Sequence-dependent anesthesia-controlled times: a retrospective study in an ophthalmology department of a single-site hospital.

Background: Anesthesia-controlled time (ACT) generally refers to the time durations before and after the period of surgery. The ACT is typically dependent on the sequence of 2 consecutive surgeries and thus adds to the complexity of operating room scheduling. We report a study on sequence-dependent ACTs at the West China Hospital (WCH), focusing on elective surgeries (also referred to as "procedures" below) performed by the ophthalmology department of WCH over a 5-year period, 2007 to 2012.

Methods: ACTs associated with 4 high-volume procedures: phacoemulsification, vitrectomy, strabismus correction, and glaucoma filtration. A total of 29,452 cases were studied, classified into 4 groups according to the sequence of the procedures involved. Specifically, P-P plots were used to determine the distributions of the ACTs, Kruskal-Wallis H test, Nemenyi test, and Student t test were performed to examine the sequence-dependent nature of the ACTs, and the t test was also applied to examine the advantage of sequencing the same procedures consecutively. Permutations were enumerated to identify the best sequence when different procedures were involved. Monte Carlo simulation was used to compute the total completion time, ACTs plus surgical periods, of any given sequence of procedures.

Results: We confirm via statistical tests that the ACTs follow lognormal distributions, and identify their corresponding means and variances. Furthermore, we verify that the ACTs are statistically different in means: they are sequence dependent in general. Using statistical tests, we conclude that it is best to sequence identical procedures consecutively, and we also identify the best sequence involving different procedures. Using Monte Carlo simulation, we compared the daily completion times using the best sequences we have identified against actual data from WCH over a 2-year period; the average reduction is 4.7% (with a standard error (SE) of ± ± 0.5%).

Conclusions: ACTs are usually sequence dependent and hence should be considered in operating room scheduling. Although identifying the best sequence in general is a difficult optimization problem, in certain departments (such as the ophthalmology department of WCH) where a set of high-volume small-variety procedures is present, the best sequences can be systematically identified using a combination of statistical tests and Monte Carlo simulation as illustrated in this study.