioperative management of appendectomy to prevent PO-IAA, and intraperitoneal irrigation has been the subject of much discussion [1] [6] [8] - [13] .

Intraperitoneal irrigation for peritonitis was first reported in 1906, since it has been performed by many surgeons [1] [9] [14] , with only 7% of pediatric surgeons in North America in 2004 not performing intraperitoneal irrigation during appendectomy [15] . We also irrigated the abdominal cavity with a large volume of saline solution during laparoscopic appendectomy (LA) for perforated appendicitis. Several studies have compared irrigation with suction alone, most of which supported the suction-alone approach, indicating that irrigation is ineffective in preventing PO-IAA [8] [10] [16] . However, most studies that supported the suction-alone approach reported irrigation volumes (IV) of 500 - 1000 mL [1] [6] [9] [11] - [13] [17] . We irrigated until the irrigation solution was clear, with an empiric IV of 500 - 1000 mL, which is insufficient. It is important to estimate the abdominal cavity volume since the amount of abdominal cavity irrigation depends on it. Yao et al. measured abdominal cavity volume using 3D CT and reported that the volume correlates with body weight [18] . Additionally, Fischbach et al. stated that the dialysate filling volume should be adjusted according to body surface area in pediatric peritoneal dialysis patients [19] . Therefore, we hypothesized that BW and BSA are factors involved in determining the IV. Contrastingly, few reports have examined the effects of a large IV on PO-IAA. Although several reports have compared irrigation versus suction alone and massive irrigation versus suction alone [1] [6] [8] - [13] [16] [17] , no reports have examined the effect of the IV.

This study aimed to compare the IV between patients with and without PO-IAA. As a secondary objective, we also compared the IV to body weight (IV/BW) and IV to body surface area (IV/BSA) ratios between the PO-IAA and PO-IAA-free groups to identify the parameters determining the IV.

A total of 1136 patients were hospitalized and treated for appendicitis between January 2014 and December 2023, and 369 patients underwent LA. Of these, 69 patients were enrolled based on the operative findings ( Figure 1 ). Forty-nine patients were assigned to the PO-IAA-free group and 20 to the PO-IAA group.

The results of this study showed that patients without PO-IAA had a significantly larger IV than those who developed PO-IAA. In terms of the secondary outcomes, IV/BW and IV/BSA were also significantly higher in the PO-IAA-free group. The lack of any significant difference in operative time or intraoperative blood loss between the PO-IAA and PO-IAA-free groups indicated that massive intraoperative rinsing could be performed safely. We generated ROC curves to determine if IV was effective for preventing PO-IAA, and the AUC values suggested that cutoff values of IV/BW 234.6 mL/kg and IV/BSA 6352.2 mL/m² may be more effective than IV 6000 mL.

Several previous studies have reviewed the use of intraoperative irrigation in patients undergoing surgery for perforated appendicitis. Shawn et al. reported no significant differences between pediatric patients treated with intraoperative irrigation and suction alone for perforated appendicitis in terms of the incidence of PO-IAA (19.1% and 18.3%, respectively, p = 1.0) and length of hospital stay (5.5 ± 3.0 and 5.4 ± 2.7 days, respectively, p = 0.93) [9] . Zhou et al. reviewed a randomized controlled study that examined the amounts of intraperitoneal washings for intraperitoneal infections. They found no advantages between intraperitoneal lavage compared with suction alone in terms of mortality (0% and 1.1%, respectively; relative risk [RR] 0.31, 95% confidence interval [CI] 0.02 - 6.39], intra-abdominal abscess (12.3% vs. 11.8%, respectively; RR 1.02, 95% CI 0.70 - 1.48; I² = 24%), incisional surgical site infections (3.3% vs. 3.8%, respectively; RR 0.72, 95% CI 0.18 - 2.86; I² = 50%), postoperative complications (11.0% vs. 13.2%, respectively; RR 0.74, 95% CI 0.39 - 1.41; I² = 64%), reoperation (2.9% vs. 1.7%, respectively; RR1.71, 95% CI 0.74 - 3.93; I² = 0%), and readmission (5.2% vs. 6.6%, respectively; RR 0.95, 95% CI 0.48 - 1.87; I² = 7%) [13] . Several other studies found that irrigation did not produce superior results in terms of PO-IAA prophylaxis compared with suction alone [1] [3] [6] [12] [17] . Similar to the current results. However, Fengbo et al. and Melanie et al. studied intraoperative massive irrigation in patients with perforated appendicitis (>2 L and 3 - 12 L, respectively) and found that both were effective in preventing PO-IAA [10] [16] .

Intraperitoneal lavage has been considered to be ineffective for several possible reasons: 1) bacteria may adhere to the peritoneal mesothelial cells, such that irrigation cannot decrease the microorganism load on the peritoneum; 2) irrigation may cause bacterial dislocation and diffuse or remote inoculation, leading to contamination by spreading microorganisms; and 3) irrigation may dilute mediators of phagocytosis, such as opsonic proteins and immunoglobulins [13] . However, IV in previous studies that recommended suction alone, rather than irrigation, was 500 - 1000 mL in each case, which was insufficient compared with the current IV, and we therefore assumed that irrigation was ineffective because of insufficient IV [1] [6] [9] [11] - [13] [17] . However, the results of this study suggest that massive irrigation is effective. We speculate that massive irrigation may reduce the number of bacteria in the abdominal cavity and dilute inflammatory mediators. The strength of this study lies in its finding that massive irrigation may effectively help to prevent PO-IAA, as a major complication of perforated appendicitis. Additionally, the generated ROC curve can estimate the amount of IV required; because pediatric appendicitis can occur at various ages, from infancy to adolescence, the definition of massive irrigation may vary depending on the patient’s size. Additionally, massive irrigation was also shown to be a safe procedure, as intraoperative blood loss and the incidence of SSI did not significantly differ between the two groups.

One limitation of this study was its retrospective case-control design. Therefore, due to the nature of data collection, potential biases may arise. Furthermore, since this study is based solely on univariate analysis, we believe that various biases can be eliminated by adding more cases and performing multivariate analysis. Additionally, the study did not compare the results with the outcomes of suction alone, which has been the standard procedure in many studies. Further studies are therefore required to compare suction alone with massive irrigation.

Massive intra-abdominal irrigation may help to prevent PO-IAA in children undergoing LA for perforated appendicitis. Additionally, IV/BW and IV/BSA may be indicators that determine the IV.

We thank Susan Furness, PhD, from Edanz ( https://jp.edanz.com/ac ) for editing a draft of this manuscript.