The primary objective of this double blind randomized study is to determine the effect of dexmedetomidine on onset and duration of the sensory and motor block and on the influence on bladder function after spinal anesthesia with chloroprocaine. Our primary hypothesis is that sensory block will significantly be prolonged by dexmedetomidine administered by both the spinal and the intravenous route. The investigators do not expect a significant difference in motor block duration between intravenous or spinal administration. Secondary outcomes are micturition problems, duration of analgesia and the occurrence of side effects such as sedation and hypotension.

The study is designed as a multicenter study and randomization and inclusion of patients will be performed in three hospitals. Each hospital has its own randomisation sequence and coordinating investigator who will be responsible for appointing blinded and unblinded participators in the study.

Patients scheduled for spinal anesthesia in ambulatory surgery will be randomized by sealed envelopes (computer generated sequence) into one of the following three study groups:

spinal chloroprocaine 40 mg with spinal dexmedetomidine (5mcg) spinal chloroprocaine 40 mg with intravenous dexmedetomidine ( 0.5 mcg/kg) spinal chloroprocaine 40 mg alone (control group)

All patients will receive the same amount of spinal solution, containing chloroprocaine 1% (4 ml) with dexmedetomidine (0.5 ml, 1 mcg/ml) or NaCl ( sodiumchloride) 0,9% (0.5 ml ) ( and 50 ml intravenous Nacl 0.9, containing dexmedetomidine (0.5 mcg/kg) or not depending on the study group. All solutions are prepared by an independent anesthesiologist and labelled as Intravenous study medication and spinal study medication. Spinal anesthesia and data registration will be done by a blinded anesthesist and/or study nurse on a data sheet designed for the study.

Spinal anesthesia is performed in the lateral decubitus at L3-L4 vertebral interspace and standard monitoring for this procedure is applied. Insufficient analgesia will be treated with sufentanil increments. Hypotension and bradycardia will be treated with ephedrine and atropine. Patients will be discharged when the block is regressed, there is no sedation or nausea and nrs scores of pain are below 4. Patients will be contacted by telephone after one week to evaluate any postoperative complaints.Micturition will receive a score based on bladder volumes measured by bladder scanning and subjective complaints . This score system was used in two of our former studies (1,2)

To calculate the power of this study, we used the mean L2 regression of a previous study after spinal chloroprocaine with its standard deviation (SD 20 min, mean 90 min) and compared it to a mean increase with 30 %. We calculated that we should include 7 patients in each group to obtain a power of 80%.

To be sure to detect a possible difference in discharge time as well, we calculated a sample size of 48 patients for each group to detect a 30-minute difference.

Statistical analysis will be done with ANOVA analysis , A kruskal-Wallis of Mann Whitney test depending on the distribution of the population and if the values are parametric or not. This will be done by spss.

Inclusion Criteria:

• ASA (American Society of Anesthesiologists) class 1 & 2,
• undergoing day-case knee arthroscopy

Exclusion Criteria:

• micturition problems,
• neurological history or
• previous lower abdominal surgery with an abnormal micturition